Friction
Nano Research
Vol. 5, No. 12, December 2012
ISSN 2223-7690
Vol. 1, No. 2, June 2013
Contents
Guest editorial: Special issue on bio-tribology / 99
Zhongmin JIN, Ming ZHOU
Review
Bio-friction / 100–113
Zhongmin JIN, Duncan DOWSON
Recent advances in gecko adhesion and friction mechanisms and the development of gecko-inspired
dry adhesive surfaces / 114–129
Ming ZHOU, Noshir PESIKA, Hongbo ZENG, Yu TIAN, Jacob ISRAELACHVILI
Skin tribology: Science friction? / 130–142
E. VAN DER HEIDE, X. ZENG, M.A. MASEN
Research Article
Use of opposite frictional forces by animals to increase their attachment reliability during
movement / 143–149
Zhouyi WANG, Yi SONG, Zhendong DAI
Influence of synovia constituents on tribological behaviors of articular cartilage / 150–162
Teruo MURAKAMI, Seido YARIMITSU, Kazuhiro NAKASHIMA, Yoshinori SAWAE, Nobuo SAKAI
Potential hydrodynamic origin of frictional transients in sliding mesothelial tissues / 163–177
Stephen H. LORING, James P. BUTLER
Damage due to rolling in total knee replacement—The influence of tractive force / 178–185
Markus A. WIMMER, Lars BIRKEN, Kay SELLENSCHLOH, Erich SCHNEIDER
Short Communication
Green tribology: Fundamentals and future development / 186–194
Si-wei ZHANG
ⅢI
Friction 1(2): 99 (2013)
DOI 10.1007/s40544-013-0016-0
ISSN 2223-7690
Guest editorial: Special issue on bio-tribology
Zhongmin JIN1,2, Ming ZHOU3
1
School of Mechanical Engineering, Xi’an Jiaotong University, China
School of Mechanical Engineering, University of Leeds, UK
3
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
Received: 30 May 2013
2
© The author(s) 2013. This article is published with open access at Springerlink.com
Tribology plays an important role in engineering as
well as in animal world and our daily life. Since the
first introduction in 1970 [1], bio-tribology has been
widely researched. More recently green tribology has
also received significant attention with the special
attention to our environment and energy consumption.
This Special Issue of Friction is intended to introduce readers to the exciting fields of bio-tribology,
covering not only the fundamental understanding of
the natural biological systems but also the application
to medical interventions, and green tribology. Review
papers, research articles and short communication
are included to demonstrate the breadth and the timeliness of the subject and to provide an opportunity
for the publication of new findings. Eights papers by
tribologists, scientists, biologists and physicians have
been selected to achieve these aims, including three
general review articles on the friction in biological
systems, the adhesion and locomotion in the animal
world, and the skin tribology, one research paper on
the attachment and movement of animals, two papers
on the subject of natural synovial joints and artificial
replacements, one on the lung (pleural), and one short
communication on green tribology.
The first paper by Jin and Dowson reviews the biofriction in a number of biological systems including
synovial joints, eye, pleurae, fat pad, skin, and oral
cavity as well as daily activities associated with shaving,
brushing, slip, etc. The role of friction studies and the
corresponding link with the understanding of the
lubrication mechanisms have been demonstrated.
Zhou et al. reviews the recent advances in gecko
adhesion and friction mechanisms and the development of gecko-inspired dry adhesive surfaces. The
importance of the gecko hierarchical structures, i.e.,
the feet, toes, setae, and spatulae on the adhesion
and friction is discussed, with the emphasis on the
understanding of the corresponding models to
ascertain the mechanical principles involved.
Skin tribology is addressed by van der Heide et al.
The current understanding of skin tribology is still
limited by the living nature of skin, subject and
anatomical sites specific, and simplified test methods.
Current predictive friction models have been shown
to be only partially capable of predicting in vivo skin
friction.
How animals use opposite frictional forces to
increase their attachment reliability during movement
is addressed by Wang et al. These opposite frictional
forces allow many animals to attach securely and
stably during movement. The coordination of different
attachment (adhesion) modes not only helps animals
adhere, but also increases the overall stability of the
attachment (adhesion) system.
The synovia constituents in synovial fluids are
important on the tribological behaviors of articular
cartilage. Murakami et al. investigate the effect of
different synovia constituents on the tribological
functioning of the intact and damaged cartilage tissues,
and the corresponding synergistic actions between
different constituents.
The role of potential elasto-hydrodynamic action on
the frictional transients in sliding mesothelial tissues
in the lung is addressed by Loring and Butler. The frictional variations seen with sliding mesothelial tissues
are found to be consistent with elasto-hydrodynamic
lubrication without direct contact between the sliding
surfaces.
Wimmer et al. consider the damage due to rolling
in total knee replacement—the influence of tractive
force. The importance of the rolling motion and its
combination with sliding of the femoral component on
the wear of the polyethylene tibial plateau is studied.
Tractive rolling has been shown to be an important
wear mechanism.
The closing paper by Zhang introduces the field of
green tribology in a short communication, including
its history, the definition, the objectives, and the
disciplinary features. The technological connotations
of green tribology are discussed comprehensively and
the future directions of this new area are highlighted.
References
[1] Dowson D. Whither tribology. Wear 16(4): 303–304 (1970)
Friction 1(2): 100–113 (2013)
DOI 10.1007/s40544-013-0004-4
ISSN 2223-7690
REVIEW ARTICLE
Bio-friction
Zhongmin JIN1,2,*, Duncan DOWSON2
1
School of Mechanical Engineering, Xi’an Jiaotong University, China
2
School of Mechanical Engineering, University of Leeds, UK
Received: 05 November 2012 / Revised: 07 January 2013 / Accepted: 01 February 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: Friction studies in biological systems are reviewed, including synovial joints (cartilage, meniscus), eye,
pleurae, fat pad, skin, and oral cavity as well as daily activities associated with shaving, brushing, slip, etc. Both
natural systems and medical interventions in terms of diagnoses and artificial replacements are considered.
Important relevant biomechanical, physiological, and anatomical factors are reviewed in conjunction with
friction studies in terms of both methodologies and friction coefficients. Important underlying tribological
mechanisms related to friction are briefly discussed. A unified view on the lubrication mechanism responsible
for the low friction in most soft biological tissues is presented.
Keywords: biofriction; soft tissues; friction
1
A brief historical context
The principles of friction have been utilized for
centuries in our daily life. For example, journal
bearings were used in chariots in China c.2698–2599
B.C. [1]. While in Egypt water or perhaps precious oil
was used as a lubricant for transporting an Egyptian
colossus from the tomb of Tehuti-Hetep, El-Bersheh,
(c.1880 B.C.) as depicted in Fig. 1. This finding was
confirmed from a simple estimation of the friction
coefficient of 0.23 for the model shown in Fig. 1 and
comparison with available modern experimental
measurements of about 0.2 between wet wood [2].
Scientific studies on friction began with Leonardo da
Vinci, as evidently from a number of his drawings.
Subsequently, Amontons, Coulomb, and others
made significant contributions to, and laid the
foundation for, the current understanding of friction.
Nevertheless, as pointed out by the late Professor
David Tabor “friction is easiest to measure, but hardest
to understand” (Private communication, Dowson).
* Corresponding author: Zhongmin JIN.
E-mail: [email protected], [email protected]
Fig. 1 Transporting an Egyptian colossus from the tomb of
Tehuti-Hetep, El-Bersheh, (c.1880 B.C.) [2].
2
Definition
Friction is loosely defined as “the resistance that
one surface or object encounters when moving
over another” in the Oxford Dictionaries
(http://oxforddictionaries.com/definition/english/frict
ion?q=friction). It is interesting to note that the
word “friction” was originated in the mid 16th
century, “denoting chafing or rubbing of the body or
limbs, formerly much used in medical treatment, via
French from Latin frictio(n-), from fricare ‘to rub’”.
Bio-friction can be defined as friction applied to
biological systems, following on a similar definition
of “bio-tribology” by Dowson [3]. It is also noted that
Friction 1(2): 100–113 (2013)
“bio-friction*” or “biofriction*” has been used much less
frequently in the literature (6 hits searched on the Web
of Science on 27th December 2012; as a comparison,
“Bio-tribolog*” or “Biotribolog*” was used in 315 hits
and “bio-lubricat*” or “biolubricat*” in 180 hits).
Friction forces can generate additional stresses that
may become important in contacting bodies. Friction
is generally low in biological systems under normal
conditions, but can become high under adverse
abnormal and diseased conditions. Friction is an
integral part of tribology and is closely related to
lubrication and wear. In general, friction measurement
is much easier to conduct than lubrication and wear.
Therefore friction studies are widely carried out to
reveal the underlying tribological mechanism. It is
important to recognize that a systematic approach
should be adopted in friction studies due to the
close links between friction, lubrication, and wear.
However, it is beyond the scope of the present review
paper to address all these tribological aspects and
therefore only bio-friction studies are reviewed, with
only brief references to related lubrication and wear
mechanisms. The importance of friction in normal
functions, as well as disease developments in selected
natural biological systems, as well as artificial
replacements is covered. Nevertheless, for each of the
biological systems in consideration, it is equally
beyond the scope of the present paper to review
comprehensively all the detailed relevant biomechanical and biotribological studies. Other general
reviews on lubrication and tribology in biological
systems can be found elsewhere [4, 5].
101
forms of a simple linear or circular motion where
the friction between the two bearing surfaces is
measured. In recent times, more and more sophisticated
functional simulators have been developed to mimic
as closely as possible the physiological environments
including loading, motion, and body fluid. Such
developments are particularly evident in friction
studies of natural synovial joints and artificial
replacements as reviewed in Section 4.1.1. Friction is
usually quantified as a coefficient of friction (  ).
There is a large variation in the reported coefficients
of friction in engineering and biological systems due
to the complexity of the underlying tribological
mechanisms. It is often convenient in engineering to
present coefficients of friction with reference to
lubrication mechanisms, including fluid-film, boundary
or mixed lubrication regime as well as the biphasic
lubrication mechanism specifically proposed for
biological tissues (Section 4.1.1). Therefore, some
values quoted in this paper should be taken as
average and representative. For each of the biological
systems considered in this paper, a common
approach to the literature review was taken; the
relevant anatomical structure and physiological/
biomechanical environment were briefly mentioned,
followed by the discussion on the importance of
friction in both normal and abnormal conditions;
selected friction studies in terms of both measurement methodologies and representative values of
coefficient of friction were presented. Finally the
underlying lubrication mechanisms were discussed.
4
3
Biological systems
Methodology
Friction is not itself a fundamental force but arises
from fundamental electromagnetic forces between
the charged particles on the contacting surfaces. It is
generally very difficult to calculate friction from first
principles due to the complexity of these interactions,
despite a number of attempts. For example, molecular
dynamics simulation has been used recently to predict
friction [6]. Instead, friction is usually measured
experimentally. Bio-friction studies are usually carried
out largely through experimental means due to
additional complexities associated with modelling of
biological tissues. Such experiments can take simple
Bio-friction studies are reviewed conveniently, according
to whether the biological system in consideration is
inside or outside the human body.
4.1
4.1.1
Inside the body
Synovial joints
The most important load bearing component inside
the human body is the natural synovial joint. Natural
synovial joints consist of articular cartilage as the
bearing surfaces, bone as the backing materials, and
synovial fluid as the lubricant, in a similar way as the
journal bearing in engineering as depicted in Fig. 2.
Friction 1(2): 100–113 (2013)
102
Table 1 Typical representative friction coefficients in
synovial joints.
References
11
Fig. 2 Comparison of a synovial joint and a journal bearing.
The loading and motion conditions in synovial
joints such as the hip are quite complex. Generally, the
load during walking is transient and the maximum
magnitude can be as high as 4 to 6 times body weight,
while the motion is reciprocating with an average
angular velocity around 2 rad/s. Most friction studies
of synovial joints reported in the literature have
utilized small cartilage specimens under simplified
loading and motion conditions. There are only a
limited number of studies where the whole joint was
considered [7, 8]. It is important to measure the
friction in synovial joints accurately, since the friction
level in these natural bearings is generally low and
also because of additional difficulties associated with
other soft tissues surrounding the joint and mechanical
factors that can contribute to the friction measurement.
Unsworth et al [9] developed a pendulum type
machine where hydrostatic bearings were adopted to
minimize the extraneous mechanical friction. This
type of pendulum friction simulator (both free and
driven) has been widely used for the friction studies
in both natural (hip and knee) and artificial joints
(hip) [10].
Healthy synovial joints exhibit friction coefficient
as low as about 0.002, despite the fact that they are
subjected to a large dynamic load and a reciprocating
motion. Table 1 summarizes representative friction
coefficients measured in synovial joints under various
conditions.
The menisci are known to play important roles in
the normal function and the development of diseases
such as osteoarthritis in the knee joint. Pickard et al.
[15] compared the time-dependent friction between
bovine meniscus and cartilage, both against a stainless
steel plate and found that the friction coefficient for
the meniscus tissue was higher, particularly during
the early stage of loading. The effect of the meniscus
Friction
coefficient
Comments
0.014 to 0.024 Pendulum; cadaveric human ankle
joint; boundary lubrication was
proposed.
12
0.0053
Arthrotripsometer; dog ankle joint;
synovial fluid.
9
0.02
Free pendulum machine with a
hydrostatic bearing; cadaveric hip
joints; fluid film to boundary was
proposed.
13
0.01
Boundary lubrication was proposed.
14
0.01 to 0.5
Cartilage specimen-on-metal; timedependent friction; biphasic lubrication
was proposed.
8
0.02
Driven pendulum machine; bovine
knee joint with cartilage-on-cartilage
and meniscus; biphasic lubrication
was proposed.
on the friction of bovine knee joint was investigated
by McCann et al. [8]. It was shown that the removal
of the meniscus significantly increased the friction
coefficient between the cartilage surfaces from 0.02 to
0.05 as a result of the increased contact pressure. Baro
et al. [16] also found a similar friction coefficient on
the order of 0.02 under migratory contacts and further
showed that the femoral apposing surface tended to
give lower friction than the tibial counterpart. It is
generally accepted that a migratory contact allowed
the re-hydration of the biphasic materials and recovery
of the fluid-load support.
The low friction inside synovial joints is generally
accepted. However, the underlying mechanism is
still not clear. It is probably a combination of various
effective lubrication mechanisms, ranging from
boundary, mixed in the form of biphasic lubrication
to fluid-film lubrication as discussed below [17].
Under normal conditions, the softness of articular
cartilage promotes the formation of fluid films and
this reduces friction markedly. Even when a fluidfilm lubrication regime is not possible, boundary
lubricating constituents of synovial fluid often reduce
friction to a level that is not much different from that
under a full fluid film lubrication condition [11].
Another friction-reduction mechanism is biphasic
lubrication in articular cartilage, which consists of
both fluid and solid phases. Immediately after loading,
the fluid phase inside cartilage is pressurized and
Friction 1(2): 100–113 (2013)
therefore the majority of the load is carried out by the
fluid phase, resulting in low friction [14, 18]. As time
increases, the load is transferred to the solid phase
and friction increases. Under a prolonged period of
loading, boundary lubrication may act as an effective
mechanism to limit friction in synovial joints. Other
lubrication mechanisms proposed for articular cartilage
include hydration or brush, which may be related to
biphasic lubrication [19] or boundary lubrication [13].
The hydration lubrication mechanism in articular
cartilage has received significant attention recently.
The essence of the hydration lubrication mechanism
is a “surface amorphous layer”, also described with
different names such as “gel layer”, “hydration layer”,
or “brush layer”, in which the condroitin- or keratan
sulphates composing the leafs of the proteoglycan
subunit are hydrated [20]. Recent studies by Klein
and colleagues [21] have revealed the remarkable
ability of phosphatidylcholine liposomes to reduce
friction coefficients on atomically smooth mica surfaces
to exceedingly low values of around 10–4 under
physiologically relevant pressures.
Consideration of friction between articulating surfaces has played an important role in the development
of artificial hip joints. The hip replacement designed
by the late Sir John Charnley utilised a material
combination with a minimum friction coefficient
under boundary lubrication (e.g., Teflon (PTFE)).
Combined with a small femoral head diameter of
approximately 22 mm, this gave a low frictional torque;
the principle for the Low Friction Arthroplasty (LFA).
Later on, PTFE was replaced by high density
polyethylene and then ultra high molecular weight
polyethylene, however, the principle of LFA has
remained. Subsequently, it has been shown by
Wroblewski et al. [22] that the loosening rate of
acetabular cups was reduced for a thicker
polyethylene cup, particularly when the linear wear
penetration exceeded 1 mm. This has been explained
on the basis of the shearing stress at the cup/cement
interface resulting from the frictional torque generated
at the articulating surfaces. A decrease in the outside
diameter and an increase in the linear wear
penetration resulted in an increase in the shear stress
and likelihood of loosening. Friction may also have
played an important role in the clinical performance
103
of large diameter metal-on-metal hip implants. The
large frictional torque in these devices under adverse
lubrication conditions due to edge loading and
micro-lateralisation may be responsible for both the
cup and the taper connections loosening and clinical
failures identified recently [23, 24]. Typical friction
coefficients in artificial hip joints with different bearing
surfaces are summarised in Table 2.
4.1.2
Fat pad and tendon
Fat pads are masses of encapsulated adipose tissue,
commonly found and strategically located within the
human body to provide mechanical advantage to the
musculo-skeletal system. Fat pads consist of water,
collagens and proteoglycans as the extracellular matrix,
and numerous unilocular adipocytes (fat cells) that
are swollen with lipid. Fat pads play an important
role in reducing friction in the musculo-skeletal
system as reviewed by Theobald [27]. Under adverse
conditions, high friction may lead to abnormity and
consequently pain. For example, one of the common
causes of anterior knee pain is known as the patellar
tendon lateral femoral condyle friction syndrome.
This is caused by patella maltracking resulting in the
impingement of the superolateral aspect of Hoffa's
fat pad between the inferior patella and the lateral
femoral condyle.
The friction between fat and bone from bovine
tissue was measured by Theobald et al. [28]. A typical
coefficient of friction of 0.01 was reported. These
authors also adopted the Sommerfeld analysis
commonly used in engineering and found that
predominant hydrodynamic lubrication was present
in their experiments. They further suggested that
one of the functions of fat pads associated with
Table 2 Typical friction coefficients (factors) for various
bearings for artificial hip joints in the presence of bovine
serum [25, 26].
Bearings
Friction factor
UHMWPE-on-Metal
0.06–0.08
UHMWPE-on-Ceramic
0.04–0.06
PEEK-on-Metal
PEEK-on-Ceramic
Metal-on-Metal
0.35
0.36
0.10–0.18
Ceramic-on-Ceramic
~0.04
Ceramic-on-Metal
~0.04
Friction 1(2): 100–113 (2013)
104
subtendinous bursae and synovial joints should be to
generate a hydrodynamic lubricating layer between
the opposing surfaces.
Tendons transfer muscular forces around the joint,
facilitating joint motion. Tendons can be subjected to
either tension (i.e., mid-substance) or compression
(i.e., fibrocartilaginous). High friction in tendon
has previously been reported in association with
cumulative trauma disorders such as carpal tunnel
syndrome and tendonitis as well as tendon suturing
failure [29]. The friction between a canine flexor
digitorum profundus tendon and its pulley was
quantified by Uchiyama et al. [30] using two force
transducers connected to each end of the tendon. A
frictional coefficient of the canine flexor tendon-pulley
was found around 0.016.
Theobald et al. [31] reported experimental data
describing the friction characteristics of the tensile
and compressive regions of bovine flexor tendon
against glass using a pin-on-plate tribometer. Under
physiological conditions, the tensile tendon region was
found to be capable of generating elastohydrodynamic
lubrication, with a coefficient of friction around 0.1
mainly as a result of viscous shearing in a fluid-film
lubrication regime. The coefficient of friction in the
equivalent region of compressive tendon was measured
as 0.008, in the mixed/boundary lubrication regime.
The surface-bound lubricin (a glycoprotein present in
the synovial fluid that specifically binds to the surface
of tendon, articular cartilage, etc.) was also found in
the compressive region, which has been shown to be an
effective boundary lubricating constituent responsible
for minimising the friction in the mixed/boundary
lubrication regime. However, such a lubricating
mechanism has not been found in a number of
synthetic grafts [32].
4.1.3
Pleurae
Friction also plays an important role between the
normal function as well as disease developments
between the lung and the chest wall. The pleurae
consist of a double membrane with a monolayer of
mesothelial cells, covering the lung (visceral pleura)
and lining the chest wall (parietal pleura) [33]. There
is a potential space between the double membrane,
the pleural cavity, where a lubricant known as
pleural (serous) fluid is found. It is important to
ensure effective lubrication between the pleural
membranes and low friction and minimum shear
stress between the two sliding membrane surfaces
during breathing [34]. However, under some adverse
conditions, friction may be significantly increased,
potentially causing damage to the tissue surfaces as
well as producing an audible sound. This latter
phenomena has been used to diagnose pleurisy and
other conditions affecting the chest cavity such as
pneumonia and pulmonary embolism, as commonly
known as a pleural friction rub, or simply pleural rub
as the pleural layers are inflamed and whenever the
patient’s chest wall moves during inspiration and
expiration.
The measurement of friction in pleural surfaces has
largely been carried out in vitro. The experimental
results have been inconsistent, mainly due to the
simplified apparatus and external conditions and the
preparation of samples. A simple inclined plane was
used in early experiments to measure starting
coefficients of friction of lung sliding on the inner
chest wall and a typical value at approximately 0.2
was found [35, 36] studied rabbit lung sliding on
chest wall pleura with pleural liquid as lubricant in
an in vitro set-up. They found the starting coefficient
of friction increased from 0.086 to 0.122 as the period
of stationary contact increased from 5 to 30 s. It is
interesting to note that such a time-dependent
friction characteristic is consistent with that observed
for articular cartilage as discussed in Section 4.1.1.
Under dynamic oscillating conditions representative
of physiological velocities and normal forces, the
average value of the coefficient of kinetic friction
was constant at 0.019. Furthermore, the friction
characteristics measured in both these experiments
were broadly consistent with boundary lubrication,
with substantial contact between the surfaces. However,
other experimental results were more consistent with
a full fluid film lubrication regime [37]. Friction was
measured in a rotational tribometer during steady state
sliding between mesothelial tissue from the peritoneal
mesothelial surface and smooth glass lubricated with
normal saline. The friction characteristics were found
to be consistent with a progression of lubrication
regimes from mixed to fully developed hydrodynamic
Friction 1(2): 100–113 (2013)
lubrication. Potential differences between these studies
were the apparatus and the samples used, as pointed
out by Loring and Butler [38]. This highlighted the
importance of large scale conformation differences
among tissue samples that promoted load support
and reduced friction to a variable extent. Alternations
to the natural system, i.e., blotting with filter paper,
can significantly increase the friction and damage the
pleural surface [39].
As with so many soft biological tissues, there
are a number of potential lubrication mechanisms
responsible for the low friction in the pleural surfaces.
Elastohydrodynamic lubrication at microscopic scales
has been proposed to be responsible for effective
lubrication and low friction between parallel pleural
surfaces. The asperities on the pleural surfaces and
subsequent deformation promotes hydrodynamic
load support and separates the two sliding surfaces
[40]. Boundary lubricating properties of the pleural
surfaces are also responsible for reducing friction. A
number of boundary lubricating constituents have
been identified, including surface active phospholipids
[39, 41], again similar to those found in synovial fluid.
However, the exact lubrication mechanisms remain
speculative and controversial.
Similar to the lungs, the heart and intestines would
probably work in a similar manner. They all need to
change their shape and size and slide against the
chest wall and other organs to function normally. A
similar effective lubrication mechanism may be
operative during this process to provide little friction
and without apparent damage or wear. Destruction
and damage of the surfaces may elevate friction and
result in diseases in all these soft tissues.
4.1.4
Eye
Normal functions of the eye depend on effective
lubrication and minimum friction and wear between
the cornea and the eyelid. The cornea is approximately
spherical in the central portion, however, its surface
is not smooth. The surface topography on the cornea
has been found to have microridges up to 0.5 m
high [42]. However these microridges are covered
with a mucus gel so the effective roughness may be
much less. Tear films also play an important role in
the lubrication of the eye. Tear films have three
105
distinct layers: the outermost being a lipid (fatty, oily)
layer having a thickness of about 0.1 m, the middle
layer being an aqueous layer of 7–10 m thick and
low viscosity, and the innermost being a viscous
mucous layer to adhere to the cornea surface. The
major biomechanical function of the eye is blinking,
which was studied in detail by Hayashi [43]. Blinking
occurs once every 5 s on average. It takes about 0.08 s
and 0.17 s during closure and opening respectively.
During closure the upper eyelid moves down with an
approximate speed of 0.15 m/s. The normal load
between the eyelid and the cornea ranges from 0.2 to
0.25 N. Loss of lubrication and increase in friction can
result in dry eye syndrome, either because of less
production of tears or more watery tears than oily or
both. High friction can result in high shear stresses,
and inflammation and damage to the anterior tissues,
leading to inconvenience to patients and scratching
and burning of the eyes. Dry eye syndrome may be
treated by using artificial tear drops.
Direct friction measurements in the natural eye have
been rather limited and most friction measurements
have been done on tear drops and contact lenses.
Cobb et al. [44] developed a low load friction
measuring apparatus and determined the coefficient
of friction between a glass pin and an intact layer of
human corneal epithelial cells of the order of 0.05.
Furthermore, they showed a direct relationship
between the coefficient of friction and the extent of
cell damage. Contact lenses are widely used to correct
eyesight. The introduction of a contact lens in the eye
results in two biotribological interfaces: the post-lens
between the posterior surface of the lens and the eye
surface (cornea) and the pre-lens between the anterior
surface of the lens and the eye-lid, with the latter
being more critical in terms of friction. Friction from
the pre-lens interface of soft contact lenses has been
measured in a number of studies. Rennie et al. [45]
used a microtribometer to measure friction in a
number of commercially available contact lenses slid
with a glass pin under a wide range of contact
pressures and speeds. The friction force was found to
consist of three components: viscoelastic dissipation,
interfacial shear, and viscous shearing. The coefficients
of friction were found to vary from 0.025 to 0.075.
Another similarly sophisticated friction apparatus
Friction 1(2): 100–113 (2013)
106
was developed by Ngai et al. [46], where a silicone
rubber eye-form that retained the contact lens was slid
against a smooth reciprocating flat glass plate.
lubrication mechanism in the natural eye, as well
as in the presence of a contact lens, has been studied
for a long time. Early studies by Ehlers [47] suggested
boundary lubrication, however Holly and Holly [42]
proposed an alternative hydrodynamic lubrication
mechanism due to the relatively thick tear film
discussed above. Extensive studies have been carried
out to measure the tear film thickness in the eye, and
the post-lens and pre-lens tear film thicknesses in the
presence of a soft contact lens. At the same time, a
number of theoretical lubrication modelling studies
on contact lenses have also been carried out [48]. All
these experimental and theoretical studies gave some
evidence supporting the role of elastohydrodynamic
lubrication in contact lens friction, broadly in agreement
with the friction studies discussed in this section.
4.1.5
Oral cavity
Human oral cavity is quite complex, consisting of
both hard and soft tissues such as palate, chin, teeth,
tongue, mucosa and glands as well as the
temporomandibular joint (TMJ) which connects the
upper temporal bone with the lower jaw bone. The
TMJ can be considered as a synovial joint and therefore
expected to behave similarly to other synovial joints
such as the hip and the knee as reviewed in Section
4.1.1. All the soft tissues in the oral cavity are covered
with mucosa, which is lined by stratified squamous
epithelium with topographic differences that correlate
with masticatory demands [49]. Another important
element in the oral cavity is saliva. Understanding of
the lubricating properties of saliva may help develop
saliva substitutes [50] to treat “dry mouth” symptoms.
As an organ, the main function of the oral cavity
is closely related to speech and food processing.
Therefore, friction can be expected to play an important
role in the oral cavity. For example, during chewing,
the movement of the teeth with the lubrication of
saliva or food slurry results in friction and wear.
Various names have been used to describe particular
examples of frictional keratosis in the oral cavity
from excessive force. Brushing the teeth may cause
toothbrush keratosis, the constant rubbing of the
tongue against the teeth may lead to tongue thrust
keratosis and injuries to the oral mucosa may result
in frictional keratosis. Another important aspect in
the oral cavity is related to oral processing. The
overall behaviour of a food in the mouth depends on
how the food interacts within the oral environment.
A number of processes are involved when food is
prepared for swallowing in the mouth, including the
mechanical breakdown of solid pieces into smaller
fragments, enzymatic reduction of starches into
sugars, molecular interaction with micro-organisms,
and mixing with saliva. This requires a wide range of
complex movement of the teeth and the tongue and
different types of shear, tensile and compressive
deformation. Furthermore, there is considerable
interest in the possible link between texture, friction,
rheology, and human perception of foodstuffs, such
as creaminess and astringency [51], in a similar
manner as the skin discussed in Section 4.2.1.
A wide range of methods has been applied to
measure friction during oral processing of food as well
as producing food as reviewed by Goh [49], including
the linear friction sledge, the pin- or ball-on-disk
tribometer as well as rheometers with specific friction
attachments. The important considerations for the
contacting surface may include hydrophobic or
hydrophilic, structures with pillars to simulate the
papillae on the tongue and in some cases using animal
tongues. The effect of surface structure on frictional
behavior of a tongue/palate tribological system was
investigated by Ranc et al. [52] under both dry and
oil and aqueous solution in a reciprocating motion
sliding tribometer. The friction was shown to be
strongly affected by the topographical structure of
the contacting surfaces. The effect of brushing on
adsorbed salivary conditioning films and friction was
investigated by Veeregowda et al. [53] using colloidal
probe atomic force microscopy under different modes
of brushing (manual, powered, rotary-oscillatory or
sonically driven). It was found that different modes
affected the friction and the mode of lubrication.
The coefficients of friction of oral tissue, including
teeth, have been shown to range from about 0.004 to
0.45, depending upon the external environment and
conditions of load, sliding speed, and counterface as
summarized by Dowson [54]. Coefficients of friction
Friction 1(2): 100–113 (2013)
in the presence of whole mouth saliva range from
0.02 to 0.2, with clear evidence of both boundary
and mixed lubrication characteristics. Under certain
conditions, when softer substrates were used, a
transition from mixed to fluid-film lubrication was
possible, with a minimum coefficient of friction of
around 0.004 in the Stribeck curve. Harvey et al. [55]
performed surface balance experiments on human
whole saliva absorbed to molecularly smooth mica
substrate and found a coefficient of fiction of 0.24 and
0.46 for the unrinsed and rinsed systems, respectively.
Metals, ceramics, and composites are generally
applied to dental restorations and implants. The
effect of friction has an important role to play in the
mechanical function of dental devices. Friction between
dental materials and bone affects the micro-motion
and consequently fixation [56], similar to the fixation
of artificial joints. Friction in fixed orthodontic
appliance systems has been known to most clinicians
to be harmful to tooth movement. Friction between
brackets with different materials such as stainless steel
etc. slid against various archwires was measured by
Tecco et al. [57] and Fidalgo [58], with considerable
differences between different designs and materials.
4.1.6
Catheter
Catheters and guidewires are widely used for medical
diagnoses and interventions by inserting into a body
cavity, duct, or vessel in order to allow drainage,
administration of fluids or gases, or access by surgical
instruments. There are numerous examples such as
wound drains, endotracheal tubes; trochars; catheters;
dilators; guide wires; angioplasty balloons; vascular,
biliary and urethral stents; patches; filters; hypodermic
or suture needles; and electrical pacemaker leads.
Friction arising from this process directly results in
shear stress that may damage the natural tissue and
affect comfort, but also may influence the ease of
insertion and manipulation in computer assisted
surgery [59]. Various materials, particularly with
coatings, have been developed over many decades to
reduce friction [60].
Both in vitro and in vivo animal models were used
to measure friction by Nickel et al. [61] and Khoury
et al. [62] for different urinary catheter materials and
coatings. In vitro measurement of static and kinetic
107
friction coefficient of a catheter surface was performed
by Kazmierska et al. [63]. Contacts between different
counter-faces (polymers, tissue) and various types of
tubes under wet conditions were simulated in order
to mimic in vivo process. Low friction was found for
super-hydrophilic biomaterials on tissue and a
hydrophobic counter-face, while slightly hydrophobic
biomaterials showed higher friction in both cases.
More hydrophobic biomaterials gave low friction
on tissue but high on hydrophobic polymer. The
smoothest friction characteristic was achieved in all
cases on tissue counter-faces. The static coefficients of
friction of catheters on bladder mucosa counter-faces
were measured as 0.15 for vinyl and siliconised latex
catheters and 0.05 for all-silicone. Hydrogel coated
catheters exhibited the lowest static and kinetic friction
factors. The use of a hydrophilic-coated catheter
during transradial cardiac catheterization was also
shown to be associated with a low incidence of radial
artery spasm [64].
4.2
4.2.1
Outside the body
Skin
Skin is the largest organ in the human body. Friction
studies on skin can provide valuable insight into how
the skin interacts with other surfaces and changes
under various conditions including age and health,
chemical treatments using lotions and moisturizers.
Friction between skin and cloth may affect how we
feel, and slips when entering or leaving a bath may
be a serious hazard particular for the elderly [65].
Blister and pressure ulcer formation are also closely
related to skin friction.
As an external surface itself, it is convenient,
relatively easy and non-invasive to measure skin
friction in vivo quantitatively. Friction studies on skin
have been carried out comprehensively. Most tests
have been performed in vivo, with a few in vitro and
on animal skins. Friction measurements have adopted
two basic designs: a probe moved across the skin in a
linear fashion or a rotating probe in contact with the
skin surface. Specific designs for friction measurements
have been comprehensively reviewed by Sivamani
and Maibach [66] and Derler and Gerhardt [67].
Coefficients of friction of skin at different anatomical
Friction 1(2): 100–113 (2013)
108
sites, against various counterfaces and in the presence
of various chemicals and under different actions have
been summarized by Sivamani and Maibach [66]. It
has been generally noted that skin friction depends
on anatomical site and skin hydration as well as
the design of the measuring instrument and the
counterface geometry and material. However, no
significant differences have been found with regard
to gender or race [68]. The effect of age on skin friction
may be linked to the increased sunlight exposure
which can affect the skin structure and, therefore,
alter the friction properties of skin. However, no
significant differences of friction have been found with
regard to age [69]. The coefficients of friction in the
normal untreated skin generally range from 0.2 to 0.5,
and under some conditions can reach as high as 2.
Representative values of the coefficient of friction for
normal dry skin from different anatomical sites range
from 0.40 (leg, hand (dorsal)), 0.49 (forehead), 0.68
(hand (palm)), 0.81 (finger), and 1.20 (foot (sole)) [70].
Despite a complex underlying tribological mechanism,
skin hydration appears to be the most important
factor, followed by the influences of surface and
material properties of the contacting materials. Friction
increases with skin hydration and decreases for dried
skin. However, the presence of a slippery layer of
water may reduce friction through hydrodynamic
action. Chemical treatments influence skin hydration
level and affect the friction coefficient.
High friction can result in skin blisters, commonly
found in active populations. Friction blisters not only
create localized discomfort but also potentially serious
secondary complications such as cellulitis and sepsis.
Most research on friction blisters has been carried out
from the military because of the nature of the
physical activity involved in this field, as well as in
the field of sports medicine. Prolonged pressure on
the skin surface such as on the heel and associated
friction and shear is related to the pathophysiology
of pressure ulcers [71].
The effect of friction on touching, sensing and
perception has received significant attention recently
in a number of studies. Tactile sensation is usually
assessed through the combination of friction
measurements with objective correlation with other
physiological parameters [72−75]. The underlying
mechano-transduction in the skin sensing has been
discussed by Zahouani et al. [76]. The mechanical
skin sensation in humans can detect and differentiate
many mechanical stimuli from the surrounding
environment, for example vibration, texture, pinching,
etc. These mechanical stimuli may exert deformations
on the nerve ending in the skin with specialized
sensitive receptors (mechanoreceptors). Friction affects
the skin deformation and hence is directly related to
this mechano-transduction process.
Friction of human hair has long been studied. The
differential friction effect has been observed for many
years when sliding direction along the hair is changed.
A differential coefficient of friction of 0.16 was
measured by Bhushan et al. [77] between polyurethane
sheet sliding against Caucasian hair. Shaving and
corresponding technologies are also closely related to
friction [70]. One of the notable developments is the
low friction PTFE coatings which are widely applied
on the cutting flanks of the built-in blades in
disposable razors.
4.2.2
Slips
Friction between feet/shoes and the floor influences
the propensity of pedestrians to slip and fall. Clarke
et al. [78] defined a pedestrian slip as occurring when
“the required friction exceeds the friction provided
from shoe-surface contact and the person fails to alter
their gait (motion) accordingly”. One of the common
sources for causing unintentional slips and falls is
bathtubs and showers. Friction studies have placed
a major role on modern footwear development.
Coefficient of friction provides a good indication of
the slip resistance between footwear and a surface.
During a gait cycle, the coefficient of friction required
by a person can be described as the ratio of the
horizontal to the vertical component which can be
measured from a force platform. The biomechanics of
slips were studied by Redfern [79]. The maximum
coefficient of friction required occurs at the heel
impact phase and the propulsion phase. Generally,
the lower the friction between shoe-floor surfaces is,
the more likely slips occur. Fiction coefficients less
than 0.24, greater than 0.36 and between 0.24 and 0.36
have been suggested to correspond to danger, safe, and
marginal risk (http://www.tribology.group.shef.ac.uk/
Friction 1(2): 100–113 (2013)
109
research/research_projects_banana.html). The presence
of a banana skin may increase the slip risk,
particularly when it becomes old, soggy, and brown.
However it is difficult to use the ground reaction
data alone to predict the likelihood of pedestrian slip
due to the subjective nature of human walking and
testing. Examples of uncertainties include large natural
variability between individual humans (age, weight,
body shape etc.) and extrinsic factors (surface and
footwear characteristics). The walking velocity, as well
as a person’s ability to adapt their gait to particular
footwear and surface conditions, are also important.
A number of mechanical testing devices have been
used in the assessment of surface slip resistance in
the form of friction coefficients, as summarized by
Clarke et al. [78]. Chang et al. [80] outlined the detailed
requirements in terms of the normal force build-up
rate, the normal pressure and sliding velocity at the
interface and the time of contact prior to and during
the friction measurement. Although these mechanical
devices can provide useful and re-producible data,
inherent complexities in mechanically simulating
subjective human gait make the validation of test
devices difficult. Nevertheless, important parameters
include shoe design, material, ground surfaces and
conditions as well as individual gait characteristics.
Table 3 summarizes typical representative coefficients
of friction in shoe-floor contacts.
Table 3 Coefficients of friction measured between a PVC sole
with a smooth PVC heel under various floor conditions [81].
5
Floor
Conditions
Coefficients of
friction
Vinyl composite tile
Carpet
Vinyl composite tile
Carpet
Vinyl composite tile
Carpet
Dry
Dry
Wet
Wet
Soapy
Soapy
1.12
1.43
0.64
0.80
0.16
0.46
Summary
In general, friction measurements are relatively easier
to conduct than lubrication and wear studies, and
therefore have been carried out widely in tribological
investigations of biological systems. Friction plays an
important role in the normal function and potential
disease development of a number of human organs
as well as the development of diagnostic and
interventional medical devices. Friction is usually
measured in simple apparatus using small samples
in vitro. The importance of these simple laboratory
experiments in revealing basic biotribological
mechanisms is widely recognized and is particularly
useful for the purpose of comparative studies.
However, this can result in a wide range of values of
coefficient of friction reported, even when a similar
tissue is considered. It is now recognised that good
simulation of the in vivo situation is essential if
laboratory observations are to be representative of
in vivo performance and in design studies and the
pre-clinical evaluation and screening of implanted
products. It is increasingly clear that the physiological
conditions should be replicated as fully as possible
in order to provide meaningful indications of in vivo
performance. Although friction studies generally
provide valuable information in terms of friction
coefficients, the underlying tribological mechanism
remains unclear in most of the organs reviewed in
this paper. It is also clear that friction measurements
in terms of magnitude alone are often insufficient
since a higher value may be associated with fluidfilm lubrication while a lower value may be a result of
some of the remarkable forms of boundary lubrication
adopted by nature.
Different lubrication mechanisms have developed
to control friction in different organs and tissues.
However, for the majority of soft tissues, such as
articular cartilage, cornea, pleura, fad pat etc., where
sliding is important, it is intriguing to recognise basic
similarities between the tissue compositions (biphasic
in terms of solid and fluid phases) and the mechanisms
of lubrication and friction adopted by these tissues
engaged in different functions. Similarly, bio-lubricants
associated with different biological tissues and
organs have similar constituents including synovial
mucin, hyaluronic acid, proteoglycans, glycoproteins
(lubricin) and lipids (dipalmitoyl phosphatidylcholine,
DPPC). Most interfaces in biological systems operate
in a mixed lubrication regime, as do many engineering
systems, with the ability to accommodate boundary,
fluid film or a mixed lubrication regime to meet
functional needs. Many of the basic mechanisms of
Friction 1(2): 100–113 (2013)
110
boundary and fluid film lubrication are operative at
different anatomical sites. Under the conditions in
favour of hydrodynamic lubrication, a fluid film
lubrication regime is responsible for low friction.
Under conditions when contacts take place, the
biphasic nature of the soft tissues takes the advantage
of the fluid pressurization and the reduction in the
load carrying proportion by the solid phase under
external loading, so that the friction remains low for
a considerably long period of time. Even when either
the fluid-film or biphasic lubrication mechanism
ceases to operate, the effective boundary lubrication
mechanism comes into play and keeps friction
adequately low. It is such a remarkable combination
of different lubrication mechanisms that are responsible
for the low friction observed in a majority of the soft
biological tissues under a wide range of operating
conditions.
Differences of the bio-friction in living biological
tissues from the mechanical counterpart in engineering
systems should be recognized. Natural biological
tissues such as articular cartilage have self-regenerating
ability, including friction and lubrication. The role of
sliding motion and frictional shear stress has been
shown to be important for regenerating functional
extra-cellular matrix of articular cartilage and
lubricating constituents (lubricin) on the surface in an
in vitro set-up [82]. Similar regenerating mechanism
may be expected for natural articular cartilage under
in vivo conditions. For hard biological tissues such as
teeth, self-repair or self-regeneration in terms of
tribological properties is also expected to be important.
Zheng et al. [83] have shown that the nanomechanical
and microtribological properties of the acid-eroded
enamel surface were significantly enhanced by
remineralization in artificial saliva. However, the loss
of the hardness and Young’s modulus of enamel
surface by acid erosion could not be fully recovered
after in vitro remineralization. The understanding of
the self-regenerating mechanism, including tribology,
of biological tissues is important for not only
understanding how our natural systems work and
diseases may develop but also providing design
guidance for developing effective tissue engineering
approaches.
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
References
[1]
Lu J Y. Tribology achievements in ancient China (in
Chinese). Lubrication Engineering 2: 6−11 (1981)
[2] Dowson D. History of Tribology, 2nd ed. New York: Wiley,
1998.
[3] Dowson D. Whither tribology. Wear 16(4): 303–304 (1970)
[4]
Jin Z M, Dowson D. Elastohydrodynamic lubrication in
biological systems. Proc Inst Mech Eng J 219: 367–380
(2005)
[5]
Jin Z M, Wimmer M. Bio-related tribology section. In
Encyclopedia of Tribology. Berlin: Springer, in press, 2013.
[6] Zhu P-Z , Hu Y-Z , Ma T-B , Wang H. Molecular dynamics
study on friction due to ploughing and adhesion in nanometric
scratching process. Tribology Letters 41(1): 41–46 (2011)
[7] O’Kelly J, Unsworth A, Dowson D, Wright V. A study of
the role of synovial fluid and its constituents in the friction
and lubrication of human hip joints. Engng Med 7(2):
77–83 (1978)
[8] McCann L, Ingham E, Jin Z, Fisher J. Influence of the
meniscus on friction and degradation of cartilage in the
natural knee joint. Osteoarthritis Cartilage 17(8): 995–1000
(2009)
[9]
Unsworth A, Dowson D, Wright V. The frictional behaviour
of human synovial joints—Part 1: Natural joints. J Lub
Techno 97(3): 369–376 (1975)
[10] Unsworth A. Tribology of artificial hip joints. Proc Inst
Mech Eng J 222: 711–718 (2006)
[11] Charnley Y J. The lubrication of animal joints in relation to
surgical reconstruction by arthroplasty. Ann Rheum Dis 19:
10–19 (1960)
[12] Linn F C. Lubrication of animal joints: I. The arthrotripsometer.
Journal of Bone and Joint Surgery 49: 1079–1098 (1967)
[13] Hills B A, Butler B D. Surfactants identified in synovial
fluid and their ability to act as boundary lubricants. Annals
of the Rheumatic Diseases 43(4): 641–648 (1984)
[14] Forster H, Fisher J. The influence of loading time and
lubricant on the friction of articular cartilage. Proc Inst
Mech Eng H 210(2): 109–119 (1996)
[15] Pickard J, Ingham E, Egan J, Fisher J. Investigation into
the effect of proteoglycan molecules on the tribological
Friction 1(2): 100–113 (2013)
properties of cartilage joint tissues. Proc Inst Mech Eng H
212(3): 177–182 (1998)
[16] Baro V J, Bonnevie E D, Lai X, Price C, Burris D L, Wang
L. Functional characterization of normal and degraded
bovine meniscus: Rate-dependent indentation and friction
studies. Bone 51(2): 232–240 (2012)
[17] Dowson D. Modes of lubrication in human joints. In
Proceedings of the Institution of Mechanical Engineers,
1966: 45–54.
[18] Ateshian G A. A theoretical formulation for boundary
friction in articular cartilage. J Biomech Eng 119(1): 81–86
(1997)
[19] Graindorge S, Ferrandez W, Jin Z M, Fisher J, Ingham E,
Grant C, Twigg P. Biphasic surface amorphous layer
lubrication of articular cartilage. Medical Engineering and
Physics 27(10): 836–844 (2005)
[20] Crockett R, Roos S, Rossbach P, Dora C, Born W, Troxler
H. Imaging of the surface of human and bovine articular
cartilage with ESEM and AFM. Tribology Letters 19(4):
311–317 (2005)
[21] Goldberg R, Klein J. Liposomes as lubricants: Beyond drug
delivery. Chem Phys Lipids 165(4): 374–381 (2012)
[22] Wroblewski B M, Siney P D, Fleming P A. The principle of
low frictional torque in the charnley total hip replacement. J
Bone Joint Surg Br 91: 855–858 (2009)
[23] Bishop N E, Waldow F, Morlock M M. Friction moments
of large metal-on-metal hip joint bearings and other modern
designs. Med Eng Phys 30(8): 1057–1064 (2008)
[24] Meyer H, Mueller T, Goldau G, Chamaon K, Ruetschi M,
Lohmann C H. Corrosion at the cone/taper interface leads to
failure of large-diameter metal-on-metal total hip arthroplasties.
Clin Orthop Relat Res 470(11): 3101–3108 (2012)
[25] Brockett C, Williams S, Jin Z, Isaac G, Fisher J. Friction of
total hip replacements with different bearings and loading
conditions. J Biomed Mater Res B Appl Biomater 81(2):
508–515 (2007)
[26] Brockett C L, John G, Williams S, Jin Z, Isaac G H, Fisher
J. Wear of ceramic-on-carbon fiber-reinforced poly-ether
ether ketone hip replacements. J Biomed Mater Res B Appl
Biomater 100(6): 1459–1465 (2012)
[27] Theobald P. Lubricating properties of the fat pad. In
Encyclopedia of Tribology. Berlin: Springer, in press, 2013.
[28] Theobald P, Byrne C, Oldfield S F, Dowson D, Benjamin
M, Dent C, Pugh N, and Nokes L D. Lubrication regime of
the contact between fat and bone in bovine tissue. Proc Inst
Mech Eng H 221(4): 351–356 (2007)
[29] Evans R B. Managing the injured tendon: Current concepts.
J Hand Ther 25(2): 173–189 (2012)
111
[30] Uchiyama S, Amadio P C, Berglund L J, An K N. Analysis
of the gliding pattern of the canine flexor digitorum profundus
tendon through the A2 pulley. J Biomech 41(6): 1281–1288
(2008)
[31] Theobald P S, Dowson D, Khan I M, Jones M D.
Tribological characteristics of healthy tendon. J Biomech
45(11): 1972–1978 (2012)
[32] Thomas J M, Beevers D, Dowson D, Jones M D, King P,
Theobald P S. The bio-tribological characteristics of synthetic
tissue grafts. Proc Inst Mech Eng H 225(2): 141–148
(2011)
[33] Finley D J, Rusch V W. Anatomy of the pleura. Thorac
Surg Clin 21(2): 157–163 (2011)
[34] Lai-Fook S J. Pleural mechanics and fluid exchange. Physiol
Rev 84(2): 385–410 (2004)
[35] Brandi G. Frictional forces at the surface of the lung. Bull
Physiopathol Respir 8: 323–336 (1972)
[36] D'Angelo E, Loring S H, Gioia M E, Pecchiari M, Moscheni
C. Friction and lubrication of pleural tissues. Respir Physiol
Neurobiol 142(1): 55–68 (2004)
[37] Loring S H, Brown R E, Gouldstone A, Butler J P.
Lubrication regimes in mesothelial sliding. J Biomech
38(12): 2390–2396 (2005)
[38] Loring S H, Butler J P. Pleural lubrication and friction in
the chest. In Encyclopedia of Tribology. Berlin: Springer, in
press, 2013.
[39] Bodega F, Sironi C, Porta C, Pecchiari M, Zocchi L,
Agostoni E. Mixed lubrication after rewetting of blotted
pleural mesothelium. Respir Physiol Neurobiol (in press)
(2012)
[40] Moghani T, Butler J P, Loring S H. Determinants of friction
in soft elastohydrodynamic lubrication. J Biomech 42(8):
1069–1074 (2009)
[41] Hills B A. Graphite-like lubrication of mesothelium by
oligolamellar pleural surfactant. J Appl Physiol 73(3):
1034–1039 (1992)
[42] Holly F J, Holly T F. Advances in ocular tribology. In
Lacrimal Gland, Tear Film, and Dry Eye Syndromes. New
York: Plenum Press, 1994: 275–283.
[43] Hayashi T T. Mechanics of contact lens motion. Ph.D.
Thesis. Berkeley (USA): University of California, Berkeley,
1977.
[44] Cobb J A, Dunn A C, Kwon J, Sarntinoranont M, Sawyer W
G, Tran-Son-Tay R. A novel method for low load friction
testing on living cells. Biotechnol Lett 30(5): 801–806 (2008)
[45] Rennie A C, Dickrell P L, Sawyer W G. Friction coefficient
of soft contact lenses: Measurements and modeling. Tribology
Letters 18(4): 499–504 (2005)
Friction 1(2): 100–113 (2013)
112
[46] Ngai V, Medley J B, Jones L, Forrest J, Teichroeb J.
Engineering & Physics 27: 443–453 (2005)
Friction of contact lenses: Silicone versus conventional
[61] Nickel J C, Olson M E, Costerton J W. In vivo coefficient of
hydrogels. Tribology and Interface Series 48: 371–379 (2005)
kinetic friction: Study of urinary catheter biocompatibility.
[47] Ehlers, N. The Precorneal Film: Biomicroscopical, Historical
and Chemical Investigations. New York: Bogtrykkeriet
Forum, 1965.
[48] Jones M B, Fulford G R, Please C P, McElwain D L S,
Collins M J. Elasto-hydrodynamics of the eyelid wiper.
Bulletin of Mathematical Biology 70: 323–343 (2008)
[49] Goh S M. Tribology of foods. In Encyclopedia of Tribology.
Berlin: Springer, in press, 2013.
[50] Zhou Z R, Zheng J. Oral tribology. Proc Inst Mech Eng J
220: 739–754 (2006)
[51] Chen J. Surface texture of foods: Perception and characterization. Crit Rev Food Sci Nutr 47(6): 583–598 (2007)
Urology 29(5): 501–503 (1987)
[62] Khoury A E, Olson M E, Villari F, Costerton J W.
Determination of the coefficient of kinetic friction of
urinary catheter materials. J Urol 145(3): 610–612 (1991)
[63] Kazmierska K, Szwast M, Ciach T J. Determination of
urethral catheter surface lubricity. Mater Sci Mater Med
19(6): 2301–2306 (2008)
[64] Koga S, Ikeda S, Futagawa K, Sonoda K, Yoshitake T,
Miyahara Y, Kohno S. The use of a hydrophilic-coated
catheter during transradial cardiac catheterization is associated
with a low incidence of radial artery spasm. Int J Cardiol
96(2): 255–258 (2004)
[52] Ranc H, Servais C, Chauvy P-F, Debaud S, Mischler S.
[65] Hills R J, Unsworth A U, Ive F A. A comparative study of
Effect of surface structure on frictional behavior of a
the frictional properties of emollient bath additives using
tongue/palate tribological system. Tribology International
39: 1518–1526 (2006)
[53] Veeregowda D H, van der Mei H C, de Vries J, Rutland M
W, Valle-Delgado J J, Sharma P K, Busscher H J. Boundary
lubrication by brushed salivary conditioning films and their
degree of glycosylation. Clin Oral Investig 16(5): 1499–1506
(2012)
[54] Dowson D. Introduction to bio-tribology. In Encyclopedia
of Tribology. Berlin: Springer, in press, 2013.
[55] Harvey N M, Yakubov G E, Stokes J R, Klein J. Lubrication
and load-bearing properties of human salivary pellicles
adsorbed ex vivo on molecularly smooth substrata. Biofouling
28(8): 843–856 (2012)
[56] Winter W, Klein D, Karl M J. Effect of model parameters on
finite element analysis of micromotions in implant dentistry.
Oral Implantol, in print (2012)
[57] Tecco S, Marzo G, di Bisceglie B, Crincoli V, Tetè S, and
Festa F. Does the design of self-ligating brackets show
different behavior in terms of friction? Orthodontics (Chic.)
12(4): 330–339 (2011)
[58] Fidalgo T K, Pithon M M, Maciel J V, Bolognese A M.
porcine skin. Br J Dermatol 130: 37–41 (1994)
[66] Sivamani R K, Maibach H I. Tribology of skin. Proceedings
of the Institution of Mechanical Engineers Part J-Journal of
Engineering Tribology 220: 729–737 (2006)
[67] Derler S, Gerhardt L C. Tribology of skin: Review and
analysis of experimental results for the friction coefficient
of human skin. Tribology Letters 45(1): 1–27 (2012)
[68] Sivamani R K, Wu G C, Gitis N V, Maibach H I.
Tribological testing of skin products: Gender, age, and
ethnicity on the volar forearm. Skin Res Technol 9(4):
299–305 (2003)
[69] Gerhardt L C, Lenz A, Spencer N D, Münzer T, Derler S.
Skin-textile friction and skin elasticity in young and aged
persons. Skin Res Technol 15(3): 288–298 (2009)
[70] Dowson D. A tribological day. Proc Inst Mech Eng J 223:
261–273 (2009)
[71] Cichowitz A, Pan W R, Ashton M. The heel: Anatomy,
blood supply, and the pathophysiology of pressure ulcers.
Ann Plast Surg 62(4): 423–429 (2009)
[72] Egawa M, Oguri M, Hirao T, Takahashi M, Miyakawa M.
The evaluation of skin friction using a frictional feel
analyzer. Research and Technology 8: 41–51 (2002)
Friction between different wire bracket combinations in
[73] Hung C, Dubrowski A, Gonzalez D, Carnahan H. Surface
artificial saliva-an in vitro evaluation. J Appl Oral Sci 19(1):
exploration using instruments: The perception of friction.
57–62 (2011)
Stud Health Technol Inform 125: 191–193 (2007)
[59] Luboz V, Zhai J, Odetoyinbo T, Littler P, Gould D, How T,
[74] Shao F, Chen X J, Barnes C J, Henson B. A novel tactile
Bello F. Guidewire and catheter behavioural simulation.
sensation measurement system for qualifying touch
Stud Health Technol Inform 163: 317–323 (2011)
perception. Proc Inst Mech Eng H 224(1): 97–105 (2010)
[60] Lawrence E L, Turner I G. Materials for urinary catheters:
[75] Skedung L, Danerlov K, Olofsson U. Tactile perception:
A review of their history and development in the UK. Medical
Finger friction, surface roughness and perceived coarseness.
Friction 1(2): 100–113 (2013)
Tribology International 44(5): 505–512 (2011)
[76] Zahouani H, Vargiolu R, Hoc T. Bio tribology of tactile
perception: Effect of mechano-transduction. In Encyclopedia
of Tribology. Berlin: Springer, in press, 2013.
[77] Bhushan B, Wei G H, Haddad P. Friction and wear studies
of human hair and skin. Wear 259: 1012–1021 (2005)
113
The role of friction in the measurement of slipperiness, Part 1:
friction mechanisms and definition of test conditions.
Ergonomics 44(13): 1217–1232 (2001)
[81] Hanson J P, Redfern M S, Mazumdar M. Predicting slips and
falls considering required and available friction. Ergonomics
42(12): 1619–1633 (1999)
[78] Clarke J D, Lewis R, Carré M J. Tribology in daily life:
[82] Grad S, Loparic M, Peter R, Stolz M, Aebi U, Alini M.
Footwear-surface interactions in pedestrian slips. In
Sliding motion modulates stiffness and friction coefficient
Encyclopedia of Tribology. Berlin: Springer, in press, 2013.
at the surface of tissue engineered cartilage. Osteoarthritis
[79] Redfern M S, Cham R, Gielo-Perczak K, Grönqvist R,
Cartilage 20(4): 288–295 (2012)
Hirvonen M, Lanshammar H, Marpet M, Pai C Y, Powers C.
[83] Zheng L, Zheng J, Weng L Q, Qian L M, Zhou Z R. Effect
Biomechanics of slips. Ergonomics 44(13): 1138–1166 (2001)
of remineralization on the nanomechanical properties and
[80] Chang W R, Grönqvist R, Leclercq S, Myung R, Makkonen
L, Strandberg L, Brungraber R J, Mattke U, Thorpe S C.
microtribological behaviour of acid-eroded human tooth
enamel. Wear 271: 2297–2304 (2011)
Zhongmin
JIN.
Currently
Distinguished Professor (Thousand
Talent Programme), School of
Mechanical Engineering, Xi’an
Jiaotong University, China and Parttime Professor of Computational
Bioengineering, School of Mechanical
Engineering, the University of Leeds,
UK. He obtained his BS degree from Xi’an Jiaotong
University in China in 1983 and PhD from the
University of Leeds, UK in 1988. He has been a
Member of the Institution of Mechanical Engineers
(UK) since 1995 and Fellow of the Chinese Tribology
Institution. His research interests include biotribology
of artificial joints, tissue engineering and finite element
modelling. He has published over 200 peer-reviewed
journal papers with an h-index of 22.
Duncan DOWSON. Emeritus
Professor, School of Mechanical
Engineering, the University of Leeds.
He obtained his BS degree and PhD
degree from the University of Leeds
in 1950 and 1952 respectively. He has
received Honorary Doctorates from
many universities and prestigious
awards from many respected bodies around the world.
He was the President of the Institution of Mechanical
Engineers, UK in 1992-3 and was elected a Fellow of
the Royal Academy of Engineering in 1982 and a
Fellow of the Royal Society of London in 1987. He
received the Order of the Commander of the British
Empire (CBE) in 1989. His research interests include
tribology and biotribology. He has published over
500 scientific papers with an h-index of 35.
Friction 1(2): 114–129 (2013)
DOI 10.1007/s40544-013-0011-5
ISSN 2223-7690
REVIEW ARTICLE
Recent advances in gecko adhesion and friction mechanisms
and development of gecko-inspired dry adhesive surfaces
Ming ZHOU1,†, Noshir PESIKA2, Hongbo ZENG3, Yu TIAN1,*, Jacob ISRAELACHVILI4
1
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
2
Chemical and Biomolecular Engineering Department, Tulane University, New Orleans, LA 70118, USA
3
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada
4
Department of Chemical Engineering, Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA
†
Present address: Institute of Mechanical Manufacturing Technology, China Academy of Engineering Physics, Mianyang, 621900, China
Received: 01 February 2013 / Revised: 10 April 2013 / Accepted: 20 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: The remarkable ability of geckos to climb and run rapidly on walls and ceilings has recently received
considerable interest from many researchers. Significant progress has been made in understanding the attachment
and detachment mechanisms and the fabrication of articulated gecko-inspired adhesives and structured
surfaces. This article reviews the direct experiments that have investigated the properties of gecko hierarchical
structures, i.e., the feet, toes, setae, and spatulae, and the corresponding models to ascertain the mechanical
principles involved. Included in this review are reports on gecko-inspired surfaces and structures with strong
adhesion forces, high ratios of adhesion and friction forces, anisotropic hierarchical structures that give rise to
directional adhesion and friction, and “intelligent” attachment and detachment motions.
Keywords: gecko feet surfaces; setae; spatulae; anisotropic dry adhesion and friction; articulated motion
1
Introduction
For millennia, the gecko has been well-known for its
fantastic climbing abilities. By using only one toe,
geckos can easily hang vertically even upside-down
from hydrophilic or hydrophobic, smooth or rough
surfaces on walls (they cannot hang from a ceiling with
only one toe). The typical step intervals of geckos are
approximately several tens of milliseconds [1]. Gecko
setal arrays have the excellent ability of self-cleaning [2],
which has a considerably wider potential application
than pressure-sensitive adhesives (PSA) in several
areas, such as robotics for rescue and detection, counterterrorism, chemical sensing, and space positioning.
A considerable number of studies have been devoted
to understanding the interrelated “frictional adhesion”
properties and mechanisms of gecko feet, hairs, and
* Corresponding author: Yu TIAN.
E-mail: [email protected]
setae in order to mimic the gecko’s swift movement
on walls and ceilings. Wall-climbing robots based on
gecko-inspired adhesives have several advantages over
those based on vacuum suction, magnetic adsorption,
or velcro, such as small size, flexible and controllable
articulation capability, self-cleaning property, and adaptability on rough surfaces [3−4]. Therefore, research on
the attachment and detachment mechanisms of gecko
feet and hairs and the overall design of gecko-inspired
adhesives is of great interest for both theoretical and
practical applications on special functional surfaces,
articulated robots, and related devices.
In this article, we review the recent advances in
gecko adhesion and friction mechanisms and the
development of gecko-inspired dry adhesive surfaces.
The essential geometric and mechanical properties
of the gecko adhesive system are first presented,
followed by an overview of the fundamental modeling
and understanding of the scientific principles of the
gecko adhesive system from the nano-scale contacts
Friction 1(2): 114–129 (2013)
to the micro-scale structures, to the macro-scale feet
and the entire animal. On the basis of the abovementioned understanding, the design principles of
gecko-inspired dry adhesive surfaces are identified and
the information they reveal about future fabrication
strategies is assessed. Finally, we discuss some future
issues in this field.
2
2.1
Adhesion and friction mechanisms of
gecko seta
Origin and measurement of adhesion forces
of gecko seta
Scanning electron microscopy has enabled the examination of the fine, hierarchical structures of setae.
There are approximately twenty rows of lamellas on
each gecko toe and approximately 20 setal arrays on
each lamella. The single seta is approximately 110-μm
long and has a diameter of approximately 4−6 μm
[5]. The seta bifurcates into approximately 100–1000
spatulae at the terminal end [6]. This branched
structure ensures close contact between the setae and
the (microscopically rough) surfaces that the gecko
climbs.
Researchers have proposed several hypotheses to
explain the origin of the strong adhesion (and friction)
forces of gecko seta, such as electrostatic interaction,
vacuum (or suction), microinterlocking (similar to
velcro), and glue, which were all eventually disproved
by experiments [7, 8]. The van der Waals and capillary
forces are the two major basic interactions between
the setae and the surface. Experimental results have
indicated that the van der Waals force between the
setae and the substrate is the main contributor to the
adhesion force, and that the presence of water vapor
in the environment may enhance this force through
an additional capillary force [9−14]. Similar adhesion
forces were observed by Autumn et al. [9] on both
hydrophobic and hydrophilic surfaces, which provided
the direct evidence for the van der Waals mechanism.
Then, many researchers found the enhanced adhesion
forces with increasing humidity [10, 11, 15]. Huber et
al. [11] also measured the gap between the spatula
and the substrate by using white light interferometry,
which is only 1–2 water monolayers even at high
115
humidity. Therefore, the authors proposed that the
presence of a monolayer or two of water would modify
the van der Waals Hamaker constant. Similarly, Pesika
et al. [16] investigated that the surface hydrophobicity
of a gecko setal array changed after prolonged
exposure to water by using a surface force apparatus
(SFA). Further, Puthoff et al. contradicted a capillary
mechanism and discovered that an increase in humidity
softens the setal β-keratin, leading to an increase in
the adhesion forces [17].
Experimental measurements of the adhesion and
friction forces of the hierarchical structures of gecko
seta have been performed over the past few years.
Hansen and Autumn found that the friction force
of an isolated setal array is 0.37 N, with an apparent
contact area of ~1.0 mm2, whereas that of a single toe
is 4.3 N, with an apparent contact area of ~200 mm2
(2 cm2) The maximum friction force of a single seta (with
approximately 100–1,000 spatulae) is approximately
200 μN, while the adhesive forces ranges from 20 to
40 μN. A single seta will spontaneously detach from
its opposing surface when the setae shaft subtends an
angle of 30° with the opposing surface [18]. Huber et
al. [19] glued an isolated seta perpendicular to the
end of an atomic force microscopy (AFM) cantilever,
cut most of the terminal branches of the seta away
to isolate a few single spatulae, and measured the
adhesion force of a single spatula to be approximately
10 nN.
The abovementioned experimental friction or shear
stresses at different structural levels (i.e., from a
single spatula to an entire toe) varied from several
kilopascals to ~1,000 kPa because the real contact area
was significantly influenced by the preload, test
conditions and the nature of the seta arrays [20–22].
These fundamental experiments laid the foundation
of subsequent theoretical analyses.
2.2
Adhesion model of gecko seta
Along with the experimental studies, significant effort
has been made to theoretically analyze the adhesion
(and friction) properties of gecko setae and spatulae
(see Fig. 1). Since the traditional Hertz model fails to
include the surface adhesion between two contacted
surfaces, the JKR model of “contact mechanics” or
“adhesion mechanics” [23], which considers the force
Friction 1(2): 114–129 (2013)
116
Fig. 1 Theoretical development of adhesion mechanism of gecko seta.
required to pull an elastic sphere of radius R from a
planar surface, is used for describing the adhesion force
of the gecko seta and spatulae. The gecko spatulae
are simplified as cylinders, each terminated with a
hemispherical end of radius R. Using the adhesion
forces measured for a single spatula, we calculated the
size of the equivalent JKR sphere to be approximately
0.13 μm, which is close to the real (imaged) size of a
spatula [11]. The principle of “contact splitting” for
bio-inspired fibrillar surfaces has been identified
[24, 25], i.e., the adhesive stress of a biological system
is inversely proportional to the radius of the terminal
structure of the attachment hairs, R. Since the adhesion
force is proportional to R while the density of contacts
Friction 1(2): 114–129 (2013)
is proportional to 1/R2 based on the self-similar scaling
assumption, the total force per unit area is inversely
proportional to R. Thus, animals with a larger mass
usually have smaller attachment structures [25].
However, a more detailed research by Peattie and
Full [26] proposed that this scaling law was invalid
in phylogenetic relationships between species, which
showed that the dynamic properties and the synergistic
effect of all hierarchical elements, not just the static
contact, should be considered.
One of the disadvantages of applying the JKR model
to describe the adhesion of gecko hairs is that the
influence of sliding deformation on the enhancement
of the adhesion force cannot be explained [27]. How
does the sliding influence the contact between the
terminal end structure of the seta and the substrate?
To address these problems, researchers have developed
a fibrillar adhesion model to consider the bending
deformation of the setal array during a normal loading
process [28]. First, the low effective elastic modulus of
the seta array must be understood and theoretically
(quantitatively) described: the gecko seta is made
of β-keratin, which has a bulk Young’s modulus
between 1.3 and 2.5 GPa based on measurements of
claws and feathers [29]. Peattie et al. [30, 31] used a
resonance technique to directly measure the Young’s
modulus and found it to lie between 1.4 and 1.6 GPa.
The complex modulus of the setal β-keratin was
measured to be 1–4 GPa depending on the environmental relative humidity [12, 26]. The 103–104-kPa
effective elastic modulus of the setal array is significantly lower [32]. Persson and Autumn modeled the
relationship between the effective elasticity of fiberarray systems and the bulk materials [32, 33], and
Schubert et al. [34] presented a similar analysis. Their
experimental and theoretical results explained the
low effective elastic modulus of the gecko setal array
and showed that this allows for good adaptability for
making good contacts on rough surfaces, while the
high bulk modulus of β-keratin itself prevents the
self-matting of the neighboring setae, provides an
efficient self-cleaning mechanism, and is fractureresistant under high stress. For more information
about the formulae, please refer to Ref. [35].
Second, the influence of bending deformations on
adhesion has been analyzed by two types of fibrillar
117
adhesion models. In the first type of model, the
adhesion force is described as the summation of the
adhesion forces of inclined cantilever beams or a
spring array of supported hemispheres on a surface
[36−38]. The second model shows that the hierarchical
structures can increase their effective adhesion energy
during detachment from a rough surface by increasing
the elastic deformation energy [33, 39].
Further, Gao et al. [40] analyzed the detachment of
a single seta using a finite element analysis method
and proposed that the peel strength can vary by one
order of magnitude as a function of the peel angle.
The results showed that the adhesion force decreased
with a decrease in the peel angle to below 30° and
that the maximum adhesion force was achieved at a
peel angle of 30°. However, these theoretical findings
did not agree with the experimental results, which
showed that a single seta had a lower adhesion force
and that it spontaneously detached from a surface at
a 30° tilt angle rather than a smaller tilt angle.
Other researchers have also analyzed the contribution of the hierarchical structures to the adhesion
energy or force [40−45]. It was reported that the
adhesion strength can change by two orders of magnitude at different tilt angles of the fiber arrays [46, 47].
However, these models focused on the deformation of
the upper supporting structures and did not consider
the actual contact shape of the terminal structures
(the adhesion junctions).
The developed fibrillar adhesion models provide a
good understanding of the strong adhesion forces of
gecko setal arrays. However, based on the JKR and
most fibrillar models, the pull-off force (the maximum
force which can be provided by the adhesion interface)
and the adhesion force (the critical force to separate
the two adhered objects) have the same magnitude,
depending only on the material properties and geometric structures of the fiber array. Thus, these models
cannot explain how the gecko can quickly detach from
surfaces. Moreover, on the basis of these models, the
effective adhesion force of the setal array ought to
decrease with an increase in the surface roughness.
In real situations, the adhesion force of the setae of
gecko, flies, and bees initially decreases and then,
increases with an increase in the surface roughness
[48, 49]. The minimum adhesion force of the gecko
Friction 1(2): 114–129 (2013)
118
spatulae is found on a surface with a roughness
of approximately 200 nm, which matches one of the
characteristic dimensions of the spatula. Since fibrillar
models do not consider the terminal structure of the
setae, they cannot fully capture the “contact mechanics”
or “adhesion mechanics” of setal arrays.
2.3
Peeling model of gecko spatulae
The adhesion models based on fiber arrays described
above do not provide an insight into why the detachment force is considerably less than the adhesion force
due to the simplification of the terminal structures of
setae as simple spheres or flat-ended cylinders, which
is very different from the actual thin fan shape of the
spatula pad. Therefore, various “peeling models” have
been developed to more appropriately demonstrate
the mechanism of gecko spatulae detachment, which
is analogous to the peeling of adhesive tapes.
The Kendall model [50] describes the peeling
strength of an adhesive tape as a function of the peel
angle. The spatula pad is simplified as a single strip of
tape [19, 20] with nanoscale dimensions. The Young’s
modulus of the spatula is ~2 GPa; thus, the elastic
energy term in the peeling spatula can be neglected,
so that w = F/γ, where F = 10 nN is the experimental
adhesion force of the spatula and γ = 50 mJ/m2. The
width of the spatula w is 200 nm, which is close to the
actual geometric dimension of the spatula. The Kendall
model has provided new insights into the peeling
mechanism of gecko detachment [51, 52]. It is also
important to note that the biomechanics of a gecko
walking on a surface reveals the use of a particular
configuration, a Y-shaped geometry/configuration
[1, 20, 52]. In this configuration, in order to take a
step forward, the gecko always has two diagonally
opposite feet on the surface while detaching the other
two. The two attached feet are angled to the surface at
a certain angle, with a tension along the feet, forming
a Y-shaped geometry and yielding a total force in the
normal direction to the surface. Further, one foot
always has five toes gripped in at different directions.
The lateral friction forces due to the toes and feet are
finally equilibrated with each other in a static staying
sate of the gecko or provide some net friction force to
drive the motion of the body.
An understanding of the mechanical behavior of
pressure-sensitive adhesive tapes is important to
describe the peeling mechanism of the gecko spatula
[53−55]. Considering the hierarchical structures of the
setae and the macroscopic articulations of gecko toes,
particularly how the friction force contributes to the
adhesion force, Tian et al. [56] theoretically analyzed
the friction and adhesion behavior of gecko pads on
the basis of tape peeling model, as shown in Fig. 2(a).
High adhesion and friction forces are predicted in the
“toe-gripping” actuation, while small release forces are
predicted in the “toe-releasing” actuation, these two
being determined by the different “peel angles,” for
gripping and releasing. The lateral friction force and
the normal adhesion force of a single seta can change
Fig. 2 (a) Tape model considering the final two levels of the hierarchical structures of setae. (b) Theoretical normal, lateral, and stretching
forces of a single spatula at different pulling angles. μ is the friction coefficient between spatula pad and substrate. In the gripping direction,
the peel angle of a spatula pad is decreased in order to approach 0°, while in the releasing direction, the peel angle of the spatula pad is
close to or more than 90°.
Friction 1(2): 114–129 (2013)
by more than three orders of magnitude during gecko
toe gripping (attachment) and releasing (detachment),
as shown in Fig. 2(b). Using a finite element model,
Peng et al. [51] analyzed the change in the peel zone
length and the peel force at different peel angles,
Young’s moduli, and spatula thicknesses. Endlein et
al. found that the adhesion forces of tree frogs can also
be explained by the peeling theory [57].
Peeling models provide a good way to explain the
experimental results of the adhesion forces of gecko
setae on surfaces with varying roughness. Fuller and
Tabor [58] developed a contact model for elastic
solids to describe the effect of roughness in reducing
the adhesion force as the real contact area decreases
with increasing surface roughness. However, surface
roughness can also increase the real contact area of a
highly compliant film, leading to the opposite result
[59]. Persson and Gorb proposed a qualitative analysis
of the effect of the elastic deformation of a spatula
on the effective adhesion energy [60]. Peng and Chen
[61] demonstrated that the normal adhesion force is
dependent on the dimensions of the film with respect
to the wavelength of the (sinusoidal) roughness of
the substrate.
The developed peeling models provide a good
theoretical basis to explain the fundamental mechanisms of seta detachment and the peeling behavior
of a single gecko spatula on surfaces with different
roughness. Future work can be conducted on the more
complex peeling behaviors of hierarchical structures
taking into consideration the different geometries and
deformations of the spatulae, setae, toe pads, and feet
at different length scales.
2.4
Coupling of friction and adhesion
Experimental results show that the friction force of
gecko setae during sliding along the setal curvature,
the gripping in this direction, is considerably higher
than the adhesion force, and that lateral sliding is
necessary to generate the strong adhesion (and friction)
forces [18, 20, 27]. The coupling of the adhesion and
friction forces, known as “frictional-adhesion” is one
of the most important mechanical properties of the
seta [12, 62, 63].
Setal arrays usually show strong friction anisotropy
depending on the shear direction. Different effective
119
elastic moduli of the setae arrays are observed in
the loading and unloading force-distance curves
when sliding along or against the seta tilt directions
(correspond to the gripping in and releasing directions,
respectively) [32]. Furthermore, friction and adhesion
forces obey different rules in the different sliding
directions. The friction force is more than four times
larger than the preload when sliding along the
gripping direction [20, 21, 64], and the adhesion force
is enhanced. In contrast, when the setal arrays are
dragged against the gripping direction, the friction
force is less than the preload and obeys Coulomb’s law,
where the normal force becomes repulsive [20, 21].
These anisotropic properties of the gecko setae are
attributed to the anisotropic structure and deformations
when the setae or toes are slid or articulated in
different directions [12, 65].
The coupling of friction and adhesion forces of the
setae is significantly influenced by the applied preload.
Wan et al. [66] experimentally showed that the preload
can decrease the tilt angle of the seta and increase the
contact number of spatulae, thereby increasing both
the adhesion and the friction forces. However, the
normal adhesion force turns into a repulsive force
when the preload is above some critical value. Ideally,
when the adhesion and friction forces are maximum,
the tilt angle is small [67]. However, crowding
considerations impose a limit on how small the tilt
angle can be before the fibers become overcrowded.
A theoretical limiting tilt angle of approximately
12.6° is consistent with the experimental compression
data [68].
The strong anisotropy and synergy between the
friction and adhesion performances of the setae arise
from the anisotropic deformations of the structures.
The friction and adhesion of single seta can reach a
stable steady-state value after sliding for several
micrometers [18], whereas for the entire setal array,
the critical sliding distance was found to be several
hundred micrometers. Numerical simulations show
that sliding causes the spatulae to become well-aligned
or ordered, leading to an increase in the real contact
area and to a more stable configuration during sliding
[69, 70]. Cheng et al. [71, 72] proposed that a pre-tension
can increase the adhesion force of the seta at small
peeling angles.
Friction 1(2): 114–129 (2013)
120
3
3.1
Gecko-inspired adhesives
Design principles
The gecko seta’s advanced performance in terms of
friction and adhesion endows the gecko with excellent
climbing abilities. The creation of a new type of dry
adhesive inspired by the gecko adhesive system has
received considerable attention. Understanding the
key properties, principles, and mechanisms of the
gecko adhesive system is essential for the design of
bio-inspired dry adhesive surfaces [73].
As discussed above, the “contact-splitting principle”
has been recognized and widely accepted [24, 25,
74−78]. On the basis of the JKR model predictions, it
can be said that the higher the extent of splitting at
the end of the setae, the higher is the adhesion force
and the better is the resistance to damage. For these
reasons, fibrillar surfaces are now widely used for
producing smart adhesive systems. Theoretical analysis
shows that fibrillar ends are not sensitive to defects
when the size of the fibrils is less than some critical
length scale [79]. However, a recent study also shows
that the adhesion force does not increase when the
number of contact elements increases while the total
contact area is constant [80].
Since the mechanical performance is highly dependent on the structures, the size and shape of the
optimum fibers is widely discussed. The effect of the
shape of the terminal ends becomes more important
with increasing size and stiffness of the materials [79].
Spolenak et al. [81] proposed that a flat punch is the
perfect shape for a bio-inspired surface, but in practical applications, the properties of the fiber array with a
flat punch end may be more easily affected adversely
by surface roughness and surface (particulate) contaminants. Gorb and Varenberg [82] proposed that
fibers with narrow necks and thin plate-shaped (or
mushroom-shaped) ends should be used for overcoming these disadvantages. Experimental results also
showed that mushroom-shaped fibril ends perform
well during loading–unloading cycles, with improved
robustness and stability [83].
An anisotropic structure for the fiber arrays is
particularly important in the design of bio-inspired
surfaces [84, 85]. Directionally angled polymer flaps
were first introduced in the fabrication of a gecko-
inspired dry adhesive surface [86]. Basically, the
anisotropic behavior of the gecko setae is due to the
asymmetric deformation and contacts. Therefore, the
asymmetric mechanical designs, including asymmetric
shapes [87] as well as different elastic moduli [88] for
the fibrillar structures are expected to provide the best
prospects for creating the desired performance (for
energy-efficient wall climbing, ceiling running, etc.).
Therefore, an anisotropic articulation is required to
make full use of these anisotropic structures.
A fiber array with a high aspect ratio promotes
contact adaptability; however, it should also be noted
that slender fibers can easily adhere to neighboring
fibers through van der Waals forces, leading to a
failure of the device (due to the so-called “crowding”
or “bunching” behavior) [89]. Sitti et al. and Hui et al.
[40, 90−92] proposed anti-self-adhesion models based
on a force and energy analysis, respectively.
Based on an understanding of the above-mentioned
design principles, some general design criteria have
been developed: The geometric parameters should be
designed taking into account the modulus of the
materials. Using numerical calculations of a fiber array
squeezed by a sphere, Aksak et al. [37] designed
the optimum length and diameter of inclined or
perpendicular fibers. Spolenak et al. [93] proposed
some general “adhesion maps” for fiber arrays with
hemisphere-shaped ends, including considerations of
condensation, adaptability, contact strength, and fiber
fracture, as shown in Fig. 3(a). The target optimum
areas are a series of triangles in the map of fibril
radius and Young’s modulus. Greiner et al. [94] further
developed adhesion design maps for fiber arrays with
different shapes. Recently, Zhou et al. [95] developed a
numerical peel-zone calculation method and proposed
an adhesion and peeling design map to evaluate
the design criteria for strong attachment and easy
detachment (peeling) forces, as shown in Fig. 3(b).
The peeling force can be changed by three orders of
magnitude with respect to the normal adhesion force
by changing the design parameters of the structures.
3.2
Fabrication
3.2.1 Fabrication of gecko-inspired surfaces with strong
adhesion
Based on the original design principle of fiber splitting,
Friction 1(2): 114–129 (2013)
121
Fig. 3 Design maps of gecko-inspired fibrillar surfaces. (a) Adhesion design maps for gecko-inspired fibrillar surfaces with hemisphereshaped ends. The triangle denotes the target area of  = 10 (reproduced from Ref. [93]). (b) Adhesion and peeling design map for
gecko-inspired fibrillar surfaces with flat ends with the typical values given in Ref. [95]. ρr denotes ρ per length at the peel angle of 90°,
and ρ is defined as the ratio of the normal adhesion force per unit width (pull-off strength) to the peel strength, which represents the
strong attachment and easy-removal properties of surfaces.
microfibrillar surfaces using microfabricated templates
with whole arrays and nanowire surfaces with relatively large aspect ratios and high Young’s modulus
have been fabricated. With further advancements
in design principles, fibrillar gecko-inspired surfaces
could be developed ranging from simple perpendicular
standing sphere-ended single-level fiber arrays to
different end-shaped fibers, inclined fiber arrays,
fibers with surface modifications, and hierarchical
structures, as shown in Fig. 4.
On the basis of the “contact-splitting principle”,
most of the early publications reported fiber array
surfaces with flat or semisphere ends. In the first trial
of a templated fabrication, an AFM pin was used
for making holes on a wax surface [89, 92]. With the
development of etching technology [25], particularly
lithographic techniques, the morphology of the fibers
could be perfectly controlled, leading to an increase
in adhesion strength [90, 96−100]. Nanowires with
self-cleaning properties have been reported, which
122
Friction 1(2): 114–129 (2013)
Fig. 4 Fabrication strategies of gecko-inspired surfaces.
shows the potential applications of nanowires and
nanotubes with a high Young’s modulus [101].
Fiber arrays with different end shapes, such as mushrooms, asymmetric spatulae, and concave structures,
have been reported [97, 102, 103]. In practice, the
mushroom-shaped fiber array is the most commonly
used [104−108]. Gorb et al. [72] reported a geckoinspired mushroom surface made of polyvinyl siloxane
(PVS) with an adhesion strength of approximately
50 kPa. Kim and Sitti fabricated a fibrillar mushroom
surface with polyurethane (PU) that generated
180 kPa [109], and Davies reported that a fibrillar
poly(dimethylsiloxane) (PDMS) surface can reach
219 kPa [106]. A combination of lithography and the
two-step molding process is now also widely used for
fabricating hierarchical fibrillar surfaces [37, 110−112].
Deep reactive ion etching [113], self-assembly [114],
anodic oxidation [113], angled etching, and mechanical
yielding techniques or methods [37, 115−117] have also
been explored to fabricate templates of gecko-inspired
dry adhesive surfaces.
The incorporation of hierarchical structures into
fibers has also been explored. Two- or three-level fiber
arrays produce higher adhesion strength than singlelevel ones [105, 118−121]. Jeong et al. [86] reported
that the adhesion of two-level fiber arrays made of
polyurethane acrylate does not decrease with an
increase in surface roughness as long as it is less than
20 μm, exhibiting better adaptabilities than a singlelevel fiber array. This gecko-inspired surface generated
an adhesion strength of 260 kPa, which can be used
for moving on large-area glass surfaces [86].
The upper supporting level that mimics the lamella
or foot is usually fabricated as the backing layer of
the fiber array [122−124]. Lee et al. [122] fabricated
a gecko-inspired polyethylene surface, combining
lamellae and nanofiber arrays by heat rolling, which
exhibited high compliance. Tian et al. [125] experimentally revealed that the soft lamellar skin of the
gecko acts as a soft spring and contributes to the
reliable control of a wide range of adhesive states
rather than a repulsive state. Further, the three-legged
hybrid clamp mimicking a lamellar skin/setae structure
was developed to transfer a horizontally placed silicon
wafer. Sameoto et al. [123] fabricated a surface that
combines the macroscale substrate and the fiber array
Friction 1(2): 114–129 (2013)
to increase adaptability. Northen et al. [124] reported
a method to actively switch between adhesion and
non-adhesion by controlling the orientation of the
cantilever by a magnetic field; the adhesion strength
was only 14 Pa, but provided the general proof-ofconcept that adhesion can be reversibly controlled
through an external stimulus.
A selection of materials for gecko-inspired adhesives,
such as polymide [89, 106], PVS [25, 74], PDMS [99, 106,
108, 126], poly(methyl methacrylate) (PMMA) [115],
polyurethane [109–110, 113], polystyrene (PS) [105,
114], silicon rubber [127], polypropylene [128], and
polyethylene has also been considered [122]. It is proposed that polyurethane with a low Young’s modulus
can generate strong adhesion because the polar groups
contribute to the enhancement of the adhesion; thus,
this material may be suitable for gecko-inspired adhesives [129]. Lee et al. [130] coated the fabricated pillars
with a mussel-adhesive-protein-mimetic polymer in
order to improve the reversible wet adhesion property
under water. This chemical coating method appears to
be effective in enhancing the adhesion of functional
surfaces.
Since the thermal and electric properties of polymer
materials are not satisfactory in certain applications,
aligned carbon nanotubes with stable electrical and
thermal properties have drawn increased attention
[124, 131−134]. Yurdumankan et al. [132, 133] first
reported a multiwalled carbon nanotube (MWCNT)
adhesive with a PMMA-backing layer in which the
adhesion stress reached 16 MPa based on experiments
conducted on the nanoscale. CNT arrays with an
adhesion strength of 110 kPa have been achieved by
Zhao et al. [134], but their durability is poor. Ge et al.
[135, 136] reported that the CNT array supports a
200-kPa shear stress over a period of 8–12 h without
any cohesive break; the adhesion strength was 30–
50 kPa. A large increase in the adhesion and friction
strength of CNT adhesives was also reported by Qu
et al. [137, 138] who achieved 100 kPa and 1 MPa,
respectively. Fiber arrays with a high Young’s modulus
can generate strong adhesion because of the compliance
of nanotubes and their strong van der Waals forces
[34, 139−141].
3.2.2 Anisotropic friction of gecko-inspired surfaces
The tribological properties of materials mainly rely on
123
the surface structures and the chemical nature of the
surfaces. Anisotropy exists widely in nature [142]. At
the macroscopic scale, for example, textured structures
are widely applied in the weaving industry, sole
decorative patterns, tires, and roads in the form of
friction anisotropy. Friction anisotropy also exists
between crystalline surfaces, such as mica, exfoliated
graphene, synthetic self-assembled monolayers, and
quasi-crystalline structures of metallic alloys, which are
assigned to the lattice structure or elastic puckering.
The skin of several animals also have anisotropic
structures, such as the feathers of birds, the scales of
fish and snakes, and the finer hierarchical micro- and
nano-scale structures of lizards, geckos, and flies.
Among these structures, the gecko with its hierarchical
structures from the macro- to the micro- and nanoscales shows great advantages in terms of anisotropic
friction and adhesion properties, which enable geckos
to rapidly switch between attachment and detachment
on both walls (requiring switchable friction) and
ceilings (requiring switchable adhesion) [22].
Two types of structures can be used for fabricating
anisotropic gecko-inspired adhesives: inclined and
asymmetric structures. According to the fibrillar
adhesion models, an inclined fiber array generates
friction anisotropy [116, 143, 144]. For example, Murphy
et al. [135] prepared an angled spatula-shaped fibrillar
surface that generated obvious anisotropic friction
and adhesion. The adhesion force along the inclined
direction of the inclined fiber array produced by Yu
et al. [116] was 6 to 7 times higher than that along
the inclined direction. Zhou et al. [139] posited that
inclined MWCNT array surfaces produce stable
friction anisotropy over several thousands of cycles.
Fiber arrays with asymmetric shapes or asymmetric
spatulae at the ends also produce frictional anisotropy
[88, 108, 145−147]. Yoon et al. [89] reported on Janusfaced fiber arrays by selectively depositing a metal
layer only on one side, leading to friction anisotropies.
4
Future research avenues
Although recently there have been significant advances
in modeling the friction and adhesion mechanisms
of geckos, some challenging issues remain to fully
understand and mimic this complex frictional-adhesive
Friction 1(2): 114–129 (2013)
124
system. The contribution of hierarchical structures is
still not fully understood. The role of the lamellae has
not been extensively explored. The experiments on a
single seta by Autumn et al. showed that sliding a
single seta on a surface at a distance of approximately
5 μm can maximize the adhesion force while a larger
distance is required for the setal array. There are still
no models that can fully explain the anisotropic
mechanical deformations of the hierarchical structures
and the peel processes during sliding. The articulation
and deformations of all the different structures during
gripping and releasing is another future research
direction. Thus, the hierarchical design principles of
the gecko-inspired dry adhesive surfaces have yet to
be fully identified and established.
Further, the effects of the sliding velocity on gecko
adhesion and friction have not been fully explored,
although a few reports are available [148]. Most friction
models are based on the Coulomb Law or Model of
Friction, which may not apply to fine biological
structures. The interfacial interactions between the
foot pad proteins and the substrate surfaces must be
further investigated in order to fully understand the
impact of the sliding velocity on adhesion and friction
forces. The stick-slip phenomena also need to be
studied further [148].
Gecko-inspired adhesive surfaces have been proposed for various applications, such as wall-climbing
robots, reversible self-adhesive labels, fixation and
fastening, and biomedical materials and sports equipment, which require remarkable properties, including
reversible attachment and detachment without
breakage, strong stability in a wide range of humidity
and temperature, and high strength and easy (low
energy) motion during adhesion. Further work
resolving these issues will no doubt allow us to realize
the full potential for the applications of gecko-inspired
adhesive surfaces.
Acknowledgments
Z.M. and Y.T. are supported by the Natural Science
Foundation of China (Grant Nos. 51175281 and
51021064). H.Z. acknowledges the support of the
University of Alberta China Opportunity Fund and a
Discovery Grant Award from the Natural Sciences and
Engineering Research Council of Canada (NSERC).
N.P. acknowledges support through a PFund grant
from the Louisiana Board of Regents. J.N.I.’s contribution to parts of this review was sponsored by the
UCSB Institute for Collaborative Biotechnologies Grant
W911NF-09-D-000 from the U.S. Army Research Office
(the content of the information does not necessarily
reflect the position or the policy of the Government,
and no official endorsement should be inferred).
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
References
[1] Autumn K, Hsieh S T, Dudeket D M, Chen J, Chitaphan C,
Full R J. Dynamics of geckos running vertically. J Exp Biol
209: 260–272 (2006)
[2] Hansen W, Autumn K. Evidence for self-cleaning in gecko
setae. P Natl Acad Sci USA 102: 385–389 (2005)
[3] Cutkosky M R. Gecko-like robot scampers up the wall. New
Sci 2252: 29–33 (2006)
[4] Menon C, Murphy M, Sitti M. Gecko inspired surface
climbing robots. In Proceedings of the IEEE International
Conference on Robotics and Biomimetics, Shenyang, China,
2004: 431–436.
[5] Ruibal R, Ernst V. The structure of the digital setae of lizards.
J Morphol 117: 271–294 (1965)
[6] Russell A P, Bauer A M, Laroiya R. Morphological correlates
of the secondarily symmetrical Pes of Gekkotan lizards. J
Zool 241: 767–790 (1997)
[7] Autumn K. How gecko toes stick—The powerful, fantastic
adhesive used by geckos is made of nanoscale hairs that
engage tiny forces, inspiring envy among human imitators.
Am Sci 94: 124–132 (2006)
[8] Autumn K, Peattie A M. Mechanisms of adhesion in geckos.
Integr Comp Biol 42: 1081–1090 (2002)
[9] Autumn K, Sitti M, Peattie A, Liang A, Hansen W, Sponberg
S, Kenny T, Fearing R, Israelachvili J, Full R. Evidence for
van der Waals adhesion in gecko setae. P Natl Acad Sci USA
99: 12252–12256 (2002)
[10] Sun W X, Neuzil P, Kustandi T S, Oh S, Samper V D. The
nature of the gecko lizard adhesive force. Biophys J 89:
L14–L17 (2005)
[11] Huber G, Mantz H, Spolenak R, Mecke K, Jacobs K, Gorb
S N, Arzt E. Evidence for capillarity contributions to gecko
Friction 1(2): 114–129 (2013)
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
adhesion from single spatula nanomechanical measurements.
P Natl Acad Sci Usa 102: 16293–16296 (2005)
Prowse M S, Wilkinson M, Puthoff J B, Mayer G, Autumn
K. Effects of humidity on the mechanical properties of
gecko setae. Acta Biomater 7: 733–738 (2011)
Kim T W, Bhushan B. The adhesion model considering
capillarity for gecko attachment system. J R Soc Interface
5: 319–327 (2008)
Niederegger S, Gorb S N. Friction and adhesion in the tarsal
and metatarsal scopulae of spiders. J Comp Physiol A 192:
1223–1232 (2006)
Niewiarowski P H, Lopez S, Ge L H, Hagan E, Dhinojwala
A. Sticky gecko feet: The role of temperature and humidity.
PLoS One 3: e2192 (2008)
Pesika N S, Zeng H B, Kristiansen K, Zhao B, Tian Y,
Autumn K, Israelachvili J. Gecko adhesion pad: A smart
surface? J Phys-Condens Mat 21: 464132 (2009).
Puthoff J B, Prowse M S, Wilkinson M, Autumn K. Changes
in materials properties explain the effects of humidity on
gecko adhesion. J Exp Biol 213: 3699–3704 (2010)
Autumn K, Liang Y A, Hsieh S T, Zesch W, Chan WP,
Kenny T W, Fearing R, Full R J . Adhesive force of a single
gecko foot-hair. Nature 405: 681–685 (2000)
Huber G, Gorb S, Spolenak R, Arzt E. Resolving the
nanoscale adhesion of individual gecko spatulae by atomic
force microscopy. Biol Lett 1: 2–4 (2005)
Autumn K, Dittmore A, Santos D, Spenko M, Cutkosky M.
Frictional adhesion: A new angle on gecko attachment. J Exp
Biol 209: 3569–3579 (2006)
Zhao B X, Pesika N, Rosenberg K, Tian Y, Zeng H,
McGuiggan P, Autumn K, Israelachvili J. Adhesion and
friction force coupling of gecko setal arrays: Implications
for structured adhesive surfaces. Langmuir 24: 1517–1524
(2008)
Wan J, Tian Y, Zhou M, Zhang X J, Meng Y G. Experimental
research of load effect on the anisotropic friction behaviors
of gecko seta array. Acta Phys Sin 61: 016202 (2012)
Johnson K L, Kendall K, Roberts A D. Surface energy and
contact of elastic solids. Proc R. Soc London Ser A 324:
301–313 (1971)
Arzt E, Gorb S, Spolenak R. From micro to nano contacts in
biological attachment devices. P Natl Acad Sci USA 100:
10603–10606 (2003)
Peressadko A, Gorb S N. When less is more: Experimental
evidence for tenacity enhancement by division of contact
area. J Adhes 80: 247–261 (2004)
Peattie A M, Full R J. Phylogenetic analysis of the scaling
of wet and dry biological fibrillar adhesives. P Natl Acad
Sci USA 104: 18595–18600 (2007)
125
[27] Autumn K, Hansen W. Ultrahydrophobicity indicates a
non-adhesive default state in gecko setae. J Comp Physiol A
193: 1205–1212 (2006)
[28] Tang T, Hui C Y, Glassmaker N J. Can a fibrillar interface
be stronger and tougher than a non-fibrillar one? J R Soc
Interface 2: 505–516 (2005)
[29] Bonser R. The Young's modulus of ostrich claw keratin. J
Mater Sci Lett 19: 1039–1040 (2000)
[30] Peattie A M, Majidi C, Corder A B, Full R J. Similar elastic
modulus of setal keratin for two species of gecko. Integr
Comp Biol 461: E109 (2006)
[31] Peattie A M, Majidi C, Corder A, Full R J. Ancestrally high
elastic modulus of gecko setal beta-keratin. J R Soc Interface
4: 1071–1076 (2007)
[32] Autumn K, Majidi C, Groff R E, Dittmore A, Fearing R.
Effective elastice modulus of isolated gecko setal arrays.
J Exp Biol 209: 3558–3568 (2006)
[33] Persson B. On the mechanism of adhesion in biological
systems. J Chem Phys 118: 7614–7621 (2003)
[34] Schubert B, Majidi C, Groff R E, Baek S, Bush B,
Maboudian R, Fearing R S. Towards friction and adhesion
from high modulus microfiberarrays. J Adhes Sci Technol
21: 1297–1315 (2007)
[35] Kwak J S, Kim T W. A review of adhesion and friction
models for gecko feet. Int J Precis Eng Manuf 11: 171–186
(2010)
[36] Campolo D, Jones S, Fearing R S. Fabrication of gecko
foot-hair like nano structures and adhesion to random
rough surfaces. In Proceedings of 2003 IEEE International
Conference on Nanotechnology, SAN FRANCISCO, USA,
2003: 856–859.
[37] Aksak B, Murphy M P, Sitti M. Adhesion of biologically
inspired vertical and angled polymer microfiber arrays.
Langmuir 23: 3322–3332 (2007)
[38] Reyes M P, Fearing R S. Macromodel for the mechanics
of gecko hair adhesion. In Proceedings of 2008 IEEE
International Conference on Robotics and Automation,
Pasadena, 2008: 1602–1607.
[39] Jagota A, Bennison S J. Mechanics of adhesion through a
fibrillar microstructure. Integr Comp Biol 42: 1140–1145
(2002)
[40] Gao H J, Wang X, Yao H M, Gorb S, Arzt E. Mechanics of
hierarchical adhesion structures of geckos. Mech Mater 37:
275–285 (2005)
[41] Yao H, Gao H. Mechanics of robust and releasable adhesion
in biology: Bottom-up designed hierarchical structures of
gecko. J Mech Phys Solids 54: 1120–1146 (2006)
[42] Yao H, Gao H. Bio-inspired mechanics of bottom-up designed
hierarchical materials: Robust and releasable adhesion systems
Friction 1(2): 114–129 (2013)
126
of gecko. Bull Pol Acad Sci-Tech 55: 141–150 (2007)
[43] Bhushan B, Peressadko A G, Kim T W. Adhesion analysis
of two-level hierarchical morphology in natural attachment
systems for 'smart adhesion'. J Adhes Sci Technol 20: 1475–
1491 (2006)
[44] Kim T W, Bhushan B. Adhesion analysis of multi-level
hierarchical attachment system contacting with a rough
surface. J Adhes Sci Technol 21: 1–20 (2007)
[45] Kim T W, Bhushan B. Effect of stiffness of multi-level
hierarchical attachment system on adhesion enhancement.
Ultramicroscopy 107: 902–912 (2007)
[46] Zeng H B, Pesika N, Tian Y, Zhao B, Chen Y, Tirrell M,
Turner K L, Israelachvili J N. Frictional adhesion of patterned
surfaces and implications for gecko and biomimetic systems.
Langmuir 25: 7486–7495 (2009)
[47] Shah G J, Sitti M. Modeling and design of biomimetic
adhesives inspired by gecko foot-hairs. In Proceedings of the
IEEE International Conference on Robotics and Biomimetics,
Shenyang, 2004: 873–878.
[48] Peressadko A G, Gorb S N. Surface profile and friction force
generated by insects. In Proceedings of the 1st International
Industrial Conference Bionik 2004, Hannover, 2004: 257–261.
[49] Huber G, Gorb S N, Hosoda N, Spolenak R, Arzt E. Influence
of surface roughness on gecko adhesion. Acta Biomater 3:
607–610 (2007)
[50] Kendall K. Thin-film peeling—The elastic term. J Phys D
Appl Phys 8: 1449–1452 (1975)
[51] Peng Z L, Chen S H, Soh A K. Peeling behavior of a
bio-inspired nano-film on a substrate. Int J Solids Struct 47:
1952–1960 (2010)
[52] Pesika N S, Tian Y, Zhao B X, Rosenberg K, Zeng H,
McGuiggan P, Autumn K, Israelachvili J N. Peel-zone model
of tape peeling based on the gecko adhesive system. J Adhes
83: 383–401 (2007)
[53] Zhang L, Wang J. A generalized cohesive zone model of
the peel test for pressure-sensitive adhesives. Int J Adhes
Adhes 29: 217–224 (2009)
[54] Lin Y Y, Hui C Y, Wang Y C. Modeling the failure of an
adhesive layer in a peel test. J Polym Sci Pol Phys 40:
2277–2291 (2002)
[55] Zhou M, Tian Y, Pesika N, Zeng H, Wan J, Meng Y G,
Wen S Z. The extended peel zone model: Effect of peeling
velocity. J Adhes 87: 1045-1058 (2011)
[56] Tian Y, Pesika N, Zeng H B, Rosenberg K, Zhao B,
McGuiggan P, Autumn K, Israelachvili J. Adhesion and
friction in gecko toe attachment and detachment. P Natl
Acad Sci USA 103: 19320–19325 (2006)
[57] Endlein T, Ji A, Samuel D, Yao N, Wang Z, Barnes W P,
Federle W, Kappl M, Dai Z D. Sticking like sticky tape:
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
[69]
[70]
[71]
[72]
[73]
Tree frogs use friction forces to enhance attachment on
overhanging surfaces. J Roy Soc Interface 10: 20120838
(2011)
Fuller K, Tabor D. Effect of surface-roughness on adhesion
of elastic solids. P Roy Soc Lond A Mat 345: 327–342 (1975)
Briggs G, Briscoe B J. Effect of surface-topography on
adhesion of elastic solids. J Phys D Appl Phys 10: 2453–2466
(1977)
Persson B, Gorb S. The effect of surface roughness on the
adhesion of elastic plates with application to biological
systems. J Chem Phys 119: 11437–11444 (2003)
Peng Z L, Chen S H. Effects of surface roughness and film
thickness on the adhesion of a bio-inspired nanofilm. Phys
Rev E 83: 051915 (2011)
Sponberg S, Hansen W, Peattie A, Autumn K. Dynamics of
isolated gecko setal arrays. Am Zool 41: 1594–1594 (2001)
Schargott M, Popov V L, Gorb S. Spring model of biological
attachment pads. J Theor Biol 243: 48–53 (2006)
Wang Z Y, Gu W H, Wu Q, Ji A, Dai Z D. Morphology and
reaction force of toes of geckos freely moving on ceilings
and walls. Sci China Technol Sci 53:1688–1693 (2010)
Zhao B X, Pesika N, Zeng H B, Wei W, Chen Y, Autumn K,
Turner K, Israelachvili J. Role of tilted adhesion fibrils
(setae) in the adhesion and locomotion of gecko-like systems.
J Phys Chem B 113: 3615–3621 (2009)
Wan J, Tian Y, Zhou M, Zhang X-J, Meng Y-G. Experimental
research of load effect on the anisotropic friction behaviors
of gecko seta array. Acta Phys Sin 61: 016202 (2012)
Hill G, Soto D, Peattie A, Full R J, Kenny T W. Orientation
angle and the adhesion of single gecko setae. J Roy Soc
Interface 8: 926–933 (2011)
Pesika N, Gravish N, Wikinson M, Zhao B, Zeng H, Tian Y,
Israelachvili J, Autumn K. The crowding model as a tool to
understand and fabricate gecko-inspired dry adhesives. J
Adhes 85: 512–525 (2009)
Filippov A, Popov V L, Gorb S N. Shear induced adhesion:
Contact mechanics of biological spatula-like attachment
devices. J Theor Biol 276: 126–131 (2011)
Kumar A, Hui C Y. Numerical study of shearing of a
microfibre during friction testing of a microfibre array. P
Roy Soc Lond A Mat 467: 1372–1389 (2011)
Cheng Q H, Chen B, Gao H J, Zhang Y W. Sliding-induced
non-uniform pretension governs robust and reversible
adhesion: A revisit of adhesion mechanisms of geckos. J Roy
Soc Interface 9: 283–291 (2012)
Chen B, Wu P D, Gao H J. Pre-tension generates strongly
reversible adhesion of a spatula pad On substrate. J Roy Soc
Interface 6: 529–537 (2009)
Boesel L F, Greiner C, Arzt E, del Campo A. Gecko-inspired
Friction 1(2): 114–129 (2013)
[74]
[75]
[76]
[77]
[78]
[79]
[80]
[81]
[82]
[83]
[84]
[85]
[86]
[87]
[88]
[89]
surfaces: A path to strong and reversible dry adhesives. Adv
Mater 22: 2125–2137 (2010)
Gorb S, Varenberg M, Peressadko A, Tuma J. Biomimetic
mushroom-shaped fibrillar adhesive microstructure. J Roy
Soc Interface 4: 271–275 (2007)
Stork N E. Experimental-analysis of adhesion of chrysolinapolita (chrysomelidae, coleoptera) on a variety of surfaces.
J Exp Biol 88: 91–107 (1980)
Chan E P, Greiner C, Arzt E, Crosby A J. Designing model
systems for enhanced adhesion. Mrs Bull 32: 496–503 (2007)
Kroner E, Arzt E. What we can learn from geckos. Nachr
Chem 57: 137–139 (2009)
Varenberg M, Peressadko A, Gorb S, Arzt E. Effect of real
contact geometry on adhesion. Appl Phys Lett 89: 121905
(2006)
Gao H J, Ji B H, Jager I L, Arzt E, Fratzl P. Materials
become insensitive to flaws at nanoscale: Lessons from
nature. P Natl Acad Sci USA 100: 5597–5600 (2003)
Varenberg M, Murarash B, Kligerman Y, Gorb S N.
Geometry-controlled adhesion: Revisiting the contact splitting
hypothesis. Appl Phys A-Mater 103: 933–938 (2011)
Spolenak R, Gorb S, Gao H J, Arzt E. Effects of contact
shape on the scaling of biological attachments. P Roy Soc
Lond A Mat 461: 305–319 (2005)
Gorb S N, Varenberg M. Mushroom-shaped geometry of
contact elements in biological adhesive systems. J Adhes
Sci Technol 21: 1175–1183 (2007)
Varenberg M, Gorb S. Close-up of mushroom-shaped fibrillar
adhesive microstructure: Contact element behaviour. J Roy
Soc Interface 5: 785–789 (2008)
Creton C, Gorb S. Sticky feet: From animals to materials.
Mrs Bull 32: 466–472 (2007)
Gorb S N. Biological attachment devices: Exploring nature's
diversity for biomimetics. Philos T R Soc A 366: 1557–1574
(2008)
Geim A K, Dubonos S V, Grigorieva I V, Novoselov K S,
Zhukov A A, Shapoval S Y. Microfabricated adhesive
mimicking gecko foot-hair. Nat Mater 2: 461–463 (2003)
Jeong H E, Lee J K, Kim H N, Moon S H, Suh K Y. A
nontransferring dry adhesive with hierarchical polymer
nanohairs. P Natl Acad Sci USA 106: 5639–5644 (2009)
Parness A, Soto D, Esparza N, Gravish N, Wilkinson M,
Autumn K, Cutkosky M. A microfabricated wedge-shaped
adhesive array displaying gecko-like dynamic adhesion,
directionality and long lifetime. J Roy Soc Interface 6:
1223–1232 (2009)
Yoon H, Jeong H E, Kim T I, Kang T JAuthor Vitae, Tahk
DAuthor Vitae, Char K, Author VSuh K Y. Adhesion
hysteresis of janus nanopillars fabricated by nanomolding
127
and oblique metal deposition. Nano Today 4: 385–392 (2009)
[90] Sitti M, Fearing R S. Synthetic gecko foot-hair micro/
nano-structures as dry adhesives. J Adhes Sci Technol 17:
1055–1073 (2003)
[91] Glassmaker N J, Jagota A, Hui C Y, Kim J. Design of
biomimetic fibrillar interfaces: 1. Making contact. J Roy
Soc Interface 1: 23–33 (2004)
[92] Hui C Y, Jagota A, Lin Y Y, Kramer E J. Constraints on
microcontact printing imposed by stamp deformation.
Langmuir 18: 1394–1407 (2002)
[93] Spolenak R, Gorb S, Arzt E. Adhesion design maps for bioinspired attachment systems. Acta Biomater 1: 5–13 (2005)
[94] Greiner C, Spolenak R, Arzt E. Adhesion design maps for
fibrillar adhesives: The effect of shape. Acta Biomaterialia
5: 597–606 (2009)
[95] Zhou M, Pesika N, Zeng H, Wan J, Zhang X, Meng Y, Wen
S, Tian Y. Design of gecko-inspired fibrillar surfaces with
strong attachment and easy-removal properties: A numerical
analysis of peel-zone. J Roy Soc Interface 9: 2424–2436
(2012)
[96] Crosby A J, Hageman M, Duncan A. Controlling polymer
adhesion with "pancakes". Langmuir 21: 11738–11743 (2005)
[97] Campo D A, Greiner C. Su-8: A photoresist for high-aspectratio and 3D submicron lithography. J Micromech Microeng
17: R81–R95 (2007)
[98] Lamblet M, Verneuil E, Vilmin T, Buguin A, Silberzan P,
Léger L. Adhesion enhancement through micropatterning at
polydimethylsiloxane-acrylic adhesive interfaces. Langmuir
23: 6966–6974 (2007)
[99] Greiner C, Campo D A, Arzt E. Adhesion of bioinspired
micropatterned surfaces: Effects of pillar radius, aspect ratio,
and preload. Langmuir 23: 3495–3502 (2007)
[100] Lu G W, Hong W J, Tong L, Bai H, Wei Y, Shi G. Drying
enhanced adhesion of polythiophene nanotubule arrays on
smooth surfaces. ACS Nano 2: 2342–2348 (2008)
[101] Lee J, Fearing R S. Contact self-cleaning of synthetic gecko
adhesive from polymer microfibers. Langmuir 24: 10587–
10591 (2008)
[102] Campo D A, Greiner C, Arzt E. Contact shape controls
adhesion of bioinspired fibrillar surfaces. Langmuir 23:
10235–10243 (2007)
[103] Campo D A, Greiner C, Alvarez I, Arzt E. Patterned surfaces
with pillars with controlled 3D tip geometry mimicking
bio-attachment devices. Adv Mater 19: 1973–1977 (2007)
[104] Aksak B, Murphy M P, Sitti M. Gecko inspired microfibrillar adhesives for wall climbing robots on micro/
nanoscale rough surfaces. In Proceedings of 2008 IEEE
International Conference on Robotics and Automation,
Pasadena, 2008: 3058–3063.
128
[105] Lee D Y, Lee D H, Lee S G, Cho K. Hierarchical geckoinspired nanohairs with a high aspect ratio induced by
nanoyielding. Soft Matter 8: 4905–4910 (2012)
[106] Davies J, Haq S, Hawke T, Sargent J P. A practical approach
to the development of a synthetic gecko tape. Int J Adhes
Adhes 29: 380–390 (2009)
[107] Haefliger D, Boisen A. Three-dimensional microfabrication
in negative resist using printed masks. J Micromech Microeng
16: 951–957 (2006)
[108] Sameoto D, Menon C. Direct molding of dry adhesives with
anisotropic peel strength using an offset lift-off photoresist
mold. J Micromech Microeng 19: 1–5 (2009)
[109] Kim S, Sitti M. Biologically inspired polymer microfibers
with spatulate tips as repeatable fibrillar adhesives. Appl
Phys Lett 89: 261911 (2006)
[110] Jin K, Tian Y, Erickson J S, Puthoff J, Autumn K, Pesika
N S. Design and fabrication of gecko-inspired adhesives.
Langmuir 28: 5737–5742 (2012)
[111] Sameoto D, Menon C. Deep UV patterning of acrylic
masters for molding biomimetic dry adhesives. J Micromech
Microeng 20: 115037 (2010)
[112] Krahn J, Sameoto D, Menon C. Controllable biomimetic
adhesion using embedded phase change material. Smart
Mater Struct 20: 015014 (2011)
[113] Kim S, Sitti M, Hui C Y, Long R, Jagota A. Effect of
backing layer thickness on adhesion of single-level elastomer
fiber arrays. Appl Phys Lett 91: 161905 (2007)
[114] Kustandi T S, Samper V D, Yi D K, Ng W S, Neuzil P,
Sun W. Self-assembled nanoparticles based fabrication
of gecko foot-hair-inspired polymer nanofibers. Adv Funct
Mater 17: 2211–2218 (2007)
[115] Yu J, Chary S, Das S, Tamelier J, Pesika N S, Turner K L,
Israelachvili J N. Gecko-inspired dry adhesive for robotic
applications. Adv Funct Mater 21: 3010–3018 (2011)
[116] Reddy S, Arzt E, Campo D A. Bioinspired surfaces with
switchable adhesion. Adv Mater 19: 3833–3837 (2007)
[117] Sato H, Houshi Y, Shoji S. Three-dimensional microstructures consisting of high aspect ratio inclined micro-pillars
fabricated by simple photolithography. Microsyst Technol
10: 440–443 (2004)
[118] Kustandi T S, Samper V D, Ng W S, Chong A S, Gao H.
Fabrication of a gecko-like hierarchical fibril array using a
bonded porous alumina template. J Micromech Microeng
17: N75–N81 (2007)
[119] Greiner C, Arzt E, Campo D A. Hierarchical gecko-like
adhesives. Adv Mater 21: 479–482 (2009)
[120] Murphy M P, Kim S, Sitti M. Enhanced adhesion by
gecko-inspired hierarchical fibrillar adhesives. ACS Appl
Mater Inter 1: 849–855 (2009)
Friction 1(2): 114–129 (2013)
[121] Bhushan B, Lee H. Fabrication and characterization of
multi-level hierarchical surfaces. Faraday Discuss 156:
235–241 (2012)
[122] Lee J, Bush B, Maboudian R, Fearing R S. Gecko-inspired
combined lamellar and nanofibrillar array for adhesion on
nonplanar surface. Langmuir 25: 12449–12453 (2009)
[123] Sameoto D, Li Y S, Menon C. Multi-scale compliant foot
designs and fabrication for use with a spider-inspired
climbing robot. J Bionic Eng 5: 189–196 (2008)
[124] Northen M T, Greiner C, Arzt E, Turner K L. A geckoinspired reversible adhesive. Adv Mater 20: 3905–3909
(2008)
[125] Tian Y, Wan J, Pesika N, Zhou M. Bridging nanocontacts
to macroscale gecko adhesion by sliding soft lamellar skin
supported setal array. Sci Rep 3: 1382 (2013)
[126] Menon C, Murphy M, Sitti M. Gecko inspired surface
climbing robots. In Proceedings of 2008 IEEE International
Conference on Robotics and Automation, Pasadena, 2004:
3058–3063.
[127] Sitti M, Fearing R S. Nanomolding based fabrication of
synthetic gecko foot-hairs. In Proceedings of the 2002 2nd
IEE Conference on Nanotechnology, Washington, 2002:
137–140.
[128] Lee J, Fearing R S. Contact self-cleaning of synthetic gecko
adhesive from polymer microfibers. Langmuir 24: 10587–
10591 (2008)
[129] Dai Z D, Yu M, Gorb S. Adhesion characteristics of
polyurethane for bionic hairy foot. J Intel Mat Syst Str 17:
737–741 (2006)
[130] Lee H, Lee B P, Messersmith P B. A reversible wet/dry
adhesive inspired by mussels and geckos. Nature 448:
334–338 (2007)
[131] Ajayan P M, Zhou O Z. Applications of carbon nanotubes.
In Carbon Nanotubes. Berlin: Springer, 2001: 391–425.
[132] Yurdumakan B, Raravikar N R, Ajayan P M, Dhinojwala
A. Synthetic gecko foot-hairs from multiwalled carbon
nanotubes. Chem Commun: 3799–3801 (2005)
[133] Dhinojwala A, Ge L H, Sethi S, Yurdumakan B, Lijie C,
Ajayan P M. Synthetic gecko foot-hairs from multiwalled
carbon nanotubes. In 2007 Spring National ACS Meeting,
Chicago, 2007: 64.
[134] Zhao Y, Tong T, Delzeit L, Kashani A, Meyyappan M,
Majumdar A. Interfacial energy and strength of multiwalledcarbon-nanotube-based dry adhesive. J Vac Sci Technol B
24: 331–335 (2006)
[135] Wirth C T, Hofmann S, Robertson J. Surface properties
of vertically aligned carbon nanotube arrays. Diam Relat
Mater 17: 1518–1524 (2008)
[136] Ge L H, Sethi S, Ci L, Ajayan P M, Dhinojwala A. Carbon
Friction 1(2): 114–129 (2013)
129
nanotube-based synthetic gecko tapes. P Natl Acad Sci
USA 104: 10792–10795 (2007)
Qu L, Dai L. Gecko-foot-mimetic aligned single-walled
carbon nanotube dry adhesives with unique electrical and
thermal properties. Adv Mater 19: 3844–3849 (2007)
Qu L, Dai L, Stone M, Xia Z, Wang Z L. Carbon nanotube
arrays with strong shear binding-on and easy normal liftingoff. Science 322: 238–242 (2008)
Majidi C S, Groff R E, Fearing R S. Attachment of fiber
array adhesive through side contact. J Appl Phys 98: 103521
(2005)
Zhou M, Liu K, Wan J, Li X, Jiang K, Zeng H, Zhang X,
Meng Y G, Shizhu Wen S Z, Zhu H W, Tian Y. Anisotropic
interfacial friction of inclined multiwall carbon nanotube
array surface. Carbon 30: 5372–5379 (2012)
Hu S H, Xia Z H, Gao X S. Strong adhesion and friction
coupling in hierarchical carbon nanotube arrays for dry
adhesive applications. ACS Appl Mater Inter 4: 1972–1980
(2012)
Zheng Y M, Gao X F, Jiang L. Directional adhesion of
superhydrophobic butterfly wings. Soft Matter 3: 178–182
(2007)
[143] Lee J H, Fearing R S, Komvopoulos K. Directional
adhesion of gecko-inspired angled microfiber arrays. Appl
Phys Lett 93: 191910 (2008)
[144] Murphy M P, Aksak B, Sitti M. Gecko-inspired directional
and controllable adhesion. Small 5: 170–175 (2009)
[145] Tamelier J, Chary S, Turner K L. Vertical anisotropic
microfibers for a gecko-inspired adhesive. Langmuir 28:
8746–8752 (2012)
[146] Sameoto D, Menon C. A low-cost, high-yield fabrication
method for producing optimized biomimetic dry adhesives.
J Micromech Microeng 19: 115002 (2009)
[147] Shon J K, Kong S S, Kim J M, Ko C H, Jin M, Lee Y Y,
Hwang S H, Yoon J A, Kim J N. Facile synthesis of highly
ordered mesoporous silver using cubic mesoporous silica
template with controlled surface hydrophobicity. Chem
Commun 14: 650–652 (2009)
[148] Gravish N, Wilkinson M, Sponberg S, Parness A, Esparza
N, Soto D, Yamaguchi T, Broide M, Cutkosky M, Creton C,
Autumn K. Rate-dependent frictional adhesion in natural
and synthetic gecko setae. J Roy Soc Interface 7: 259–269
(2010)
Yu TIAN, born 1975, is the Professor
and Associate Director of the State
Key Laboratory of Tribology at
Tsinghua University in China.
Tian gained his BA and PhD in
Mechanical Engineering at Tsinghua
University in 1998 and 2002,
respectively. Then he became a faculty of the State
Key Laboratory of Tribology at Tsinghua University.
He did his postdoc at the University of California,
Santa Barbara with Professor Jacob Israelachvili
(from 2005 to 2007). He has been a visiting associate
professor at Nanyang Technology University for five
months. His research interest is the science and
technology at the interface of physics, materials,
engineering and biology to understand the physical
laws of adhesion, friction and rheology. He has
published over 70 peer-reviewed journal papers. He
has received the Wen Shizhu-Maple Award-Young
Scholar award (2012), the Young Scholar Achievement
Award of the Society of Mechanical Engineering of
China (2011), Outstanding Young Scholar Award of the
Chinese Tribology Institute (2009), and the National
Excellent Doctoral Dissertation of China (2004).
Ming ZHOU, graduated in 2007
from Tsinghua University, and
received her PhD degree in
2013 from the SKLT, Tsinghua
University. Now she is working
as an engineer in the Institute
of Mechanical Manufacturing
Technology, China Academy of Engineering Physics.
She has published 9 papers. Her research interests
include the mechanism and application of gecko
adhesion and the gecko-inspired surfaces, nano
contact mechanics, nano-tribology, and more recently,
the techniques and mechanisms of ultra-precision
machining.
[137]
[138]
[139]
[140]
[141]
[142]
Friction 1(2): 130–142 (2013)
DOI 10.1007/s40544-013-0015-1
ISSN 2223-7690
REVIEW ARTICLE
Skin tribology: Science friction?
E. VAN DER HEIDE1,2,*, X. ZENG1, M.A. MASEN1
1
Laboratory for Surface Technology and Tribology, Faculty of Engineering Technology, University of Twente, Drienerlolaan 5, 7522 NB
Enschede, The Netherlands
2
TNO, De Rondom 1, 5612 AP Eindhoven, The Netherlands
Received: 01 February 2013 / Revised: 30 March 2013 / Accepted: 23 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: The application of tribological knowledge is not just restricted to optimizing mechanical and chemical
engineering problems. In fact, effective solutions to friction and wear related questions can be found in our
everyday life. An important part is related to skin tribology, as the human skin is frequently one of the interacting
surfaces in relative motion. People seem to solve these problems related to skin friction based upon a trial-anderror strategy and based upon on our sense for touch. The question of course rises whether or not a trained
tribologist would make different choices based upon a science based strategy? In other words: Is skin friction
part of the larger knowledge base that has been generated during the last decades by tribology research groups
and which could be referred to as Science Friction? This paper discusses the specific nature of tribological systems
that include the human skin and argues that the living nature of skin limits the use of conventional methods.
Skin tribology requires in vivo, subject and anatomical location specific test methods. Current predictive friction
models can only partially be applied to predict in vivo skin friction. The reason for this is found in limited
understanding of the contact mechanics at the asperity level of product–skin interactions. A recently developed
model gives the building blocks for enhanced understanding of friction at the micro scale. Only largely
simplified power law based equations are currently available as general engineering tools. Finally, the need for
friction control is illustrated by elaborating on the role of skin friction on discomfort and comfort. Surface
texturing and polymer brush coatings are promising directions as they provide way and means to tailor friction
in sliding contacts without the need of major changes to the product.
Keywords: friction; bio-tribology; skin; soft tissue; surface texture; brush coatings
1
Skin friction in daily life
The application of tribological knowledge, i.e., knowledge on the science and technology of interacting
surfaces in relative motion, is not restricted to
optimizing mechanical and chemical engineering
problems. In fact, effective solutions to tribology related
questions are evident in our everyday life, as illustrated
in fascinating examples described by D. Dowson’s
“A tribological day” [1]. An important part of the
effective solutions in daily life situations is related to
skin tribology, as the human skin is frequently one
* Corresponding author: E. VAN DER HEIDE.
E-mail: [email protected]
of the interacting surfaces in relative motion. These
questions are typically related to optimizing friction
and lubrication problems in skin–product interactions,
rather than to optimising wear. Take for example the
swimming pool or bathroom where material selection
and application of anti-slip coatings prevent us from
falling when the floor gets wet. Yet, if such coatings
do not sufficiently increase friction, one will optimize
the tribological system, e.g., by pressing our full foot
to the floor and subsequently increasing the true area
of contact or by changing the operational conditions,
e.g., by minimising the sliding velocity, in order to
prevent falling. Another striking example of optimising
the frictional response of a skin–product interaction
Friction 1(2): 130–142 (2013)
in the bathroom is found in shaving. The application
of tailored operational conditions during shaving, i.e.,
person specific pressure and sliding velocity during
the shaving action, combined with tailored boundary
layers—shaving soap—gives a close shave. Another
modern-day typical aspect of our current lifestyle is the
interaction with touch screens, which are dominantly
present around us world-wide, especially among the
younger generation of consumers. Touching screens
with the index finger clearly illustrates the relative
importance of skin friction: reduced control over
friction during the interaction, e.g., because of the
environmental conditions, will reduce the ability to
manipulate the device. People will change the
operational conditions, i.e., sliding velocity or contact
pressure, in such a situation to regain control based
on a trial-and-error strategy. This probably holds for
more skin–product interactions such as selection of
clothing and textiles. People seem to solve these problems related to skin friction based upon a trial-anderror strategy and based upon on our sense for touch.
The question rises whether or not a trained tribologist
would make different choices based upon a science
based strategy? In other words: Is skin friction part of
the larger knowledge base that has been generated
during the last decades by tribology research groups
and which could be referred to as Science Friction? This
paper tries to formulate an answer to this question by
elaborating on the specific nature of the tribological
system, by elaborating on the feasibility of current
friction models to skin tribology, and by the possibilities
to influence friction in skin–product interactions by
surface texturing and polymer coatings.
2
2.1
131
situations is reduced in the case of skin tribology to
the interaction between a product surface and a skin
surface in the presence of a possible “lubricant” and
surrounded by a specific environment (see Fig. 1).
In product–skin interactions, the function of the
systems is related to the application, i.e., sports or personal care with a process that depends on the selected
product, like for example making a sliding on artificial
turf or wet-shaving, respectively. The connections
between the system and the rest of the application can
generally be reduced to input: the operating variables,
and output: friction and wear. In the case of skin tribology one of the contacting surfaces is a living material.
The implication of this condition is only limitedly
explored in current engineering practise [4]. Emphasis
is put on the connection with the human somatosensory
system, see Refs. [5−9] for touch related literature and
on the characteristics of individual subjects [10] in
relation to best practises in panel testing.
The human somatosensory system has a tribological
aspect. In fact, the exploratory procedure that is used to
touch a surface is similar to experimentally determining
friction in a reciprocating test. By pressing your
finger(s) at the surface of interest and sliding to feel
specific features, friction is generated in the contact.
Friction in skin–product interactions
The systems approach and living materials
A well accepted method for analysing the tribological
performance is based upon the so-called systems
approach [2, 3]. Basically this means that a tribological
contact situation is separated from the application
studied, by using a hypothetical system envelope. The
contact situation separated by this envelope is regarded
as a system, that is, a set of elements interconnected by
structure and function. Hence, the structure of contact
Fig. 1 Schematic presentation of the tribological system in
skin−product contacts, showing the interaction of the product’s
surface with the top layer of the human skin, in the presence of a
lubricant and surrounded by the environment. The input, i.e., the
operating variables and the output, i.e., friction and wear, connect
the tribo system with the rest of the application. Histology by P.
van Erp, Dermatology, Nijmegen, NL.
Friction 1(2): 130–142 (2013)
132
Pressure in this contact is linked to the applied normal
force of for example the finger that “feels” the surface
and sliding velocity is related to the exact exploratory
procedure that is selected for feeling. A key aspect of
the human sense for touch is formed by a group of
sensory cells, an assortment of morphologically and
functionally distinct mechanosensory cell types that
are tuned to selectively respond to various mechanical
stimuli, such as vibration, stretch and pressure. In
glabrous skin of the palms and fingertips, Pacinian
corpuscles, rapidly adapting Meissner’s corpuscles,
Merkel cell-neurite complexes, Ruffini corpuscles make
up the majority of touch receptors [11, 12]. From the
tribological action, signals are produced by the
sensory cells that are transmitted by the nerve system,
through the spinal cord, to the thalamus and from
there to the somatosensory part of the brain. Next, the
sensory information is processed by the brain, i.e.,
organised, identified, and interpreted in order to
fabricate a mental representation, which essentially
determines the touch perception or tactility of a
surface. The relation between finger ridges, vibrations,
friction and surface texture is subject of research in
Refs. [13, 14], yet a straightforward translation to comfort during use [15] or an application to for example
touch perception of robotic fingers is at the very
beginning of development [9].
The set of operating variables, involved in tribological
contact situations in skin–product interactions and their
relative importance strongly depends on the actual
application. Sliding velocity and the load or interfacial
pressure are usually taken as main operating variables.
The loss-output of a tribo-system is described by
measuring and classifying the friction and wear
characteristics of the system. Wear is typically discussed in terms of removal of the stratum corneum,
the presence of scratches or wounds or by indirect
measures such as trans epidermal water loss, skin
irritation and redness or the occurrence of blisters
[16, 17]. Friction data and models are presented by
Refs. [13, 18−20] and are discussed in more detail in
Section 2.
The systems approach is designed to handle
complex processes that influence wear or unexpected
friction levels in industrial practice and shows a way
to simulate critical aspects of the operation at a
laboratory scale. By changing the operating variables
and studying the tribological characteristics it becomes
possible to optimise the function of the system, without
necessarily understanding the structure of the system
in detail. Secondly, it is possible to study the structure
of a system by varying the elements and comparing
the performance at given operational conditions. Both
techniques are used in skin tribology.
2.2
Modeling and predicting friction
The science of friction typically starts with theempirical rules formulated by Amontons and Coulomb for
elastically deforming, dry contacts, i.e., the force of
friction is directly proportional to the applied load,
the force of friction is independent of the apparent
area of contact and the force of dynamic friction is
independent of the sliding velocity. These empirical
rules are summarized by Eq. (1) in which µ is the
coefficient of friction, Ff the friction force and Fn the
normal force.

Ff
Fn
(1)
The coefficient of friction given by Eq. (1) can be determined experimentally, maintaining a sliding contact
with the contacting surfaces of interest and using a
limited range of operating variables.
In vivo experimental research on skin friction is
conducted basically with four contact set-ups, i.e., the
contact material moves with respect to skin linearly,
the contacting material rotates with the axis of rotation
parallel to the skin or rotates with the axis of rotation
perpendicular to the skin, or the skin moves linearly
in contact with a non-moving surface. A summary of
the experimental research on skin friction, given by
Derler and Gerhardt [21], and recently by Veijgen [4]
reveals a large range of values for the coefficient of
dynamic friction [4], i.e., from 0.07 [22] to 5.0 [23]. This
is also found for the coefficient of static friction [4]
that ranges from 0.11 [24] to 3.4 [25]. Based on these
results it is concluded that the coefficient of friction in
skin–product interaction is not constant and depends
greatly upon the operational conditions, the environmental conditions, materials selection and possibly
upon the type of motion that is used for the study, see
Table 1 for an overview extracted from Ref. [4]. This
Friction 1(2): 130–142 (2013)
Table 1
133
Coefficient of (a) dynamic friction and (b) static friction from experimental research, extracted from Ref. [4].
(a)
Reference used in Ref. [4]
Asserin et al. [26]
Location at human body*
Forearm (V)
Counter surface
Ruby
Bobjer et al. [27]
Finger
PC
Hand (D)
PTFE
PA, sheet
PE
Wool
PA, knitted
Terylene
Comaish & Bottoms [28]
Cua et al. [29, 30]
Derler et al. [31]
Forehead
Upper arm
Forearm (V)
Forearm (D)
Postauricular
Hand (P)
Abdomen
Upper back
Lower back
Thigh
Ankle
Finger
Forearm (V)
El-Shimi [22]
Forearm (D)
Gee et al. [32]
Finger
PTFE
Wool
Polished steel
Polished steel
Rough steel
Rubber
PC
Steel
Glass
PE
Paper
Scar tissue
Li et al. [33, 34]
PE
Prosthetic / healthy skin
Naylor [35]
Lower leg (V)
PE
Pailler-Mattei et al. [23]
Forearm (V)
Steel
Forearm (V)
Ramalho et al. [36]
Glass
Palm
Sivamani & Maibach [37]
Finger (D)
* (V) ventral, (D) dorsal and (P) palmar side
Stainless steel
μdynamic
0.7
2.22
0.85
0.61−1.21
0.11−0.30
0.09−0.28
0.10−0.72
0.20
0.47
0.30−1.3
0.40
0.37
0.40
0.34
0.23
0.26
0.23
0.34
0.21
0.12
0.25
0.19
0.15
0.21
0.27−0.71
0.31
0.07−0.38
0.37
0.12
2.4
2.7
1.8
1.2
1.6
0.6
0.8
0.6
0.72
0.47
0.17
0.5−0.6
Max 1.1
Max 1.1
1.1−1.4
0.15−1.07
0.17−0.87
0.10−0.84
0.5−1.35
0.8−1.4
1.21
0.90
1.24
0.45−0.7
0.8−1.4
1.1
0.55
0.3−0.9
Remarks
–
1 N normal load
20 N normal load
Sweat
Glycerol
Paraffin oil
Lard
Untreated
Silicone oil, velocity
Dry
Dry
0.1 N normal load
0.7 N normal load
0.1 N normal load
0.7 N normal load
8.0 N normal load
Wweating
Cleaned skin
Standard
Washed
Alcohol
Glycerine
Petrolatum
Standard
Washed
Alcohol
Glycerine
Petrolatum
0.05 N normal load
0.45 normal load
Cream
Friction 1(2): 130–142 (2013)
134
(b)
Reference used in Ref. [4]
Location at human body*
Counter surface
PTFE
PA, sheet
PE
Wool
PA, knitted
Terylene
PE
PE
Hand (D)
Comaish & Bottoms [28]
Hand (P)
Lower leg
Al (lacquered)
Lewis et al. [24]
Finger
Label paper
Mossel & Roosen, adapted
from Ref. [4]
Mossel, adapted from Ref. [4]
μstatic
0.25
0.55
0.43
0.45
0.42
0.45
0.62.1
0.6−1.3
0.26
0.54
0.11
0.29
0.41
0.13
Finger
Stainless steel
0.35−1.13
Finger
Stainless steel
0.35−0.94
Remarks
0.03−10 N normal load
Dry
Wet
Oil
Dry
Wet
Oil
* (V) ventral, (D) dorsal and (P) palmar side
dependence of friction on the system characteristics is
consistent with the non-linear, visco-elastic mechanical
behavior of the skin and with the strong dependence
of the mechanical properties of the outermost layers
of the skin with the environmental conditions [21].
An explanation for the nonlinear relation between
the friction force and the normal force in skin–object
interactions could be found in analyzing the frictional
response with the two term (non-interacting) model
of friction [13, 18−21]. The friction force in skin–object
interactions is seen as the sum of the forces required
to break the adhesive bonds between the two surfaces
at the asperity level, Ff, adh, and the forces related to the
deformation of the bodies in contact, Ff, def. This concept
was recently applied to the contact of a regularly
patterned surface in contact with in vivo skin by van
Kuilenburg et al. [13]. The regular pattern consisted
of an array of summits of equal height with a common
radius Rsummit at a distance λ in both x and y direction,
made by direct laser texturing. The term related to adhesion in the contact between the summits and the skin,
is assumed to be proportional to the real area of contact
for each summit individually, Areal, summit, see Eq. (2).
Ff ,adh  τAreal ,summit
(2)
The interfacial shear strength, τ, depends on subject
specific or anatomical location specific “lubricating” properties of the skin, like the sebum content, hydration
of the skin, the amount of sweat, any effects due to
treatments of the skin, such as the use of creams and
conditioners [26] and possibly the hair density [4]. The
deformation related term is assumed to be determined
by the indentation of an individual summit into the
skin, see Eq. (3) [38],
Ff ,adh 
3 
 Fn
16 R
(3)
in which β is the visco elastic loss fraction,  the radius
of the contact area and R the radius of the individual
summit present at the textured surface.
Expressions for the area of contact H and the indentation depth  H in the Hertzian case for an individual
summit–skin contact are depicted in Eqs. (4) and (5),
respectively.
n 
 H  
*
 4 E 
3 RF
1/ 3
 9 Fn 2 
H  
*2 
 16 RE 
(4)
1/ 3
(5)
in which E* equals the reduced elastic modulus given
by Eq. (6):
1 1  vskin 2 1  vproduct


E*
Eskin
Eproduct
2
(6)
Friction 1(2): 130–142 (2013)
135
with Eskin, Eproduct, vskin, and vproduct the Young’s moduli
and Poisson’s ratios of the skin and product surface,
respectively at the asperity level. As the elastic
moduls of skin is not a material property but a system
property—values depend e.g., on the indentation depth
and the indentors radius, see Ref. [39] —it is necessary
to use values that are measured with indenter that have
equal or similar dimensions as the summits of interest.
Values for Eskin and vskin could therefore be taken from
representative experimental research presented in
Ref. [40]. Although the viscous character of skin is not
incorporated in this contact model yet, it is possible
to improve the quality of the model greatly by adding
adhesion to the Hertzian contact model. As demonstrated by Ref. [13], the normal force acting on an individual summit must be corrected to an effective normal
force, Feff,summit to correctly estimate the increased contact
area for that specific summit–skin contact.
Feff,summit  Fn  2 Fadh  2 Fadh ( Fn  Fadh )
(7)
with the adhesive force Fadh based on the JKR theory
of adhesion [41],
Fadh 
3
 RW12
2
(8)
The work of adhesion at the asperity level, W12, gives
the opportunity to fine tune the overall contact by
tailoring individual summits to the presence of specific
layers. The feasibility of this approach however, is to
be validated by future research. From Eqs. (3)−(8) one
can construct an expression for the real or true area of
contact, as a function of the material properties of the
skin and product, as a function of the two controlling
roughness parameters and the nominal contact area
A0, see Eq. (9):
2
Areal
2
2
3 3 R 3  E 3
   *     eff  A0
 4 E      A0 
(9)
Similarly, an expression for the deformation related
term of friction for an individual summit–skin contact
with radius asummit-skin relative to the radius of that
specific summit R can be constructed, see Eq. (10).
1
Equations (9) and (10) can be used as building blocks
for predicting skin-friction, as shown in more detail
in the work of Van Kuilenburg et al. [13].
The presented approach , although developed for a
specific texture, could possibly be extended to rough
product surfaces in general, as it is based on the
contact behavior of individual summits.
An alternative approach that circumvents these
issues has been followed by Veijgen et al. [4, 10], who
used multivariable statistical analyses to develop a
quantitative model for the friction of human skin
based on a large dataset composed of several hundred
friction measurements and recording the associated
tribo- system properties, including contact conditions
and the environment, but also subject characteristics,
and dietary habits.
However, a complete physics-based model describing the friction behaviour of human skin is still a
subject of debate and research and is not expected to be
ready for engineering purposes at short notice. In the
meantime a power law expression given by Eq. (11)
is frequently suggested as simplified model for the
coefficient of friction:
2
1
asummit-skin  3  3    3  Eeff  3
 *   

R
 4 E   R   A0 
(10)
  c1  Fnc
2 1
(11)
One could start with c2 = 2/3 for contact situations
where adhesion is dominant, compare Eqs. (9) and (1),
and with c2 = 4/3 for situation where deformation is
dominant, compare Eqs. (10) and (2) and fine-tune
with c1.
3
3.1
Engineering skin friction
The role of skin friction in comfort perception
Materials selection by manufacturers of sports and care
products includes optimising the complex interaction
of manufacturing costs, functionality, durability and
product specific aspects like colour. The degree of
comfort or the degree of discomfort, important from
the user’s point of view, is incorporated as well in this
selection process. Analysis of comfort and discomfort
in skin–product interactions that involve sliding
actions—thinking of making a sliding on artificial
turf—clearly reveals the relative importance of skin
friction in relation to comfort and discomfort.
Friction 1(2): 130–142 (2013)
136
Deformation of the skin during sliding could cause
discomfort. A threshold for that is given by Xu et al.
[42] as the threshold for stress at the nociceptor
location and is assumed to be 0.2 MPa. The depth of
the nociceptor varies in the range of 75 to 200 μm below
the skin surface. Below the threshold values for mechanical damage, σcrit, tactile sensation is determined by
the subsurface stresses and strains at the locations of
the mechanoreceptors in the skin:
 Merkel cells—points, edges and curvatures;
 Meissner corpuscles—slip, friction and vibrations
(10−200 Hz);
 Ruffini endings—(direction of) motion;
 Pacinian corpuscles—surface roughness, vibrations
(70−1000 Hz).
A linear relation between the firing rate of the nerve
endings and the subsurface stress and strain distribution in the skin is known to exist as shown by Sripati
et al. [43]. Innervation density and psychophysical
thresholds of defined stimuli at the skin surface have
been investigated thoroughly within the scope of for
example haptics and plastic surgery [44].
The subsurface stress and strains within the skin
are influenced by skin friction. For estimation of the
influence of friction load at the surface on the magnitude of stresses within the skin explicit equations
are available [45]. For example, the maximum tensile
stress beneath a sliding spherical contact occurs at the
skin surface at the back edge of the contact and contains
a term that increases linearly with the coefficient of
friction and with the maximum contact pressure, pmax.
In other words, the absolute stress value at the skin
surface could rise an order of magnitude if friction
changes from µ = 0.1 to µ = 1, e.g., due to changes in
environmental conditions. As such, it is important
to characterize the mechanical intensity of a contact,
e.g., by defining a dimensionless mechanical intensity
number MI given by
MI 
 pmax
 crit
was found and modelled successfully using an
Arrhenius equation by Tropea and Lee [46]. Tissue
specific values are found experimentally by calibration.
Non-invasive tests with a thermal imager confirmed
that the temperature of the skin surfacs rises after
friction testing [47]. A solution for local surface temperature rise presented in Ref. [48] and summarized
by Eq. (3) can now be used to predict skin temperature
rise by frictional heating in real asperity contacts.
Tf 
  Fn  v
a  Keff
(13)
with Keff the effective thermal conductivity that takes
into account the operational conditions and the thermal
properties of the contacting materials. From Eq. (13)
it is clear that the local temperature increases linearly
with the coefficient of friction and is equally sensitive
for an increase in sliding velocity. In other words,
higher sliding velocities require low friction forces
in skin–product interactions. From Eq. (3) one can
construct a thermal intensity number given by
TI 
  Fn  v
Tcrit  a  Keff
(14)
in which Tcrit represents the critical contact temperature.
Combining the MI and TI parameters with a
measure that represents comfort during use, enables
the construction of a skin comfort map, which can
serve as a design diagram. A conceptual version of
such a diagram is given in Fig. 2. No experimental
evidence exists yet for this diagram, but nevertheless
(12)
Secondly, frictional heating during sliding is strongly
associated with discomfort. Temperature and exposure
time determine to a great extent of the severity of skin
burns [46]. From pathologic examination a reciprocal
relationship between temperature and exposure time
Fig. 2 Conceptual version of a comfort diagram based on the
mechanical and thermal intensity of a sliding contact.
Friction 1(2): 130–142 (2013)
137
it clearly illustrates the need to predict and control
friction. Two promising directions to influence friction
in a controlled way are the use of surface textures
and the use of brush coatings.
3.2
Changing friction by surface texture
In “hard” tribological contacts, the (macroscopic)
apparent area of contact is significantly larger than
the real area of contact and there is only a negligible
influence of the surface roughness on the friction force.
When one of the contact partners is a compliant
material, such as an elastomeric material or skin, the
area of real contact may approach the area of apparent
contact, which means that the adhesion component
of friction can be quite substantial, particularly when
the surface has a low roughness. Indeed, in describing
the friction behaviour of human skin, any effects due
to deformation (e.g., viscoelastic losses and mechanical
interlocking) are often ignored, and only adhesion
phenomena are taken into account, see Ref. [22].
The relation between the surface roughness and
the adhesive component of the friction force has been
be described as
Ff,adh  Rq h
(15)
in which Rq represents the root mean square roughness
of the counter surface and the exact value of the
exponent h is, as yet, unknown. Hendriks and Franklin
[49] reported a factor 5 decrease in the coefficient
of friction measured on skin when the roughness of
the counter material was increased from 0.1 to 10 μm,
from which the exponent h can be estimated to
be approximately −2. In contrast, based on a fully
elastic approximation combined with a GreenwoodWilliamson-like statistical approach, Masen [50]
estimated h to range between −0.66 and −1. However,
this latter estimate is an over-simplification because
the mechanical properties of skin vary with the size
of the contact [39], and a deterministic approach to
account for the effects of surface roughness seems
more appropriate.
For surfaces with a roughness Rq in the order of
micrometres and more, the adhesive model gives
rather low coefficients of friction, and such low values
are not obtained in experiments. The increased surface
roughness will result in a larger separation between
the mean planes of the two contacting surfaces causing
a reduction in the amount of adhesion, provided that
the lateral spacing between the asperities is small
enough so that the skin does not fill the valleys, which
would result in an increased area of contact and, hence,
high friction. Indeed Peressadko et al. [51] showed
that the lateral geometry such as the wavelength or
the spacing between the individual asperities can play
an important role. One could visualise the influence
of the spacing of the micro-geometry by imagining
the skin surface wrapping itself around the roughness
asperities of the rigid surface, meaning that full
surface-to-surface contact also occurs inside the valleys
of the rough surface. When the asperities are too high,
or positioned too close to each other, the valleys will
not be filled and only partial contact occurs.
The deformation component of friction in skinobject interactions is often neglected. For surfaces
with high roughness and waviness, the ploughing of
the roughness asperities through the skin causes
viscoelastic losses as well as mechanical interlocking
between the asperities and the friction ridges of the
finger pad. This contribution can be substantial and
provide an opportunity to create high friction and
increased grip. The viscoelastic loss factor β is often
estimated to amount to about 24% of the total energy
involved in the deformation process and, as a general
guideline, for skin interactions with surfaces with a
roughness Rq in the order of several micro-meters
and more, the deformation component can be used
to change the frictional response of a product–skin
interaction substantially.
3.3
Changing friction by brush coatings
Brush coatings, a relatively new and promising strategy
for boundary lubrication, is a way to control the
friction in skin–product interactions. Brush coatings
represent polymer layers developed on a supporting
surface by tethering long polymer chains with a sufficiently high grafting density. A schematic illustration
of a polymer brush coating in an aqueous solution is
shown in Fig. 3. When in good solvent, the end-grafted
polymer chains allow the fixation of a large number
of solvent molecules to form brush-like structure [52].
Many experimental and computer-simulation studies
138
Fig. 3 Schematic illustration of a polymer brush coating on a
glass surface in an aqueous solution.
have been performed to investigate the lubrication
mechanism of polymer-bearing surfaces and it was
thought that the origin of the low frictional forces
between brush-bearing surfaces is attributed both to
the steric repulsion between the polymers supporting
high normal loads and to intermolecular interactions
between the polymer brushes and the solvent
molecules which maintain a lubricating fluid layer at
the sheared interfacial region [52, 53]. By varying the
polymer architecture, such brushes can profoundly
modify interfacial properties and change surface
properties like wettability, surface energy, adhesion
and friction to desirable state [54−57].
Friction and lubrication of skin play a major role in
product development for cosmetics, textiles, artificial
turf, medical devices, floor, etc. Some of these systems
are in aqueous environment, like wet shaving,
showering in bathroom, playing football on artificial
turf after raining, etc. To enhance skin comfort during
these activities, hydration lubrication by hydrophilic
polymer brushes can be applied. Most tribological
studies concerned with brush coatings have been
performed at the nano-scale in a very low-load regime
[58−60]. A translation of these results to engineering
applications is one of the challenges of current skin
tribological research.
Application-oriented studies on macroscopic scale
contacts have been conducted to develop appropriate
Friction 1(2): 130–142 (2013)
surfaces for the control of skin–product interactions
[61, 62], in which the contact pressures applied were
higher than 0.004 MPa, reported as clinically realistic
for supine person on a foam mattress, and lower than
0.23 MPa, measured for highly stressed local contact
at the forefoot during walking. A study on the effect
of polyacrylic acid (PAA) grafted with poly(ethylene
glycol) (PEG) (PAA-g-PEG) on friction was carried out
using a reciprocating flat-on-flat test setup involving
silicone skin L7350 [63]. The result shows that effective
lubrication by water is able to reduce friction
coefficient from above 1 to below 0.01 at low sliding
velocities. The great friction reduction of more than
one order of magnitude is contributed to the change
of the hydrophobic-hydrophobic tribopair to the
hydrophobic-hydrophilic tribopair with PAA-g-PEG
brush coating, which can bind water in its structure
and result in a lubricating water layer to remain in
the contact. Thus, the sliding between two surfaces
can be accommodated by shearing of a thin water
film that is created in the contact area by applying a
normal load. Such a layer is able to effectively separate
the two tribological surfaces during sliding contact
and as a consequence minimize the high adhesive
contribution to friction that occurs for dry contact.
Another study with hydrophilic brush coatings was
conducted using a rotating pin-on-plate test setup
involving polyurethane as mechanical skin equivalent.
In this study, the influence of end group type (hydroxyl,
methyl, lactide) and hydrophilicity (PEG, polyglycerol
(PGO)) was evaluated. Result indicates that the friction
coefficient is in the order of methyl>lactide>hydroxyl
and PGO<PEG, which correlates to the hydrophilicity,
that is, the higher the hydrophilicity, the lower the
friction coefficient in aqueous environment. In addition,
with the increasing of normal load, the friction coefficient increases and the difference is more obvious
for brush coating with hydrophobic end group. This
may be because the hydrophobic end group makes
the polymer chains less densely packed, leading to
weak steric repulsion, which cannot support high
normal load. Therefore, under high normal load, the
bound water molecule can be easily squeezed out,
causing the increasing of friction. Further studies on
the effect of skin temperature, the interactions between
brush coatings and emulsions are under investigation.
Friction 1(2): 130–142 (2013)
4
Conclusions
139
[8] Lui X, Yue Z, Cai Z, Chetwynd D G, Smith S T. Quantifying
touch-feel perception: Tribological aspects. Measurement
This paper shows the relative importance of skin
friction, not only for everyday situations but also in
the design process of consumer products. Skin friction
has a clear and distinct role in the perception of
discomfort and comfort. For that, modelling of skin
friction is important. Current friction models can only
partially be applied to predict in vivo skin friction
and are not ready yet to serve as general engineering
tools. The specific nature of the tribological system
limits furthermore, the use of conventional methods
and stresses the need for in vivo, subject and anatomical location specific test methods. The need to control
friction especially in product–skin interactions with a
sliding component is evident. For that, surface texturing
and polymer coatings are promising directions.
Science and Technology 19: 084007 (2008)
[9]
Mathew Mate C, Carpick R W. A sense for touch. Nature
480: 189–190 (2011)
[10] Veijgen N K, Masen M A, van der Heide E. A multivariable
model for predicting the frictional behaviour and hydration
of the human skin. Skin Res Technol, in press, DOI:
10.1111/srt.12053
[11] Johnson K O. The roles and functions of cutaneous mechanoreceptors. Current Opinion in Neurobiology 11: 455−461
(2001)
[12] Lumpkin E A, Marshall K L, Nelson A M. The cell biology
of touch. J Cell Biol 19: 237–248 (2010)
[13] Van Kuilenburg J, Masen M A, van der Heide E. The role
of the skin microrelief in the contact behaviour of human
skin: Contact between the human finger and regular surface
textures. Tribology International, in press, DOI: 10.1016/
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
j.triboint.2012.11.024
[14] Fagiani R, Massi F, Chatelet E, Berthier Y, Akay A. Tactile
perception by friction induced vibrations. Tribology
International 44: 1100–1110 (2011)
[15] Kuijt-Evers L F M, Twisk J, Groenesteijn L, De Looze M P,
Vink P. Identifying predictors of comfort and discomfort in
References
[1]
[16] Davis B L. Foot ulceration: Hypotheses concerning shear
Dowson D. A tribological day. Proc IMechE Part J: J
and vertical forces acting on adjacent regions of the skin.
Engineering Tribology 223: 261–272 (2009)
Med Hypotheses 40: 44–47 (1993)
[2] Czichos H. Tribology: A Systems Approach to the Science and
Technology of Friction, Wear and Lubrication. Amsterdam
(The Netherlands): Elsevier Scientific Publishing Company,
1978.
[3] Salomon G. Application of systems thinking to tribology.
[4]
using hand tools. Ergonomics 48: 692–702 (2005)
[17] Comaish J S. Epidermal fatigue as a cause of friction blisters.
Lancet 301: 81–83 (1973)
[18] Wolfram L. Friction of skin. Journal of the Society of Cosmetic
Chemists 34: 465–476 (1983)
[19] Johnson S A, Gorman D M, Adams M J, Briscoe B J. The
ASLE Trans 17: 295–299 (1974)
friction and lubrication of human stratum corneum. In
Veijgen N K. Skin friction—A novel approach to measuring
Proceedings of the 19th Leeds-Lyon Symposium on Tribology
in vivo human skin. PhD Thesis. Enschede (The Netherlands):
held at the Institute of Tribology, University of Leeds, 1993:
University of Twente, 2013.
663–672.
[5] Hollins M, Risner S R. Evidence for the duplex theory of
[20] Adams M J, Briscoe B J, Johnson S A. Friction and
tactile perception. Perception & Psychophysics 62: 695–705
lubrication of human skin. Tribology Letters 26: 239–253
(2000)
[6]
(2007)
Hollins M, Bensmaia S J. The coding of roughness.
[21] Derler S, Gerhardt L. Tribology of skin: Review and analysis
Canadian journal of experimental psychology 61: 184–195
of experimental results for the friction coefficient of human
(2007)
skin. Tribology Letters 45: 1–27 (2011)
[7] Smith A M, Chapman C E, Deslandes M, Langlais J S,
Thibodeau M P. Role of friction and tangential force
variation in the subjective scaling of tactile roughness. Exp
Brain Res 144: 211–223 (2002)
[22] El-Shimi A F. In vivo skin friction measurements. J Soc
Cosmet Chem 28: 37–52 (1977)
[23] Pailler-Mattéi C, Pavan S, Varigiolu R, Pirot F, Falson F,
Zahouani H. Contribution of stratum corneum in determining
Friction 1(2): 130–142 (2013)
140
bio-tribological properties of the human skin. Wear 263:
1038–1043 (2007)
[24] Lewis R, Menardi C, Yoxall A, Langley J. Finger friction:
Grip and opening packaging. Wear 263: 1124–1132 (2007)
[25] Koudine A A, Barquins M, Anthoine P, Aubert L, Lévèque
J L. Frictional properties of skin: Proposal of a new approach.
Int J Cosmet Sc 22: 11–20 (2000)
[26] Asserin J, Zahouani H, Humbert P, Couturaud V, Mougin D.
Measurement of the friction coefficient of the human skin
in vivo. Colloids Surf B Biointerfaces 19: 1–12 (2000)
[27] Bobjer O, Johansson S E, Piguet S. Friction between hand
and handle. Appl Ergon 24: 190–202 (1993)
[28] Comaish S, Bottoms E. The skin and friction: Deviations
from Amonton’s laws. Br J Dermatol 84: 37–43 (1971)
[29] Cua A B, Wilhelm K P, Maibach H I. Frictional properties
[40] Yuan Y, Verma R. Measuring microelastic properties of
stratum corneum. Colloids and Surfaces B: Biointerfaces
48: 6–12 (2006)
[41] Johnson K L, Kendall K, Roberts A D. Surface energy
and the contact of elastic solids. Proc R Soc Lond A 324:
301–313 (1971)
[42] Xu F, Li T J, Seffen K A. Skin thermal pain modeling—A
holistic method. J Therm Biol 33: 223–237 (2008)
[43] Sripati A P, Bensmaia S J, Johnson K O. A continuum
mechanical model of mechanoreceptive afferent responses
to indented spatial patterns. J Neurophysiol 95: 3852–3864
(2006)
[44] Johansson R S, Vallbo Å B. Tactile sensory coding in the
glabrous skin of the human hand. Trends in Neurosciences
6: 27–32 (1983)
of human skin: Relation to age, sex and anatomical region,
[45] Hamilton G M. Explicit equations for the stresses beneath
stratum corneum hydration and transepidermal water loss.
a sliding spherical contact. Proc Instn Mech Engrs 197C:
Br J Dermatol 123: 473–479 (1990)
53–59 (1983)
[30] Cua A B, Wilhelm K P, Maibach H I. Skin surface lipid and
[46] Tropea B I, Lee R C. Thermal injury kinetics in electrical
skin friction: Relation to age, sex and anatomical region.
trauma. Journal of Biomechanical Engineering 114: 241–244
Skin Pharmacol 8: 246–251 (1995)
(1992)
[31] Derler S, Schrade U, Gerhardt L-C. Tribology of human
[47] Li W, Pang Q, Jiang Y S, Zhai Z H, Zhou Z R. Study of
skin and mechanical skin equivalents in contact with textiles.
physiological parameters and comfort sensations during
Wear 263: 1112–1116 (2007)
friction contacts of the human skin. Tribology Letters 48:
[32] Gee M G, Tomlins P, Calver A, Darling R H, Rides M. A
new friction measurement system for the frictional component
of touch. Wear 259: 1437–1442 (2005)
[33] Li W, Qu X, Zhou Z R. Reciprocating sliding behaviour of
human skin in vivo at lower number of cycles. Tribology
Letters 23: 165–170 (2006)
[34] Li W, Kong M, Liu X D, Zhou Z R. Tribological behavior of
scar skin and prosthetic skin in vivo. Tribol Int 41: 640–647
(2007)
[35] Naylor P F D. The skin surface and friction. Br J Dermatol
67: 239–248 (1955)
293–304 (2012)
[48] Van der Heide E, Schipper D J. On the frictional heating
in single summit contacts: towards failure at asperity level
in lubricated systems. Journal of Tribology 126: 275–280
(2004)
[49] Hendriks C P, Franklin S E. Influence of surface roughness,
material and climate conditions on the friction of human
skin. Tribol Lett 37: 361–373 (2010)
[50] Masen M A. A system based experimental approach to
tactile friction. J Mech Behav Biomed Mater 4: 1620–1626
(2011)
[36] Ramalho A, Silva C L, Pais A A C C, Sousa J J S. In vivo
[51] Peressadko A G, Hosoda N, Persson B N J. Influence of
friction study of human skin: Influence of moisturizers on
surface roughness on adhesion between elastic bodies. Physical
different anatomical sites. Wear 263: 1044–1049 (2007)
[37] Sivamani R K, Maibach H I. Tribology of skin. Proc Inst
Mech Eng J 220: 729–737 (2006)
[38] Greenwood J A, Tabor D. The friction of hard sliders on
lubricated rubber: The importance of deformation losses. In
Proceedings of the Physical Society, 1958: 989–1001.
[39] Van Kuilenburg J, Masen M A, van der Heide E. Contact
Review Letters 95: 124301 (2005)
[52] Brittain W J, Minko S. A structure definition of polymer
brushes. Journal of Polymer Science: Part A: Polymer
Chemistry 45: 3505–3512 (2007)
[53] Raviv U, Tadmor R, Klein J. Shear and frictional interactions
between adsorbed polymer layers in a good solvent. J Phys
Chem B 105: 8125–8134 (2001)
modelling of human skin: What value to use for the modulus
[54] Grest G S. Interfacial sliding of polymer brushes: A
of elasticity? Proc IMechE Part J: J Engineering Tribology
molecular dynamics simulation. Physical Review Letters
227: 349–361 (2012)
76(26): 4979–4982 (1996)
Friction 1(2): 130–142 (2013)
[55] Muller M, Lee S, Spikes H A, Spencer N D. The influence
of molecular architecture on the macroscopic lubrication
141
brush surfaces using QCM-D and AFM. Colloids and Surfaces
B: Biointerfaces 74: 350–357 (2009)
properties of the brush-like co-polyelectrolyte poly(L-lysine)-
[60] LeMieux M C, Lin Y H, Cuong P D, Ahn H S, Zubarev E R,
g-poly(ethylene glycol) (PLL-g-PEG) adsorbed on oxide
Tsukruk V V. Microtribological and nanomechanical pro-
surfaces. Tribology Letters 15: 395–405 (2003)
perties of switchable Y-shaped amphiphilic polymer brushes.
[56] Vyas M K, Schneider K, Nandan B, Stamm M. Switching of
friction by binary polymer brushes. Soft Matter 4: 1024–1032
(2008)
[57] Nordgren N, Rutland M W. Tunable nanolubrication between
dual-responsive polyionic grafts. Nano Letters 9: 2984–2990
(2009)
[58] Zhang Z, Morse A J, Armes S P, Lewis A L, Geoghegan
Advanced Functional Materials 15: 1529–1540 (2005)
[61] Drobek T, Spencer N D. Nanotribology of surface-grafted
PEG layers in aqueous environment. Langmuir 24: 1484–
1488 (2008)
[62] Van Der Heide E, Lossie C M, Van Bommel K J C, Reinders
S A F, Lenting H B M. Experimental investigation of a
polymer coating in sliding contact with skin-equivalent silicone
M, Leggett G J. Effect of brush thickness and solvent
rubber in an aqueous environment. Tribology Transactions
composition on the friction force response of poly(2-
53: 842–847 (2010)
(methacryloyloxy)ethylphophorylcholine) brushes. Langmuir
27: 2514–2521 (2011)
[59] Kitano K, Inoue Y, Matsuno R, Takai M, Ishihara K.
Nanoscale evaluation of lubricity on well-defined polymer
[63] Zeng X, Van Der Heide E. Bio-inspired tribological interfaces
design for reducing friction in sliding contacts with skin
equivalent in aqueous lubrication system. In Proceedings of
Bio-Inspired Materials, Potsdam, Germany, 2012: 26.
Emile VAN DER HEIDE holds
the chair “Skin Tribology” at the
Laboratory for Surface Technology
and Tribology, Faculty of Engineering Technology at the University of
Twente. His current research focuses
on skin friction fundamentals, sensing
& control of friction in product-skin interactions and
on bio inspired interfaces. He worked for the Dutch
Organization for Applied Scientific Research TNO as
researcher, programme leader and senior scientist in
Tribology since 1995. In 2002 he received his PhD from
the University of Twente in Tribology on lubricant
failure in sheet metal forming processes. He is currently
active in more than 15 European and national projects
on materials and tribology as Principal Investigator
or Coordinator.
Marc MASEN is a senior lecturer
in mechanical engineering and
industrial design engineering at the
University of Twente. He obtained
his PhD in 2004 on wear mechanisms
in sheet metal forming processes.
After this, he worked as a research
scientist for Hydro Aluminium Extrusion, in the
UK and Belgium. His research interests include the
tribology of viscoelastic materials, wear mechanisms
and friction of the human skin, with special attention
to the prevention of decubitus ulcers. As a Principal
Investigator, he has obtained over 2Meuros in research
funding, e.g., from the Dutch Technology Foundation
STW, the European Union or directly by Industry. He
has delivered over 40 papers in scientific journals and
to international conferences.
142
Xiangqiong ZENG. Assistant professor, obtained her Master degree
in Applied Chemistry in 2003 and
PhD degree in Material Science and
Engineering in 2006 from Shanghai
Jiao Tong University (SJTU). Her
PhD research was on the design and
tribological study of environmental
friendly boundary lubrication additives. She worked
for the Emerging Market Innovation Center of Johnson
& Johnson (China) and Johnson & Johnson Asia Pacific
R&D Center (Singapore) during 2006–2010 as staff
scientist in Skin Care technology. Since 2011, she is
Friction 1(2): 130–142 (2013)
appointed as a tenure track assistant professor at the
University of Twente (UT). She is currently active in
the research on bio-inspired tribological interfaces
design, tribo-mechanical and tribo-chemical modeling
of product-tissue interactions, skin comfort prediction
and surface/interface layers in hydration lubrication,
with grants from UT UTWIST program, UT Incentive
Fund, European FP7 Marie Curie Career Integration
Grant and Double Degree program between UT and
SJTU. She has published around 20 papers in peerreviewed international journals, 5 patents, one book
and one book chapter.
Friction 1(2): 143–149 (2013)
DOI 10.1007/s40544-013-0009-z
ISSN 2223-7690
RESEARCH ARTICLE
Use of opposite frictional forces by animals to increase their
attachment reliability during movement
Zhouyi WANG1,2, Yi SONG1,3, Zhendong DAI1,*
1
Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China
2
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, Nanjing, 210016, China
3
College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, Nanjing, 210016, China
Received: 08 January 2013 / Revised: 04 March 2013 / Accepted: 13 March 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: Many animals have the natural ability to move on various surfaces, such as those having different
roughness and slope substrates, or even vertical walls and ceilings. Legged animals primarily attach to surfaces
using claws, soft and hairy pads, or combinations of them. Recent studies have indicated that the frictional
forces generated by these structures not only control the movement of animals but also significantly increase the
reliability of their attachment. Moreover, the frictional forces of various animals have opposite characteristics
and hierarchical properties from toe-to-toe and leg-to-leg. These opposite frictional forces allow animals to
attach securely and stably during movement. The coordination of several attachment (adhesion) modes not
only helps animals adhere, which would be impossible in single mode, but also increases the overall stability of
the attachment (adhesion) system. These findings can help the design of highly adaptable feet for bionic robots
in the near future.
Keywords: opposite frictional forces; claw interlocks; soft pad adhesion; hairy pad adhesion; attachment reliability
1
Introduction
Motion is a fundamental animal characteristic that
defines behavioral traits such as predation, escape,
courtship, and reproduction. Although there are
numerous legged species with various types of complex movements, all of them require the animals to
overcome their body weight, and adapt to complex
land morphology and environment. Therefore, legged
animals have evolved different morphological
structures, topological structures, and movement
modes to adapt to their living environment [1] and
have developed superb abilities to maintain dynamic
stability [2], climb obstacles [3, 4], and achieve shock
absorption and antifriction [5, 6]. In particular, many
animals move with easiness in a variety of complex
surfaces using highly evolved feet. For example, the
ability of geckos [7–9], insects [10–14], and spiders
* Corresponding author: Zhendong Dai.
E-mail: [email protected]
[15, 16] to move on different types of surfaces is
valuable to bionic design. Moreover, the stability,
flexibility, robustness, adaptability, and use of energy
displayed by animals are still challenges for bionic
robots [17]. Previous studies have indicated that the
first obstacle a robot must overcome is the fast and
reliable attachment (adhesion) of the robot’s feet to the
surface. The contact between the robot’s feet and the
surface should generate frictional or adhesive forces,
which the robot uses to move. This study discusses
how animals increase their attachment reliability by
using opposite frictional forces. Finally, the principle
behind the mechanism of opposite frictional forces is
introduced into the design of highly adaptable robot
soles.
2 Opposite friction and animal adhesion
Animals primarily attach to surfaces using claw
interlocking, pad adhesion, and setal adhesion;
144
Friction 1(2): 143–149 (2013)
opposite frictional forces were found in all these
attachment modes.
2.1 Opposite friction and claw structures
Animals use claw attachment to balance the gravitational and inertial forces with the help of the frictional
force between the chitin-based claws and the surface,
which has a microprofile that can be regarded as
spheres compared with the animal’s claws [18]. The
frictional mechanism of a single claw on a spherical
surface closely resembles that of a point on a surface
with the stable margin located in the friction cone
(Fig. 1(a)). The stability of this type of attachment
mode depends on the physical properties of the claws
and the contact surface. For example, the stability of
a beetle’s attachment on a rough surface is determined
by the friction coefficient of its claws with the corresponding contact surface and the curvature of the
contact tips [18] (Fig. 1(a)). Locusts can safely attach
on rough ceilings where the diameters of the rough
peaks are larger than the radii of the claw tips but
they fail when they try to attach on smooth substrates
where the diameters of the smooth peaks are equivalent to or smaller than the tips [13]. A single-claw
interlock has poor anti-interference ability and can
easily become unstable. Thus, most wall-climbing
animals have two claws on the terminals of their legs;
a feature that enhances their attachment stability
(Fig. 1(b)) because of the generated opposite forces
[19]. Figures 1(c) and 1(d) show how the opposite
frictional forces at two different contact points prevent
the claws from lateral slipping, which extends the
contact model proposed by Dai et al. [18]. Assuming
that the attachment system is laterally self-balanced
according to its specific geometrical structure; that is
FQx  FPx , the load along the leg axis Fxz will strengthen
the mechanical locking and result in good contact,
when the force angle is smaller than the friction angle,
which helps animals attach to rough and inclined
surfaces. On the other hand, a small disturbance force
against the leg axis will eliminate locking, showing the
evident asymmetric character of this attachment mode.
Moreover, animals regulate the shared loads on two
claws to generate unequal opposite frictional forces and
increase the antirollover ability within a safe frictional
margin (Fig. 1(d)).
Fig. 1 (a) Model of single claw contact with micro surface particle,
where α is the contact angle, R is the radius of the particle, F is the
force acting on the claw, N is the normal load, and fN is the tangential
force. The shaded area is the frictional cone [18]. (b)–(d) Threedimensional models of an insect claw attaches to a micro surface
granule. (The coordinate origin is the midpoint of line between two
contact points; x axis is along the line between two contact points; z
axis is parallel to the substrate and perpendicular to the x axis; x,
y, z axis accord with the Cartesian coordinate system.)
2.2 Opposite friction and soft pad structures
Animals reduce the impact force during attachment
and generate capillary-based adhesion by using soft
pads [20]. Dendrocola ants can resist separating forces
of 40–150 times their body weight on smooth surfaces.
The elastic deformation of oscules also contributes to
frictional forces because the adhesive and frictional
forces produced by mucus alone are not strong enough
to secure the movements of ants as determined by
interference reflection microscopy (IRM) analyses and
estimates of the thickness and viscosity of the mucous
membranes [21]. When Lycorma delicatula specimens
contact glass, the contact areas in the tangential contact
state are typically larger than those in the normal
contact state with frictional and adhesive forces per
unit area of 312–900 mN/mm2 and 83–119 mN/mm2,
respectively [14]. The deformable epidermis of locust
claws has different material and mechanical properties,
and microstructure compared with the neighboring
epidermis. Such differences lead to different mechanical
Friction 1(2): 143–149 (2013)
properties [22]. The geometry, structure, material design,
and plasma and other internal tissues give locust pads
very low contact stiffness (Fig. 2(a)) [20]. As a result,
locusts have large contact areas. In locusts, because the
directions of the grovy structures on the endosexine
of the epidermis of the pads are parallel to the
primary cuticula, the grovy structures cannot restrict
the deformation of the epidermis in these directions.
Hence, flexible pads have large contact areas and adapt
to contact surfaces well (Fig. 2(a)) [23]. Consequently,
animals create large adhesive forces and opposite
frictional forces by increasing their contact areas
through the elastic deformation of their pads if pad
adhesion is used [20]. In addition, opposite frictional
forces significantly contribute to adhesion in this mode.
According to finite element method (FEM) results [20]
(Fig. 2(b)), the biggest pulling stresses in the entire pad
were located at the grovy structures. In addition, the
deformation of the contact structures showed that there
were lateral displacements at projecting parts during
the contact process, indicating that the presence of
opposite frictional forces in these parts. Similar to the
interlocks of double claws, the scalar sum of FQy and FPy
equals the normal load while their vector sum balances
the tangential load (Fig. 2(c)). Thus, the safety margin
for adhesion and friction, and the antisideslip ability
are enhanced. The difference derives from the fact
that the contact areas obviously change depending on
the loads because of the special structure of the pads,
and these changes help increase the friction coefficient,
frictional force, and adhesive stability.
2.3 Opposite friction and hairy structures
Flies, geckos, and some beetles have the ability to move
on various inclined substrates by using hairy pads
145
and the so-called dry adhesion. Previous studies
have shown that the pads of the soles of flies have an
elliptical profile and primarily consist of an elastic
epidermis. They are covered with setae, which increase
the actual adsorption areas [24]. Furthermore, the
direction of the setae arrays helps flies control the
adhesive and friction forces, and thus generate
opposite frictional forces on the right and left pads.
Geckos’ setae exhibit anisotropic features in opposite
directions as well. First, Young’s modulus differ along
the direction that the setae bend and the opposite
direction [25]. Second, the deformation of the setae
creates crush and friction forces with the rubbing
surfaces, whereas the normal forces obey Coulomb’s
friction law in the direction opposite to the direction
that the setae bend. For preloads, the adhesive and
friction forces were measured four times along the
bending direction of the setae [26, 27] (Fig. 3(a)). A
friction force of about 200 μN and a maximum adhesive
force of about 40 μN were measured for the adhesion
of a micron-sized single-sheared seta that detaches
around 30° [28]. The setae arrays and toes of geckos
also display asymmetric friction. Moreover, the friction
forces along the setae arrays and toes are larger than
those in the opposite direction [26]. The angles 
between the tangential forces on the contact plane and
the toes on the vertical walls and ceilings are 12.6° and
3.1°, respectively, whereas the angles between the
reaction forces and motion planes are approximately
equal to 20°, thus securing attachment (Fig. 3(b)). The
adhesive forces perpendicular to the surfaces are
sufficient to balance the animals’ weight and the
moments caused by weight. The opposite friction forces
at the first and fifth toe of the geckos form an interlock
on the contact plane, which increases the stability and
reliability of the attachment [29] (Fig. 3(c)).
Fig. 2 (a) Cross-sectional structural representation of locust’s pad. EXO is the pad epidermis, which contains rod-shaped tissues and appeared
to be smooth when observed with a light microscope. (b) Vectorial deformation field of locust pad [20]. (c) Force analysis of locust pad.
Friction 1(2): 143–149 (2013)
146
Fig. 3 (a) Experiments on opposite frictions of setae arrays [26].
(b) Measurements of lateral forces of a single gecko toe along its
direction [29]. (c) Patterns of gecko sole on ceiling.
3
of their feet to stably attach to different substrates.
Geckos catch surface particles with their claws on
rough surfaces while they use their setae to attach to
smooth inclined surfaces [8, 26]. Gampsocleis gratiosa
[30] creeps along vertical glass using flexible pads,
where the tangential forces (friction forces) are much
larger than the normal adhesive forces because they
can insert pad cuticles into the microstructures of the
glass surface. Locusts, which belong to Orthoptera just
like Gampsocleis gratiosa, can reliably grasp the surface
particles on sandpaper with microsurface profiles containing spheres of 12–41 μm in diameter [21]. Animals
can move on surfaces with a roughness comparable
to their critical microscale by coordinating the opposite
frictions generated by the different attachment modes.
Geckos can securely attach to a smooth glass ceiling
by overcoming the adhesive angle increment caused
by gravity and keep this angle smaller than the critical
angle at all times by using the opposite friction forces
at their two toes [9, 31, 32] (Fig. 4(a)). Clearly, these
findings will increase the operating range of bionic
robots.
The coordination of the different attachment modes
ensures that the animals have the ability to attach to
multiple surfaces. Without considering their internal
microstructures and reciprocities, the setae-and-setae
Opposite friction between different
adhesion modes
No attachment mode is completely versatile because
the physical properties of contact surfaces heavily
influence attachment. For example, the stability of the
claw interlock is limited by the roughness and friction
coefficient of the substrates, and the relative scales of
the claws and surface particles [18]. Pad adhesion is
highly influenced by the actual microcontact areas,
whereas setae adhesions are affected by the actual
microcontact angles of the setae [26, 28]. Many animals
have more than one tool for attaching on the various
substrates. The soles of the toes of geckos are covered
with setae even though each toe has a terminal claw;
soft pads or setae pads and claws exist concurrently
on the tarsal extremities of many insects. Animals
make intelligent use of the different adhesion modes
Fig. 4 (a) Pattern of gecko sole on incline. (b) Electron micrograph of Erthesina fullo sole. (c) Mechanical model of coupled
attachment modes.
Friction 1(2): 143–149 (2013)
147
(Fig. 4(a)) and claw-and-pads (Fig. 4(b)) attachment
modes can be simplified to the module shown in
Fig. 4(c).
Equivalent frictional angles are introduced for the
attachment modes that are different at the two contact
areas. The equivalent frictional angle  P and  Q at
points P and Q, respectively, satisfies the inequality.
 FQy  FQx  tan(   Q )
 F  F  tan(   )
Py
P
 Px
(1)
Hence, the system can be also described by
tan(   ) 
FPy  tan(   P )  FQy / tan(   Q )
FPy  FQy
(2)
where  is the total equivalent frictional angle and
from Formula (2)    Q , and    P . These two inequalities imply that because of the opposite friction
forces, the total equivalent frictional angle of the
synergetic attachment modes is larger than that of any
single mode, which is the most unique trait of opposite
frictional attachment, and increases the attachment
safety margin. Combined with earlier findings, it is easy
to see that animals form stable triangles on contact
planes and stable tetrahedra using the reciprocities
of the leg mechanisms and surfaces because the
directions of the frictional forces at different contact
areas are different. Two of the toes of geckos can exert
a couple of opposite frictional forces, whereas all toes
and the surface form a tridimensional stable area.
The combined attachments not only are more reliable
and safer than single-mode attachments but also show
high antijamming ability.
In addition, the research regarding the climbing
ability of geckos [8], tree frogs [33], and locusts [13]
show that their left and right legs in the stance phase
need to generate opposite lateral forces, or sometimes
opposite shear forces, to increase the stability of the
attachment on an inclined surface. This suggests the
contribution of opposite forces at different scales, from
the basic-level—toe-to-toe in geckos, claw-to-claw in
beetles, and left-to-right projections in the soft pads
of locusts—to higher-level legs between the left and
right side of the animals. Therefore, the movements
of animals are processes in which opposite frictional
forces operate from the micro- to the macro-level.
This means that multiscale opposite frictional forces
guarantee the stability and reliability of the locomotion
of animals.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (Grant No. 60910007)
and the Fundamental Research Funds for the Central
Universities (Grant Nos. CXZZ11_0198 and BCXJ10_10).
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
Reference
[1] Lu Y X. Significance and progress of bionics. J Bionic Eng
1(1): 1–3 (2004)
[2] Jindrich D L, Full R J. Dynamic stabilization of rapid
hexapedal locomotion. J Exp Biol 205: 2803–2823 (2002)
[3] Lammers A R, Earls K D, Biknevicius A R. Locomotor
kinetics and kinematics on inclines and declines in the gray
short-tailed opossum Monodelphis domestica. J Exp Biol
209: 4154–4166 (2006)
[4] Lammers A R. Locomotor kinetics on sloped arboreal and
terrestrial substrates in a small quadrupedal mammal. Zoology
110: 93–103 (2007)
[5] Zhou Q, He B, Qian M G, Yue J G. Analysis on friction and
adhesive force of insects pads. J Uni Shanghai Sci Technol
2(30): 143–146 (2008)
[6] Zhou Q, He B, Qian M G, Yue J G. Testing of wet adhesive
forces of ants and ANSYS analysis. J Tongji Uni (Nat Sci)
35: 670–673 (2008)
[7] Chen J J, Peattie A M, Autumn K, Full R G. Differential leg
function in a sprawled-posture quadrupedal trotter. J Exp
Biol 209: 249–259 (2006)
[8] Wang Z Y, Wang J T, Ji A H, Dai Z D. Locomotion
behavior and dynamics of geckos freely moving on the
ceiling. Chin Sci Bull 55: 3356–3362 (2010)
[9] Pesika N S, Tian Y, Zhao B X, Rosenberg K, Zeng H,
McGuiggan P, Israelachvili J N. Peel-zone model of tape
peeling based on the gecko adhesive system. J Adhesion 83:
383–401 (2007)
Friction 1(2): 143–149 (2013)
148
[10] Chen D H, Tong J, Sun J Y, Ren L Q. Tribological behavior
[22] Jiao Y, Gorb S, Scherge M. Adhesion measured on the
of Gampsocleis gratiosa foot pad against vertical flat surfaces.
attachment pads of tettigonia viridissima (Orthoptera, insecta).
J Bionic Eng 2(4): 187–194 (2005)
J Exp Biol 203(12) : 1887–1895 (2000)
[11] Gorb S N. Attachment Devices of Insect Cuticle. Berlin:
Kluwer Academic Publishers, 2002.
[12] Gorb S N, Jiao Y, Scherge M. Ultrastructural architecture
[23] Gorb S. N. The design of the fly adhesive pad: Distal tenent
setae are adapted to the delivery of an adhesive secretion.
Proc R Soc Lond B 265: 747–752 (1998)
and mechanical properties of attachment pads in Tettigonia
[24] Eisner T, Aneshansley D J. Defense by foot adhesion in a
viridissima (Orthoptera Tettigoniidae). J Comp Physiol A
beetle (Hemisphaerota cyanea). PNAS 97(12): 6568–6573
186: 821–831 (2000)
(2000)
[13] Han L B, Wang Z Y, Ji A H, Dai Z D. Grip and detachment
[25] Autumn K, Majidi C, Groff R E, Dittmore A, Fearing R.
of locusts on inverted sandpaper substrates. Bioinspir Biomim
Effective elastice modulus of isolated gecko setal arrays.
6: 386–392 (2011)
J Exp Biol 209: 3558–3568 (2006)
[14] Frantsevich L, Ji A H, Dai Z D, Wang J, Frantsevich L,
[26] Autumn K, Dittmore A, Santos D, Spenko M, Cutkosky M.
Gorb S N. Adhesive properties of the arolium of a lantern-
Frictional adhesion: A new angle on gecko attachment. J Exp
fly, Lycorma delicatula (Auchenorrhyncha, Fulgoridae).
Biol 209: 3569–3579 (2006)
J Insect Physio 54: 818–827 (2008)
[27] Zhao B X, Pesika N, Rosenberg K, Tian Y, Zeng H,
[15] Niederegger S, Gorb S N. Friction and adhesion in the tarsal
McGuiggan P, Autumn K, Israelachvili J. Adhesion and
and metatarsal scopulae of spiders. J Comp Physiol A 192:
friction force coupling of gecko setal arrays: Implication for
1223–1232 (2006)
structured adhesive surfaces. Langmuir 24: 1517–1524 (2008)
[16] Wang Z Y, Wang J T, Ji A H, Li H K, Dai Z D. Movement
[28] Autumn K, Liang Y A, Hsieh S T, Zesch W, Chan W P,
behavior of a spider on a horizontal surface. Chin Sci Bull
Kenny T W, Fearing R, Full R J. Adhesive force of a single
56(25): 2748–2757 (2011)
gecko foot-hair. Nature 405: 681–684 (2000)
[17] Dickinson M H, Farley C T, Full R J, Koehl M A R, Kram R,
[29] Wang Z Y, Gu W H, Wu Q, Ji A H, Dai Z D. Morphology
Lehman S. How animals move: An Integrative view. Science
and reaction force of toes of geckos freely moving on ceilings
288(7): 100–106 (2000)
and walls. Sci China Technol Sci 53: 1688–1693 (2010)
[18] Dai Z D, Gorb S N and Schwarz U. Roughness-dependent
[30] Chen D H, Tong J, Sun J Y, Ren L Q. Tribological behavior of
friction force of the tarsal claw system in the beetle Pachnoda
gampsocleis Gratiosa foot pad against vertical flat surfaces.
marginata (Coleoptera, Scarabaeidae). J Exp Biol 205: 2479–
2488 (2002)
[19] Dai Z D, Yu M, Ji A H. Study on tribological characteristics
of animals’ driving pads and their bionic design (in Chinese).
Chin Mech Eng 8: 1454–1457 (2005)
J Bionic Eng 2: 187–194 (2005)
[31] Autumn K, Hsieh S T, Dudek D M, Chen J, Chitaphan C,
Full R J. Dynamics of geckos running vertically. J Exp Biol
209: 260–272 (2006)
[32] Wang Z Y, Wang J T, Ji A H, Zhang Y Y, Dai Z D.
[20] Dai Z D, Gorb S N. A study on contact mechanics of grass-
Behavior and dynamics of gecko locomotion: The effects of
chopper's pad (Insecta: Orthoptera) by finite element methods.
moving directions on vertical surface. Chin Sci Bull 56:
Chin Sci Bull 54(4): 549–555 (2009)
573–583 (2010)
[21] Federle W, Riehle M, Curtis A S G, Full R G. An
[33] Endlein T, Ji A H, Samuel D, Yao N, Wang Z H, Barnes W
integrative study of insect adhesion: Mechanics and wet
J, Federle W, Kappl M, Dai Z D. Sticking like sticky tape:
adhesion of pretarsal pads in ants. J Integr Comp Biol 42:
Tree frogs use friction forces to enhance attachment on
1100–11061 (2002)
overhanging surfaces. J R Soc Interface 10: 1742 (2013)
Friction 1(2): 143–149 (2013)
149
Zhouyi WANG Doctor, obtained his
Master degree in 2009 from Nanjing
University of Aeronautics and
Astronautics (NUAA). He studied
as a PhD Candidate since 2009 at
Institute of Bio-inspired Structure
and Surface Engineering, NUAA. His interested
research areas include tribology, bionics, animal
kinematics and dynamics. He has participated in
many research projects and has published 12 papers
on international journals.
Zhendong DAI Professor and tutor
of PhD students, obtained his
doctor degree in 1999 from College
of Mechanical and Electrical
Engineering, NUAA. He is one of the
Chinese delegates of International
Institute of Bionic Engineering,
an executive member of the council of Chinese
Mechanical Engineering in Tribology, and a member of
the academic committee of State Key Laboratory of
Solid Lubrication. He also is a member of editorial
board of many academic journals such as Journal
of Bionic Engineering, International Journal of Vehicle
Autonomous System, Tribology and so on. He was
invited to attend the Advisory Seminar about the
development planning of American science foundation
and invited to give lectures in Case Western Reserve
University, UC San Diego, GIT, Kyoto University,
Yonsei University, Cambridge University in 2010. His
research areas include bionics, light material, control
of bionics, bio-robots, and biological robots. He has
successively presided and participated in many
research projects and has published more than 200
papers and gotten more than 15 patents.
Friction 1(2): 150–162 (2013)
DOI 10.1007/s40544-013-0010-6
ISSN 2223-7690
RESEARCH ARTICLE
Influence of synovia constituents on tribological behaviors of
articular cartilage
Teruo MURAKAMI1,*, Seido YARIMITSU1, Kazuhiro NAKASHIMA2, Yoshinori SAWAE2, Nobuo SAKAI3
1
Research Center for Advanced Biomechanics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
2
Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
3
Department of Applied Science for Integrated System Engineering, Graduate School of Engineering, Kyushu Institute of Technology,
1-1, Sensuicho, Tobata-ku, Kitakyushu, 804-8550, Japan
Received: 31 December 2012 / Revised: 18 February 2013 / Accepted: 16 March 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: The extremely low friction and minimal wear in natural synovial joints appear to be established by
effective lubrication mechanisms based on appropriate combination of articular cartilage and synovial fluid.
The complex structure of cartilage composed of collagen and proteoglycan with high water content contributes
to high load-carrying capacity as biphasic materials and the various constituents of synovial fluid play important
roles in various lubrication mechanisms. However, the detailed differences in functions of the intact and damaged
cartilage tissues, and the interaction or synergistic action of synovia constituents with articular cartilage have
not yet been clarified. In this study, to examine the roles of synovia constituents and the importance of cartilage
surface conditions, the changes in friction were observed in the reciprocating tests of intact and damaged
articular cartilage specimens against glass plate lubricated with lubricants containing phospholipid, protein
and/or hyaluronic acid as main constituents in synovial fluid. The effectiveness of lubricant constituents and
the influence of cartilage surface conditions on friction are discussed. In addition, the protectiveness by synovia
constituents for intact articular cartilage surfaces is evaluated.
Keywords: articular cartilage; synovial fluid; synovial joint; lubrication; biotribology
1
Introduction
In various biotribological systems, it is widely known
that the healthy synovial joints maintain superior
load-carrying capacity and lubricating properties with
extremely low friction and minimal wear even in
heavily loaded hip, knee and ankle joints. The synovial
joints are prominent natural bearings different in
geometric congruity depending on joint positions/
movements and are in general covered with soft layers
of biphasic articular cartilage lubricated with synovial
fluid containing appropriate lubricating constituents.
The superior tribological properties of synovial joints
appear to be established by a well-suited combination
* Corresponding author: Teruo Murakami.
E-mail: [email protected]
of articular cartilage and synovial fluid. However,
the detailed cooperative and/or interactive behaviors
between articular cartilage and synovial fluid under
various rubbing conditions have not yet been clarified.
In this paper, we will focus on the influence of main
synovia constituents such as phospholipid, protein and
hyaluronic acid on tribological behaviors of articular
cartilage different in surface conditions particularly
as related to lubrication mechanism.
The operating conditions in human synovial joints
change under variable loading and motions including
sliding and rolling depending on joint types in various
daily activities. Therefore, the superior lubricating
performance of natural synovial joints is likely to be
actualized not by a single lubrication mode but by
the synergistic combination of various modes from
fluid film lubrication to boundary lubrication [1, 2].
Friction 1(2): 150–162 (2013)
Other specific lubrication mechanisms such as weeping
lubrication [3], boosted lubrication [4], biphasic
lubrication [5], micro-elastohydrodynamic lubrication
(micro-EHL) [6] and so on have been proposed. The
ingenious lubrication mechanism as the synergistic
combination of various modes depending on the
severity of operating conditions was called the adaptive
multimode lubrication mechanism [7, 8]. For example,
during normal walking, fluid film lubrication mechanisms such as soft-EHL and/or micro-EHL play
major roles to maintain low friction and minimize
wear. In contrast, in thin film conditions such as at
slow motion or at movement after standing for a long
time, it is expected that adsorbed films [9–12], surface
gel films [13], hydration lubrication [14] and polymeric
brush-like layers [15, 16] contribute to keep friction
low and protect rubbing surfaces.
Another new development in lubrication theory is
the elucidation of the biphasic lubrication mechanism.
Since an experimental finding [17] and a proposal of
boundary friction model based on biphasic lubrication
by Ateshian [18], the important phenomena on the
effectiveness of biphasic lubrication with interstitial
fluid pressurization have been demonstrated on the
basis of the biphasic finite element (FE) analyses
and experimental observations [19, 20]. The articular
cartilage has high water content from 70% to 80% in
tissue as porous media composed of type II collagen,
proteoglycan and chondrocytes, and thus exhibits a
time-dependent biphasic behavior due to the simultaneous coexistence of solid and liquid phases [21].
When articular cartilage as biphasic material with
low permeability is applied by compressive load, the
fluid content in the tissue is trapped within contact
area and the collagen matrix network resists interstitial
fluid pressure in aggregate solid matrix. Thus, the
interstitial fluid pressure supports significant proportion of total load in contact area and this situation
consequently causes the reduction of contact force
of solid phase for a considerable time. The timedependent change in load support by interstitial fluid
pressure in biphasic cartilage depends on the extent
of exudation from cartilage tissue and rehydration of
cartilage. If the fluid load support is maintained at
high level for a long time, the low friction is maintained
because of low level for solid-to-solid contact [20].
151
For reciprocating sliding under constant load,
Pawaskar et al. [22] introduced sliding motion into
their FE model and indicated the importance of
migrating contact area for the sustainability of the
biphasic lubrication in their biphasic FE analysis.
Sufficient stroke for rehydration of cartilage tissue in
reciprocating motion maintained the high level of
load support by interstitial fluid pressure. Sakai et al.
[23] examined the compressive response of the articular
cartilage by high precision testing machine with a
feedback-controlled servomotor and estimated material
properties in physiological condition for the biphasic
FE model, which included (1) the depth-dependence
of apparent Young’s modulus of solid phase, (2) straindependent permeability as compaction effect, and
(3) collagen reinforcement in tensile strain. These
properties (parameters) were estimated by the curve
fitting between the experimental time-dependent
compressive behavior and simulation in indentation
tests for cartilage specimens with cylindrical rigid
indenter of 5 mm radius. In the reciprocating test, the
load of 0.5 N/mm was applied at the center of the
cylindrical indenter in 1 s and then the reciprocating
motion was introduced with the speed of 4 mm/s over
a stroke length of 8 mm. FE analyses using commercial
package ABAQUS (6.8-4) showed that the tensile
reinforcement by spring elements representing the
collagen network and the depth-dependent elastic
properties improved the proportion of the fluid load
support especially in the sliding condition. The
compaction effect on permeability of solid phase was
functional in a condition without the migrating contact
area, whereas under sliding condition the compaction
effect showed a little effect in terms of the proportion
of the fluid load support.
In the next stage, the influence of operating conditions on the effectiveness of biphasic lubrication in
reciprocating sliding was examined. The differences
in frictional behaviors between the reciprocation with
migration of contact zone, i.e., at on-off loading on
articular cartilage (model A) as described above, and
without migration of contact zone, i.e., at continuous
loading on cartilage (model B), shown in Fig. 1 were
compared in FE analysis [24]. In this simulation of
reciprocating test with similar method to the previous
study [23], the load of 0.5 N/mm was applied by
Friction 1(2): 150–162 (2013)
152
Fig. 1 Time-dependent frictional behaviors estimated by biphasic
theory for cartilage.
the rigid cylindrical indenter against flat cartilage
specimen or by the rigid flat plate against cylindrical
cartilage specimen with a ramp time of 1 s and then
the load was held constant during reciprocation. The
reciprocation of rigid cylinder or flat plate at 4 mm/s
was started immediately after loading and continued
for 508 s, 127 cycles at period of 4 s. The initial fluid
load support percentages are very high as 90% and
91% for models A and B, respectively. After 127 cycles,
it is noticed that the high percentage of fluid load
support (83%) was maintained even after 508 s in
model A, but the percentage of fluid load support
was remarkably decreased to 27% in the model B.
The time-depending changes in friction coefficient
eff were estimated for eq as coefficient of friction for
solid-to-solid contact using the following formula by
Ateshian et al. [20, 25].
eff = eq (1 – (1 – ) Wp /W )
(1)
where W is the total load support, Wp the load support by fluid pressure and  the fraction of contact
area of solid phase.
In Fig. 1, the time-dependent changes in friction
estimated from total traction force in biphasic FE
analysis for assumption of eq = 0.2 [24] are shown.
It is worth noting that the lower friction level is
maintained due to the sustainability of interstitial fluid
pressure in the reciprocating sliding for model A. In
contrast, significant gradual increase to high level in
friction is observed in reciprocation for model B. It is
supposed that the tribological problems are more
likely to occur for model B with high friction level
and thus the method to suppress friction increase is
required.
In this study, the combination of cartilage-on-glass
was used to simplify the frictional condition, although
articular cartilage is rubbed against cartilage or
meniscus in natural synovial joints. The glass plate
has very smooth, hard and non-porous/impermeable
surface compared with articular cartilage but hydrophilic surface with negatively charged property
similar to proteoglycan on superficial cartilage layer
in wet condition [12]. The adsorption of synovia
constituents on glass plate appears to be considerably
similar to boundary film formation on intact cartilage
as shown by in situ observation for fluorescent images
of adsorbed molecules during reciprocating rubbing
process [26], while the interaction to the smooth,
hard and non-porous/impermeable glass surface may
induce certain different behaviors. Smooth glass surface
minimizes ploughing resistance, but may enhance
the adhesive resistance by interaction with adsorbed
protein molecules at intimate contacts in very thin
film condition. However, the intrinsic tribological
properties of compliant and biphasic articular cartilage
are expected to be reflected appropriately in the
effectiveness of lubricant constituents even in sliding
pair of articular cartilage and glass plate. As a matter
of course, the difference in tribological behaviors
between for cartilage-cartilage and cartilage-glass
combinations should be explored. The influence of
glass plate on frictional behaviors is discussed in
Section 4. Thus, the frictional behaviors in a sliding
pair of ellipsoidal articular cartilage specimens and
reciprocating glass plate were examined in the sliding
condition for model B without migration of contact
zone for cartilage.
2 Materials and methods
The reciprocating test for the sliding pair of the upper
stationary ellipsoidal articular cartilage specimen
and the lower reciprocating flat glass plate was
conducted in the reciprocating tester shown in Fig. 2.
The continuous loading condition without migration
of contact zone for articular cartilage corresponds to
the severe operating condition for cartilage (model
B) as described above in related to the biphasic FE
analysis.
Friction 1(2): 150–162 (2013)
153
was added for protein solutions. The combinations of
lubricant constituents used in reciprocating tests are
shown in Table 1.
Table 1
1
2
3
4
5
6
7
8
9
10
11
12
13
Fig. 2 Reciprocating apparatus.
2.1
Materials
An upper intact cartilage specimen with subchondral
layer was prepared from a femoral condyle in a
porcine knee joint (6 to 7 months old). The damaged
cartilage specimen was prepared by wiping 15 times
with a wiping tissue (Kimwipe), where the partial
removal of surface gel layer was confirmed by observation with atomic force microscopy (AFM). AFM
images in tapping mode (in Dimension Icon, Bruker
Corpration, USA) in saline solution for intact and
damaged specimens are shown in Fig. 3. On the
damaged cartilage surface, the partial removal of
surface gel-like layer is recognized with some exposed
collagen fibers. The glass plate as a lower specimen is
a slide glass.
The lubricants are saline solution containing 0.15 M
NaCl (Otsuka Pharmaceutical Factory Inc., Japan),
saline solution of 0.5 wt% sodium hyaluronate (HA,
molecular weight: 9.2 × 105), HA solutions containing
0.7 wt% or 1.4 wt% bovine serum albumin (Wako
Pure Chemical Industries Ltd., Japan) and/or 0.7 wt%
human serum -globulin (Wako Pure Chemical
Industries Ltd., Japan) and/or 0.01 wt% L-dipalmitoyl
phosphatidylcholine (L-DPPC) as an phospholipid
liposome. In order to prevent bacterial growth in
protein solutions as lubricant, 0.3 wt% sodium azide
2.2
0
0
0
0.5
0.5
0.5
0
0
0
0.5
0.5
0.5
0.5
0
0
0
0
0
0
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0
0.7
0
0
0.7
0
0
0.7
0
0
0.7
0
1.4
0
0
0.7
0
0
0.7
0
0
0.7
0
0
0.7
0.7
Experimental methods
The reciprocating test was conducted at a sliding
speed of 20 mm/s for rectangular reciprocating mode
and at a stroke of 35 mm at a constant load of 9.8 N.
The glass plate was cleaned ultrasonically in a solution
of 0.5 vol% Triton X-100, distilled water and ethanol,
and then dried. The lubricants were supplied in liquid
bath. At room temperature, the reciprocating sliding
was started immediately after loading, and interrupted
after 514 cycles at sliding distance of 36 m for running
time of 30 min, and then the unloading state was
maintained for 5 min. Subsequently, the reciprocating
test was restarted immediately after reloading and
continued for a further sliding distance 36 m. The
restarting processes after unloading were repeated
three times. The changes in friction force were continuously monitored to compare the differences in
time-dependent frictional behaviors. The number of
tests under the same condition was three.
3
Fig. 3 AFM images of articular cartilage surfaces in saline
solution: (a) intact cartilage and (b) damaged cartilage.
Compositions of lubricants (wt%) as saline solutions.
HA:sodium
DPPC
Albumin
-globulin
hyaluronate
Results
Time-dependent frictional behaviors for intact cartilage
lubricated with saline, saline solutions of albumin,
-globulin, HA and DPPC are shown in Fig. 4. It is
noted that the initial friction is very low between 0.01
154
Fig. 4 Influence of lubricant constituents on frictional behaviors
for intact articular cartilage.
and 0.02 as coefficient of friction for all lubricants, as
typical for intact natural articular cartilage. However,
the friction gradually increases with sliding distance
until the sliding stops at 36 m. The final values are
different as the order of -globulin > saline > albumin >
HA > DPPC. The addition of a single constituent into
saline usually reduced the friction level at the final
stage except -globulin. For -globulin, initial friction is
lower than saline but the friction gradually increases
to a higher level with thinning of lubricating film. At
reloading-restarting after 5 min unloading at 36 m
sliding, the restarting friction is remarkably reduced
from the previous high level at interruption, but it is
slightly higher than the initial friction, as reported by
Murakami et al. [26, 27]. This friction reduction was
considered to be brought by the recovery of both the
hydration and some deformation of articular cartilage,
in which the hydration lubrication and biphasic
lubrication becomes partly effective accompanied with
adsorbed film formation although initial adsorbed
film may have been partly removed. In the second
reciprocating sliding process, the friction again
gradually increases with sliding distance. In the second
to fourth processes where the cartilage surface was
partly injured, albumin showed higher friction than
the saline (Fig. 4).
The results mentioned above indicate the limitation
of effectiveness of single additive for improvement of
steady or final friction at each 36 m sliding. Therefore,
it is required to examine the possibility in which the
Friction 1(2): 150–162 (2013)
combination of different synovia constituents should
be effective. As reported by our previous study [26],
on the reduction of final friction at each 36 m sliding,
the synergistic effect of -globulin and HA was
confirmed, but the coexistence of albumin and HA
showed the adverse interaction for intact and damaged
cartilage. It was considered that the combination of
HA and -globulin form adsorbed film cooperatively,
and furthermore HA as a viscosity improver is likely
to alleviate the friction resistance by its viscous
property to improve the fluid film formation in a
mixed lubrication regime. It was pointed out for the
combination of albumin and HA that the repulsive
properties of negatively charged molecules prevented
the lubricating adsorbed film formation. In this study,
the lubricity in the combination of DPPC and albumin
or -globulin was examined for intact and damaged
cartilage specimens. As shown in Fig. 5 for intact
articular cartilage, the coexistence of DPPC and
albumin or -globulin reduced friction compared with
DPPC alone in saline. In contrast, some interaction
between DPPC and proteins brought increase in
friction for damaged roughened cartilage surfaces with
partially removed gel-like layer as shown in Fig. 6.
Next, the effectiveness of DPPC to HA solutions with
and without proteins is evaluated. Figure 7 shows
frictional behaviors for intact cartilage. It is noteworthy
that even the addition of 0.01 wt% DPPC alone to
HA solution exhibited a remarkable reduction in
friction. Furthermore, the addition of 0.01 wt% DPPC
accompanied with 1.4 wt% albumin, 0.7 wt% -globulin
Fig. 5 Influence of DPPC and proteins on frictional behaviors
for intact articular cartilage.
Friction 1(2): 150–162 (2013)
Fig. 6 Influence of DPPC and proteins on frictional behaviors
for damaged articular cartilage.
Fig. 7 Influence of DPPC, proteins and HA on frictional behaviors
for intact articular cartilage.
in HA solution demonstrated the lowest frictional
behaviors as about 0.01 without gradual increase
until each 36 m sliding. In this lubricant composition,
it is confirmed that the friction does not increase but
maintains a very low steady level. On the contrary, the
addition of DPPC with albumin alone or -globulin
alone in HA solution shows higher friction than
DPPC alone in HA solution but lower than saline
solution. In Fig. 8 the frictional behaviors of these
combinations for damaged cartilage are shown.
Compared with intact cartilage, the friction levels are
generally increased and the order of friction level is
partly changed, i.e., HA solution containing DPPC
155
Fig. 8 Influence of DPPC, proteins and HA on frictional behaviors
for damaged articular cartilage.
becomes higher than HA solution containing DPPC
and -globulin. HA solution with 0.01 wt% DPPC,
1.4 wt% albumin and 0.7 wt% -globulin maintains
minimum friction but friction gradually increases until
the level of 0.05 as coefficient of friction at each 36 m
sliding for damaged cartilage.
The comparison of friction at restart and at steady
state for both intact and damaged cartilage is
summarized in Fig. 9. It is noteworthy for intact
articular cartilage that the lubricant of HA solution
with 0.01 wt% DPPC, 0.14 wt% albumin and 0.7 wt%
-globulin showed the minimum coefficient of
friction 0.003 and 0.01 at restart and at steady state,
respectively. For damaged cartilage, these values
showed 0.004 and 0.05, respectively. It was confirmed
that the optimum combination of DPPC, albumin
and -globulin with HA for minimum friction is
common (No. 13 in Table 1) for intact and damaged
cartilage specimens.
To sustain superior tribological properties of
articular cartilage, not only low friction but minimum
wear are required in various daily activities. Therefore,
wear on cartilage surfaces was evaluated. The articular
cartilage contains plenty of water, therefore, it is
difficult to measure actual changes due to wear in
weight. In this study, the changes in surface photographs were compared with a surface before testing.
Representative photographs are shown in Fig. 10. Intact
Friction 1(2): 150–162 (2013)
156
articular cartilage has smooth surface with some
irregularity (left picture). Tests lubricated with No. 10
lubricant (HA solution containing 0.01 wt% DPPC)
and No. 12 (HA solution containing 0.01 wt% DPPC
and 0.7 wt% -globulin) exhibited the low friction
and mild wear with scratches on surfaces. On the
contrary, minimum friction as 0.01 and little wear
were confirmed for lubricant No. 13 (HA solution
containing 0.01 wt% DPPC, 1.4 wt% albumin and 0.7
wt% -globulin), where superficial gel-like layer may
have been slightly removed without scratching during
rubbing.
4 Discussion
Fig. 9 Friction levels at restart and steady state for intact and
damaged cartilage (error bars indicate standard deviation): (a) friction
at restart and (b) friction at steady state.
For natural synovial joint systems, the synergistic
action between articular cartilage and synovial fluid
appears to play an important role in minimizing friction
and wear. In this study, the repeated reciprocating tests
including interrupting-unloading periods for 5 min
for ellipsoidal cartilage specimen against flat glass plate
were conducted, where the contact zone of articular
cartilage was not migrated and thus the effect of the
interstitial fluid pressurization in articular cartilage
Fig. 10 Average friction levels at steady state and cartilage surface photographs for intact cartilage (error bars indicate standard
deviation).
Friction 1(2): 150–162 (2013)
was gradually diminished. Under such severe rubbing
conditions as model B in Fig. 1, the effectiveness of
lubricant constituents and the influence of cartilage
surface conditions on tribological behaviors were
evaluated.
The common features in frictional behaviors of
articular cartilage in the reciprocating tests are as
follows.
(1) Initial low friction is established by biphasic/
hydration and/or mixed lubrication for cartilage
surface with sufficient adsorbed films.
(2) Time-dependent gradual increase in friction
during rubbing process is controlled by biphasic
property of cartilage, interaction of adsorbed molecules
and/or slight removal of cartilage surfaces.
(3) Reduction in restarting friction is brought by
the recovery of hydration and biphasic property with
recovery of deformation accompanied with adsorbed
film formation after unloading for 5 min.
As indicated by the Eq. (1) in FE analysis, we can
estimate the frictional behaviors of various cartilage
surfaces different in adsorbed film formation, i.e.,
coefficient of friction for solid-to-solid contact eq. In
Fig. 11, the changes in friction estimated from total
traction force in biphasic FE analyses during rubbing
process under constant load are shown for eq = 0.01
and 0.2. Most cases of frictional behaviors in this study
except for addition of a single protein seem to be
located between the upper (high friction) and lower
(low friction) curves in Fig. 11, although FE analysis
was conducted for two dimensional model.
Fig. 11 Influence of eq on time-depending frictional behaviors
estimated by biphasic theory for cartilage.
157
In cases of addition of a single constituent into saline
solution (Fig. 4), the frictional features as described
above are observed, but the friction levels change
depending on the properties of lubricant constituents.
The addition of protein, i.e., albumin or -globulin
into saline solution improved the restarting friction
but increased the final friction at each 36 m sliding.
Particularly, the addition of -globulin brought a
remarkable lowering in restarting friction but higher
final friction than albumin. The reason why two kinds
of proteins show different friction levels was considered that -globulin has stronger adsorption ability
on cartilage than albumin as indicated by fluorescent
images [27], and thus -globulin showed lower
restarting friction with appropriate adsorbed film formation in mild condition immediately after reloading,
but exhibited higher friction due to molecular
interaction as a bonding effect in very thin film
condition after each 36 m sliding.
In the in situ observation of the rubbing pair of
poly(vinyl-alcohol) (PVA) hydrogel and glass plate
by Yarimitsu et al. [28], the fluorescent images for
proteins adsorbed on glass plate, protein aggregates
between rubbing surfaces and proteins on PVA
hydrogel surface were discriminately observed in
reciprocating tests for boundary lubrication regime at
low sliding speed of 0.2 mm/s and the average contact
pressure of 0.104 MPa. This reciprocating apparatus
was constructed on the stage of the inverted fluorescent
microscope. In saline solution of albumin, the easy
peeling of albumin was observed, but in saline solution
of -globulin, quick adsorption and uniform adsorbed
film formation were observed. These phenomena
indicate the differences in adsorption abilities for both
proteins. In binary protein solutions with coexistence
of albumin and -globulin, the relative ratio and
concentration of proteins had an intense influence
on adsorbed film formation [29]. Furthermore, the
observation of adsorbed molecules in the evanescent
field within about 200 nm from surface by using the
total internal reflection fluorescence (TIRF) microscopy
indicated in binary protein solutions that the bottom
layer of stable protein adsorbed film is mainly
composed of -globulin and the friction-induced
enhancement of forming protein adsorbed film occurs
158
in lubricant with appropriate protein composition
[30, 31]. The competitive adsorption of albumin
and -globulin appears to affect these behaviors as
indicated in study of adsorption and desorption of
both proteins with TIRF spectroscopy by Tremsina et
al. [32]. Furthermore, the differences in adsorption
behaviors of serum proteins depend on the changes
in conformation, molecular weight, charge condition,
hydrophobic/hydrophilic properties of proteins and
solid surfaces, pH of lubricant, and so on. Particularly
under rubbing, denatured proteins change their
conformations and adsorption properties, and thus
affect the tribological behaviors [33–35]. Therefore,
overall viewpoints are required to elucidate the actual
adsorption behaviors of serum proteins.
The addition of HA with viscous property in
lubricants was expected to improve the fluid film
thickness, and subsequently improved friction level
compared with saline [26]. The addition of DPPC
alone is the most effective in reduction of friction but
the final coefficient of friction is not so low (about 0.1)
in Fig. 4.
Therefore, the effect of combination of different
constituents was evaluated. The influences of
coexistence of protein with HA on friction were
examined in our previous study [26]. The coexistence
of -globulin and HA showed the lowering of both
the restarting and final or steady friction compared
with HA solution. However, albumin exhibited higher
final friction than HA solution although it showed a
little lower restarting friction than HA solution.
These facts suggest the synergistic effect of -globulin
and HA, but indicate the adverse interaction of
albumin and HA for intact cartilage. It is reported
that albumin and HA show repulsive interaction [36]
and the HA-protein complexes in natural synovial
fluid contain globulin but almost no albumin at pH
7–8 [37]. These frictional trends for both proteins are
similar for damaged cartilage with partially removed
surface proteoglycan gel layer. The suppressive action
between negatively charged albumin and negatively
charged HA molecules was observed in fluorescent
images of sparsely distributed adsorbed films, compared with intimate adsorbed films for -globulin
and HA [26].
In this study, the effect of addition of neutral
Friction 1(2): 150–162 (2013)
phospholipid DPPC with and without protein was
examined. It should be noted that the coexistence
of DPPC with protein is effective for intact cartilage
(Fig. 5), but increases friction for damaged cartilage
(Fig. 6). This difference appears to be brought about
by changes in adsorbed film formation on damaged
cartilage surface. For reciprocating tests of PVA
hydrogel and glass plate lubricated with saline
solution of DPPC alone, the Janus-faced property for
high or low friction was affirmed in accord to either
irregular adsorbed film or uniform DPPC adsorbed
film formation in AFM images [38]. It is pointed out by
Hills [10] that even only the oligolamellar phospholipid
plays an effective lubricating role in natural synovial
joints. By in situ fluorescent observation of forming
adsorbed films for sliding pair of PVA hydrogel and
glass plate in coexistence of DPPC and albumin [38],
it was clarified that the formation of albumin-DPPC
sheet-like composite film was found and therefore the
friction was reduced. It is pointed out that DPPC with
a neutral charge is likely to bind to albumin [39].
Next, the influence of addition of DPPC in HA
solution with and without proteins was examined.
The addition of DPPC alone in HA solution was
considerably effective in reduction of friction for intact
cartilage compared with coexistence of DPPC and
either albumin or -globulin in HA solution (Fig. 7).
This fact may suggest the formation of lubricating
complex materials as membrane-like and roller
structures composed of DPPC and HA [40]. Mirea
et al. [41] indicated that HA has high affinity to
phospholipid bilayer in the force-distance curve in
AFM study. The detailed structure of HA-DPPC
complex has not yet been clarified but the coexistence
of DPPC and HA is likely to act synergistically as
lamellar lubrication or related mechanism. Furthermore,
for coexistence of DPPC and HA, HA-DPPC composite boundary film was visually confirmed [38]
and friction was remarkably lowered, where the
lubricating ability by HA-DPPC complex as gel-like
film is supposed to become effective with high water
retention ability of HA. However, HA solution
containing DPPC showed an effective but limited
protective property with local scratching as shown in
Fig. 10.
On the contrary, albumin-DPPC composite was not
Friction 1(2): 150–162 (2013)
found in coexistence of three constituents, i.e., DPPC,
HA and albumin [38], probably due to repulsive
interaction between albumin and HA. This fact
corresponds to the phenomenon in which the friction
for HA solution with DPPC and albumin (Fig. 7) is
higher than saline solution with DPPC and albumin
(Fig. 5).
However, the supply of both albumin and -globulin
as definite ratio into HA solution containing DPPC
(lubricant No. 13) could remarkably improve the
friction at very low level of 0.01 as final coefficient of
friction (Fig. 7) and high wear resistance (Fig. 10). For
damaged cartilage, the friction level increased in
general but No.13 lubricant showed the minimum
friction (Fig. 8).
In natural synovial joints, various lubricating
constituents such as HA, proteins, glycoproteins and
phospholipds different in molecular properties and
sizes play different roles. Therefore, the interaction
and/or synergistic action between phospholipids and
other constituents seem to control the adsorbed film
formation and tribological behavior. The influences
of lubricants as HA solutions containing DPPC with
or without proteins on the friction at restart and at
steady state are summarized in Fig. 9 for intact and
damaged cartilage specimens. The effectiveness of
adsorbed film on reduction in restarting friction and
steady friction is clearly demonstrated compared
with saline solution. Particularly, it is noticed that the
lubricant No.13 (HA solution with 1.4 wt% albumin,
0.7 wt% -globulin and 0.01 wt% DPPC) provided very
low restarting friction for both intact and damaged
cartilage specimens (Fig. 9(a)). This lubricant maintained
very low friction until each 36 m sliding for intact
cartilage, but the friction gradually increased until
0.05 as coefficient of friction for damaged cartilage
(Fig. 9(b)). In the study by Nakashima et al. [29],
HA solution with 1.4 wt% albumin and 0.7 wt%
-globulin (albumin/globulin = A/G ratio of 2:1) or
0.7 wt% albumin and 1.4 wt% -globulin (A/G ratio
of 1:2) showed very low wear for rubbing of PVA
hydrogel against itself. For low wear condition in
the latter, the layered adsorbed film formation was
observed by the fluorescent method. In these cases, it
is suggested that the -globulin forms protective
adsorbed layer on cartilage surface and albumin plays
159
as low shearing layers. On the contrary, HA solution
with 1.4 wt% albumin and 1.4 wt% -globulin (A/G
ratio of 1:1) formed the heterogeneous adsorbed film
and showed higher wear.
The lubricant No.13 has similar composition to that
in natural synovial fluid as hyaluronate solution
containing lubricating constituents such as 1.25 wt%
albumin, 0.75 wt% globulin (including --, and
-globulins) as medium values [42], 1.1 wt% albumin
and 0.7 wt% globulin [36], or 1.9 wt% albumin,
1.1 wt% globulin and 0.01 wt% DPPC [12]. In this
lubricant, the lubricating layered structure in adsorbed
film is expected for low friction and minimal wear,
but the detailed elucidation of this mechanism is
required in the future study. As exhibited in Figs. 7
and 9, lubricant No. 13 showed very low and steady
friction in repeated reciprocating test at 20 mm/s. In
situ fluorescent observation at very slow speed with
this lubricant [38] showed the stable mixed adsorbed
film containing albumin and -globulin but friction is
not so low probably due to very thin film condition
at 0.2 mm/s condition. Therefore, we plan to observe in
situ the actual adsorbed film formation and frictional
behavior at 20 mm/s or so. In various daily activities,
synovia constituents appear to play their appropriate
roles depending on the severity of operating conditions.
DPPC and albumin are likely to act as low shearing
layer, and -globulin acts as the protective film as
strongly adsorbed on cartilage surface. HA has ability
to thicken the lubricating fluid film and form some
lubricating gel-like layer. Although some of synergistic
mechanisms between lubricating constituents were
shown in this study, the overall mechanisms are
expected to be clarified from the viewpoint of multiscale level in future. On the role of lubricin as another
lubricating constituent, Mirea et al. [41] suggested
that it anchors lipid layers on the cartilage. We
confirmed that the addition of lubricin in HA solution
could reduce friction for intact cartilage in the preliminary test. In future study, we plan to evaluate the
effective roles of all influential synovia constituents.
For damaged cartilage specimens with partially
removed proteoglycan brush-like layer, the best
composition in lubricant for low friction is the same
lubricant No. 13 which is the best for intact cartilage,
but the second one was changed to the HA solution
Friction 1(2): 150–162 (2013)
160
containing DPPC with -globulin from the HA containing DPPC solution without protein as the second
one for intact cartilage. It is suggested for damaged
cartilage that the protective role of -globulin with
strong adsorption ability becomes important.
As discussed above, the effectiveness of lubricant
constituents changes depending on rubbing cartilage
properties in reciprocating tests of cartilage-on-glass.
To evaluate rigorously the influence of synovia constituents on tribological behaviors of articular cartilage
in natural synovial joints, the rubbing pair of cartilageon-cartilage [8, 43] or cartilage-on-meniscus [44] should
be used, and therefore the influence of glass plate
on tribological behaviors in this study should be
discussed. As mentioned in Section 1, the glass plate
surface possesses hydrophilic characteristics with
negatively charged property similar to proteoglycan
on superficial cartilage layer in wet condition, whilst
it is hard, smooth and nonporous/impermeable
material. The adsorption of synovia constituents on
glass plate is expected to be considerably similar to
boundary film formation on intact cartilage but the
interaction to the smooth, hard and nonporous/
impermeable glass surface may be different. HA and
albumin (at pH > 4.7) are negatively charged but
-globulin is positively charged (at pH < 7.5). These
electrostatic properties of adsorbed molecules have
an influence on adsorption. On the contrary, the
ploughing friction may be minimized for smooth
surface, but adsorbed proteins on very smooth surface
may induce high friction by their intense adhesive
effect as hydrophobic bonding in watery system in
very thin film condition. However, the effectiveness
of lubricant constituents on tribological behaviors of
compliant and biphasic articular cartilage appear to
be reflected appropriately even in sliding pair of
articular cartilage and glass plate. In pendulum friction
tests for cartilage-on-cartilage of porcine shoulder
joints composed of humerus head and glenoid cavity
(cup) [8], the effectiveness in friction reduction by
addition of 0.01 wt% DPPC or 1.0 wt% -globulin to
HA solution for cartilage treated with detergent had
been confirmed as similar effect to cartilage-on-glass
combination. In contrast, the addition of 1.0 wt% or
3.0 wt% albumin to HA solution did not improve
friction of cartilage-on-cartilage, which corresponds to
adverse interaction of albumin and HA for cartilageon-glass [26]. In contrast, the sliding pair of cartilage
and clean glass plate showed higher friction in HA
solution than that of cartilage and glass plate treated
with Langmuir–Blodgett (LB) film as 5 to 10 bilayer of
DPPC alone or mixed LB film of DPPC and -globulin
[12]. As mentioned above, common features and/or
some differences seem to occur in frictional behavior
for cartilage-glass combination compared with
cartilage-cartilage. In the next stage, therefore, further
studies for cartilage-on-cartilage or cartilage-onhydrogel (artificial cartilage) are required to elucidate
strictly the influence of synovia constituents on
tribological behaviors of articular cartilage in natural
synovial joints. The sustaining of the synergistic
mechanism of various synovia constituents on matched
cartilage surfaces in natural synovial joints is expected
to maintain the healthy condition.
5
Conclusions
In this study, at repeated reciprocating tests including
restarting after interrupting-unloading process, the
changes in friction were observed for intact and
damaged articular cartilage specimens against glass
plate lubricated with lubricants containing phospholipid, protein and hyaluronic acid as synovia
constituents. The optimum composition in lubricants
for low friction and minimum wear of both intact
and damaged cartilage specimens was exhibited to
be similar composition to natural synovial fluid.
Furthermore, it was shown that the effectiveness
of lubricant constituents changes depending on the
surface conditions of articular cartilage.
Acknowledgements
Financial support was given by the Grant-in-Aid for
Specially Promoted Research of Japan Society for the
Promotion of Science (23000011).
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
Friction 1(2): 150–162 (2013)
References
[1] Dowson D. Modes of lubrication in human joints. Proc Inst
Mech Engrs 181: 45–54 (1966)
[2] Unsworth A, Dowson D, Wright V. Some new evidence on
human joint lubrication. Ann Rheum Dis 34(4): 277–285
(1975)
[3] McCutchen CW. The frictional properties of animal joints.
Wear 5: 1–17 (1962)
[4] Walker P S, Dowson D, Longfield M D, Wright V. Boosted
lubrication in synovial joints by fluid entrapment and
enrichment. Ann Rheum Dis 27(6): 512–520 (1968)
[5] Mansour J M, Mow V C. On the natural lubrication of
synovial joints: Normal and degenerate. J Tribol 99:
163–173 (1977)
[6] Dowson D, Jin Z M. Micro-elastohydrodynamic lubrication
of synovial joints. Eng Med 15: 65–67 (1986)
[7] Murakami T. The lubrication in natural synovial joints and
joint prostheses. JSME International Journal Ser III 33(4):
465–474 (1990)
[8] Murakami T, Higaki H, Sawae Y, Ohtsuki N, Moriyama S,
Nakanishi Y. Adaptive multimode lubricaion in natural
synovial joints and artificial joints. Proc Inst Mech Eng
Part H 212: 23–35 (1998)
[9] Swann D A, Hendren R B, Radin E L, Sotman S L, Duda E
A. The lubricating activity of synovial fluid glycoproteins.
Arthritis Rheum-US 24: 22–30 (1981)
[10] Hills B A. Oligolamellar lubrication of joints by surface
active phospholipids. J Rheum 16(1): 82–91 (1989)
[11] Higaki H, Murakami T. Role of constituents in synovial
fluid and surface layer of articular cartilage in joint lubrication
(part 2) the boundary lubricating ability of proteins. Jpn
J Tribol 40(7): 691–699 (1996)
[12] Higaki H, Murakami T, Nakanishi Y, Miura H, Mawatari T,
Iwamoto Y. The Lubricating ability of biomembrane models
with dipalmitoyle phosphatidylcholine and γ-globulin. Proc
Inst Mech Eng Part H 212: 337–346. (1998)
[13] Murakami T, Sawae Y, Horimoto M, Noda M. Role of
surface layers of natural and artificial cartilage in thin film
lubrication. In Lubrication at Frontier. A msterdam: Elsevier,
1999: 737–747.
[14] Ikeuchi K. Origin and future of hydration lubrication. Proc
Inst Mech Eng Part J 221: 301–305 (2007)
[15] Klein J. Molecular mechanisms of synovial joint lubrication.
Proc Inst Mech Eng Part J 220: 691–710 (2006)
[16] Zappone B, Ruths M, Greene W G, Jay G D, Israelachvili J
N. Adsorption, lubrication, and wear of lubricin on model
surfaces: Polymer brush-like behavior of a glycoprotein.
Biophysical J 92: 1693–1708 (2007)
161
[17] Forster H, Fisher J. The influence of loading time and
lubricant on the friction of articular cartilage. Proc Inst Mech
Eng Part H 210: 109–119 (1996)
[18] Ateshian G A. Theoretical formulation for boundary friction
in articular cartilage. J Biomech Eng 119(1): 81–86 (1997)
[19] Krishnan R, Kopacz M, Ateshian G A. Experimental
verification of the role of interstitial fluid pressurization in
cartilage lubrication. J Orthop Res 22: 565–570 (2004)
[20] Ateshian G A. The role of interstitial fluid pressurization in
articular cartilage lubrication. J Biomech 42: 1163–1176 (2009)
[21] Mow V C, Kuei S C, Lai W M, Armstrong C G. Biphasic
creep and stress relaxaion of articular cartilage in compression: Theory and experiments. ASME J Biomech Eng
102: 73–84 (1980)
[22] Pawaskar S S, Jin Z M, Fisher J. Modelling of fluid support
inside articular cartilage during sliding. Proc Inst Mech Eng
Part J 221: 165–174 (2007)
[23] Sakai N, Hagihara Y, Furusawa T, Hosoda N, Sawae Y,
Murakami T. Analysis of biphasic lubrication of articular
cartilage loaded by cylindrical indenter. Tribol Int 45:
225–236 (2012)
[24] Murakami T. Importance of adaptive multimode lubrication
mechanism in natural and artificial Joints. Proc Inst Mech
Eng Part J 216: 827–837 (2012)
[25] Ateshian G A, Wang H, Lai W M. The role of interstitial
fluid pressurization and surface porosities on the boundary
friction of articular cartilage. ASME J Tribol 120: 241–248
(1998)
[26] Murakami T, Nakashima K, Yarimitsu S, Sawae Y, Sakai N.
Effectiveness of adsorbed film and gel layer in hydration
lubrication as adaptive multimode lubrication mechanism
for articular cartilage. Proc Instn Mech Eng Part J 225:
1174–1185 (2011)
[27] Murakami T, Nakashima K, Sawae Y, Sakai N, Hosoda N.
Roles of adsorbed film and gel layer in hydration lubrication
for articular cartilage. Proc Inst Mech Eng Part J 223:
287–295 (2009)
[28] Yarimitsu S, Nakashima K, Sawae Y, Murakami T. Study
on mechanisms of wear reduction of artificial cartilage
through in situ observation on forming protein boundary
film. Tribol Online 2(4): 114–119 (2007)
[29] Nakashima K, Sawae Y, Murakami T. Study on wear
reduction mechanisms of artificial cartilage by synergistic
protein boundary film formation. JSME Int J 48(4): 555–561
(2005)
[30] Yarimitsu S, Nakashima K, Sawae Y, Murakami T. Effect
of lubricant composition on adsorption behavior of proteins
on rubbing surface and stability of protein boundary film.
Tribol Online 3(4): 238–242 (2008)
Friction 1(2): 150–162 (2013)
162
[31] Yarimitsu S, Nakashima K, Sawae Y, Murakami T.
Influences of lubricant composition on forming boundary
film composed of synovia constituents. Tribol Int 42:
1615–1623 (2009)
[32] Tremsina Y S, Sevastianov V I, Petrash S, Dando W, Foster
M D. Competitive adsorption of human serum albumin
and gamma-globulin from a binary protein mixture onto
hexadecyltrichlorosilane coated glass. J Biomater Sci Polym
Ed 9(2): 151–162 (1998)
[33] Heuberger M P, Widmer M R, Zobeley E, Glockshuber R,
Spencer N D. Protein-mediated boundary lubrication in
arthroplasty. Biometarials 26: 1165–1173 (2005)
[34] Nakashima K, Sawae Y, Murakami T. Influence of protein
conformation on frictional properties of poly)vinyl alcohol)
hydrogel for artificial cartilage. Tribol Lett 26: 145–151
(2007)
[35] Nakashima K, Sawae Y, Murakami T. Effect of conformational changes and differences of proteins on frictional
properties of poly(vinyl alcohol) hydrogel. Tribol Int 40:
1423–1427 (2007)
[36] Oates K M N, Krause W E, Jones R L, Colby R H.
Rheopexy of synovial fluid and protein aggregation. J R
Soc Interface 3: 167–174 (2006)
[37] Curtain C C. The nature of protein in the hyaluronic complex
Teruo MURAKAMI. Professor at
Research Center for Advanced Biomechanics, Kyushu University. He
graduated from Kyushu University
in 1970 and received his PhD degree
from Kyushu University in 1978.
He was appointed a professor of
[38]
[39]
[40]
[41]
[42]
[43]
[44]
of bovine synovial fluid. Biochem J 61(4): 688–697 (1955)
Yarimitsu S, Nakashima K, Sawae Y, Murakami T.
Influences of synovia constituents on frictional behavior
of artificial cartilage material and formation of boundary
lubricating film (in Japanese). Tribologist 55(7): 489–498
(2010)
Hernández-Caselles T, Villalaín J, Gómez-Fernáindez J C.
Influence of liposome charge and composition on their
interaction with human blood serum proteins. Mol Cell
Biochem 120: 119–126 (1993)
Pasquali-Ronchetti I, Quaglino D, Mori G, Bacchell B.
Hyaluronan-phospholipid interactions. J Struct Biol 120:
1–10 (1997)
Mirea D A, Trunfio-Sfarghiu A-M, Matei C I, Munteanu B,
Piednoir A, Rieu J P, Blanchin M G, Berthie Y. Role of the
biomolecular interactions in the structure and tribological
properties of synovial fluid. Tribol Int 59: 302–311 (2013)
Sasada T, Tsukamoto Y, Mabuchi K. Biotribology (in
Japanese). Sangyo Tosho, 1988.
Roberts B J, Unsworth A, Mian N. Modes of lubrication in
human hip joints. Ann Rheum Dis 41: 217–224 (1982)
McCann L, Ingham E, Jin Z, Fisher J. Influence of the
meniscus on friction and degradation of cartilage in natural
knee joint. Osteoarthr Cartilage 17: 995–1000 (2009)
Mechanical Engineering in 1988 and a distinguished
professor in 2011 at Kyushu University. Research fields
are biotribology, biomechanics and bionic design. He
is a research leader of a Grant-in–Aid for Scientific
Research on artificial hydrogel cartilage with super
lubricity as Specially Promoted Research supported
by Japan Society for the Promotion of Science.
Friction 1(2): 163–177 (2013)
DOI 10.1007/s40544-013-0013-3
ISSN 2223-7690
RESEARCH ARTICLE
Potential hydrodynamic origin of frictional transients in sliding
mesothelial tissues
Stephen H. LORING1,*, James P. BUTLER2
1
Department of Anesthesia, Critical Care and Pain Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston
MA 02215, USA
2
Department of Medicine, Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston MA 02215, USA
Received: 26 January 2013 / Revised: 26 April 2013 / Accepted: 20 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: Steady-state and transient variations in frictional force observed in tribological experiments of
mesothelial tissues sliding in lubricant were analyzed with a mathematical model to test the hypothesis that
such phenomena are manifestations of elastohydrodynamic lubrication and, importantly, do not require physical
contact between the sliding surfaces. The model incorporates three phenomena characteristic of elastohydrodynamic
lubrication: thinning of the liquid layer between sliding surfaces under a normal load (“squeeze-out”),
thickening of the liquid layer due to hydrodynamic pumping, and smoothing of the elastic surfaces caused by
hydrodynamic pressure gradients. Observations in soft mesothelial tissues sliding in lubricant showed variations
in steady state friction with velocity, load, and lubricant viscosity. In non-steady sliding, the decay rate of
frictional transients at the start of rotation varied with velocity, the amplitudes of these transients varied with
the preceding periods without rotation, and frictional force varied during sinusoidal sliding. Model simulations
were qualitatively similar to experimental results, supporting these mechanisms. Higher lubricant viscosity
increased lubricating layer thickness and lowered friction at low speeds and increased friction at high speeds,
supporting hydrodynamic pumping. We conclude that the frictional variations seen with sliding mesothelial
tissues are consistent with elastohydrodynamic lubrication without contact between the sliding surfaces.
Keywords: pleural space; breathing; pericardium; lubrication; model
1
Introduction
Throughout life, the mesothelial surfaces within the
body slide against each other, lubricated by a thin layer
of serous fluid. The relationships between frictional
force and tribological parameters such as sliding
velocity, normal load, and viscosity have been used
to infer the nature of the physical interactions between
these tissues [1–5]. For example, D’Angelo et al. [4]
studied pleural tissues during oscillatory rectilinear
sliding and found the friction coefficient, defined as
the ratio of shear stress to normal stress on the sliding
surfaces, to be nearly independent of the frequency
* Corresponding author: Stephen H. LORING.
E-mail: [email protected]
of oscillation at a constant displacement amplitude.
They concluded that this independence with respect
to velocity was consistent with boundary lubrication
in which contact or near-contact between asperities
bears the normal load [4]. By contrast, in a subsequent study using a rotating tribometer that
measured the friction coefficient as torque divided by
normal load, Loring et al. [5] found that the friction
of wetted pleural tissues on a rotating plate varied
with rotation rate and hence circumferential velocity,
which is inconsistent with boundary lubrication
but consistent with mixed elastohydrodynamic or
fully-developed hydrodynamic lubrication, in which
hydrodynamic pressure in the fluid bears much or all
of the normal load. In a similar rotational apparatus,
Lin et al. [6] measured the thickness of the fluid layer
Friction 1(2): 163–177 (2013)
164
between sliding mesothelial tissues and a rotating
glass plate and found that the thickness of the fluid
layer beneath the tissue surface depended on sliding
velocity. In particular, higher velocities resulted in
greater fluid thickness and a faster approach to
the steady state fluid thickness, consistent with
hydrodynamic lubrication. In addition, soft materials
sliding in lubricant are smoothed by hydrodynamic
pressure gradients, making the lubricant layer more
uniform in thickness [7]. Considered together, the
latter three studies suggest that soft materials with
statically uneven surfaces sliding in lubricants are
smoothed and deformed so as to become load-bearing,
thus maintaining a layer of lubricant between sliding
surfaces [8, 9].
The latter studies and subsequent observations
suggest that the tribological behavior of sliding tissue
samples in these experiments can be largely explained
by the following three phenomena characteristic of
elastohydrodynamic lubrication. (1) Squeeze-out: thinning of the liquid layer between the sliding surfaces
due to centrifugal flow driven by load-dependent
pressure gradient. An example of this effect in vivo
is the flow of pleural liquid away from the region
between a convex rib indenting the lung towards
the surrounding regions of the pleural space where
the normal stress and fluid pressure are lower. (2)
Hydrodynamic pumping: thickening of the fluid layer
between the sliding surfaces due to centripetal fluid
flow driven by local hydrodynamic pressure gradients
caused by the sliding of slightly uneven surfaces
that are nearly, but not completely, parallel. This
phenomenon is responsible for load support in
hydrodynamic lubrication. An example of this effect
in vivo is the hydrodynamically driven flow of pleural
fluid from a relatively thick layer of liquid near a
lobar margin into the pleural space covering the
costal surface of the lung [10]. (3) Elastohydrodynamic
smoothing: reversible deformation and flattening of
the sliding surfaces caused by hydrodynamic pressure
gradients that result in a decrease in the spatial nonuniformity of thickness of the fluid layer. An example
of this effect in vivo is the small-scale reversible
deformation that would smooth mesothelial surfaces
wherever they slide relative to each other in the
presence of serous fluid.
In this report, examples of steady state and transient
friction observed experimentally in vitro are presented
and then simulated with a mathematical model that
synthesizes the three phenomena above: thinning of
the fluid layer due to squeeze-out, thickening due to
hydrodynamic pumping, and elastohydrodynamic
smoothing. The model is based on geometrical and
physical parameters of our experimental preparations,
variations in velocity similar to those in the experiments, and a free parameter relating the velocity of
sliding to the magnitude of the hydrodynamic pressure
opposing squeeze-out. Simulations were compared
with experimental results to quantify the importance
of these mechanisms to the transient phenomena
observed under different experimental conditions.
2
2.1
Methods
Experimental preparation and apparatus
Tribological experiments on mesothelial tissues were
conducted using the peritoneal surface of rat belly wall
sliding on a rotating glass plate (Fig. 1) as described
previously [5, 6] or on a rotating plate covered with
tissue. Briefly, a tissue sheet consisting of the inner
layer of the abdominal wall, including the peritoneum
and attached muscle layers, was dissected free and
mounted, mesothelial side outward, at the mouth of
a shallow tissue-cup (3.2 cm diameter). In the top-fixed
experimental apparatus [5], the mesothelial tissue and
cup, mounted on a shaft and bearing for measurement
of rotational torque, were held close (< 1 mm) to
the surface of a rotating glass plate lubricated with
physiologic saline. The tissue sheet was then pressed
against the glass plate using air pressure within the
tissue cup, thus applying a known normal stress on
the sliding mesothelial surface. Data from those
experiments were reanalyzed to characterize the
transient changes in torque at the onset of motion. In
new experiments using a top-weighted apparatus, the
torque measuring apparatus was mounted on a balance
arm and pressed against the rotating glass plate by an
applied weight. With this apparatus, friction between
tissue surfaces was also measured by mounting mesothelial tissue, prepared as above, on a rotating platform
slightly larger than the stationary tissue and cup.
Friction 1(2): 163–177 (2013)
165
Fig. 1 Schematic of apparatus illustrating some of the features
of the rotational tribometers used in the experiments.
The glass plate or tissue platform was rotated by a
computer-controlled stepper motor, and the applied
pressure, torque, and angular displacement of the drive
system were recorded at > 750 Hz (Dataq Instruments,
Akron, OH). Friction was quantified by rotational
torque measured with a strain gauge [5] and divided
by normal load to produce a rotational coefficient of
friction (CF). Details of individual experiments are
described in Section 3.
2.2
Fig. 2 Model of a soft tissue disk with an uneven bottom surface
pressed by a normal load against a rotating flat disk, all bathed
in lubricant. The minimum fluid thickness (hmin), amplitude of
unevenness (hamp) and characteristic wavelength (L) are not to
scale.
Table 1
Constant
Description
Value
Rdisk
disk radius (R in the equations above)
1.5 cm
0
hamp
initial undeformed amplitude of unevenness
70 µm
Model simulation
A mathematical model was used to simulate the
experiments and explore mechanism. The model incorporates a disc of soft tissue with an undulating bottom
surface pressed against a flat plate rotating in the
presence of lubricating fluid (Fig. 2). The unevenness
of the bottom tissue surface decreases as the tissue
approaches the opposing plate. The parameters of
the model are similar to those of the experiment and
are presented in Table 1. Experimental input variables
include rotation rate and normal pressure load as
functions of time. Output variables include frictional
force (quantified by torque) and fluid thickness. The
dynamic behavior of the system is determined by the
three phenomena introduced above and further defined
below: (1) thinning of the fluid layer due to squeezeout of fluid caused by the normal load, (2) thickening
of the fluid layer due to hydrodynamic pumping of
fluid from the surrounding reservoir, and (3) elastic
deformation and flattening of the surface unevenness
causing elastohydrodynamic smoothing of the tissue surface, decreasing the amplitude of surface roughness.
*
Constants of the model simulations.
L
wavelength of unevenness

lubricant viscosity
k pump
constant of hydrodynamic pumping*
hmin, crit
hmin below which Phydro progressively
decreases*
1000 µm
0.01
poise
100
8 µm
Parameters used for fitting simulation to experimental data
2.3
2.3.1
Mechanisms of hydrodynamic lubrication
Squeeze-out
Squeeze-out is based on the physics of a piston sinking
under a normal load in lubricant toward a flat surface
(see Eq. (9) below). The normal load is balanced by the
effective hydrodynamic pressure caused by downward
movement of the piston. The rate of descent (fluid
thinning) is directly proportional to the load and
inversely proportional to fluid viscosity, the cube of
the harmonic mean fluid layer thickness, and the
radius of the disk to the 4th power.
Friction 1(2): 163–177 (2013)
166
2.3.2
Hydrodynamic pumping
The load-supporting pressure due to hydrodynamic
pumping is added to the hydrodynamic pressure due
to squeeze-out. When the pressure due to the normal
load is greater than that produced by hydrodynamic
pumping, fluid flow is centrifugal (from disk center)
and the fluid layer gets thinner. Conversely, when the
hydrodynamic pumping pressure is greater than the
normal load, fluid flow is centripetal and the fluid
layer gets thicker. Hydrodynamic pumping pressure
is the global effect of highly local hydrodynamic
pressures generated between sliding surfaces that are
nearly but not completely parallel. Consistent with the
major features of lubrication theory [11], hydrodynamic
pumping is assumed to be proportional to sliding
velocity and fluid viscosity and inversely proportional
to the minimum fluid thickness, hmin . To simulate the
increase in CF at very high sliding speeds, hydrodynamic pumping pressure is assumed to diminish
as the fluid thickness gets large compared with the
characteristic wavelength of the surface unevenness.
2.3.3
Elastohydrodynamic smoothing
Relative flattening of the surface undulations due to
hydrodynamic pressure is assumed to occur as the
uneven tissue approaches the flat plate. The amplitude
( hamp ) of the unevenness of the tissue disk remains
relatively constant near its initial undeformed amplitude
0
when the mean fluid thickness (h) is large and
hamp
decreases smoothly and progressively as the minimum
thickness hmin  0 , or equivalently, as h approaches
0
. The harmonic mean thickness ( hhm ) and hmin of
hamp
0
the fluid are derived from h and hamp
. Flattening of
the surface to an extreme degree is assumed to reduce
hydrodynamic pumping, leading to an increase in CF
at very low sliding speeds.
Simulations of the model, including fluid thickness
and coefficient of rotational friction, were compared
to experimental data obtained with the following
velocity−time protocols: (1) steady-state sliding at
different constant rates, loads, and lubricant viscosities;
(2) a step increase in rotation rate from zero to
various constant speeds following 30 s without motion;
(3) constant speed rotation at different rates interrupted by different periods without rotation; and (4)
sinusoidal rotation at various frequencies and peak
rotation rates.
2.4
Model details
Conceptually, the model is a disk of tissue with an
uneven bottom surface pressed against a coaxial
rotating flat surface, the “bottom plate” (Fig. 2). The
flow of fluid within the space between tissue disk
and bottom plate depends on the pressure gradient
caused by the combined effects of the normal force
(load) applied to the tissue disk, which causes centrifugal flow and fluid layer thinning (squeeze-out) and
the pressure generated by hydrodynamic pumping,
which causes centripetal flow and fluid layer thickening.
When the tissue is pressed close to the bottom plate,
it is smoothed (flattened) by normal stresses caused
by hydrodynamic pressures. Whereas the physics
underlying squeeze-out is well understood and a
rigorous solution exists, the physics of hydrodynamic
pumping and soft tissue smoothing are only partially
understood, and a rigorous solution in general is
lacking.
The theoretical basis for the model starts with a
dynamical argument to find the relationship between
normal force, Fn , in relation to the rate of change
in the height of the fluid channel between the disk
and the plate. In what follows, we assume circular
symmetry and deal only with the radial dependence
of variables.
The volume flow rate (or flux) of fluid inward at
any radius r is
dh
Q (r )   r 2
dt
(1)
where h is the mean height of the tissue surface above
the plate. In particular, the flow rate, Q , into the
region between the disk and the bottom plate from
the reservoir is
dh
Q  Q ( R)   R2
dt
(2)
where R is the radius of the tissue disk.
The cylindrical cross-sectional area of the fluid
channel at radius r is A(r )  2  rh , and so the mean
fluid radial velocity U (r ) is given by
U(r ) 
Q (r )
r dh

A(r ) 2 h dt
(3)
Friction 1(2): 163–177 (2013)
167
dP
, the
dr
pressure gradient driving flow in the channel. In
steady state, the fully developed fluid velocity is
quadratic in height, y, and vanishes at the bottom
and top surfaces secondary to a no slip condition.
4
Thus, U ( y , r )  U (r )max 2 y( h  y) , where U (r )max is the
h
This is used at each r to determine
velocity at mid-channel. The average velocity U (r ) 
h
1
2
U ( y , r )  U max (r ) , and so, from Eq. (3), we have

3
h0
U max (r ) 
3 r dh
4 h dt
(4)
The gradients in U ( y , r ) with respect to y induce
a viscous drag on the boundary, which must be
balanced by radial gradients in pressure. The force
on a differential volume of fluid between r and
r  dr of angular width d due to the two boundaries
is thus twice product of the shear stress and the area,
given by 2  dU / dy y  0 rdrd  (8 U max / h)rdrd .
The force on the differential element due to a pressure
gradient is simply hr d dP . Equating these yields
dP / dr  6 
r dh
h 3 dt
(5)
hmin 
The pressure at any radius is
dP
3 dh 2 2
 R  r 
(r )  P0  3
d
r
h dt 
R
r
P(r )  P0  
(6)
where P0 is pressure in the surrounding reservoir.
The normal force (load) is
R
Fn  2  P(r ) r dr  
0
3 R4 dh
2 h 3 dt
(7)
and therefore
2 h 3 Fn
dh

dt
3 R 4
(8)
or, in terms of average pressure ( P ),
dh
2h3 P

dt
3 R2
rate of change in thickness of the fluid layer is a
function of the effective average pressure, i.e., the
sum of the average pressure load, acting to decrease
thickness, and a hydrodynamic pressure ( Phydro ) due
to sliding of the uneven tissue surface against the
rotating disk, acting to increase thickness. Phydro is
assumed to have the same spatial distribution as that
of the pressure load and to oppose the gradients
caused by the load or downward movement of the
tissue disk. Constants of the model include the disk
radius (R) and fluid viscosity (  ).
The pressure load and rotation rate ( f , signed
such that f  0 when rotation is counterclockwise,
typical units are revolutions per second) are input
variables. Hydrodynamic pressure caused by tissue
sliding is due to wedge-like deformations of these
otherwise symmetrical undulations of the tissue surface
of amplitude, hamp . The wavelength (L) and initial
undeformed amplitude of microscopic sinusoidal
0
unevenness of the tissue surface ( hamp
) are specified.
0
0
When h  hamp , hamp  hamp . As the tissue approaches
the bottom plate, hamp is reduced by adjusting the local
minimum thickness of the liquid layer ( hmin  h  hamp )
to maintain hmin  0 according to the following
equation,
The rate of increase or decrease in the average
h (Eq. (9)) is calculated using the harmonic mean
thickness hhm , given by
1/ hhm  1 2 (1/ hmin  1/ hmax ) ,
where 1 2 ( hmax  hmin )  h . The equation for Phydro is a
simplification based on lubrication theory [11]. In
essence, the normal (lifting) force is assumed to be
proportional to velocity, viscosity, the area of the
uneven sliding surface, and roughly inversely proportional to the minimum fluid thickness,
Phydro 
(9)
The model simulations are based on Eq. (9). The
h
.
0
h
1  hamp
k pump  f 2 R 2
hmin
,
where k pump is a constant. We modified the formula
to simulate a reduction of lift when the tissue surface
is far separated from the disk ( h  hamp ),
Friction 1(2): 163–177 (2013)
168
Phydro 
k pump  f 2 R 2
hmin

L
,
L  hmin
where L is the undulation wavelength. We further
modified this formula to simulate a reduction in lift
when the undulations in the tissue surface flatten in
close proximity to the bottom plate,
Phydro 
k pump  f 2 R2
0.25
hmin  h min, crit hmin
L

,
L  hmin
where hmin, crit is a constant. Eq. (9) is integrated to
determine changes in h .
Shear stress on a differential area ( dS ) of the disk
(based on hhm ) is
dS 

hhm
rf ,
which is integrated to obtain torque
Torque 
3
3.1

hhm

R
0
rfSr dr 
  fR
.
hhm
2
4
Results
Steady state rotation at constant rates: effects
of velocity, load, and viscosity
3.1.1 Experimental: tissue sliding on tissue, effects of
rotation rate and load
Seven experiments were performed with the topweighted apparatus with tissue rotating on tissue
under loads of 5 or 10 g, the upper tissue disk being
pressurized with 100 or 200 Pa. The bottom tissue
surface was rotated at various rates in alternating
directions with a square-wave pattern that allowed
steady state CF determinations after transients died
out. The coefficient of friction (torque/load) vs. velocity
curves were reminiscent of classical Stribeck curves,
with relatively high CF at the lowest rotation rates,
lower CF at transitional rates and increasing CF at
higher rates, but there was substantial variability
among experiments, both in the magnitude of CF and
the pattern of its variation with velocity. These results
were similar to those of tissue sliding on glass published previously using the top-fixed apparatus [5]. CF
did not vary systematically with load in these experiments (Fig. 3). In general, the tribological behavior of
tissue sliding on tissue was similar to that of tissue
sliding on glass, suggesting a common mechanism.
3.1.2 Experimental: tissue sliding on glass, effects of
rotation rate and viscosity
Seven experiments were performed with the topweighted apparatus with tissue rotating on flat glass
under loads of 0.10 or 0.15 N, with the tissue disk
pressurized with 200 or 300 Pa. The CF measured
with saline lubricant, viscosity ~1 centipoise (cp), was
compared to that with solutions of 4.0 or 6.5 mg/ml
carboxymethylcellulose in saline, viscosity ~0.020 or
~0.033 cp (measured with a U-tube viscometer). At
the lowest rotation rates, CF tended to be lower with
the higher viscosity lubricant, whereas at the highest
rotation rates, the opposite was true (Fig. 4).
3.1.3 Model simulations and mechanism: effects of
velocity, load, and viscosity
In steady state simulations (Fig. 5), CF is relatively
constant at lower velocities, decreasing slightly at
intermediate speeds and increasing at higher speeds,
reproducing the experimental changes in CF with
speed.
The mechanism of CF variation in the simulation
can be related to the changes in h at low and high
velocities. In lubrication theory and in our model, the
normal force supporting the load above a wedge
sliding with velocity U and the shear force both vary
roughly as h 1 . In our simulations, h varies as U 0.4
throughout the range of velocities studied. At very
low velocities, decreases in velocity cause fractional
decreases in h that re-establish the steady state
hydrodynamic pressure to balance the normal load
without causing large changes in shear stress. When h
0
, hydrodynamic smoothing degrades
approaches hamp
the hydrodynamic pumping, causing a large decrease
in h and consequent increase in CF at very low speeds.
Conversely, at very high speeds, increases in velocity
cause relatively small fractional increases in h, and
shear force increases with speed.
Figure 5 also shows the effects of lubricant viscosity
and load. Doubling of the load decreases h and increases CF at low speeds while slightly reducing CF
Friction 1(2): 163–177 (2013)
169
Fig. 3 Coefficient of friction (CF) of tissue rotating on tissue. Rotation was alternating clockwise (CW) and counter-clockwise (CCW,
CF shown as negative) under loads (L) of 0.05 or 0.10 N (See Section 3.1.1)
170
Friction 1(2): 163–177 (2013)
Fig. 4 CF of tissue rotating on glass under loads (L) of 0.10 or 0.15 N with saline lubricants of viscosity () of 1, 2, or 3.3 cp. Rotation
alternated direction as in Fig. 3.
Friction 1(2): 163–177 (2013)
171
(rps), torque increased progressively during rotation
without reaching a maximum during 30 s rotation. At
rotation rates of 0.05–0.8 rps, peak torque was reached
soon after the onset of rotation and then decayed
to the steady state value, this decay being faster at
higher rotation rates (Fig. 6(a)). The normalized rate
of transient decay (the characteristic slope of torque
decrease with time after peak torque divided by the
peak-steady state torque difference) was significantly
correlated with rotation rate (Fig. 7(a)).
Fig. 5 Simulations of steady state CF (a) and fluid thickness (b)
as a function of rotation rate with pressure load (P) of 100 or 200
Pa and lubricant viscosity () of 1 or 3 cp.
at high speeds without changing CF over the entire
mid-range of rotation rates (compare with Fig. 3).
Increasing viscosity increases h and reduces CF at low
speeds while increasing CF at high speeds, reproducing
the experimental results in Fig. 3.
3.2
3.2.1
Decay of frictional transients at the onset of
rotation at various rates
Experimental observations
Data from 8 experiments in an earlier study of steady
state friction [5] were re-analyzed to determine how
the rate at which torque decayed from its peak after
the onset of rotation varies with speed of rotation. In
a top-fixed apparatus, pressure loads of 100–200 Pa
were applied 30 s before the start of rotation. At
rotation speeds below ~0.02 revolutions per second
Fig. 6 (a) An example of the torque transients at the onset of
rotation at various rates. The peak torque was greater and the
decay of the torque transient was quicker at higher rotation rates.
(b) Simulated transients at the start of rotation. There is a greater
rate-dependence of peak torque in the simulations than in the
experimental results. We speculate that deformation of mesothelial
tissue by stresses at the onset of rotation reduce the initial torque
and redistribute fluid beneath the tissue, whereas peak torques in
the simulation are not limited by this mechanism.
Friction 1(2): 163–177 (2013)
172
Fig. 7 (a) Characteristic rate of decrease from peak torque normalized by the peak-to-steady state difference in torque at various
rotation rates under normal stress of 100 or 200 Pa in 8 experiments. The normalized rate of torque decrease was significantly related to
rotation rate in 5 of 8 experiments and in the group as a whole (ANOVA, p < 0.0001). The rate of decay was also related to the individual
tissue preparation (P < 0.0001) and pressure ( p = 0.0054). (b) Simulation: normalized rate of torque decrease after the onset of rotation
in simulations at various speeds after a squeeze-out period of 30 s. Normalized decay rate was calculated as the inverse of the initial
half-time of the decay. Increasing rotation rate increases the rate of torque decay from the peak to the steady state value.
3.2.2 Model simulations and mechanism
The model simulations (Figs. 6(b), 7(b)) started with
an initial fluid thickness of 0.1 mm and a 30 s period
with 100 Pa pressure load to cause squeeze-out before
rotation. At the onset of rotation at lower rotation
rates, torque increased relatively slowly to a peak and
decreased slowly to the steady state value, whereas
at higher rates, torque increased and decreased more
rapidly. Peak torque and the rate of decay of torque
were greater at higher rotation rates because the
shear stress and lifting force due to hydrodynamic
pumping pressure were greater, and higher rates of
fluid thickening increased h and reduced torque more
quickly at higher rotation rates. As in the experiments,
the onset of rotation at rates below 0.04 rps caused
a progressive increase in torque (and h) without a
subsequent torque decrease, because h had not
decreased to its steady state value in the 30 s before
rotation began.
3.3
3.3.1
Transient peaks in friction after different periods
of squeeze-out
Experimental observations
In six experiments with the top-fixed apparatus, we
measured peak torque at the onset of rotation after
periods without rotation ranging from 0.1 s to 128 s.
Rotation rates ranged from 0.002 to 2 rps at pressure
loads of 100 or 200 Pa. The peak torque increased with
increasing length of the preceding period without
rotation in all runs in all experiments.
The data were fit to an equation derived from the
rudimentary squeeze-out model described in Section
2.4, together with the additional assumption that the
initial peak force is inversely proportional to the
thickness of the fluid layer at the onset of rotation.
Without rotation, thickness decreases from the steady
state thickness at the end of the previous rotation
due to squeeze-out. As given by Eq. (8), the height
Friction 1(2): 163–177 (2013)
173
of the tissue disk above the bottom plate (h) changes
2 h 3 Fn
dh
at the rate

, where Fn is the normal force
dt 3 R4
applied, R is the disk radius, and  is fluid viscosity.
2 Fn
Integration yields h(t ) 
where ti is
3 R 4 t  ti
3.3.2 Model simulations and mechanism
The simulation results were similar to the experimental data. Figure 9 shows the peak torque and
thickness of the liquid layer at the end of intervals
without motion. The basic squeeze-out model (Eq. (1))
fits data at individual speeds well.
the time that would have been required for squeezeout to reduce h from an initial large value to its
previous steady state value, which in turn is the initial
value during pure squeeze-out. The equation of peak
torque has the form,
PeakTorque  k t  t i
(10)
where k is a constant. Figure 8 shows two representative experimental results.
Fig. 9 Simulated peak torques following intervals without rotation
of various durations (left axis) and h values at the end of the
intervals (right axis) with rotation rates of 0.1 or 1 rps with a
pressure load of 100 Pa. The progressive increase in torque with
increasing time without motion can be attributed to the progressive
reduction in h due to squeeze-out. Note that the initial difference
in h at the two rotation rates almost disappears by 128 s, and the
curves rapidly converge, consistent with the h3 dependence of the
squeeze-out rate (Eq. (9)).
3.4 Sinusoidal rotation
3.4.1
Fig. 8 Peak torque in a top-fixed apparatus at the onset of rotation
after various intervals without rotation. In Expt 0121, pressure
was 200 Pa and rotation rates were 0.05, 0.1, and 0.2 rps. One
squeeze-out model simulation (Eq. (1)) fits all the data relatively
well for these 5 runs with a limited range of rotation rate. The
constant ti (see Section 3.3.1) ranged from 2.1 to 3.7 s in theses
experiments. In Expt 1126, pressure was 100 Pa, and there was a
wider range of rotation rates. Although Eq. (1) continued to fit
individual curves relatively well, no single simulation could fit
all curves. The large difference in the toque magnitudes between
the two experiments is typical of previous results [5], and we
speculate that it arises from differences in surface topography and
mechanical properties of individual tissue preparations.
Experimental observations
Sinusoidal rotation was applied in 9 experiments with
the top-fixed apparatus at frequencies from 0.005 to
1.25, with peak rotation rates from 0.006 to 1.25 rps
and pressures of 100 or 200 Pa. Figure 10 shows
examples of the force-displacement and force-velocity
plots at high and low frequencies in one experiment.
At higher frequencies, torque vs. displacement plots
were rounded (Fig. 10(a)) and force vs. velocity plots
were nearly linear (Fig. 10(b)), indicating that torque
varied with velocity. By contrast, at low frequencies,
force displacement plots were relatively rectangular
(Fig. 10(c)), indicating that torque was relatively
constant with speed, changing sign with direction of
rotation (Fig. 10(d)).
Friction 1(2): 163–177 (2013)
174
Fig. 10 Torque during sinusoidal oscillation at 1 Hz (a, b) and 0.05 Hz (c, d), with 1 rps peak velocity and a pressure load of 100 Pa. Torque
was nearly in phase with velocity at the higher frequency and nearly invariant with velocity at the lower frequency. These results were typical.
3.4.2 Model simulations and mechanism
Model simulations showed similar variations in the
shapes of torque-displacement and force-velocity
characteristics with frequency. At high frequencies
and velocities, torque is nearly proportional to velocity
(Figs. 11(a) and 11(b)). This can be attributed to the
fact that h is relatively constant because the short period
of the oscillatory cycle does not permit substantial
squeeze-out (Fig. 11(c)).
At low frequencies, (Figs. 11(d) and (e)), torque
was nearly independent of velocity and the forcedisplacement characteristic was more rectangular. At
high velocities and low frequencies, the relatively
constant torque can be attributed to relatively large
variation in both h and hmin due to the dynamic
equilibrium between hydrodynamic pumping and
squeeze-out that stabilizes shear stress as velocity
changes (Figs. 11(c) and (f)). The asymmetry of the
torque-displacement curves at high frequency and
the cusps on torque-displacement and torque-velocity
curves at low frequency (Figs. 11(a), (d), and (e)) are
caused by the transiently low values of h following
squeeze-out during the preceding low velocity
(Figs. 11(c) and (f)). These asymmetries were sometimes
observed in the experimental data (note the similar
asymmetry in Fig. 10(a)).
4
Discussion
Here, we have interpreted steady state and transient
behavior of tissues sliding in saline with a model that
synthesizes physical principles and phenomena of
elastohydrodynamic lubrication. This analysis extends
earlier theoretical work suggesting a hydrodynamic
mechanism that could explain a relatively constant
CF with decreasing velocity [12] to transient and nonsteady state sliding. Although elastohydrodynamic
mechanisms are able to explain many experimental
phenomena, their importance in mesothelial lubrication
in vivo is debated.
Conventionally, a transient peak in force at the onset
of sliding has been interpreted as implying that the
surfaces initially in contact exhibit static friction [13].
Force increases with increasing elastic deformation
until the surfaces break free and slide at a lower
frictional force in the steady state; if contact returns
Friction 1(2): 163–177 (2013)
175
Fig. 11 Simulations of oscillatory rotation at 2 or 0.02 Hz with a load of 100 Pa and velocity amplitude of 1 rps (See Section 3.4.2).
and this cycle repeats, it constitutes the classical
“stick-slip” behavior. By contrast, in our simulations
surfaces are never in contact, and there is no elastic
deformation caused by shear stress of tissues before
sliding begins. Instead, the transient variations in
torque are due to history-dependent variations in
the thickness of the lubricating fluid layer, which is
decreased by squeeze-out, increased by hydrodynamic
pumping, and influenced by elastic smoothing of the
surface roughness. Without motion, the thickness of the
liquid layer decreases at a rate inversely proportional
to the square root of time.
The coefficients of friction observed in the experiments described are variable, and, in general, much
greater than those in the model simulation. In a
previous study [5], we noted a remarkable variability
among tissue specimens in the steady state torque
and patterns of torque variation with velocity, and we
speculated that such variability is due to topographical
features peculiar to individual tissue specimens that
augment or diminish the load bearing effects of
hydrodynamic lubrication. We infer that the model’s
regular and homogeneous surface unevenness results
in lower shear stress with relatively homogeneous
fluid thickness. Alternatively, the higher torques in
experimental preparations may reflect regions of
contact between surfaces. This is the view held by
Agostoni, D’Angelo and others [3, 4] who have
Friction 1(2): 163–177 (2013)
176
interpreted the velocity-invariant friction observed
with mesothelial tissues during rectilinear sliding
experiments as evidence for boundary lubrication.
However, the velocity-dependent behavior we observed
during rotational tribometry [5] was reproduced in
experiments using rectilinear tribometry of tissue on
glass (unpublished), suggesting that the difference
in speed-dependence of CF is not due to rectilinear
versus rotational tribometry, but rather to differences
in the normal loads and sliding velocities employed.
The three elements of the model are based to varying
degrees on theory and experimental observation.
Squeeze-out is an obvious phenomenon when lubricated surfaces are pressed together without sliding
motion, as described by the physics of a piston
sinking under a load (see Section 2.4). Hydrodynamic
pumping is based on the two-dimensional lubrication
theory [11], whereby fluid thickness is increased
between sliding surfaces that are nearly parallel.
In three dimensions, hydrodynamic pumping is a
general phenomenon exhibited by soft shapes sliding
in lubricant [9], and has been demonstrated during
rotational sliding in biomechanical experiments [6]
and in finite element models [8]. Elastohydrodynamic
pumping lacks a rigorous general mathematical
description, as does hydrodynamic smoothing. The
biological relevance of elastic smoothing of mesothelial tissue surfaces has been recently explored
through mechanical surface characterization [14].
In conclusion, the friction of mesothelial tissues
during steady state and time-varying sliding can be
largely reproduced by a synthesis of three phenomena characteristic of elastohydrodynamic lubrication.
These phenomena are thinning of the lubricating fluid
layer by squeeze-out under a normal load, thickening
of the fluid layer through hydrodynamic pumping,
and elastic smoothing of surface unevenness by local
hydrodynamic stresses. These phenomena provide a
plausible mechanism to explain tribological experiments in sliding mesothelial tissues, but they do not
preclude the possibility of other mechanisms, including
tissue-tissue contact.
inappropriately influence this investigation.
Acknowledgements
The authors are indebted to Richard E. Brown for
physiological insights and experimental data presented
here, and we thank Jae Hun Kim for help in revision.
The work is supported by grant HL-63737 from the
National Institutes of Health.
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
References
[1] Brandi G. Determinazione del coefficiente di attrito statico tra
le due pleure. Boll Soc Ital Biol Sper 46(8): 427−429 (1970)
[2] Brandi G. Frictional forces at the surface of the lung. Bull
Physiopathol Respir (Nancy) 8(2): 323−336 (1972)
[3] D'Angelo E. Stress-strain relationships during uniform and
non uniform expansion of isolated lungs. Respir Physiol
23(1): 87−107 (1975)
[4]
D'Angelo E, Loring S H, Gioia M E, Pecchiari M, Moscheni
C. Friction and lubrication of pleural tissues. Respir Physiol
Neurobiol 142(1): 55−68 (2004)
[5] Loring S H, Brown R E, Gouldstone A, Butler J P. Lubrication
regimes in mesothelial sliding. J Biomech 38(12): 2390−2396
(2005)
[6] Lin J L, Moghani T, Fabry B, Butler J P, Loring S H.
Hydrodynamic thickening of lubricating fluid layer beneath
sliding mesothelial tissues. J Biomech 41(6): 1197−1205
(2008)
[7]
Gouldstone A, Brown R E, Butler J P, Loring S H.
Elastohydrodynamic separation of pleural surfaces during
breathing. Respir Physiol Neurobiol 137(1): 97−106 (2003)
[8]
Moghani T, Butler J P, Lin J L, Loring S H. Finite element
simulation of elastohydrodynamic lubrication of soft biological
tissues. Comput Struct 85(11−14): 1114−1120 (2007)
[9]
Skotheim J M, Mahadevan L. Soft lubrication: The
elastohydrodynamics of nonconforming and conforming
Conflict of interest statement
The authors have no financial or personal relationship with other people or organizations that could
contacts. Physics of Fluids 17(9): 092101−092123 (2005)
[10] Butler J P, Huang J, Loring S H, Lai-Fook S J, Wang P M,
Wilson T A. Model for a pump that drives circulation of
pleural fluid. J Appl Physiol 78(1): 23−29 (1995)
Friction 1(2): 163–177 (2013)
[11] Batchelor G K. An Introduction to Fluid Dynamics.
Cambridge, UK: Cambridge University Press, 1967.
177
[13] Bowden F P, Talbor D. The Friction and Lubrication of
Solids. Oxford, UK: Oxford University Press, 2001.
[12] Butler J P, Loring S H. A potential elastohydrodynamic origin
[14] Kim J H, Butler J P, Loring S H. Influence of the softness
of load-support and coulomb-like friction in lung/chest wall
of the parietal pleura on respiratory sliding mechanisms.
lubrication. J Tribol 130(4): 041201 (2008)
Respir Physiol Neurobiol 177(2): 114−119 (2011)
Stephen H. LORING received his
M.D. from Harvard Medical School
in 1973 and joined the Physiology
Department at Harvard School of
Public in 1977. In 1991 he joined
the Department of Anesthesia and
Critical Care at Beth Israel Deaconess Medical Center
and Harvard Medical School, where his current
position is Scientific Director of Respiratory Medicine.
His research has centered on respiratory physiology
and medicine and the biomechanics and physiology
of the pleural space.
James P. BUTLER received his
Ph.D. in physics from Harvard Univ.
in 1974, and for the past 4 decades
has worked in a wide variety of
applications of physics to respiratory
physiology. At the whole organ
and integrated level, his particular interests include
lung mechanics, gas exchange, aerosol transport, and
sleep disordered breathing; at the cellular and tissue
level he works in the rheological properties of single
cells, migrating monolayers, and elastohydrodynamic
fluid/tissue interactions.
Friction 1(2): 178–185 (2013)
DOI 10.1007/s40544-013-0014-2
ISSN 2223-7690
RESEARCH ARTICLE
Damage due to rolling in total knee replacement—The influence
of tractive force
Markus A. WIMMER1,*, Lars BIRKEN2, Kay SELLENSCHLOH2, Erich SCHNEIDER2
1
Section of Tribology, Rush University Medical Center, Chicago, IL 60612, USA
2
Section of Biomechanics, Hamburg University of Technology, Hamburg, 21071, Germany
Received: 05 March 2013 / Revised: 30 April 2013 / Accepted: 20 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: The femoral condyles of a knee prosthesis articulate with a combination of rolling and sliding on the
tibial polyethylene plateau. Little is known about potential polyethylene damage due to rolling motion. Since
rolling does not exclude the presence of tangential surface loads, this study sought to investigate the influence
of tractive rolling on the wear of polyethylene. A “wheel-on-flat” apparatus, consisting of a metal wheel and a
polyethylene flat, mimicked contact conditions present in total knee replacement. An increasingly tractive force
under conditions of pure rolling was applied. It was found that under rolling kinematics a tangential surface load
of up to 17% of the normal load could be transferred through the contact. Surface damage was dependent on
the amount of tractive force and appeared more severe with higher forces. In the region of highest tractive force,
wear features were identified that resembled perpendicular ridges on surfaces of retrieved tibial polyethylene
devices. This suggests that tractive rolling may be a relevant wear mode in total knee replacement.
Keywords: polyethylene; wear; knee prosthesis; tractive rolling
1
Introduction
The knee is the largest and one of the most mechanically complex joints in the human body. The
medial and lateral condyles of the distal part of the
femur articulate against the medial and lateral compartments of the tibial plateau, similarly to cylindrical
wheels on a flat surface. The primary motion of the
knee joint is flexion-extension, i.e., the rotation around
the “epicondylar axis” of the femoral condyles, but
secondary motions, as for example translations on
the tibial plateau, occur as well. Hence, the kinematic
interaction of the surfaces is best described as a rollingsliding action. Modern, successful designs in total
knee replacement mimic the anatomy of the natural
joint and allow the same degrees of freedom. Typically,
the surfaces of the femoral condyles are replaced with
metal that articulate against polyethylene anchored
* Corresponding author: Markus A. WIMMER.
E-mail: [email protected]
on the tibial plateau. This polyethylene component
is prone to wear and has been subject of many
investigations since wear jeopardizes the longevity
of the implant.
In many studies, cyclic sliding has been assumed to
be the most relevant kinematic action in the generation
of polyethylene wear debris and was investigated using
a variety of test set-ups [1−3]. Blunn et al. [4] were the
first to investigate the influence of rolling versus sliding.
For this purpose, they modified a reciprocating pin-on
flat device to provide rolling and sliding under cyclic
load. The pin was replaced with a polished sphere that
rolled or slid over the flat UHMWPE disc. To produce
rolling, the “tibial” disc was reciprocated and the
“femoral” sphere was rotated in synchrony. To achieve
sliding, the sphere was blocked. Rolling resulted in
the generation of shallow wear tracks without major
damage, while sliding produced deeper impressions
with evidence of subsurface cracking. The reduced
amount of wear during rolling was attributed to the
lack of frictional shear forces across the surface.
Friction 1(2): 178–185 (2013)
Using the terminology introduced by Johnson [5],
“free rolling” had been applied in the experiment
mentioned above. Under conditions of free rolling,
tangential surface loads are zero. “Tractive rolling”, in
contrast, describes dynamic conditions where tangential surface loads are present under pure rolling
conditions. In total knee replacements, tractive forces
can be substantial and it has been estimated that
they can reach up to 20 % of the contact load during
walking activity [6]. The purpose of this study was to
experimentally study the traction coefficient, i.e., the
ratio of tractive to normal force during rolling motion,
and to investigate the influence of increasing tractive
force on surface wear.
2
Materials & methods
A special testing apparatus was designed to simulate
increasing tractive forces under conditions of pure
rolling. The oscillating “wheel-on-flat” configuration
consists of a wheel that drives a polyethylene (PE)
flat with increasing tractive force against a pneumatic
cylinder (Figs. 1(a) and 1(b)).
The following describes one full operating cycle:
After wheel and PE flat are in starting position and
the desired constant normal load is reached, the wheel
starts turning at constant angular velocity. It drives
the PE flat, which is mounted on a sledge supported
by roller bearings, against a tangential cylinder. The
force in the cylinder is regulated via closed loop
control and increases linearly with sledge displacement.
179
Thus, an increasing tractive force at the rolling contact
between wheel and flat is generated. Tournament of
the wheel and cylinder force are stopped immediately
when sliding is detected. Wheel and PE flat are then
separated from each other and set back to the starting
positions to begin a new cycle. This was done in an
attempt to avoid the superposition of surface features,
which would have been generated during the return
stroke of wheel and PE flat.
The testing apparatus was designed to provide easy
access to the articulating components, the wheel and
the PE flat (Fig. 1(b)). The sledge is supported by
high precision bearings underneath, which account
for negligible friction forces (3–5 N under 1000 N of
normal load). A pneumatic short-stroke cylinder,
driven by a 3/2 electric directional control valve and a
pressure regulator, supplies the normal load in order
to bring wheel and flat into contact. The wheel itself is
driven by a pneumatic rotary actuator. Both, wheel and
actuator are mounted onto a frame being constrained
to z-motion and assuring that the rotational axis of
the wheel runs parallel to the y-axis of the sledge
(Fig. 1(a)). This arrangement ensures correct alignment
of the cylindrical wheel on the polyethylene surface
and compensates irregularities within the articulation.
A tensile spring supports the weight of the free-hanging
wheel and actuator unit. A high precision valve,
designed for closed loop control, adjusts the air
pressure in the rotary cylinder. Two additional 3/2
directional control valves on each side of the doubleacting cylinder are used for quick air release. This
Fig. 1 (a) Mechanical concept of the “wheel-on-flat” apparatus: the sledge is driven by the wheel against a tangential force, actively
produced by a pneumatic cylinder. (b) Photograph of the wear testing device.
Friction 1(2): 178–185 (2013)
180
allows for a quick stop of the system once sliding
is detected. An identical arrangement of valves was
applied to the linear cylinder which provides the
tangential force to the sledge.
Ultra-high molecular weight polyethylene (UHMWPE)
flats in dimensions of 100 mm × 40 mm × 8 mm were
compression molded using Himont 1 900 powder by
Zimmer Inc. (Warsaw, IN, USA). UHMWPE powder,
pressure, and temperature protocols were similar to
those used for Miller–Galante knee inserts and, thus,
facilitated the comparison to retrieved components
of such design. Cylindrical wheels of 100 mm in diameter and 20 mm in width were manufactured from
cast cobalt–chromium–molybdenum alloy (CoCrMo)
according to ASTM F75 by Implantcast GmbH
(Buxtehude, Germany). A polishing surface finish
similar to commercially available prostheses was
applied (Ra = 0.04 µm).
Three experiments were executed at room temperature using the compressive force mode of the
tangential cylinder without lubricant. This was done
in an attempt to achieve worst case conditions and the
highest tractive forces possible. A constant normal
force of 900 N was applied to the wheel. This reflects
about 50 % of the average normal load during stance
phase of gait of a 60−75 kg person (half of the load is
applied because only one of the two femoral condyles
is modeled in this wear test). The load characteristics
of the tangential cylinder were adjusted so the load
increased linearly with 2.3 N/mm starting from zero.
Thus, the length of the wear track covered at least
70 mm before the force got too high for pure rolling.
This approach made it possible to relate specific
locations on the polyethylene flat to precise loading
characteristics. 0.5 million cycles were performed.
Each pass over the PE flat lasted approximately 1 s.
All tests were run under a positive pressure hood to
keep contamination to a minimum. Throughout the
experiment, tractive force Ft (sensor range 500 N,
error < ± 0.1%), normal force Fn (2 kN, ±0.5%), PE flat
displacement (100 mm, ±0.1%), and wheel motion
(120°, ±0.05%) were measured. The data acquisition
was set to 150 Hz per channel and measurements were
taken every 5 000 cycles for an interval of 5 cycles.
Thereby, the digital resolution was set to approx. 1% of
the measured values. The traction coefficient µt = Ft /Fn
was calculated from the determined maximum tractive
force Ft during rolling movement.
At the end of each wear test, CoCrMo wheel and
PE flat were rinsed with distilled water to remove
any loose wear particles from the surfaces. Then, the
specimens were dried for 24 h. Initially, wear characteristics of wheel and PE flat were analyzed and
mapped under polarized contrast using a light
microscope (Orthoplan, Leitz, Germany). Afterwards,
to obtain more detail of characteristic sections, a low
voltage scanning electron microscope (LVSEM: S-4500,
Hitachi, Japan) was used. No conductive coating was
applied during this initial stage of the investigation.
Later, samples were carbon sputtered to allow energy
dispersive spectroscopy at 15 kV (Link ISIS, Oxford
Instruments, England) and to identify any system
inherent or extrinsic contaminants which could have
influenced the wear process.
3
Results
As intended, rolling motion of the wheel was initiated
without major tractive force on the polyethylene flat,
but plastic deformation at the resting position created
constrained forces. This raised the initial force reading
to approximately 25−50 N. Once this dip was cleared,
the tractive force increased as a linear function of sledge
displacement, however, plotted as a time function,
the behavior was non-linear (Fig. 2).
Fig. 2 Five consecutive cycles showing the tractive force, sledge
displacement and wheel displacement. Note that the velocity of the
driving wheel was constant during the first 35 mm of displacement
and then diminished. A time relay, effective at cycle start, delayed
motion initiation until vibrations due to air filling of cylinders
faded. In this example, the surface deformation at start was large
enough to create an offset in force reading (see text).
Friction 1(2): 178–185 (2013)
This non-linearity was caused by deceleration of
the wheel due to an increasing traction moment with
sledge displacement. Maximum traction coefficients
yielded 0.13 to 0.17 and were maintained throughout
testing. The transition from rolling to sliding was
abrupt, but did not cause any instability in the control
algorithm.
The CoCrMo wheel showed only minor surface
damage to the unaided eye after testing. Under the
microscope, uni-directional, mild scratching was
observed (Fig. 3). For all tested wheels, scratches
looked similarly throughout the wear track on the
metal surfaces, i.e., an influence of tractive force was
not identified. Using LVSEM, it was found that the
scratches were up to 0.6 µm in width. The smooth
appearance suggested a micro-plowing rather than a
micro-cutting mechanism. No foreign material could
be identified inside the scratches.
Repeated rolling of the wheel across the PE flat
generated a macroscopically visible deformed path,
mostly from plastic deformation of the polyethylene
material. The severity of damage on the polyethylene
181
surface increased with increasing tractive force (Fig. 4).
The whole wear track of the polyethylene plateau
was covered with fine, longitudinal scratches following
the direction of motion. Using microscopy, no
differences in morphological appearance could be
found along the course. Also, pitting and transferred
polyethylene particles were observed over the whole
Fig. 3 Fine scratches on the CoCrMo wheel that were aligned
in direction of motion.
Fig. 4 The appearance of wear on the tibial plateau changes with applied tractive force. The wheel moved from left to right over the
PE flat.
Friction 1(2): 178–185 (2013)
182
wear track. Pit sizes were usually confined to a
diameter of 10 to 20 µm, while re-embedded particles
could gain several hundred microns in width.
Longitudinal scratching also occurred, however, its
appearance varied with location on the wear track.
Within the first 30 mm of the wear track, random
oriented scratches deviating 0° to 20° from the principal
direction of motion were found. From 30 to 50 mm,
these scratches became more indented and more
oriented (in the direction of movement). Within the
last 20 mm of the wear track, scratching as a damage
feature disappeared and another wear feature
occurred: ridges perpendicular to the direction of
motion came into view, pronounced in height and
frequency within the last few millimeters of the wear
track. In that area the ridges reached about 5 to 10 µm
in length and seemed to be built up rather than
separated from the surface (Fig. 5). Loose particles with
varying sizes, rarely exceeding 1 µm, were found. After
carbon coating, the particles were analyzed. Their
elemental composition was determined to sodium,
potassium and chlorine. In some cases, calcium and
phosphor were identified, too.
Fig. 5 A close-up of the perpendicular ridges occurring at locations
of highest tractive force at the tibial plateau.
4
Discussion
This study attempted to relate tractive rolling contact
conditions of the knee joint to the appearance of
surface wear of the contacting bodies. The specific
approach, namely to control the tractive force at the
interface by contact displacement (rather than time),
allowed to classify the observed wear features relative
to the tractive force magnitude. In addition, the traction
coefficient during rolling motion of CoCrMo alloy on
UHMWPE was studied.
The coefficient of traction is an important factor
influencing the kinematics of the tibio-femoral articulation [6]. Once the maximum traction coefficient is
reached, rolling stops and gross sliding takes place.
In this study, maximal traction coefficients of 0.17
occurred. Since no lubricant was used in this study,
the value can be considered as the upper limit in
total knee replacement. Under lubricated conditions
the coefficient should be lower; however, as will be
shown below, damage features that occurred under
high tractive force in this study were also present on
retrieved tibial polyethylene components. This is an
indication that tractive rolling is occurring in artificial
knee joints. These kinematic characteristics are in
contrast to the natural knee where friction is 10 to
100-fold lower [7] and the tractive force negligible.
Tractive rolling facilitates antero-posterior translation
of the femoral condyles on the tibial polyethylene
plateau in the absence of cruciate ligaments1; thus,
contributing to femoral rollback of the artificial joint.
Femoral rollback is an important mechanism to enable
proper knee mechanics. It increases the lever arm
between contact point (i.e., the instantaneous center
of knee rotation) and extensor muscles during flexion
maneuvers as for example during seating onto and
rising from a chair. This minimizes the necessary
muscle force to flex and extend the knee. Hence,
femoral rollback comprises an important functional
feature necessary for easy execution of daily activities.
In the light of this discussion, a recent manuscript
about a new type of a rolling artificial knee joint that
has been based on the mechanics of rolling friction is
noteworthy [9].
Figure 4 shows the wear appearances of the whole
track generated due to tractive rolling. Note that the
different damage features are concentrated in certain
areas of the PE flat and, thus, are related to tractive
1
In the natural knee joint, the cruciate ligaments form a crossed
four-bar linkage facilitating femoral rollback [8]. One or two
ligaments are surgically removed during total knee arthroplasty
destroying the linkage.
Friction 1(2): 178–185 (2013)
force. Scratching is present along most of the wear
track. Shallow, non-oriented scratches are found within
the first 20 mm of the scar, where the tractive forces
were low. Longitudinal scratches along the principal
direction of motion are found thereafter and are related
to tractive forces between 50 and 110 N (or traction
coefficients from 0.06 to 0.12). These scratches are more
pronounced and deeper than those at the beginning of
the wear track. They seem to correspond in appearance
and dimension to scratches found on the wheel
suggesting third-body wear. Indeed, particles in a size
were found matching the dimensions of the scratches.
Their elemental composition points towards intrinsic
salt contaminants occurring in UHMWPE [10].
Perpendicular ridges are present at the end of the
wear track, where the tractive forces were highest. At
least 100 N of tractive force (or a traction coefficient
of 0.9) were necessary to produce ridges. However,
their frequency increases and their appearance becomes
more pronounced when the traction coefficient reached
values beyond 0.12. Due to the specific (moving) stress
field, i.e., compressive stress in front and tensile stress
behind the rolling wheel, high cyclic compressivetensile tangential stresses were induced to the
polyethylene in that region of the wear scar [11].
Maximum shear stresses moved closer to the surface
in that area, too. The applied stress field may have
caused plastic deformation of near-surface polyethylene
layers, thus leading to the formation of ridges perpendicular to the direction of tractive force. Plumlee
and Schwartz [12], using finite element models in
a recent study, found areas of plastic deformation
within the metal-UHMWPE contact area that were of
the same size scale as the ridges observed herein. It
should be noted that partially released surface layers
and perpendicular ridges are capable of forming particulate debris and/ or initiating progressive delamination
if the material embrittles due to oxidation.
Ridges similar to those observed in this study were
previously found on retrieved UHMWPE tibial plateaus
using white light interferometry [13]. Interestingly,
they are present on top of antero-posterior oriented
“striations”, which protrude from the polyethylene
surface and form troughs in between (Fig. 6). On top
of the striated hills, the ridges run perpendicular to
183
Fig. 6 Height image taken by white light interferometry. Note
the perpendicular ridges (arrow) on the summits of the striated
pattern.
the main direction of rolling (anerior-posterior). The
striated pattern represents a typical wear pattern in
total knee replacement and has been reported by
several investigators [14−16].
Their specific feature is a topographical profile that
has similarities to the threads of a vehicle rubber tire.
It is therefore hypothesized that the striated pattern
enables the drainage of lubricant through its troughs
during rolling contact and thus facilitates high traction
coefficients. The occurrence of the perpendicular ridges
on top of this surface feature seems to provide indirect
proof of this concept.
The study has several limitations which shall be
briefly highlighted. The “wheel-on-flat” design is a
very crude approximation of total knee prosthesis,
which is much more complex in shape than a single
radius contacting a flat surface. The studied motions
are therefore not directly translatable to the interface
of an artificial joint. The experiment was conducted
under dry conditions at room temperature. This may
be looked at as a worst case scenario, but under all
likelihood the in vivo contact is wet, thus lowering the
coefficient of friction and therefore the tractive force.
Despite that, similar surface features produced in this
study have been observed in vivo.
It should be mentioned that this is an early study
which triggered follow-up studies from the same
author group [17–19]. None of these studies, however,
focuses on surface features as related to tractive force.
In general, it is surprising how little scientific effort
has been spent to disentangle the mechanistic interactions of sliding and rolling at the knee joint since
Friction 1(2): 178–185 (2013)
184
the landmark study of Blunn et al. [4]. Two studies
which further contributed to the field are from
Cornwall et al. [20], who studied the three basic
kinematic contact conditions (sliding, gliding, rolling)
in a reciprocating wear tester, and from Keller et al.
[21], who studied the microstructural re-organization
of polyethylene after sliding-rolling contact.
[6]
Wimmer M A, Andriacchi T P. Tractive forces during
rolling motion of the knee: implications for wear in total
knee replacement. J of Biomechanics 30: 131−137 (1997)
[7] Unsworth A. Lubrication of human joints. In Mechanics of
Human Joints: Physiology, Pathophysiology, and Treatment.
Wright V, Radin E L, Eds. New York: Marcel Dekker Inc.,
1993: 137−162.
[8] O'Connor J J, Zavatsky A. Kinematics and mechanics of
5
Conclusions
Tractive rolling is a likely kinematic condition in total
knee replacement. Tractive coefficients up to 0.17 may
occur, stressing the polyethylene surface. This study
demonstrated that the damage features due to rolling
are dependent on tractive force and culminate in
perpendicular ridges, most likely generated due to
flow of near-surface polyethylene layers.
the cruciate ligaments of the knee. In Biomechanics of
Diathrodial Joints, Vol II. Mow V C, Ratelitte A, Woo S
L-Y, Eds. New York: Springer, 1990: 197−241.
[9] He Y, Yu Z, Chen M, Wang C. Rolling friction: A design
of artificial knee joint (in Chinese). Journal of Biomedical
Engineering 22(4): 840−843 (2005)
[10] Loos J, Wimmer M A. Observation of salt impurities in ultrahigh-molecular-weight polyethylene (UHMWPE). Journal
of Materials Science 34: 3327−3333 (1999)
[11] Natarajan R N, Hussain M, Wimmer M A, Rosenberg A G,
Jacobs J J. Wear of UHMWPE total knee components of
Acknowledgements
This work was conducted at the TUHH Hamburg
University of Technology, Germany within the framework of a doctoral thesis [13].
total knee replacement depend upon walking pattern. Trans
Orthop Res Soc 29: 1035 (2004)
[12] Plumlee K G, Schwartz C J. Surface layer plastic deformation
as a mechanism for UHMWPE wear, and its role in debris size.
Wear, in press, http://dx.doi.org/10.1016/j.wear.2012.11.081.
[13] Wimmer M A. Wear of the polyethylene component created
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
by rolling motion of the artificial knee joint. Ph.D. Thesis.
Aachen (Germany): Technische Universitaet HamburgHarburg, 1999: 81−117.
[14] Wimmer M A, Andriacchi T P, Natarajan R N, Loos J,
Karlhuber M, Petermann J, Schneider E, Rosenberg A G.
A striated pattern of wear in ultrahigh-molecular-weight
References
[1] White S E, Whiteside L A, McCarthy D S, Anthony M,
Poggie R A. Simulated knee wear with cobalt chromium
and oxidized zirconium knee femoral components. Clin
Orthop 309: 176–184 (1994)
[2] Blunn G W, Lilley A P, Walker P S. Variability of the wear
of ultra high molecular weight polyethylene in simulated
TKR. Trans Orthop Res Soc 19: 177 (1994)
[3] Walker P S, Ben-Dov M, Askew M J, Pugh J. The deformation
and wear of plastic components in artificial knee joints—An
experimental study. Eng Med 10: 33–38 (1981)
[4] Blunn G W, Walker P S, Joshi A, Hardinge K. The dominance
of cyclic sliding in producing wear in total knee replacements.
Clin Orthop 273: 253–260 (1991)
[5] Johnsen K L. Contact Mechanics, 2nd Editon. Cambridge
(UK): Cambridge University Press, 1985.
polyethylene components of Miller–Galante total knee
arthroplasty. J Arthroplasty 13: 8−16 (1998)
[15] Harman M K, DesJardins J, Benson L, Banks S A, LaBerge
M, Hodge W A. Comparison of polyethylene tibial insert
damage from in vivo function and in vitro wear simulation.
J Orthop Res 27(4): 540−548 (2009)
[16] Heyse T J, Davis J, Haas S B, Chen D X, Wright T M,
Laskin R S. Retrieval analysis of femoral zirconium
components in total knee arthroplasty: Preliminary results.
J Arthroplasty 26(3): 445−450 (2011)
[17] Schwenke T, Borgstede L L, Schneider E, Andriacchi T P,
Wimmer M A. The influence of slip velocity on wear of total
knee arthroplasty. Wear 259(7–12): 926−932 (2005)
[18] Galetz M C, Uth T, Wimmer M A, Adam P, Glatzel U.
Determination of the temperature rise within UHMWPE tibial
components during tribological loading. Acta Biomaterialia
6(2): 552−562 (2010)
Friction 1(2): 178–185 (2013)
185
[19] Schwenke T, Wimmer M A. Cross-shear in metal-onpolyethylene articulation of orthopaedic implants and its
relationship to wear. Wear, in press, http://dx.doi.org/10.1016/
j.wear.2013.01.069.
[20] Cornwall G B, Bryant J T, Hansson C M. The effect of
kinematic conditions on the wear of ultra-high molecular
weight polyethylene (UHMWPE) in orthopaedic bearing
applications. Proc Inst Mech Eng H 215(1): 95−106 (2001)
[21] Keller T F, Engelhardt H, Adam P, Galetz M C, Glatzel U,
Jandt K D. Near-surface microstructural reorganization
of UHMWPE under cyclic load—A pilot study. Advance
Engineering Materials 13(12): 476−482 (2011)
Markus A. WIMMER is an Associate
Professor of orthopedics and directs
the Motion Analysis and Tribology
Laboratories at Rush University
Medical Center. He is also an Adjunct
Professor of bioengineering at the
University of Illinois at Chicago.
He studied mechanical engineering at the Technical
University of Munich, Germany and received his
diploma in 1992. After a post-graduate year in the
laboratory of Dr. Thomas Andriacchi at Rush University
(Chicago, USA), he started as a Research Associate at
the Hamburg University of Technology and worked
towards a doctoral degree in biomechanics under the
supervision of Dr. Erich Schneider. In 1997, Markus
Wimmer moved from Germany to Switzerland and
started to work at the AO Research Institute in Davos.
He returned to Rush University in 2001 to join the
faculty of the Medical College. His current research
focuses on wear of natural and artificial joints and
includes, among other topics, polyethylene damage
in knee prostheses.
Friction 1(2): 186–194 (2013)
DOI 10.1007/s40544-013-0012-4
ISSN 2223-7690
SHORT COMMUNICATION
Green tribology: Fundamentals and future development
Si-wei ZHANG*
School of Mechanical and Storage and Transportation Engineering, China University of Petroleum, 20 Xueyuan Rd, PO Box 902, Beijing
100083, China
Received: 30 October 2012 / Revised: 23 Jaruary 2013 / Accepted: 20 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com
Abstract: As green tribology is a new field of tribology still in its infancy, understanding its fundamentals is
essential for its further development. In this article, a brief historical retrospective on the emergence of green
tribology is introduced first, and then the definition, objectives, and disciplinary features of green tribology are
clarified. In particular, the technological connotations of green tribology are expounded comprehensively. Also,
the developing directions of this new area are envisaged. These findings may contribute to laying the foundation
of further advancement in green tribology.
Keywords: green tribology; technological connotations; environmental impact; biological impact; developing
directions
1 Background
Since the 1980s, energy and environmental problems on
a global scale have increased in severity year by year.
Against this background, the objectives of tribology
expanded gradually, and then some tribologists successively put forward several new notions: “tribology
for energy conservation” (1997), “environmental
friendly tribology” (2000), and “ecological tribology
or ecotribology” (2000). In particular, after entering
the new millennium, tribology was expected to have
an increasingly important role to play along with
the crises of resources, energy, and environment
being aggravated in the world. Its basic objectives of
“controlling friction, reducing wear, and improving
lubrication” have extended to “saving energy and
materials, reducing emissions, shock absorption,
decreasing noise pollution, developing bio- and
ecolubrication and improving quality of life.” It is
noticeable that tribology has developed into a new
phase. In 2001, to mirror this change in objectives of
* Corresponding author: Si-wei ZHANG.
E-mail: [email protected]
tribology, a term/area “green tribology” was advanced
by the present author at a national symposium on
tribology in China. A paper was published from this
conference [1], in which the concept and objectives of
green tribology were clarified. Soon after, another two
expressions also emerged successively, namely, “total
tribology” (2001–2002) and “lifecycle tribology” (2004).
In 2008, the concept of green tribology was raised
again by the present author at the 5th China International Symposium on Tribology in a plenary lecture,
which was intended to replace similar notions as
mentioned above [2]. In the lecture, an investigation
of the industrial application of tribology in China was
presented. It was found that the industrial enterprises
of the whole nation can save 414.8 hundred million
USD per year (a lowest figure selected) (1.55% of
gross national product, GNP, 2006) by means of the
industrial application of tribology. Just based on this
investigation, it was concluded that “making tribology
green”/green tribology is now able to provide full
technical support to the preservation of resources and
energy, environmental protection, and improvement
of quality of life, and even to reduce natural disasters,
and so it is certainly an important way forward to a
Friction 1(2): 186–194 (2013)
sustainable society [2]. This lecture had every attention
of Professor Jost.
In the following year, Jost promoted the Chinese
Tribology Delegation’s visit to the UK and suggested
that the subject of this visit be green tribology: saving
energy and materials, improving the environment and
the quality of life [3]. During this visit, a keynote
address was delivered by the present author, in which
the definition, major tasks, and main contents of
green tribology were explained. Moreover, the view
that “green” embodies an ideology, sense, and values
was advanced [4]. In this very period, after repeated
deliberating with Jost, an exact definition of green
tribology was determined.
Soon after, Jost delivered an opening address under
the distinct title of “Green Tribology—A footprint
where economies and environment meet” at the Fourth
World Tribology Congress in Kyoto. He introduced the
definition and main objectives of green tribology, and
indicated that the expression “green tribology” was
first used by Professor Si-weir Zhang about 2 years
previously and was launched as a tribology policy in
London on June 8 of the same year. This date can be
regarded as the acknowledged birthday of green
tribology as an international concept [5]. It has since
spread rapidly and has become an integral part of
tribology in several major countries [6]. The above is
a brief historical retrospective on the emergence of
green tribology.
In recent years, a number of papers, academic books,
and reports related to this area have been presented
[1, 6−12]. However, up to now, there have been few
articles expounding the concepts, objectives, disciplinary
features, and technological connotations of green
tribology in precise terms and in an all-round way.
As green tribology is a new field of tribology still in its
infancy, an exact understanding of its fundamentals
is essential for further development. Connected with
this, the aim of this current work is to clarify the
fundamental basics and the developing directions of
green tribology based on the analysis, generalization,
and summation of the research achievements of green
tribology, but it is not intended to review progress in
this new field.
187
2
Concepts, definition, and objectives
The basic objectives of tribology are “controlling
friction, reducing wear, and improving lubrication.”
Therefore, the saving of energy and materials is
certainly one of the main objectives. Obviously, in
this respect, tribology is much better able to meet the
demands of a sustainable society. However, it did not
consider the ecological balance and environmental
impact at that time owing to the limitations of the
times. Thereupon, green tribology emerged to keep
abreast of the sustainable developments of nature
and society. “Green” is meant as a new mode of
thinking that represents views on ecological balance
and environmental protection, and so embodies the
ideology of the sustainable developments of nature and
society perfectly. The main task of green tribology is to
study and develop the tribological theories, methods,
and technologies with the new mode of thinking and
a completely new angle of view as stated above. Green
tribology is defined as the science and technology of
the tribological aspects of ecological balance and of
environmental and biological impacts [3−5].
Green tribology might be regarded as a subdiscipline
of tribology, such as nanotribology and biotribology.
These subdisciplines are all within the general concept
of tribology but place more emphasis on their specific
characteristics. However, compared with the other
subdisciplines of tribology, green tribology is an
interdisciplinary subject intersected with a wide range
of subjects, such as energy science, environmental
science, ecology, science of materials, life science,
geosciences, and green chemistry (environmental
benign chemistry) [9].
In a broad sense, green tribology involves tribology
for life (human biotribology), biomimetic tribology, renewable energy tribology, and a part of geotribology [9].
Guided by the viewpoint of sustainable development of resources and environment, the main objectives of green tribology are the environmental-friendly
saving of energy and materials, and the enhancement
of the environment and quality of life [3−5]. Thus,
the concepts and objectives of green tribology might
be summarized into 3L + 1H, namely, low energy
consumption, low discharge (CO2), low environmental
Friction 1(2): 186–194 (2013)
188
cost, and high quality of life. The mission of green
tribology is researching and developing tribological
technologies to reach the main objectives, thus making
the sustained artificial ecosystems of the tribological
parts and tribo-systems in the course of a lifecycle [9].
In view of this, green tribology is a subdiscipline
of tribology mainly concerning the environmentally
friendly consumption of resources and energy, and
the environmental and biological impacts. Moreover,
it is also an independent science and technology with
definite disciplinary character, namely, researching and
applying the tribological theories and technologies
forward to a sustainable society and nature. Therefore,
in a sense, green tribology could be defined as the
science and technology of research on the tribological
theories and technologies and the practices related to
a sustainable society and nature, and might also be
termed “tribology for sustainability” or “sustainable
tribology.”
Fig. 1 Features of developed tapered roller bearing (TRB) [16].
hybrid bushings in automotive aggregates [17]. It
exhibited a much lower coefficient of friction and
specific wear rate in comparison to the commercial
product, leading to a pronounced reduction in fuel
consumption and a better engine efficiency.
3.1.2 Technologies for super-low friction and wear resistance
3
3.1
Technological connotations (research
contents/areas)
Sustainable tribo-techniques for saving energy
and materials, and increasing the lifetime of
tribological parts and tribo-systems
3.1.1 Technologies for improving the fuel economy of
engine systems
Environmental impact and energy consumption have
made the improvement of the fuel economy of
engine systems an important issue. For this purpose,
a number of new lubricants were developed, such as
PAO(Polyalpholefin)-based lubricants [13] and new
types of synthetic esters [14].
In addition, DLC-Si coating with diesel fuel lubrication has a larger effect on friction reduction than
coating with engine oil lubrication [15].
A super-low friction torque tapered roller bearing
(TRB) applied to the rear axle differential for passenger
cars was developed [16], which obtained a friction
torque reduction of up to 75% compared with the
conventional low friction torque TRB. Its three features
are shown in Fig. 1.
A new nanoparticle-modified polyetheretherketone
(PEEK) composite was used as the thin coating for
A novel fullerene-like hydrogenated carbon film was
prepared by pulse bias-assisted plasma enhanced
chemical vapor deposition, and its mechanical and
tribological properties were investigated [18]. This
film exhibited super-low friction and wear in both
dry inert and humid ambient atmospheres and less
sensitivity to H2O and O2 molecules in air.
The mechanism responsible for excellent tribological
properties in AlMgB14-TiB2 nanocomposite coatings
was identified as oxidation of the TiB2 phase and subsequent reaction of the oxide with moisture to produce
a surface layer of boric acid, B(OH)3 [19]. These
coatings show sustained friction coefficient values as
low as 0.02 in water-glycol-based lubricants, and offer
a unique combination of excellent wear resistance and
low friction when combined with the high hardness
of the mixed-phase composite (30–35 GPa).
The wear behaviors of ultra-high molecular weight
polyethylene (UHMWPE) coated with hydrogenated
diamond like carbon (DLCH) layers were investigated
[20]. It was found that the surface hardness and the
wear resistance of coated materials were increased
compared to that of an uncoated one. The DLCH
coatings could be a potential method to reduce backside
wear in modular implants.
Friction 1(2): 186–194 (2013)
3.2
3.2.1
Sustainable tribo-techniques for removing or
reducing the harmful effects on ecological
balance (including human health) produced
by both tribological parts and tribo-systems in
the course of a lifecycle
Eco-/bio-lubricants
Environmental issues are leading to a growing interest
in eco- and bio-lubricants. However, the perfect ecoand bio-lubricants should be eco-non-toxic and biodegraded quickly, and moreover, capable of sustainable
large-scale production.
SAPS (phosphorus and sulfur)-free additive KWF012122 derived from natural resource “amino acids”
was developed [21]. More recently, the use of chitin,
chitosan, and acylated derivatives as thickener agents
of vegetable oils has been explored [22].
Sliding friction was analyzed for titanium covered
with mixed biofilms consisting of Streptococcus mutans
and Candida albicans [23]. The structure of biofilms
consisted of microbial cells, and their hydrated
exopolymeric matrix acts as a lubricant. Very low
friction was found on titanium immersed in artificial
saliva and sliding against alumina in the presence of
biofilms. This result is of particular significance for
dental implant connections and prosthetic joints.
Hydration lubrication is a new area explored
recently. Klein [24] pointed out that combining the
supramolecular benefits of polymer brushes together
with the highly hydrated nature of zwitterionic phosphorylcholine monomers could provide important
advantages in designing extremely efficient boundary
lubricants.
Recently, much research and the development of
new bio-based metal working fluid based on various
vegetable oils have been engaged. From the viewpoint
of the qualities required of metal working fluids, the
advantages and disadvantages of vegetable oils as
lubricants were listed (Table 1) [25].
More recently, Winter and coworkers [26] described
the use of ecologically benign lubricants as cutting fluid
and hydraulic fluid. They analyzed the usability and
resulting technological and ecological consequences
of water miscible biopolymers as a substitute, and
confirmed the good performance of the polymer fluid
as an optimal ecologically benign lubricant for metal
processing and hydraulic systems.
189
Table 1 Advantages and disadvantages of vegetable oils
as lubricants [25].
Advantages
Disadvantages
High biodegradability
Low thermal stability
Low pollution of the environment
Low oxidative
Compatibility with additives
High freezing points
Low production cost
Poor corrosion protection
Wide production possibilities
Low toxicity
High flash points
Low volatility
High viscosity indices
Tribological study and case analyses of the elastomeric bearings lubricated with seawater for marine
propeller shaft systems were conducted [27].
3.2.2 Biomimetic tribological materials and tribo-techniques
As living beings have natural adaptability to ecological
environments, biomimetic tribological materials and
tribo-techniques became an important area of green
tribology.
To obtain a much adhesive erosion-protection
surface on the hydraulic construction (flood-way
concrete structure), UHMWPE was applied as an
erosion-resistant material under the condition of high
sand-content slurry erosion, and a bionic surface
structure based on the epidermis of sandfish and the
clamp of a dragonfly’s wing was developed by Jian
Li and Chengqing Yuan (Fig. 2). This technique has
Fig. 2 Bionic surface structure based on the epidermis of sandfish
and the clamp of dragonfly’s wing.
Friction 1(2): 186–194 (2013)
190
provided the concrete structure with good protection
after three flood seasons (Personal communication,
2010).
More recently, the mechanisms of sand erosion
resistance of the desert scorpion (Androctonus australis)
were investigated to improve the erosion resistance of
tribo-components [28]. It was found that the functional
surfaces used for sand erosion resistance of the desert
scorpion were constructed by the special microtextures such as bumps and grooves.
3.2.3
Noise reduction
Brake squeal is a very complex phenomenon. There
is as yet neither a complete understanding of the problem nor a generalized theory of squeal mechanism.
Recently, an investigation on brake squeal noise was
carried out on simplified experimental rigs [29]. It
was concluded that a squeal-free design of a brake
system should consider not only the out-of-plane
dynamics but also the in-plane dynamics, and the role
of damping must be thoroughly considered. Moreover,
it was found that the stick–slip, detachment between
disc and pad, and other nonlinear characteristics of
the brake, did not affect the squeal propensity of the
brake but played a relevant role on the amplitude of
the radiated sound.
3.2.4 Application of lifecycle assessment (LCA) to
tribological technologies
Recently, Bartz [30] advanced that the green automobile has to be green from the cradle to grave. This
means that the lifecycle assessment (LCA) should be
the required approach.
A procedure based on the digraph and matrix
method was developed for modeling and evaluation
of the LCA of a tribo-element [31]. This procedure is
not only useful for the evaluation of LCA of triboelements at the operational stage, but can also be used
for the design and development of tribo-elements at
the system design stage from the viewpoint of LCA.
An environmental approach to environmentally
friendly hydraulic fluid was conducted by MullerZermini and Gaule [32]. They pointed out that the
facts of environmental law can be visualized with
the sustainability pyramid (Fig. 3), and only a comprehensive lifecycle assessment can show to which
category a product belongs.
Fig. 3 Sustainability pyramid for hydraulic fluids [32].
3.3
Research on the tribological aspects of the
natural ecological environment (including
atmosphere, water, and soil, etc.) and natural
disaster (including earthquake, landslide, mudrock flow and volcanic eruption, etc.), which
mainly focused on the role, mechanisms, and
effects of friction
Xianshuihe fault located in the Tibetan plateau in
China is a highly active strike-slip fault. To understand
its historical seismicity characteristics and to obtain
insight into its seismic potential, a numerical simulation
of seismic activity was performed using a rate- and
state-dependent friction law [33]. It was found that
the cumulative distribution function of the recurrence
intervals of simulated earthquakes at each segment
approximately obeys a Brownian passage time distribution or a lognormal distribution.
Later on, using a rate-, state-, and temperaturedependent friction law, a numerical method was
developed to investigate the effects of frictional heating
and thermal advection on pre-seismic sliding [34].
Han and coworkers [35] investigated the ultra-low
friction of carbonate faults caused by thermal decomposition. They demonstrated that thermal decomposition of calcite due to frictional heating induces
pronounced fault weakening with steady-state friction
coefficients as low as 0.06; moreover, this thermal
decomposition may be an important process for the
dynamic weakening of faults.
3.4
Tribological technologies for providing technological support to the equipment of both
renewable and clean energy
Meeting future world energy needs while addressing
Friction 1(2): 186–194 (2013)
climate change requires deployment of various renewable and clean energies as alternatives to traditional
fossil energy; examples include wind energy, solar
energy, marine energy, nuclear energy, geothermal
energy, and so on.
Frictional contacts in wind energy plants are found
in gears and bearings. Due to severe operating conditions, the average lifetime of main bearings and
multiplier gears is between 5–7 years in Western
Europe and about 2–3 years in Asian countries [36].
REWITEC nanocoatings is a metal treatment that can
be applied to gearboxes and bearings during regular
operation for restoration of its efficiency and economy
[37]. The documented tests and evaluations of gearbox
operation were reviewed for the REWITEC technology.
It has been proven that this technology was validated
for improvement of operation in wind turbine gears.
Taking specific areas with micro-pitting in the metal
surfaces of a wind turbine gear before the application
of REWITEC and after 6 months of treatment it was
found that the surface damages are filled and the
asperities smoothened, and thus the surfaces were
smoother with higher surface contact area (Fig. 4).
Heemskerk [38] pointed out that wind turbines are
complex dynamic systems that require advanced
technologies (including tribology) to yield answers
to service life and early failure issues. He suggested
focusing on nine areas for tribology R&D applied to
wind turbine rolling bearing.
A six-degree-of-freedom dynamic bearing model
(DBM) has been used to simulate roller-raceway slip
for a cylindrical roller bearing used in an intermediate
shaft location of a wind turbine gearbox [37]. It was
found that significant slip occurred during rapid
transient accelerations and decelerations, but these
high slip conditions decayed to a much lower level of
slip at steady state. Moreover, extreme slip occurred
for low load and high speed conditions because of
concomitant contact area reduction and traction loss
at the roller-raceway interfaces.
Based on a literature review related to the study
of the phenomenon of micro-pitting in wind turbine
gearbox roller bearings, a proposed test scheme could
be created from which a method to predict the risk of
micro-pitting might be determined [39].
191
Fig. 4 Images in 3D for the metal surface before and after
treatment with REWITEC with 6 months difference [36].
The tribology of three marine energy conversion
devices, namely, offshore wind turbines, tidal turbines,
and wave machines has been reviewed [40]. These
devices are sensitive to operation and maintenance
costs and thus rely on functioning tribological parts
and lubrication.
4
Developing directions
Faced with a great number of tribological problems
demanding prompt solution, which relate to the
earth-scale environmental pollution and crisis of energy,
green tribology should be extended in the following
areas.
4.1
Large-scale deployment of existing knowledge,
methods, and technologies of green tribology
More recently, three tribological case studies (micro
Combined Heat and Power systems, slipways, recycled
plastics) presented by Tzanakis and coworkers are
the outstanding examples in this aspect [12].
Friction 1(2): 186–194 (2013)
192
4.2 Research and development of novel green
tribological technologies
(1) Low-carbon bio- and ecolubricants
Halogen-free and biodegradable oils
Carbon-neutral vegetable oils
(2) Environmental-friendly tribological materials and
coatings
Tribological applications of ecomaterials
Biological coatings applied on the surface of
implants or medical devices
(3) New tribo-techniques based on bionics
4.3
Making the traditional tribo-materials and
lubricating materials “green” in the course of a
lifecycle, namely, realizing cleaner production
or ecodesign of the above materials
4.4
Research and development of tribo-techniques
to support diversification and hybridization of
renewable and clean energy
4.5
Building up the theory and methodology of
green tribology
(1) Setting up the theories and methods of analyses
and evaluation of sustainability (including value
of environmental and ecological impacts, value of
saving energy, etc.) for tribological parts, tribosystems, and tribo-techniques.
(2) Research on the theories and methods of integration of different green tribological techniques,
and of the effects of coupling and coordinating
among various areas of green tribology.
5
Concluding remarks
Green tribology plays a unique role in developing a
low-carbon economy, dealing with environmental
pollution, the energy crisis, and climate change on a
global scale. Therefore, it is one of the important ways
forward to a sustainable society.
Just as Jost pointed out, “…the cause of Green
Tribology is indeed a worthy cause for all tribologists
and their organizations to pursue, as it will help
tribology to play its rightful part, not only for the
benefit of science and technology, but much more
importantly, for the benefit of mankind…” [6].
Consequently, tribologists ought to dedicate their best
efforts to the development and application of green
tribology, thus adding valuable contributions to the
existence and development of humanity.
In this paper, the definition, disciplinary features,
objectives, mission, technological connotations, and
the future developing directions of green tribology
have been expounded comprehensively. These findings
may contribute to laying the foundations for further
advancement in green tribology.
Acknowledgments
The author wishes to express his sincere thanks to
Prof. Jost for his energetic support, fruitful discussions,
and valuable suggestions while green tribology was
advanced. Also, the author would like to acknowledge
Prof. Jian Li from Wuhan Research Institute of Materials
Protection and Prof. Chengqing Yuan from Wuhan
University of Technology for generously affording
their unpublished achievements in scientific research
to be quoted in this article. Assistance provided by Dr.
Huiqing Lan of Beijing Jiaotong University during
manuscript preparation is appreciated.
Open Access: This article is distributed under the terms
of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any medium, provided
the original author(s) and source are credited.
References
[1] Zhang S W. Investigation concerning the developing directions
of tribology in China (in Chinese). Tribology 21: 321–323
(2001)
[2] Zhang S W. Current industrial activities of tribology in China.
Plenary Lecture to the 5th China International Symposium
on Tribology (CIST) 2008, Beijing, 2008.
[3] 30th Anniversary and “Green Tribology”—Report of a successful Chinese Mission to the United Kingdom (7th to 14th
June 2009), Tribology Network of Institution of Engineering
and Technology, 2009.
[4] Zhang S W. Tribological application in China and green
tribology. Institution of Engineering and Technology. London
UK, 2009.
[5] Jost H P. Green tribology—A footprint where economies
Friction 1(2): 186–194 (2013)
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
and environment meet. Address to the 4th World Tribology
Congress, Kyoto, 2009.
Jost H P. Development of green tribology—An overview.
Seminar—New Direction in Tribotechnology, Moscow, 2010.
Nasonovsky M, Bhushan B. Green Tribology. Phil Trans R
Soc A 368: 4675–4676 (2010)
Nasonovsky M, Bhushan B. Towards the “Green Tribology”:
Biomimetic surfaces, biodegradable lubrication, and renewable
energy. In Proceedings of STLE/ASME International Joint
Tribology Conference, San Francisco, 2010.
Zhang S W. Green tribology—The way forward to a
sustainable society. In Proceedings of the International
Tribology Congress—ASIATRIB 2010, Perth, Australia,
2010: 6.
Tzanakis I, Hadfield M, Thomas B, Noya S M, Henshaw I,
Austen S. Future perspectives on sustainable tribology.
Renewable and Sustainable Energy Reviews 16: 4126–4140
(2012)
Nasonovsky M, Bhushan B (Eds). Green Tribology:
Biomimetics, Energy Conservation and Sustainability.
Berlin: Spring, 2012.
Assenova E, Majstovovic V, Vencl A, Kandeva M. Green
tribology and quality of life. International Journal of
Advanced Quality 40: 1–6 (2012)
Deitz T G. Advanced PAO based industrial lubricants for
improved energy efficiency. In Proceedings of the 4th World
Tribology Congress, Kyoto, 2009: 39.
Hirao K, Yamada M, Kijki T, Maekawa N. Environment and
energy saving by use of synthetic esters. In Proceedings of
the 4th World Tribology Congress. Kyoto, 2009: 38.
Koyamaishi N, Murakami M, Komiya K, Moritani H. Study
of future oil. In Proceedings of the 4th World Tribology
Congress. Kyoto, 2009: 577.
Matsuyama H, Kawaguchi K, Uemura A, Masuda N.
Development of super-low friction torque tapered roller
bearing for high efficiency axle differential. In Proceedings
of the 4th World Tribology Congress, Kyoto, 2009: 591.
Friedrich K, Almajid A A. Polymer composites in triboapplications with elongated maintenance intervals and
reduced energy consumption. In Proc International Tribology
Congress—ASIATRIB 2010, Perth, Australia, 2010, K3.
Ji L, Li H, Zhou F, Quan W, Chen J, Zhou H. Fullerene-like
hydrogenated carbon films with super-low friction and wear,
and low sensitivity to environment. J Physics D: Applied
Physics 43: 015404 (2010)
Higdon C, cook B, Harrings J, Russell A, Gddsmith J, Qu J,
Blan P. Friction and wear mechanisms in AlMgB14-TiB2
nanocoatings. Wear 273: 2111–2115 (2011)
Puértolas J A, Martínez-Nogués V, Martínez-Morlanes M J,
193
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
Mariscal M D, Medel F J, López-Santos C, Yubero F.
Improved wear performance of ultra high molecular weight
polyethylene coated with hydrogenated diamond like
carbon. Wear 269: 458–465 (2010)
Numazaki R, Nakayama S, Inayama T, Nakayama S,
Inayama T, Isogai Y, Minami I, Mori S. Contribution for
energy saving by novel environmental lubricants containing
derivatives from natural resource. In Proceedings of the 4th
World Tribology Congress, Kyoto, 2009: 41.
Sanchez R, Stringari G B, Franco J M, Valencia C, Gallegos
C. Use of chitin, chitosan and acylated derivatives as thickener
agents of vegetable oils for bio-lubricant applications.
Carbohydrate Polym 85: 705–714 (2011)
Souza J C M, Henriques M, Oliveira R, Teughels W, Celis
J-P Rocha L A. Biofilms inducing ultra-low friction on
Titanium. J Dent Res 89: 1470 (2010)
Klein J. Hydration lubrication: Exploring a new paradigm.
In Proceedings of the China 6th International Symposium on
Tribology, Lanzhou, China, 2011: 1–2.
Shashidhara Y M, Jayaram S R. Vegetable oil as a potential
cutting fluid—An evolution. Tribol Int 43: 1073–1081 (2010)
Winter M, Herfellner T, Malberg A, Eisner P, Dwuletzki H,
Zein A, Bock R, Hermann C. Mineral oil free machine
tool-the usage of ecologically benign lubricants as coolant
and fluid. In Proceeding of the 18th International Colloquium
Tribology, Stuttgart/Ostfildern, Germany, 2012: 143.
Hirani H, Verma M. Tribological study of elastomeric
bearings for marine propeller shaft system. Tribology
International 42: 378−390 (2009)
Han Z, Zhang J, Ge C, Wen L, Lu R. Erosion resistance of
bionic functional surfaces inspired from desert scorpions.
Langmuir 28: 2914–2921 (2012)
Akay A, Giannini O, Massi F, Sestieri A. Disc brakes squeal
characterization through simplified test rigs. Mechanical
Systems and Signal Processing 23: 2590–2607 (2009)
Bartz W J. The green automobile–definition and realization.
In Proceedings of the China 6th International Symposium on
Tribology, Lanzhou, China, 2011: 73.
Wani M F, Anand A. Life-cycle assessment modelling and
life-cycle assessment evaluation of a triboelement. Proc
IMechE Part J: J Engineering Tribology 224: 1209–1220
(2010)
Muller-Zermini B, Goule G. Environmental approach to
hydraulic fluids. In Proceeding of the 18th International
Colloquium Tribology, Stuttgart/Ostfildern, Germany, 2012.
Kato N, Lei X, Wen X. A synthetic seismicity model for the
Xianshuihe fault, southwestern China: simulation using a
rate-dependent friction law. Geophys J Int 169: 286–300
(2007)
194
Friction 1(2): 186–194 (2013)
[34] de Lorenzo S, Lodds M. Effect of frictional heating and
thermal and advection on pre-seismic sliding: a numerical
simulation using a rate-, state- and temperature-dependent
friction law. J Geodynamics 49: 1−13 (2010)
[35] Han R, Shimamoto T, Hiroset Ree J H, Ando J. Ultralow
friction of carbonate faults caused by thermal decomposition.
Science 316: 878−881 (2007)
[36] Bill S. BEWITEC surface technology-Reconditioning and
durable wear protection for high loaded gearboxes and bearings
in wind turbines. In Proceedings of the 18th International
Colloquium Tribology, Stuttgart/Ostfildern, Germany, 2012.
[37] Kang Y S, Evans R D, Doll G L. Roller-raceway slip
simulations of wind turbine gearbox bearings using dynamic
bearing model. In Proceedings of STLE/ASME International
Joint Tribology Conference, San Francisco, USA, 2010:
407–409.
[38] Heemskerk R S. Tribology for wind turbines: R&D needs
to increase sustainability of the technology. In Proceedings
of the 37th Leeds—Lyon Symposium on Tribology, Leeds,
UK, 2010.
[39] Harris T A, Kotzalas M N. Predicting micro-pitting
occurrence in wind turbine gearbox roller bearings. In
Proceedings of STLE/ASME International Joint Tribology
Conference, San Francisco, 2010.
[40] Wood B J K, Bahaj A S, Turnock S R, Wang L, Evans M.
Tribological design constraints of marine renewable energy
systems. Philosophical Transactions of the Royal Society A
368: 4807–4827 (1929)
Si-wei ZHANG is the professor
of Tribology and Mechanical
Engineering of the China University
of Petroleum in Beijing. From 1982
to 1984, as a visiting professor, he
studied in tribology of polymers at
the Institute of Polymer Science,
University of Akron, and then at
the Tribology Laboratory, Department of Mechanical
Engineering and Applied Mechanics, University of
Michigan in USA. In 1987, he conducted researches
in the field of elastomer tribology at the Department
of Chemical Engineering, Imperial College in UK.
His professional interests include tribology and
oilfield equipment. He mostly conducted and supervised in tribology of polymers, friction materials,
nanotribology and tribological components. And
more recently, he is interested in green tribology and
interface engineering.
He is the Chairman of the Advisory Board of Chinese
Tribology Institution, the Associate Editor-in-Chief for
both journals Advances in Tribology (USA) and Tribology
(China). Also, he is the member of Advisory Editorial
Board of journal Petroleum Science (English edition,
China) and the member of International Advisory
Editorial Board of journal Tribology International (UK).