Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Phil. Trans. R. Soc. A 370, 2321–2347
doi:10.1098/rsta.2011.0483
Bioadhesion: a review of concepts and
applications
BY MANUEL L. B. PALACIO*
AND
BHARAT BHUSHAN∗
Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics,
The Ohio State University, Columbus, OH 43210, USA
Bioadhesion refers to the phenomenon where natural and synthetic materials adhere
to biological surfaces. An understanding of the fundamental mechanisms that govern
bioadhesion is of great interest for various researchers who aim to develop new
biomaterials, therapies and technological applications such as biosensors. This review
paper will first describe various examples of the manifestation of bioadhesion along with
the underlying mechanisms. This will be followed by a discussion of some of the methods
for the optimization of bioadhesion. Finally, nanoscale and macroscale characterization
techniques for the efficacy of bioadhesion and the analysis of failure surfaces are described.
Keywords: bioadhesion; bioadhesives; cell adhesion; mucoadhesion
1. Introduction
The term ‘bioadhesion’ refers to widely diverse phenomena, which all involve
the adherence of materials (natural or synthetic) to biological surfaces. As an
interface phenomenon, bioadhesion is similar to conventional adhesion, except
for the special characteristics of biological organisms and surfaces. As such, this
term covers the adhesive properties of both synthetic components and the natural
surfaces (such as cells). Bioadhesion could also refer to the use of bioadhesives to
bond two surfaces together, which is relevant in drug delivery, dental and surgical
applications. Therefore, the wide interest in bioadhesion research is due to its
implications for the development of new biomaterials, therapies and technological
products such as biosensors [1–6].
A few examples illustrating various manifestations of bioadhesion are shown
in figure 1. Pillar-directed cell growth is an area of interest for researchers
investigating the role of substrate micro-roughness on cell behaviour in the
development of new biomaterials. In figure 1a, fibroblasts attach on top of the
silicon pillar array substrate, and then extend to reach the surrounding pillars [7].
The height and spacing between pillars affect cell shape and thickness, which
in turn, has implications for biological functions, such as cell growth and gene
expression. Bioadhesion can also refer to the application of adhesives for biological
interfaces in clinical use. Figure 1b is an example of the application of a medical
adhesive. In this case, fibrin glue was used to adhere a surgical mesh on an
*Authors for correspondence ([email protected]; [email protected]).
One contribution of 10 to a Theme Issue ‘Biosensors: surface structures and materials’.
2321
This journal is © 2012 The Royal Society
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2322
(a)
M. L. B. Palacio and B. Bhushan
(c)
(b)
(d)
(e)
fatty material
on blood
vessel wall
Figure 1. Examples of bioadhesion. (a) Cell adhesion on a silicon-based micropillar array [7].
(b) Example of a medical adhesive application where fibrin glue was used to attach a surgical mesh
after a laparoscopic surgery procedure [8]. (c) Tooth enamel and dentin fracture and reattachment
of the tooth fragment using a dentin adhesive [9]. (d) Adhesion of a mussel on a glass slide,
where multiple threads composed of the mussel adhesive protein were secreted by the animal to
attach itself to the glass surface [10]. (e) Sketch depicting fat deposition on a blood vessel wall in
atherosclerosis (adapted from Anonymous [11]). (Online version in colour.)
animal peritoneum after a laparoscopy [8]. Meanwhile, figure 1c shows the use
of adhesives in dental applications. In this example, a dentin adhesive was used
to reattach a tooth fragment onto a fractured area [9].
Examples of naturally occurring bioadhesion are shown in figure 1d,e. The
phenomenon of mucoadhesion is illustrated in figure 1d, which shows the
attachment of mussels to underwater surface, such as corals or ship surfaces.
The foot of the animal extends from its shell to secrete a pad of the mussel
adhesive protein (MAP) on the underwater surface. It then retreats to the shell
interior, leaving a thread that secures the mussel to the pad. This process is
repeated multiple times, creating a strong bond [10]. The MAP has been used
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2323
50 µm
Figure 2. Biofilm development on the inner surface of a polyurethane stent. Adapted from
Tenke et al. [12].
as the model for the development of mucoadhesive materials. Lastly, a sketch is
shown in figure 1e depicting the build-up of fats on the walls of animal arteries,
which is part of the mechanism that leads to atherosclerosis [11].
The bioadhesion between materials can also be detrimental, and is referred to
as biofouling. A biofilm is an aggregation of micro-organisms that form on a solid
substrate, which may form during the implantation of biomaterials into the body.
For humans and animals, the formation of biofilms on both living (cells) and nonliving (stents and catheters, for example) surfaces has been identified as a cause of
disease. Bacterial biofilms are known to cause diseases such as endocarditis, cystic
fibrosis and other infections that are largely resistant to antibiotics. Figure 2
shows an example of biofilm formation on a stent used for urological applications
[12]. In other applications, dental plaque is a biofilm that can consist of more
than 500 bacterial strains [13].
This review paper is focused on the positive aspects of bioadhesion, and thus,
will not discuss biofouling in detail. There are many different situations where
bioadhesion is beneficial and significant, such as cell adhesion, mucoadhesion and
the use of bioadhesives for surgical and dental applications. Examples from these
broad applications will be cited in order to discuss mechanisms that control
the bioadhesion process, and identify ways to optimize bioadhesion. Finally,
nanoscale and macroscale experimental techniques to characterize bioadhesion
will be discussed.
2. Mechanisms that control bioadhesion
The factors that determine biological adhesion are diverse, and can be classified
into the effects of surface morphology, chemical interactions, physiological
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2324
M. L. B. Palacio and B. Bhushan
factors and physical–mechanical interactions. These aforementioned determinants
usually work in concert with one another in order to achieve adhesion between
the interfaces of interest.
(a) Surface morphology effects
Cell adhesion on synthetic biomaterial surfaces is widely studied owing to its
direct implications for the design and clinical performance of body implants. The
adhesion of cells on biomaterials is an example that illustrates the importance of
the micro/nanotopography of the substrate surface where the cells are cultured.
During the implantation of a biomaterial into a living host, cells are not directly
attached to the biomaterial surface. Instead, the biomaterial is rapidly coated
with a protein layer. Other proteins will displace the initial layer in a process
known as the ‘Vroman effect’. Various extracellular matrix (ECM) and serum
proteins are involved, such as fibronectin, fibrinogen, albumin and vitronectin.
The conformation of the adsorbed protein is partly determined by the morphology
of the biomaterial surface, which in turn, influences the cell adhesion and
proliferation process [14–18].
Model synthetic surfaces have been used to investigate the effect of surface
morphology on protein conformation and cell adhesion. These surfaces have
well-defined microscale or nanoscale topography, and are typically created using
top-down (e.g. lithographic or dry etching methods) or bottom-up (self-assembly
processes or block copolymers) approaches [19–23].
Accounts vary on the optimal feature size required for effective cell
adhesion. Accounts of bone cell adhesion on patterned titanium indicate that
submicrometre (greater than 100 nm) features are better than nanometre-sized
(less than 100 nm) features in facilitating cell adhesion [24]. Lehnert et al.
[25] performed cell adhesion and spreading studies on patterns with distances
ranging from 1 to 30 mm. They found that for patterns with small areas of
the adhesive ECM proteins (0.1 mm2 ), pattern spacings larger than 5 mm show
cellular adhesion, but do not support cell spreading. However, for larger dot
dimensions (9 mm2 ), cells can effectively connect non-adhesive regions during
the spreading process as long as the distances between features do not exceed
25 mm. This variation in cell spreading as a function of the substrate geometry
is shown in figure 3, where for 25 mm-spaced features, the spreading was limited
and the cells were observed to appear as either triangular, ellipsoidal or round. In
another study, using lithographically created patterns with deposited fibronectin,
it was found that cellular adhesion increases as the spacing between features
increases up to the optimal distance of 11 mm [26]. However, for cell adhesion
on gold nanoparticles, it was found that for spacings less than 100 nm, closer
patterns (ca 60 nm) allowed for greater cell adhesion [20]. Even though these
literature accounts are not definitive, it is apparent that dimensional thresholds
that enhance cell adhesion are present.
(b) Chemical interactions
Various interactions between two chemically active surfaces (i.e. not inert)
facilitate the bioadhesion process. Strong adhesion can occur if the two surfaces
are capable of forming either covalent, ionic or metallic bonds. At the same
time, weaker forces, such as polar (dipole–dipole), hydrogen bonding or van
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
(a)
(b)
(d )
(c)
<1 µm
hs
(e)
10 µm
(g)
2 µm
(f)
15 µm
(h)
25 µm
2325
20 µm
(i)
25 µm
10 µm
25 µm
Figure 3. Fluorescence microscopy images showing the effect of substrate geometry on cell adhesion
and spreading: (a) homogeneous substrate (hs), (b) 0.1 mm2 dots, approximately 1 mm apart,
(c) 1 mm2 dots, 2 mm apart, (d) 9 mm2 dots, 10 mm apart, (e) 9 mm2 dots, 15 mm apart, (f ) 9 mm2
dots, 20 mm apart, (g–i) 9 mm2 dots, 25 mm apart (adapted from Lehnert et al. [25]). (Online version
in colour.)
der Waals interactions (induced dipoles) also aid in bonding the two surfaces
[27–29]. A schematic of possible protein–solid surface interactions is shown in
figure 4. Here, it is assumed that the surface is uniform and that there is
only one main type of interaction between the protein and the surface. The
strength of protein adsorption depends on the net charge and polarity of the
side of the protein available for adsorption and the composition of the substrate
surface. The protein and substrate could either be predominantly hydrophobic,
positively charged, negatively charged or neutral hydrophilic. The interaction
of a neutral hydrophilic side of the protein with a surface having a similar
polarity tends to lead to weak adsorption. Ionic interactions between proteins
and substrate surfaces will lead to relatively moderate adsorption. Meanwhile,
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2326
M. L. B. Palacio and B. Bhushan
–
–
–
–
–
–
+
+
+
+
+
+
–
–
–
protein with + , – , neutral hydrophilic (
and hydrophobic (
) faces.
– – –
+ + +
+ + +
– – –
+
+
+
–
neutral, hydrophilic
polymer—
weak adsorption
),
hydrophobic
polymer—
strong adsorption
–
moderate
adsorption
f(I.S., pH,...)
–
+
+
+
moderate
adsorption
f(I.S., pH,...)
Figure 4. Schematic of a protein adsorbing on surfaces with different net charge and polarity
characteristics. The strength of adsorption depends on the net charge and polarity of the
side of the protein (whether it is mainly hydrophobic, positively charged, negatively charged
or neutral hydrophilic) available for adsorption and the charge and polarity of the substrate
surface [30].
the interactions between hydrophobic protein moieties and the substrate lead
to strong adsorption [30]. This illustrates schematically the role of surface
chemistry in protein adsorption. However, proteins contain complex arrangements
of hydrophobic, charged and neutral hydrophilic groups, such that the resulting
interactions will be combinations of the cases presented. A discussion of protein
composition is provided in appendix A for completeness.
Specific examples, illustrating chemical interactions in the formation of
bioadhesive bonds, are discussed below.
(i) Mussel adhesion
The case of mussel adhesion on underwater surfaces is an example of how the
chemical composition of the contacting surfaces affects the adhesion mechanism.
The initial interaction between the mussel and the underwater surface involves
the removal of weak boundary layers (mostly water). If the underwater surface
is non-polar, then the water boundary layer interacts through weak dispersive
forces. Because the MAP is larger than a water molecule, the protein experiences
an entropic gain and greater dispersive interactions with the non-polar surface
relative to water, leading to the displacement of the water boundary layer, and
adhesion of the protein. In the case of polar underwater surfaces, the water
boundary layer cannot be easily displaced. In this case, the MAP uses its
hydrophilic amino acid side chains, which contain aminoalkyl, hydroxyalkyl and
phenolic groups (such as 3,4-dihydroxyphenylalanine or DOPA), all of which
are capable of forming strong hydrogen bonds. Hence, the MAP is able to
displace water and the mussel is able to adhere to polar underwater surfaces as
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2327
well [31,32]. The unique chemical composition and properties of the MAPs have
led to the development of synthetic analogues for potential use as mucoadhesives
for drug-delivery systems.
(ii) Cell adhesion to biomaterials
Cell adhesion to biomaterial surfaces is a complex phenomenon. As discussed
in §2a, the micro/nanoscale surface topography has a direct impact on cell
adhesion and proliferation. But aside from morphology, the chemical composition
of the biomaterial surface has also been established to play a significant role
in the cell adhesion and proliferation process. For instance, cell adhesion as
mediated by the integrin group of cell surface receptors has been shown to
depend on the conformation of the ECM protein fibronectin, which in turn is
sensitive to the chemical composition of the synthetic surface. To demonstrate
this concept, Keselowsky et al. [33] used various self-assembled monolayers
(SAMs) with OH, COOH, NH2 and CH3 termini in order to create surfaces
that are hydrophilic and neutrally charged, hydrophilic and acidic, hydrophilic
and basic, and hydrophobic, respectively. They found that the adhesion strength
of cell binding (as determined by a centrifugation assay) followed the trend:
OH > COOH > NH2 > CH3 . This trend was found to correlate with the integrinbinding profile on the SAMs. In another study, these researchers found that other
events related to cell differentiation, such as osteoblast-specific gene expression,
alkaline phosphatase enzyme activity and matrix mineralization, were also surface
chemistry dependent, such that OH- and NH2 -terminated surfaces were more
advantageous compared with COOH and CH3 SAMs [34].
So, how does the surface charge and hydrophilicity/hydrophobicity influence
cell adhesion? The answer lies in the effect of the surface chemistry on the
adsorption of the ECM proteins such as fibronectin, vitronectin, collagen and
laminin. In the context of the four model surfaces (OH-, COOH-, NH2 - and
CH3 -terminated SAMs), it has been found that fibronectin undergoes the largest
extent of denaturation on the CH3 -terminated SAM, which corresponds to
poor cell adhesion characteristics. The denaturation of fibronectin prevents the
surface exposure of the arginine–glycine–aspartic acid (RGD) groups on its
cell-binding domain, which are known to mediate cell adhesion. In contrast,
the hydrophilic, neutrally charged SAM (OH terminus) has been found to
induce the least extent of unfolding or denaturation, leading to a good cell
adhesion on the fibronectin/SAM surface [35]. The extent of denaturation of
fibronectin on the OH and CH3 surfaces is attributed to functional group
dehydration and water-restructuring effects brought about by the substrate
surface [36,37].
It has also been established that the surface chemistry of the substrate plays
a role in the formation of focal adhesions, which are adhesive complexes with
signalling molecules responsible for cell migration, survival and differentiation.
The focal adhesion complexes (or plaques) have been found to contain integrin
receptors and cytoplasmic proteins, such as talin, vinculin and a-actinin. An
example is shown in figure 5 of the localization of talin on fibronectin-coated
SAMs [38]. In these SAMs, the termini were also modified to possess OH,
COOH, NH2 and CH3 termini. Talin formed large clusters on the OH and
COOH SAMs, fewer structures on NH2 SAMs, and the least amount on the
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2328
M. L. B. Palacio and B. Bhushan
CH3
OH
COOH
NH2
Figure 5. Localization of the structural protein talin on self-assembled monolayers, illustrating how
varying the surface chemistry influences the formation of focal adhesion complexes [38].
CH3 surface. The extent of cluster formation is directly related to the amount of
focal adhesions formed, which is indicative of the success of cell adhesion on the
substrate surface.
The studies on integrin binding, cell adhesion, fibronectin conformation and
focal adhesion formation are all consistent in demonstrating that modulating
the surface chemistry of the biomaterial has a large impact on optimizing the
cell adhesion process. A schematic illustrating the effect of surface chemistry on
the cell adhesion process is shown in figure 6 [39]. Figure 6a illustrates the case
for cell adhesion to a hydrophobic surface. Because the ECM protein (such as
fibronectin) is denatured, specific amino acid sequences (such as the RGD groups
of fibronectin) are inaccessible for the integrin receptors. The receptors cannot
cluster into focal adhesion complexes and bind to cytoplasmic proteins (such
as talin), which is required for cell adhesion. In contrast, when the biomaterial
surface is moderately hydrophilic (figure 6b), the adsorbed ECM proteins are
more flexible and not denatured, such that the receptors can form focal adhesions
and link the integrins to the actin cytoskeleton of the cells [39].
(iii) Cyanoacrylate bioadhesives
Bioadhesives for surgical applications is another area where chemical
interactions are important. There are two main types of surgical tissue adhesives,
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2329
Bioadhesion: concepts and applications
(a)
(b)
alpha-actinin
alpha-actinin
actin
talin
actin
vinculin
cell
membrane
vinculin
paxillin
paxillin
adhesion
receptors
adhesion
receptor
adhesion ligands
talin
adsorbed protein
material
cell
membrane
adhesion ligands
adsorbed protein
material
Figure 6. (a) Cell adhesion to a hydrophobic surface. (b) Cell adhesion to a moderately hydrophilic
surface. The effect of surface chemistry on the formation of focal adhesion complexes is shown [39].
(Online version in colour.)
namely cyanoacrylates and fibrin tissue adhesives (the latter will be discussed
in a separate section later). The main difference between the two is that
while cyanoacrylates are synthetic compounds (the same type of materials
found in ‘super glue’), fibrin glues are composed of endogenous biomolecules.
Cyanoacrylates are used as surgical adhesives owing to their wound-sealing
properties and antimicrobial activity [4].
Cyanoacrylate monomers polymerize with the aid of water molecules available
on the surface or present as relative humidity in the air, leading to rapid
polymerization. The first cyanoacrylate compound to be considered as a
surgical adhesive was methyl-2-cyanoacrylate. However, this compound has been
found to cause histotoxic (toxicity to biological tissues) reactions, and other
cyanoacrylate compounds have been investigated since. The latest generation
of cyanoacrylate adhesives used in surgery is based on octyl-2-cyanoacrylate,
and is sold under the brand name Dermabond Topical Skin Adhesive (Ethicon,
Somerville, NJ, USA). This adhesive is deemed non-toxic and has been
approved by the US Food and Drug Administration for topical wound closure
applications [4].
(iv) Solubility parameters and dental restoratives
The solubility parameter concept, which is used as a guide for the miscibility of
polymer and solvent systems, can be used to predict the bonding of systems such
as resin composites to dentin. Initially introduced by Hildebrand, the solubility
parameter (d) of a liquid is the square root of the energy of vaporization per unit
volume, expressed as [40,41]
DHv − RT 1/2
d=
,
(2.1)
V
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2330
M. L. B. Palacio and B. Bhushan
where DHv is the molar enthalpy of vaporization, R is the universal gas constant,
T is the temperature and V is the molar volume. In this concept, liquids
with comparable solubility parameter values are predicted to be miscible with
each other. As applied to dental biomaterials, it has been found that there is
a correlation between the solubility parameters of adhesive monomers and the
dentin and the ability of the monomers to permeate into the substrate [42–44].
(c) Physiological factors
Biological adhesion can also take place through specific physiologically related
mechanisms. Two examples are discussed, namely fibrin glues and the plant
lectin adhesive.
(i) Fibrin tissue adhesive
The fibrin tissue adhesive is one of the two major categories of adhesives
for surgical applications (the other type being cyanoacrylates). Fibrin tissue
adhesives can be applied below the dermis as a sealant (or haemostatic clamp)
for skin grafts and flaps, and for laparoscopic surgeries [4,8]. These adhesives
are typically packaged as two components, which are then mixed together
during surgery. The first component contains fibrinogen, plasma glutaminase (also
called Factor XIII) and CaCl2 . The second component contains thrombin and
antifibrinolytic agent. Fibrin tissue adhesives work based on the physiology of
the blood coagulation process. Thrombin cleaves the protein fibrinogen during
clotting into smaller fibrin subunits, which then go through end-to-end and sideto-side polymerization. Factor XIII is responsible for the cross-linking of the
subunits into a stable fibrin clot in the presence of calcium [4].
(ii) Lectins
Lectins are carbohydrate-binding proteins found in both animals and plants.
One of the common animal lectins is the C-type or calcium-dependent binding
lectins. One of these C-type lectins, the asialoglycoprotein lectins, is specific
to liver cells and is involved in animal biological function. Meanwhile, plant
lectins are abundant and can be found in common plants such as tomatoes and
in the seeds of legumes. Some plant lectins are regarded as toxins when they
bind to animal cells [27,45]. However, owing to their ability to bind specifically
to glycosylated cell membrane components, lectins are being investigated for
their ability to transport macromolecules, with implications for drug delivery
to the gastrointestinal tract [46,47]. Aside from lectins, other molecules that are
being considered for bioadhesive-based drug delivery are bacterial fimbrins and
invasins [3].
(d) Physical and mechanical effects
The physical and mechanical factors that influence bioadhesion were originally
developed for polymer–polymer adhesion, but are directly applicable to biological
adhesion as well [1]. The mechanisms of wetting and interpenetration will be
discussed as follows.
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2331
P
gPL
liquid
gML
P
gPM
M
M
Figure 7. Schematic of the relevant interfacial energies (g) that determine the formation of a
bioadhesive bond, as illustrated for the adhesion between a polymer (P) and the mucous layer
(M) on a biological membrane in the presence of a liquid medium (L) (adapted from Buckton [2]).
(i) Wetting phenomenon
The interfacial energy is an important determinant of successful bioadhesion. In
systems exhibiting bioadhesion, the liquid environment influences the spreading of
one material phase over another. Consider the case of a polymer surface adhering
to the mucous gel layer on a biological membrane immersed in liquid medium (e.g.
in drug-delivery applications). Figure 7 is a schematic illustrating the interfacial
energy components that should be considered in evaluating the thermodynamic
work of adhesion of this system. During adhesion, a unit interface between the
polymer and the liquid and between the mucus and the liquid vanishes, whereas
an interface between the polymer and mucus forms. The thermodynamic work
adh
) or the energy of adhesion per unit area between the polymer
of adhesion (WPM
and mucus is then defined by the Dupre equation as follows [2,28]:
adh
= gPM − (gPL + gML ),
WPM
(2.2)
where g represent surface energies and the subscripts P, M and L are for polymer,
adh
mucus and liquid, respectively. A positive value for WPM
is regarded to be a
necessary condition to achieve successful bonding between the two surfaces.
(ii) Interpenetration
While interfacial contact and chemical bonding interactions are needed for the
initial stages of bioadhesion, the interpenetration or interdiffusion between the
molecules of the two contacting surfaces will maintain the adhesive bond. If a
polymer is one of the contacting systems involved in the bioadhesion process,
then the interpenetration process involves the mobility of the individual chains
and their entanglement in the opposing biological membrane. A related concept is
the swelling capacity of the polymer, which is the ratio of the wet to dry weights.
A high swelling capacity for a given polymer implies that it has greater chain
mobility, and a higher tendency towards interpenetration [2].
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2332
M. L. B. Palacio and B. Bhushan
For bioadhesion in dental prostheses, mechanical interlocking or interpenetration is the primary adhesive mechanism. When bonding an acrylic resin
restorative to tooth enamel, surface etching is performed to create micropores
on the surface that will increase the penetration of the restorative material.
This method was pioneered by Buonocore [48], who discovered that conditioning
human enamel with 85 per cent phosphoric acid improves the adhesion of
acrylic resin to the enamel surface. This is an illustration of how a surface
chemical treatment roughens the surface and facilitates adhesion via mechanical
interlocking effects.
The tooth dentin, on the other hand, contains tubules that radiate from the
pulp. These tubular structures facilitate the penetration of resin monomers into
the dentin and their retention once the resin has been polymerized [42]. As
discussed earlier, chemical effects (i.e. solubility) play a role in the adhesion
between a restorative and dentin. However, the other dimension in this adhesive
event is the permeation of the adhesive into the collagen fibrillar network found in
the tooth dentin [5]. A list of hydrophilic monomers that have been investigated
for this purpose is given in table 1. It is thought that the entanglement of
the polymer network formed during the polymerization of the monomer on the
collagen network of dentin is responsible for the observed adhesion.
3. Optimization of bioadhesion
The bioadhesion between two surfaces in contact may be enhanced using various
physical, chemical and mechanical processing techniques.
(a) Physical processes
Various methods are used to modify surfaces, such as exposure to plasma,
corona discharge, ions and ultraviolet (UV)-ozone. Plasma processing is a widely
used technique for surface cleaning and modification of the surface chemical
composition. The plasma, which is generated by applying an electric field to
a low-pressure gas in vacuum, produces reactive species such as ions and free
radicals. For surface cleaning purposes, the plasma breaks most organic bonds,
such as C−H, C−C, C=C, C−O and C−N. In addition, if oxygen plasma is
used, another cleaning mode is possible. The activated oxygen species (O•,
−
O+ , O− , O+
2 , O2 , O3 , among others) can combine with organic compounds to
form H2 O, CO, CO2 and low molecular weight hydrocarbons, thereby removing
organic contaminants on the surface [49]. Surface modification through plasma
processing can render the surface more hydrophilic or hydrophobic. If the gas used
is oxygen or carbon dioxide, polar groups are introduced to the surface (such
as C−OH, C=O and COOH), enhancing surface hydrophilicity. The presence
of more polar surface groups is valuable in enhancing adhesion, e.g. between
proteins and polymeric biomaterials used in tissue engineering applications. The
application of this concept is shown schematically in figure 8 [39]. Two examples
are shown to illustrate how plasma treatment leads to an activated surface
with free radicals. After the plasma processing, molecules can be grafted onto
the activated surface, such as polyethylene glycol in figure 8a or dithiol and
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2333
Table 1. Monomers used as priming agents for tooth dentin adhesives [5].
common abbreviation
full name
HEMA
GLUMA
NPG-GMA
NTG-GMA
GDMA
GPDM
NMBu
NMGlu
NAAsp
NMHyp
NMGly
DIPENTA
PMDM
MEM
MMEM
MMPM
MDPM
MBP
MOP
ACE
BMEP
EGMP
MDP
PhenylP
4-META
PA
PM
SBMA
SEMA
MSPMA
2-hydroxyethyl methacrylate
adduct of glutaraldehyde and HEMA
N -phenyl glycine/glycidyl methacrylate (adduct)
N -(p-tolyl)glycine and glycidyl methacrylate
glyceryl dimethacrylate
glycerophosphoric acid dimethacrylate
N -methacryloyl butyric acid
N -methacryloyl glutamic acid
N -acryloyl aspartic acid
N -methacryloyl hydroxyproline
N -methacryloyl glycine
pentaacryloyldipentaerythritol phosphoric acid
diadduct of pyromellitic anhydride with 2-hydroxyethyl methacrylate
2-methacryloyloxyethyl hydrogen maleate
mono-2-(methacryloyloxy)ethyl maleate
mono-2-(methacryloyloxy)propyl maleate
3-methacryloxypropyl phosphate
4-methacryloyloxybutyl phosphoric acid
8-methacryoyloxyoctyl phosphoric acid
acryloyloxyethyl citraconate
bis[2-(methacryloyloxy)-ethyl]phosphate
ethylene glycol methacrylate phosphate
10-methacryloyldeacamethylene phosphoric acid
2-methacryloyloxy phenyl phosphate
4-methacryloxyethyl trimellitate anhydride
2-acryloyloxyethyl phosphate
2-methacryloyloxy ethyl phosphate
3-sulpho-2-butyl methacrylate
2-sulphoethyl methacrylate
3-methoxy-1-sulpho-2-propyl methacrylate
Au nanoparticles in figure 8b. In these modified polymer surfaces, successful
adhesion of fibroblast cells was achieved, indicating the cytocompatibility of the
plasma-processed polymer [39,50].
It should be noted that a similar effect on enhancing surface hydrophilicity
can be obtained by using UV-ozone surface treatment on polydimethylsiloxane
(PDMS). Compared with the untreated polymer surface, the surface-treated
PDMS tends to be rougher and more hydrophilic. An increase in the cell
attachment as well as an enhancement in cell spreading was observed with the
use of the UV-ozone-treated PDMS [51].
On the other hand, the use of fluorinated gases such as CF4 in the plasma
results in the introduction of a low surface energy polytetrafluoroethylenelike structure, enhancing surface hydrophobicity. This approach has been used
to prevent the undesirable protein adhesion and the unwanted generation of
inflammatory cells on poly(methyl methacrylate) (PMMA) contact lenses [52].
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2334
M. L. B. Palacio and B. Bhushan
(b)
polymer
(a)
plasma
polymer
R
R
plasma
R
R
R
R
R
R
R
R
polymer
R
R
dithiol grafting
polymer
polymer
PEG grafting
Au nanoparticles
grafting
polymer
cell culturing
polymer
cell culturing
polymer
polymer
Figure 8. Schematic showing the effect of plasma processing on a polymer surface, leading to
its activation and capacity to graft (a) polyethylene glycol (PEG) or (b) dithiol followed by Au
nanoparticles. Cells are able to grow onto the grafted surfaces [39]. (Online version in colour.)
(b) Chemical processes
Specific functional groups can be covalently attached to substrate surfaces
using chemical synthesis techniques to attach biomolecules and construct model
systems for use in adhesion studies. Covalent bonding will lead to enhanced
retention of the biomolecule relative to simply relying on its innate adsorption to
the substrate. An example is shown in figure 9, illustrating how a silica surface is
chemically functionalized with the protein streptavidin [53]. In this illustration,
the functionalization process consists of multiple steps, starting with the addition
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2335
Bioadhesion: concepts and applications
O
HN NH
O
streptavidin
has four
biotin-binding
pockets. Two
or one may be
attached to the
biotin on the
surface, with
the remaining
two or three
available to
bind the
biotin analyte
S
3-aminopropyltriethoxysilane (3-APTES)
O
O
SiO2
O
H
O
H
O
H
HN
NH
S
CH2CH2O
CH2CH2O Si
OR
CH2CH2CH2NH3+
O
CH2CH2O
O
HN
NH
S
O
O
HN
sulpho-NHS biotin
O
Si
CH2CH2CH2NH3+
O
Si
CH2CH2CH2NH3+
O
O
O
S
O
OR
O
O
Na+
O
N
O
O
HN
S
NH
O
O
CH2CH2CH2NH
Si
CH2CH2CH2NH3+
O
HN
OR
O
Si
NH
S
O
O
NH
S
O
CH2CH2CH2NH
O Si
O
O
SiO2
SiO2
O
O Si
O
O
CH2CH2CH2NH
HN
NH
S
O
Figure 9. Schematic showing the steps in the functionalization of a silica surface with the protein
streptavidin. Adapted from Bhushan et al. [53].
of the aminosilane 3-aminopropyltriethoxysilane (APTES) in order to introduce
amine groups to the silica surface. This is then followed by reacting the surface
with sulpho-N -hydroxysuccin-imido-biotin (abbreviated as biotin), which forms a
covalent bond with the amine group of the APTES. In the final step, the protein
streptavidin will bind to biotin. Streptavidin has a high affinity to biotin, and
this interaction is regarded as one of the strongest non-covalent bonds known.
This technique can be implemented for the functionalization of the silica surface
with other proteins as well [22,23].
(c) Mechanical interactions
Surface modification through mechanical texturing is a simple and
cost-effective method to roughen biomaterials and to create anchor points for
enhanced adhesion. In this method, abrasive particles (such as silicon carbide
or alumina) impinge on the biomaterial, and the resulting impact increases its
surface roughness [28,54]. This method can be used on orthopaedic implant
materials such as titanium or cobalt/chrome.
In the design of artificial cartilage scaffold, the surface modification of
a non-degradable polymer could be undertaken to enhance cell adhesion.
This involves the addition of a non-woven mesh composed of polymer fibres
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2336
M. L. B. Palacio and B. Bhushan
Figure 10. Cross-section scanning electron microscopy image showing the growth and
interpenetration of a cell matrix (long arrows) on a poly(L-lactic acid) scaffold (short arrows) [55].
bonded onto a non-porous surface. The presence of the mesh on the surface
creates a porous surface on which cells can proliferate. As the ECM develops,
the fibres remain interpenetrated, and ensuring the bonding of the scaffold onto
the cartilage. An image is shown in figure 10 illustrating this concept [55]. The
short arrows point to the fibres (made of poly(L-lactic acid)), whereas the long
arrows indicate the presence of cartilage growth on the fibres.
4. Characterization techniques
In this section, nanoscale and macroscale experimental techniques for quantifying
bioadhesion are discussed.
(a) Nanoscale
Fundamental studies of bioadhesion at the nanoscale are conducted with
atomic force microscopy (AFM). Aside from its high-resolution surface imaging
capability, AFM can measure forces in the pico- to nanonewton range, allowing for
the detection of single-molecule interactions [6,56]. In AFM, force–distance curves
for the contact between the probe and the sample surface can be obtained. An
example is shown in figure 11. The force–distance curve consists of two segments,
namely the advancing curve (corresponding to AFM piezo extension), which
shows how the probe approaches the surface, and the retracting curve, which
illustrates how the probe detaches from the surface. In the retracting force curve,
a distinct snap-off point is observed, which corresponds to the force necessary to
separate the tip from the sample surface. This is the measured adhesive force.
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2337
Bioadhesion: concepts and applications
4000
force (pN)
3000
2000
1000
extending
0
–1000
retracting
0
Fadh
100
200
300
400
tip and sample separation (nm)
500
Figure 11. Atomic force microscopy force–distance curve between a tip functionalized with a
fibronectin antibody and fibronectin deposited on a polymer surface obtained in phosphate-buffered
saline medium.
adhesive force (nN)
2.5
2.0
1.5
1.0
0.5
0
9.1
4.4
7.4
pH values of phosphate-buffered saline solution
Figure 12. Effect of pH on the adhesion between streptavidin and biotin as measured by atomic
force microscopy. Adapted from Bhushan et al. [57].
The force resolution of AFM enables it to detect differences in the net charge
of biomolecules brought about by a change in the surrounding ionic environment.
In another AFM study, Bhushan et al. [57] were able to demonstrate that the
adhesion between streptavidin and biotin increases with the pH of the buffer
solution, as shown in figure 12. This is related to the charge interactions between
the two biomolecules. Streptavidin has an isoelectric point of pH 5.5, such that
it has a net positive charge at pH 4.4 and net negative charge at pH 7.4 and 9.1.
The net negative charge in streptavidin facilitates adhesive interactions with the
biotin molecule.
Force–distance curves can also be taken over a predefined area divided into an
array of points, where the average adhesive force can be determined. This method
is useful when analysing the adhesive properties of next-generation biomaterials
such as block copolymers, which have heterogeneous surface properties. Their
morphology is a function of the composition and molecular weight of the
individual blocks, as well as the spatial relationship of the blocks; for instance,
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2338
M. L. B. Palacio and B. Bhushan
10
PMMA
PMMA-b-PAA (random 1/1)
PMMA-b-PAA (1/1)
PAA-b-PMMA-b-PAA (0.5/1/0.5)
8
adhesive force (nN)
pH 7.4
pH 6.2
pH 7.4
pH 6.2
pH 7.4
pH 6.2
6
4
2
0
fibronectin
BSA
collagen
Figure 13. Atomic force microscopy (AFM) data on the average adhesive force for the interaction
between block copolymers containing poly(methyl methacrylate) (PMMA)/poly(acrylic acid)
(PAA) with the proteins fibronectin, BSA and collagen (attached to functionalized AFM tips) in pH
7.4 (phosphate-buffered saline) and pH 6.2 buffer media, with data for PMMA as a reference [22].
A–B block copolymers (or diblock) will have a different morphology from A–B–A
block copolymers (or triblock), and so on. This variation in the morphology
translates to observable differences in the adhesive force. Figure 13 is a summary
of the average adhesive force for the interactions between three proteins, namely
fibronectin, bovine serum albumin (BSA) and collagen, on block copolymers
composed of PMMA and poly(acrylic acid) (PAA), but with different block
arrangements (random, diblock and triblock) [22].
The data in figure 13 show the variation of the adhesive force at two ionic
environments, pH 7.4 (phosphate-buffered saline, PBS) and pH 6.2, where it
is seen that the liquid environment affects the adhesive force. For both buffer
media, the highest average adhesive force was observed in the triblock copolymer
(PAA-b-PMMA-b-PAA, where ‘b’ denotes block copolymer), and the lowest force
was obtained from the PMMA reference. The higher average adhesive force in
the triblock copolymer surface can be due to greater ordering on its surface,
which would expose more PAA-rich areas than it would for the diblock or
random copolymer.
The average adhesive forces measured at pH 6.2 are higher throughout the
entire series, relative to the PBS medium (pH 7.4) data. This is attributed
to higher repulsion at pH 7.4 (or conversely, increased attractive forces at the
lower pH). Chemical interactions between the protein and the polymer surface
are influenced by the surface charges of the protein and the polymer itself. The
amine groups of the proteins are protonated at both pH 7.4 and 6.2. But these
proteins carry a net negative charge when immersed in either the pH 7.4 or 6.2
buffer media, because the isoelectric points of fibronectin, BSA and collagen are
all lower than 6.2 [22]. There are more negative charges on the protein surface
at pH 7.4 than at pH 6.2. Also, the acrylic acid chains are ionized (to acrylates)
during the adhesive force mapping experiment as they are immersed in aqueous
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
pH 7.4
pH 6.2
––– –––
–––
–
2339
protein
O
O–
–
–
O–
reduction in
charge
O
polymer
Figure 14. Schematic illustrating the effect of pH on protein–block copolymer interactions (focusing
on the effect of poly(acrylic acid), which is ionized at pH 7.4 and 6.2), showing negative charge
reduction at lower pH conditions, which leads to decreased repulsion [22].
medium. The presence of negative charges on both the tip and sample surface
leads to repulsive interactions. As illustrated in figure 14, the reduced repulsion
between the acrylate groups in the acrylic acid and the smaller amount of surface
negative charges in fibronectin, BSA and collagen at the lower pH increase the
measured adhesion between the two surfaces [22]. At the higher pH (7.4), the
proteins carry a greater net negative charge. Because the acrylic acid in the block
copolymer is present in its ionized form, the repulsion from the negative charges
present on both the protein and the polymer surface is more significant at pH
7.4 than at pH 6.2. This accounts for the higher adhesive force measured at pH
6.2. The results demonstrate how AFM is able to measure the effect of the ionic
environment on bioadhesion.
(b) Macroscale
Traditional adhesion characterization techniques, such as crack growth, peel
and shear tests, could be applied to evaluate bioadhesion. A few examples of the
use of these methods are provided below.
For dental restorations, the Griffith energy balance model could be used
to characterize the crack growth using a bending test, where the cracks
originate from the shrinking of the implanted dental composite material upon
polymerization [58]. The energy balance concept, as applied to restorations, deals
with the balance between the elastic energy in the tooth (which is actually
composed of the individual elastic energies of the tooth and the restorative
material) and the surface energy associated with the crack. Experimentally,
either the stress intensity factor (K , also referred to as the fracture toughness)
or the strain energy release rate (G, also referred to as the fracture energy)
is evaluated from the measurement of the crack growth. Fracture toughness is
obtained from [58]
K = s(ap)1/2 ,
(4.1)
where s is the applied stress and a is half the length of the crack. A tabulation
of the fracture toughness of adhesive joints, tooth components, and restorative
materials is presented in table 2. The fracture energy can be obtained as long as
the modulus and Poisson’s ratio of the material are known.
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2340
M. L. B. Palacio and B. Bhushan
(a)
(b)
*
*
2 µm
5 µm
Figure 15. Fractography of tooth–dental restorative interfaces using scanning electron microscopy:
(a) an example of predominantly adhesive failure, except for the region marked with an asterisk,
where cohesive failure occurred and (b) an example of cohesive failure (adapted from Yang
et al. [59]).
Table 2. Fracture toughness of selected dental materials [58].
material
fracture toughness (MPa m1/2 )
adhesives bonded to dentin
adhesives bonded to enamel
enamel
dentin
dentin–enamel junction
composite resins
amalgams
0.1–1.2
1.1
0.65–2.5
1.0–4.0
0.8–3.4
0.7–1.9
1.4–2.4
As a qualitative tool, fractographic analysis through either scanning electron
microscopy (SEM) or transmission electron microscopy is usually conducted
on dental restoration–dentin interfaces analysed in vitro using tensile testing.
Examples of fracture surface SEM images are shown in figure 15, where it is
seen that either adhesive or cohesive failure can take place. The determination
of the failure mode is useful when comparing between different adhesive resin
formulations [59].
Standard American Society for Testing and Materials (ASTM) characterization
procedures, such as the tensile test (ASTM D368), peel test (ASTM D903),
shear strength (ASTM D3654) and shear adhesion failure temperature (SAFT,
ASTM D4498), are conducted on interfaces where an adhesive is applied to the
biomaterial. Schematics for the peel and shear tests are shown in figure 16. These
tests are applicable to systems such as fibrin glues used in surgical operations,
PAA hydrogels and pressure-sensitive adhesives (PSAs) intended for transdermal
drug-delivery applications [8,54,60,61]. Figure 17 shows examples of macroscale
adhesion data on PSAs. The adhesive system investigated was PDMS. It was
loaded with an organo-clay based on montmorillonite (MMT) to form a composite
material. The addition of the organo-clay was shown to improve the shear strength
and the SAFT, with a minimal reduction in the peel strength [61].
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2341
Bioadhesion: concepts and applications
(a)
(b)
adhesive tape
coating
adhesive tape
substrate
substrate
(c)
tape and coating
cut to shape shown
coating
(d)
flexible substrate
flexible
substrate
coating
backing
plate
90°
double stick
adhesive tape
(e)
coating
double stick
adhesive tape
backing plate
linear slide
glass
normal load
coating
adhesive
substrate
Figure 16. Schematics of macroscale adhesion tests. (a, b) The scotch tape test, (c, d) examples of
the peel test and (e) the lap shear test [54].
The flow-through method, which measures the flow rate needed to remove a
bioadhesive-coated sphere, is a common macroscale experiment suited for the
characterization of mucoadhesion of potential drug-delivery systems. A widely
used biophysical assay involves the monitoring of molecular weight change
through the variation in the sedimentation coefficient as measured by an
analytical ultracentrifuge [62]. The Wilhelmy plate experiment is another
technique for evaluating bioadhesion at the macroscale. Instead of having a liquid
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2342
M. L. B. Palacio and B. Bhushan
(a) 1000
(b) 3.0
2.5
180° peel (lbs per in.)
500 g shear (min)
800
600
400
200
0
2.0
1.5
1.0
0.5
2
4
clay loading (weight %)
6
0
2
4
clay loading (weight %)
6
(c) 180
SAFT (°C)
165
150
135
120
0
2
4
clay loading (weight %)
6
Figure 17. Data from macroscale testing of bioadhesion. (a) Lap shear method (ASTM D3654)
used to determine shear time versus clay loading in pressure-sensitive adhesive (PSA) loaded with
an organosilicate nanocomposite; (b) 180◦ peel strength versus clay loading in the PSA (ASTM
D903); (c) lap shear method used to determine shear adhesion failure temperature (SAFT) versus
clay loading (ASTM D4498). Circles with lines denote C18-MMT and squares (Na-MMT) denote
the pure clay used as the reference material [61].
medium such as water (as used in surface tension measurements), it is replaced
with either natural or synthetic mucus and the plate is coated with the polymer
of interest [2,54]. This method is useful for determining interfacial forces between
mucus and polymer, as well as for the investigation of changes in bioadhesion
properties over time.
5. Conclusion
The subject of bioadhesion covers many different concepts, mechanisms and
applications. To illustrate the diversity of bioadhesion processes, relevant
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2343
Bioadhesion: concepts and applications
chemical stuctures of the 20 common amino acids
O
O
O
H2N
OH
H2N
glycine (Gly, G)
OH
OH
serine (Ser, S)
O
O
H2N
OH
OH
OH
threonine (Thr, T)
valine (Val, V)
OH
alanine (Ala, A)
O
H2N
H2N
H2N
OH
O
H2N
OH
SH
cysteine (Cys, C)
O
H2N
O
HO
aspartic acid (Asp, D)
OH
O
H2N
asparagine (Asn, N)
O
H2N
O
H2N
OH
leucine (Leu, L)
O
H2N
OH
O
OH
H2N
OH
isoleucine (Ile, I)
NH
NH3+
lysine (Lys, K)
H2N
NH2+
arginine (Arg, R)
O
O
O
H2N
OH
H2N
OH
H2N
O
OH
OH
H
NH
O
O
glutamic acid (Glu, E)
glutamine (Gin, Q)
O
methionine (Met, M)
OH
H2N
O
O
O
H2N
proline (Pro, P)
S
NH2
OH
H2N
OH
OH
HN
H 2N
OH
NH
N
histidine (His, H)
phenylalanine (Phe, F)
OH
tyrosine (Tyr, Y)
tryptophan (Trp, W)
Figure 18. Chemical structures of the 20 amino acids found in proteins. Adapted from [63].
examples, such as cell adhesion to biomaterials, dental restorative adhesives and
mucoadhesives, were discussed in this paper. Bioadhesion can be beneficial, as it
can facilitate the desired adhesion of cells and biomolecules on various natural and
synthetic substrates, which then leads to the development of novel biomaterials,
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2344
M. L. B. Palacio and B. Bhushan
DAHKSE VAHRFKDLGE ENFKALVLIA FAQYLQQCPF
EDHVKLVNEV TEFAKTCVAD ESAENCDKSL HTLFGDKLCT
VATLRETYGE MADCCAKQEP ERNECFLQHK DDNPNLPRLV
RPEVDVMCTA FHDNEETFLK KYLYEIARRH PYFYAPELLF
FAKRYKAAFT ECCQAADKAA CLLPKLDELR DEGKASSAKQ
RLKCASLQKF GERAFKAWAV ARLSQRFPKA EFAEVSKLVT
DLTKVHTECC HGDLLECADD RADLAKYICE NQDSISSKLK
ECCEKPLLEK SHCIAEVEND EMPADLPSLA ADFVESKDVC
KNYAEAKDVF LGMFLYEYAR RHPDYSVVLL LRLAKTYETT
LEKCCAAADP HECYAKVFDE FKPLVEEPQN LlKQNCELFE
QLGEYKFQNA LLVRYTKKVP QVSTPTLVEV SRNLGKVGSK
CCKHPEAKRM PCAEDYLSVV LNQLCVLHEK TPVSDRVTKC
CTESLVNRRP CFSALEVDET YVPKEFNAET FTFHADICTL
SEKERQIKKQ TALVELVKHK PKATKEQLKA VMDDFAAFVE
KCCKADDKET CFAEEGKKLV AASQAALGL
Figure 19. Amino acid sequence of the protein human serum albumin (HSA) [65].
therapies and technologies such as biosensors. Various processing techniques that
could enhance bioadhesion are available. The characterization of bioadhesion
must be performed at the length scale appropriate to the phenomenon (either at
the nanoscale or macroscale), as it relates to the application where the adherence
between two interfaces is desired.
Appendix A. The composition of proteins
Proteins are known as the most abundant class of macromolecules in cells. Each
protein molecule can be envisaged as a polymer composed of amino acids, which
are molecules that contain an amine group, a carboxylic acid group and a variable
side chain. The chemical structures of the 20 common amino acids found in
proteins, along with their three-letter and one-letter abbreviations, are shown in
figure 18 [63]. On the basis of their chemical composition, the amino acids alanine,
valine, leucine, isoleucine, proline, phenylalanine, tryptophan and methionine
are classified as non-polar. Glycine, serine, threonine, cysteine, asparagine and
glutamine are considered as uncharged polar amino acids. Aspartic acid, glutamic
acid, lysine, arginine and histidine have charged side groups [64].
The amino acid sequence of human serum albumin is shown in figure 19 to
illustrate the side chain composition variation found on a typical protein. This
protein contains 585 amino acids in the form observed in the blood [65]. A close
examination of this sequence reveals that charged, uncharged polar and non-polar
amino acids are present in varying arrangements throughout this protein.
References
1 Peppas, N. A. & Buri, P. A. 1985 Surface, interfacial and molecular aspects of polymer
bioadhesion on soft tissues. J. Control. Release 2, 257–275. (doi:10.1016/0168-3659(85)90050-1)
2 Buckton, G. 1995 Interfacial phenomena in drug delivery and targeting. Chur, Switzerland:
Harwood Academic.
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2345
3 Woodley, J. 2001 Bioadhesion: new possibilities for drug administration. Clin. Pharmakokinet.
40, 77–84. (doi:10.2165/00003088-200140020-00001)
4 Mobley, S. R., Hilinski, J. & Toriumi, D. M. 2002 Surgical tissue adhesives. Facial Plast. Surg.
Clin. North Am. 10, 147–154. (doi:10.1016/S1064-7406(02)00014-7)
5 Vaidyanathan, T. K. & Vaidyanathan, J. 2009 Recent advances in the theory and mechanism
of adhesive resin bonding to dentin: a critical review. J. Biomed. Mater. Res. B 88, 558–578.
(doi:10.1002/jbm.b.31253)
6 Bhushan, B. 2010 Springer handbook of nanotechology, 3rd edn. Heidelberg, Germany: Springer.
7 Su, W.-T., Liao, Y.-F., Lin, C.-Y. & Li, L.-T. 2010 Micropillar substrate influences the cellular
attachment and laminin expression. J. Biomed. Mater. Res. A 93, 1463–1469. (doi:10.1002/jbm.
a.32643)
8 Eriksen, J. R., Bech, J. I., Linnemann, D. & Rosenberg, J. 2008 Laparoscopic intraperitoneal
mesh fixation with fibrin sealant (Tisseel ) versus titanium tacks: a randomized control
experimental study in pigs. Hernia 12, 483–491. (doi:10.1007/s10029-008-0375-z)
9 Prashanth, K. D., Yiu, C. K.-Y. & King, N. M. 2007 A review of dentin adhesives in pediatric
dentistry. Hong Kong Dent. J. 4, 80–89.
10 Burkett, J. R., Wojtas, J. L., Cloud, J. L. & Wilker, J. J. 2009 A method for measuring
the adhesion strength of marine mussels. J. Adhesion 85, 601–615. (doi:10.1080/00218460902
996903)
11 Anonymous. 2011 Hardening of the arteries. A.D.A.M. Medical Encyclopedia. See http://www.
ncbi.nlm.nih.gov/pubmedhealth/PMH0001224.
12 Tenke, P., Riedl, C. R., Jones, G. L., Williams, G. J., Stickler, D. & Nagy, E. 2004 Bacterial
biofilm formation on urologic devices and heparin coating as preventive strategy. Int. J.
Antimicrob. Agents 23, S67–S74. (doi:10.1016/j.ijantimicag.2003.12.007)
13 Kroes, I., Lepp, P. W. & Relman, D. A. 1999 Bacterial diversity within the human subgingival
crevice. Proc. Natl Acad. Sci. USA 96, 14 547–14 552. (doi:10.1073/pnas.96.25.14547)
14 Vroman, L. 1962 Effect of absorbed proteins on the wettability of hydrophilic and hydrophobic
solids. Nature 196, 476–477. (doi:10.1038/196476a0)
15 Turbill, P., Beugeling, T. & Poot, A. A. 1996 Proteins involved in the Vroman effect during
exposure of human blood plasma to glass and polyethylene. Biomaterials 17, 1279–1287.
(doi:10.1016/0142-9612(96)88673-X)
16 Jung, S. Y., Lim, S. M., Albertorio, F., Kim, G., Gurau, M. C., Yang, R. D., Holden, M. A. &
Cremer, P. S. 2003 The Vroman effect: a molecular level description of fibrinogen displacement.
J. Am. Chem. Soc. 125, 12 782–12 786. (doi:10.1021/ja037263o)
17 Krishnan, A., Siedlecki, C. A. & Vogler, E. A. 2004 Mixology of protein solutions and the
Vroman effect. Langmuir 20, 5071–5078. (doi:10.1021/la036218r)
18 Noh, H. & Vogler, E. A. 2007 Volumetric interpretation of protein adsorption: competition
from mixtures and the Vroman effect. Biomaterials 28, 405–422. (doi:10.1016/j.biomaterials.
2006.09.006)
19 Arnold, M., Cavalcanti-Adam, E. A., Galss, R., Blummel, J., Eck, W., Kantlehner, M.,
Kessler, H. & Spatz, J. P. 2004 Activation of integrin function by nanopatterned adhesive
interfaces. ChemPhysChem 5, 383–388. (doi:10.1002/cphc.200301014)
20 Arnold, M., Schwieder, M., Blummel, J., Cavalcanti-Adam, E. A., Lopez-Garcia, M.,
Kessler, H., Geiger, B. & Spatz, J. P. 2009 Cell interactions with hierarchically structured
nano-patterned adhesive surface. Soft Matter 5, 72–77. (doi:10.1039/b815634d)
21 Biggs, M. J. P., Richards, R. G., Gadegaard, N., Wilkinson, C. D. W. & Dalby, M. J. 2007
Regulation of implant cell adhesion: characterization and quantification of s-phase primary
osteoblast adhesions on biomimetic nanoscale substrates. J. Orthop. Res. 25, 273–282.
(doi:10.1002/jor.20319)
22 Palacio, M. L. B., Schricker, S. R. & Bhushan, B. 2011 Bioadhesion of various proteins on
random, diblock and triblock copolymer surfaces and the effect of pH conditions. J. R. Soc.
Interface 8, 630–640. (doi:10.1098/rsif.2010.0557)
23 Palacio, M. L. B., Schricker, S. R. & Bhushan, B. 2011 Protein conformation changes on
block copolymer surfaces detected by antibody-functionalized atomic force microscope tips.
J. Biomed. Mater. A 100A, 18–25. (doi:10.1002/jbm.a.33219)
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
2346
M. L. B. Palacio and B. Bhushan
24 Khang, D., Lu, J., Yao, C., Haberstroh, K. M. & Webster, T. J. 2008 The role of nanometer
and sub-micron surface features on vascular and bone cell adhesion on titanium. Biomaterials
29, 970–983. (doi:10.1016/j.biomaterials.2007.11.009)
25 Lehnert, D., Wehrle-Haller, B., David, C., Weiland, U., Ballestrem, C., Imhof, B. A. &
Bastmeyer, M. 2004 Cell behaviour on micropatterned substrata: limits of extracellular matrix
geometry for spreading and adhesion. J. Cell Sci. 117, 41–52. (doi:10.1242/jcs.00836)
26 Kim, C.-H., Kim, G.-W. & Chun, H. J. 2010 Submicron-patterned fibronectin controls the biological behavior of human dermal fibroblasts. J. Nanosci. Nanotechnol. 10, 6864–6868.
(doi:10.1166/jnn.2010.2989)
27 Yang, X. & Robinson, J. R. 1998 Bioadhesion in mucosal drug delivery. In Biorelated polymers
and gels (ed. T. Okano), pp. 135–192. San Diego, CA: Academic Press.
28 Bhushan, B. 2002 Introduction to tribology. New York, NY: Wiley.
29 Bhushan, B. 2003 Adhesion and stiction: mechanisms, measurement techniques, and methods
for reduction. J. Vac. Sci. Technol. B 21, 2262–2296. (doi:10.1116/1.1627336)
30 Andrade, J. D., Hlady, V. & Wei, A. P. 1992 Adsorption of complex proteins at interfaces. Pure
Appl. Chem. 64, 1777–1781. (doi:10.1351/pac199264111777)
31 Waite, J. H. 1987 Nature’s underwater adhesive specialist. Int. J. Adhesion Adhes. 7, 9–14.
(doi:10.1016/0143-7496(87)90048-0)
32 Dalsin, J. L., Hu, B.-H., Lee, B. P. & Messersmith, P. B. 2003 Mussel adhesive protein
mimetic polymers for the preparation of nonfouling surfaces. J. Am. Chem. Soc. 125, 4253–4258.
(doi:10.1021/ja0284963)
33 Keselowsky, B. G., Collard, D. M. & Garcia, A. J. 2003 Surface chemistry modulates fibronectin
conformation and directs integrin binding and specificity to control cell adhesion. J. Biomed.
Mater. Res. A 66, 247–259. (doi:10.1002/jbm.a.10537)
34 Keselowsky, B. G., Collard, D. M. & Garcia, A. J. 2005 Integrin binding specificity regulates
biomaterial surface chemistry effects on cell differentiation. Proc. Natl Acad. Sci. USA 102,
5953–5957. (doi:10.1073/pnas.0407356102)
35 Michael, K. E., Vernekar, V. N., Keselowsky, B. G., Meredith, J. C., Latour, R. A. & Garcia,
A. J. 2003 Adsorption-induced conformational changes in fibronectin due to interactions with
well-defined surface chemistries. Langmuir 19, 8033–8040. (doi:10.1021/la034810a)
36 Latour Jr, R. A. & Rini, C. J. 2002 Theoretical analysis of adsorption thermodynamics for
hydrophobic peptide residues on SAM surfaces of varying functionality. J. Biomed. Mater. Res.
60, 564–577. (doi:10.1002/jbm.10052)
37 Basalyga, D. M. & Latour Jr, R. A. 2003 Theoretical analysis of adsorption thermodynamics
for charged peptide residues on SAM surfaces of varying functionality. J. Biomed. Mater. Res.
64, 120–130. (doi:10.1002/jbm.a.10360)
38 Keselowsky, B. G., Collard, D. M. & Garcia, A. J. 2004 Surface chemistry modulates focal
adhesion composition and signaling through changes in integrin binding. Biomaterials 25, 5947–
5954. (doi:10.1016/j.biomaterials.2004.01.062)
39 Bacakova, L., Filova, E., Parizek, M., Ruml, T. & Svorcik, T. 2011 Modulation of cell adhesion,
proliferation and differentiation on materials designed for body implants. Biotechnol. Adv. 29,
739–767. (doi:10.1016/j.biotechadv.2011.06.004)
40 Hildebrand, J. H. 1936 The solubility of non-electrolytes. New York, NY: Reinhold.
41 Young, R. J. & Lovell, P. A. 1991 Introduction to polymers, 2nd edn. London, UK: Chapman
and Hall.
42 Asmussen, E., Hansen, E. K. & Peutzfeldt, A. 1991 Influence of the solubility parameter
of intermediary resin on the effectiveness of the gluma bonding system. J. Dent. Res. 70,
1290–1293. (doi:10.1177/00220345910700091101)
43 Asmussen, E. & Uno, S. 1993 Solubility parameters, fractional polarities, and bond strengths of
some intermediary resins used in dentin bonding. J. Dent. Res. 72, 558–565. (doi:10.1177/0022
0345930720030101)
44 Chappelow, C. C., Power, M. D., Bowles, C. Q., Miller, R. G., Pinzino, C. S. & Elick, J. D.
2000 Novel prining and crosslinking systems for use with isocyanomethacrylate dental adhesives.
Dent. Mater. 16, 396–405. (doi:10.1016/S0109-5641(00)00034-8)
Phil. Trans. R. Soc. A
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
Bioadhesion: concepts and applications
2347
45 Lord, J. M. 1985 The structure and synthesis of plant lectins. New Phytol. 101, 351–366.
(doi:10.1111/j.1469-8137.1985.tb02842.x)
46 Lehr, C.-M., Bouwstra, J. A., Kok, W., Noach, A. B. J. & Junginger, H. E. 1992 Bioadhesion
by means of specific binding of tomato lectin. Pharm. Res. 9, 547–553. (doi:10.1023/A:1015804
816582)
47 Lehr, C.-M. 2000 Lectin-mediated drug delivery: the second generation of bioadhesives.
J. Control. Release 65, 19–29. (doi:10.1016/S0168-3659(99)00228-X)
48 Buonocore, D. H. 1955 A simple method of increasing the adhesion of acryl filling materials to
enamel surfaces. J. Dent. Res. 34, 849–853. (doi:10.1177/00220345550340060801)
49 Hozbor, M. A., Hansen, W. P. & McPherson, M. 1994 Plasma cleaning of metal surfaces. Precis.
Cleaning 2, 46.
50 Svorcik, V. et al. 2009 Cytocompatibility of Ar plasma treated and Au nanoparticle-grafted
PE. Nucl. Instrum. Methods Phys. Res. B 267, 1904–1910. (doi:10.1016/j.nimb.2009.03.099)
51 Toworfe, G. K., Composto, R. J., Adams, C. S., Shapiro, I. M. & Ducheyne, P. 2004 Fibronectin
adsorption on surface-activated poly(dimethylsiloxane) and its effect on cellular function.
J. Biomed. Mater. Res. A 71, 449–461. (doi:10.1002/jbm.a.30164)
52 Legeay, G. & Poncin-Epaillard, F. 2005 Surface engineering by coating of hydrophilic
layers: bioadhesion and biocontamination. In Adhesion: current research and applications (ed.
W. Possat), pp. 175–188. Weinheim, Germany: Wiley-VCH.
53 Bhushan, B., Tokachichu, D. R., Keener, M. T. & Lee, S. C. 2005 Morphology and
adhesion of biomolecules on silicon based surfaces. Acta Biomater. 1, 327–341. (doi:10.1016/
j.actbio.2005.01.002)
54 Bhushan, B. & Gupta, B. K. 1991 Handbook of tribology: materials, coatings and surface
treatments. New York, NY: McGraw-Hill.
55 Romito, L. & Ameer, G. A. 2006 Mechanical interlocking of engineered cartilage to an
underlying polymeric substrate: towards a biohybrid tissue equivalent. Ann. Biomed. Eng. 34,
737–747. (doi:10.1007/s10439-006-9089-5)
56 Bhushan, B. 2011 Nanotribology and nanomechanics I: measurement techniques and
nanomechanics. Part II nanotribology, biomimetics and industrial applications, 3rd edn.
Heidelberg, Germany: Springer.
57 Bhushan, B., Tokachichu, D. R., Keener, M. T. & Lee, S. C. 2006 Nanoscale adhesion,
friction and wear studies of biomolecules on silicon based surfaces. Acta Biomater. 2, 39–49.
(doi:10.1016/j.actbio.2005.08.010)
58 Soderholm, K.-J. 2010 Review of the fracture toughness approach. Dent. Mater. 26, e63–e77.
(doi:10.1016/j.dental.2009.11.151)
59 Yang, B., Ludwig, K., Adelung, R. & Kern, M. 2006 Micro-tensile bond strength of three luting
resins to human regional dentin. Dent. Mater. 22, 45–56. (doi:10.1016/j.dental.2005.02.009)
60 Bromberg, L., Temchenko, M., Alakhov, V. & Hatton, T. A. 2004 Bioadhesive properties and
rheology of polyether-modified poly(acrylic acid) hydrogels. Int. J. Pharm. 282, 45–60.
(doi:10.1016/j.ijpharm.2004.05.030)
61 Shaikh, S., Birdi, A., Qutubuddin, S., Lakatosh, E. & Baskaran, H. 2007 Controlled release
in transdermal pressure sensitive adhesives using organosilicate nanocomposites. Ann. Biomed.
Eng. 35, 2130–2137. (doi:10.1007/s10439-007-9369-8)
62 Harding, S. E. 2006 Trends in mucoadhesive analysis. Trends Food Sci. Technol. 17, 255–262.
(doi:10.1016/j.tifs.2005.12.007)
63 Anonymous. 2010 Amino acid. See http://en.citizendium.org/wiki/Amino_acid.
64 Zubay, G. L. 1998 Biochemistry, 4th edn. Dubuque, IA: W.C. Brown.
65 Anonymous. 2011 Human serum albumin. See http://en.wikipedia.org/wiki/Human_serum_
albumin.
Phil. Trans. R. Soc. A