Comparison of frictional and
mechanical properties of human
skin and synthetic materials in
dry and moist skin conditions
Malgorzata Nachman & Steve Franklin
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Aim of study
Investigation of a synthetic materials to simulate in-vivo friction behavior of
human skin in dry and moist conditions
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September 22, 2015 Malgorzata Nachman & Steve Franklin
Why a non-human test material is needed?
Disadvantages of in vivo testing:
 Poor reproducibility: Person-to-person variability
 Involuntary human movement during testing
 Last too long or they are destructive
 Necessary regulations: increase the effort and lead-time of experiments
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September 22, 2015 Malgorzata Nachman & Steve Franklin
Effect of skin hydration
Disadvantages of currently available Skin Substitutes:
 SynTissue™ from SynDaver Labs : Decrease in friction with water content
Fluid squeeze out of the porous structure and form a lubricating layer
 Silicone elastomers are hydrophobic and are not able to absorb water
 Designed to imitate the biological properties of skin with no regard for
their mechanical or frictional similarity
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September 22, 2015 Malgorzata Nachman & Steve Franklin
Human skin
The combination of layers is:
• anisotropic
• a non-linear force-displacement
relationship
• viscoelastic
The different mechanical properties of the individual skin layers influence and
determine the deformation behavior and the global mechanical response of skin
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September 22, 2015 Malgorzata Nachman & Steve Franklin
The requirements for the synthetic skin model
The artificial skin model should be built up with different layers: a very soft layer on the bottom
and a stronger layer at the top
The top layer
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•
•
•
•
•
•
Viscoelastic
Hydrophilic
Absorbs and releases moisture
Elastic modulus decrease with water content
Friction should increase with water content
very thin layer
Surface texture
The bottom layer
•
•
•
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Very soft
Viscoelastic
shouldn’t absorb water
EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
The mechanical properties of the different layers in human skin measured from
indentation experiments
J. v. Kuilenburg et al., 2012 In vitro indentation to determine the mechanical properties of epidermis, M. Geerligs et al., 2012
Skin layer, tissue
Stratum corneum
dry
wet
Viable epidermis
Dermis
Hypodermis
Thickness, mm
500 (3.5 – 1000)
0.025 (0.01 – 0.04)
30 (10-50)
1.5
0.095 (0.04 – 0.15)
0.02 (8-35 x 10-3)
1.4 (0.8 – 2)
2 x 10-3
0.8
The top layer simulating the epidermis
(S.C + Viabe epidermis)
The bottom layer simulating
dermis and hypodermis
•
•
Elastic modulus:
2 – 35 kPa
•
Thickness:
1.6 – 2.8 mm
•
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Elastic modulus,
MPa
Elastic modulus: dry > 1.5 MPa
wet < 1.5 MPa
Thickness:
50 - 200 µm
EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising material for the bottom layer
The mechanical properties of the bottom layer should be similar to that of human skin
Silicone rubber 3 ShA
Polyurethane gel
Technogel
SynDaver skin
Human skin
A force-displacement curve on the human forearm was measured for indentation of a steel
ball and then compared with the various synthetic materials
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising material for the bottom layer
The bottom layer simulating
dermis and hypodermis
W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by
instrumented indentation, 2003
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•
Elastic modulus:
•
Thickness:
2 – 35 kPa
1.6 – 2.8 mm
Polyurethane gel system:
part A - polyIsocyanate prepolymer extended with polyether polyol
part B- Curing agent based on a blend of polyether polyols
6.1
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising materials for the bottom layer
W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation, 2003
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising materials for the top layer
Silicone elastomers
Synthetic Skin Simulant Platform for the
Investigation of Dermal Blistering
Mechanics
Positive replica of human right index finger
Silicone elastomers are hydrophobic and are not able to absorb water; this could lead to effective
surface lubrication and reduction of friction which does not occur with actual human skin
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising materials for the top layer
A new class of hydrophilic silicones has been developed at Philips that show a strong
water uptake
Patent application:
 Medical and non-medical devices made from hydrophilic rubber materials
US 20140134416 A1
Dirk Burdinski, Joyce Van Zanten, Lucas Johannes Anna Maria Beckers, Cornelis Petrus Hendriks,
Willem Franke Pasveer, Nicolaas Petrus Willard, Mareike Klee, Biju Kumar Sreedharan Nair, David
Smith
 Water-absorbing elastomeric material US 20140113986 A1
Dirk Burdinski, Joyce Van Zanten, Lucas Johannes Anna Maria Beckers, Cornelis Petrus HENDRIKS,
Willem Franke Pasveer, Nicholaas Petrus Willard, Mareike Klee, Biju Kumar Sreedharan Nair,
David W. Smith
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Selection of the most promising materials for the top layer
Hydrophilic silicones are based on standard silicones modified with strongly
hydrophilic alpha-olefin sulfonate
Alpha-olefin sulfonate
Requirements for top layer
•
•
•
•
•
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Viscoelastic
Hydrophilic
Absorbs and releases moisture
Elastic modulus decrease with water content
Friction increase with water content
EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Standard silicone rubber with hardness of 40 ShA modified with sodium alpha-olefin
sulfonate
Thickness
200 µm
Elastomeric replica
An elastomeric replica of human arm was pressed against the
surface of the silicone sample.
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Water uptake in time
The water capacity of the top layer was determined to be 120 % after 24h
immersed in water and 25% after 24h in a climatic room
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
A new synthetic substitute of human skin
200 µm
200 µm
2.8 mm
2.8 mm
100 µm
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EUROMAT 2015
September 22, 2015 Malgorzata Nachman & Steve Franklin
Friction and indentation test
Carried out using a CETR-UMT Tribometer on human skin in vivo (volar forearm)
and then compared with the synthetic skin under:
 “dry”: 23°C, 37% Rh and
 “moist” skin hydration conditions:
Human skin - cleaned and wrapped in transparent plastic (kitchen) foil
Synthetic skin - left for 24 hours in a humidity chamber 28°C, 80% Rh
Hydration values were monitored using a Corneometer®
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Friction results
Synthetic skin
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Human skin
Indentation results
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Conclusions
A new synthetic skin substitute has been developed
 Provide a good simulation of the friction behavior of human skin in dry and
moist conditions
 The friction coefficient increases when conditions are changed from dry to wet
This is the same with human skin!
 Provide a good simulation of the deformation behavior of human skin
(Elastic Modulus in the same range, decrease in water content)
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September 22, 2015 Malgorzata Nachman & Steve Franklin
Acknowledgements
This work was supported by EU Marie Curie Industry-Academia Partnerships and
Pathways: UNITISS, Understanding Interactions of Human Tissue with Medical
Devices, FP7-PEOPLE-2011-IAPP/286174.
Author M.N. would like to acknowledge the Polish Ministry of Science and High
Education for financial support for the research within the co-financed
international project in the years 2012-2016.
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September 22, 2015 Malgorzata Nachman & Steve Franklin