comparison of frictional and mechanical properties …/file/malgorzata... · comparison of...

EUROMAT 2015 September 22, 2015 Malgorzata Nachman & Steve Franklin 1

Comparison of frictional and mechanical properties of human skin and synthetic materials in dry and moist skin conditions

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


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

Why a non-human test material is needed?


Disadvantages of currently available Skin Substitutes:

SynTissue™ from SynDaver Labs : Decrease in friction with water contentFluid 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

Effect of skin hydration


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

Human skin


The requirements for the synthetic skin model

The artificial skin model should be built up with different layers: a very soft layer on the bottomand a stronger layer at the top

The bottom layer

• Very soft• Viscoelastic• shouldn’t absorb water

The top layer

• Viscoelastic• Hydrophilic• Absorbs and releases moisture• Elastic modulus decrease with water content• Friction should increase with water content• very thin layer• Surface texture


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

The top layer simulating the epidermis(S.C + Viabe epidermis)

• Elastic modulus: dry > 1.5 MPawet < 1.5 MPa

• Thickness: 50 - 200 µm

The bottom layer simulatingdermis and hypodermis

• Elastic modulus: 2 – 35 kPa

• Thickness: 1.6 – 2.8 mm

Skin layer, tissue Elastic modulus,

MPa

Thickness, mm

Stratum corneum dry

wet

Viable epidermis

Dermis

Hypodermis

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


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 SynDaver skin Human skinTechnogel

A force-displacement curve on the human forearm was measured for indentation of a steel ball and then compared with the various synthetic materials


Selection of the most promising material for the bottom layer

W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus byinstrumented indentation, 2003

The bottom layer simulatingdermis and hypodermis

• Elastic modulus: 2 – 35 kPa

• Thickness: 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


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


Silicone elastomers

Silicone elastomers are hydrophobic and are not able to absorb water; this could lead to effectivesurface lubrication and reduction of friction which does not occur with actual human skin

Positive replica of human right index finger

Selection of the most promising materials for the top layer

Synthetic Skin Simulant Platform for theInvestigation of Dermal BlisteringMechanics


A new class of hydrophilic silicones has been developed at Philips that show a strongwater uptake

Patent application:

Medical and non-medical devices made from hydrophilic rubber materials US 20140134416 A1Dirk Burdinski, Joyce Van Zanten, Lucas Johannes Anna Maria Beckers, Cornelis Petrus Hendriks,

Willem Franke Pasveer, Nicolaas Petrus Willard, Mareike Klee, Biju Kumar Sreedharan Nair, DavidSmith

Water-absorbing elastomeric material US 20140113986 A1Dirk 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



Requirements for top layer

• Viscoelastic• Hydrophilic• Absorbs and releases moisture• Elastic modulus decrease with water content• Friction increase with water content

Hydrophilic silicones are based on standard silicones modified with strongly hydrophilic alpha-olefin sulfonate

Alpha-olefin sulfonate



Standard silicone rubber with hardness of 40 ShA modified with sodium alpha-olefin sulfonate

An elastomeric replica of human arm was pressed against the surface of the silicone sample.

Elastomeric replica

Thickness200 µm


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

Water uptake in time


A new synthetic substitute of human skin

2.8 mm

200 µm

200 µm

100 µm2.8 mm


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®


Human skinSynthetic skin

Friction results


Indentation results


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 wetThis 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)


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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