Tribology of Skin: Review and Analysis of Experimental Results for the Friction Coefficient of Human Skin

Abstract

In this review, we discuss the current knowledge on the tribology of human skin and present an analysis of the available experimental results for skin friction coefficients. Starting with an overview on the factors influencing the friction behaviour of skin, we discuss the up-to-date existing experimental data and compare the results for different anatomical skin areas and friction measurement techniques. For this purpose, we also estimated and analysed skin contact pressures applied during the various friction measurements. The detailed analyses show that substantial variations are a characteristic feature of friction coefficients measured for skin and that differences in skin hydration are the main cause thereof, followed by the influences of surface and material properties of the contacting materials. When the friction coefficients of skin are plotted as a function of the contact pressure, the majority of the literature data scatter over a wide range that can be explained by the adhesion friction model. The case of dry skin is reflected by relatively low and pressureindependent friction coefficients (greater than 0.2 and typically around 0.5), comparable to the dry friction of solids with rough surfaces. In contrast, the case of moist or wet skin is characterised by significantly higher (typically [1) friction coefficients that increase strongly with decreasing contact pressure and are essentially determined by the mechanical shear properties of wet skin. In several studies, effects of skin deformation mechanisms contributing to the total friction are evident from friction coefficients increasing with contact pressure. However, the corresponding friction coefficients still lie within the range delimited by the adhesion friction model. Further research effort towards the analysis of the microscopic contact area and mechanical properties of the upper skin layers is needed to improve our so far limited understanding of the complex tribological behaviour of human skin.

Skin tribology: Science friction?

“…[4] Location at human body * Counter surface μ dynamic Remarks Asserin et al [26] Forearm (V) Ruby 0. 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].…”

“…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].…”

A mechanistic insight into the mechanical role of the stratum corneum during stretching and compression of the skin

“…The cells eventually die, release their glycolipids into the intracellular space, become flat and finally keratinise to form the stratum corneum (Marieb and Hoehn, 2010;Shimizu, 2007). These strong keratinised cells, bonded by desmosomes, form the so-called "brick and mortar" structure (Derler and Gerhardt, 2012;Geerligs et al, 2011) that characterises the stratum corneum as a stiff mechanical barrier. In a biotribological context, the nature of physical interactions between the skin and the external environment is strongly conditioned by the propensity of the stratum corneum to absorb liquids such as water and lubricants (Bhushan, 2012;Wu et al, 2006) which modify its mechanical and chemical properties, which in turn affect the physical appearance of the skin surface (Raab, 1990) and its mechanical response to deformations.…”

Comparison of the Friction Behavior of Occluded Human Skin and Synthetic Skin in Dry and Moist Conditions

“…The material used for the commercial SynTissue™ synthetic skin tested here is unknown. However, the dielectric constants of common polymers that have been used elsewhere in synthetic skin substitutes such as polyurethane elastomer (4-10) [26] and silicone rubber (3)(4)(5)(6)(7)(8)(9)(10) [27] are significantly less than water at about 80. The dielectric constant of SynTissue™ is reported by the manufacturer to be validated against that of human skin [24].…”

“…Because skin-contacting devices may be used in different climates, it is important that the synthetic skin model is capable of representing human skin friction behaviour under different environmental and other usage conditions. In a humid climate the skin hydration ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 3 increases and the friction can be dramatically higher than with dry skin conditions; an increase of up to a factor 10 in coefficient of friction is not unusual [6][7][8].…”