Skin Microstructure is a Key Contributor to Its Friction Behaviour

Due to its multifactorial nature, skin friction remains a multiphysics and multiscale phenomenon poorly understood despite its relevance for many biomedical and engineering applications (from superficial pressure ulcers, through shaving and cosmetics, to automotive safety and sports equipment). For example, it is unclear whether, and in which measure, the skin microscopic surface topography, internal microstructure and associated nonlinear mechanics can condition and modulate skin friction. This study addressed this question through the development of a parametric finite element contact homogenisation procedure which was used to study and quantify the effect of the skin microstructure on the macroscopic skin frictional response. An anatomically realistic two-dimensional image-based multilayer finite element model of human skin was used to simulate the sliding of rigid indenters of various sizes over the skin surface. A corresponding structurally idealised multilayer skin model was also built for comparison purposes. Microscopic friction specified at skin asperity or microrelief level was an input to the finite element computations. From the contact reaction force measured at the sliding indenter, a homogenised (or apparent) macroscopic friction was calculated. Results demonstrated that the naturally complex geometry of the skin microstructure and surface topography alone can play as significant role in modulating the deformation component of macroscopic friction and can significantly increase it. This effect is further amplified as the ground-state Young’s modulus of the stratum corneum is increased (for example, as a result of a dryer environment). In these conditions, the skin microstructure is a dominant factor in the deformation component of macroscopic friction, regardless of indenter size or specified local friction properties. When the skin is assumed to be an assembly of nominally flat layers, the resulting global coefficient of friction is reduced with respect to the local one. This seemingly counter-intuitive effect had already been demonstrated in a recent computational study found in the literature. Results also suggest that care should be taken when assigning a coefficient of friction in computer simulations, as it might not reflect the conditions of microscopic and macroscopic friction one intends to represent. The modelling methodology and simulation tools developed in this study go beyond what current analytical models of skin friction can offer: the ability to Electronic supplementary material The online version of this article (doi:10.1007/s11249-016-0794-4) contains supplementary material, which is available to authorized users. & Georges Limbert [email protected] 1 National Centre for Advanced Tribology at Southampton (nCATS), Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK 2 Bioengineering Science Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK 3 Institute of Fundamental Technological Research, Polish Academy of Sciences (IPPT PAN), ul. Pawinskiego 5B, 02-106 Warsaw, Poland 4 Biomedical Imaging Unit, Faculty of Medicine, University of Southampton, Southampton General Hospital, Southampton SO16 6YDJ, UK 5 Computational Engineering and Design Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK 6 Laboratory of Biomechanics and Mechanobiology, Division of Biomedical Engineering, Department of Human Biology, Faculty of Health Sciences, University of Cape Town, Observatory, Cape Town 7935, South Africa 123 Tribol Lett (2017) 65:12 DOI 10.1007/s11249-016-0794-4