Changes in the three-dimensional microscale topography of human skin with aging impact its mechanical and tribological behavior

Abstract

Human skin enables interaction with diverse materials every day and at all times. The ability to grasp objects, feel textures, and perceive the environment depends on the mechanical behavior, complex structure, and microscale topography of human skin. At the same time, abrasive interactions, such as sometimes occur with prostheses or textiles, can damage the skin and impair its function. Previous theoretical and computational efforts have shown that skin’s surface topography or microrelief is crucial for its tribological behavior. However, current understanding is limited to adult surface profiles and simplified two-dimensional simulations. Yet, the skin has a rich set of features in three dimensions, and the geometry of skin is known to change with aging. Here we create a numerical model of a dynamic indentation test to elucidate the effect of changes in microscale topography with aging on the skin’s response under indentation and sliding contact with a spherical indenter. We create three different microrelief geometries representative of different ages based on experimental reports from the literature. We perform the indentation and sliding steps, and calculate the normal and tangential forces on the indenter as it moves in three distinct directions based on the characteristic skin lines. The model also evaluates the effect of varying the material parameters. Our results show that the microscale topography of the skin in three dimensions, together with the mechanical behavior of the skin layers, lead to distinctive trends on the stress and strain distribution. The major finding is the increasing role of anisotropy which emerges from the geometric changes seen with aging.

Fractal surface-based three-dimensional modeling to study the role of morphology and physiology in human skin friction

“…It should be noted that to account for the effect of humidity, we only consider a change the Youngʼs modulus of the SC in this study. Though, this is consistent with the previously conducted mechanical modeling of human skin in [4,[29][30][31], in practice, a change in the humidity can also affect the chemical bonds at molecular level [66] and physiological properties of skin like lipid barriers [67]. Therefore, while our results point to the importance of humidity as a factor affecting skin friction, the question of humidity needs to be investigated further.…”

“…Limbert and Kuhl [46] use this material model to understand the effect of compression induced wrinkling using a three-dimensional model. Recently Diosa et al [31] use this approach for three-dimensional modeling of human skin contact with a spherical indenter.…”

“…They find that skin roughness and its layered structure indeed influence its contact behavior. Diosa et al [31] show the importance of roughness in how aging impacts the mechanical and tribological behavior of human skin using an image-based computational model. Given the randomness of roughness over skin and the complexities involved in obtaining actual skin surfaces in large numbers, representative models of rough surfaces come as a convenient tool.…”

RVE based Finite Element Modelling of the Contact Mechanics Between Skin and Indenter

“…The stratum corneum (SC), being the topmost layer of skin, has a major role in the contact mechanics between skin and object. Though it has the least thickness (≈20 μ m ) (Diosa, Moreno, Chica, Villarraga, & Tepole, 2021), it has a considerably higher elastic modulus compared to other layers of skin (Van Kuilenburg, Masen, & Van Der Heide, 2013; Diosa, Moreno, Chica, Villarraga, & Tepole, 2021). Further, its mechanical properties vary over a large range and humidity or water content plays a big role in determining its behavior (Kim & Yun, 2019; Dzidek, Bochereau, Johnson, Hayward, & Adams, 2017; Mojumdar, Pham, Topgaard, & Sparr, 2017; Bernard, et al, 2001; Derler, Rossi, & Rotaru, 2015; Tomlinson, Lewis, Liu, Texier, & Carré, 2011).…”

“…Among the various in-silico modelling approaches, finite element models have been extensively used over the past decade to study the biomechanics of skin (Flynn & McCormack, Simulating the wrinkling and aging of skin with a multi-layer finite element model, 2010; Leyva-Mendivil, Page, Bressloff, & Limbert, 2015; Leyva-Mendivil, Lengiewicz, Page, Bressloff, & Limbert, 2017; Santoprete & Querleux, 2014; Nandamuri, 2016; Diosa, Moreno, Chica, Villarraga, & Tepole, 2021) for a variety of applications including wrinkling, subcutaneous drug delivery, wound healing and robotics assisted surgery. Recent advancements in the development of commercial and open-source FEM packages have allowed researchers to develop robust in-silico models of skin that can a) include property variation across different skin layers b) model contact mechanics between skin and indenter for large deformations and include microscopic details of skin structure into the modelling framework.…”