In vitro investigation of skin damage due to microscale shearing

Despite several studies dealing with the mechanical and tribological properties of skin, the majority of these investigations have been performed at macroscopic levels. However, because of the multilayer structure of skin, it is necessary to perform studies at microscopic scales to reveal the effect of individual layer constituents on the overall skin response to mechanical stimuli. To bridge the gap in knowledge of the micromechanical behavior of skin, a custom-made mechanical tester, optical microscopy, and cross-sectional histology were used to examine the deformation and tribological behavior of porcine skin subjected to various normal and shear loadings. Representative friction and wear results of skin tested under unidirectional and reciprocating (cyclic) shearing (scratching) conditions are interpreted in terms of the scratching speed, normal load, and number of scratch cycles to illustrate the effects of stratum corneum, cellular epidermis, and dermis on the friction and wear characteristics of skin. Depending on the normal load and scratch time (cycles), different friction mechanisms (i.e., adhesion, plowing, and squeeze-film lubrication) and wear mechanisms (i.e., surface plasticity/plowing, bulk shearing, cohesive failure, tearing, and delamination) were found to control shear-induced skin damage. The results of this study provide insight into microscale friction and wear processes influencing the mechanical response of skin subjected to normal and shear surface tractions.