Mimicking the tribo-mechanical performance of human skin: a scale dependent approach based on poly (vinyl alcohol) hydrogel

The development of an appropriate substitute to simulate the frictional performance of human skin at different conditions is required for the design and optimization of products in contact with the skin. With this purpose, the composition, structure and mechanical properties of the skin need to be analyse to produce a suitable surrogate. In addition, the frictional performance of human skin is subjected to the influence of several factors, such as the surface condition, hydration level or the operational conditions. Thus, the influence of these factors in the frictional performance of an artificial skin substitute must be evaluated and the results compared to in vivo and/or ex vivo skin results. Therefore, this thesis presents a skin substitute with a layered structure and similar tribo-mechanical performance as the human skin. The mechanical model is composed of two layers with different composition and properties. The top layer is a thin film of Poly (vinyl alcohol) mixed with rapeseed oil and crosslinked with Glutaraldehyde which exhibits similar roughness and adhesive properties as the human skin. The bottom layer is made of a water solution of PVA crosslinked by freezing and thawing cycles which displays similar viscoelastic properties as the human dermis. The validation of the mechanical model was done in comparison to ex vivo skin results from abdominal skin and other results on in vivo skin from the literature. Additionally, the influence of the layered structure of the skin and the effect of the capillary forces in the frictional behavior of the skin were introduced as an input in the classic Hertz's model and the modified JKR model. The comparison of the theoretical and the experimental results on ex vivo skin indicated two different frictional regimes due to the relative influence of the adhesive and the elastic properties of the skin depending on the applied force. Moreover, the effect of the capillary forces introduced slight changes at wet conditions which effect on skin friction, especially at low forces, should be analysed further