ABSTRACT
Stratum corneum (SC), the typically 15 micron-thick outermost layer of the skin on most parts of the body except palms and soles can be mechanically described as a brick and mortar structure corresponding to a stiff tissue composed of a series of layers of anucleated dead cells, the corneocytes, embedded in an intercellular lipid organization. SC cohesiveness between corneocytes is due to proteic structures named corneodesmosomes.After reviewing its morphological and mechanical properties at different scales, this chapter details the structurally and physically-based model we have developed. A particular focus is given on the level of complexity of the numerical model regarding the availability and variability of the experimental data.Results mainly concern (i) a method to extract the unknown mechanical properties of certain SC components by a reverse engineering approach in good agreement with experimental data
obtained on other epithelial tissues providing a first validation of the model, (ii) a quantitative analysis of the relative impact of the three major SC components upon its mechanical properties at the macroscopic scale, demonstrating that corneodesmosomes should have the largest impact, a few times higher than the impact of lipids, while the corneocytes should have a rather smaller impact; (iii) a better understanding of the role of relative humidity (RH) on SC mechanical properties, by demonstrating that the intercellular spaces, and in particular the corneodesmosomes, are the main responsible for the observed variation of the SC Young’s modulus. All these results will give us a better insight on the impact of hydration on SC mechanical properties at different scales.In brief, our approach aims at providing additional information on new targets to be addressed for better treating some skin diseases as well as designing new skincare products. 6.1 IntroductionOne major function of the skin is to act as a protective barrier toward environmental factors such as physicochemical penetration of exogenous compounds, microbial invasion, or mechanical insults [1,2]. If this latter aspect is related to the biomechanical properties of the skin, increasing our understanding in this domain also concerns their changes with intrinsic factors such as aging [3,4], ethnic origin [5,6] and/or external environmental factors such as sun exposure [7,8]. It should also be noticed that many dermatological disorders often lead to alterations in the biomechanical properties of the skin [9-11].Many authors have reported on skin biomechanics using experimental methods [12,13], or numerical modeling [14-16]. Recent results have demonstrated the important role of the outermost layer, the stratum corneum, on the mechanical properties of the whole skin [17-20].This chapter is devoted to the study of the mechanical properties of the SC, and in particular their respective dependence on the morphological and mechanical properties of its main constituents at a cellular scale. It has to be acknowledged that related literature is rather scarce mainly due to the difficulty in assessing the
mechanical properties of the constituents and relate them to overall tissue properties.The approach chosen here aims at combining mechanical and morphological experimental data at different scales to a multi-scale biomechanical model. The method allows us to (i) extract the unknown mechanical properties of certain constituents by inverse analysis, (ii) provide a quantitative link between constituent’s prop-erties at a cellular scale and the overall properties of macroscopic tissue, and (iii) better understand the role of relative humidity (RH) on SC mechanical properties and, at the same time, to better highlight the effect of some common moisturizers.
