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Biomechanics of human stratum corneum [electronic resource] : dry skin conditions, tissue damage and alleviation

The outermost layer of human skin, the stratum corneum (SC), is subject daily to variable ambient moisture and temperature conditions as well as application of potentially damaging cleansing agents. The inevitable results of these exposures are "tightness" of the skin which is directly related to the buildup of tensile residual drying stresses in the SC layer. In this work, we first describe the application of the substrate curvature technique to quantitatively measure the magnitude of these stresses and their relationship to selected drying environments and times. The SC drying stresses were observed to be very sensitive to the relative humidity and temperature of the drying environment as well as harshness of the chemical treatment. There was a strong correlation with the SC drying stresses and the chemical potential of water in the drying environment. The evolution of drying stresses in SC is discussed in relation to the effects of hydration and damage caused by chemical treatments on the underlying SC structure. We also describe the application of the substrate curvature technique to characterize stresses in occlusive topical coatings. We then extend the substrate curvature technique to measure the combined effects of the coating applied to human stratum corneum (SC) where the overall drying stresses may have contributions from the coating, the SC and the interaction of the coating with the SC. We show how these separate contributions in the coating and SC layers can be differentiated. Using this methodology, we characterize the effect of a range of moisturizing treatments on the drying stresses in human stratum corneum. Following moisturizer treatment, the SC was observed to have distinctive stress profiles with drying time depending on the effectiveness of the treatment. The stress values of specimens treated with the humectant moisturizers were observed to increase and stabilize after a few hours in the drying environment where they remained relatively constant until the end of exposure to the drying environment whereas the stress values of specimens treated with the emollient treatments were observed to rise rapidly to a peak stress value and relax to a final stress value. The effect of moisturizing treatments on the SC drying stresses was rationalized in terms of SC water loss and the chemical state of the SC components. Finally, we employ a fracture mechanics approach to understand the implications of the drying stresses in SC as a mechanical driving force for damage propagation (e.g. cracking and chapping) in the tissue. The crack driving force G was found for several cracking configurations and compared with the intercellular delamination energy, Gc, which is a property of the tissue that provides a measure of the resistance to cracking. Using this approach, we demonstrate how damaging treatments enhance and moisturizing treatments alleviate the propensity for dry skin damage.