The top 15μm of human skin, known as the stratum corneum (SC), has the essential role of providing a barrier between the human body and surrounding environment. This barrier plays multiple key functions including moderating the diffusion of water and other chemical species through the skin, protecting the skin from infectious microbes, and adding mechanical strength to the weaker underlying layers of the skin. A key measure of the driving force for damage of this critical barrier is the stress that develops in the skin during drying. By measuring the drying stress over time of SC samples exposed to different skin treatments and environmental conditions, we in the Dauskardt group are able to quantify the effects of these treatments on the integrity of the SC barrier.
Figure 1: Schematic showing the structure of skin and potential treatments such as sunscreen and UV exposure
While this technique is useful for probing the effects of simple molecules, the use of complex chemical formulae by skin care companies has created the need for a more nuanced understanding of the interactions the ingredients of these formulations have with each other and the SC structure while they diffuse into the skin. Confocal Raman spectroscopy is a useful technique for filling this gap in knowledge as it provides information about the structures of proteins and lipids in the SC, as well as how these structures change due to the presence of skin creams or other treatments.
Figure 2: Confocal Raman spectrum of human stratum corneum with some regions of interest indicated
In order to connect the changes in SC microstructure observed via confocal Raman measurements with the drying stress profile of SC, we have begun developing a model of the drying stress in the SC. Our model is based off of the diffusion of water and other treatment molecules through the SC, a process that is highly structure dependent. By studying the diffusion of chemical ingredients along with their effect on the structure and drying stress profile of the SC, we will arrive at a deeper understanding of the biomechanical mechanisms operating in the SC, which will be valuable in predicting the response of SC to exposure to any number of molecules.
Figure 3: Drying stress change and stress model plotted over 13 hours for a piece of initially fully hydrated SC placed in a dry environment