Cleansers' mildness: Stratum corneum lipid organization and water uptake after a single wash

The modern marketplace for personal cleansing demands effective but also gentle products. The acute effects of skin exposure to personal cleansers are a key driver in product mildness. Yet many current clinical methods of assessing this mildness rely upon exaggerated test conditions that do not match the brief contact time with the skin in typical use. Here, we present an approach to quantitatively measure changes in composition, water uptake, and spacing of lipid lamellar structures in human stratum corneum after a single similar to 2-minute wash protocol. The approach is based upon in vivo confocal Raman spectroscopy and small-angle X-ray scattering (SAXS) of ex vivo stratum corneum. Raman data, collected using cleansers with varying degrees of harshness, reveal changes in water uptake as a function of skin depth and detect depth of damage upon cleansing, a conduit to barrier integrity. By treating isolated human stratum corneum with cleansers for similarly short exposures, SAXS reveals structural perturbations associated with interactions of surfactants and skin lipid lamellar structure, also a measure of barrier strength. Disruption of the characteristic periodicity phases of the lipid lamellae or formation of new phases with intermediate periodicity due to surfactant intercalation are observed for different formulations. This approach provides molecular scale characterization of skin-product interactions and resulting skin barrier effects in more realistic consumer treatment conditions. It also provides feedback about cleansers' mildness in a rapid fashion and at lower cost, widening a bottleneck of identification and assessment of new ingredients in the early stages of formula design.