Effect of detergents on the physicochemical properties of skin stratum corneum: a two-photon excitation fluorescence microscopy study.

OBJECTIVE: Understanding the structural and dynamical features of skin is critical for advancing innovation in personal care and drug discovery. Synthetic detergent mixtures used in commercially available body wash products are thought to be less aggressive towards the skin barrier when compared to conventional detergents. The aim of this work is to comparatively characterize the effect of a mild synthetic cleanser mixture (SCM) and sodium dodecyl sulphate (SDS) on the hydration state of the intercellular lipid matrix and on proton activity of excised skin stratum corneum (SC). METHOD: Experiments were performed using two-photon excitation fluorescence microscopy. Fluorescent images of fluorescence reporters sensitive to proton activity and hydration of SC were obtained in excised skin and examined in the presence and absence of SCM and SDS detergents. RESULTS: Hydration of the intercellular lipid matrix to a depth of 10 µm into the SC was increased upon treatment with SCM, whereas SDS shows this effect only at the very surface of SC. The proton activity of SC remained unaffected by treatment with either detergent. CONCLUSION: While our study indicates that the SC is very resistant to external stimuli, it also shows that, in contrast to the response to SDS, SCM to some extent modulates the in-depth hydration properties of the intercellular lipid matrix within excised skin SC.

Surfactant treatments influence drying mechanics in human stratum corneum.

We describe a high-throughput method capable of quantifying the elastic modulus and drying stress of ex vivo samples of human stratum corneum. Spatially resolved drying deformations in circular tissue samples are measured, azimuthally averaged and fitted with a profile based on a linear elastic model. Our approach enables the comparison of the physical effects of different cleansers. We find that cleansing can cause dramatic changes to the mechanical properties of stratum corneum. In some cases, cleansing can lead to an order of magnitude increase in elastic modulus and drying stress. We expect that these mechanical properties have a direct impact on cracking and chapping skin as well as the milder sensation of perceived tightness often experienced after washing. Mechanical drying studies are also combined with drop wetting studies and pyranine staining experiments. This combination of techniques allows one to establish a multidimensional profile of stratum corneum including stiffness, susceptibility to drying, hydrophilicity and barrier functionality.

Heterogeneous drying stresses in stratum corneum.

We study the drying of stratum corneum, the skin's outermost layer and an essential barrier to mechanical and chemical stresses from the environment. Even though stratum corneum exhibits structural features across multiple length-scales, contemporary understanding of the mechanical properties of stratum corneum is based on the assumption that its thickness and composition are homogeneous. We quantify spatially resolved in-plane traction stress and deformation at the interface between a macroscopic sample of porcine stratum corneum and an adherent deformable elastomer substrate. At length-scales greater than a millimeter, the skin behaves as a homogeneous elastic material. At this scale, a linear elastic model captures the spatial distribution of traction stresses and the dependence of drying behavior on the elastic modulus of the substrate. At smaller scales, the traction stresses are strikingly heterogeneous and dominated by the heterogeneous structure of the stratum corneum.