Three-dimensional chemical imaging of skin using stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) microscopy is used to generate structural and chemical three-dimensional images of native skin. We employed SRS microscopy to investigate the microanatomical features of skin and penetration of topically applied materials. Image depth stacks are collected at distinct wavelengths corresponding to vibrational modes of proteins, lipids, and water in the skin. We observed that corneocytes in stratum corneum are grouped together in clusters, 100 to 250 μm in diameter, separated by 10- to 25-μm-wide microanatomical skin-folds called canyons. These canyons occasionally extend down to depths comparable to that of the dermal-epidermal junction below the flat surface regions in porcine and human skin. SRS imaging shows the distribution of chemical species within cell clusters and canyons. Water is predominately located within the cell clusters, and its concentration rapidly increases at the transition from stratum corneum to viable epidermis. Canyons do not contain detectable levels of water and are rich in lipid material. Oleic acid-d34 applied to the skin surface lines the canyons down to a depth of 50 μm below the surface of the skin. This observation could have implications on the evaluation of penetration profiles of bioactive materials measured using traditional methods, such as tape-stripping.