Lipid organization and stratum corneum thickness determined in vivo in human skin analyzing lipid-keratin peak (2820-3030 cm−1) using confocal Raman microscopy

While the intercellular lipid structure of the stratum corneum (SC) plays an important role in the skin barrier function, the depth‐dependent profile of the intercellular lipids contributes decisively to deepen the understanding of the skin barrier function, drug penetration, development of skin diseases and their medication. The depth‐dependent profile of the lipids' chemico‐physical properties, such as the solid–fluid phase transition and the order–disorder transition, can exclusively be measured in human skin in vivo by means of confocal Raman microscopy. In the present paper, the lipid–keratin peak (2820–3030 cm−1) was investigated. The lipid‐related Raman peaks centered at 2850 cm−1 and 2880 cm−1 were deconvoluted using Gaussian functions and investigated for their depth‐dependent shape and positional changes. Different fitting procedures show that even an additional Gaussian function cannot be used to fully characterize the lipid's polymethylene chains around 2880 cm−1, which justifies the introduction of the sharpness of the peak centered near 2880 cm−1. The results show that the 2880 cm−1 peak sharpness might be used for determining the SC thickness. The concentration of the lipids with long‐chain carbon backbone (free fatty acids and ceramides) semi‐quantitatively decreases from 10 µm to 20 µm (SC thickness is 19.8 µm). The maximum position and broadness of the Gaussian peak centered at 2850 cm−1 show that near the surface and in the deeper layers of the SC, the state of the lipids is more fluid and disordered compared to the medium layers of the SC. Copyright © 2016 John Wiley & Sons, Ltd. In healthy human skin, the sharpness of the peak centered near 2880 cm−1, which characterizes the polymethylene backbone of long‐chain lipids, might be used for determining the stratum corneum thickness. The maximum position and broadness of the Gaussian peaks centered at 2850 cm−1 show that near the surface and in the deep‐located layers of the stratum corneum, the state of the lipids is more fluid and disordered compared to the medium layers (20–40% of the stratum corneum thickness).