Infrared spectroscopy of the skin: influencing the stratum corneum with cosmetic products.

The stratum corneum is the outermost layer of the skin and, as such, represents the decisive barrier between the body and the environment. The combination of horny cells and lamellar lipid layers prevents water loss from the body and protects it against penetration by exogenous substances. For selective investigation of the thin outermost stratum corneum layer ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopy has proved itself in practice. It provides information about the sebum content, type of fatty substances, water content and degree of order of the lamellar lipid film in the stratum corneum. Different types of skin (dry, normal and greasy) differ mainly in their sebum content but also in the composition of their fats and the degree of order of their lipids. A comparison with model lipid mixtures shows that the stratum corneum lipids are mainly present in a fixed gel form under physiological conditions. In cleansing processes a large part of the sebum is removed from the skin. The skin reacts to the defatting process by the rapid secretion of sebum. After cleansing with a mild face cleansing emulsion the re-achievement of the same fat status takes longer than with a surfactant shower gel. Skin creams cause alterations to the skin moisture and in the degree of order of the stratum corneum lipids. A lamellar cream with a structure similar to that of the stratum corneum lipids increases the degree of order of the alkyl chains of the skin lipid film (biomimetic principle), whereas a conventional w/o cream reduces this degree of order. Skin moisture increases after the use of the creams.