tMCR-ALS method for the determination of water concentration profiles in the stratum corneum of untreated and treated skin in vivo

Abstract

Skin moisturizing is important in dermatology and cosmetics. Raman microspectroscopy is a well-suited method to determine water concentration profiles in the stratum corneum (SC) of untreated and formulation-treated skin in vivo. Until now, the water in the SC was quantified using the ratio of water to proteins, and other SC components were neglected. Here, we apply the tailored multivariate curve resolution-alternating least squares (tMCR-ALS) method to improve the quantitative determination of water in the SC of cosmetic oil-treated skin accounting Raman spectra of proteins, lipids, and oils in the high wavenumber region. We compared the water concentration profiles in the SC of petrolatum-treated skin calculated using the tMCR-ALS method and two existing conventional and extended methods, which are both based on the analysis of the water-to-protein-ratio. The results show that in petrolatum-treated skin, the conventional method calculates an incorrect water profile in the depth of 0-70% of the SC thickness; the extended method shows an incorrect profile in the depth of 0-10% of the SC thickness; the applied tMCR-ALS method show results, which coincide well with the modern knowledge of the moisturizing effect of cosmetic oils on the SC. Almond, jojoba, and paraffin oils exhibit a similar moisturizing effect that manifests itself in an increase of water concentration in the intermediate SC depth. However, the effect of petrolatum is more pronounced. In the context of treated skin, the proposed method calculates water concentration profiles throughout the SC correctly and thus has a great potential in dermatology and cosmetics.