The skin barrier function is mostly provided by the stratum corneum (SC), the uppermost layer of the epidermis. To noninvasively analyze the physiological properties of the skin barrier function in vivo, it is important to determine the SC thickness. Confocal Raman microscopy (CRM) is widely used for this task. In the present in vivo study, a new method based on the determination of the DNA concentration profile using CRM is introduced for determining the SC thickness. The obtained SC thickness values are compared with those obtained using other CRM-based methods determining the water and lipid depth profiles. The obtained results show almost no significant differences in SC thickness for the utilized methods. Therefore, the results indicate that it is possible to calculate the SC thickness by using the DNA profile in the fingerprint region, which is comparable with the SC thickness calculated by the water depth profiles (ANOVA test p = 0.77) and the lipid depth profile (ANOVA test p = 0.74). This provides the possibility to measure the SC thickness by using the DNA profile, in case the water or lipid profile analyses are influenced by a topically applied formulation. The increase in DNA concentration in the superficial SC (0-2 µm) is related to the DNA presence in the microbiome of the skin, which was not present in the SC depth below 4 µm.

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.