Studying the effects of suberythemal UV doses on the human stratum corneum by in vivo confocal Raman spectroscopy

The stratum corneum (SC) plays an important role in skin barrier function. It acts as a protective barrier against water loss, eliminates foreign substances and micro-organisms and acts against harmful effects of UVR.Our aim was to study the impact of suberythemal doses of UVA and UVB exposure on the molecular structure, organization and barrier function of the SC by following different Raman descriptors.Twenty female volunteers, aged 20-30 years, with healthy skin were enrolled. Doses of 95 mJ/cm² UVA and 15 mJ/cm² UVB were applied to volunteers’ forearms. In vivo Raman measurements were performed at irradiated and control regions.The impact of UVA and UVB irradiation was observed following several Raman descriptors, i.e. the ratio of vasymCH2/vsymCH2 (2885 cm-1/2850 cm-1) corresponding to the organizational order of the lipid bilayer. Water content and mobility descriptors were obtained by calculating vOH/vCH ratio. Finally, protein secondary structure was evaluated based on the 1670 cm-1/1650 cm-1 ratio related to β sheets and α helices, respectively.UVA induced a loosening of the lateral packing of lipids immediately after irradiation. In contrast, delayed impact caused a tightening of the lipid barrier, an increase in water content -mainly in the unbound water fraction and a higher relative amount of β sheets in SC proteins. Overall, these observations may explain the thickening of the SC observed in previous studies. A UVB dose of 15 mJ/cm² was apparently below the threshold necessary to induce significant changes despite the trends observed in this study.