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Skin plays a protective role against the loss of water and external aggression, including mechanical stresses. These crucial functions are ensured by different cutaneous layers, particularly the stratum corneum (SC). During aging, the human skin reveals some apparent modifications of functionalities such as a loss of elasticity. Our investigations aimed at demonstrating that Raman microspectroscopy, as a label-free technique with a high molecular specificity, is efficient to assess in vivo the molecular composition of the skin and the alterations underwent during aging. Our approach was based on a search for correlation between Raman data collected on healthy female volunteers of different ages (from 21 to 70 years old) by means of a remote confocal Raman and skin firmness measurements used as a reference method. Raman and biometric data were then submitted to a partial least square (PLS)-based data processing. Our experiments demonstrated the potential of Raman microspectroscopy to provide an objective in vivo assessment of the skin "biological age" that can be very different from the "chronological age" of the person. In addition, Raman features sensitive to the elasticity and the fatigability of the SC were highlighted. Thereafter, calibration transfer functions were constructed to show the possibility to compare the results obtained during two distinct measurement campaigns conducted with two Raman probes of the same conception. This approach could lead to several interesting prospects, in particular by objectifying the effects of dermocosmetic products on the superficial layers of the skin and by accessing some underlying molecular mechanisms.

During chronological skin aging, alterations in dermal structural proteins cause morphological modifications. Modifications are probably due to collagen fiber (type I collagen) rearrangement and reorientation with aging that have not been researched until now. FTIR microspectroscopy appears as an interesting method to study protein structure under normal and pathological conditions. Associated with a polarizer, this vibrational technique permits us to probe collagen orientation within skin tissue sections, by computing the ratio of integrated intensities of amide I and amide II bands. In this study, we used the polarized-FTIR imaging to evaluate molecular modifications of dermal collagen during chronological aging. The data processing of polarized infrared data revealed that type I collagen fibers become parallel to the skin surface in aged skin dermis. Our approach could find innovative applications in dermatology as well as in cosmetics.