Combining confocal Raman microscopy and freeze-drying for quantification of substance penetration into human skin

In the area of dermatological research, the knowledge of rate and extent of substance penetration into the human skin is essential not only for evaluation of therapeutics, but also for risk assessment of chemicals and cosmetic ingredients. Recently, confocal Raman microscopy emerged as a novel analytical technique for analysis of substance skin penetration. In contrast to destructive drug extraction and quantification, the technique is non-destructive and provides high spatial resolution in three dimensions. However, the generation of time-resolved concentration depth profiles is restrained by ongoing diffusion of the penetrating substance during analysis. To prevent that, substance diffusion in excised human skin can instantly be stopped at defined time points by freeze-drying the sample. Thus, combining sample preparation by freeze-drying with drug quantification by confocal Raman microscopy yields a novel analytical platform for non-invasive and quantitative in vitro analysis of substance skin penetration. This work presents the first proof-of-concept study for non-invasive quantitative substance depth profiling in freeze-dried excised human stratum corneum by confocal Raman microscopy.