<i>In Vivo</i> Two-Dimensional Correlation Raman Spectroscopy Unveiling the Reversible Microstructural Changes and Spatially Differential Swelling of the Human Stratum Corneum in Response to Glycerin as a Temporary Penetration Enhancer

The human stratum corneum (SC) serves as a key barrier against exogenous molecules and is a living, heterogeneous system whose composition and microstructure vary along its depth. Noninvasive <i>in vivo</i> monitoring of its spatiotemporal microstructural changes is thus crucial yet challenging in many areas, such as transdermal drug delivery and cosmetics. Herein, we directly employed confocal Raman microspectroscopy on forearm skin to characterize the SC and its dynamic response to glycerin. To overcome low spectral resolution from signal overlap, two-dimensional correlation spectroscopy (2DCOS), moving window 2DCOS (MW2D), and perturbation-correlated MW2D (PCMW2D) were used to monitor the (1) trends and (2) sequence of Raman signal variations with depth. These served as unique Raman indicators to describe the multiscale structural features of SC, determine its thickness, and clarify glycerin's effects on lipid, keratin, and water molecular structures within SC. Further, a three-layer stratification of SC was proposed, revealing its spatially differential swelling upon adding glycerin. These glycerin-induced changes, from molecular structure to microscale swelling, would enhance the penetration of retinyl palmitate. Finally, the recovery of SC over time was assessed by the depth-dependent variations in the molecular conformations of SC components and the SC thickness, uncovering an unexpected application of glycerin as a temporary and mild skin penetration enhancer (SPE). Consequently, two-dimensional correlation Raman spectroscopy is promising in characterizing the SC and exploring qualified SPEs.