Quantification of skin penetration of caffeine and propylene glycol applied topically in a mixture by tailored multivariate curve resolution-alternating least squares of depth-resolved Raman spectra

The quantitative determination of topically applied substances in the skin is severely limited and represents a challenging task. The porcine skin ex vivo was topically treated with a gel containing caffeine (CF) and propylene glycol (PG), and depth-resolved Raman spectra were recorded with two confocal Raman micro-scopes. We applied a novel tailored multivariate curve resolutional ternating least squares method to the selected spectral regions (512-604 and 778-1148 cm(-1)) of gel-treated skin and quantitatively determined the concentrations of CF and PG in the stratum corneum (SC). The highest concentration of CF (181 mg/cm(3)) was found at the surface, while PG (384 mg/cm(3)) was found at 10% SC depth, indicating the formation of a reservoir at the superficial SC. The concentrations of CF and PG decreased monotonically and reached the detection limit at approximate to 60% and approximate to 80% SC depth, respectively, indicating that neither permeate the SC.

Automated summary

This study quantitatively determined the distribution of topically applied substances in a pig skin model using a new method, revealing that the concentration of caffeine was highest at the surface, while propylene glycol reached its highest concentration at a depth of 10% in the skin. Both substances showed a monotonic decrease in concentration in deeper layers of the skin, reaching the detection limit at around 60% and 80% depth in the stratum corneum, respectively.