Quantification of the Inhomogeneous Distribution of Topically Applied Substances by Optical Spectroscopy: Definition of a Factor of Inhomogeneity

The inhomogeneous distribution of topically applied substances due to decisive differences in the skin structure (furrows and wrinkles) affects the efficacy of cosmetic products, in particular sunscreens. The combination of tape stripping and optical spectroscopy results in absorption data, which reflect ex vivo the inhomogeneity of the in vivo distribution of topically applied substances. Based on these data, a factor of inhomogeneity is defined describing the individual distribution of formulations on the skin surface of volunteers. Thus, the influence of different skin surface structures and the influence of different formulations on the distribution of the topically applied substances can be determined. Analyzing the inhomogeneity data on 6 volunteers (5 sunscreens per volunteer), it was found that the influence on the distribution of sunscreens caused by the formulation was higher than the inhomogeneity originating from the differences in the skin surface structure of the volunteers. The method is well suited to characterize, for example, sunscreens and antiaging creams in the process of development, as well as for the evaluation of the final products.