Investigation of differences in follicular penetration of particle- and nonparticle-containing emulsions by laser scanning microscopy

Hair follicles represent a long-term storage of topically applied drugs and cosmetics in the skin. Analyzing the penetration of particle- and nonparticle-containing formulations by laser scanning microscopy, it was found, surprisingly, that particles at a size similar to the thickness of the keratin cells of the hair penetrate more efficiently into the hair follicles. These results were obtained from in vitro and in vivo investigations. It seems that the moving hairs in the follicles act as a geared pump because of the zigzag structure of the surface of the hairs. This pumping effect probably pushes particles with the corresponding size deep into the hair follicles.