Penetration and storage of particles in human skin: perspectives and safety aspects.

The application of particles in dermatology and cosmetology represents an emerging field and is closely connected with the question of risk assessment as the potential for, and consequences of, penetration of such particles into the living tissue has not been determined conclusively. In the medical sector, extensive research activities are in progress to develop particles, which can be used as efficient carriers for drug delivery through the skin barrier. In contrast, in cosmetic products, particles are mostly required to remain on the skin surface to fulfill their beneficial effect. Whereas the intercellular penetration of particles seems to be unlikely, the hair follicle has been shown to be a relevant penetration pathway for particles as well as an important long-term reservoir. It has been demonstrated that the penetration depth of the particles can be influenced by their size resulting in the possibility of a differentiated targeting of specific follicular structures. In the present review, the follicular penetration mechanisms and storage properties of particles are discussed.