Percutaneous absorption after twenty-five years: or "old wine in new wineskins"

Two developments have greatly added to our knowledge of percutaneous absorption over the past quarter century: (1) proof that the entire stratum corneum is the effective barrier layer of the skin, and (2) the advent of highly sensitive analytical techniques. The first enabled us to measure permeation rates on excised skin and the second greatly extended the kinds of substances that could be studied in vivo and in vitro. The process of percutaneous absorption is beginning to be understood more quantitatively. New light has been shed on the elementary mechanisms of permeation, the site of diffusional resistance at the molecular level, the effect of specific functional groups on solute permeation, the role of skin appendages, and the effects of vehicles.