The barrier function of the skin in relation to percutaneous absorption of drugs.

There is currently a high level of interest in using the skin as a route for delivering drugs. The skin, however, provides an efficient barrier against percutaneous absorption of drugs. This barrier function can be ascribed to the macroscopical structure of the stratum corneum, which consists of alternating lipoidal and hydrophylic regions. For this reason, physico-chemical characteristics of the drug, such as partition coefficient and molecular weight, play an important role in determining the facility of percutaneous absorption. Another factor to consider in transdermal drug delivery, is the vehicle in which the drug is formulated as it acts on the release of drug from the formulation. Moreover, vehicles may also interact with human stratum corneum, thereby affecting its barrier function. Surfactants and penetration enhancers are well-known examples of the latter. Subsequently, dosing conditions, such as humidity, temperature and occlusion, also have their impact on the actual input (rate) of drug through human skin. Finally, all bits of information are combined to form a reasonably faithful picture of percutaneous absorption.