Percutaneous absorption: theoretical description.

Equations are derived to describe the percutaneous absorption of a substance through the epidermal barrier. The treatment includes interfacial barriers and allows for the depletion of the substance in the external phase. The equations are derived both for the continuous application and for pulse experiments where the drug is applied for a time, then removed, and the response occurs some time after the removal of the drug. Competition between the drug diffusing through the keratinized cells (transcellular route) and diffusing in the interstitial channels around the cells (intercellular route) is also considered.