Transdermal refers to the process in which the drug diffuses through the intact skin for the purpose of systematic distribution (as opposed to topical distribution). In order to achieve this route of administration, utilities such as transdermal patches, transdermal gels, and transdermal implants can be used for medical purposes.
The transcellular pathway is the more direct path in which drugs can make its way across the skin. In this route, drugs are transported across the skin by proceeding through the membranes of dead kertinocytes (the predominatn cell type within the epidermis) that make up the top layer of skin, which is known as the stratum corneum. Even though the path distance for this method is the shortest, drugs can experience resistance due to the phospholipid membranes that they have to cross.
The intercellular pathway is the more common pathway in which the drugs pass through the skin by navigating through small spaces that exist between the cells of the skin.
Even though the skin is easily accessible for the purpose of drug delivery, its main function of preventing foreign substances from entering the body can limit the types and amounts of drugs that are taken in through this route of administration. In order to reach the microcirculation of the dermis, the drug must manage to go through the Epidermis and the Dermis, which are two significant layers of the skin.
- McCarley, K.D & Bunge, A.L. (2001). "Review of pharmacokinetic models of dermal absorption." J Pharmaceut Sci. 90: 1699–1719.
- Hadgraft, J. (2001). "Modulation of the barrier function of the skin." Skin Pharmacol Appl Skin Physiol. 14(1): 72-81.