Skin is the largest organ and represents the body’s protective surface as the first contact site for topically applied substances. It plays a key role in controlling not only the penetration and distribution but also the metabolism of topically applied chemicals and is thus a first pass organ for penetrating substances.

Human skin tissue acts as the first line of defense against outside stimuli attempting to enter the body. It also provides an important role in maintaining an ongoing level of internal homeostasis. Separated into the Epidermis, Dermis, and Hypodermis, each layer performs a specific function essential to the maintenance of ongoing bodily function.