<P> Although the skin is a large and logical target for drug delivery, its basic functions limit its utility for this purpose . The skin functions mainly to protect the body from external insults (e.g. harmful substances and microorganisms) and to contain all body fluids . It must be tough, yet flexible enough to allow for movement . The lipids in our skin serve as poor conductors of electricity and can hence protect us from electrical currents if the need so arises . </P> <P> There are two important layers to the human skin: (1) the Epidermis and (2) the Dermis . For transdermal delivery, drugs must pass through the two sublayers of the epidermis to reach the microcirculation of the dermis . </P> <P> The Stratum corneum is the top layer of the skin and varies in thickness from approximately ten to several hundred micrometres, depending on the region of the body . It is composed of layers of dead, flattened keratinocytes surrounded by a lipid matrix, which together act as a brick - and - mortar system that is difficult to penetrate . </P> <P> The stratum corneum provides the most significant barrier to diffusion . In fact, the stratum corneum is the barrier to approximately 90% of transdermal drug applications . However, nearly all molecules penetrate it to some minimal degree . Below the stratum corneum lies the viable epidermis . This layer is about ten times as thick as the stratum corneum; however, diffusion is much faster here due to the greater degree of hydration in the living cells of the viable epidermis . Below the epidermis lies the dermis, which is approximately one millimeter thick, 100 times the thickness of the stratum corneum . The dermis contains small vessels that distribute drugs into the systemic circulation and to regulate temperature, a system known as the skin's microcirculation . </P>

Routes of administration refers to crossing the skin barrier