Transdermal drug delivery: overcoming the skin's barrier function.

The skin represents an extraordinary evolutionary feat. Not only does it physically encapsulate the organism and provide a multifunctional interface between us and our surroundings, but it is perpetually engaged in the assembly of a highly efficient homeostatic barrier to the outward loss of water(1). In so doing, it furnishes a membrane that is equally adept at limiting molecular transport both from and into the body. Overcoming this barrier function then, for the purpose of transdermal drug delivery, has been a necessarily challenging task for the pharmaceutical scientist, and one that boasts significant progress.