ABSTRACT
I. INTRODUCTION Over the past few decades there have been many advances in our understanding of the physicochemical properties of both formulation systems and their ingredients. These have led to the ability to develop physically, chemically, and biologically stable products. There has also been a significant increase in our knowledge of the properties of skin and the processes that control skin permeation. The ground rules for skin permeation were laid down by Scheuplein and Blank in the late 1960s and early 1970s (1), and these have been updated on a reasonably regular basis (2-8). We have learned, for example, that the permeation of compounds across intact skin is controlled fundamentally by the stratum corneum, and it is the chemical composition and morphology of this layer that usually determines the rate and extent of absorption (9,10). Similarly, we have discovered how to modify this barrier, by chemical or physical means and, thereby, alter the rate of diffusion of many permeating molecules (11,12).
