Skin: the ultimate interface

The outer layer of the skin, the stratum corneum is a unique barrier membrane. On average it is only 20 mm thick (about a quarter the thickness of a normal sheet of paper) but it prevents us from losing excessive water and it protects us from our environment. It forms a special interface between our body, the air, water and various solids. In order to understand the barrier properties of the skin we need to determine its structure at various levels ranging from the macroscopic scale to the molecular level. This has been made easier by the advances that there have been over the recent decade. However, the amount of a material that is capable of penetrating this excellent barrier and reaching the underlying systemic circulation is still only of the order of 1 or 2 per cent of the total applied dose. The purpose of this publication is to explore the strategies currently employed to promote skin permeation and to consider the most exciting approaches currently under investigation. The limitations of current methodology to examine the problem are discussed. New opportunities to fill the gaps in our current knowledge are identified and the importance of interdisciplinary research in the field is emphasised.