ABSTRACT
Skin penetration of chemicals The skin has many different functions: it can be the site of action of a product or the reservoir for a product; it can simply be a transit site or it can be an impermeable barrier. The skin’s main role is to form a barrier limiting the transfer of molecules in and out. It is especially adapted for this thanks to the multiple external layers of corneocytes. For the stratum corneum to be impermeable to both chemical and physical agents,1 it is essential for it to be intact. It is this layer that forms the main barrier to the penetration of active products through the skin. Corneocytes are made up of α-keratin filaments, buried in an amorphous matrix rich in disulfide bonds. There is a thick protein envelope of involucrin round each corneocyte, and the intercellular spaces are full of extremely hydrophobic lipids. The outer layer of the plasma membrane is hydrophilic. The outer layer of skin can be described as a hydrophilic ‘brick wall’ sealed with hydrophobic ‘mortar’.2 The whole forms an efficient barrier against both hydrophilic and hydrophobic molecules. This barrier is so efficient that chemicals can penetrate living or dead skin to the same degree, just as there is the same degree of penetration through the whole skin or the stratum corneum alone!2
