ABSTRACT
Because transdermal absorption is relatively slow, there has been a large amount of work concerned with finding materials which will increase the penetration rate of drugs through the skin. Such materials are called penetration enhancers. Penetration enhancers are believed to operate by increasing the permeability of the stratum corneum, either in the lipid or the keratinised protein regions. It is unlikely that many materials penetrate as far as the epidermis in sufficient concentration to increase its ability to transport hydrophobic drugs. The largest class of penetration enhancers appear to fluidise the lipid channels. These include dimethyl sulphoxide (DMSO) at high concentrations, decylmethyl sulphoxide, and azone. These materials are known to influence lipid structure31 but are also polar and capable of swelling proteinaceous regions. Lipid fluidity appears to be the most important factor since at low concentrations substances such as DMSO swell keratin but do not appreciably improve absorption. However, it is only high surface concentrations (>60%) which affect skin lipid fluidity and cause an increase in drug penetration.
