At the very early stages of understanding the roles of the body’s organs, we understood that the skin functions as a barrier between our body and our surroundings. Therefore it was clear that the skin would challenge penetration of compounds and repel outside insults. Having such a nature it was not surprising to discover that skin’s upper layer, the stratum corneum (SC), is a subtissue that very effi ciently limits penetration of compounds.
Over the years scientists have attempted to find compounds or systems that will allow overcoming this barrier and interaction with deeper subtissues or tolerating permeation to the circulation system. After years of research, it is now clear that there are ways to allow permeation of compounds to and through the skin. The focus has shifted toward understanding the microstructure of the skin, as well as the mechanism of action of these enhancers.
Because skin penetration enhancers provoke structural changes in the SC, they often trigger undesired immune system reactions such as irritation, allergy, or infl ammation. Most of the enhancers are not specific and will allow penetration of any compound that is small and lipophilic enough to penetrate. When dealing with cosmetic formulations this means that compounds such as fragrance components and preservatives will penetrate in conjunction with the active compound. Moreover, the skin is a very viable tissue. It includes many metabolic systems that were originally aimed to drive biochemical processes such as desquamation, creation of extracellular lamellar sheets, programmed cell death (apoptosis) and sebum or sweat secretion. These enzymes may attack active or inactive compounds as they penetrate, and convert them into an inactive, active or toxic form.