The Historical Challenges of Transdermal Delivery
The skin is the body’s natural protective layer, with two primary barriers to passage: the stratum corneum on the surface and restrictive junctional proteins deeper in the skin. The stratum corneum is composed of flattened corneocytes set within a waxy, lipid matrix. This barrier is very effective at keeping water and nutrients inside the body and pathogens and chemicals out, making it the largest hurdle for most transdermal delivery technologies.
Of the 10,000+ drugs currently approved by the FDA, less than 30 are approved for transdermal delivery. This limited success has only been achieved with drugs that fall within very restricted chemical characteristic boundaries; they are low in molecular weight, moderately lipophilic, and highly potent.