Formation of Ion Pairs and Complex Coacervates

Over the past several decades, there have been important advances in the field of transdermal drug delivery. However, for most drugs, the successful transdermal delivery is still limited due to the barrier function of the skin. The skin, especially its outermost layer, the stratum corneum (SC), poses a formidable barrier to the penetration of exogenous substances into the skin and only allows the penetration of drugs with low molecular weight, suitable solubility in oil and water, moderate partition coefficient, and low melting point. Generally, hydrophilic ionized drugs and other drugs with unfavorable physicochemical properties cannot pass adequately the skin barrier to exert their pharmacological effect. Ion pairs and complex coacervates are effective strategies to facilitate the transdermal penetration of this type of drugs.