Transport efficiency in transdermal drug delivery: What is the role of fluid microstructure?

Interaction of microstructured fluids with skin is ubiquitous in everyday life, from the use of cosmetics, lotions, and drugs, to personal care with detergents or soaps. The formulation of microstructured fluids is crucial for the control of the transdermal transport. In biomedical applications transdermal delivery is an efficient approach, alternative to traditional routes like oral and parenteral administration, for local release of drugs. Poor skin permeability, mainly due to its outer layer, which acts as the first barrier against the entry of external compounds, greatly limits the applicability of transdermal delivery. In this review, we focus on recent studies on the improvement of skin transport efficiency by using microemulsions (ME). Quantitative techniques, which are able to investigate both skin morphology and penetration processes, are also reviewed. ME are increasingly used as transdermal systems due to their low preparation cost, stability and high bioavailability. ME may act as penetration enhancers for many active principles, but ME microstructure should be chosen appropriately considering several factors such as ratio and type of ingredients and physic-chemical properties of the active components. ME microstructure is strongly affected by the flow conditions applied during processing, or during spreading and rubbing onto skin. Although the role played by ME microstructure has been generally recognized, the skin transport mechanisms associated with different ME microstructures are still to be elucidated and further investigations are required to fully exploit the potential of ME in transdermal delivery.