Microemulsions as Transdermal Drug Delivery Systems

Microemulsions are nanosized thermodynamically stable transparent, single optically isotropic liquid systems of water, oil and surfactants. The thermodynamic stability, ease of production, high solubilization capacity and small droplet size made them ideal for transdermal drug delivery. Microemulsions can be prepared using various oils, surfactants and cosurfactants. This diversity in composition will affect the type of microemulsion and influence the transdermal drug delivery potential. The use of microemulsions in skin drug delivery has been reviewed previously but a lack of consecutive studies hampered drawing general conclusions on the effect of components and properties on the drug delivery rate. In addition, alternative mechanisms were reported for transdermal delivery from microemulsion. The first is related to the high drug loading capacity. The second is the penetration enhancing effect of the microemulsion components. The third depends on possible entrance of microemulsion components into the skin as monomers, increasing the solubility of the drug in the skin. The fourth depends on the microstructure of the system which provides large surface area of drug transfer. The last mechanism relies on the phase transition of microemulsions which provides a possibility for producing supersaturated system with high thermodynamic activity. Safety is another important factor which can influence the formulation development. This manuscript will critically evaluate skin drug delivery potential of microemulsions. The assessment will also highlight possible mechanisms of action before finally reporting on the safety and the feasibility of microemulsions for scaling up.