The term “microemulsion” often used to describe a variety of micellar structures from simple droplets to complex cubic compositions. In this discussion, the classic definition is assumed: a thermodynamically stable colloidal dispersion of water and oil stablized by a surfactant, and often, a co-surfactant. In comparison with coarse emulsions, microemulsions form spontaneously, are clear or transparent, have smaller droplet size (<150nm), and have lower interfacial energy.

Interest in microemulsion formulations spans the cosmetic and pharmaceutical industry because of potential for lower skin irritation, drug permeation enhancement, cosmetic properties, and drug solubilization. With increased interest in microemulsions, formulation improvements are being reported at an increasing rate. For example, microemulsions have historically included a low-to-medium-chain-length alcohol as a co-surfactant. When applied to skin, these formulations often caused irritation and dehydration. In recent years, more cases of alcohol-free microemulsions have been reported with much lower irritation potential.

The thermodynamic stability of microemulsions can contribute to improved formulation stability relative to conventional coarse emulsions by their lower potential for phase separation, creaming, flocculation, or increased droplet size (”Ostwald ripening”). This advantage can yield increased formulation robustness when challenged by prolonged temperature excursions. The favorable thermodynamic stability of microemulsions also can lead to simplified pharmaceutical product development, scale-up, technolology transfer, and manufacture. This is due partially to the lower energy input requirement to achieve the microemulsion. Simpler, low-energy mixing equipment may be adequate to prepare microemulsion formulations. In contrast, coarse emulsions often require the addition of heat, preparation of multiple side phases, and the use of rotor-stator homogenizers or high pressure microfluidization process equipment. In some cases, the low viscosity of microemulsions presents a challenge when formulating topical products because of the need to retain the drug at the application site. In these cases, it may be necessary to add viscosity modifying excipients to form a microemulsion gel formulation.

The mechanism by which microemulsions may enhance drug permeation in skin is not clear. Increased permeation may be due to several mechanisms, including: higher drug solubility in the vehicle providing a greater reservoir of drug in the vehicle; permeation enhancement properties due to one of the formulation components; and improved wetting of the skin surface due to lower interfacial tension.

In summary, microemulsion technology has advanced significantly and is a viable formulation option for topical and transdermal drug delivery products for drugs with appropriate physicochemical characteristics and therapeutic endpoints.