Chapter 33 - Phase transition microemulsions as drug delivery systems

Abstract

Microemulsions (MEs) are isotropic transparent liquid systems of water, oil, and amphiphile. Three types of MEs have been identified, including oil-in-water (O/W ME) in which water represents the continuous phase; water-in-oil (W/O ME), which has water as the internal phase; and the water and oil bicontinuous ME, which comprises comparable amounts of aqueous and oily phases. MEs can be developed by mixing the oily phase with water in the presence of suitable surfactant/cosurfactant system. The relative proportions of these components affect the phase behavior of the developed systems. The composition and phase behavior of any ME can be modified by dilution with water or loss of volatile constituent such as volatile oil or cosurfactant. Dilution can take place after administration whereby ME can mix with biological fluids such as the tears or the sweat after ocular or skin application, respectively. Dilution can take place after intramuscular injection or oral administration as well. The volatile component can evaporate after topical application. The resulting change in the phase behavior can be associated with a change in the viscosity and/or solubilizing power of the system. The later may lead to supersaturation which together with the viscosity change can at least affect the rate of drug release with a potential variation in drug delivery efficiency. These effects attracted the attention of scientists to employ phase transition MEs as promising drug delivery systems. This chapter will summarize the attempts made in this direction.