ABSTRACT
Although the majority of pharmaceutical particulates are larger than 1 μm in diameter, some systems consist of considerably smaller particles. The most well-known examples of these are parenteral fat emulsions, which have mean diameters of 200-300 nm. For some years, there has been increasing interest in using very small particles to solve a range of drug delivery problems. Liposomes were among the first submicrometre systems to be investigated, and they typically have mean diameters of 20-1000 nm. It has taken nearly 30 years of development to achieve a marketable product using liposome technology (the antifungal Ambisome is being investigated in extensive trials at present, and appears certain to reach the market). At least part of this delay can be attributed to the unfamiliarity of the pharmaceutical industry with submicrometre or colloidal systems. Many investigators are now experimenting with liposomes, emulsions, microemulsions, nanoparticles made from every conceivable polymer, and even submicrometre nanocapsules, and we can only expect more colloidal and submicrometre formulations to reach the marketplace in coming years.
