The discovery of emulsion evaporation technology came at a time when the development of systems for long-term drug delivery was proceeding rapidly. A major difficulty during this period was the problem that systems for long-term drug delivery systems made from synthetic materials such as wax, silicone networks, or hydrogels triggered histopathological reactions when they came into contact with tissue. In this context, Gurny and coworkers conducted a study in 1981 in which they investigated the possibility of using biodegradable and biocompatible polymers (e.g., polylactic acid, polylactic glycolic acid, glycerides, polysaccharides, etc.) in the same manner because their degradation products would not exhibit significant toxicity (polylactic acid, for example, degrades to lactic acid, which is the end product of anaerobic glucose metabolism). With this in mind, the researchers prepared drug carriers of polylactic acid into which testosterone (a model drug) was encapsulated. Rather than disclose all the experimental details of this work, we will content ourselves with explaining the basics of the procedure. Because of the opposing dissolution properties of the polymer and the drug in an aqueous environment, the encapsulation process must be carried out in an oil-inwater emulsion in the presence of surfactants. When the organic solvent is removed, encapsulated polymeric materials are obtained .