protein microspheres are formed by phase separation in a non - solvent followed by solvent removal . the preferred proteins are prolamines , such as zein , that are hydrophobic , biodegradable , and can be modified proteolytically or chemically to endow them with desirable properties , such as a selected degradation rate . composite microspheres can be prepared from a mixture of proteins or a mixture of proteins with one or more bioerodible polymeric materials , such as polylactides . protein coatings can also be made . compounds are readily incorporated into the microspheres for subsequent release . the process does not involve agents which degrade most labile proteins . the microspheres have a range of sizes and multiple applications , including drug delivery and delayed release of pesticides , fertilizers , and agents for environmental cleanup . selection of microsphere size in the range of less than five microns and mode of administration can be used to target the microparticles to the cells of the reticuloendothelial system , or to the mucosal membranes of the mouth or gastrointestinal tract . larger implants formed from the microspheres can also be utilized , especially for agricultural applications .