Microencapsulation by solvent evaporation: state of the art for process engineering approaches

Microencapsulation by solvent evaporation technique is widely used in pharmaceutical industries. It facilitates a controlled release of a drug, which has many clinical benefits. Water insoluble polymers are used as encapsulation matrix using this technique. Biodegradable polymer PLGA (poly(lactic-co-glycolic acid)) is frequently used as encapsulation material. Different kinds of drugs have been successfully encapsulation: for example hydrophobic drugs such as cisplatin, lidocaine, naltrexone and progesterone; and hydrophilic drugs such as insulin, proteins, peptide and vaccine. The choice of encapsulation materials and the testing of the release of drug have been intensively investigated. However process-engineering aspects of this technique remain poorly reported. To succeed in the controlled manufacturing of microspheres, it is important to investigate the latter. This article reviews the current state of the art concerning this technique by focusing on the influence of the physical properties of materials and operating conditions on the microspheres obtained. Based on the existing results and authors' reflection, it gives rise to reasoning and suggested choices of materials and process conditions. A part of this paper is also dedicated to numerical models on the solvent evaporation and the solidification of microspheres. This review reveals also the surprising lack of knowledge on certain aspects, such as the mechanism of formation of pores in the microspheres and the experimental study on the solidification of microspheres.