Fig. 1

Mechanism of drug release from the polymer-based drug delivery systems discussed in this paper. IR, immediate release; SR, sustained release. Spheres represent drug molecules. In the matrix and biodegradable systems shown, the polymer is uniformly distributed with the drug. Drug release occurs by diffusion through the matrix and gel erosion (e.g., hypromellose-based tablets) or biodegradation (e.g., poly(lactide-co-glycolide)-based systems). In coated or encapsulated systems, a polymer forms a film or shell around drug particles. In coated IR systems, the film dissolves rapidly, while in coated SR systems, drug release occurs gradually by diffusion through an insoluble polymer film