ABSTRACT
This chapter examines additional methodologies for modifying drug release from matrix devices. It emphases on the mechanism and analyses drug-release kinetics and examples relating to the application of these approaches. Drug release from such matrix systems is governed mainly by the diffusion process, although other processes, such as swelling and erosion, may also take place simultaneously, depending on the nature of the drug and the matrix material. The mathematical descriptions of diffusion problems involving moving boundaries are known as moving-boundary problems, free-boundary problems, or simply Stefan problems. The chapter reviews the effect of nonuniform initial drug concentration distribution on the release kinetics for both the diffusion-controlled and surface erosion-controlled matrix systems, and examines various methods for achieving nonuniform drug loading. Diffusion- and erosion-controlled matrix systems where the drug is uniformly dissolved or dispersed in a polymer generally exhibit release rates that continuously diminish with time.
