ABSTRACT
The pharmaceutical community shows increasing interest in designing systems that will deliver a drug at a constant or near-constant rate. Various mechanisms can control the drug release rate. This chapter discusses control by the processes of diffusion or osmosis. Diffusional matrix systems may be one of the earliest and most utilized means of delivering bioactive agents, at least on an experimental level. Pilocarpine ocular therapeutic systems are the first pharmaceuticals to specify in their labeling the rate at which they deliver drug in vivo. Imbibition of water by the reservoir will decrease the drug concentration and hence drug diffusion rate with time. The transdermal therapeutic systems-scopolamine, recently introduced in the US by Ciba Pharmaceuticals, is the first generally available rate-controlled transdermal drug delivery system. Scopolamine is one of many biologically active agents for which selectivity of physiological action relates to the rate of administration.
