Sustained delivery of an active agent using an implantable system

An implantable, fluid-imbibing device for delivering an active agent to a fluid environment of use, said device comprising the following components:

• an impermeable reservoir comprising a material selected from the group consisting of non-reactive polymer, a biocompatible metal, and a biocompatible metal alloy, the reservoir having an interior surface;
• a piston that divides the reservoir into a water-swellable agent chamber and an active agent chamber, wherein the water-swellable agent chamber has an open end and the active agent formulation chamber has an open end;
• a back diffusion regulating outlet received in the open end of the active agent formulation chamber of the reservoir for delivering fluid from the active agent formulation chamber to the fluid environment;
• and
• a water-swellable semipermeable plug received in sealing relationship with the interior surface of the open end of the water-swellable agent chamber of the reservoir.