An implantable osmotically driven delivery system having a dynamic, two-way valve
and a self-adjusting, variable geometry fluid flow channel. As pressure within
the agent delivery system increases, the fluid channel narrows, thereby restricting
flow. At exceptionally high pressures, the valve can be designed to close altogether
at the orifice or delivery end, or it can provide a minimal leak path so that a
maximum fluid flow is never exceeded. At zero or very low pressures, the valve
will close completely at the beneficial agent reservoir end, isolating the beneficial
agent formulation from external fluid infiltration and thereby eliminating diffusion
of external fluid into the beneficial agent formulation.