Membrane-reservoir controlled release systems have been used for the delivery of a wide variety of therapeutic agents. Drug permeation is typically achieved by a solution-diffusion type of mechanism from a loaded reservoir region through a liquid-filled pore, and the rate of release is generally dependent on membrane porosity, tortuosity, and surface area. We have exploited the partition coefficient (between the reservoir liquid and the pore liquid phases) as an additional rate-controlling mechanism, and we have also extended the applications of membrane-reservoir systems to include delivery of microstructures such as liposomes.

If the fluid that fills the pore of the membrane is chosen to be immiscible with the liquid in the reservoir phase, the drug partitions between the two phases at the interface between the reservoir and the pore. The partition coefficient may be exploited to provide rate control by reducing the driving force for diffusion across the fluid-filled membrane pore. The performance of this type of system is evaluated using benzoic acid in an organic reservoir phase, diffusing through water-filled membrane pores. Both hydrophilic and hydrophobic membrane materials were investigated. A hydrophilic membrane material afforded the advantage of a more stable water-filled pore phase.

Controlled delivery of liposomal structures may also be achieved using a porous membranereservoir device. The pores of the membrane must be large enough to permit the passage of the liposomes, which are transported by simple Brownian motion. This concept is demonstrated using a porous polycarbonate membrane system, which was chosen to minimize interaction between the membrane and the liposomes.