an osmotic dispenser comprised of a water permeable membrane forming part or all of the walls of an enclosure surrounding an active agent , and in some cases an additional compound known as an osmotic attractant which together exhibits an osmotic pressure against water . when placed in an aqueous environment water is osmotically drawn into the enclosure by the combined action of the agents and which distends and swells the enclosure until the membrane or some other part of the enclosure wall reaches the point of ultimate elongation and a portion of the wall yields and ruptures releasing the contents of the enclosure to the environment .