Abstract

An osmotic device for releasing a useful agent at different release rate patterns from the device in a selected environment is disclosed. The device is comprised of a wall surrounding and forming a compartment as a means for containing a useful agent and having a passageway for releasing the agent. The wall is comprised in at least a part of a material permeable to an external fluid and impermeable to the agent. A layer of a pattern rate controlling material that erodes in a selected environment for changing the release rate pattern of the device is carried on the semipermeable wall distant from the compartment. The agent is soluble in the fluid and exhibits an osmotic pressure gradient across the semipermeable wall against the fluid, or the agent has limited solubility and is admixed with an osmotically effective solute soluble in the fluid that exhibits an osmotic pressure gradient across the semipermeable wall against the fluid. In operation, agent is released from the device at different release rate patterns by the pattern rate controlling material eroding at a controlled rate to increase the amount of fluid available to the semipermeable wall for imbibition, which fluid is imbibed through the wall into the compartment producing a solution of the soluble agent or a solution of the osmotically effective solute containing the agent which is released through the passageway to the exterior of the device at a rate controlled by the permeability of the semipermeable wall and the osmotic pressure gradient across the wall of the device.