Osmotic pump with self-retaining, fast-start membrane plug

An osmotic pump, comprising:

• a capsule having a first end and a second end and an interior chamber disposed between the first end and the second end, the capsule having at least one delivery port formed at a the first end;
• a membrane plug made of a semipermeable material inserted in an opening at the second end of the capsule remote from the delivery port to provide a fluid-permeable baffler between the interior chamber and an exterior of the capsule;
• wherein the membrane plug has a columnar body bounded externally by a side surface and a base surface and at least one slot formed in the columnar body, the slot being in communication with the base surface and the base surface being in communication with the interior chamber, the slot extending from the base surface and terminating at a non-basal point on the side surface distanced from the base surface to vent pressure from the interior chamber to the exterior of the capsule when the columnar body extends a predetermined distance relative to the second end of the capsule, thereby preventing expulsion of the membrane plug from the second end;
• wherein an orifice traverses an entire length of the membrane plug such that exterior fluid flows into and through the columnar body through the orifice until the orifice becomes occluded due to swelling of the semipermeable material.