osmotic pump

An osmotic pump is a tiny tablet-sized water engine. It is a core surrounded by a semipermeable membrane — one that lets water in but not the dissolved drug out — with a single laser-drilled hole. Water is drawn in by osmosis, the inside pressure rises, and that pressure squeezes drug solution out through the hole at a steady drip, much like pressing water out of a syringe.

Because water keeps entering at a near-constant rate as long as the core stays saturated, the push stays constant, and the device delivers close to true zero-order release. The elegant part is that gut acidity, food, and movement barely affect it — the membrane only cares about water, so release is unusually independent of where the tablet is in the digestive tract.

The best-known version is the elementary osmotic pump and its refinement the push-pull OROS system, which adds an expanding osmotic layer to handle poorly soluble drugs. Limitations include higher manufacturing cost, the need for laser drilling, and the fact that the rigid outer shell passes out intact in the stool, which can alarm patients who are not warned.