A pump with no moving parts
The osmotic pump is the most elegant way to get true zero-order release from a swallowed tablet, and it uses nothing but water and a tiny hole. The tablet core holds the drug plus an osmotic agent (often just salt or sugar), wrapped in a semipermeable membrane that lets water in but holds solutes back. A single laser-drilled orifice pierces the coat.
- In the gut, water is drawn across the membrane by osmosis — driven by the osmotic pressure of the concentrated core.
- The core swells and builds pressure inside the rigid coat.
- Pressure pushes saturated drug solution out through the orifice — at a rate set by how fast water enters, which stays constant as long as the core remains saturated.
- The result is a steady, near-constant output for hours — clean zero-order release — that is largely indifferent to stomach pH or stirring.
Delayed release: timing the start
Delayed release is a different problem entirely — not slowing the rate, but holding fire until the dose reaches the right place. The most common version is the enteric coating: a polymer film that is insoluble in the acid of the stomach but dissolves once it reaches the more neutral small intestine.
Why bother? Two reasons. To protect the drug from stomach acid that would destroy it before it could be absorbed; or to protect the stomach from a drug that irritates its lining. The mechanism is a neat application of acid–base chemistry: enteric polymers carry weakly acidic groups that stay un-ionized (and insoluble) at low pH but ionize and dissolve as pH rises — the same pH-dependent behaviour that governs so much of drug absorption. This is the simplest member of the broader family of pH-responsive delivery.