reservoir system

A reservoir system is like a water bottle with a tiny pinhole in the cap: a big store of liquid inside, and a small opening that limits how fast it can escape. Here the store is a drug core and the cap is a thin rate-controlling membrane wrapped around it.

Drug must dissolve and then diffuse across the surrounding membrane before it reaches the body, and that membrane is the bottleneck that sets the pace. If the core is kept saturated, the concentration just inside the membrane stays constant, so release can approach the prized zero-order rate — a key advantage over matrix systems. Coated pellets, microcapsules, and many transdermal patches work this way.

The big caveat is safety: because nearly the whole dose sits as a concentrated core behind one membrane, any breach — a cracked coating, a chewed pellet, or membrane failure — can release everything at once, the textbook setup for dose dumping. Reservoir designs therefore demand a defect-free, robust membrane.