diffusion-controlled release

Diffusion-controlled release relies on the slow, spontaneous wandering of molecules from where they are crowded to where they are sparse. The drug seeps out through a polymer or membrane the way a drop of ink spreads through still water — driven only by the difference in concentration, with no pump pushing it.

The governing physics is Fick's first law: the release rate rises with the surface area and the concentration difference across the barrier, and the diffusion coefficient of the drug, and falls as the barrier gets thicker. In a reservoir device the membrane is the diffusion barrier; in a matrix device, drug diffuses out through water-filled channels or the swollen polymer itself.

Because the dose depletes from the outside in, the diffusion path usually lengthens over time, so a pure matrix tends toward Higuchi (square-root-of-time) or first-order release rather than constant rate. A reservoir with a saturated core keeps the gradient steady and can therefore deliver closer to zero order. The whole approach assumes the polymer does not erode meaningfully — when it does, erosion takes over.