matrix system

A matrix system is like a raisin loaf: the raisins (drug) are scattered evenly throughout the bread (polymer). To get out, each raisin near the surface leaves first, and the deeper ones must work their way through the surrounding dough — so the drug is released gradually rather than all at once.

Formally, in a matrix (or monolithic) system the drug is dispersed uniformly within a rate-controlling material, usually a polymer. Hydrophilic matrices such as hydroxypropyl methylcellulose swell into a gel layer that the drug diffuses through; insoluble matrices such as ethylcellulose or wax let drug leach out through pores; and erodible matrices slowly dissolve away to liberate their drug.

Matrix tablets are cheap, robust, and easy to make by ordinary granulation and compression, which is why they are the most common controlled-release platform. Their main limitation is that release usually slows over time (often following Higuchi or first-order kinetics) rather than staying truly constant, because the diffusion path lengthens as outer drug is depleted.