erosion-controlled release

Erosion-controlled release works like a sucking on a hard sweet: the candy shrinks layer by layer from the outside, and whatever is embedded in each layer is freed as that layer wears away. The drug is let out as the surrounding matrix gradually dissolves or breaks down, not because the drug itself diffuses through an intact barrier.

Two flavours exist. Surface (heterogeneous) erosion eats the dosage form from the outside inward while the core stays dry — like the sweet — and if the surface area held constant this could give near zero-order release. Bulk (homogeneous) erosion lets water soak the whole matrix so it degrades throughout at once. Materials range from soluble waxes and lipids to biodegradable polymers such as polylactic-co-glycolic acid used in implants and microspheres.

Erosion is attractive because the carrier disappears, leaving nothing to retrieve — valuable for injectable depots and implants. The catch is that erosion and diffusion almost always happen together, and erosion rate can vary with pH, enzymes, and individual physiology, making truly constant release hard to guarantee.