Which statement best describes diffusion-controlled release (Higuchi) in polymer matrices?

• A. Release is driven by erosion of the polymer
• B. Release is governed by diffusion of drug through the matrix
• C. Release depends on chemical reaction with medium
• D. Release is independent of surface area

Answer: (B)

The main idea here is that the release rate in a Higuchi-type system is controlled by diffusion of the drug through the polymer matrix. In this model, the drug is initially dispersed within a non-eroding, relatively uniform matrix, and the surrounding medium acts as a sink. Drug molecules migrate out of the matrix by Fickian diffusion, so the amount released increases with the square root of time as the diffusion path length grows. Because diffusion is the rate-limiting step, the release depends on how easily the drug can move through the polymer (its diffusivity) and the available surface area for transfer, rather than on polymer erosion or chemical reactions with the medium. It also implies that changing the surface area will alter the release rate, which would not be the case if erosion or a chemical reaction dominated.