Modelling drug release from inert matrix systems: from moving-boundary to continuous-field descriptions.

The purpose of this review is to provide a comprehensive overview of mathematical procedures that can be used to describe the release of drugs from inert matrix systems. The review focuses on general principles rather than particular applications. The inherent multiscale nature of the drug-release process is pointed out and multiscale modelling is exemplified for inert porous matrices. Although effects of stagnant layers and finite volumes of release media are briefly discussed, the systematic analysis is restricted to systems under sink conditions. When the initial drug loading exceeds the drug solubility in the matrix, Higuchi-type moving-boundary descriptions continue to be highly valuable for obtaining approximate analytical solutions, especially when coupled with integral balance methods. Continuous-field descriptions have decisive advantages when numerical solutions are sought. This is because the mathematical formulation reduces to a diffusion equation with a nonlinear source term, valid over the entire matrix domain. Solutions can thus be effortlessly determined for arbitrary geometries using standard numerical packages.