  We propose a mathematical model for computing drug release from multi-layer
capsules. The diffusion problem in such heterogeneous layer-by-layer composite
medium is described by a system of coupled partial differential equations,
which we solve analytically using separation of variables. In addition to the
conventional interlayer continuous mass transfer boundary conditions, we
consider also the case of finite mass transfer resistance, which corresponds to
the case of a coated capsule. The drug concentration in the core and through
all layers, as well as in the external release medium, is then given in terms
of a Fourier series that we compute numerically to characterize the drug
release mechanism.
