An advanced model for composite planar three-layer matrix-controlled release devices. Part II. Devices with non-uniform material properties and a practical example

In this series of papers, a new model has been presented for symmetrical, planar, three-layer (ABA) matrix-controlled release (MCR) devices, wherein all the advanced features of a previous monolithic MCR model have been incorporated. An extensive parametric study was performed to explore the possibilities afforded by the ABA configuration to alleviate, or even practically eliminate the undesirable features of initially very high, and subsequently continuously declining, release rates which normally characterize diffusion-limited monolithic devices. ABA matrices with uniform material properties (UMP) and layers A and B carrying different solute loads were examined in Part I. The results presented here refer to the more general case, where A and B may also represent non-uniformity in sorption and transport properties (non-UMP ABA devices). It is shown that judicious choice of two different polymeric materials for layers A and B may further improve the favorable results previously obtained for ABA-UMP matrices to the point where demands of very narrow limits for dose rate uniformity in conjunction with very high efficiency, can be met. The applicability and utility of the ABA, non-UMP model was demonstrated in a real experimental situation concerning surface-modified PVA matrices. Parameterization of the model on the basis of the experimental information provided and on literature data, resulted in successful interpretation of the effect of two different degrees of surface crosslinking on the relative rates of water uptake and proxyphylline release.