Two-scale, three-phase theory for swelling drug delivery systems. Part I: Constitutive theory

Polymeric carriers used in drug delivery applications, such as hydroxypropyl methylcellulose, that swell significantly upon coming in contact with water (or biological fluid) have been historically difficult to model due to the complex interplay of forces. This article seeks to introduce a thermodynamically consistent framework in which to address such problems. Here, a constitutive theory is developed that is applicable to viscoelastic polymers carrying an initially elastic drug that subsequently dissolves when exposed to a viscous fluid. The theoretical model consists of three phases, the polymer, drug, and fluid. A novel form of Darcy's law is reported that clearly distinguishes between distortional and dilatational forces and accounts for the affect of polymer relaxation on fluid transport. A standard form of Fick's law is also derived. (c) 2007 Wiley-Liss, Inc. and the American Pharmacists Association.