A unified mathematical model for diffusion from drug–polymer composite tablets

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Abstract
The derivation and experimental verification of a unified mathematical model for the estimation of drug release rate from drug–polymer composite tablets are presented. Cylindrical coordinates are utilized in the solution of the diffusion equation for a three-dimensional system. The model is applicable to tablets that range from the shape of a flat disk (radius > thickness) to that of a cylindrical rod (radius < thickness). The general solution for the fraction of drug released at a time t is This approach to a three-dimensiona system, utilizing cylindrical coordinates, presents a comprehensive method for the estimation of drug release rates from sustained release tablets with drug distributed homogeneously throughout a polymer matrix. The calculated and experimental drug diffusion rate of pyrimethamine from pyrimethamine-silicone rubber composite tablets that range in shape from that of a disk to a cylinder, and of hydrocortisone from EVA, poly-caprolactone, and PVA terpolymer, are compared.