The effect of erosion and swelling on the dissolution of theophylline from low and high viscosity sodium alginate matrices.

The extent of dissolution of theophylline from sodium alginate matrices has been studied in water and HCl. Surprisingly, it was observed that the dissolution rate was very fast for the low viscosity (LV) material in water and for the high viscosity (HV) in acid, whereas slow release was seen for LV in acid with very slow release for the HV in water. The extent of erosion and swelling of the matrices (with and without drug and with and without stirring) was determined from simple weighing experiments. Based on the extent of erosion and swelling, along with visual observations, it was concluded that the rapid release from LV in water was due to rapid and complete erosion of a weak gel layer. The rapid release of HV in HCl was due to an unexpected lamination of the tablet during swelling, which resulted in exposure of more of the tablet core, and caused increased erosion of the matrix. The slow release seen with the LV in HCl was due to the acid gel being tough and resistant to erosion. The very slow release observed with the HV in water was due to very substantial swelling, with the presence of near zero-order kinetics for this system being due to a balance of swelling and erosion, keeping the diffusion process at a constant rate. Furthermore, an anisotropic swelling behavior is indicated by the preferential expansion in the axial dimension relative to the radial dimension for both LV and HV alginates.