PROBING THE MECHANISMS OF SWELLING OF HYDROXYPROPYLMETHYLCELLULOSE MATRICES

Hydroxypropylmethylcellulose is often used as a component of sustained release matrix formulations. It is known that the swelling of HPMC results in the formation of a gel through which the drug slowly diffuses. It has been shown that HPMC compacts swell predominantly in the axial, rather than the radial, direction on exposure to water. Other workers have considered the possibility of applying impermeable coats to different aspects of HPMC compacts in order to alter drug release mechanism. The aim of this study was, given the axial rather than radial relaxation, to investigate whether the faces of cylindrical compacts of HPMC were of different nature to the edge. This was studied by coating the edges and faces of different compacts with paraffin, in order to prevent water access. The interpretation of the results would be different depending on the method of data presentation. with plots of normalised increases in height, diameter and area all suggesting a difference in properties between uncoated, edge coated and face coated tablets. However, if the areas were corrected for the paraffin coating, the plots of normalised area were found to tend towards being superimposed consequently, within experimental error, it can be concluded that the faces and edges of the HPMC compacts behaved in an identical manner. The axial relaxation must simply relate to the relief of stresses induced during compaction.