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 re sults in the formation of a gel through which the drug slowly diffuses . It has been shown that HPMC compacts swell predominantly in the axia l, rather than the radial, direction on exposure to water. Other worke rs have considered the possibility of applying impermeable coats to di fferent aspects of HPMC compacts in order to alter drug release mechan ism. The aim of this study was, given the axial rather than radial rel axation, to investigate whether the faces of cylindrical compacts of H PMC were of different nature to the edge. This was studied by coating the edges and faces of different compacts with paraffin, in order to p revent water access. The interpretation of the results would be differ ent depending on the method of data presentation. with plots of normal ised increases in height, diameter and area all suggesting a differenc e in properties between uncoated, edge coated and face coated tablets. However, if the areas were corrected for the paraffin coating, the pl ots of normalised area were found to tend towards being superimposed c onsequently, within experimental error, it can be concluded that the f aces and edges of the HPMC compacts behaved in an identical manner. Th e axial relaxation must simply relate to the relief of stresses induce d during compaction.