Polymer particle erosion controlling drug release. II. Swelling investigations to clarify the release mechanism.

The aim of the study was a comprehensive swelling investigation of hydrocolloid tablets with drug release by diffusion, erosion and polymer particle erosion, respectively, in order to reveal differences in the swelling behaviour responsible for the diverging drug release mechanisms. Four different methods were applied to study swelling of the tablets: determination of the expansion factor, texture analysis, visual swelling observation of dye containing tablets sandwiched between plexiglas discs and photomicroscopy. Altogether they allowed the investigation of dimensional changes, swelling velocity, thickness, appearance and strength of the gel layer and front movements. However, none of the methods included a determination of all these factors. A combination of the different techniques proved to be helpful to provide information necessary for a broad understanding of the complex phenomenon of swelling. Intensive swelling was observed for matrices with diffusion controlled release (e.g. MHPC 100000), while erosion controlled systems (e.g. Pharmacoat 606) were characterized by limited swelling and fast polymer erosion. In the case of tablets exhibiting polymer particle erosion (e.g. MHEC 10000 B) the importance of the amount of insoluble fibres was confirmed. Insoluble fibres were clearly visible in the swelling zone of these tablets. They impeded the swelling, weakened the gel layer and caused attrition of polymer material, thus only a thin gel layer was formed. Synchronization of the movement of swelling and erosion fronts occurred during the swelling of tablets with a high content of insoluble fibres. The freely soluble drug proxyphylline was found to promote swelling while the poorly soluble acetophenetidin hindered the hydration of the tablet. Furthermore, the swelling study confirmed the low robustness to hydrodynamic stress of tablets with erosion control compared to tables with polymer particle erosion.