Comparative imaging of a slow-release starch excipient tablet: Evidence of membrane formation

A controlled release excipient made of chemically modified high-amylose starch (HAS) has been used to explore the surface membrane responsible for the slow release characteristics. Using scanning electron microscopy (SEM) and X-ray computed microtomography (CMT), the radial and axial surfaces of dry, swollen and freeze-dried sections observed were arranged in concentric domains. The presence of water enhances contrast in CMT imaging of the swollen tablet compared to the non-hydrated state. Freeze-dried SEM images showed a twin domain structure, thus defining membrane thickness. CMT quantified the average global porosity of the dry and swollen domains. The bird's eye view of the latter defined a central-core with high contrast and 34% porosity while the average global porosity was 19% and the dry porosity was 10%. The overall membrane after 15 min swelling is best described as concentric hydrogel domains.