Visualization and analysis of the release mechanism of shellac coated ascorbic acid pellets

Shellac coated sustained release ascorbic acid containing pellets were investigated using scanning electron microscopy in order to visualize the release mechanism and to establish a correlation to dissolution data. Scanning electron micrograph pictures revealed different drug release profiles of individual pellets. Single pellet dissolution measurements demonstrated that the release profile of the encapsulated dosage form, containing approximately 400 pellets per capsule, is a combination of different release profiles of all individual pellets. The release of ascorbic acid occurred only in some small spots on the surface area of the pellets and could be visualized by the reduction of silver ions from an aqueous silver nitrate solution. These spots could be identified as defects in the shellac sustained release film using scanning electron microscopy. In further trials, the dissolution rate of an individual pellet could be related to the number and dimension of holes in its membrane. In conclusion, the release is characterized by surface defects and scanning electron microscopy studies are a useful tool to get new information for a better understanding of the drug release.