Permeability and Stability of Microcapsules

Microcapsules consisting of natural polysaccharide hydrogels such as pectinates have a large potential for the transportation of liquid drugs with food. If a controlled and site specific release of the encapsulated drugs is desired, they must be stored sufficiently long under different environmental conditions. Here, we optimize the capsules with respect to the retention time of the encapsulated drugs and the chemical stability under gastrointestinal conditions. To achieve this, we coat the pure hydrogel capsules with the natural resin shellac. NMR microscopy is used for the characterization of the structure and suitability of these microcapsules. We measure the permeability of differently prepared capsules by using paramagnetic probe molecules. By an acquisition of T₁-weighted images as a function of time we can monitor the diffusion of the molecules into the capsules. Diffusion coefficients for the probe molecules in the capsule membranes can be extracted from these measurements for quantizing the permeabilities. The obtained permeabilities show that shellac can seal pure pectinate capsules under acidic conditions. In addition, we also monitor the behavior of the capsules under simulated gastrointestinal conditions. These measurements show that the capsules do not undergo changes under gastric and intestinal conditions, but dissolve under colonic conditions on timescales between 1 and 28 hours, depending on the preparation process.