Materials for microencapsulation: what toroidal particles (doughnuts) can do better than spherical beads

The vast majority of matrices used in microencapsulation are spherical. This geometry has some drawbacks due to its low surface/volume ratio: for example the difficult diffusion of oxygen to the bulk of a cell-containing microbead may lead to hypoxia or necrosis of the encapsulated cells. Here we focused on toroidal microparticle (doughnuts); these structures have a higher surface area than spherical beads and dispersed objects will experience a lower average distance from the surface. The toroidal morphology is the result of the flattening of water drops impacting on a liquid surface, and is then quenched by the occurrence of a rapid gelation due to polyelectrolyte complexation. We have produced toroidal microparticles using chitosan with an appropriately low molecular weight in combination with triphosphate (TPP), optimizing the conditions of pH and concentration of the two components. We also present here an evaluation of the process used for chitosan depolymerisation and of the cytotoxicity of the resulting polymers, which marginally decreases with decreasing molecular weight. Under the conditions adopted, the doughnuts result more densely cross-linked than spherical microparticles and show a considerably improved physical stability.