Reversible buckling and diffusion properties of silica-coated hydrogel particles

The structure and diffusion properties of composite particles consisting of a calcium alginate hydrogel core and a thin SiO(2) surface layer have been investigated. The composite particles were formed by depositing a silica layer onto calcium alginate cores using a sol-gel process starting from alkoxysilane precursors. The composite particles were found to have a remarkable ability to reversibly rehydrate and return to their original size and shape after partial drying. The organo-silica skin was able to sustain large local deformations (such as complete folding) without the formation of cracks or defects. Such mechanical properties are uncharacteristic of pure silica and they can be attributed to the specific microstructure of the alginate-silica composite. The structure and composition of the alginate-silica particles were characterised by SEM, X-ray micro-tomography, Laser Scanning Confocal Microscopy and Thermo-gravimetry. In order to quantify the effect of the organo-silica layer on the diffusional transport into and out of the alginate particles, the uptake and release rates of several test molecules with increasing molecular weight were measured for both un-coated and silica-coated particles. While the diffusion rate of small and medium-size molecules (water, vitamin B12) was essentially unaffected by the presence of the silica layer, the diffusion rate of a larger biomolecule (lysozyme) was found to be slowed down by the presence of the surface layer. The flexibility of the organo-silica layer combined with the ability of even large biomolecules to diffuse through it indicate that the silica layer is macroporous, formed by individual SiO(2) nanoparticles dispersed and immobilised in the surface layer of the alginate hydrogel.