Synthesis and properties of chitosan–silica hybrid aerogels

Chitosan, a polymer that is soluble in dilute aqueous acid, is derived from chitin, a natural polyglucosamide. Aquagels, where the solid phase consists of both chitosan and silica, can be easily prepared by using an acidic solution of chitosan to catalyze the hydrolysis and condensation of tetraethylorthosilicate. Gels with chitosan/tetraethoxysilane (TEOS) mass ratios of 0.1–1.1 have been prepared by this method. Standard drying processes using CO2 give the corresponding aerogels. The amount of chitosan in the gel plays a role in the shrinkage of the aerogel during drying. Gels with the lowest chitosan/silica ratios show the most linear shrinkage, up to 24%, while those with the highest ratios show only a 7% linear shrinkage. Pyrolysis at 700°C under nitrogen produces a darkened aerogel due to the thermal decomposition of the chitosan, however, the aerogel retains its monolithic form. The pyrolyzed aerogels absorb slightly more infrared radiation in the 2–5 μm region than the original aerogels. B.E.T. surface areas of these aerogels range from 470 to 750 m2/g. Biocompatibility screening of this material shows a very high value for hemolysis, but a low value for cytotoxicity.