New way to crosslink chitosan in aqueous solution

In this paper, the reaction between o-phthaldialdehyde and free NH₂ of chitosan is investigated; at a very low molar ratio between the two reactants ([dialdehyde]/[NH₂] ∼ 2.5 × 10⁻⁴), an increase of the apparent molecular weight is obtained as evidenced from the rheological behaviour. Then, three non-ionic polysaccharides (galactomannan, maltodextrins, methylcellulose) are oxidised to 10% with sodium metaperiodate to obtain polyaldehydic derivatives able to react with free NH₂ of chitosan after their direct dissolution into chitosan solution at a molar ratio [monosaccharide units]/[NH₂ ] ∼ 0.6. Stable swollen porous gels are obtained with an excellent yield in the presence of a reducing agent (NaBH₃CN) chosen to reduce the Schiff base; nearly no influence of the structure of the initial non-ionic polysaccharides is observed when the polysaccharides are oxidized in the same conditions. Different parameters for the reaction of oxidized methylcellulose (Me-ox) with chitosan are tested: influence of the degree of oxidation (up to 50%), and of the oxidised methylcellulose concentration. The larger is the degree of oxidation or the Me-ox concentration, the lower is the degree of swelling (i.e., the larger is the degree of chitosan cross-linkage). The swollen gels formed immediately after reaction are isolated and re-swell in aqueous acidic conditions, a good solvent of initial chitosan, to purify the gel and determine the yield of the reaction and the swelling degree. At the end, preliminary tests of biodegradability of these new gels are performed using specific enzymatic degradation with lysozyme and cellulase in the case of chitosan/Me-ox cogels chosen as example.