Self-healing chitosan/vanillin hydrogels based on Schiff-base bond/hydrogen bond hybrid linkages

Natural vanillin, the primary component of vanilla bean extract, was used as crosslinking agent to fabricate self-healing chitosan hydrogel. The aldehyde group of vanillin reacts with the amino group of one chitosan molecule through a Schiff-base reaction while its hydroxyl group forms hydrogen bonds with the hydroxyl or the amino groups in another chitosan molecule, which provides the basis for construction of a reversible network. We found that the self-healing effect of the chitosan/vanillin hydrogel mainly originated from the reconstruction of the Schiff-base bond, while the hydrogen bonds were relatively stable at room temperature. It was found that 10 ml 5% (w/v) chitosan solution gelated with 0.3 g vanillin achieved a good balance between self-healing capability and mechanical strength, reaching a moderate gelation time of 6 min. When the vanillin dosage exceeded 0.3 g, a compact network was formed. In this case, the chitosan molecule chains were unable to move freely due to the fact that the linkages were fixed by the massive hydrogen bonds, which reduced the formation of new hybrid linkages through the dynamic exchange of Schiff-base bonds. This made the chitosan/vanillin hydrogel lose its self-healing capability.