Hydrogels constitute a growing field of research in the uses of chitosan, which is investigated for biomedicine, agriculture, food industry and drug delivery applications. Glutaraldehyde and formaldehyde have been commonly used as chitosan crosslinkers, although these aldehydes have a certain degree of toxicity. In this framework, trans-2-hexenal, a natural volatile responsible for the green odour of plants, can be a green alternative agent.
The aim of this research was to investigate trans-2-hexenal as a new crosslinking agent for the rapid in situ formation of chitosan hydrogels. As far as known, this aldehyde has not been used as chitosan crosslinker, however other studies have showed that it can react with primary amines of chitosan giving raise to condensation products as imines or Schiff bases. In addition, being an α,β-unsaturated aldehyde, it can undergo 1.4- nucleophilic addition of primary amine groups of chitosan (Michael addition), acting as a bridge between the chitosan chains and linking them.
The formation of hydrogels due to the presence of these two covalent bonds was evaluated by modifying the aldehyde concentration in chitosan solutions of 4% (w/v). Elemental analysis was used to confirm the conversion of the amino groups into imine bonds by determining the degree of substitution (DS) of lyophilized chitosan hydrogels. Aldehyde incorporation was dependent on DS, increasing from 21% for 4:0.125 (chitosan g : aldehyde g)  to 59% for 4:1 of the primary chitosan amino groups attached to the aldehyde. Moreover, rheological and morphological properties were characterised to check the effect of trans-2-hexenal concentration on the gelation properties of the polymer, which showed that stronger hydrogels were obtained as the trans-2-hexenal concentration increased. The lyophilized hydrogels presented a porous structure as observed by SEM, which allowed great water uptakes (> 500%), making them suitable for absorbent applications.