Restorable, high-strength poly(N-isopropylacrylamide) hydrogels constructed through chitosan-based dual macro-cross-linkers with rapid response to temperature jumps

To address the poor mechanical performance of conventional thermo-responsive poly(N-isopropylacrylamide) (pNIPAM) hydrogels, methacryloyl covalently-modified chitosan (methacryloylchitosan, MACS) was conceived as the chemical macro-cross-linker and employed cooperatively with chitosan, as the physical macro-cross-linker, to construct dually macro-cross-linked pNIPAM hydrogels for the first time. The employed MACS was synthesized through acylation of chitosan using methacrylic acid activated by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxy-succinimide (NHS). The influence of the preparation conditions, including the solids content, the dosage and component of the cross-linkers as well as the amount of the initiator, on the mechanical performance and swelling/deswelling properties of the obtained hydrogels was investigated systematically. The prepared dual-cross-linked pNIPAM hydrogels exhibit high mechanical strength, manifested in the achieved compressive stress of 8.75 MPa at 98% strain and tensile strength of 0.105 MPa. The high recovery efficiency (beyond 95%) from cyclic tensile tests reveals the good recoverability of these hydrogels. Moreover, they exhibit excellent thermo-responsiveness, to be more specific, rapid deswelling rates, i.e., most can release 50% of the absorbed water in ∼10 minutes on the temperature jump across the phase transition temperature. In particular, the deswelling rate increases with the increase of either the cross-linkers or MACS. The prominent mechanical performance and rapid responsiveness of the novel pNIPAM hydrogels is highly conductive for their future applications.