Doubly-crosslinked, emulsion-templated hydrogels through reversible metal coordination

Double crosslinking can be used to significantly improve the mechanical properties of gels. Here, doubly-crosslinked hydrogel polyHIPEs (DC-PHs), which exhibited rapid water absorption, enhanced mechanical properties, and shape memory behaviour, were generated by forming metal coordination crosslinks within covalently crosslinked hydrogel polyHIPEs (HG-PHs), hydrogels formed by templating within oil-in-water high internal phase emulsions (HIPEs). The HG-PHs were based on acrylamide (AAm) and sodium acrylate (NaA), which provided −COO⁻ groups, and were covalently crosslinked with N,N'-methylenebisacrylamide. The metal coordination crosslinking between the −COO⁻ groups and Fe³⁺ was generated by adding FeCl₃ to the HG-PHs. The interconnected macroporous structures and the rapid water absorption of the HG-PHs were preserved in the DC-PHs, while the mechanical properties, in both the swollen and dry states, were significantly enhanced. The metal coordination crosslinking was easily removed through the light-induced reduction of the Fe³⁺ in the presence of citric acid. The reversibility of the metal coordination crosslinking endowed the DC-PHs with a multiple-cycle shape memory behaviour.