A shape memory hydrogel induced by the interactions between metal ions and phosphate

A novel ferric-phosphate induced shape memory (SM) hydrogel is prepared by the one-step copolymerization of isopropenyl phosphonic acid (IPPA) and acrylamide (AM) in the presence of a crosslinker polyethylene glycol diacrylate (PEGDA). Different from the traditional SM hydrogels, our SM hydrogel can be processed into various shapes as needed and recovers to its original form in 'multi-conditions' such as in the presence of a reducing agent or in the presence of a competitive complexing agent. This unique feature is attributed to the fact that the oxidized ferric ions show a high complexation ability with phosphate groups of IPPA, which acts as a physical crosslinker to form the secondary networks within the hydrogels to induce the shape memory effect. The memory behavior was totally reversible, owing to Fe3+ that can be reduced to Fe2+ and extracted by the complexing agent. Particularly, the SM hydrogels exhibit controllable and good mechanical characteristics by introduction of the ferric ions, i.e., the elastic modulus can increase from 2 kPa to 70 kPa dramatically. Learning from biological systems, phosphate-metal ion based hydrogels could become an attractive candidate for various biomedical and environmental applications.