Dual-responsive fiber-reinforced hydrogel actuators by direct ion patterning

Nature inspired deformable heterogeneous smart hydrogels have attracted much attention in many fields such as biomedicine devices and soft actuators. However, normal spatial heterogeneous hydrogel structures can only respond to single factor and take one action as set in fabrication. Herein, we report a pre-stretched metal-liganded shape memory hydrogel with fiber reinforced, P(AAc-co-AAm)/CCNFs-Fe³⁺ (CCNFs: carboxylated cellulose nanofibers, AAc: acrylic acid, AAm: acrylamide), which can conduct shape deformation by solvent induction and ultraviolet (UV) light. The deformation pattern could be programmed by the deposing of ferroin ions. Also, the pre-stretched shape memory hydrogels could effectively produce cyclic actuation or complex shape actuation by UV light. More importantly, combining the solvent response with the light response enabled complex reversible actuations, such as simulating the bending and unfolding of fingers. The addition of CCNFs significantly enhanced the mechanical properties of the hydrogels. The hydrogels with 3 wt.% CCNFs showed an elongation at break of about 500% and a significant increase in tensile strength of 8.7-fold to 1.55 MPa after coordination with metal ions, which was able to meet the mechanical requirements of the bionic actuated hydrogels. This work demonstrated that combining light-programmed and light-responsive shape-memory hydrogels, complemented by another independent response property, could achieve complex and reversible programmed actuations.