Cold-induced shape memory hydrogels for strong and programmable artificial muscles

Thermo-responsive shape memory hydrogels generally achieve shape fixation at low temperatures, and shape recovery at high temperatures. However, these hydro-gels usually suffer from poor mechanical properties. Herein, we present a unique poly(acrylic acid)/calcium acetate shape memory hydrogel with cold-induced shape recovery performances as ultrastrong artificial muscles. Since the acetate groups could form aggregate at high temperatures and thus induce the association of the hydrogel network, the hydrogel can be fixed into a temporary shape upon heating and recover to its original shape in a cold environment. Moreover, a programmable shape recovery process is realized by adjusting the shape fixing time. In addition, the unique shape memory process enables the application demonstration as bio-inspired artificial muscles with an ultrahigh work density of 45.2 kJ m−3, higher than that of biological muscles (∼8 kJ m−3).