4D printing of Metal-Reinforced double network granular hydrogels

Recent developments in soft actuation demand for resilient, responsive materials with locally varying compositions that are sufficiently stiff to exhibit significant actuation forces. Hydrogels are inherently responsive to certain stimuli. Yet, they typically suffer from a stiffness-toughness compromise such that those that are soft and show adequate flexibility display limited actuation forces. This compromise can be partially addressed if hydrogels are formulated as double networks. However, their involved processing prevents controlled variations in the local composition. The composition of hydrogels can be varied down to the 100 & mu;m length scale through 3D printing. A wide range of hydrogels can be 3D printed if formulated as microparticles. Yet, the resulting granular hydrogels are soft. They can be reinforced with a percolating hydrogel network, resulting in double network granular hydrogels (DNGHs) that, however, are rather brittle. Here, we introduce 3D printable metal-reinforced DNGHs (mrDNGHs) that combine three seemingly contradictory traits: stiffness, toughness, and processability. Our mrDNGHs can bear loads up to 3 MPa while displaying a fracture energy up to 12 MJ & BULL;m  3, a value exceeding that of any of the previously 3D printed hydrogels at least 20-fold. We leverage the different degrees of swelling of the mrDNGHs to 3D print shape morphing structures.