Programmed Deformations of 3D-Printed Tough Physical Hydrogels with High Response Speed and Large Output Force

Shape-morphing hydrogels have emerging applications in biomedical devices, soft robotics, and so on. However, successful applications require a combination of excellent mechanical properties and fast responding speed, which are usually a trade-off in hydrogel-based devices. Here, a facile approach to fabricate 3D gel constructs by extrusion-based printing of tough physical hydrogels, which show programmable deformations with high response speed and large output force, is described. Highly viscoelastic poly(acrylic acid-co-acrylamide) (P(AAc-co-AAm)) and poly(acrylic acid-co-N-isopropyl acrylamide) (P(AAc-co-NIPAm)) solutions or their mixtures are printed into 3D constructs by using multiple nozzles, which are then transferred into FeCl3 solution to gel the structures by forming robust carboxyl-Fe3+ coordination complexes. The printed gel fibers containing poly(N-isopropyl acrylamide) segment exhibit considerable volume contraction in concentrated saline solution, whereas the P(AAc-co-AAm) ones do not contract. The mismatch in responsiveness of the gel fibers affords the integrated 3D gel constructs the shape-morphing ability. Because of the small diameter of gel fibers, the printed gel structures deform and recover with a fast speed. A four-armed gripper is designed to clamp plastic balls with considerable holding force, as large as 115 times the weight of the gripper. This strategy should be applicable to other tough hydrogels and broaden their applications.