In this study, tough and conductive hydrogels were printed by 3D printing method. The combination of thermo-responsive agar and ionic-responsive alginate can highly improve the shape fidelity. With addition of agar, ink viscosity was enhanced, further improving its rheological characteristics for a precise printing. After printing, the printed construct was cured via free radical polymerization, and alginate was crosslinked by calcium ions. Most importantly, with calcium crosslinking of alginate, mechanical properties of 3D printed hydrogels are greatly improved. Furthermore, these 3D printed hydrogels can serve as ionic conductors, because hydrogels contain large amounts of water that dissolve excess calcium ions. A wearable resistive strain sensor that can quickly and precisely detect human motions like finger bending was fabricated by a 3D printed hydrogel film. These results demonstrate that the conductive, transparent, and stretchable hydrogels are promising candidates as soft wearable electronics for healthcare, robotics and entertainment.

Tough swelling nanocomposite hydrogels have been fabricated via introducing graphene oxide (GO) and in situ synthesis. Upon addition of GO, the swelling properties of a calcium alginate/polyacrylamide (CA/PAAm) double network (DN) are considerably suppressed. Moreover, the mechanical properties of CA/PAAm DN gels and GO reinforced DN gels in swelling and as-prepared states have been systematically explored. The results affirm that the swelling largely damages the mechanical properties of gels, due to the osmotic pressure introducing solvents into the gaps between polymer chains and cross-linkers, thereby leading to weak spots in the gels. GO demonstrates the ability to improve the mechanical properties of pristine DN gels in both swelling and as-prepared states by offering large numbers of physical interpenetrations and chemical bindings between long polymeric chains and GO nanosheets, and building a Ca²⁺ coordination-induced GO network. The toughness of swelling nanocomposite gels is around 106 kJ m⁻³ that is comparable to that of cartilage in large animal joints. Furthermore, the consecutive loading–unloading tests and cytotoxicity tests show that the swelling nanocomposite gels, respectively, obtain superior fatigue resistance and high biocompatibility. Therefore, these swelling nanocomposite gels are promising as substitutes for load-bearing tissues.