**4. Conclusion**

In summary, a tough and conductive hydrogel was developed by 3D printing technology. The combination of alginate and agar guarantee high ink viscosity resulting in high printing precision. A double network structure that combined covalent crosslinking and Ca2+-alginate coordination was employed to achieve conductive, transparent and stretchable hydrogels. The results demonstrated that the double network structure affords a smooth stress-transfer and recoverable energy dissipation to gift the hydrogels with superior mechanical strength and toughness. In addition, the 3D printed hydrogels containing a large amount of water that dissolves calcium ions, could work as ionic conductors. Conductive hydrogels depict quick, steady and repeated deformation toward strain to change the ionic transport, leading to rapid sensing response, high sensing stability and strain sensitivity. A wearable resistive strain sensor was fabricated connecting one 3D printed hydrogel film with conductive tape, which can rapidly and precisely detect the joint motions of finger bending. These results support the possibility for developing conductive, transparent, and stretchable hydrogels as wearable resistive strain sensor for human motion detection or sensory skin employed in a soft robot.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/11/1873/s1, Figure S1: Design of 3D printed hydrogels with different infill angles (a) 0◦, (b) 45◦ and (c) 90◦, Figure S2: (a) Stress-Stain curve of 3D printed gel with different alginate concentration, (b) strength of 3D printed gels with different agar content (P > 0.05), means with different letters are statistically different at P < 0.05, (c) Stress-stain curve of 3D printed gel with different agar concentration, and (d) strength of 3D printed gels with alginate content (P > 0.05), means with different letters are statistically different at P < 0.05, Figure S3: The fitted stress-strain curve of both notched and unnotched sample (a) A1S2, (b) A1C2, (c) A2S2, and (d) A2C2, Figure S4: (a) swelling ratio of gel with different agar content, (b) swelling ratio of gel with different alginate content, and (c) Swelling ratio of A2C2 gel with different infill method, Figure S5: Live and dead cell image (a) control, and (b) A2S2, Figure S6: High transparency and conductivity of 3D printed hydrogels, Figure S7: (a) Conductivity of hydrogels (sodium *Polymers* **2019**, *11*, 1873

alginate (SA) 200 mg and acrylamide (AAm) 1200 mg) by injection molding method with various concentration of calcium chloride, means with different letters are statistically different at P < 0.05, (b) Conductivity of hydrogels (AAm 1200 mg and CaCl2 100 mM) by injection molding method various alginate content, (P > 0.05), means with different letters are statistically different at P < 0.05, and (c) Conductivity of hydrogels (SA 200 mg and CaCl2 100 mM) by injection molding method various concentration of acrylamide, (P > 0.05), means with different letters are statistically different at P < 0.05, Table S1: the formula of printing ink in 10 mL DI water, Table S2: the mechanical properties of printed hydrogels with different printing parameter.

**Author Contributions:** Data curation, S.E.R. and F.N.; Formal analysis, J.W., S.E.R., F.N., G.C. and W.C.; Investigation, J.W., Y.L. and S.S.; Methodology, J.W.; Project administration, J.Q.; Resources, G.C., W.C. and J.Q.; Writing—original draft, J.W.; Writing—review & editing, J.W., Y.L., S.S. and J.W.

**Funding:** Fundamental Research Funds for the Central Universities: 19D110112

**Acknowledgments:** The authors thank the financial support from the Fundamental Research Funds for the Central Universities (19D110112). This research is also supported by the Initial Research Funds for Young Teachers of College of Textiles, Donghua University.

**Conflicts of Interest:** The authors declare no conflict of interest.
