*2.9. Statistical Analysis*

A one-way analysis of variance (ANOVA) with Fisher's pair-wise multiple comparison was employed to analyze the data. A *P*-value smaller than 0.05 was considered statistically significant.

#### **3. Results and Discussions**

#### *3.1. Viscosity and Printability*

To achieve 3D printable tough and conductive hydrogels, pre-gel solution was prepared by mixing sodium alginate (SA), agar (Ag), acrylamide (AAm), *<sup>N</sup>*,*<sup>N</sup>*'-methylenebis (acrylamide) (MBAA) and Irgacure 2959 into DI water. To well understand printability of hydrogels, the viscosity of pre-gel solution was well characterized by an AR-G2 stress-controlled rheometer. As shown in Figure 2a, the pure SA solution shows a low zero-shear viscosity that is below 30 mPa s that is regarded as minimum solution viscosity for extrusion printing in previous literature, leading to poor shape fidelity [23]. The increased viscosity can help to improve shape fidelity during printing. To improve the shape fidelity of SA, thermo-responsive agar gel solution was added into the 3D printing ink formulation. Compared to SA, agar is a thermo-responsive polymer, which exhibits high viscosity at low temperature. The mixture agar and SA exhibited an extremely higher viscosity at low shear rate and was shear thinning. With temperature decreasing, the viscosity of mixture dramatically increased because of agar gelation (the insert in Figure 2a). After adding a large amount of acrylamide, the printing ink was prepared, and the change of viscous behavior was negligible. Figure 2b showed storage and loss modulus, and tan δ of ink solution (SA 200 mg, Ag 200 mg, and AAm 3000 mg). Tan δ is the ratio of loss modulus to storage modulus. The printing ink solution had a tan δ value below 1, indicating this printing ink is more gel-like than liquid [16].

**Figure 2.** (**a**) Rheological data for ink formulation, the insert: Viscosity of Sodium alginate (SA) 200/Ag 200 with different temperature at shear rate 1/s, and (**b**) Tan δ of the ink solution, the inset: the storage and loss modulus of ink solution.
