*2.1. Materials*

Sodium alginate was received from FMC biopolymer (Rockland, ME, USA). Agar, acrylamide, *<sup>N</sup>*,*<sup>N</sup>*'-methylenebis (acrylamide) (MBAA), Irgacure 2959 and calcium chloride were ordered from Sigma-Aldrich (St. Louis, MO, USA) without further purification. An acrylic elastomer (VHB 4905) was received from 3M (St. Paul, MN, USA). Copper tape was used to connect conductive gels and electric wires.

#### *2.2. 3D Printing System*

As shown in Figure 1, a modified Leapfrog 3D printer was employed to fabricate tough hydrogels as described in our previous literature [18]. In brief, a syringe pump (NE-500 OEM, New Era, Gawler, Australia) was installed onto 3D printer to extrude pre-gel ink at a controlled infusion velocity (0.73 μL h−1–2100 mL <sup>h</sup>−1), and a thermal pad (HEATER-KIT-5SP, New Era, Gawler, Australia) was used to wrap up syringe to maintain printing temperature. The blunt tip needles (gauge 14–26) were used to inject continuous hydrogel solution. A commercial software "Simplify 3D" was applied to control printing process.

**Figure 1.** Schematic diagram of a 3D printing procedure.

#### *2.3. 3D Printing Hydrogels Fabrication*

The 3D printing procedure was performed on a modified Leapfrog 3D printer that is similar to our previous literature [17]. SA 200 mg was first dissolved in 10 mL DI water with continuous stirring overnight. Then the SA solution was heated to 95 ◦C in an oil bath, and 200 mg of agar was added into the solution. After agar was fully dissolved in the water and 3000 mg acrylamide and corresponding MBAA, Irgacure 2959 were added, the hybrid ink was cooled to 55 ◦C and ready for printing. As shown in Figure S1, three different infill angles were used to achieve 3D printing constructs. The width and length of design were set at 100 mm. The thickness of 3D printing constructs was around 1 mm. After printing, the printed construct was exposed to UV light (365 nm) for 1 h. Then 100 mM CaCl2 solution was used to crosslink sodium alginate for 15 min. To achieve conductive printed hydrogels, the printed gels was soaking in 100 mL 100 mM CaCl2 solution overnight to increase the concentration of Ca2+ ions in printed gels. These excess Ca2+ ions can be used as ions carriers. This sample was labelled as A2C2. The formula and labels of other samples were summarized in Table 1.


**Table 1.** The formula of printing ink in 10 mL Deionized (DI) water.

### *2.4. Mechanical Test*

The tensile test and pure shear test were performed on a universal tensile machine (AGS-X, SHIMADZU, Kyoto, Japan) at room temperature. Each sample was measured in triplicate. The tensile measurement was performed at a crosshead speed of 10 mm min−1. The stress with 0 to 10% strain was used to calculate the elastic modulus (E). The stress σ was calculated by the following equation [34].

$$
\sigma = \frac{F}{WT} \tag{1}
$$

where *F* is force, *W* and *T* mean width and thickness of sample. The pure shear test is used to calculate the fracture energy. Two same samples are used at one test, one notched sample and one un-notched sample, and the notched sample is measured at first to ge<sup>t</sup> the point at which crack propagation began, while the un-notched sample is measured to ge<sup>t</sup> the force-displacement curve. The fracture energy is calculated by the following equation [35].

$$\text{Fracture\\_energy} = \frac{\int\_{l\_0}^{l\_c} \text{Fdl}}{\text{WT}} \tag{2}$$

where *W* and *T* mean width and thickness of sample, *lc* represents critical distance, at which point crack propagation occurs, lo means initial length of sample.
