*3.6. Conductivity Analysis of Hydrogels*

The composite hydrogel not only possessed excellent mechanical properties and self-healing ability but also possessed outstanding electrical conductivity due to the existence of CNTs and PANI in the hydrogels. The conductivity was quantitatively characterized by the *I–V* measurement at potential ranging from −4 to 4 V. In Figure 6a, the *I–V* curves of composite hydrogels are all linear and non-hysteretic, indicating the excellent electro-conductive character. The conductivity of the TOCNF-CNT@PANI/PVA-3, TOCNF-CNT@PANI/PVA-2, TOCNF-CNT@PANI/PVA-1, and TOCNF-CNT/PVA composite hydrogels were 15.3, 12.8, 8.2, and 6.4 S m−1, respectively. The value was superior to phytic acid cross-linked polyaniline/poly(N-isopropylacrylamide) (PANI/PNIPAM) conductive hydrogels (~0.8 S m−1) [15]. polyaniline-poly(styrene sulfonate) (PANI-PSS) hydrogels (~10−<sup>2</sup> S m<sup>−</sup>1) strengthened by sorbitol derivatives (DBS) supramolecular nanofibers [40]. Theoretically, CNTs and PANI were the main active material of conductive network within the composite hydrogels. With PANI as the shell and CNTs as the core, a composite fiber with a "core-shell" structure was formed. The electrical conductivity of the composite fiber was higher than that of bare CNTs fibers [44]. The conductivity of hydrogel increased rapidly when the mass ratio of ANI to TOCNF-CNTs increased from 1:1 to 2:1. However, the conductivity of hydrogel increased slowly, when the mass ratio of ANI to TOCNF-CNTs changed from 2:1 to 3:1. It could be concluded that a 2:1 ratio of ANI and TOCNF-CNTs could form the most perfect conductive network. As the ratio of ANI to TOCNF-CNT increased to 3:1, the TOCNF-CNT skeleton framework was insufficient to load excess PANI, which resulted in a slow increase in conductivity. Consequently, the well-integrating and stability of the TOCNF-CNT@PANI conductive network with the "core-shell" structure offered an effective electron-transfer pathway in the hydrogel. TOCNF-CNT@PANI/PVA-2 was selected for the next experiment based on the previous mechanical test results.

**Figure 6.** (**a**) Current-voltage (*I–V*) curves of TOCNF-CNT@PANI/PVA with different doping concentrations of PANI. (**b**) *I–V* curves of TOCNF-CNT@PANI/PVA-2 hydrogel after multiple cutting/healing cycles. (**c**) Cycling of the cutting/healing processes for TOCNF-CNT@PANI/PVA-2 at the same location under ambient conditions. (**d**) Time dependence of the electrical healing process by *I-V* measurements under ambient conditions. (**e**) Optical images of TOCNF-CNT@PANI/PVA-2 hydrogel under a cutting/healing cycle in a circuit with a light-emitting diode (LED) bulb.

The electrical conductivity's self-healing efficiency of composite hydrogels was further investigated. The conductivity of original, cutting, and self-healed TOCNF-CNT@PANI/PVA-2 hydrogel was characterized by the *I–V* measurement in Figure 6b. After 10th, 20th, and 30th self-healing, the conductivity of TOCNF-CNT@PANI/PVA-2 hydrogel was 12.8, 11.6, 10.0, and 8.0 S m<sup>−</sup>1, respectively. The self-healing efficiency was calculated by σr/σ<sup>i</sup> (σ<sup>r</sup> is the healing conductivity, and σ<sup>i</sup> is the original conductivity) [45]. After 10th, 20th, and 30th self-healing, the self-healing efficiency of TOCNF-CNT@PANI/PVA-2 hydrogel was 90.6%, 78.1%, and 62.5%, respectively. The average efficiency was 99.1% for each self-healing cycle, indicating the composite hydrogel possessed significant and repeatable electrical restoration performance.

By repeating the complete cutting/self-healing process, without any external force at room temperature, the conductivity of TOCNF-CNT@PANI/PVA-2 hydrogel was tested through *I-V* measurement. Figure 6c shows the time-current flow of TOCNF-CNT@PANI/PVA-2 hydrogel at the same location during repeated cutting/healing processes. In Figure 6d, when the hydrogel was completely cut in half to form an open circuit, the current dropped to zero. Then, the two fractured parts contacted each other, and the current quickly recovered to the initial value through a 20 s in situ self-healing. The conductivity of the hydrogel sample remained stable during the cycle, indicating that the conductivity had a high self-healing efficiency during the cutting-healing process.

As shown in Figure 6e, the self-healing conductive performance of the composite hydrogel was visually displayed through a closed-loop composed of light-emitting diode (LED), TOCNF-CNT/PVA-2 hydrogel, and power components. The LED indicator was lighted with a voltage of 5 V. The LED indicator was extinguished when the TOCNF-CNT/PVA-2 hydrogel was completely separated. However, the LED indicator lit up again, after pushing the two separated parts together for self-healing, illustrating the excellent self-healing conductive property of the composite hydrogel. The hierarchical 3D network consisting of PANI, CNTs, and TOCNFs formed a continuous conducting pathway for electron transport. The dynamically reversible cross-linking points from different borate-induced complexes provided inherent and repeatable self-healing capabilities for hydrogels, exhibiting promise for the self-healing electrode materials [46].
