**1. Introduction**

The skin is the largest organ of the human body, and has the characteristics of flexibility, self-repairability, and tactile sensitivity [1–3]. Bionic tactile sensor devices that mimic the characteristics and functions of human skin are referred to as electronic skin [4]. With the rapid development of electronic technology in recent years, increasingly more electronic devices are developing towards the directions of achieving miniaturization, flexibility, and lightweight properties [5,6]. Electronic skin has also been widely used in wearable devices, health monitoring, intelligent robots, and bionic prostheses [7–10].

In order to perceive deformations in real time, electronic skin must be highly flexible and resilient, and should be able to generate specific response signals to stimuli and provide timely feedback [11,12]. Conductive hydrogels are soft materials with good flexibility and biocompatibility, whose structure is similar to natural living tissue [13–15]. Conductive hydrogels are considered to constitute an ideal material for the preparation of bionic electronic skin due to their functional designability [16–18]. Bionic electronic skin requires a high level of electrical conductivity and sensitivity, which are often achieved by introducing conductive filling materials [19]. Examples of such incorporations include the addition of conductive polymers (polypyrrole (PPy) and polyaniline (PANI)) and inorganic nanomaterials (carbon nanotubes, graphene, and metal nanoparticles) [13,20–22]. Among

**Citation:** Chen, X.; Zhang, H.; Cui, J.; Wang, Y.; Li, M.; Zhang, J.; Wang, C.; Liu, Z.; Wei, Q. Enhancing Conductivity and Self-Healing Properties of PVA/GEL/OSA Composite Hydrogels by GO/SWNTs for Electronic Skin. *Gels* **2023**, *9*, 155. https://doi.org/ 10.3390/gels9020155

Academic Editor: Tal Dvir

Received: 31 January 2023 Revised: 7 February 2023 Accepted: 13 February 2023 Published: 15 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

them, graphene and carbon nanotubes have excellent electrical, mechanical, and surface properties, and have shown the greatest potential for use in bionic electronic skin [23,24]. Compared with conductive polymers, the excellent mechanical properties of graphene and carbon nanotubes can improve the overall mechanical properties (especially the toughness) of hydrogels, while building stable conductive networks within the hydrogels [25–27]. For example, Wu et al. [28] prepared polyvinyl alcohol (PVA)/graphene oxide (GO) composite hydrogels by introducing metal ion coordination bonds. The tensile strength of the PVA-GO hydrogel could reach 11.10 MPa, which is 175% higher than that of pure PVA. Wu et al. [29] successfully prepared a composite hydrogel based on PVA, carboxymethyl chitosan (CMCS), oxidized sodium alginate (OSA), and oxidized multi-walled carbon nanotubes (OMWCNTs). The addition of OMWCNTs increased the fracture strength and electrical conductivity of the hydrogel to 0.8 MPa and 70.2 mS/m, respectively, which were 2.5 and 4 times higher than those of the previous hydrogel. The change in the resistance of the composite hydrogel after 200 cycles of 20% constant strain was almost identical to the initial value, thus demonstrating the excellent stability of the composite hydrogel. Li et al. [30] successfully constructed gelatin (GEL)-OSA-PVA ternary hydrogels by mixing PVA with GEL and OSA. By adjusting the concentration of PVA and the degree of oxidation of OSA, the hydrogel obtained adjustable mechanical properties with a maximum compressive modulus of 75 kPa. In the 300 s fatigue compression test, a large amount of PVA only reduced the stress of the hydrogel by 30.99%. In addition, the hydrogel exhibits good electrical conductivity (10.68 mS/m) due to the presence of free ions.

In addition to the mechanical and electrical properties mentioned above, the selfhealing properties and biocompatibility of hydrogels are also of interest. Self-healing hydrogels can spontaneously repair damage without any external stimulation, which can effectively extend the lifetime of hydrogel electronic skins and expand their applications in extreme environments [31,32]. Self-healing is mainly achieved by constructing reversible dynamic bonds (non-covalent and dynamic covalent bonds) in hydrogel networks. The non-covalent bonds include hydrogen bonds, metal coordination bonds, etc. Dynamic covalent bonds include imine/hydrazone bonds, Diels–Alder reactions, borate ester bonds, and disulfide bonds [33,34]. In addition, electronic skin needs to be biocompatible to avoid damaging human skin when employed as health-monitoring devices [25,35]. Therefore, a kind of hydrogel with excellent properties (conductivity, mechanics, self-healing, biocompatibility, etc.) used for electronic skin still needs to be found, and this pursuit is attracting a considerable amount of attention.

Herein, we report the construction of a conductive and self-healing composite hydrogel using PVA, GEL, OSA, GO, and single-walled carbon nanotubes (SWNTs). The formation of imine bonds between GEL and OSA allows the hydrogel to heal itself without any external stimulation. Two kinds of nanomaterials, GO and SWNTs, were applied to enhance the properties of the composite hydrogel, such as effecting a higher compression modulus, better elastic behavior, and enhanced electrical conductivity. Finally, the prepared composite hydrogel proved to have excellent biocompatibility and sensitivity. These characteristics show that the prepared composite hydrogel has great potential in application scenarios regarding electronic skin such as wearable devices, health monitoring, and voice recognition.

#### **2. Results and Discussion**

#### *2.1. Microtopography of Hydrogels*

As shown in Figure 1, which presents scanning electron microscopy (SEM) images of the freeze-dried PGO and PGO-GA hydrogels, it can be seen that all the freeze-dried hydrogels show the characteristics of a porous structure. Particularly, the GO sheets and SWNTs were evenly distributed without agglomeration in the PGO-GS3 hydrogel framework (Figure 1c). Compared with the PGO hydrogel, the PGO-GS3 hydrogel has a porous three-dimensional network structure and more uniform pore size, which is conducive to the conductivity and mechanical strength of hydrogels.

the conductivity and mechanical strength of hydrogels.

**Figure 1.** The SEM images of (**a**) PGO hydrogel, (**b**) PGO-GS1 hydrogel, (**c**) PGO-GS3 hydrogel, and (**d**) PGO-GS5 hydrogel. **Figure 1.** The SEM images of (**a**) PGO hydrogel, (**b**) PGO-GS1 hydrogel, (**c**) PGO-GS3 hydrogel, and (**d**) PGO-GS5 hydrogel.

As shown in Figure 1, which presents scanning electron microscopy (SEM) images of the freeze-dried PGO and PGO-GA hydrogels, it can be seen that all the freeze-dried hydrogels show the characteristics of a porous structure. Particularly, the GO sheets and SWNTs were evenly distributed without agglomeration in the PGO-GS3 hydrogel framework (Figure 1c). Compared with the PGO hydrogel, the PGO-GS3 hydrogel has a porous three-dimensional network structure and more uniform pore size, which is conducive to

The density and porosity of the hydrogels were also calculated, as shown in Table 1. Notably, the PGO-GS3 hydrogel has the highest porosity and the lowest density; this is because they are closely related to the structure of the hydrogel. This result is consistent with the results observed using electron microscopy. The density and porosity of the hydrogels were also calculated, as shown in Table 1. Notably, the PGO-GS3 hydrogel has the highest porosity and the lowest density; this is because they are closely related to the structure of the hydrogel. This result is consistent with the results observed using electron microscopy.

**Table 1.** Density and porosity of hydrogels. **Table 1.** Density and porosity of hydrogels.


Figure S1 shows the swelling ratios of the hydrogels. The swelling ratio of the PGO hydrogel reaches 13.91, which is significantly higher than that of the PGO-GS hydrogel. This indicates that the PGO hydrogel has large internal pores and loose structures, and this finding is consistent with the SEM images. The reason behind this is that the addition of GO and SWNTs may increase the viscosity of the hydrogel, resulting in a denser network, and the denser hydrogel network prevents water molecules from diffusing into the Figure S1 shows the swelling ratios of the hydrogels. The swelling ratio of the PGO hydrogel reaches 13.91, which is significantly higher than that of the PGO-GS hydrogel. This indicates that the PGO hydrogel has large internal pores and loose structures, and this finding is consistent with the SEM images. The reason behind this is that the addition of GO and SWNTs may increase the viscosity of the hydrogel, resulting in a denser network, and the denser hydrogel network prevents water molecules from diffusing into the hydrogel.
