With the substantial development of electronic skins and personalized healthcare monitoring technologies, the use of wearable strain sensors has received tremendous attention. (Meng et al., 2022; Li, Zhou, Sarkar, Gagnon-Lafrenais, & Cicoira, 2022; Zhang et al., 2022; Su et al., 2022). As one of the emerging materials, conductive hydrogels have been widely investigated for the preparation of flexible wearable strain sensors (Nie et al., 2022; Zheng et al., 2022) due to their excellent stress-strain adaptability (He et al., 2022; Zhang et al., 2022), tunable mechanical properties, and remarkable biological characteristics (Ye et al., 2022; Yao et al., 2022). Nowadays, hydrogel materials used in strain sensing are increasingly required to meet the diversity of the function, such as a quick and reliable real-time signal response with high and stable conductivity, work efficiently under various conditions, high mechanical performance, strong adhesion to different substrates and easily manipulated (Huang et al., 2022; Yang et al., 2021). Unfortunately, many of the hydrogel-based sensors available today (such as ionic gel, organogel, and composite hydrogels containing conductive nanomaterials) (Li et al., 2022) are unable to adequately satisfy all the requirements listed above. Previous hydrogel systems had some drawbacks, such as poor structural stability, limited mechanical strength and elasticity, unconformable interaction with skin, and a significantly reduced service life. Therefore, it is essential, but also difficult, to create self-adhesive, electrically conductive hydrogel materials using a simple method (Yu et al., 2022; Wang et al., 2022).
In materials science, two-dimensional (2D) sheet-based nanomaterials have already demonstrated interesting and practical potential in various applications including sensing, electronics and optoelectronics. Among them, MXene, as a two-dimensional inorganic substance, has been extensively studied due to its excellent conductivity, good biocompatibility and high hydrophilicity, and is an excellent choice for the preparation of conductive hydrogels (Wang et al., 2021). For example, Huang et al. used ammonium polyphosphate (APP) to interact with MXene sheets at the multimolecular level (hydrogen bond, coordination bond, electrostatic), and the conductivity of the synthesized MXene/PAA composite could reached 8312.4 S cm− 1 (Huang et al., 2022). However, most of the reported studies focus on the improvement of mechanical or electrical properties, and to some extent ignore other properties that are equally important for practical applications of conductive organic hydrogels, such as adhesion and self-healing properties (Shi et al., 2021). In addition, for the flexible and stretchable hydrogels based on MXene nanosheets, these water-insoluble nanosheets will inevitably aggregate during gelation, leading to serious degradation of the conductive path of the hydrogel network, as well as irreversible damage to the sensor performance. Thus, it is still desirable to further improve the interfacial interaction as well as the versatility of hydrogels, so as to become high performance candidate materials for wearable/stretchable electronics.
Recently, the development of conductive networks with lower electrical conductivity percolation thresholds should prefer supporting or template materials that can establish strong interfacial interaction with conductive fillers, boosting the flexibility and sensitivity (Yang et al., 2022; Qin et al., 2022). For example, Yang et al. obtained a highly conductive graphene/polyvinyl alcohol composite by regulating the interfacial interactions, and the conductivity could reach 25 S m− 1 at the graphene content of only 6.25% (Yang et al., 2019). Combining the advantages of interpenetrating networks, especially for the typical double-network (DN) system, is an effective method to prepare polysaccharide hydrogel sensors, resulting in optimized mechanical properties, self-healing ability and interface adhesion. In search of substrate materials for this purpose, biomass bacterial cellulose nanofiber (BCNF), biologically synthesized by Acetobacter xylinum, appears to be an ideal candidate due to the ultra-thin nanofibers (30–50 nm) and unique three-dimensional (3D) interconnected network structure. Such advantages in combination with their higher aspect ratio and higher mechanical strengths have imparted BCNF with unique features that can be harnessed for excellent mechanical enhancement in polymer matrix (Chibrikov et al., 2022; Yang et al., 2021). For instance, Blaker et al. prepared self-reinforced PLA composites using only 2 wt% of modified BCNF, which showed a 175% increase in viscoelastic properties in terms of bending storage modulus (Blaker et al. 2014). More attractively, the abundant hydroxyl groups exposed on the surface of the BCNF allow uniformly dispersion and polymerization of monomers along the fibers for the construction of conductive network with high electrical properties. Recently, we have reported the use of BCNF as network pathway construction substrates and reinforcement materials to induce the formation of a nacre MXene-based film with remarkable electrical conductivity (2848 Scm –1) (Sun et al., 2021). Therefore, we expect such design of continuous conductive pathways with high aspect ratio and multiple network structure could grantee suitable mechanical properties at low loading of fillers, thus balancing the permeability threshold and strain-sensor behavior obtained.
On the basis of the aforementioned considerations, we used modified bacterial cellulose nanofibers (BCNF) with 30–50 nm diameter as double network hydrogel-reinforced substrates to prepare MXene-based strain sensor. The prepared hydrogel sensor exhibited high stretchability, shape adaptability, adhesion and rapid self-healing ability. The high aspect ratio as well as the strong inter-fiber connections of BCNF fillers are believed to reduce the contact resistance between organic-inorganic interfaces, and simultaneously endow the formation of more efficient mechanical interlocking. Therefore, a tightly packed 2D/1D structure was built in PVA matrix with well-percolated BCNF/PVA reinforcing skeleton and continuous MXene-MXene conductive paths. As a result, over a strain range of more than 250%, a measurement factor of 46.64 can be achieved, which is much better than most reported MXene-based stretchable strain sensors. The MPCB sensors thus have been shown to be useful in a variety of wearable motion monitoring ranges, including finger movements, elbow bending, and knee bending. Moreover, the sensor also exhibits multiple characteristics, i.e., ideal EMI, tunable flexibility, self-healing and self-adhesive performance, indicating its tremendous potential applications in future intelligent electronics.