Recently, in order to meet the demand of personalized monitoring of physiological signals (heart-beating rate (Ye et al, 2020), movement (Wu et al, 2019) and vocalization (Pan et al, 2019)), abundant wearable devices have emerged in the market, such as smart glasses, smart watches and medical smart wristbands. However, the problems of uncomfortable wearing and poor adaptability to complex surfaces have not been solved yet. Nowadays, flexible wearable electronic devices (flexible sensors (Bao et al, 2021; Chen et al, 2020), implantable medical devices (Li et al, 2019; Zheng et al, 2019), bionic skin (Ma et al, 2019; Zheng et al, 2021), etc.) have attracted extensive attention due to their wearability, real-time feedback, remote control and other advantages (Huang et al, 2020). Acting as the indispensable unit of flexible wearable devices, flexible sensors, the component that can accept external stimuli (strain (Mao et al, 2020; Zha et al, 2020), pressure (Liu et al, 2020; Wang et al, 2021), temperature (Chen et al, 2021; Feng et al, 2021), humidity (Pan et al, 2020; Wu et al, 2019b) and gas (Wu et al, 2020; Wu et al, 2021)) and convert them into detectable electrical signals, overcome the rigidity of traditional metal-based or semiconductor-based sensors and exhibit higher flexibility, ductility and adaptability. Defined as flexible materials with three-dimensional porous structure and high-water content, hydrogels are similar to human skin in many aspects, hence they are widely application in the fields of drug carriers (Mandal et al, 2017) and tissue engineering (Henn et al, 2022). Additionally, conductive hydrogels are supposed to be desirable choice for sensors because of their excellent stretchability and biocompatibility, and yet the sensitivity of most hydrogel-based sensors is lower than that of others (fiber-based (Pan et al, 2020), elastomer-based (Gong et al, 2017; Zhang et al, 2020), mixed (Han et al, 2019)). Therefore, exploiting highly sensitive conductive hydrogels without sacrificing physical properties is of great significance to the promotion of flexible wearable devices.
Since all external stimuli are ultimately reflected in the form of electrical signals, the selection and distribution of conductive materials is the top priority of wearable sensors. Based on different conductive principles, the selectable conductive materials include ionic conductors (electrolyte (Chen et al, 2021; Liu et al, 2021; Lu et al, 2021), ionic liquid (Izawa et al, 2009; Lee et al, 2008)) and electronic conductors (intrinsic conductive polymer (Jiao et al, 2021; Zhou et al, 2021), carbon-based materials (Liao et al, 2019; Sun et al, 2021) and metal-based materials (Wang et al, 2019; Zhang et al, 2022)). Among them, AgNWs have attracted extensive attention because of their good conductivity and bactericidal properties (Pan et al, 2021). At present, conductive hydrogels are usually prepared by a simple one-pot process to obtain the homogeneous dispersion in the hydrogel (Chen et al, 2018; Hu et al, 2022; You et al, 2021). But it is not economical since a large number of conductive fillers are needed, and the cost problem is intensified when the expensive conductors like AgNWs and MXene are used. Moreover, electronic conductors tend to aggregate with the movement of water molecules, resulting in the deteriorated conductivity during long-term storage. The effect of conductive fillers on the mechanical properties and biocompatibility of hydrogels is also a latent question worth considering as well. The Janus structure (Zhao et al, 2017) can be obtained by depositing conductive materials on the upper surface of hydrogels, which not only can decrease the quantity of fillers, but also can enhance the sensitivity through the conductivity difference between the surface layer and the hydrogel itself. In view of strain sensors, the conductivity network varies with the overall stretch of the hydrogel, and the surface structure hardly affects the sensitivity for homogeneous hydrogels. For instance, Wang et al. (Wang et al, 2019) prepared a poly (3, 4-ethylenedioxythiophene): sulfonated lignin/PAA hydrogel and then succeed in constructing self-wrinkled structure by solvent replacement, but the improvement of GF was not obvious enough. However, the change of the surface structure for the stretched hydrogel with Janus structure will directly affect the conductive network, thereby improving the sensitivity of strain sensing.
As an all-in-one conductive hydrogel, the adhesion of conductive material and hydrogel body is important to the service life of the sensor. The conventional method is encapsulating to prevent the surface conductors from falling off, but the commonly used packaging materials such as PDMS (Gong et al, 2017) will greatly influence the sensitivity and extensibility of the hydrogels. To address the issue of good adhesion, the combination of fillers and polymer network by coordination bonds, hydrogen bonds or others interactions needs designing. Inspired by mussels, researchers found that substances with catechol structure (polydopamine (Rahim et al, 2016; Teixeira et al, 2012; Wu et al, 2020), tannic acid (He et al, 2021; Sun et al, 2021)) combine matrix via π-π interactions, metal coordination, covalent crosslinks and hydrogen bonds (Xie et al, 2020). The use of adhesive matrix materials is an easier method to make adhesive hydrogels, though the adhesion of these materials is relatively weak (Xia et al, 2019). Fu et al (Fu et al, 2021) and Lu et al. (Lu et al, 2020) prepared PAA/PEDOT hydrogels and PAA/CNS hydrogels respectively, and proved their adhesion. In addition, good adhesion is also conducive to tight fit between hydrogels and skins, so that their deformations are precisely matched and the accuracy of sensors reflecting physiological signal is improved (Pan et al, 2021; Sun et al, 2021).
Herein, we demonstrated a simple two-step method to fabricate hydrogel-based strain sensors with self-healing, water retention, and ultra-high sensitivity as flexible wearable devices. Briefly speaking, the hydrogel body is comprised of PAA and CS via a one-pot process, and the wrinkled surface of AgNWs layer is obtained by simple dripping-drying (Fig. 1). The Janus structure and the conductivity difference between AgNWs and PAA/CS hydrogels endows the outstanding sensing sensitivity (GF = 191.2, 7413, 18720). In addition, the good self-adhesive of the PAA/CS hydrogel ensures the adhesive strength of the AgNWs and the precise matching of the sensor to external strain stimuli. Good physical properties and excellent sensing performance show the potential application in electronic skins, soft robots and interactive devices.