Hydrogels, as classic soft materials, are important materials for tissue engineering and biosensing with unique properties, such as good biocompatibility, high stretchability, strong adhesion, excellent self-healing, and self-recovery. Conductive hydrogels possess the additional property of conductivity, which endows them with advanced applications in actuating devices, biomedicine, and sensing. In this review, we provide an overview of the recent development of conductive hydrogels in the field of strain sensors, with particular focus on the types of conductive fillers, including ionic conductors, conducting nanomaterials, and conductive polymers. The synthetic methods of such conductive hydrogel materials and their physical and chemical properties are highlighted. At last, challenges and future perspectives of conductive hydrogels applied in flexible strain sensors are discussed.

Recently, hydrogel flexible sensors have attracted wide attention in the field of wearable electronic devices. However, they need an external solid-state power supply to output stable signals. The huge modulus difference between solid-state power supplies and hydrogel flexible sensors is not conducive to the integration, portability, and application of hydrogel flexible sensors. Therefore, it is necessary to design power supplies with a similar modulus to those of flexible sensors and assemble them with flexible sensors to develop self-powered flexible sensing systems, which is important to facilitate the integration, portability, and application of flexible sensors. In this work, polyvinyl alcohol (PVA), sodium chloride (NaCl), and N-hydroxyethyl acrylamide (HEAA) were used to synthesize a fully physically crosslinked double network hydrogel (PNH DN hydrogel) with excellent mechanical performance (tensile stress/strain of 3.76 MPa/872.15%), high ionic conductivity (1.3 S m⁻¹), and rapid self-recovery (energy dissipation/peak stress recovery efficiency of 57.08%/99.06% after a rest time of 10 min). Based on PNH DN hydrogels, a hydrogel strain sensor and a flexible solid-state supercapacitor were prepared, respectively. The hydrogel strain sensor based on the PNH DN hydrogel showed good sensitivity (gauge factor = 1.74), a wide detection range (0–735%), and a negligible response delay time (0.06 s), which are helpful for timely and reliable detection of various mechanical deformations. Meanwhile, the supercapacitor based on the PNH DN hydrogel exhibited an areal capacitance of 2.9 mF cm⁻², a high cycle stability (areal capacitance retention rate of 85.71% after 2000 galvanostatic charge–discharge cycles at a current density of 0.5 mA cm⁻²), and mechanical stability (cyclic voltammetry curves were almost overlapped under 0°, 45°, 90°, 135°, and 180° bending). Finally, the hydrogel strain sensor and supercapacitor were integrated into a self-powered flexible sensing system, which could stably and effectively detect human motions, such as finger bending, neck bending, and frowning. Overall, this work provides a new idea to improve the integration, portability, and application of hydrogel flexible sensors and can broaden the application of hydrogels in the field of new energy.

Flexible sensors based on conductive hydrogels have been of wide interest in the field of smart wearable electronics due to the excellent stretchability and strain-responsive ability. However, lacking harsh environment tolerance and self-recovery properties seriously limit their practical applications. Therefore, the development of anti-fatigue hydrogels with anti-freezing and water-retaining abilities is urgently required. In this study, we constructed a fully physically cross-linked gelatin/poly(N-hydroxyethyl acrylamide)/glycerin/lithium chloride double network (gelatin/pHEAA/Gly/LiCl DN) organohydrogel based on a hydrogen bond crosslinking strategy using a facial one-pot method. The dynamic hydrogen bond in the DN organohydrogels provided an effective energy dissipation pathway, which produced gels with high tensile strength/strain (2.14 MPa/1637.49%), fast self-recovery properties and strong interfacial toughness. The introduction of binary solvents of water and glycerin endowed the DN organohydrogels with excellent anti-freezing and water-retaining properties. Furthermore, a simple flexible sensor was fabricated based on the organohydrogel for detecting human motions. The sensor not only showed remarkable sensitivity (GF = 14.54), broad strain range (0–1600%) and high response speed (0.2 s), but also presented accurate and reliable signals under different mechanical deformations and low temperature (−20 °C). This work provides a feasible way to build high mechanical and sensing performance organohydrogel-based sensors with anti-freezing and water-retaining abilities, which greatly promotes the application of flexible sensors in the field of smart wearable electronic devices, electronic skin and human/machine interface.

A facile strategy to fabricate an advanced AIE fluorescence-based information encryption platform with self-erasure ability by combining the shape and fluorescence change of the TPE3N ³⁺ -hydrogel/PAA-hydrogel bilayer hydrogel was demonstrated.