Zwitterionic Osmolyte‐Based Hydrogels with Antifreezing Property,High Conductivity,and Stable Flexibility at Subzero Temperature

Conductive hydrogels have emerged as fascinating materials applied in flexible electronics because of their integrated conductivity and mechanical flexibility. However, the large amounts of water in conductive hydrogels inevitably freeze at subzero temperature, causing a reduction of their ionic transport ability and elasticity. Herein, the bioinspired antifreezing agents—zwitterionic osmolytes (e.g., betaine, proline) are first proposed to prevent ammonium chloride‐containing Ca‐alginate/polyacrylamide hydrogels from freezing. With a facile one‐pot solvent displacement method, the zwitterionic osmolytes can displace the water molecules inside the hydrogels. Due to the excellent freeze tolerance of zwitterionic osmolytes, the resulting zwitterionic osmolyte‐based hydrogels exhibit outstanding ionic conductivity (up to ≈2.7 S m^?1) at ?40 °C, which exceeds the conductivities of most reported conductive hydrogels. Meanwhile, they present stable mechanical flexibility over a wide temperature range (?40 to 25 °C). More importantly, two types of the resulting hydrogel‐based flexible electronics, including a capacitive sensor and a resistive sensor, can maintain their response function at ?40 °C. This work offers a new solution to fabricate conductive hydrogels with antifreezing ability, which can broaden the working temperature range of flexible electronics.     

Environment Tolerant Conductive Nanocomposite Organohydrogels as Flexible Strain Sensors and Power Sources for Sustainable Electronics

Conductive hydrogels (CHs) have been highlighted in the design of flexible strain sensors and stretchable triboelectric nanogenerators (TENGs) on the basis of their excellent physicochemical properties such as large stretchability and high conductivity. Nevertheless, the incident freezing and drying behaviors of CHs by using water solvent as the dispersion medium limit their application scopes significantly. Herein, an environment tolerant and ultrastretchable organohydrogel is demonstrated by a simple solvent-replacement strategy, in which the partial water in the as-synthesized polyacrylamide/montmorillonite/carbon nanotubes hydrogel is replaced with the glycerol, leading to excellent temperature toleration (−60 to 60 °C) and good stability (30 days under normal environment) without sacrificing the stretchability and conductivity. The organohydrogel exhibits an ultrawide strain sensing range (0–4196%) with a high sensitivity of 8.5, enabling effective detection and discrimination of human activities that are gentle or drastic under various conditions. Furthermore, the organohydrogel is assembled in a single-electrode TENG, which displays excellent energy harvesting ability even under a stretchability of 500% and robustness to directly power wearable electronics in harsh cold conditions. This work inspires a simple route for multifunctional organohydrogel and promises the practical application of flexible and self-powered wearable devices in extreme environments.     

Conductive MXene Nanocomposite Organohydrogel for Flexible,Healable, Low‐Temperature Tolerant Strain Sensors

Conductive hydrogels are attracting tremendous interest in the field of flexible and wearable soft strain sensors because of their great potential in electronic skins, and personalized healthcare monitoring. However, conventional conductive hydrogels using pure water as the dispersion medium will inevitably freeze at subzero temperatures, resulting in the diminishment of their conductivity and mechanical properties; meanwhile, even at room temperature, such hydrogels suffer from the inevitable loss of water due to evaporation, which leads to a poor shelf‐life. Herein, an antifreezing, self‐healing, and conductive MXene nanocomposite organohydrogel (MNOH) is developed by immersing MXene nanocomposite hydrogel (MNH) in ethylene glycol (EG) solution to replace a portion of the water molecules. The MNH is prepared from the incorporation of the conductive MXene nanosheet networks into hydrogel polymer networks. The as‐prepared MNOH exhibits an outstanding antifreezing property (?40 °C), long‐lasting moisture retention (8 d), excellent self‐healing capability, and superior mechanical properties. Furthermore, this MNOH can be assembled as a wearable strain sensor to detect human biologic activities with a relatively broad strain range (up to 350% strain) and a high gauge factor of 44.85 under extremely low temperatures. This work paves the way for potential applications in electronic skins, human?machine interactions, and personalized healthcare monitoring.     

Ultra-Sensitive and Stretchable Ionic Skins for High-Precision Motion Monitoring

Stretchability and sensitivity are essential properties of wearable electronics for effective motion monitoring. In general, increasing the sensitivity of strain sensors based on ionic conductors trades off elasticity, which results in low sensitivity of the strain sensors at large mechanical deformations. To address this, ion-permeable conducting polymer electrodes with low contact resistance are utilized in ionic gel-based strain sensors. Using a rectangular-shaped ionic gel and ion-permeable electrodes significantly increase the gauge factor of the strain sensor, similar to the theoretical value at a given strain. To further increase the sensitivity of the strain sensor, the ionic gel is patterned with zigzagged tracks that gap apart as the gel stretches, and the gaps close as the gel contracts, leading to a large variation in the relative resistance upon stretching. By combining the zigzagged ionic gel and the ion-permeable electrodes, highly sensitive stretchable sensors are realized with a record-high gauge factor of 173, compared to existing ionic conductor-based stretchable strain sensors. The zigzag-patterned ionic sensor can successfully monitor various motions when attached to the human body. These results are expected to afford promising strategies for developing highly sensitive, stretchable sensing systems for E-skin sensors and soft robotics.     

Highly Oxidation-Resistant and Self-Healable MXene-Based Hydrogels for Wearable Strain Sensor

Very recently, MXene-based wearable hydrogels have emerged as promising candidates for epidermal sensors due to their tissue-like softness and unique electrical and mechanical properties. However, it remains a challenge to achieve MXene-based hydrogels with reliable sensing performance and prolonged service life, because MXene inevitably oxidizes in water-containing system of the hydrogels. Herein, catechol-functionalized poly(vinyl alcohol) (PVA-CA)-based hydrogels is proposed to inhibit the oxidation of MXene, leading to rapid self-healing and superior strain sensing behaviors. Sufficient interaction of hydrophobic catechol groups with the MXene surface reduces the oxidation-accessible sites in the MXene for reaction with water and eventually suppresses the oxidation of MXene in the hydrogel. Furthermore, the PVA-CA-MXene hydrogel is demonstrated for use as a strain sensor for real-time motion monitoring, such as detecting subtle human motions and handwriting. The signals of PVA-CA-MXene hydrogel sensor can be accurately classified using deep learning models.     

Wearable Force Touch Sensor Array Using a Flexible and Transparent Electrode

Transparent electrodes have been widely used for various electronics and optoelectronics, including flexible ones. Many nanomaterial‐based electrodes, in particular 1D and 2D nanomaterials, have been proposed as next‐generation transparent and flexible electrodes. However, their transparency, conductivity, large‐area uniformity, and sometimes cost are not yet sufficient to replace indium tin oxide (ITO). Furthermore, the conventional ITO is quite rigid and susceptible to mechanical fractures under deformations (e.g., bending, folding). In this study, the authors report new advances in the design, fabrication, and integration of wearable and transparent force touch (touch and pressure) sensors by exploiting the previous efforts in stretchable electronics as well as novel ideas in the transparent and flexible electrode. The optical and mechanical experiment, along with simulation results, exhibit the excellent transparency, conductivity, uniformity, and flexibility of the proposed epoxy‐copper‐ITO (ECI) multilayer electrode. By using this multi‐layered ECI electrode, the authors present a wearable and transparent force touch sensor array, which is multiplexed by Si nanomembrane p‐i‐n junction‐type (PIN) diodes and integrated on the skin‐mounted quantum dot light‐emitting diodes. This novel integrated system is successfully applied as a wearable human–machine interface (HMI) to control a drone wirelessly. These advances in novel material structures and system‐level integration strategies create new opportunities in wearable smart displays.     

Muscle-Inspired MXene Conductive Hydrogels with Anisotropy and Low-Temperature Tolerance for Wearable Flexible Sensors and Arrays

Conductive hydrogels as flexible electronic devices, not only have unique attractions but also meet the basic need of mechanical flexibility and intelligent sensing. How to endow anisotropy and a wide application temperature range for traditional homogeneous conductive hydrogels and flexible sensors is still a challenge. Herein, a directional freezing method is used to prepare anisotropic MXene conductive hydrogels that are inspired by ordered structures of muscles. Due to the anisotropy of MXene conductive hydrogels, the mechanical properties and electrical conductivity are enhanced in specific directions. The hydrogels have a wide temperature resistance range of −36 to 25 °C through solvent substitution. Thus, the muscle-inspired MXene conductive hydrogels with anisotropy and low-temperature resistance can be used as wearable flexible sensors. The sensing signals are further displayed on the mobile phone as images through wireless technology, and images will change with the collected signals to achieve motion detection. Multiple flexible sensors are also assembled into a 3D sensor array for detecting the magnitude and spatial distribution of forces or strains. The MXene conductive hydrogels with ordered orientation and anisotropy are promising for flexible sensors, which have broad application prospects in human–machine interface compatibility and medical monitoring.     

Stretchable,Transparent, and Thermally Stable Triboelectric Nanogenerators Based on Solvent‐Free Ion‐Conducting Elastomer Electrodes

The development of stretchable/soft electronics requires power sources that can match their stretchability. In this study, a highly stretchable, transparent, and environmentally stable triboelectric nanogenerator with ionic conductor electrodes (iTENG) is reported. The ion‐conducting elastomer (ICE) electrode, together with a dielectric elastomer electrification layer, allows the ICE‐iTENG to achieve a stretchability of 1036% and transmittance of 91.5%. Most importantly, the ICE is liquid solvent‐free and thermally stable up to 335 °C, avoiding the dehydration‐induced performance degradation of commonly used hydrogels. The ICE‐iTENG shows no decrease in electrical output even after storing at 100 °C for 15 h. Biomechanical motion energies are demonstrated to be harvested by the ICE‐iTENG for powering wearable electronics intermittently without extra power sources. An ICE‐iTENG‐based pressure sensor is also developed with sensitivity up to 2.87 kPa^?1. The stretchable ICE‐iTENG overcomes the strain‐induced performance degradation using percolated electrical conductors and liquid evaporation‐induced degradation using ion‐conducting hydrogels/ionogels, suggesting great promising applications in soft/stretchable electronics under a relatively wider temperature range.     

Self‐Healing,Flexible, and Tailorable Triboelectric Nanogenerators for Self‐Powered Sensors based on Thermal Effect of Infrared Radiation

Self‐healing triboelectric nanogenerators (TENGs) with flexibility, robustness, and conformability are highly desirable for promising flexible and wearable devices, which can serve as a durable, stable, and renewable power supply, as well as a self‐powered sensor. Herein, an entirely self‐healing, flexible, and tailorable TENG is designed as a wearable sensor to monitor human motion, with infrared radiation from skin to promote self‐healing after being broken based on thermal effect of infrared radiation. Human skin is a natural infrared radiation emitter, providing favorable conditions for the device to function efficiently. The reversible imine bonds and quadruple hydrogen bonding (UPy) moieties are introduced into polymer networks to construct self‐healable electrification layer. UPy‐functionalized multiwalled carbon nanotubes are further incorporated into healable polymer to obtain conductive nanocomposite. Driven by the dynamic bonds, the designed and synthesized materials show excellent intrinsic self‐healing and shape‐tailorable features. Moreover, there is a robust interface bonding in the TENG devices due to the similar healable networks between electrification layer and electrode. The output electric performances of the self‐healable TENG devices can almost restore their original state when the damage of the devices occurs. This work presents a novel strategy for flexible devices, contributing to future sustainable energy and wearable electronics.     

Metal-Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel-Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human-Machine Interfaces

Flexible triboelectric nanogenerators (TENGs) with multifunctional sensing capabilities offer an elegant solution to address the growing energy supply challenges for wearable smart electronics. Herein, a highly stretchable and durable electrode for wearable TENG is developed using ZIF-8 as a reinforcing nanofiller in a hydrogel with LiCl electrolyte. ZIF-8 nanocrystals improve the hydrogel's mechanical properties by forming hydrogen bonds with copolymer chains, resulting in 2.7 times greater stretchability than pure hydrogel. The hydrogel electrode is encapsulated by microstructured silicone layers that act as triboelectric materials and prevent water loss from the hydrogel. Optimized ZIF-8-based hydrogel electrodes enhance the output performance of TENG through the dynamic balance of electric double layers (EDLs) during contact electrification. Thus, the as-fabricated TENG delivers an excellent power density of 3.47 Wm^–2, which is 3.2 times higher than pure hydrogel-based TENG. The developed TENG can scavenge biomechanical energy even at subzero temperatures to power small electronics and serve as excellent self-powered pressure sensors for human-machine interfaces (HMIs). The nanocomposite hydrogel-based TENG can also function as a wearable biomotion sensor, detecting body movements with high sensitivity. This study demonstrates the significant potential of utilizing ZIF-8 reinforced hydrogel as an electrode for wearable TENGs in energy harvesting and sensor technology.     

Multiscale Structural Nanocellulosic Triboelectric Aerogels Induced by Hofmeister Effect

The advent of self-powered wearable electronics will revolutionize the fields of smart healthcare and sports monitoring. This technological advancement necessitates more stringent design requirements for triboelectric materials. The triboelectric aerogels must enhance their mechanical properties to address the issue of structural collapse in real-world applications. This study fabricates stiff nanocellulosic triboelectric aerogels with multiscale structures induced by the Hofmeister effect. The aggregation and crystallization of polymer molecular chains are enhanced by the Hofmeister effect, while ice crystal growth imparts a porous structure to the aerogel at the micron scale. Therefore, the triboelectric aerogel exhibits exceptional stiffness, boasting a Young's modulus of up to 142.9 MPa and a specific modulus of up to 340.6 kN m kg^–1, while remaining undeformed even after supporting 6600 times its weight. Even after withstanding an impact of 343 kPa, highly robust wearable self-powered sensors fabricated with triboelectric aerogels remain operational. Additionally, the self-powered sensor is capable of accurately detecting human movements, particularly in abnormal fall postures detection. This study provides considerable research and practical value for promoting material design and broadening application scenarios for self-powered wearable electronics.     

A Puncture-Resistant and Self-Healing Conductive Gel for Multifunctional Electronic Skin

Flexible electronic skins (e-skins) play a very important role in the development of human–machine interaction and wearable devices. To fully mimic the functions of human skin, e-skins should be able to perceive multiple external stimuli (such as temperature, touch, and friction) and be resistant to injury. However, both objectives are highly challenging. The fabrication of multifunctional e-skins is difficult because of the complex lamination scheme and the integration of different sensors. The design of skin-like materials is hindered by the trade-off problem between flexibility, toughness, and self-healing ability. Herein, flexible sodium methallyl sulfonate functionalized poly(thioctic acid) polymer chains are combined with rigid conductive polyaniline rods through ionic bonds to obtain a solvent-free polymer conductive gel. The conductive gel has a modulus similar to that of skin, and shows good flexibility, puncture-resistance, notch-insensitivity, and fast self-healing ability. Moreover, this conductive gel can convert changes in temperature and strain into electrical signal changes, thus leading to multifunctional sensing performance. Based on these superior properties, a flexible e-skin sensor is prepared, demonstrating its great potential in the wearable field and physiological signal detection.     

MXene Functionalized,Highly Breathable and Sensitive Pressure Sensors with Multi-Layered Porous Structure

Breathable, flexible, and highly sensitive pressure sensors have drawn increasing attention due to their potential in wearable electronics for body-motion monitoring, human-machine interfaces, etc. However, current pressure sensors are usually assembled with polymer substrates or encapsulation layers, thus causing discomfort during wearing (i.e., low air/vapor permeability, mechanical mismatch) and restricting their applications. A breathable and flexible pressure sensor is reported with nonwoven fabrics as both the electrode (printed with MXene interdigitated electrode) and sensing (coated with MXene/silver nanowires) layers via a scalable screen-printing approach. Benefiting from the multi-layered porous structure, the sensor demonstrates good air permeability with high sensitivity (770.86–1434.89 kPa⁻¹), a wide sensing range (0–100 kPa), fast response/recovery time (70/81 ms), and low detection limit (≈1 Pa). Particularly, this sensor can detect full-scale human motion (i.e., small-scale pulse beating and large-scale walking/running) with high sensitivity, excellent cycling stability, and puncture resistance. Additionally, the sensing layer of the pressure sensor also displays superior sensitivity to humidity changes, which is verified by successfully monitoring human breathing and spoken words while wearing a sensor-embedded mask. Given the outstanding features, this breathable sensor shows promise in the wearable electronic field for body health monitoring, sports activity detection, and disease diagnosis.     

Mechanically Robust and Highly Conductive Ionogels for Soft Ionotronics

Ionogels are promising materials for flexible electronics due to their continuous conductive phase, high thermal and chemical stability. However, a large amount of ionic liquid is required to get high conductivity, resulting in a sharp decline in the mechanical properties. Therefore, it is a great challenge to prepare ionogels with both high conductivity and mechanical properties, which is important for their practical applications. Herein, ionogels with high mechanical strength and stretchability, extraordinary ionic conductivity, excellent transparency, outstanding durability, and stability are fabricated with crosslinked polymer, ionic liquid, and lithium salt. The adoption of lithium salt can significantly improve both the mechanical strength and stretchability, which is a common dilemma in material science, and simultaneously, address the conflict between mechanical strength and ionic conductivity in ionogels. It is primarily corresponding to the microphase-separation effects induced by the lithium bonds formed between lithium ions and carbonyl groups on the polymer networks. Ionotronics including resistance-type sensors for strain and temperature sensing and triboelectric nanogenerators with stable output performance are fabricated. Moreover, ionogel-based microcircuit and sensing arrays with high resolution and accuracy are fabricated through digital light processing printing technology. The ionogels have great promise for various ionotronics in many fields.     

Polyelectrolyte Dielectrics for Flexible Low‐Voltage Organic Thin‐Film Transistors in Highly Sensitive Pressure Sensing

Organic thin‐film transistors (OTFTs) can provide an effective platform to develop flexible pressure sensors in wearable electronics due to their good signal amplification function. However, it is particularly difficult to realize OTFT‐based pressure sensors with both low‐voltage operation and high sensitivity. Here, controllable polyelectrolyte composites based on poly(ethylene glycol) (PEG) and polyacrylic acid (PAA) are developed as a type of high‐capacitance dielectrics for flexible OTFTs and ultrasensitive pressure sensors with sub‐1 V operation. Flexible OTFTs using the PAA:PEG dielectrics show good universality and greatly enhanced electrical performance under a much smaller operating voltage of ?0.7 V than those with a pristine PAA dielectric. The low‐voltage OTFTs also exhibit excellent flexibility and bending stability under various bending radii and long cycles. Flexible OTFT‐based pressure sensors with low‐voltage operation and superhigh sensitivity are demonstrated by using a suspended semiconductor/dielectric/gate structure in combination with the PAA:PEG dielectric. The sensors deliver a record high sensitivity of 452.7 kPa^?1 under a low‐voltage of ?0.7 V, and excellent operating stability over 5000 cycles. The OTFT sensors can be built into a wearable sensor array for spatial pressure mapping, which shows a bright potential in flexible electronics such as wearable devices and smart skins.     

Flexible MXene-Based Hydrogel Enables Wearable Human–Computer Interaction for Intelligent Underwater Communication and Sensing Rescue

Conductive hydrogels have recently attracted extensive attention in the field of smart wearable electronics. Despite the current versatility of conductive hydrogels, the balance between mechanical properties (tensile properties, strength, and toughness) and electrical properties (electrical conductivity, sensitivity, and stability) still faces great challenges. Herein, a simplified method for constructing hydrophobic association hydrogels with excellent mechanical and electrical properties is proposed. The prepared conductive hydrogels exhibit high tensile properties (≈1224%), high linearity in the whole-strain–range (R² = 0.999), and a wide strain sensing range (2700%). The conductive hydrogel can realize more than 1000 cycles of sensing under 500% tensile strain. As an application demonstration, an underwater communication device is assembled in combination with polydimethylsiloxane/Triton X-100 film coating, which successfully transmits underwater signals and provides warning of potential hazards. This study provides a new research method for developing underwater equipment with excellent mechanical properties and sensing properties.     

Flexible Transparent Reduced Graphene Oxide Sensor Coupled with Organic Dye Molecules for Rapid Dual‐Mode Ammonia Gas Detection

Flexible chemical sensors utilizing chemically sensitive nanomaterials are of great interest for wearable sensing applications. However, obtaining high performance flexible chemical sensors with high sensitivity, fast response, transparency, stability, and workability at ambient conditions is still challenging. Herein, a newly designed flexible and transparent chemical sensor of reduced graphene oxide (R‐GO) coupled with organic dye molecules (bromophenol blue) is introduced. This device has promising properties such as high mechanical flexibility (>5000 bending cycles with a bending radius of 0.95 cm) and optical transparency (>60% in the visible region). Furthermore, stacking the water‐trapping dye layer on R‐GO enables a higher response as well as workability in a large relative humidity range (up to 80%), and dual‐mode detection capabilities of colorimetric and electrical sensing for NH₃ gas (5–40 ppm). These advantageous attributes of the flexible and transparent R‐GO sensor coupled with organic dye molecules provide great potential for real‐time monitoring of toxic gas/vapor in future practical chemical sensing at room conditions in wearable electronics.     

Flexible and Stretchable Fiber-Shaped Triboelectric Nanogenerators for Biomechanical Monitoring and Human-Interactive Sensing

Wearable, flexible, and even stretchable tactile sensors, such as various types of electronic skin, have attracted extensive attention, which can adapt to complex and irregular surfaces, maximize the matching of wearable devices, and conformally apply onto human organs. However, it is a great challenge to simultaneously achieve breathability, permeability, and comfortability for their development. Herein, mitigating the problem by miniaturizing and integrating the sensors is tried. Highly flexible and stretchable coaxial structure fiber-shaped triboelectric nanogenerators (F-TENGs) with a diameter of 0.63 mm are created by orderly depositing conductive material of silver nanowires/carbon nanotubes and encapsulated polydimethylsiloxane onto the stretchable spandex fiber. As a self-powered multifunctional sensor, the resulting composite fiber can convert mechanical stimuli into electrical signals without affecting the normal human body. Moreover, the F-TENGs can be easily integrated into traditional textiles to form tactile sensor arrays. Through the tactile sensor arrays, the real-time tactile trajectory and pressure distribution can be precisely mapped. This work may provide a new method to fabricate fiber-based pressure sensors with high sensitivity and stretchability, which have great application prospects in personal healthcare monitoring and human–machine interactions.     

A Highly Stretchable Fiber‐Based Triboelectric Nanogenerator for Self‐Powered Wearable Electronics

The development of flexible and stretchable electronics has attracted intensive attention for their promising applications in next‐generation wearable functional devices. However, these stretchable devices that are made in a conventional planar format have largely hindered their development, especially in highly stretchable conditions. Herein, a novel type of highly stretchable, fiber‐based triboelectric nanogenerator (fiber‐like TENG) for power generation is developed. Owing to the advanced structural designs, including the fiber‐convolving fiber and the stretchable electrodes on elastic silicone rubber fiber, the fiber‐like TENG can be operated at stretching mode with high strains up to 70% and is demonstrated for a broad range of applications such as powering a commercial capacitor, LCD screen, digital watch/calculator, and self‐powered acceleration sensor. This work verifies the promising potential of a novel fiber‐based structure for both power generation and self‐powered sensing.     

A Self-Powered,Rechargeable, and Wearable Hydrogel Patch for Wireless Gas Detection with Extraordinary Performance

Flexible gas sensors play an indispensable role in diverse applications spanning from environmental monitoring to portable medical electronics. Full wearable gas monitoring system requires the collaborative support of high-performance sensors and miniaturized circuit module, whereas the realization of low power consumption and sustainable measurement is challenging. Here, a self-powered and reusable all-in-one NO₂ sensor is proposed by structurally and functionally coupling the sensor to the battery, with ultrahigh sensitivity (1.92%/ppb), linearity (R² = 0.999), ultralow theoretical detection limit (0.1 ppb), and humidity immunity. This can be attributed to the regulation of the gas reaction route at the molecular level. The addition of amphiphilic zinc trifluoromethanesulfonate (Zn(OTf)₂) enables the H₂O-poor inner Helmholtz layer to be constructed at the electrode–gel interface, thereby facilitating the direct charge transfer process of NO₂ here. The device is then combined with a well-designed miniaturized low-power circuit module with signal conditioning, processing and wireless transmission functions, which can be used as wearable electronics to realize early and remote warning of gas leakage. This study demonstrates a promising way to design a self-powered, sustainable, and flexible gas sensor with high performance and its corresponding wireless sensing system, providing new insight into the all-in-one system of gas detection.