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.     

电子皮肤用纤维素水凝胶的研究进展

综述了纤维素水凝胶用于电子皮肤器件近期取得的研究进展.指出水凝胶可穿戴电子未来的发展和应用方向:一是在智能医疗领域实现健康管理和动态治疗;二是实现人类感知支配的物联网.先从拉伸性、抗冻性和自愈合三方面分析了电子皮肤用纤维素水凝胶所需的基本特性.然后举例阐述了纤维素水凝胶在药物负载和释放两方面的独特优势,揭示了其在智能医...     

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.     

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.     

Antifreezing Hydrogel with High Zinc Reversibility for Flexible and Durable Aqueous Batteries by Cooperative Hydrated Cations

Hydrogels are widely used in flexible aqueous batteries due to their liquid‐like ion transportation abilities and solid‐like mechanical properties. Their potential applications in flexible and wearable electronics introduce a fundamental challenge: how to lower the freezing point of hydrogels to preserve these merits without sacrificing hydrogels' basic advantages in low cost and high safety. Moreover, zinc as an ideal anode in aqueous batteries suffers from low reversibility because of the formation of insulative byproducts, which is mainly caused by hydrogen evolution via extensive hydration of zinc ions. This, in principle, requires the suppression of hydration, which induces an undesirable increase in the freezing point of hydrogels. Here, it is demonstrated that cooperatively hydrated cations, zinc and lithium ions in hydrogels, are very effective in addressing the above challenges. This simple but unique hydrogel not only enables a 98% capacity retention upon cooling down to ?20 °C from room temperature but also allows a near 100% capacity retention with >99.5% Coulombic efficiency over 500 cycles at ?20 °C. In addition, the strengthened mechanical properties of the hydrogel under subzero temperatures result in excellent durability under various harsh deformations after the freezing process.     

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.     

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.     

From Glutinous-Rice-Inspired Adhesive Organohydrogels to Flexible Electronic Devices Toward Wearable Sensing,Power Supply,and Energy Storage

Flexible electronic devices (FEDs) based on hydrogels are attracting increasing interest, but the fabrication of hydrogels for FEDs with adhesiveness and high robustness in harsh-temperature conditions and long-term use remains a challenge. Herein, glutinous-rice-inspired adhesive organohydrogels are developed by introducing amylopectin into a copolymer network through a “one-pot” crosslinking procedure in a glycerol–water mixed solvent containing potassium chloride as the conductive ingredient. The organohydrogels exhibit excellent transparency (>90%), conductivity, stretchability, tensile strength, adhesiveness, anti-freezing property, and moisture retention ability. The wearable strain sensor assembled from the organohydrogels achieves a wide working range, high sensitivity (gauge factor: 8.82), low response time, and excellent reversibility, and properly responds in harsh-temperature conditions and long-time storage (90 days). The strain sensor is further integrated with a Bluetooth transmitter and receiver for fabricating wireless wearable sensors. Notably, a sandwich-structured capacitive pressure sensor with organohydrogels containing reliefs as electrodes records a new gauge factor of 9.43 kPa^?1 and achieves a wide response range, low detection limit, and outstanding reversibility. Furthermore, detachable and durable batteries and all-in-one supercapacitors are also fabricated utilizing the organohydrogels as electrolytes. Overall, this work offers a strategy to fabricate adhesive organohydrogels for robust FEDs toward wearable sensing, power supply, and energy storage.     

High-Throughput Screening of Self-Healable Polysulfobetaine Hydrogels and their Applications in Flexible Electronics

Hydrogels are promising materials in the applications of wound adhesives, wearable electronics, tissue engineering, implantable electronics, etc. The properties of a hydrogel rely strongly on its composition. However, the optimization of hydrogel properties has been a big challenge as increasing numbers of components are added to enhance and synergize its mechanical, biomedical, electrical, and self-healable properties. Here in this work, it is shown that high-throughput screening can efficiently and systematically explore the effects of multiple components (at least eight) on the properties of polysulfobetaine hydrogels, as well as provide a useful database for diverse applications. The optimized polysulfobetaine hydrogels that exhibit outstanding self-healing and mechanical properties, have been obtained by high-throughput screening. By compositing with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), intrinsically self-healable and stretchable conductors are achieved. It is further demonstrated that a polysulfobetaine hydrogel-based electronic skin, which exhibits exceptionally fast self-healing capability of the whole device at ambient conditions. This work successfully extends high-throughput synthetic methodology to the field of hydrogel electronics, as well as demonstrates new directions of healable flexible electronic devices in terms of material development and device design.     

Super-Stretchable,Anti-Freezing,Anti-Drying Organogel Ionic Conductor for Multi-Mode Flexible Electronics

Due to their intrinsic flexibility, tunable conductivity, multiple stimulus-response, and self-healing ability, ionic conductive hydrogels have drawn significant attention in flexible/wearable electronics. However, challenges remain because traditional hydrogels inevitably faced the problems of losing flexibility and conductivity because of the inner water loss when exposed to the ambient environment. Besides, the water inside the hydrogel will freeze at the water icing temperatures, making the device hard and fragile. As a promising alternative, organogels have attracted wide attention because they can, to some extent, overcome the above drawbacks. Herein, a kind of organogel ionic conductor (MOIC) by a self-polymerization reaction is involved, which is super stretchable, anti-drying, and anti-freezing. Meanwhile, it can still maintain high mechanical stability after alternately loading/unloading at the strain of 600% for 600 s (1800 cycles). Using this MOIC, high-performance triboelectric nanogenerator (TENG) is constructed (MOIC-TENG) to harvest small mechanical energy even the MOIC electrode underwent an extremely low temperature. In addition, multifunctional flexible/wearable sensors (strain sensor, piezoresistive sensor, and tactile sensor) are realized to monitor human motions in real time, and recognize different materials by triboelectric effect. This study demonstrates a promising candidate material for flexible/wearable electronics such as electronic skin, flexible sensors, and human-machine interfaces.     

An Anti-Freezing,Ambient-Stable and Highly Stretchable Ionic Skin with Strong Surface Adhesion for Wearable Sensing and Soft Robotics

Natural living systems such as wood frogs develop tissues composed of active hydrogels with cryoprotectants to survive in cold environments. Recently, hydrogels have been intensively studied to develop stretchable electronics for wearables and soft robots. However, regular hydrogels are inevitably frozen at the subzero temperature and easily dehydrated, and have weak surface adhesion. Herein, a novel hydrogel-based ionic skin (iSkin) capable of strain sensing is demonstrated with high toughness, high stretchability, excellent ambient stability, superior anti-freezing capability, and strong surface adhesion. The iSkin consists of a piece of ionically and covalently cross-linked tough hydrogel with a thin bioadhesive layer. With the addition of biocompatible cryoprotectant and electrolyte, the iSkin shows good conductivity in wide ranges of relative humidity (15–90%) and temperature (−95–25 °C). In addition, the iSkin can adhere firmly to diverse material surfaces under different conditions, including cloth fabric, skin, and elastomers, in both dry and wet conditions, at subzero temperature, and/or with dynamic movement. The iSkin is demonstrated for applications including strain sensing on both human body and winter coat, human–machine interaction, motion/deformation sensing on a soft gripper and a soft robot at extremely cold conditions. This work provides a new paradigm for developing high-performance artificial skins for wearable sensing and soft robotics.     

Fatigue-Resistant Conducting Polymer Hydrogels as Strain Sensor for Underwater Robotics

Conducting polymer hydrogels are widely used as strain sensors in light of their distinct skin-like softness, strain sensitivity, and environmental adaptiveness in the fields of wearable devices, soft robots, and human-machine interface. However, the mechanical and electrical properties of existing conducting polymer hydrogels, especially fatigue-resistance and sensing robustness during long-term application, are unsatisfactory, which severely hamper their practical utilities. Herein, a strategy to fabricate conducting polymer hydrogels with anisotropic structures and mechanics is presented through a combined freeze-casting and salting-out process. The as-fabricated conducting polymer hydrogels exhibit high fatigue threshold (>300 J m⁻²), low Young's modulus (≈100 kPa), as well as long-term strain sensing robustness (over 10 000 cycles). Such superior performance enables their application as strain sensors to monitor the real-time movement of underwater robotics. The design and fabrication strategy for conducting polymer hydrogels reported in this study may open up an enticing avenue for functional soft materials in soft electronics and robotics.     

Skin-Inspired Double-Hydrophobic-Coating Encapsulated Hydrogels with Enhanced Water Retention Capacity

Traditional hydrogels always lose their flexibility and functions in dry environments because the internal water inevitably undergoes evaporation. In this study, a skin-inspired, facile, and versatile strategy for developing encapsulated hydrogels with excellent water retention capacity through a double-hydrophobic coating is proposed. The robust double-layer coating, which integrates a hydrophobic polymer coating with a hydrophobic oil layer simultaneously, can provide a barrier to prevent the evaporation of water. To overcome the weak interfacial strength between the hydrophilic hydrogel surface and the double-hydrophobic coating, (3-aminopropyl) triethoxysilane (APTES) is utilized as a chemical binding agent. Furthermore, the overall mechanical properties of the bulk hydrogel are not significantly affected, because the coating is only anchored to the surface and its thickness is much lower than that of the native hydrogel. Moreover, it is demonstrated that this proposed strategy particularly holds the capability of encapsulating various types and different shapes of hydrogels, leading to enhanced stability and a prolonged lifetime in air. Therefore, the proposed technology provides new insights for multifarious surface functionalization of hydrogel and broadens the range of hydrogel applications.     

Supplementary Networking of Interpenetrating Polymer System (SNIPSy) Strategy to Develop Strong & High Water Content Ionic Hydrogels for Solid Electrolyte Applications

To synthesize hydrogels that possess tensile strength and modulus together in MPas along with extensibility at high equilibrium water content (≥90 wt%) is challenging but important from the application perspective. Especially, such hydrogel compositions are useful for fabricating flexible electronics devices for subsea applications, where underwater risk-free implementation and optimum device performance at low temperature (≈0 °C) and high hydrostatic pressure (≤20 bar) conditions is desirable. The high water content of hydrogel is necessary to facilitate ion transportation, and mechanical strength is desirable to maintain a stable electrode–electrolyte interface under load. In this study, supplementary networking of an interpenetrating polymer system strategy is utilized to develop ionic hydrogels with tensile strength and Young's modulus values up to 2 and 1.67 MPa, respectively, at high equilibrium water content value up to 96%. Cost-effective, durable, rechargeable, and flexible batteries are fabricated using the Zn & Li ion soaked hydrogel as solid electrolyte without barrier. These batteries display minimal loss in capacity when immersed in water, deformed, exposed to flame, put under high load, and operated under low-temperature conditions suggesting the viability for subsea application.     

Self-Shaping Soft Electronics Based on Patterned Hydrogel with Stencil-Printed Liquid Metal

Hydrogel-based soft electronics (HSE) is promising as implantable devices due to the similarity of hydrogel substrates to biologic tissues. Most existing HSE devices are based on conducting hydrogels that usually have weak mechanical properties, low conductivity, and poor patternability. Reported here is an HSE with good mechanical performance, high sensitivity, and versatile functions by stencil printing of liquid metal on a tough hydrogel, facilitating integration of multiple sensing units. Self-shaping ability is imparted to the HSE by creating gradient structure in the hydrogel substrate. The resultant HSE actively deforms into 3D configurations with zero or nonzero Gaussian curvature to fix on objects or organs with sophisticated geometries and maintains the sensing functions. The versatilities and potential applications of this HSE are demonstrated by monitoring motions of a rice field eel and beatings of a rabbit heart. Such HSE based on morphing substrate should pave the way for implantable electronics with better fixation and interfacial contact with the organs. The concept of morphing hydrogel devices can be extended to other soft electronics with responsive polymer films or elastomers as the substrates.     

Protein Gel Phase Transition: Toward Superiorly Transparent and Hysteresis‐Free Wearable Electronics

The next generation of wearable electronics for health monitoring, Internet‐of‐Things system, “interface‐on‐invisible,” and green energy harvesting require electrically conductive material that is superiorly transparent, negligibly hysteretic, industrially feasible, and highly stretchable. The practical potential of ionic hydrogel is challenged with obvious hysteresis and a limited sensing range due to relative delamination and viscoelastic performance. Herein, a novel liquid conductor, termed as egg white liquid, is developed from self‐liquidation of egg white hydrogel, and the liquid not only inherits the designed architecture from a hydrogel predecessor but also achieves comparable conductivity (20.4 S m^?1) to the ionic hydrogel and ultrahigh transparency (up to 99.8%) . Moreover, the 3D‐printed liquid–elastomer hybrid exhibits excellent conformability, remarkable sensitivity with negligible hysteresis (0.77%), and the capability of monitoring human motions and dynamic moduli is further demonstrated. The liquid nature inspires a gesture‐controlled touchless user interface for front‐end electronic systems. Furthermore, mechanical energy harvesting and pressure sensing are evidenced by exploiting this liquid conductor into a triboelectric nanogenerator. Notably, the as‐prepared liquid via subsequent phase transition possessing superior transparency, ultralow hysteresis, economic benefit, and unique liquid phase may potentially fuel the development of a new class of wearable electronics, human–machine interface, and clean energy.     

Multifunctional Organohydrogel with Ultralow-Hysteresis,Ultrafast-Response,and Whole-Strain-Range Linearity for Self-Powered Sensors

The conductive hydrogels always suffered from high internal friction, large hysteresis, and low capability of accurately predicting physical deformation, which seriously restricted their application in smart wearable devices. To address these problems, solvent molecules are directionally inserted into the polymer molecule chains via bridge effect to effectively reduce the molecular internal friction. Moreover, swelling is also combined to eliminate the temporary entanglements in the hydrogel system. The cooperation between the bridge and swollen effect endows the prepared polyacrylamide (PAM)/laponite/H₃BO₃/ethylene glycol (Eg) organohydrogel (PLBOH) ultralow hysteresis (1.38%, ε = 100%), ultrafast response (≈10 ms), and high linearity in the whole-strain-range (R² = 0.996) with a great sensitivity (GF = 2.68 at the strain range of 0–750%). Meanwhile, the prepared PL₁₀B₃₀OH exhibits long-term stability, excellent stretchability, and low dissipated energy. Furthermore, the assembled triboelectric nanogenerator (TENG) displays an outstanding energy harvesting performance with an output voltage of 200 V with the size of 20 mm × 20 mm. The assembled strain sensors can monitor the small strain of facial expressions and large strain of human movements, indicating the tremendous applications in self-powered intelligent and flexible wearable electronics under harsh environmental conditions.     

Photocurable 3D Printing of High Toughness and Self-Healing Hydrogels for Customized Wearable Flexible Sensors

Currently, most customized hydrogels can only be processed via extrusion-based 3D printing techniques, which is limited by printing efficiency and resolution. Here, a simple strategy for the rapid fabrication of customized hydrogels using a photocurable 3D printing technique is presented. This technique has been rarely used because the presence of water increases the molecular distance between the polymer chains and reduces the monomer polymerization rate, resulting in the failure of rapid solid-liquid separation during printing. Although adding cross-linkers to printing inks can effectively accelerate 3D cross-linked network formation, chemical cross-linking may result in reduced toughness and self-healing ability of the hydrogel. Therefore, an interpenetrated-network hydrogel based on non-covalent interactions is designed to form physical cross-links, affording fast solid-liquid separation. Poly(acrylic acid (AA)-N-vinyl-2-pyrrolidone (NVP)) and carboxymethyl cellulose (CMC) are cross-linked via Zn²⁺-ligand coordination and hydrogen bonding; the resulting mixed AA-NVP/CMC solution is used as the printing ink. The printed poly(AA-NVP/CMC) hydrogel exhibited high tensile toughness (3.38 MJ m⁻³) and superior self-healing ability (healed stress: 81%; healed strain: 91%). Some objects like manipulator are successfully customized by photocurable 3D printing using hydrogels with high toughness and complex structures. This high-performance hydrogel has great potential for application in flexible wearable sensors.     

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.     

Neuron-Inspired Self-Powered Hydrogels with Excellent Adhesion,Recyclability and Dual-Mode Output for Multifunctional Ionic Skin

Hydrogels are promising materials for electronic skin due to their flexibility and modifiability. Reported hydrogel electronic skins can recognize stimulations and output signals, but the single output signal and the requirement of external power source limit their further applications. In this study, inspired by the neuron system, the self-powered neuron system-like hydrogels based on gelatin, water/glycerin and ionic liquid modified metal organic frameworks (MOFs) are prepared. The optimized hydrogel exhibits excellent adhesion (40 kPa), stretchability (0%–100%), water retention (>92% at 0% relative humidity (RH) atmosphere), ionic conductivity (>10⁻³ S m⁻¹) and stability (>30 days). Besides, the neuron system-like hydrogels are highly sensitive to pressure (0—10 N) and humidity (0%–75% RH) with dual-modal output, without external power source. Finally, the optimized hydrogel ionic skin is applied in human motion detection, energy harvesting, and low humidity sensing. This study provides a preliminary exploration of self-powered ionic skin for multi-application scenarios.