Environment-Resistant Organohydrogel-Based Sensor Enables Highly Sensitive Strain, Temperature, and Humidity Responses

Flexible and low-temperature-resistant double-network hydrogel with a bionic octopus-tentacle-like structure for integrated supercapacitor and nanogenerator sensor fabrication

Flexible and stretchable hydrogels are important components of flexible electronics; however, they are typically easily detached upon repeated high-strain stretching because of their smooth surfaces and cannot be used at subfreezing temperatures because of ice formation. To address these shortcomings, we prepared a low-temperature-resistant and flexible double-network hydrogel with a bionic octopus-tentacle-like structure composed of polyvinyl alcohol and sodium alginate. We also verified its suitability for developing high-performance, flexible, stretchable, and environmentally durable supercapacitors and nanogenerator sensors. The influence of melting temperature on the hydrogel's surface morphology decreased the interfacial resistance. The fabricated supercapacitor demonstrated exceptional performance, with 1326.5 mF cm-2 (areal capacitance) at 1 mA cm-2, a maximum energy and power densities of 172.3 μWh cm-2, and 708.6 mW cm-2, respectively, outperforming most integrated supercapacitors previously reported. The corresponding nanogenerator sensor demonstrated outstanding suitability for energy harvesting and low-temperature sensing, with potential applications in underwater information transmission using international Morse code. The results of this study paves the way for the fabrication of intelligent wearable electronics and solves the problems associated with the fabrication of flexible and low-temperature-resistant hydrogels.

Enzyme-mediated hydrogelation for biomedical applications: A review

Hydrogels possess significant potential for biomedical applications due to their flexibility and biocompatibility. However, current physical or chemical methods for their preparation often fail to balance biocompatibility and mechanical properties, limiting their application scope. Enzymatic preparation of hydrogels offers advantages including mild reaction conditions, absence of toxic substances, and superior biocompatibility. This review focuses on the enzymatic preparation systems of hydrogels and its application in the fast-growing biomedical field. Firstly, the mechanisms of enzyme-mediated hydrogel preparation can be categorized into four classes: enzyme cross-linking, enzyme polymerization, enzyme-mediated self-assembly of small molecular gelators, and enzyme-induced pH changes. Hydrogels prepared through the first two mechanisms retain the mechanical advantages of chemically cross-linked hydrogels while preserving the inherent biocompatibility. Additionally, hydrogels prepared via the latter two mechanisms exhibit rapid responses to external stimuli similar to physically crosslinked hydrogels while maintaining high biocompatibility. Furthermore, we discuss their application in biomedical scope and analyze the correlation between the mechanism of enzyme-mediated hydrogels and their respective application domains. Finally, the current challenges faced by enzymatically mediated hydrogelation are summarized; notably that enzymes incorporated and immobilized during hydrogel preparation remain active, resulting in catalytic activity exhibited by these enzymatically mediated hydrogels, which broadens their potential applications.

Cationic Carbon Dot Reinforced Highly Tensile, Tough, Dehydration Resistant Polyelectrolyte Hydrogels with Fluorescence for Flexible Sensing and Information Anti-Counterfeiting

With the rapid development of wearable devices, there is an increasing demand for multifunctional conductive soft materials. Nanocomposite hydrogels containing carbon nanofillers such as carbon dots (CDs) composite gels emerge as promising candidates. However, traditional CDs nanocomposite hydrogels face limitations in terms of mechanical strength, stability and elasticity. To overcome these critical challenges, in this work, a cationic carbon dots (CCDs)-reinforced polyelectrolyte hydrogel engineered through synergistic electrostatic assembly and salting-out strategies is developed. The polyacrylic acid/sodium hyaluronate/cationic carbon point glycerol-water binary solvent fluorescent organohydrogel (PAH-CG) is fabricated. The resulting organohydrogel PAH-CG successfully overcame the plasticizing effect of glycerol, resulting in a significant enhancement of mechanical properties, with a 149-fold increase in Young's modulus compared to the control hydrogel. Specifically, the PAH-CG hydrogel exhibited high tensile strain (1200%-2734%), tensile strength (234 kPa), and modulus (275 kPa), alongside excellent elasticity, fluorescence, and dehydration resistance. The improvement in mechanical properties leads to excellent performance in flexible sensor applications. Concurrently, glycerol incorporation not only amplifies fluorescence intensity but also improves dehydration resistance and moisture absorption. Applications for encrypted transmission of information and anti-counterfeiting have been developed based on these properties, making PAH-CG hydrogels a promising platform for advanced smart devices.

Liquid Metal Composite Organohydrogel Based on Water-Soluble Starch Stabilizer with Supertoughness, Self-Healing, and Harsh-Environmental Tolerance for an Advanced Strain Sensor

In this work, a supertough, self-healable, and extreme-environment-tolerable liquid metal (LM) composite organohydrogel was fabricated by dispersing LM particles (LMPs) with water-soluble starch (WS) and leveraging multilevel hydrogen-bonding interactions. Attributed to the cooperation of the strong dual-hydrogen bonds and weak monohydrogen bonds, the organohydrogel obtained an outstanding tensile strength of 2.0 ± 0.13 MPa and toughness of 16.0 ± 1.0 MJ m-3, as well as desirable self-healing ability. The organohydrogel strain sensor has a high gauge factor (GF) of 15.08 along with a large detection range (0-1159%), demonstrating its outstanding sensitivity. It was successfully applied for manipulator gesture detection in harsh environments, showing excellent detection resolution and sensing stability in a wide temperature range (-20 to 50 °C). This work provides a new avenue for preparing multifunctional LM composite gels, showing great promise for next-generation wearable electronics.

Natural polysaccharides-based smart sensors for health monitoring, diagnosis and rehabilitation: A review

With the rapid growth of multi-level health needs, precise and real-time health sensing systems have become increasingly pivotal in personal health management and disease prevention. Natural polysaccharides demonstrate immense potential in healthcare sensors by leveraging their superior biocompatibility, biodegradability, environmental sustainability, as well as diverse structural designs and surface functionalities. This review begins by introducing a variety of natural polysaccharides, including cellulose, alginates, chitosan, hyaluronic acid, and starch, and analyzing their structural and functional distinctions, which offer extensive possibilities for sensor design and construction. Further, we summarize several principal sensing mechanisms, such as piezoresistivity, piezoelectricity, capacitance, triboelectricity, and hygroelectricity, which provide a theoretical and technological foundation for developing high-performance healthcare sensing devices. Additionally, the review discusses the most recent applications of natural polysaccharide-based sensors in diverse healthcare contexts, including human body motion tracking, respiratory and heartbeat monitoring, electrophysiological signal recording, body temperature variation detection, and biomarker analysis. Finally, prospective development directions are proposed, such as the integration of artificial intelligence for real-time data analysis, the design of multifunctional devices that combine sensing with therapeutic functionalities, and advancements in remote monitoring and smart wearable technologies. This review aims to provide valuable insights into the development of next-generation healthcare sensors and propose novel research directions for personalized medicine and remote health management.

Skin-Mountable Thermo-responsive Structured Hydrogel for Optical and Adhesion Coupled Functional Sensing

Smart hydrogel sensors with intrinsic responsiveness, such as pH, temperature, humidity, and other external stimuli, possess broad applications in innumerable fields such as biomedical diagnosis, environmental monitoring, and wearable electronics. However, it remains a great challenge to develop wearable structural hydrogels that possess simultaneously body temperature-responsive, adhesion-adaptable, and transparency-tunable. Herein, an innovative skin-mountable thermo-responsive hydrogel is fabricated, which endows tunable optical properties and switchable adhesion properties at different temperatures. Interestingly, it is able to exhibit lower critical solution temperature (LCST) to adapt to the human body temperature by altering the acrylic acid(AAc) content in the hydrogel network. The hydrogel also displays high transparency and strong adhesion at low temperatures, while it becomes opaque and feeble adhesion at high temperatures. Furthermore, a wearable and highly sensitive hydrogel sensor array structure is constructed by harnessing vat photopolymerization three-dimensional (3D) printing. As a proof of concept, a wearable hydrogel sensor attached to the back of the human hand is capable of detecting simultaneously temperature and strain differences, and also integrating high-temperature monitoring and alarm functions with visual transparency alteration. This work provides an advanced manner to fabricate structured responsive hydrogels, which have potential application prospects in the field of smart medical patches and wearable devices.

Design of super stretchability, rapid self-healing, and self-adhesion hydrogel based on starch for wearable strain sensors

Since hydrogels are conductive, easily engineered, and sufficiently flexible to imitate the mechanical properties of human skin, they are seen as potential options for wearable strain sensors. However, it is still a great challenge to prepare a hydrogel through simple and straightforward methods that integrate excellent stretchability, ionic conductivity, toughness, self-adhesion, and self-healing. Herein, an acrylamide/3-acrylamide phenylboronic acid cross-linked network is modified to produce a semi-interpenetrating cross-linked hydrogel in just one easy step by adding starch. The prepared hydrogel contains dynamic boronic ester bonds and hydrogen bonds, which endow the exceptional stretchability (5769-13,976 %, 20-50 wt%), ideal transmittance (>90 %), self-adhesiveness (0.636 ± 0.060 kPa, 30 wt%), and self-healing properties. Notably, the self-healing process is completed instantly, achieving a healing strength of up to 81.21 %. Additionally, the aforementioned hydrogel exhibits a broad working strain range (≈ 500 %) and high sensitivity (gauge factor = 1.99) as a strain sensor, allowing it to record and track human actions precisely. This work provides a novel approach to synthesizing hydrogels with optimal overall mechanical characteristics, with the potential to facilitate the development of wearable strain sensing system based on hydrogels for real-world applications.

Self-healing and transparent ionic conductive PVA/pullulan/borax hydrogels with multi-sensing capabilities for wearable sensors

Conductive hydrogels as wearable sensors have been used for numerous applications in human motion detection, personal healthcare monitoring and other diverse scenarios. However, it remains a challenge to integrate self-healing ability, multiple sensing capabilities, and transparency in one single unit. In this work, multifunctional polyvinyl alcohol (PVA)/Pullulan/Borax conductive hydrogels were fabricated by introducing borate ester bonds and hydrogen bonds. The described hydrogels showed fast self-healing properties, which could autonomously completely recover within 15 s. The hydrogels possessed high optical transparency (92.9%) in the visible light range and had multi-sensing capabilities, such as strain, temperature and humidity sensing. The assembled hydrogel sensor displayed a high strain sensitivity of 2.74 within the strain range of 300%, and it could be used to monitor human motions such as finger and wrist bending. In addition, the hydrogel sensor could sense temperature variations with a temperature coefficient of resistance of -0.914 °C-1 over 28-46 °C. Besides, the hydrogel sensor demonstrated the humidity sensing ability and can recognize human inhale and exhale. The overall sensing performance of the PVA/Pullulan/Borax hydrogel was satisfactory and repeatable. This conductive hydrogel shows great potential in wearable electronics and personal healthcare and inspires a new generation of multifunctional hydrogel sensors.

A synergistic enhancement strategy for mechanical and conductive properties of hydrogels with dual ionically cross-linked κ-carrageenan/poly(sodium acrylate-co-acrylamide) network

Applying conductive hydrogels in electronic skin, health monitoring, and wearable devices has aroused great research interest. Yet, it remains a significant challenge to prepare conductive hydrogels simultaneously with superior mechanical, self-recovery, and conductivity performance. Herein, a dual ionically cross-linked double network (DN) hydrogel is fabricated based on K+ and Fe3+ ion cross-linked κ-carrageenan (κ-CG) and Fe3+ ion cross-linked poly(sodium acrylate-co-acrylamide) P(AANa-co-AM). Benefiting from the abundance of hydrogen bonds and metal coordination bonds, the conductive hydrogel has excellent mechanical properties (fracture strain up to 1420 %, fracture stress up to 2.30 MPa, and toughness up to 20.63 MJ/m3) and good self-recovery performance (the recovery rate of the toughness can reach 85 % after waiting for 1 h). Meanwhile, due to the introduction of dual metal ions of K+ and Fe3+, the ionic conductivity of conductive hydrogel is up to 1.42 S/m. Furthermore, the hydrogel strain sensor has good sensitivity with a gauge factor (GF) of 2.41 (0-100 %). It can be a wearable sensor that monitors different human motions, such as sit-ups. This work offers a new synergistic strategy for designing a hydrogel strain sensor with high mechanical, self-recovery, and conductive properties.

Recent advances in hydrogel-based flexible strain sensors for harsh environment applications

Flexible strain sensors are broadly investigated in electronic skins and human-machine interaction due to their light weight, high sensitivity, and wide sensing range. Hydrogels with unique three-dimensional network structures are widely used in flexible strain sensors for their exceptional flexibility and adaptability to mechanical deformation. However, hydrogels often suffer from damage, hardening, and collapse under harsh conditions, such as extreme temperatures and humidity levels, which lead to sensor performance degradation or even failure. In addition, the failure mechanism in extreme environments remains unclear. In this review, the performance degradation and failure mechanism of hydrogel flexible strain sensors under various harsh conditions are examined. Subsequently, strategies towards the environmental tolerance of hydrogel flexible strain sensors are summarized. Finally, the current challenges of hydrogel flexible strain sensors in harsh environments are discussed, along with potential directions for future development and applications.

Gelatin/sodium alginate-based strongly adhesive, environmentally resistant, highly stable hydrogel for 3D printing to prepare multifunctional sensors and flexible supercapacitors

The increasing demand for environmentally friendly performance materials in the field of wearable electronics has brought renewable and low-cost hydrogels based on natural polymers into the research spotlight. As a biodegradable natural polymer, sodium alginate (SA) shows great promise for applications in wearable electronics. Here, we report a hydrogel with printability, adhesion, and is highly stable based on gelatin (Gel) and SA. SA improves the viscosity of the hydrogel, which can bond iron products weighing up to 20 kg due to metal coordination with the material, and the hydrogel binder is recyclable and reusable. The presence of glycerin allowed the hydrogel sensor device to maintain sensitivity after exposure to air at 25 °C for up to 35 days, and printed hydrogel samples retained their compressive resilience after exposure to air (25 °C, 55 % RH) for 30 days. Hydrogel-based supercapacitors show good stability after 58 h of charge/discharge cycling. This paper provides research ideas for the preparation of hydrogels with strong adhesion properties, as well as hydrogel 3D printing technology for the preparation of flexible sensor devices and flexible energy storage devices.

Flexible Supercapacitor with Wide Electrochemical Stable Window Based on Hydrogel Electrolyte

Hydrogel electrolyte can endow supercapacitors with excellent flexibility, which has developed rapidly in recent years. However, the water-rich structures of hydrogel electrolyte are easy to freeze at subfreezing and dry at high temperatures, which will affect its energy storage characteristics. The low energy density of micro supercapacitors also hinders their development. Herein, a strategy is proposed to reduce the free water activity in the hydrogel to improve the operating voltage and the energy density of the device, which is achieved through the synergistic effect of the hydrogel skeleton, N, N'-dimethylformamide (DMF), NaClO4 and water. High concentrations of DMF and NaClO4 are introduced into sodium alginate/polyacrylamide (SA/PAAM) hydrogel through solvent exchange to obtain SA/PAAM/DMF/NaClO4 hydrogel electrolyte, which exhibited a high ionic conductivity of 82.1 mS cm-1, a high breaking strength of 563.2 kPa, and a wide voltage stability window of 3.5 V. The supercapacitor devices are assembled by the process of direct adhesion of the hydrogel electrolyte and  laser induced graphene (LIG). The micro-supercapacitor exhibited an operating voltage of 2.0 V, with a specific capacitance of 2.41 mF cm-2 and a high energy density of 1.34 µWh cm-2, and it also exhibit a high cycle stability, good flexibility, and integration performance.

High Ion-Conducting PVA Nanocomposite Hydrogel-Based Wearable Piezoelectric and Triboelectric Sensors for Harsh Environments

Traditional hydrogel-based wearable sensors with flexibility, biocompatibility, and mechanical compliance exhibit potential applications in flexible wearable electronics. However, the low sensitivity and poor environmental resistance of traditional hydrogels severely limit their practical application. Herein, high-ion-conducting poly(vinyl alcohol) (PVA) nanocomposite hydrogels were fabricated and applied for harsh environments. MXene ion-conducting microchannels and poly(sodium 4-styrenesulfonate) ion sources contributed to the directional transport of abundant free ions in the hydrogel, which significantly improved the sensitivity and mechanical-electric conversion of the nanocomposite hydrogel-based piezoelectric and triboelectric sensors. More importantly, the glycerol as an antifreezing agent enabled the hydrogel-based sensors to function in harsh environments. Therefore, the nanocomposite hydrogel exhibited high gauge factor (GF) at -20 °C (GF = 3.37) and 60 °C (GF = 3.62), enabling the hydrogel-based sensor to distinguish different writing letters and sounding words. Meanwhile, the hydrogel-based piezoelectric and triboelectric generators showed excellent mechanical-electric conversion performance regardless of low- (-20 °C) or high- (60 °C) temperature environments, which can be applied as a visual feedback system for information transmission without external power sources. This work provides self-powered nanocomposite hydrogel-based sensors that exhibit potential applications in flexible wearable electronics under harsh environmental conditions.

Research Progress and Application of Multimodal Flexible Sensors for Electronic Skin

In recent years, wearable electronic skin has garnered significant attention due to its broad range of applications in various fields, including personal health monitoring, human motion perception, human-computer interaction, and flexible display. The flexible multimodal sensor, as the core component of electronic skin, can mimic the multistimulus sensing ability of human skin, which is highly significant for the development of the next generation of electronic devices. This paper provides a summary of the latest advancements in multimodal sensors that possess two or more response capabilities (such as force, temperature, humidity, etc.) simultaneously. It explores the relationship between materials and multiple sensing capabilities, focusing on both active materials that are the same and different. The paper also discusses the preparation methods, device structures, and sensing properties of these sensors. Furthermore, it introduces the applications of multimodal sensors in human motion and health monitoring, as well as intelligent robots. Finally, the current limitations and future challenges of multimodal sensors will be presented.

Recent Progress of Anti-Freezing, Anti-Drying, and Anti-Swelling Conductive Hydrogels and Their Applications

Hydrogels are soft-wet materials with a hydrophilic three-dimensional network structure offering controllable stretchability, conductivity, and biocompatibility. However, traditional conductive hydrogels only operate in mild environments and exhibit poor environmental tolerance due to their high water content and hydrophilic network, which result in undesirable swelling, susceptibility to freezing at sub-zero temperatures, and structural dehydration through evaporation. The application range of conductive hydrogels is significantly restricted by these limitations. Therefore, developing environmentally tolerant conductive hydrogels (ETCHs) is crucial to increasing the application scope of these materials. In this review, we summarize recent strategies for designing multifunctional conductive hydrogels that possess anti-freezing, anti-drying, and anti-swelling properties. Furthermore, we briefly introduce some of the applications of ETCHs, including wearable sensors, bioelectrodes, soft robots, and wound dressings. The current development status of different types of ETCHs and their limitations are analyzed to further discuss future research directions and development prospects.

Wearable Sensor Based on a Tough Conductive Gel for Real-Time and Remote Human Motion Monitoring

The usage of a conductive hydrogel in wearable sensors has been thoroughly researched recently. Nonetheless, hydrogel-based sensors cannot simultaneously have excellent mechanical property, high sensitivity, comfortable wearability, and rapid self-healing performance, which result in poor durability and reusability. Herein, a robust conductive hydrogel derived from one-pot polymerization and subsequent solvent replacement is developed as a wearable sensor. Owing to the reversible hydrogen bonds cross-linked between polymer chains and clay nanosheets, the resulting conductive hydrogel-based sensor exhibits outstanding flexibility, self-repairing, and fatigue resistance performances. The embedding of graphene oxide nanosheets offers an enhanced hydrogel network and easy release of wearable sensor from the target position through remote irradiation, while Li+ ions incorporated by solvent replacement endow the wearable sensor with low detection limit (sensing strain: 1%), high conductivity (4.3 S m-1) and sensitivity (gauge factor: 3.04), good freezing resistance, and water retention. Therefore, the fabricated wearable sensor is suitable to monitor small and large human motions on the site and remotely under subzero (-54 °C) or room temperature, indicating lots of promising applications in human-motion monitoring, information encryption and identification, and electronic skins.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Ionic Liquid/Water Binary Solvent Anti-Freezing Hydrogel for Strain and Temperature Sensors

Hydrogels are widely applied in the flexible wearable electronic devices field owing to their skin-like stretchability, superb biocompatibility, and high conductivity retention under mechanical deformations. Nevertheless, hydrogels are prone to freezing at low temperatures and losing water at high temperatures, which seriously limits their practical applications. Herein, a binary solvent system of ionic liquid (1-ethyl-3-methylimidazolium chloride) and water was prepared to endow the ionic hydrogel high ionic conductivity (0.28 S m-1 at 25 °C), high transparency (94.26%), and superior freezing tolerance (-50 °C). The multiple hydrogen bonds formed among polymer chains, water, and ionic liquids significantly improved the mechanical properties of the ionic hydrogel, enabling excellent tensile properties (strain >1800%) and durability (1000 times at 100% strain). Moreover, the ionic hydrogel was further assembled into a dual-response sensor, which exhibited satisfactory sensitivity to both tension (gauge factor = 2.15 at 200% strain) and temperature (temperature coefficient of resistance = -1.845%/°C) and can be applied for human motion and body temperature monitoring. This study provides a versatile method for preparing multifunctional hydrogels with a wide range of applications and lays the groundwork for human movement detection and smart health care.

A Dual-Mode Pressure and Temperature Sensor

The emerging field of flexible tactile sensing systems, equipped with multi-physical tactile sensing capabilities, holds vast potential across diverse domains such as medical monitoring, robotics, and human-computer interaction. In response to the prevailing challenges associated with the limited integration and sensitivity of flexible tactile sensors, this paper introduces a versatile tactile sensing system capable of concurrently monitoring temperature and pressure. The temperature sensor employs carbon nanotube/graphene conductive paste as its sensitive material, while the pressure sensor integrates an ionic gel containing boron nitride as its sensitive layer. Through the application of cost-effective screen printing technology, we have successfully manufactured a flexible dual-mode sensor with exceptional performance, featuring high sensitivity (804.27 kPa-1), a broad response range (50 kPa), rapid response time (17 ms), and relaxation time (34 ms), alongside exceptional durability over 5000 cycles. Furthermore, the resistance temperature coefficient of the sensor within the temperature range of 12.5 °C to 93.7 °C is -0.17% °C-1. The designed flexible dual-mode tactile sensing system enables the real-time detection of pressure and temperature information, presenting an innovative approach to electronic skin with multi-physical tactile sensing capabilities.

Highly stretchable anti-freeze hydrogel based on aloe polysaccharides with high ionic conductivity for multifunctional wearable sensors

Conductive hydrogels have limitations such as non-degradability, loss of electrical conductivity at sub-zero temperatures, and single functionality, which limit their applicability as materials for wearable sensors. To overcome these limitations, this study proposes a bio-based hydrogel using aloe polysaccharides as the matrix and degradable polyvinyl alcohol as a reinforcing material. The hydrogel was crosslinked with borax in a glycerol-water binary solvent system, producing good toughness and compressive strength. Furthermore, the hydrogel was developed as a sensor that could detect both small and large deformations with a low detection limit of 1 % and high stretchability of up to 300 %. Moreover, the sensor exhibited excellent frost resistance at temperatures above -50 °C, and the gauge factor of the hydrogel was 2.86 at 20 °C and 2.12 at -20 °C. The Aloe-polysaccharide-based conductive hydrogels also functioned effectively as a wearable sensor; it detected a wide range of humidities (0-98 % relative humidity) and exhibited fast response and recovery times (1.1 and 0.9 s) while detecting normal human breathing. The polysaccharide hydrogel was also temperature sensitive (1.737 % °C-1) and allowed for information sensing during handwriting.

3D Printable Organohydrogel with Long-Lasting Moisture and Extreme-Temperature Tolerance for Flexible Electronics

Hydrogels with high electrical conductivity and mechanical stretchability are promising materials for flexible electronics. However, traditional hydrogels are applied in short-term usage at room temperature or low temperature due to their poor water-retention ability and freezing-tolerance property. Here, a dually cross-linked glycerol-organohydrogel (GL-organohydrogel) based on GL and acrylic acid was synthesized in a GL-water binary solvent. Fe3+ ions working as an electrolyte were added to improve the conductivity of the organohydrogel and form coordination interactions between Fe3+ ions and carboxyl groups of acrylic acid. The strong hydrogen bonding between GL and water molecules firmly lock water in the organohydrogel network, thereby endowing the GL-organohydrogel with the antifreezing property, long-term stability, and moisture lock-in capability. Our organohydrogel could endure extremely low temperature (-80 °C) over 30 days without freezing and retain its water content (almost 100% of its initial state) after being stored at room temperature (25 °C, 54% humidity) for 30 days. It also demonstrated desired stretchable properties, conductivity, three-dimensional (3D) printability, and self-healing ability. A wearable data glove was constructed by using the GL-organohydrogel and digital light processing technology. This work opens a new avenue for developing hydrogels with long-term stability, moisture lock-in capability, and extreme-temperature tolerance for stretchable electronics.

Bioinspired Fast Room-Temperature Self-Healing, Robust, Adhesive, and AIE Fluorescent Waterborne Polyurethane via Hierarchical Hydrogen Bonds and Use as a Strain Sensor

Developing a new generation of ecofriendly water-based polymeric materials that integrate mechanical robustness, fast room-temperature self-healing, adhesive, and fluorescence remains a formidable challenge. Herein, inspired by titin protein, a series of novel waterborne polyurethanes (WPU-CHZ-NAGA) containing irregular 6-fold and diamide hydrogen bonds are synthesized by introducing carbohydrazide (CHZ) and N,N-bis(2-hydroxyethyl)-3-amino propionyl glycinamide (HO-NAGA-OH) groups. The representative WPU-CHZ₂-NAGA₃ exhibits outstanding mechanical properties (tensile strength of 36.58 MPa, tearing energy of 81.2 kJ m^-2, and toughness of 125.82 MJ m^-3) and fast room-temperature self-healing ability with the aid of ethanol (≥90% within 8 h) originated from hierarchical hydrogen bonds. These properties are superior to those of most of the reported room-temperature self-healing polymer materials. Benefiting from plentiful hydrogen bonds, the WPU matrix achieves excellent adhesive properties without heating or adding curing agents. Interestingly, WPU-CHZ₂-NAGA₃ film emits inherent blue fluorescence due to the aggregation-induced emission effect of tertiary amine groups, and its potential applications in information encryption and anticounterfeiting are further demonstrated. Specially, a eutectic gel strain sensor is also fabricated with WPU-CHZ₂-NAGA₃ and deep eutectic solvent by a simple physical blending method, which can be used to monitor the movement of human fingers and wrists as well as the change in body temperature. In summary, this work provides new insight into the design and synthesis of multifunctional WPU with fast room-temperature self-healing and high mechanical properties.

Sodium alginate reinforced polyacrylamide/xanthan gum double network ionic hydrogels for stress sensing and self-powered wearable device applications

Strong and ductile sodium alginate (SA) reinforced polyacrylamide (PAM)/xanthan gum (XG) double network ionic hydrogels were constructed for stress sensing and self-powered wearable device applications. In the designed network of PXS-Mⁿ⁺/LiCl (short for PAM/XG/SA-Mⁿ⁺/LiCl, where Mⁿ⁺ stands for Fe³⁺, Cu²⁺ or Zn²⁺), PAM acts as a flexible hydrophilic skeleton, and XG functions as a ductile second network. The macromolecule SA interacts with metal ion Mⁿ⁺ to form a unique complex structure, significantly improving the mechanical strength of the hydrogel. The addition of inorganic salt LiCl endows the hydrogel with high electrical conductivity, and meanwhile reduces the freezing point and prevents water loss of the hydrogel. PXS-Mⁿ⁺/LiCl exhibits excellent mechanical properties and ultra-high ductility (a fracture tensile strength up to 0.65 MPa and a fracture strain up to 1800%), and high stress-sensing performance (a high GF up to 4.56 and pressure sensitivity of 0.122). Moreover, a self-powered device with a dual-power-supply mode, i.e., PXS-Mⁿ⁺/LiCl-based primary battery and TENG, and a capacitor as the energy storage component was constructed, which shows promising prospects for self-powered wearable electronics.

A one-pot approach to prepare stretchable and conductive regenerated silk fibroin/CNT films as multifunctional sensors

Silk fibroin (SF)-based materials are characterized by their outstanding biocompatibility and biodegradability and are considered as the most promising candidates for next-generation flexible electronics. In order to generate such devices, SF can be mixed with carbon nanotubes (CNTs) which feature excellent mechanical, electrical, and thermal properties. However, obtaining regenerated SF with homogeneous dispersion of CNTs in a sustainable manner represents a challenging task, mainly due to the difficulty in overcoming van der Waals forces and strong π-π interactions that hold together the CNT structure. In this study, a one-pot strategy for fabricating SF/CNT films is proposed by designing SF as a modifier of CNTs through non-covalent interactions with the assistance of aqueous phosphoric acid solution. Glycerol (GL) was introduced, endowing the SF/GL/CNT composite film with excellent flexibility and stretchability. The sustainable strategy greatly simplifies the preparation process, avoiding dialysis of SF and the use of artificial dispersants. The as-fabricated SF/GL/CNT films showed an excellent mechanical strength of 1.20 MPa and high sensitivity with a gauge factor of up to 13.7 toward tensile deformation. The composite films had a sensitive monitoring capability for small strains with detection limits as low as 1% and can be assembled into versatile sensors to detect human movement. Simultaneously, the composite films showed a superb thermosensitive capacity (1.64% °C^-1), which satisfied the requirement of real-time and continuous skin temperature monitoring. We anticipate that the presented one-pot strategy and the prepared composite films could open a new avenue for forthcoming technologies for electronic skins, personal health monitoring, and wearable electronics.

Environmentally Stable, Robust, Adhesive, and Conductive Supramolecular Deep Eutectic Gels as Ultrasensitive Flexible Temperature Sensor

It is essential and of great significance to impart high mechanical performance, environmental stability, and high sensitivity to emerging flexible temperature sensors. In this work, polymerizable deep eutectic solvents are designed and prepared by simply mixing N-cyanomethyl acrylamide (NCMA) containing an amide group and a cyano group in the same side chain with lithium bis(trifluoromethane) sulfonimide (LiTFSI), and obtain supramolecular deep eutectic polyNCMA/LiTFSI gels after polymerization. These supramolecular gels exhibit excellent mechanical performance (tensile strength of 12.9 MPa and fracture energy of 45.3 kJ m^-2 ), strong adhesion force, high-temperature responsiveness, self-healing ability, and shape memory behavior due to the reversible reconstruction ability of amide hydrogen bonds and cyano-cyano dipole-dipole interactions in the gel network. In addition, the gels also demonstrate good environmental stability and 3D printability. To verify its application potential as a flexible temperature sensor, the polyNCMA/LiTFSI gel-based wireless temperature monitor is developed and displays outstanding thermal sensitivity (8.4%/K) over a wide detection range. The preliminary result also suggests the promising potential of PNCMA gel as a pressure sensor.

Liquid-Free Ion-Conducting Elastomer with Environmental Stability for Soft Sensing and Thermoelectric Generating

Ionic conductors are promising candidates for fabricating soft electronics, but currently applied ionic hydrogels and organogels suffer from liquid leakage and evaporation issues. Herein, we fabricated a free-liquid ionic conducting elastomer (LFICE) with dry lithium bis(trifluoromethane sulfonimide) and elastomeric waterborne polyurethane. The resultant versatile LFICE exhibits superior tensile strength (∼4.5 MPa), satisfactory stretchability (>900%), excellent ionic conductivity (8.32 × 10^-4 S m^-1 at 25 °C), and sensitive strain (3.21) and temperature (2.22% °C^-1) response. The LFICE also presents durable environmental stability due to the all-solid-state feature. In the exploration of application prospects, the as-assembled LFICE sensor can precisely and repeatedly detect human motion and temperature changes, demonstrating its potentials in digital medical diagnosis and monitoring; the as-assembled LFICE thermoelectric generator (TEG) shows a high ionic thermovoltage of 4.41 mV K^-1, paving a bright path for the advent of self-powered soft electronics. It is believed that this research boosts the facile fabrication of environmental stable stretchable ionic conductors holding great promise in next-generation soft electronics integrated with dual thermo- and strain-response and energy harvesting.