Development of an Ultrastretchable Double-Network Hydrogel for Flexible Strain Sensors

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0-900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

High stretchable and self-adhesive multifunctional hydrogel for wearable and flexible sensors

Ionic conductive hydrogel has recently garnered significant research attention due to its potential applications in the field of wearable and flexible electronics. Nonetheless, the integration of multifunctional and synergistic advantages, including reliable electronic properties, high swelling capacity, exceptional mechanical characteristics, and self-adhesive properties, presents an ongoing challenge. In this study, we have developed an ionic conductive hydrogel through the co-polymerization of 4-Acryloylmorpholine (ACMO) and sodium acrylate using UV curing technology. The hydrogel exhibits excellent mechanical properties, high conductivity, superior swelling capacity, and remarkable self-adhesive attributes. The hydrogel serves as a highly sensitive strain sensor, enabling precise monitoring of both substantial and subtle human motions. Furthermore, the hydrogel demonstrates the capability to adhere to human skin, functioning as a human-machine interface for the detection of physiological signals, including electromyogram (EMG) signals, with low interfacial impedance. This work is anticipated to yield a new class of stretchable and conductive materials with diverse potential applications, ranging from flexible sensors and wearable bio-electronics to contributions in the field of artificial intelligence.

Machine Learning Assisted Electronic/Ionic Skin Recognition of Thermal Stimuli and Mechanical Deformation for Soft Robots

Soft robots have the advantage of adaptability and flexibility in various scenarios and tasks due to their inherent flexibility and mouldability, which makes them highly promising for real-world applications. The development of electronic skin (E-skin) perception systems is crucial for the advancement of soft robots. However, achieving both exteroceptive and proprioceptive capabilities in E-skins, particularly in terms of decoupling and classifying sensing signals, remains a challenge. This study presents an E-skin with mixed electronic and ionic conductivity that can simultaneously achieve exteroceptive and proprioceptive, based on the resistance response of conductive hydrogels. It is integrated with soft robots to enable state perception, with the sensed signals further decoded using the machine learning model of decision trees and random forest algorithms. The results demonstrate that the newly developed hydrogel sensing system can accurately predict attitude changes in soft robots when subjected to varying degrees of pressing, hot pressing, bending, twisting, and stretching. These findings that multifunctional hydrogels combine with machine learning to decode signals may serve as a basis for improving the sensing capabilities of intelligent soft robots in future advancements.

Modulus-tunable multifunctional hydrogel ink with nanofillers for 3D-Printed soft electronics

Seamless integration and conformal contact of soft electronics with tissue surfaces have emerged as major challenges in realizing accurate monitoring of biological signals. However, the mechanical mismatch between the electronics and biological tissues impedes the conformal interfacing between them. Attempts have been made to utilize soft hydrogels as the bioelectronic materials to realize tissue-comfortable bioelectronics. However, hydrogels have several limitations in terms of their electrical and mechanical properties. In this study, we present the development of a 3D-printable modulus-tunable hydrogel with multiple functionalities. The hydrogel has a cross-linked double network, which greatly improves its mechanical properties. Functional fillers such as XLG or functionalized carbon nanotubes (fCNT) can be incorporated into the hydrogel to provide tunable mechanics (Young's modulus of 10-300 kPa) and electrical conductivity (electrical conductivity of ∼20 S/m). The developed hydrogel exhibits stretchability (∼1000% strain), self-healing ability (within 5 min), toughness (400-731 kJ/m3) viscoelasticity, tissue conformability, and biocompatibility. Upon examining the rheological properties in the modulated region, hydrogels can be 3D printed to customize the shape and design of the bioelectronics. These hydrogels can be fabricated into ring-shaped strain sensors for wearable sensor applications.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Ultrafast Self-Healing, Superstretchable, and Ultra-Strong Polymer Cluster-Based Adhesive Based on Aromatic Acid Cross-Linkers for Excellent Hydrogel Strain Sensors

Synthetic hydrogel strain sensors rarely exhibit a comprehensive combination of mechanical properties such as ultra-stretchability, ultrafast self-healing, and high sensitivity. Herein, seven small molecule enhanced mechanical behaviors of polymer-cluster based hydrogels are demonstrated. The oxidized polyethyleneimine/polymeric acrylic acid (ohPEI/PAA) hydrogels with aromatic formic acids as supramolecular cross-linkers are prepared by simultaneous formation of ohPEI polymer clusters and PAA upon the addition of ammonium persulfate. The optimized hydrogel adhesive exhibits comprehensive excellent properties, such as high extensibility (up to 12 298%), real-time mechanical self-healing capability (<1 s, 93% efficiency), high uniformity, underwater adhesivity, and water-sealing ability. The proper binding strength of hydrogel and skin (47 kPa) allows the hydrogel to be utilized as highly sensitive (gauge factor:16.08), highly conductive (2.58 mS cm-1), and underwater strain sensors. Specially, the adhesive strength of the adhesive to wood after dehydration is extremely high, reaching up to 29.59 MPa. Additionally, when glycerol is introduced, the obtained gel maintains the physical properties even at harsh-temperature conditions (-40 to 80 °C). It presents that multiple and hierarchical non-covalent interactions including multiple hydrogen bonding interactions, π-π stacking, electrostatic interactions, and dipole-dipole interactions of polymer clusters, allow for the energy dissipation and contribute to the excellent performance of the hydrogel.

A High-Stretching, Rapid-Self-Healing, and Printable Composite Hydrogel Based on Poly(Vinyl Alcohol), Nanocellulose, and Sodium Alginate

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (<30 s) and achieves mechanical properties of 33.92 kPa. Additionally, it exhibits high tensile strain performance (4000%). The abundant internal ions and functional groups of SPBC hydrogels provide support for the good electrical conductivity (0.62 S/cm) and electrical response properties. In addition, the SPBC hydrogel can be attached to surfaces such as fingers and wrists to monitor human movements in real time, and its good rheological property supports three-dimensional (3D) printing molding methods. In summary, this study successfully prepared a self-healing, conductive, printable, and mechanically superior SPBC hydrogel. Its suitability for 3D-printing personalized fabrication and outstanding sensor properties makes it a useful reference for hydrogels in wearable devices and human motion monitoring.

Self-Healable and 4D Printable Hydrogel for Stretchable Electronics

Materials with high stretchability and conductivity are used to fabricate stretchable electronics. Self-healing capability and four-dimensional (4D) printability are becoming increasingly important for these materials to facilitate their recovery from damage and endow them with stimuli-response properties. However, it remains challenging to design a single material that combines these four strengths. Here, a dually crosslinked hydrogel is developed by combining a covalently crosslinked acrylic acid (AAC) network and Fe3+ ions through dynamic and reversible ionically crosslinked coordination. The remarkable electrical sensitivity (a gauge factor of 3.93 under a strain of 1500%), superior stretchability (a fracture strain up to 1700%), self-healing ability (a healing efficiency of 88% and 97% for the mechanical and electrical properties, respectively), and 4D printability of the hydrogel are demonstrated by constructing a strain sensor, a two-dimensional touch panel, and shape-morphing structures with water-responsive behavior. The hydrogel demonstrates vast potential for applications in stretchable electronics.

Fabrication of anti-freezing and self-healing nanocomposite hydrogels based on phytic acid and cellulose nanocrystals for high strain sensing applications

For hydrogel-based flexible sensors, it is a challenge to enhance the stability at sub-zero temperatures while maintaining good self-healing properties. Herein, an anti-freezing nanocomposite hydrogel with self-healing properties and conductivity was designed by introducing cellulose nanocrystals (CNCs) and phytic acid (PA). The CNCs were grafted with polypyrrole (PPy) by chemical oxidation, which were used as the nanoparticle reinforcement phase to reinforce the mechanical strength of hydrogels (851.8%). PA as a biomass material could form strong hydrogen bond interactions with H2O molecules, endowing hydrogels with prominent anti-freezing properties. Based on the non-covalent interactions, the self-healing rate of the hydrogels reached 92.9% at -15 °C as the content of PA was 40.0 wt%. Hydrogel-based strain sensors displayed high sensitivity (GF = 0.75), rapid response time (350 ms), good conductivity (3.1 S m-1) and stability at -15 °C. Various human movements could be detected by using them, including small (smile and frown) and large changes (elbow and knee bending). This work provides a promising method for the development of flexible wearable sensors that work stably in frigid environments.

Mussel-inspired adhesive and anti-swelling hydrogels for underwater strain sensing

The application of hydrogels in an underwater environment is limited due to their swelling behavior and the existence of a hydration layer. In this study, a hydrogel based on poly(sulfobetaine methacrylate) (PSBMA), tannic acid (TA) and montmorillonite (MMT) was prepared with excellent anti-swelling properties and underwater self-adhesion properties. The PSBMA hydrogel has excellent anti-swelling properties due to the strong electrostatic interaction between charged groups of PSBMA chains. Inspired by marine mussels, tannic acid modified montmorillonite (TA@MMT) was introduced. Natural polyphenol tannic acid, as a catechol donor, provides a large number of catechol groups for hydrogels. Montmorillonite acts as the physical cross-linking point of PSBMA chains through electrostatic interaction to improve the cohesion of the hydrogel. By combining the adhesion mechanism of zwitterions and catechol, the hydrogel maintains adhesion in air and underwater environments. In addition, a strain sensor was prepared based on the PSBMA/TA@MMT hydrogel, which can closely fit the human skin and stably monitor different movements in air and in underwater environments. Through a Bluetooth communication system, long-distance information transmission can be achieved. Therefore, the PSBMA/TA@MMT hydrogel broadens the application prospect of wearable devices in the underwater environment.

Enhancing the Toughness of PAA/LCNF/SA Hydrogel through Double-Network Crosslinking for Strain Sensor Application

Lignin-containing nanocellulose fibers (LCNF) have been considered as a valuable enhancer for polyacrylic acid (PAA)-based hydrogels that can form rigid porous network structures and provide abundant polar groups. However, the PAA-LCNF hydrogel is dominated by a single-network (SN) structure, which shows certain limitations when encountering external environments with high loads and large deformations. In this paper, sodium alginate (SA) was introduced into the PAA-LCNF hydrogel network to prepare a double-network (DN) hydrogel structure of the SA-Ca2+ and PAA-LCNF through a two-step process. The covalent network of PAA-LCNF acts as the resilient framework of the hydrogel, while the calcium bridging networks of SA, along with the robust hydrogen bonding network within the system, function as sacrificial bonds that dissipate energy and facilitate stress transfer. The resulting hydrogel has porous morphologies. Results show that SA can effectively improve the mechanical properties of DN hydrogels and endow them with excellent thermal stability and electrical conductivity. Compared with pure PAA-LCNF hydrogel, the elongation at break of DN hydrogel increased from 3466% to 5607%. The good electrical conductivity makes it possible to use the flexible sensors based on DN hydrogel to measure electrophysiological signals. Our results can provide a reference for developing multifunctional hydrogels that can withstand ultra large deformation.

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.

Pathways toward wearable and high-performance sensors based on hydrogels: toughening networks and conductive networks

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors.

Skin-adhesive lignin-grafted-polyacrylamide/hydroxypropyl cellulose hydrogel sensor for real-time cervical spine bending monitoring in human-machine Interface

Developing a straightforward method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, and biocompatibility remains a significant challenge. While current approaches aim to enhance mechanical performance, they often require additional steps or external forces for fixation, leading to increased production time and limited practicality. A novel lignin-grafted polyacrylamide/hydroxypropyl cellulose hydrogel (L-g-PAM/HPC hydrogel) with a semi-interpenetrating polymer network structure had been developed in this research that boasted exceptional adhesion to the skin (∼68 kPa) and stretchability properties (∼1637 %) compared to PAM-based hydrogels. By incorporating conductive additives such as silver nanowires and carbon nanocages to construct a bridge-like structure within the hydrogel matrix, the resulting AgC@L-g-PAM/HPC hydrogel exhibited impressive electrical conductivity, surpassing that of other PAM-based hydrogels relying on MXene, with a maximum value of 0.76 S/m. Furthermore, the AgC@L-g-PAM/HPC hydrogel retained its efficient electrical signal transmission capability even under mechanical stress. These make it an ideal flexible strain sensor capable of detecting various human motions. In this study, a smart real-time monitoring system was successfully developed for tracking cervical spine bending, serving as an extension for monitoring human activities.

Electrochemical Wearable Biosensors and Bioelectronic Devices Based on Hydrogels: Mechanical Properties and Electrochemical Behavior

Hydrogel-based wearable electrochemical biosensors (HWEBs) are emerging biomedical devices that have recently received immense interest. The exceptional properties of HWEBs include excellent biocompatibility with hydrophilic nature, high porosity, tailorable permeability, the capability of reliable and accurate detection of disease biomarkers, suitable device-human interface, facile adjustability, and stimuli responsive to the nanofiller materials. Although the biomimetic three-dimensional hydrogels can immobilize bioreceptors, such as enzymes and aptamers, without any loss in their activities. However, most HWEBs suffer from low mechanical strength and electrical conductivity. Many studies have been performed on emerging electroactive nanofillers, including biomacromolecules, carbon-based materials, and inorganic and organic nanomaterials, to tackle these issues. Non-conductive hydrogels and even conductive hydrogels may be modified by nanofillers, as well as redox species. All these modifications have led to the design and development of efficient nanocomposites as electrochemical biosensors. In this review, both conductive-based and non-conductive-based hydrogels derived from natural and synthetic polymers are systematically reviewed. The main synthesis methods and characterization techniques are addressed. The mechanical properties and electrochemical behavior of HWEBs are discussed in detail. Finally, the prospects and potential applications of HWEBs in biosensing, healthcare monitoring, and clinical diagnostics are highlighted.

Low-Temperature Adaptive Dual-Network MXene Nanocomposite Hydrogel as Flexible Wearable Strain Sensors

Flexible electronic devices and conductive materials can be used as wearable sensors to detect human motions. However, the existing hydrogels generally have problems of weak tensile capacity, insufficient durability, and being easy to freeze at low temperatures, which greatly affect their application in the field of wearable devices. In this paper, glycerol was partially replaced by water as the solvent, agar was thermally dissolved to initiate acrylamide polymerization, and MXene was used as a conductive filler and initiator promoter to form the double network MXene-PAM/Agar organic hydrogel. The presence of MXene makes the hydrogel produce more conductive paths and enforces the hydrogel's higher conductivity (1.02 S·m-1). The mechanical properties of hydrogels were enhanced by the double network structure, and the hydrogel had high stretchability (1300%). In addition, the hydrogel-based wearable strain sensor exhibited good sensitivity over a wide strain range (GF = 2.99, 0-200% strain). The strain sensor based on MXene-PAM/Agar hydrogel was capable of real-time monitoring of human movement signals such as fingers, wrists, arms, etc. and could maintain good working conditions even in cold environments (-26 °C). Hence, we are of the opinion that delving into this hydrogel holds the potential to broaden the scope of utilizing conductive hydrogels as flexible and wearable strain sensors, especially in chilly environments.

Ultrastrong and Tough Urushiol-Based Ionic Conductive Double Network Hydrogels as Flexible Strain Sensors

Ionic conductive hydrogels have attracted increasing research interest in flexible electronics. However, the limited resilience and poor fatigue resistance of current ionic hydrogels significantly restrict their practical application. Herein, an urushiol-based ionic conductive double network hydrogel (PU/PVA-Li) was developed by one-pot thermal initiation polymerization assisted with freeze-thaw cycling and subsequent LiCl soaking. Such a PU/PVA-Li hydrogel comprises a primary network of covalently crosslinked polyurushiol (PU) and a secondary network formed by physically crosslinked poly(vinyl alcohol) (PVA) through crystalline regions. The obtained PU/PVA-Li hydrogel demonstrates exceptional mechanical properties, including ultrahigh strength (up to 3.4 MPa), remarkable toughness (up to 1868.6 kJ/m3), and outstanding fatigue resistance, which can be attributed to the synergistic effect of the interpenetrating network structure and dynamic physical interactions between PU and PVA chains. Moreover, the incorporation of LiCl into the hydrogels induces polymer chain contraction via ionic coordination, further enhancing their mechanical strength and resilience, which also impart exceptional ionic conductivity (2.62 mS/m) to the hydrogels. Based on these excellent characteristics of PU/PVA-Li hydrogel, a high-performance flexible strain sensor is developed, which exhibits high sensitivity, excellent stability, and reliability. This PU/PVA-Li hydrogel sensor can be effectively utilized as a wearable electronic device for monitoring various human joint movements. This PU/PVA-Li hydrogel sensor could also demonstrate its great potential in information encryption and decryption through Morse code. This work provides a facile strategy for designing versatile, ultrastrong, and tough ionic conductive hydrogels using sustainable natural extracts and biocompatible polymers. The developed hydrogels hold great potential as promising candidate materials for future flexible intelligent electronics.

Bone-inspired (GNEC/HAPAAm) hydrogel with fatigue-resistance for use in underwater robots and highly piezoresistive sensors

A novel bone-inspired fatigue-resistant hydrogel with excellent mechanical and piezoresistive properties was developed, and it exhibited great potential as a load and strain sensor for underwater robotics and daily monitoring. The hydrogel was created by using the high edge density and aspect ratio of graphene nanosheet-embedded carbon (GNEC) nanomaterials to form a three-dimensional conductive network and prevent the expansion of microcracks in the hydrogel system. Multiscale progressive enhancement of the organic hydrogels (micrometer scale) was realized with inorganic graphene nanosheets (nanometer scale). The graphene nanocrystals inside the GNEC film exhibited good electron transport properties, and the increased distances between the graphene nanocrystals inside the GNEC film caused by external forces increased the resistance, so the hydrogel was highly sensitive and suitable for connection to a loop for sensing applications. The hydrogels obtained in this work exhibited excellent mechanical properties, such as tensile properties (strain up to 1685%) and strengths (stresses up to 171 kPa), that make them suitable for use as elastic retraction devices in robotics and provide high sensitivities (150 ms) for daily human monitoring.

Multifunctional Starch-Based Sensor with Non-Covalent Network to Achieve "3R" Circulation

With the consumption of disposable electronic devices increasing, it is meaningful but also a big challenge to develop reusable and sustainable materials to replace traditional single-use sensors. Herein, a clever strategy for constructing a multifunctional sensor with 3R circulation (renewable, reusable, pollution-reducing biodegradable) is presented, in which silver nanoparticles (AgNPs) with multiple interactions are introduced into a reversible non-covalent cross-linking network composed of biocompatible and degradable carboxymethyl starch (CMS) and polyvinyl alcohol (PVA) to simultaneously obtain high mechanical conductivity and long-term antibacterial properties by a one-pot method. Surprisingly, the assembled sensor shows high sensitivity (gauge factor up to 4.02), high conductivity (0.1753 S m^-1 ), low detection limit (0.5%), long-term antibacterial ability (more than 7 days), and stable sensing performance. Thus, the CMS/PVA/AgNPs sensor can not only accurately monitor a series of human behavior, but also identify handwriting recognition from different people. More importantly, the abandoned starch-based sensor can form a 3R circulation. Especially, the fully renewable film still shows excellent mechanical performance, achieving reusable without sacrificing its original function. Therefore, this work provides a new horizon for multifunctional starch-based materials as sustainable substrates for replacing traditional single-use sensors.

Functionalized Hydrogel-Based Wearable Gas and Humidity Sensors

Breathing is an inherent human activity; however, the composition of the air we inhale and gas exhale remains unknown to us. To address this, wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks, and for the early detection and treatment of diseases for home healthcare. Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable. Functionalized hydrogels are intrinsically conductive, self-healing, self-adhesive, biocompatible, and room-temperature sensitive. Compared with traditional rigid vapor sensors, hydrogel-based gas and humidity sensors can directly fit human skin or clothing, and are more suitable for real-time monitoring of personal health and safety. In this review, current studies on hydrogel-based vapor sensors are investigated. The required properties and optimization methods of wearable hydrogel-based sensors are introduced. Subsequently, existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized. Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented. Moreover, the potential of hydrogels in the field of vapor sensing is elucidated. Finally, the current research status, challenges, and future trends of hydrogel gas/humidity sensing are discussed.

Intrinsically Nonswellable Multifunctional Hydrogel with Dynamic Nanoconfinement Networks for Robust Tissue-Adaptable Bioelectronics

Developing bioelectronics that retains their long-term functionalities in the human body during daily activities is a current critical issue. To accomplish this, robust tissue adaptability and biointerfacing of bioelectronics should be achieved. Hydrogels have emerged as promising materials for bioelectronics that can softly adapt to and interface with tissues. However, hydrogels lack toughness, requisite electrical properties, and fabrication methodologies. Additionally, the water-swellable property of hydrogels weakens their mechanical properties. In this work, an intrinsically nonswellable multifunctional hydrogel exhibiting tissue-like moduli ranging from 10 to 100 kPa, toughness (400-873 J m^-3 ), stretchability (≈1000% strain), and rapid self-healing ability (within 5 min), is developed. The incorporation of carboxyl- and hydroxyl-functionalized carbon nanotubes (fCNTs) ensures high conductivity of the hydrogel (≈40 S m^-1 ), which can be maintained and recovered even after stretching or rupture. After a simple chemical modification, the hydrogel shows tissue-adhesive properties (≈50 kPa) against the target tissues. Moreover, the hydrogel can be 3D printed with a high resolution (≈100 µm) through heat treatment owing to its shear-thinning capacity, endowing it with fabrication versatility. The hydrogel is successfully applied to underwater electromyography (EMG) detection and ex vivo bladder expansion monitoring, demonstrating its potential for practical bioelectronics.

A high-pressure resistant ternary network hydrogel based flexible strain sensor with a uniaxially oriented porous structure toward gait detection

Gait abnormalities have been widely investigated in the diagnosis and treatment of neurodegenerative diseases. However, it is still a great challenge to achieve a comfortable, convenient, sensitive and high-pressure resistant flexible gait detection sensor for real-time health monitoring. In this work, a polyaniline (PANI)@(polyacrylic acid (PAA)-polyvinyl alcohol (PVA)) (PANI@(PVA-PAA)) ternary network hydrogel with a uniaxially oriented porous featured structure was successfully prepared using a simple freeze-thaw method and in situ polymerization. The PANI@(PVA-PAA) hydrogel shows excellent compressive mechanical properties (423.44 kPa), favorable conductivity (2.02 S m^-1) and remarkable durability (500 loading-unloading cycle), and can sensitively detect the effect of pressure with a fast response time (200 ms). The PANI@(PVA-PAA) hydrogel assembled into a flexible sensor can effectively identify the movement state of the shoulder, knee and even the sole of the plantar for gait detection. The uniaxially oriented porous structure enables the hydrogel-based sensor to have a high rate of change in the longitudinal direction and can effectively distinguish various gaits. The construction of a hydrogen bond between PANI and the PVA-PAA hydrogel ensures the uniform distribution of PANI in the hydrogel to form a ternary network structure, which improves the pressure resistance and conductivity of the PANI@(PVA-PAA) hydrogel. Thus, PANI@(PVA-PAA) hydrogel flexible sensor for gait detection can not only effectively monitor some serious diseases but also detect some unscientific exercise in people's daily life.

Highly stretchable, self-healing elastomer hydrogel with universal adhesion driven by reversible cross-links and protein enhancement

Engineered hydrogels with excellent mechanical properties and multi-functionality have great potential as soft electronic skins, tissue substitutes and flexible robotic joints. However, it has been a challenge to construct multifunctional hydrogels, especially when integrating high stretchability, toughness and strength, low hysteresis, good self-healing and adhesion abilities into a hydrogel system simultaneously. Here, we successfully developed a structural hydrogel composed of a reversible covalently cross-link-based poly-N-(2-hydroxyethyl)acrylamide (PHEMAA) network and available plastically deformable casein micelles. Such a design enabled the reversible covalent cross-links and casein micelles to enhance energy dissipation and toughen the PHEMAA/casein hybrid hydrogel synergistically. More importantly, the hydrogel could respond to the imposed strains reversibly by cross-link and micelle deformation induced-network reconstitution, which led to low hysteresis of the hydrogels. The recoverable gel networks still exhibited their effects on energy dissipation at the stress-focused area, endowing the hydrogels with fatigue resistance. As a result, the hydrogels exhibited a compressive strength of 36.5 MPa, high stretchability (1460%), high toughness (∼5.98 MJ m^-3), low hysteresis (<30%) and fatigue resistance with almost completely overlapped hysteresis curves during 10 loading cycles. In addition, the introduction of casein micelles and reversible covalent bonding endowed the elastomer hydrogels with high adhesivity, self-healing abilities and biocompatibility.

Synthesis and Characterization of a Liquid Crystal-Modified Polydimethylsiloxane Rubber with Mechanical Adaptability Based on Chain Extension in the Process of Crosslinking

The mechanical adaptive material is a kind of functional material that can effectively dissipate energy and suppress the increase of its stress under continuous strain in a large deformation area, which are vital in artificial muscles, connection devices, soft artificial intelligence robots, and other areas. Scientists have been working to broaden the platform of the material's mechanical adaptive platform and improve its mechanical strength by specific structure design. Based on it, we expect to introduce a mechanism of energy dissipation from the molecular chain scale to further improve mechanical adaptability. We developed a liquid crystal-modified polydimethylsiloxane rubber with mechanical adaptability based on chain extension in the process of crosslinking. Results showed that liquid crystal (0.7 mol %)-modified silicone rubber can obviously dissipate energy to achieve mechanical adaptive function, and the energy dissipation ratio of polydimethylsiloxane rubber (MQ), 4-propyl-4'-vinyl-1,1'-bi(cyclohexane)-modified polydimethylsiloxane rubber (3CCV-MQ), and 4-methoxyphenyl-4-(3-butenyloxy) benzoate-modified polydimethylsiloxane rubber (MBB-MQ) gradually decreases from 30 to 24%. Excessive thiol groups of liquid crystal-modified polydimethylsiloxane react with its vinyl group to achieve the chain extension, which significantly improves the mechanical strength from 2.74 to 5.83 MPa and elongation at break from 733 to 1096%. This research offers some new insights into improving the mechanical strength of silicone rubber and is of great significance for the application of the mechanical adaptive material.

Ultrastretchable and adhesive agarose/Ti3C2Tx-crosslinked-polyacrylamide double-network hydrogel for strain sensor

A novel agarose/Ti₃C₂T_x-crosslinked-polyacrylamide (AG/T-PAM) double-network (DN) hydrogel is synthesized by combining heating-cooling and γ-ray radiation-induced polymerization. The AG/T-PAM DN hydrogel possesses excellent mechanical properties with 4250% stretchability, and good adhesion to different substrates, such as an adhesive strength of 1148 kPa to copper at 30 °C. The resultant hydrogel also exhibits excellent tensile and compression sensing properties due to the variation of conductive network within hydrogel. The flexible and wearable strain sensor composed of the AG/T-PAM DN hydrogel presents rapid response to strain withstand 1000 cycles, and can monitor various movements of human body with a high sensibility. The AG/T-PAM DN hydrogel-based strain sensor will have broad application in large-scale strain detection scenarios requiring high sensitivity and adhesion.

Effect of Amylose and Crystallinity Pattern on the Gelatinization Behavior of Cross-Linked Starches

Starches from normal maize (NM), normal potato (NP), waxy maize (WM), and waxy potato (WP) were cross-linked with seven different concentrations (0.01, 0.05, 0.1, 0.5, 1, 5, 10%) of sodium trimetaphosphate and sodium tripolyphosphate. The use of low-amylose WM and WP as well as A-crystalline maize and B-crystalline potato starches can determine the influence of the amylose content and crystallinity pattern on the cross-linking of starches. The results showed that the viscosity of the cross-linked starch (CLs) first increased and then deceased, and finally no viscosity was detected; WM showed no viscosity at 5% and NP at 1%. In addition, the viscosity of NM first increased and then became undetectable at 0.5%. Strikingly, the WP developed viscosity even at a 10% reagent level (RL), and it developed the highest viscosity of all samples at 1%. The starch-iodine method was a facile and high-performance method for the characterization of the cross-linking degree (CL%), having been applied to normal starches, because the increase in the CL% resulted in a decrease of iodine-complexed amylose and blue intensity. In this study, the starch-iodine method was extended to waxy starches, which stained brown with iodine, and the brown intensity decreased with the increase of the CL%. Moreover, the CL% and RL showed a linear-log relationship.

A Highly Mechanical, Conductive, and Cryophylactic Double Network Hydrogel for Flexible and Low-Temperature Tolerant Strain Sensors

Due to their stretchability, conductivity, and good biocompatibility, hydrogels have been recognized as potential materials for flexible sensors. However, it is still challenging for hydrogels to meet the conductivity, mechanical strength, and freeze-resistant requirements in practice. In this study, a chitosan-poly (acrylic acid-co-acrylamide) double network (DN) hydrogel was prepared by immersing the chitosan-poly (acrylic acid-co-acrylamide) composite hydrogel into Fe₂(SO₄)₃ solution. Due to the formation of an energy-dissipative chitosan physical network, the DN hydrogel possessed excellent tensile and compression properties. Moreover, the incorporation of the inorganic salt endowed the DN hydrogel with excellent conductivity and freeze-resistance. The strain sensor prepared using this DN hydrogel displayed remarkable sensitivity and reliability in detecting stretching and bending deformations. In addition, this DN hydrogel sensor also worked well at a lower temperature (−20 °C). The highly mechanical, conductive, and freeze-resistant DN hydrogel revealed a promising application in the field of wearable devices.

Dual-Sensing, Stretchable, Fatigue-Resistant, Adhesive, and Conductive Hydrogels Used as Flexible Sensors for Human Motion Monitoring

Hydrogel-based sensors serve as an ideal platform for developing personalized wearable electronics due to their high flexibility and conformability. However, the weak stretchability and inferior conductivity of hydrogels have severely restricted their large-scale application. Herein, a natural polymer-based conductive hydrogel integrated with favorable mechanical properties, good adhesive performance, and excellent fatigue resistance was fabricated via interpenetrating tannic acid (TA) into a chitosan (CS) cross-linked network in an acidic aqueous solution. The hydrogel was composed of a regular hierarchical porous structure, which was built by the hydrogen bonding between TA and CS. In addition, the hydrogels exhibited adjustable mechanical properties (maximum yield stress of 7000 Pa) and good stretchability (strain up to 320%). Benefiting from the abundant catechol groups of TA, the proposed hydrogels could repeatedly adhere to various material surfaces and could be easily peeled off without residue. Moreover, the hydrogel exhibited stable conductivity, high stretching sensitivity (gauge factor of 2.956), rapid response time (930 ms), and excellent durability (>300 cycles), which can be assembled as a strain sensor to attach to the human body for precise monitoring of human exercise behavior, distinguishing physiological signals, and recognizing speech. Furthermore, the prepared hydrogels also exhibited stable sensing performance to temperature. As a result, the hydrogels exhibited dual sensory performance for both temperature and strain deformation. It is anticipated that the incorporation of strain sensors and thermal sensors will provide theoretical guidance for developing multifunctional conductive hydrogels and pave a way for the versatile application of hydrogel-based flexible sensors in wearable devices and soft actuators.

Nanocage Ferritin Reinforced Polyacrylamide Hydrogel for Wearable Flexible Strain Sensors

Biocomposite hydrogels are promising for applications in wearable flexible strain sensors. Nevertheless, the existing biocomposite hydrogels are still hard to meet all requirements, which limits the practical application. Here, inspired by the structure and composition of natural ferritin, we design a PAAm-Ferritin hybrid hydrogel through a facile method. Ferritin is uniformly distributed in the cross-linking networks and acts as a nanocage spring model, leading to the enhanced tensile strength of the hydrogel. The fracture stress is 99 kPa at 1400% maximum elongation. As fabricated PAAm-Ferritin hybrid hydrogels exhibit high toughness and low elastic modulus (21 kPa). The PAAm-Ferritin hybrid hydrogels present excellent biocompatibility and increased conductivity compared with PAAm hydrogel. Impressively, as a wearable flexible strain sensor, the PAAm-Ferritin hybrid hydrogels have high sensitivity (gauge factor = 2.06), excellent reliability, and cycling stability. This study indicates the feasibility of utilizing ferritin to synthesize functional materials, which is conducive to expanding the use of protein synthesis of materials technology and application fields.

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

Conductive hydrogels have been extensively used in wearable skin sensors owing to their outstanding flexibility, tissuelike compliance, and biocompatibility. However, the dehydration and embrittlement of hydrogels can result in sensitivity loss or even invalidation, restraining their wearable applications in external environments, especially at low temperatures and in arid environments. Herein, an environment-resistant organohydrogel is developed for multifunctional sensors. A double-network organohydrogel based on hyaluronic acid and poly(acrylic acid-co-acrylamide) is developed, and glycerol is introduced into the organohydrogel network via a solvent displacement strategy. Owing to the water-locking effects of glycerol and tough polymeric backbone, the resultant organohydrogel not only exhibits stable tensibility but also maintains excellent flexibility and stable conductivity with the environment-resistant properties, including freezing resistance against -30 °C and moisture retention at 4% relative humidity in a high temperature of 60 °C. Moreover, a series of organohydrogel-based sensors and an array device are developed to achieve highly sensitive strain, temperature, and humidity responses and exhibit a high gauge factor of 10.79 in the strain-sensitive test. This work develops a universal ionic skin based on organohydrogels to be applied to wearable sensors for health monitoring.

Recent Advances in Design Strategies of Tough Hydrogels

Hydrogels are a fascinating class of materials popular in numerous fields, including tissue engineering, drug delivery, soft robotics, and sensors, attributed to their 3D network porous structure containing a significant amount of water. However, traditional hydrogels exhibit poor mechanical strength, limiting their practical applications. Thus, many researchers have focused on the development of mechanically enhanced hydrogels. This review describes the design considerations for constructing tough hydrogels and some of the latest strategies in recent years. These tough hydrogels have an up-and-coming prospect and bring great hope to the fields of biomedicine and others. Nonetheless, it is still no small challenge to realize hydrogel materials that are tough, multifunctional, intelligent, and zero-defect. This article is protected by copyright. All rights reserved.

Double-Network Tough Hydrogels: A Brief Review on Achievements and Challenges

This brief review attempts to summarize research advances in the mechanical toughness and structures of double-network (DN) hydrogels. The focus is to provide a critical and concise discussion on the toughening mechanisms, damage recoverability, stress relaxation, and biomedical applications of tough DN hydrogel systems. Both conventional DN hydrogel with two covalently cross-linked networks and novel DN systems consisting of physical and reversible cross-links are discussed and compared. Covalently cross-linked hydrogels are tough but damage-irreversible. Physically cross-linked hydrogels are damage-recoverable but exhibit mechanical instability, as reflected by stress relaxation tests. This remains one significant challenge to be addressed by future research studies to realize the load-sustaining applications proposed for tough hydrogels. With their special structure and superior mechanical properties, DN hydrogels have great potential for biomedical applications, and many DN systems are now fabricated with 3D printing techniques.

Mechanical Integrity in a Dynamic Interpenetrating Hydrogel Network of Supramolecular Peptide-Polysaccharide Supports Enhanced Chondrogenesis

Tissue engineering demands intelligently designed scaffolds that encompass the properties of the target tissues in terms of mechanical and bioactive properties. An ideal scaffold for engineering a cartilage tissue should provide the chondrocytes with a favorable 3D microarchitecture apart from possessing optimal mechanical characteristics such as compressibility, energy dissipation, strain stiffening, etc. Herein, we used a unique design approach to develop a hydrogel having a dynamic interpenetrating network to serve as a framework to support chondrocyte growth and differentiation. An amyloid-inspired peptide amphiphile (1) was self-assembled to furnish kinetically controlled nanofibers and incorporated in a dynamic covalently cross-linked polysaccharide network of carboxymethyl cellulose dialdehyde (CMC-D) and carboxymethyl chitosan (CMCh) using Schiff base chemistry. The dynamic noncovalent interaction played a pivotal role in providing the desired modulation in the structure and mechanical properties of the double-network hydrogels that are imperative for cartilage scaffold design. The adaptable nature supported shear-induced extrusion of the hydrogel and facilitated various cellular functions while maintaining its integrity. The potential of the as-developed hydrogels to support in vitro chondrogenesis was explored using human chondrocytes. Evidence of improved cell growth and cartilage-specific ECM production confirmed the potential of the hydrogel to support cartilage tissue engineering while reaffirming the significance of mimicking the biophysical microenvironment to induce optimal tissue regeneration.

Strong, tough, anti-freezing, non-drying and sensitive ionic sensor based on fully physical cross-linked double network hydrogel

Ionic conductive double network (DN) sensors have attracted increasing attention in wearable electronic devices. However, their low mechanical and sensing properties as well as poor moisture retention and freezing resistance restrict severely their applications. Herein, we synthesized a fully physical cross-linked poly (N-hydroxymethyl acrylamide)/agar/ethylene glycol (PHA/Agar/EG) ionic conductive DN hydrogel exhibiting high strength and toughness, fast self-recovery, good fatigue resistance and good self-healing. Agar could form a physical network via reversible sol-gel transition, and interact with physical cross-linked poly (N-hydroxymethyl acrylamide) and sodium chloride (NaCl) via hydrogen bonds and salting-out effect, respectively. Meanwhile, ethylene glycol and NaCl improved the mechanical properties, long-lasting moisture retention and anti-freezing ability. The PHA/Agar/EG gel-based flexible sensor possessed excellent long-lasting and fatigue resistant sensing properties, and could monitor various human activities stably and sensitively. Therefore, this work would provide a simple and promising strategy to fabricate flexible sensors with integrated high performances for smart wearable devices.

Polyacrylamide/Chitosan-Based Conductive Double Network Hydrogels with Outstanding Electrical and Mechanical Performance at Low Temperatures

Hydrogel-based electronics have received growing attention because of their great flexibility and stretchability. However, the fabrication of conductive hydrogels with high stretchability, excellent toughness, outstanding sensitivity, and low-temperature stability still remains a great challenge. In this study, a type of conductive hydrogels consisting of a double network (DN) structure is synthesized. The dynamically cross-linked chitosan (CS) and the flexible polyacrylamide network doped with polyaniline constitute the DN through the hydrogen bonds between the hydroxyl, amide, and aniline groups. This type of hydrogels displays excellent mechanical performance, striking conductivity, and remarkable freezing tolerance. The flexible electronic sensors based on the double-network hydrogels demonstrate superior strain sensitivity and linear response on various deformations. Additionally, the good antifreezing property of the hydrogels allows the sensors to exhibit excellent performance at -20 °C.