Bio-Based Hydrogel Transducer for Measuring Human Motion with Stable Adhesion and Ultrahigh Toughness

Enhanced Mechanical Properties and Sensing Performance of MXene-Based Dual-Crosslinked Hydrogel via EGCG Coating and Dynamic Covalent Bond

MXenes hold great promise for flexible sensors due to their outstanding electrical and mechanical properties. However, their practical application in aqueous environments is often compromised by oxidation susceptibility. Here, epigallocatechin gallate (EGCG), a naturally derived compound is introduced, as a protective coating for Ti₃C₂Tx MXene nanosheets. The catechol groups in EGCG form strong hydrogen bonds with MXene, significantly enhancing its oxidation resistance in aqueous environments for up to 40 days. By incorporating EGCG-coated MXene (MXene@EGCG) to form a dual-crosslinked hybrid network, a tough hydrogel with exceptional properties, including enhanced compressibility (>95%), high compressive strength (5.43 MPa), minimal residual strain (<1%), and rapid recovery within seconds is developed. Furthermore, the hydrogel exhibits superior sensing capabilities with a compressive gauge factor exceeding 10 and a stretch gauge factor of up to 3.72. This well-designed structure also endows it a low degree of hysteresis in compressive sensing. In vitro experiments confirm its great biocompatibility, desired self-adhesion properties, and practical utility as a sensing platform. This approach pioneers a versatile and transformative strategy for enhancing MXene stability and engineerability, unlocking new possibilities for fabricating high-performance hydrogel-based sensors capable of effectively sensing dynamic strains, which may find broad applications in the fields of multifunctional bioelectronics.

Highly adhesive conductive hydrogels fabricated by catechol lignin/liquid metal-initiated polymerization of acrylic acid for strain sensors

Conductive hydrogels have emerged as promising candidates for next-generation flexible electronics owing to their unique combination of electrical conductivity and mechanical compliance. However, the development of an eco-friendly and efficient polymerization strategy to simultaneously achieve robust adhesion and superior functionality remains a challenge. In the work, catechol lignin (DAL)/liquid metal (LM) were utilized as initiators for the polymerization of acrylic monomers (PAA), resulting in the preparation of conductive hydrogels (PAA-DAL-LM). The engineered DAL component serves dual functions of establishing an interfacial stabilization layer for LM nanoparticles while participating in radical generation for polymerization initiation, and this synthesis protocol eliminates conventional toxic initiators through LM-mediated radical generation mechanisms. The resultant PAA-DAL-1.6 %LM hydrogel demonstrated remarkable performance characteristics, including exceptional compressive strength (688.5 KPa), good self-healing properties, and high electrical conductivity (0.24 S/m). Structural modification of alkali lignin through catechol incorporation significantly improved both the water solubility and interfacial adhesion strength (16.23 KPa). Systematic characterization revealed stable strain-responsive electrical behavior with high strain sensing accuracy as well as stable electrical output. These multifunctional hydrogels not only hold significant potential for advancing flexible sensor technologies but also pave the way for sustainable valorization of lignin biopolymers in advanced material applications.

CNC-mediated functionalized MWCNT-reinforced double-network conductive hydrogels as smart, flexible strain and epidermic sensors for human motion monitoring

Soft, stretchable, and smart strain-sensing hydrogels have attracted significant attention due to their broad applicability in emerging fields. However, developing hydrogel-based strain-sensing materials with finely tuned mechanical and sensing properties remains challenging, primarily due to the inherent brittleness of traditionally fabricated hydrogels. In this study, a novel flexible strain- and epidermis-sensitive sensor was designed using a cellulose nanocrystal (CNC)-mediated acid functionalized multiwalled carbon nanotube (A-MWCNT)-reinforced double-network conductive hydrogel. This dual-network hydrogel system was fabricated by integrating a covalently crosslinked acrylamide (Amm) and [2-(acryloyloxy) ethyl] trimethyl-ammonium chloride (AETAC) with a physically crosslinked network of A-MWCNTs, which were uniformly dispersed via CNCs. Incorporating hydrogen bonding and strong electrostatic interactions within the physical network introduced reversible sacrificial bonds, significantly enhancing the hydrogel's mechanical strength. The hydrogel exhibited mechanical and sensing performance, including sufficient stretchability (431.6%), remarkable sensitivity, a gauge factor (GF) of 4.32 at 400% strain, toughness of 65.6 kJ m-3, Young's modulus of 1.5 kPa, and rapid response and recovery times of 100 msec. Furthermore, it demonstrated excellent cycling stability over 100 cycles and effective sensing capabilities across a broad strain range, from small deformations (5%) to large strains (400%). The conductivity of 0.09 S m-1, facilitated by the formation of conduction pathways through the AETAC and A-MWCNTs, further enhanced its performance. Moreover, the hydrogel exhibited practical applicability in detecting various large-scale and physiological human movements. Functioning as a wearable electronic skin, it represents a highly flexible and adaptable material suitable for applications in soft robotics, flexible sensors, and health monitoring devices.

Ion-conductive hydrogel sensor prepared with alginate crosslinker for wide-range motion and temperature monitoring

Flexible wearable electronic devices have gained widespread attention due to their comfort and real-time information transmission capabilities. However, they often exhibit poor stretchability, low sensitivity, and limited functionality, which restricts their practical applications. Herein, a conductive hydrogel (PNAL, a polymer synthesized from polyvinyl alcohol, nipaam, acrylic acid, and lithium chloride) that responds to both strain and temperature simultaneously was developed using a natural polysaccharide composite (SA-AGE) synthesized from sodium alginate (SA) and allyl glycidyl ether (AGE) as a crosslinking agent. These hydrogels were fabricated into multifunctional sensors capable of detecting strain and temperature with high sensitivity (gauge factor up to 1.12 and temperature coefficients of resistance up to -1.20 %/°C) and reliable stability (1000 cycles). Importantly, by adjusting the molar ratio of NIPAAm to AA, the volume phase transition temperature (VPTT) of the hydrogel can be easily controlled. When the temperature increases, the hydrogel changes from translucent to white and opaque within 20 s; when the temperature decreases, it returns to its original state. Based on the color-changing property of the hydrogel in response to temperature variations, it can serve as a visible color-changing sticker, providing early warning for patients with fever, thus significantly improving traditional temperature measurement methods.

Highly Efficient Electrophoretic Deposition of Durable, Corrosion-Resistant Chitosan-PEG Composites on Metallic Implants

In this work, the electrophoretic deposition (EPD) of uniform and mechanically stable chitosan/poly(ethylene glycol) (PEG) films from a biphase H2O/CO2 medium under high pressure was proposed. This solvent has a pronounced sterilizing ability and spontaneously self-neutralizes during decompression, becoming a fully biocompatible medium. In such a medium, chitosan aggregates much less than in traditional acid solutions, which allows greater electrophoretic mobility of the macromolecules during EPD of the coating and contributes to an increase in the efficiency of the coating application. The addition of PEG improves the insufficient mechanical strength of the chitosan films, increasing their hardness and adhesion to the substrate. The average peel strengths were 0.15 ± 0.09 N mm-1 for the chitosan coating and 0.7 ± 0.2 N mm-1 for the chitosan/PEG composite. The interaction of chitosan and PEG, which form intermolecular complexes due to hydrogen bonding, was studied by Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray diffraction, and X-ray photoelectron spectroscopy methods. The contact angles of water droplets on the obtained coatings are in the optimal range for bone cell proliferation: 35-85°. High biocompatibility of the coatings was confirmed by evaluating the cytotoxicity using the methyl-thiazol tetrazolium assay with C2C12 osteo-like cells. The coatings show good corrosion resistance due to their high adhesion to metal. Composite coating demonstrated a shift in the corrosion potential toward positive values by 0.1 V, and a drop in corrosion current density by an order of magnitude as compared to the bare titanium substrate. The morphology of the coatings was evaluated by atomic force microscopy, scanning electron microscopy, and profilometry methods, and it was shown that the addition of PEG leads to a significant decrease in the coating thickness (2 μm), while increasing the uniformity. The roughness of the coatings is in the submicron range.

3D printing of wearable sensors with strong stretchability for myoelectric rehabilitation

Myoelectric biofeedback (EMG-BF) is a widely recognized and effective method for treating movement disorders caused by impaired nerve function. However, existing EMG-feedback devices are almost entirely located in large medical centers, which greatly limits patient accessibility. To address this critical limitation, there is an urgent need to develop a portable, cost-effective, and real-time monitoring device that can transcend the existing barriers to the treatment of EMG-BF. Our proposed solution leverages polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) as core materials, ingeniously incorporating wood pulp nano celluloses (CNF-P)-Na+ to enhance the structural integrity. Additionally, the inclusion of nano-silica particles further augments the sensor's capabilities, enabling the creation of a stress-sensitive mineral ionization hydrogel sensor. This innovative approach not only capitalizes on the superior rheological properties of the materials but also, through advanced 3D printing technology, facilitates the production of a micro-scale structural hydrogel sensor with unparalleled sensitivity, stability, and durability. The potential of this sensor in the realm of human motion detection is nothing short of extraordinary. This development can potentially improve the treatment landscape for EMG-BF offering patients more convenient and efficient therapeutic options.

Instantaneous self-recovery and ultra-low detection limit hydrogel electronic sensor for temporomandibular disorders intelligent diagnosis

Temporomandibular disorders (TMD) intelligent diagnosis promises to elevate clinical efficiency and facilitate timely TMD management for patients. However, development of TMD intelligent diagnostic tools with high accuracy and sensitivity presents challenges, particularly in sensing minute deformations and ensuring rapid self-recovery. Here we report a biocompatible hydrogel electronic sensor with instantaneous self-recovery (within 2.1 s) and ultra-low detection limit (0.005% strain). It could efficiently diagnose disc displacement with reduction (DDwR) with satisfactory accuracy of 90.00%, and also had a clear indication of the typical clinical manifestations of DDwR and the timing of temporomandibular joint (TMJ) clicking, with a sensitivity of up to 100% in human compared to the diagnostic criteria for TMD (DC/TMD). Furthermore, a predictive model based on waveform features achieved 84.4% accuracy and 86% sensitivity, reducing dependence on physicians. In summary, the hydrogel sensor is expected to become a radiation-free, non-invasive, practical and effective tool for future TMD diagnosis.

Progress in Protein-Based Hydrogels for Flexible Sensors: Insights from Casein

In recent years, the rapid advancement of flexible sensors as the cornerstone of flexible electronics has propelled a flourishing evolution within the realm of flexible electronics. Unlike traditional flexible devices, hydrogel flexible sensors have characteristic advantages such as biocompatibility, adhesion, and adjustable mechanical properties and have similar properties to human skin. Especially, biobased hydrogels have become the preferred substrate material for flexible sensors due to increased environmental pressures caused by the scarcity of petrochemical resources. In this regard, proteins possess advantages such as diverse amino acid compositions, adjustable advanced structures, chemical modifiability, the application of protein engineering techniques, and the ability to respond to various external stimuli. These enable the hydrogels constructed from them to have greater designability, flexibility, and adaptability. As a result, their applications in manufacturing various types of sensors have experienced rapid growth. This work systematically reviews the sensing mechanism of protein-based hydrogels, focusing on the preparation of protein-based hydrogels and the optimization of flexible sensors mainly from the perspective of a typical type of animal-derived protein casein. In addition, while the potential of casein is recognized, the limitations of casein-based hydrogels in flexible sensor applications are explored, and insights are provided into the development trends of next-generation sensors based on casein-based hydrogel materials.

Lignin hydrogel sensor with anti-dehydration, anti-freezing, and reproducible adhesion prepared based on the room-temperature induction of zinc chloride-lignin redox system

In recent years, flexible sensors constructed mainly from hydrogels have received increasing attention. However, conventional hydrogels need to be prepared by high-temperature or radiation-induced polymerization reactions, which limits their practical applications due to their suboptimal electrical conductivity and weak mechanical properties. In this paper, using sodium lignosulfonate as the raw material, a dynamic catechol-quinone redox system formed by lignin‑zinc ions was constructed to initiate rapid free radical polymerization of acrylamide (AM) monomer at room temperature. In addition, Deep eutectic solvent (DES) can form a strong hydrogen bonding network within the molecules and between the molecules of the hydrogel, resulting in a hydrogel with good tensile properties (hydrogel elongation at break of 727.19 %, breaking strength of 84.09 kPa), and provides the hydrogel with high electrical conductivity, anti-dehydration, anti-freezing, and anti-bacterial properties. Meanwhile, the addition of lignin also improved the adhesion and UV resistance of the hydrogel. This hydrogel assembled into a flexible sensor can sense various small and large amplitude movements such as nodding, smiling, frowning, etc., and has a wide range of applications in flexible sensors.

Electronic skin based on natural biodegradable polymers for human motion monitoring

The wearability of the flexible electronic skin (e-skin) allows it to attach to the skin for human motion monitoring, which is essential for studying human motion and especially for assessing how well patients are recovering from rehabilitation therapy. However, the use of non-degradable synthetic materials in e-skin may raise skin safety concerns. Natural biodegradable polymers with advantages such as biodegradability, biocompatibility, sustainability, natural abundance, and low cost have the potential to be alternative materials for constructing flexible e-skin and applying them to human motion monitoring. This review summarizes the applications of natural biodegradable polymers in e-skin for human motion monitoring over the past three years, focusing on the discussion of cellulose, chitosan, silk fibroin, gelatin, and sodium alginate. Finally, we summarize the opportunities and challenges of e-skin based on natural biodegradable polymers. It is hoped that this review will provide insights for the future development of flexible e-skin in the field of human motion monitoring.

Highly conductive, rapid self-healing, and anti-freezing poly(3,4-ethylenedioxythiophene)/lignosulfonate-cationic guar gum ionogels for multifunctional sensors

Soft ionic conductors exhibit immense potential for applications in soft ionotronics, including ionic skin, human-machine interface, and soft luminescent device. Nevertheless, the majority of ionogel-based soft ionic conductors are plagued by issues such as freezing, evaporation, liquid leakage, and inadequate self-healing capabilities, thereby constraining their usability in complex environments. In this study, we present a novel strategy for fabricating conductive ionogels through the proportionally mixing cationic guar gum (CGG), water, 1-butyl-3-methylimidazolium chloride (BmimCl)/glycerol eutectic-based ionic liquid, and poly(3,4-ethylenedioxythiophene)/lignosulfonate (PEDOT/LS). The resultant benefits from strong hydrogen bonding and electrostatic interactions among its constituents, endowing it with an ultrafast self-healing capability (merely 30 s) while sustaining high electrical conductivity (~16.5 mS cm-1). Moreover, it demonstrates exceptional water retention (62 % over 10 days), wide temperature tolerance (-20 to 60 °C), and injectability. A wearable sensor fabricated from this ionogel displayed remarkable sensitivity (gauge factor = 17.75) and a rapid response to variations in strain, pressure, and temperature, coupled with both long-term stability and wide working temperature range. These attributes underscore its potential for applications in healthcare devices and flexible electronics.

Casein-based hydrogels: Advances and prospects

Casein-based hydrogels (Casein Gels) possess advantageous properties, including mechanical strength, stability, biocompatibility, and even adhesion, conductivity, sensing capabilities, as well as controlled-releasing behavior of drugs. These features are attributed to their gelation methods and functionalization with various polymers. Casein Gels is an important protein-based material in the food industry, in terms of dairy and functional foods, biological and medicine, in terms of carrier for bioactive and sensitive drugs, wound healing, and flexible sensors and wearable devices. Herein, this review aims to highlight the importance of the features mentioned above via a comprehensive investigation of Casein Gels through multiple directions and dimensional applications. Firstly, the composition, structure, and properties of casein, along with the gelation methods employed to create Casein Gels are elaborated, which serves as a foundation for further exploration. Then, the application progresses of Casein Gels in dairy products, functional foods, medicine, flexible sensors and wearable devices, are thoroughly discussed to provide insights into the diverse fields where Casein Gels have shown promise and utility. Lastly, the existing challenges and future research trends are highlighted from an interdisciplinary perspective. We present the latest research advances of Casein Gels and provide references for the development of multifunctional biomass-based hydrogels.

Lysine-Triggered Polymeric Hydrogels with Self-Adhesion, Stretchability, and Supportive Properties

Hydrogels, recognized for their flexibility and diverse characteristics, are extensively used in medical fields such as wearable sensors and soft robotics. However, many hydrogel sensors derived from biomaterials lack mechanical strength and fatigue resistance, emphasizing the necessity for enhanced formulations. In this work, we utilized acrylamide and polyacrylamide as the primary polymer network, incorporated chemically modified poly(ethylene glycol) (DF-PEG) as a physical crosslinker, and introduced varying amounts of methacrylated lysine (LysMA) to prepare a series of hydrogels. This formulation was labeled as poly(acrylamide)-DF-PEG-LysMA, abbreviated as pADLx, with x denoting the weight/volume percentage of LysMA. We observed that when the hydrogel contained 2.5% w/v LysMA (pADL2.5), compared to hydrogels without LysMA (pADL0), its stress increased by 642 ± 76%, strain increased by 1790 ± 95%, and toughness increased by 2037 ± 320%. Our speculation regarding the enhanced mechanical performance of the pADL2.5 hydrogel revolves around the synergistic effects arising from the co-polymerization of LysMA with acrylamide and the formation of multiple intermolecular hydrogen bonds within the network structures. Moreover, the acid, amine, and amide groups present in the LysMA molecules have proven to be instrumental contributors to the self-adhesion capability of the hydrogel. The validation of the pADL2.5 hydrogel's exceptional mechanical properties through rigorous tensile tests further underscores its suitability for use in strain sensors. The outstanding stretchability, adhesive strength, and fatigue resistance demonstrated by this hydrogel affirm its potential as a key component in the development of robust and reliable strain sensors that fulfill practical requirements.

Fast gelling, high performance MXene hydrogels for wearable sensors

Hydrogel-based functional materials had attracted great attention in the fields of artificial intelligence, soft robotics, and motion monitoring. However, the gelation of hydrogels induced by free radical polymerization typically required heating, light exposure, and other conditions, limiting their practical applications and development in real-life scenarios. In this study, a simple and direct method was proposed to achieve rapid gelation at room temperature by incorporating reductive MXene sheets in conjunction with metal ions into the chitosan network and inducing the formation of a polyacrylamide network in an extremely short time (10 s). This resulted in a dual-network MXene-crosslinked conductive hydrogel composite that exhibited exceptional stretchability (1350 %), remarkably low dissipated energy (0.40 kJ m-3 at 100 % strain), high sensitivity (GF = 2.86 at 300-500 % strain), and strong adhesion to various substrate surfaces. The study demonstrated potential applications in the reliable detection of various motions, including repetitive fine movements and large-scale human body motions. This work provided a feasible platform for developing integrated wearable health-monitoring electronic systems.

Cartilage-on-a-chip with magneto-mechanical transformation for osteoarthritis recruitment

Osteoarthritis (OA) is a prevalent joint disease primarily induced by overstrain, leading to disability and significantly impacting patients' quality of life. However, current OA studies lack an ideal in vitro model, which can recapitulate the high peripheral strain of the joint and precisely model the disease onset process. In this paper, we propose a novel cartilage-on-a-chip platform that incorporates a biohybrid hydrogel comprising Neodymium (NdFeB)/Poly-GelMA-HAMA remote magneto-control hydrogel film. This platform facilitates chondrocyte culture and stress loading, enabling the investigation of chondrocytes under various stress stimuli. The Neodymium (NdFeB)/Poly-GelMA-HAMA hydrogel film exhibits magneto-responsive shape-transition behavior, further dragging the chondrocytes cultured in hydrogels under magnetic stimulation. It was investigated that inflammation-related genes and proteins in chondrocytes are changed with mechanical stress stimulation in the cartilage-on-a-chip. Especially, MMP-13 and the proportion of collagen secretion are upregulated, showing a phenotype similar to that of real human osteoarthritis. Therefore, we believed that this cartilage-on-a-chip platform provides a desired in vitro model for osteoarthritis, which is of great significance in disease research and drug development.

Fabrication of flexible accelerated-wound-healing chitosan/dopamine-based bilayer hydrogels for strain sensors

Flexible conductive hydrogels have great potential for healthcare and human motion sensing. However, it is difficult to simultaneously achieve conductive hydrogel epidermal sensors with reliable adhesion capabilities and excellent sensing properties, as well as accelerated wound healing performance in wearable hydrogels. Here, an epidermal sensor with excellent adhesion (0.6 kPa) and tensile strain (218.0 %) properties was assembled from an easy-to-prepare bilayer antimicrobial hydrogel, which effectively accelerates wound healing, as well as for human motion sensing. The upper hydrogel layer was composed of PVA, which could effectively enhance the mechanical properties of the bilayer hydrogel. The lower hydrogel layer consisted of polyacrylamide (PAm) and chitosan-dopamine (CC-DA). PAm with good adhesion properties adhered effectively to the skin surface. CC-DA not only had adhesion properties, but also has good antibacterial effects. It inhibited the growth of bacteria, which assisted in wound healing and infection prevention. Therefore, the design of the bilayer hydrogel combined the mechanical enhancement of PVA with the adhesion properties and antimicrobial effect of PAm and CC-DA to provide better wound repair. In addition, the double-layer hydrogel with good electrical conductivity (1.65 S·m-1) could sensitively monitor the tiny electrophysiological signals emitted by the human body during exercise rehabilitation training.

Easy-to-Prepare Flexible Multifunctional Sensors Assembled with Anti-Swelling Hydrogels

Recent years have witnessed the development of flexible electronic materials. Flexible electronic devices based on hydrogels are promising but face the limitations of having no resistance to swelling and a lack of functional integration. Herein, we fabricated a hydrogel using a solvent replacement strategy and explored it as a flexible electronic material. This hydrogel was obtained by polymerizing 2-hydroxyethyl methacrylate (HEMA) in ethylene glycol and then immersing it in water. The synergistic effect of hydrogen bonding and hydrophobic interactions endows this hydrogel with anti-swelling properties in water, and it also exhibits enhanced mechanical properties and outstanding self-bonding properties. Moreover, the modulus of the hydrogel is tissue-adaptable. These properties allowed the hydrogel to be simply assembled with a liquid metal (LM) to create a series of structurally complex and functionally integrated flexible sensors. The hydrogel was used to assemble resistive and capacitive sensors to sense one-, two-, and three-dimensional strains and finger touches by employing specific structural designs. In addition, a multifunctional flexible sensor integrating strain sensing, temperature sensing, and conductance sensing was assembled via simple multilayer stacking to enable the simultaneous monitoring of underwater motion, water temperature, and water quality. This work demonstrates a simple strategy for assembling functionally integrated flexible electronics, which should open opportunities in next-generation electronic skins and hydrogel machines for various applications, especially underwater applications.

Preparation and properties of lignin-based dual network hydrogel and its application in sensing

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is now difficult to develop conductive hydrogels including excellent self-healing and mechanical properties, mainly due to their inherent trade-off between dynamic cross-linked healing and stable cross-linked mechanical strength. In this work, alkali lignin-Polyvinyl alcohol-polyacrylic acid double network conductive hydrogels with high mechanical strength and good self-healing properties were prepared. We formed the primary network structure by hydrogen bonding interaction between polyvinyl alcohol, alkali lignin and polyacrylic acid, and the secondary network structure by coordination interaction with polyacrylic acid through the addition of Fe3+. The added lignin acts as a dynamic linkage bridge in a porous network mediated by multiple ligand bonds, imparting superior mechanical properties to the hydrogels. The relationships between the alkali lignin and iron ion dosage and the comprehensive properties of hydrogels (adhesion, antibacterial, self-healing, electrical conductivity and mechanical properties) were studied in detail. On this basis, the hydrogels explored the role of lignin in the regulation of hydrogels properties and revealed the self-healing and conductive mechanism.

Tremella polysaccharide-based conductive hydrogel with anti-freezing and self-healing ability for motion monitoring and intelligent interaction

The application of conductive hydrogels in flexible wearable devices has garnered significant attention. In this study, a self-healing, anti-freezing, and fire-resistant hydrogel strain sensor is successfully synthesized by incorporating sustainable natural biological materials, viz. Tremella polysaccharide and silk fiber, into a polyvinyl alcohol matrix with borax cross-linking. The resulting hydrogel exhibits excellent transparency, thermoplasticity, and remarkable mechanical properties, including a notable elongation (1107.3 %) and high self-healing rate (91.11 %) within 5 min, attributed to the dynamic cross-linking effect of hydrogen bonds and borax. A strain sensor based on the prepared hydrogel sensor can be used to accurately monitor diverse human movements, while maintaining exceptional sensing stability and durability under repeated strain cycles. Additionally, a hydrogel touch component is designed that can successfully interact with intelligent electronic devices, encompassing functions like clicking, writing, and drawing. These inherent advantages make the prepared hydrogel a promising candidate for applications in human health monitoring and intelligent electronic device interaction.

Understanding Gel-Powers: Exploring Rheological Marvels of Acrylamide/Sodium Alginate Double-Network Hydrogels

This study investigates the rheological properties of dual-network hydrogels based on acrylamide and sodium alginate under large deformations. The concentration of calcium ions affects the nonlinear behavior, and all gel samples exhibit strain hardening, shear thickening, and shear densification. The paper focuses on systematic variation of the alginate concentration-which serves as second network building blocks-and the Ca²⁺-concentration-which shows how strongly they are connected. The precursor solutions show a typical viscoelastic solution behavior depending on alginate content and pH. The gels are highly elastic solids with only relatively small viscoelastic components, i.e., their creep and creep recovery behavior are indicative of the solid state after only a very short time while the linear viscoelastic phase angles are very small. The onset of the nonlinear regime decreases significantly when closing the second network (alginate) upon adding Ca²⁺, while at the same time the nonlinearity parameters (Q₀, I₃/I₁, S, T, e₃/e₁, and v₃/v₁) increase significantly. Further, the tensile properties are significantly improved by closing the alginate network by Ca²⁺ at intermediate concentrations.

Synthesis and Sensing Performance of Chitin Fiber/MoS2 Composites

In this study, chitin fibers (CFs) were combined with molybdenum sulfide (MoS₂) to develop high-performance sensors, and chitin carbon materials were innovatively introduced into the application of gas sensing. MoS₂/CFs composites were synthesized via a one-step hydrothermal method. The surface properties of the composites were greatly improved, and the fire resistance effect was remarkable compared with that of the chitin monomer. In the gas-sensitive performance test, the overall performance of the MoS₂/CFs composite was more than three times better than that of the MoS₂ monomer and showed excellent long-term stability, with less than 10% performance degradation in three months. Extending to the field of strain sensing, MoS₂/CFs composites can realize real-time signal conversion in tensile and motion performance tests, which can help inspectors make analytical judgments in response to the analysis results. The extensive application of sensing materials in more fields is expected to be further developed. Based on the recycling of waste chitin textile materials, this paper expands the potential applications of chitin materials in the fields of gas monitoring, biomedicine, behavioral discrimination and intelligent monitoring.

In situ rapid synthesis of hydrogels based on a redox initiator and persistent free radicals

The development of fast and economical hydrogel manufacturing methods is crucial for expanding the application of hydrogels. However, the commonly used rapid initiation system is not conducive to the performance of hydrogels. Therefore, the research focuses on how to improve the preparation speed of hydrogels and avoid affecting the properties of hydrogels. Herein, a redox initiation system with nanoparticle-stabilized persistent free radicals was introduced to rapidly synthesize high-performance hydrogels at room temperature. A redox initiator composed of vitamin C and ammonium persulfate rapidly provides hydroxyl radicals at room temperature. Simultaneously, three-dimensional nanoparticles can stabilize free radicals and prolong their lifetime, thereby increasing the free radical concentration and accelerating the polymerization rate. And casein enabled the hydrogel to achieve impressive mechanical properties, adhesion, and electrical conductivity. This method greatly facilitates the rapid and economical synthesis of high-performance hydrogels and presents broad application prospects in the field of flexible electronics.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Highly Stretchable and Biocompatible Wrinkled Nanoclay-Composite Hydrogel With Enhanced Sensing Capability for Precise Detection of Myocardial Infarction

It is challenging to balance high biocompability with good mechanical-electrical sensing performance, especially when triggering inflammatory stress response after in vivo implantation. Herein, a bioinspired wrinkle-reinforced adaptive nanoclay-interlocked soft strain-sensor based on a highly stretchable and elastic ionic-conductive hydrogel is reported. This novel nanoclay-composite hydrogel exhibits excellent tensile properties and high sensing capacity with steady and reliable sensing performance due to the structural-mechanical-electrical integrity of the nanoclay crosslinked and nano-reinforced interpenetrating network. The incorporation of amphiphilic ions provides the hydrogel with significant protein resistance, reducing its non-specific adsorption to proteins upon implantation, improving its biosafety as an implanted device, and maintaining the authenticity of the sensing results. Based on the revealed sensing enhanced mechanism based on hierarchical ordered structures as a proof-of-concept application, this hydrogel sensor is demonstrated to be able to accurately localize the region where myocardial infarction occurs and may become a novel strategy for real-time monitoring of pathological changes in heart disease.

Construction of Carboxymethyl Chitosan Hydrogel with Multiple Cross-linking Networks for Electronic Devices at Low Temperature

On the basis of the original hydrogen bonding interaction and physical entanglement, covalent cross-linking and ionic cross-linking were additionally introduced to construct a carboxymethyl chitosan/allyl glycidyl ether conductive hydrogel (CCH) through a one pot method by a graft reaction, an addition reaction, and simple immersion, successively. The multiple cross-linking networks significantly increased the strength of CCHs and endowed them with ionic conductivity and an antifreezing property at -40 °C, which showed stable, durable, and reversible sensitivity to finger bending activity at subzero temperature. The CCHs could even be assembled into a triboelectric nanogenerator (TENG) to provide electric energy, which demonstrated stability against temperature variation, multiple drawing, long-term storage, or large quantities of contact-separation motion cycles. CCH-TENG can also be used as a tactile sensor within the pressure range from 0.4 kPa to higher than 8000 kPa. This work provided a simple route to fabricate antifreezing conductive hydrogels based on carboxymethyl chitosan and to find potential applications in soft sensor devices under a low temperature environment.

Fabrication of lignin reinforced hybrid hydrogels with antimicrobial and self-adhesion for strain sensors

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is still challenging to prepare hydrogels with both high toughness and self-healing ability. In this study, lignin-based β-CD-PVA (LCP) self-healing conductive hydrogels with high tensile properties were prepared by one-step method using alkali lignin as a plasticizer. Compared with PVA hydrogel, the maximum storage modulus and elongation were increased by 2.5 and 20.0 times, respectively. Uniform distribution of lignin can increase the fluidity and distance of polymer molecular chains, thus improving the viscoelastic and tensile properties of the LCP self-healing hydrogel. LCP hydrogels can maintain self-healing ability in both high (45 °C) and low temperature (0 °C) environments, and the self-healing ability is not affected by pH. Moreover, it also has good conductivity, anti-bacterial, thermostability, and anti-UV property, which has a good application prospect in the field of 3D printing and wearable electronic devices, which expands the efficient utilization of lignin in biorefinery.

Temperature-Responsive Ionic Conductive Hydrogel for Strain and Temperature Sensors

Flexible wearable devices have achieved remarkable applications in health monitoring because of the advantages of multisignal collecting and real-time wireless transmission of information. However, the integration of bulky sensing elements and rigid metal circuit components in traditional wearable devices may lead to a mechanical and signal-conducting mismatch between wearable devices and biological tissues, thus restricting their wide applications in the human body. The excellent mechanical properties, conductivity, and high tissue resemblance of conductive hydrogel contribute to its application in flexible electronic sensors to monitor human health. In this work, a dual-network, temperature-responsive ionic conductive hydrogel with excellent stretchability, fast temperature responsiveness, and good conductivity was developed by introducing a polyvinylpyrrolidone (PVP)/ tannic acid (TA)/ Fe3+ cross-linked network into the N,N-methylene diacrylamide (MBAA) cross-linked poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AM)) network. Furthermore, the introduction of the PVP/TA/Fe3+ cross-linked network endowed the hydrogel with excellent stretchability and conductivity. By adjusting the molar ratio of TA and Fe3+ to 3:5, a hydrogel with a maximal stretching ratio of 720% and sensitive strain response (GF = 3.61) was achieved, showing a promising application in wearable strain sensors to monitor both large and fine human motions. Moreover, by introducing PNIPAAm with a lower critical solution temperature (LCST), the hydrogel may be used to monitor the environmental temperature through the temperature-conductivity responsiveness, which can be applied as a wearable temperature sensor to detect fever or tissue hyperthermia in the human body.

A self-healing, recyclable and conductive gelatin/nanofibrillated cellulose/Fe3+ hydrogel based on multi-dynamic interactions for a multifunctional strain sensor

Conductive hydrogels have emerged as promising material candidates for multifunctional strain sensors, attributed to their similarity to biological tissues, good wearability, and high accuracy of information acquisition. However, it is difficult to simultaneously manufacture conductive hydrogel-based multifunctional strain sensors with the synergistic properties of reliable healability for long-term usage and environmental degradability/recyclability for decreasing the electronic waste. This work reports a facile strategy to engineer a self-healing, recyclable and conductive strain sensor by virtue of molecular-level multi-dynamic interactions (MMDIs) including Schiff base complexes, hydrogen bonds, and coordination bonds, which were fabricated using a dialdehyde TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanofibrillated cellulose (DATNFC) pre-reinforced gelatin nanocomposite hydrogel (gelatin/DATNFC hydrogel, GDH) followed by dipping in an Fe³⁺ aqueous solution. The MMDI strategy allows synchronous regulation of both bulk and interfacial interactions to obtain exciting properties that outperform those of conventional hydrogels, including extraordinary compressive stress (1310 kPa), intriguing self-healing abilities, and remarkable electrical conductivity. With these outstanding merits, the as-prepared gelatin/DATNFC/Fe³⁺ hydrogel (GDIH) is developed to be a multifunctional strain sensor with appealing strain sensitivity (GF = 2.24 under 6% strain) and compressive sensitivity (S = 1.14 kPa^-1 under 15 kPa), which can be utilized to manufacture electronic skin and accurately discern subtle bodily motions, handwriting and personal signatures. Notably, this GDIH-based sensor also exhibited reliable self-healing properties for long-term usage, environmental degradability and complete recyclability for decreasing the electronic waste. In consideration of the extremely facile preparation process, biocompatibility, satisfactory functionalities, remarkable self-healing properties and recyclability, the emergence of the GDIH-based sensor is believed to propose a new strategy for the development of sustainable-multifunctional strain sensors and healthcare monitoring.

β-Cyclodextrin-Based Ultrahigh Stretchable, Flexible, Electro- and Pressure-Responsive, Adhesive, Transparent Hydrogel as Motion Sensor

In the present work, a multiple-stimuli-responsive hydrogel has been synthesized via polymerization of acrylamide (AAm) and N-hydroxy methyl acrylamide (HMAm) on β-cyclodextrin (β-CD). The synthesized hydrogel β-CD-g-(pAAm/pHMAm) exhibited various striking features like ultrahigh stretchability (>6000%), flexibility, stab resistivity, self-recoverability, electroresponsiveness, pressure-responsiveness, adhesiveness, and high transparency (>90%). Besides, the hydrogel has demonstrated enhanced biocompatibility, UV resistance, and thermoresponsive shape memory behaviors. On the basis of these attractive characteristics of the hydrogel, a flexible pressure sensor for the real-time monitoring of human motion with superior biocompatibility and transparency was fabricated. Moreover, due to the nanofibrillar surface morphology of the β-CD-g-(pAAm/pHMAm) hydrogel, the sensor based on the gel exhibited high sensitivity (0.053 kPa^-1 for 0-3.3 kPa). The flexible sensor demonstrates very fast response time (130 ms-210 ms) with adequate stability (5000 cycles). Interestingly, the sensor can rapidly sense both robust (index finger and wrist) motions as well as tiny (swallowing and phonation) physiological actions. In addition, this adhesive hydrogel patch also acts as a potential carrier for the sustained topical release of (∼80.8% in 48 h) the antibiotic drug gentamicin sulfate.

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding

Adjustable interfacial adhesion is of great significance in smart-hydrogel-related engineering fields. This study presents an electroadhesion strategy for universal and ultrastrong hydrogel bonding with electrically programmable strength. An ionic hydrogel containing lithium ions is designed to achieve hydrated-ion-diffusion-mediated interfacial adhesion, where external electric fields are employed to precisely control spatiotemporal dynamics of the ion diffusion across ionic adhesion region (IAR). The hydrogel can realize a universal, ultrastrong, efficient, tough, reversible, and environmentally tolerant electroadhesion to diverse hydrogels, whose peak adhesion strength and interfacial adhesion toughness are as high as 1.2 MPa and 3750 J m^-2 , respectively. With a mechanoelectric coupling model, the dominant role of the hydrated ions in IAR played in the interfacial electroadhesion is further quantitatively revealed. The proposed strategy opens a door for developing high-performance adhesion hydrogels with electrically programmable functions, which are indispensable for various emerging fields like flexible electronics and soft robotics.

Multifunctional bacterial cellulose-based organohydrogels with long-term environmental stability

Sensitive strain sensors have attracted more attention due to their applications in health monitoring and human-computer interaction. However, the problems existing in conventional hydrogels, such as inherent brittleness, freezing at low temperature, low toughness, and water evaporation, have greatly hindered the practical applications. In order to solve the above problems, herein, we designed dual network multifunctionality organohydrogels using polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) covalent cross-linking polymer as the first network, the bacterial celluloses (BCs) and calcium chloride by ligand binding as the second network. The prepared organohydrogels showed good conductivity and sensitivity over a wide temperature range (-20 ∼ 40 ℃), and maintained long-term stability (>15 days) in the air. In addition, the dynamic combination of BCs-Ca^{2 +} and hydrogen bonds in the binary system further endows the organohydrogels with excellent tensile strength (≈1.0 MPa), tensile strain (≈1300%), toughness (≈6.2 MJ m^-3), conductivity (3.4 S m^-1), gauge factor (≈1.24), adhesion (≈0.3 MPa), and self-healing properties (self-healing tensile strain to 632%). Therefore, this organohydrogel has potential candidates for flexible electronic skin, motion monitoring, and soft robotics.

Relationship between Structure and Rheology of Hydrogels for Various Applications

Hydrogels have gained a lot of attention with their widespread use in different industrial applications. The versatility in the synthesis and the nature of the precursor reactants allow for a varying range of hydrogels with different mechanical and rheological properties. Understanding of the rheological behavior and the relationship between the chemical structure and the resulting properties is crucial, and is the focus of this review. Specifically, we include detailed discussion on the correlation between the rheological characteristics of hydrogels and their possible applications. Different rheological tests such as time, temperature and frequency sweep, among others, are described and the results of those tests are reported. The most prevalent applications of hydrogels are also discussed.