Stretchable, compressible, and conductive hydrogel for sensitive wearable soft sensors

An adhesive, antibacterial, conductive zwitterionic cellulose nanofibers-containing hydrogel for flexible strain sensors and super-capacitors

Cellulose-based hydrogels are promising materials for constructing flexible supercapacitors and energy storage devices due to their environmental sustainability and resource renewability. However, preparing cellulose-based hydrogel electrolytes with super flexibility, high conductivity, and high specific capacitance for practical applications is still challenging. Herein, an adhesive, antibacterial, conductive zwitterionic cellulose nanofibers-reinforced poly(sulfobetaine methacrylate-acrylic acid-acrylamide (ZCNF/PSAA) composite hydrogel was fabricated by a blue light-triggered free radical polymerization of 2-methacryloyloxy ethyl dimethyl-3-sulfopropyl ammonium hydroxide (SBMA), acrylic acid (AA), acryl amide (AM), dopamine methacrylamide (DMA) and zwitterionic cellulose nanofibers (ZCNF). The prepared hydrogel exhibited excellent mechanical properties with tensile strength of 0.17 MPa, compressive strength of 0.87 MPa, and shear strength of 1.25 MPa, respectively. The zwitterionic groups significantly enhanced the hydrogel's conductivity (5.8 S/m). Moreover, the hydrogel with electrically sensitive perception of external strain (GF = 2.5), can withstand large bending and compression deformations and can be used as a motion sensor to monitor human movements such as arm and finger bending, pressing, and subtle fist clenching. The resulting hydrogel presented excellent antibacterial activity against Escherichia coli and Staphylococcus aureus. As the hydrogel was applied as electrolyte, the developed super-capacitor exhibited a desirable specific capacitance of 404.5 mF·cm-2, with a maximum energy density of 53.93 Wh·kg-1 and capacitance retention of 80.3 % after 2000 consecutive charge-discharge cycles. The ZCNF/PSAA hydrogel has great potential for applications in flexible strain sensors and energy storage devices.

Utilizing carboxymethyl cellulose sodium/sodium lignosulfonate hydrogel to build efficient electrolyte/electrode interfaces for multifunctional high-performance supercapacitors

Although hydrogel electrolytes have attracted much attention in flexible supercapacitors, the interfacial contact between the electrolyte and electrode is a key factor restricting the development of supercapacitors. In this work, we constructed a hydrogel supercapacitor by endowing the hydrogel electrolyte with excellent adhesion properties and adopting an interfacial integration strategy, which effectively improves the interfacial contact problem. A multifunctional PC4L hydrogel was successfully prepared with excellent tensile properties (1976 %) and self-healing ability through the semi-interpenetrating network formed by carboxymethyl cellulose sodium and polyacrylamide chains, as well as multiple physical interactions. Meanwhile, the catechol groups in the hydrogel conferred strong adhesion. Subsequently, a flexible integrated supercapacitor was successfully prepared through in situ polymerization of polypyrrole/ammonium aurintricarboxylate/sodium lignosulfonate composite electrode materials on the PC4L hydrogel. The supercapacitor exhibited a high specific capacitance of 624.36 mF/cm2 at 1 mA/cm2. Due to its strong adhesion, the supercapacitor demonstrated excellent stability under severe deformations, without delamination or displacement between the electrolyte and electrode, thereby maintaining excellent capacitance retention. The supercapacitor exhibits excellent self-healing performance with 81.4 % capacitance retention even after 5 cut-and-heal cycles. This flexible supercapacitor with stable electrochemical properties shows promising applications in wearable electronic devices.

Lignosulfonate-enhanced dispersion and compatibility of liquid metal nanodroplets in PVA hydrogel for improved self-recovery and fatigue resistance in wearable sensors

Stretchable and resilient conductive hydrogels, incorporating flowable liquid metals (LM) into polyvinyl alcohol (PVA), have emerged as promising materials for wearable sensors due to their exceptional mechanical properties and sustainability. However, the fluidity and compatibility of LM with the hydrogel matrix limit the construction and performance of LM/PVA conductive hydrogels. This study aimed to develop a flexible, high-performance hydrogel for advanced wearable sensors by introducing LM nanoparticles encapsulated in sodium lignosulfonate (LS-LM) into the PVA matrix. The renewable natural macromolecule LS, rich in functional groups, enhanced the compatibility between LM and the PVA matrix. Moreover, LS formed a stable shell around the LM droplets, preventing rupture and leakage of LM, ensuring uniform dispersion within the hydrogel and significantly improving its durability by preventing phase separation. The optimized conductive lignosulfonate-liquid metal/polyvinyl alcohol hydrogel (LS-LM/PVA) exhibited a tensile stress of 1.60 MPa, a compressive strength of 0.53 MPa under 70 % strain, and electrical conductivity (4.87 S m-1). The hydrogel-based sensor demonstrated excellent sensitivity (GF = 2.40) and outstanding fatigue resistance (over 500 cycles). A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impacts of LS-LM/PVA hydrogel production. The composite hydrogel-based sensor shows significant promise for advancing human motion tracking and information recognition.

Anti-Swelling Aramid-Nanofiber-Reinforced Zwitterionic Polymer Hydrogel for Strain Sensors

Zwitterionic polymer hydrogels have great application prospects in wearable electronic devices due to their antifouling and excellent biocompatibility. However, its strong hydrophilicity often leads to easy swelling and poor mechanical properties. In this study, aramid nanofiber (ANF)-reinforced zwitterionic ion hydrogels were synthesized by the one-step free radical polymerization of N-acryloyl glycinamide (NAGA), N-[Tris (hydroxymethyl) methyl] acrylamide (THMA) and sulfobetaine methacrylate (SBMA) monomers in the presence of ANFs. A large number of hydrogen bonds were formed between the amide groups of the ANFs and the amide groups of the NAGA units/the hydroxyl groups of the THMA units/the sulfonic groups of the SBMA units, which improved the internal interface force of the hydrogel. The obtained ANF-reinforced hydrogel had an anti-swelling property, and its swelling ratio and tensile strength were 25% and 170% of those of the hydrogel without the addition of ANFs. By introducing lithium chloride as an electrolyte to improve its ion conductivity and subsequently assembling it into strain sensors, it exhibited a high sensitivity (GF = 1.12), short response and recovery times (100 ms and 150 ms), and excellent cycling stability. This work provides a feasible strategy for anti-swelling wearable strain sensors.

Self-healing, deformable and safe integrated supercapacitor enabled by synergistic effect of multiple physical interactions in gel polymer electrolyte with dual-role Co2

With the rapid development of wearable electronic devices, flexible supercapacitors have gained strong interest. However, traditional sandwich supercapacitors have weak interfacial binding, resulting in high interface resistance and poor deformability. Herein, a self-healing integrated supercapacitor based on a polyacrylic acid-polyisodecyl methacrylate-CoSO4 gel polymer electrolyte (GPE) was developed. By incorporating ion coordination into a hydrophobic association network, a double network structure was formed, endowing the GPE with remarkable mechanical properties and self-healing abilities. Specifically, Co2+ ions functioned both as charge carrier and crosslinker, simultaneously enhancing the electrochemical (2.87 S/m) and mechanical (0.262 MPa) properties of the GPE. In situ growth of polyaniline electrode material on the GPE surface resulted in an integrated supercapacitor with a continuous morphology at the electrode/electrolyte interface, minimizing interface resistance and improving electrochemical performance. The supercapacitor exhibits high specific capacitance, exceptional cyclic stability, superior deformability and security due to the unique integrated structure. Furthermore, it demonstrates remarkable electrochemical and self-healing properties even at quite low temperature. Overall, this work offers a promising approach for reliable self-healing energy storage devices with high performance and adaptability to complex usage conditions.

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

Engineering Hydrogels with Enhanced Adhesive Strength Through Optimization of Poly(Ethylene Glycol) Molecular Weight

Hydrogels are extensively utilized in biomedical fields because of their remarkable properties, including biocompatibility, high water content, flexibility, and elasticity. However, despite substantial progress in hydrogel research, creating a hydrogel adhesive that integrates high stretchability, fatigue resistance, and reversible adhesion continues to pose significant challenges. In this study, we aimed to address these challenges by preparing hydrogels using a combination of acrylic acid, acrylamide, carboxymethylcellulose methacrylate, thiol-functionalized polyhedral oligomeric silsesquioxane, and poly(ethylene glycol) dimethacrylate (PEGDM). By systematically varying the molecular weight of PEG, we were able to precisely adjust the mechanical and adhesive properties of the hydrogels. Our research revealed that a PEG molecular weight of 2000 (resulting in P1 hydrogel) provided a notable adhesive strength of 717.2 kPa on glass surfaces. This performance is particularly impressive given the challenges associated with achieving high adhesive strength while maintaining other desirable hydrogel properties. Beyond its strong adhesive capabilities, the P1 hydrogel also demonstrated exceptional stretchability, support, and fatigue resistance. These characteristics are crucial for applications where the adhesive needs to endure repeated stress and deformation without losing effectiveness. The successful development of P1 hydrogel underscores its potential as a multifunctional adhesive material with a broad range of applications. The ability to tailor the properties of hydrogels through molecular weight adjustments offers a promising approach to creating advanced adhesive solutions that meet the demanding requirements of modern biomedical and industrial applications.

Antifreeze proteins and surface-modified cellulose nanocrystals for designing anti-freezing conductive hydrogel sensors

Antifreeze proteins (AFPs) are a type of protein capable of inhibiting ice crystal growth, lowering the freezing point, and protecting organisms from cold-induced damage. In this study, cellulose nanocrystals (CNCs) are chemically modified to enhance the hydrogel's performance. The synergistic effect with AFPs further regulates its mechanical properties, antifreeze performance, and high sensing sensitivity. First, based on the principles of mussel-inspired chemistry, dopamine (DA) was introduced to increase adhesion and chemical crosslinking points, successfully modifying zwitterionic proline (ZP) onto the CNCs surface to obtain cellulose nanocrystals@polydopamine@zwitterionic proline (CNCs@PDA@ZP). Subsequently, CNCs@PDA@ZP and AFPs were incorporated into a multi-component network of polyacrylic acid/guar gum (PAA/GG) and FeCl3 to construct an antifreeze, self-healing nanocomposite hydrogel-based flexible sensor. Through the synergistic effect of its components, along with AFPs inhibiting ice crystal growth by binding to H2O, the hydrogel achieves an 81.7 % self-healing rate in 1.3 h at -20 °C and a fracture stress of 3.0 MPa. Additionally, the hydrogel's high sensitivity (GF = 4.0) enhances its suitability for motion detection in flexible sensors. This work provides a method for modifying nanomaterials and contributes an approach for antifreeze, self-healing nanocomposite hydrogels.

Bioinspired Conductivity-Enhanced, Self-Healing, and Renewable Silk Fibroin Hydrogel for Wearable Sensors with High Sensitivity

The development of silk fibroin-based hydrogels with excellent biocompatibility, aqueous processability, and facile controllability in structure is indeed an exciting advancement for biological research and strain sensor applications. However, silk fibroin-based hydrogel strain sensors that combine high conductivity, high stretchability, reusability, and high selectivity are still desired. Herein, we report a simple method for preparing double-network hydrogels including silk fibroin and poly(acrylic acid) sodium-polyacrylate (PAA-PAAS) networks. The conformation and aggregate of silk fibroin could be facilely tuned by both ions and pH resulting from the PAA-PAAS network. The optimized hydrogel exhibits intriguing properties, such as high conductivity (3.67 S/m) and transparency, high stretchability (1186%) with a tensile strength of 110 kPa, good adhesion properties, reversible compression, self-healing, and high sensitivity (GF = 10.71). This hydrogel strain sensor can detect large-scale and small human movements in real time, such as limb movements, heartbeats, and pulse. Additionally, its ability to adsorb water and recover effectiveness after losing water from air with 90% humidity along with the capability for low-temperature motion detection facilitated by ethylene glycol further enhance its practical utility. This work offers a novel and simple approach to design flexible bionic strain sensors.

Highly Stretchable Conductive Hydrogel-Based Flexible Triboelectric Nanogenerators for Ultrasensitive Tactile Sensing

Wearable electronic devices have shown great application prospects in the fields of tactile sensing, electronic skin, and soft robots. However, the existing wearable electronic devices face limitations such as power supply challenges, lack of portability, and discomfort, which restrict their applications. The invention of triboelectric nanogenerators (TENGs) with dual functions of energy harvesting and sensing provides an innovative solution to address these issues. This study prepared a highly stretchable conductive hydrogel using doped conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a strain sensor, demonstrating high sensitivity (GF = 4.31), an ultra-wide sensing range (0-1690%), ultra-fast response speed (0.15 s), excellent durability, and repeatability. A high-performance triboelectric nanogenerator was constructed using the hydrogel as an electrode, achieving an output performance of up to 192 V. Furthermore, the TENG fixed in the hands, wrists, legs, and feet of the human body can be used as a wearable electronic device to monitor human motion, which is conducive to promoting the development of triboelectric nanogenerators based on conductive hydrogels in strain sensors and self-powered wearable devices.

Design of self-healing nanocomposite hydrogels and the application to the detection of human exercise and ascorbic acid in sweat

Hydrogel sensors have broad application prospects in human motion monitoring and sweat composition detection. However, hydrogel-based sensors are faced with challenges such as low accuracy and poor mechanical properties of analytes detection. Based on mussel-inspired chemistry, we synthesized mesoporous silica@polydopamine-Au (MPS@PDA-Au) nanomaterials and designed a self-healing nanocomposite hydrogel to monitor human movement and ascorbic acid detection in sweat. Mesoporous silica (MPS) possess orderly mesoporous structure. Dopamine (DA) polymerized on the surface of MPS to generate polydopamine (PDA), forming the composite material MPS@PDA-Au. This composite was then embedded into polyvinyl alcohol (PVA) hydrogels through a simple freeze-thaw cycle process. The hydrogels have achieved excellent deformable ability (508.6%), self-healing property (90.5%) and mechanical strength (2.9 MPa). The PVA/MPS@PDA-Au hydrogel sensors had the characteristics of fast response time (123.2 ms), wide strain sensing range (0-500%), excellent fatigue resistance and stability in human detection. The detection range of ascorbic acid (AA) in sweat was wide (8.0 μmol/L-100.0 μmol/L) and the detection limit was low (3.3 μmol/L). Therefore, these hydrogel sensors have outstanding application prospects in human motion monitoring and sweat composition detection.

Epidermal Sensors Constructed by a Stabilized Nanosilver Hydrogel with Self-Healing, Antimicrobial, and Temperature-Responsive Properties

The development of conductive hydrogels has garnered significant attention in the field of wearable devices and smart sensors. However, the fabrication of hydrogels that possess both multifunctionality and structural stability remains a challenging task. In this study, a novel hydrogel, PAgHCB, was synthesized using a mild method and exhibited outstanding characteristics such as electrical conductivity, self-healing capability, antimicrobial activity, dimensional stability, and temperature sensitivity. The exceptional mechanical performance (∼120 kPa at a strain of 450%) of PAgHCB is attributed to the incorporation of hydroxypropylmethylcellulose (HPMC) and the mechanical reinforcement of the gel network by carboxylated carbon nanotubes (CNT-COOH). The borate bonds between or within poly(vinyl alcohol) (PVA) chains confer self-healing capabilities upon PAgHCB, with a healing efficiency of 74.1%. The in situ reduction of silver nanoparticles through ultraviolet irradiation imparts antimicrobial characteristics to the hydrogel [against Escherichia coli, zone of inhibition (ZOI) = 3.7 mm; against Staphylococcus aureus, ZOI = 6.3 mm]. The linear temperature responsiveness of the PAgHCB hydrogel (R = -3.99T + 608.84 and COD = 0.9988) arises from the migration of silver ions within the gel matrix and the dissociation of borate bonds. Furthermore, PAgHCB was seamlessly integrated into sensors designed for monitoring human motion. The gel-based sensors exhibited three distinct sensing strain ranges corresponding to three different gauge factors (GF1 = 2.976, GF2 = 1.063, and GF3 = 2.97). Notably, PAgHCB gel sensors demonstrated the capability to detect electrical signals generated by finger and wrist joint movements and even discerned signals arising from subtle deformations induced by activities such as speaking. Additionally, the PAgHCB gel was utilized as a pressure sensor to detect external pressures applied to the skin (from 0.373 to 15.776 kPa). This work expands the avenues for designing and synthesizing multifunctional conductive hydrogels, promoting the application of hydrogel sensors with comfortable wear and high sensitivity.

Fabricating a SFMA/BAChol/PAA/ZnCl2 Hydrogel with Excellent Versatile Comprehensive Properties and Stable Sensitive Freezing-Tolerant Conductivity for Wearable Sensors

Flexible wearable sensors have obtained tremendous interest in various fields and conductive hydrogels are a promising candidate. Nevertheless, the insufficient mechanical properties, the low electrical conductivity and sensitivity, and the limited functional properties prevent the development of hydrogels as wearable sensors. In this study, an SFMA/BAChol/PAA/ZnCl2 hydrogel was fabricated with high mechanical strength and versatile comprehensive properties. Specifically, the obtained hydrogel displayed excellent adhesion and mechanical stability, cryophylactic ability, stable sensitive freezing-tolerant conductivity, and feasible electrical conduction under a wide temperature range, demonstrating the high application potential as a flexible wearable sensor for movement behavior surveillance, even under harsh environments. Furthermore, the mechanical strength of the hydrogel could easily be regulated by varying the copolymer content. The molecular mechanisms of the hydrogel formation and the reversible adhesion during the wet-dry transition were proposed. The non-covalent interactions, including the electrostatic interaction, hydrogen bond interaction and hydrophobic association, and coordination interaction, were dynamically presented in the hydrogel network and hence supported the versatile comprehensive properties of the hydrogel. This study provides a strategy for designing novel hydrogels to promote the development of flexible sensors with stable sensitive freezing-tolerant conductivity.

An adhesive, highly stretchable and low-hysteresis alginate-based conductive hydrogel strain sensing system for motion capture

A strain sensor stands as an indispensable tool for capturing intricate motions in various applications, ranging from human motion monitoring to electronic skin and soft robotics. However, existing strain sensors still face difficulties in simultaneously achieving superior sensing performance sufficing for practical applications like high stretchability and low hysteresis, as well as seamless device fabrication like desirable interfacial adhesion and system-level integration. Herein, we develop a highly stretchable and low-hysteresis strain sensor with adhesive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polyacrylamide (PAAm)-sodium alginate (SA) composite hydrogel, allowing the successful construction of a wireless motion capture sensing system that can provide precise data collection within a large deformation range. The resultant composite hydrogel displays favorable interfacial adhesion and robust mechanical stability, and the fabricated strain sensor demonstrates a wide working strain range (up to 500%) with high sensitivity (gauge factor = 11) and ultra-low hysteresis (1.52%), outperforming previous PEDOT-based hydrogel strain sensors. Enabled by the intriguing material properties and high sensing performance, we further demonstrate the fabrication and integration of a wireless motion capture sensing system for diverse applications like human motion monitoring, gesture recognition, and interactive communication.

Attapulgite-Reinforced Cellulose Hydrogels with High Conductivity and Antifreezing Property for Flexible Sensors

Ionic conductive cellulose hydrogels are some of the most promising candidates for flexible sensors. However, it is difficult to simultaneously prepare cellulose hydrogels with high mechanical strength, good ionic conductivity, and antifreeze performance. In this work, a natural clay (attapulgite)-reinforced cellulose hydrogel was fabricated. Through a one-pot method, cellulose and attapulgite were dispersed in a concentrated ZnCl2 solution. The obtained hydrogel exhibited a dual network of hydrogen bonds and Zn2+-induced ionic interactions. Attapulgite serves as an inorganic filler that can regulate the hydrogen-bonding density among cellulose molecules and provides abundant channels for fast ion transport. By optimizing the attapulgite loading, a mechanically strong (compressive strength up to 1.10 MPa), tough (fracture energy up to 0.36 MJ m-3), highly ionic conductive (4.15 S m-1), and freezing-tolerant hydrogel was prepared. These hydrogels can be used for sensitive and stable human motion sensing, demonstrating their great potential for healthcare applications.

From lab to wearables: Innovations in multifunctional hydrogel chemistry for next-generation bioelectronic devices

Functional hydrogels have emerged as foundational materials in diagnostics, therapy, and wearable devices, owing to their high stretchability, flexibility, sensing, and outstanding biocompatibility. Their significance stems from their resemblance to biological tissue and their exceptional versatility in electrical, mechanical, and biofunctional engineering, positioning themselves as a bridge between living organisms and electronic systems, paving the way for the development of highly compatible, efficient, and stable interfaces. These multifaceted capability revolutionizes the essence of hydrogel-based wearable devices, distinguishing them from conventional biomedical devices in real-world practical applications. In this comprehensive review, we first discuss the fundamental chemistry of hydrogels, elucidating their distinct properties and functionalities. Subsequently, we examine the applications of these bioelectronics within the human body, unveiling their transformative potential in diagnostics, therapy, and human-machine interfaces (HMI) in real wearable bioelectronics. This exploration serves as a scientific compass for researchers navigating the interdisciplinary landscape of chemistry, materials science, and bioelectronics.

Electronic Skin for Health Monitoring Systems: Properties, Functions, and Applications

Electronic skin (e-skin), a skin-like wearable electronic device, holds great promise in the fields of telemedicine and personalized healthcare because of its good flexibility, biocompatibility, skin conformability, and sensing performance. E-skin can monitor various health indicators of the human body in real time and over the long term, including physical indicators (exercise, respiration, blood pressure, etc.) and chemical indicators (saliva, sweat, urine, etc.). In recent years, the development of various materials, analysis, and manufacturing technologies has promoted significant development of e-skin, laying the foundation for the application of next-generation wearable medical technologies and devices. Herein, the properties required for e-skin health monitoring devices to achieve long-term and precise monitoring and summarize several detectable indicators in the health monitoring field are discussed. Subsequently, the applications of integrated e-skin health monitoring systems are reviewed. Finally, current challenges and future development directions in this field are discussed. This review is expected to generate great interest and inspiration for the development and improvement of e-skin and health monitoring systems.

Ionic Conductive Cellulose-Based Hydrogels with Superior Long-Lasting Moisture and Antifreezing Features for Flexible Strain Sensor Applications

Nowadays, wearable devices derived from flexible conductive hydrogels have attracted enormous attention. Nevertheless, the utilization of conductive hydrogels in practical applications under extreme conditions remains a significant challenge. Herein, a series of inorganic salt-ion-enhanced conductive hydrogels (HPE-LiCl) consisting of hydroxyethyl cellulose, hydroxyethyl acrylate, lithium chloride, and ethylene glycol/water binary solvent were fabricated via a facile one-pot method. Apart from outstanding self-adhesion, high stretchability, and remarkable fatigue resistance, the HPE-LiCl hydrogels possessed especially excellent antifreezing and long-lasting moisture performances, which could maintain satisfactory flexibility and electric conductivity over extended periods of time, even in challenging conditions such as extremely low temperatures (as low as -40 °C) and high temperatures (as high as 80 °C). Consequently, the HPE-LiCl-based sensor could timely and accurately monitor various human motion signals even in adverse environments and after long-term storage. Hence, this work presents a facile strategy for the design of long-term reliable hydrogels as smart strain sensors, especially used in extreme environments.

Recent advances in fabricating injectable hydrogels via tunable molecular interactions for bio-applications

Hydrogels with three-dimensional structures have been widely applied in various applications because of their tunable structures, which can be easily tailored with desired functionalities. However, the application of hydrogel materials in bioengineering is still constrained by their limited dosage flexibility and the requirement of invasive surgical procedures. Compared to traditional hydrogels, injectable hydrogels, with shear-thinning and/or in situ formation properties, simplify the implantation process and reduce tissue invasion, which can be directly delivered to target sites using a syringe injection, offering distinct advantages over traditional hydrogels. These injectable hydrogels incorporate physically non-covalent and/or dynamic covalent bonds, granting them self-healing abilities to recover their structural integrity after injection. This review summarizes our recent progress in preparing injectable hydrogels and discusses their performance in various bioengineering applications. Moreover, the underlying molecular interaction mechanisms that govern the injectable and functional properties of hydrogels were characterized by using nanomechanical techniques such as surface forces apparatus (SFA) and atomic force microscopy (AFM). The remaining challenges and future perspectives on the design and application of injectable hydrogels are also discussed. This work provides useful insights and guides future research directions in the field of injectable hydrogels for bioengineering.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (-20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

Multifunction E-Skin Based on MXene-PA-Hydrogel for Human Behavior Monitoring

Hydrogels have attracted significant attention in various fields, such as smart sensing, human-machine interaction, and biomedicines, due to their excellent flexibility and versatility. However, current hydrogel electronic skins are still limited in stretchability, and their sensing functionality is often single-purpose, making it difficult to meet the requirements of complex environments and multitasking. In this study, we developed an MXene nanoplatelet and phytic acid-coreinforced poly(vinyl alcohol) (PVA) composite, denoted as MXene-PA-PVA. The strong hydrogen bonds formed by the interaction of the different components and the enhancement of chain entanglement result in a significant improvement in the mechanical properties of the PVA/PA/MXene composite hydrogel. This improvement is reflected in an increase of 271.43% in the maximum tensile strain and 35.29% in the maximum fracture stress. Moreover, the composite hydrogel exhibits excellent adhesion, water retention, heat resistance, and conductivity properties. The PVA/PA composite material combined with MXene demonstrates great potential for use as multifunctional sensors for strain and temperature detection with a strain-sensing sensitivity of 3.23 and a resistance temperature coefficient of 8.67. By leveraging the multifunctional characteristics of this composite hydrogel, electronic skin can accurately monitor human behavior and physiological reactions. This advancement opens up new possibilities for flexible electronic devices and human-machine interactions in the future.

Stretchable, Adhesive, and Biocompatible Hydrogel Based on Iron-Dopamine Complexes

Hydrogels' exceptional mechanical strength and skin-adhesion characteristics offer significant advantages for various applications, particularly in the fields of tissue adhesion and wearable sensors. Herein, we incorporated a combination of metal-coordination and hydrogen-bonding forces in the design of stretchable and adhesive hydrogels. We synthesized four hydrogels, namely PAID-0, PAID-1, PAID-2, and PAID-3, consisting of acrylamide (AAM), N,N'-methylene-bis-acrylamide (MBA), and methacrylic-modified dopamine (DA). The impact of different ratios of iron (III) ions to DA on each hydrogel's performance was investigated. Our results demonstrate that the incorporation of iron-dopamine complexes significantly enhances the mechanical strength of the hydrogel. Interestingly, as the DA content increased, we observed a continuous and substantial improvement in both the stretchability and skin adhesiveness of the hydrogel. Among the hydrogels tested, PAID-3, which exhibited optimal mechanical properties, was selected for adhesion testing on various materials. Impressively, PAID-3 demonstrated excellent adhesion to diverse materials and, combined with the low cytotoxicity of PAID hydrogel, holds great promise as an innovative option for biomedical engineering applications.

Enhanced Mechanical Strength of Metal Ion-Doped MXene-Based Double-Network Hydrogels for Highly Sensitive and Durable Flexible Sensors

Development of conductive hydrogels with high sensitivity and excellent mechanical properties remains a challenge for constructing flexible sensor devices. Herein, a universal strategy is presented for enhancing the mechanical strength of Mxene-based double-network hydrogels through metal ion coordination effects. Polyacrylamide (PAM)/sodium alginate (SA)/Mxene double-network (PSM-DN) hydrogels were prepared by metal ion impregnation of PAM/SA/Mxene (PSM) hydrogels. High electrical conductivity is achieved due to MXene nanosheets, while the strong coordination bond between metal ions and SA constructs a second network that increases the mechanical strength of the hydrogel by an order of magnitude. Mechanical tests demonstrated that the elastic modulus of hydrogels matches that of human tissues. Hence, they can be used as a highly sensitive electronic skin sensor to recognize the movement of different joints in humans and also as a pressure sensing interface to recognize characters for anticounterfeiting and information transfer. This work can promote the practical application of conductive hydrogels in high-tech fields, such as flexible electronic skin and interface interaction.

A novel multi-scale pressure sensing hydrogel for monitoring the physiological signals of long-term bedridden patients

For long-term bedridden patients who need to wear diapers, the timely replacement of diapers is very important to ensure their quality of life. Therefore, it is urgent to develop a pressure sensor that can monitor the physiological conditions of patients in real time. Inspired by the multi-scale network structure of the multi-fiber protein in the muscle, a multi-scale hydrogel as a pressure sensor was prepared by introducing micron-scale hydrogel microspheres as physical crosslinking agents. Compared with the traditional polyacrylamide hydrogel (0.17 MPa of compressive strength), the multi-scale hydrogel showed a higher compressive strength of up to 1.37 MPa. Meanwhile, the hydrogel exhibited better pressure sensitivity (0.59 kPa-1) than the existing hydrogels (0.27-0.40 kPa-1). The sensor prepared by this hydrogel could monitor the patient's physiological condition (urine outflow and urinary filling) in real time through the conductivity response to ion concentration and pressure, and then transmit the signal to the caregivers in time to avoid skin damage. This multi-scale hydrogel provided a great convenience for the physiological monitoring of long-term bedridden patients by acting as a pressure sensor.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

Bio-Inspired Homogeneous Conductive Hydrogel with Flexibility and Adhesiveness for Information Transmission and Sign Language Recognition

The wearable electronic technique is increasingly becoming an effective approach to overcoming the communication obstacles between signers and non-signers. However, the efficacy of conducting hydrogels currently proposed as flexible sensor devices is hindered by their poor processability and matrix mismatch, which frequently results in adhesion failure at the combined interfaces and deterioration of mechanical and electrochemical performance. Herein, we propose a hydrogel composed of a rigid matrix in which the hydrophobic and aggregated polyaniline was homogeneously embedded, while quaternate-functionalized nucleobase moieties endowed the flexible network with adhesiveness. Accordingly, the resulting hydrogel with chitosan-graft-polyaniline (chi-g-PANI) copolymers exhibited a promising conductivity (4.8 S·m^-1) because of the uniformly dispersed polyaniline components and a high strain strength (0.84 MPa) because of the chain entanglement of chitosan after soaking. In addition, the modified adenine molecules not only realized synchronization in improving the stretchability (up to 1303%) and exhibiting a skin-like elastic modulus (≈184 kPa), but also provided a durable interfacial contact with various materials. The hydrogel was further fabricated into a strain-monitoring sensor for information encryption and sign language transmission based on its sensing stability and strain sensitivity of up to 2.77. The developed wearable sign language interpreting system provides an innovative strategy to assist auditory or speech-impaired people in communicating with non-signers using visual-gestural patterns including body movements and facial expressions.

Reversible adhesives with controlled wrinkling patterns for programmable integration and discharging

Switchable and minimally invasive tissue adhesives have great potential for medical applications. However, on-demand adherence to and detachment from tissue surfaces remain difficult. We fabricated a switchable hydrogel film adhesive by designing pattern-tunable wrinkles to control adhesion. When adhered to a substrate, the compressive stress generated from the bilayer system leads to self-similar wrinkling patterns at short and long wavelengths, regulating the interfacial adhesion. To verify the concept and explore its application, we established a random skin flap model, which is a crucial strategy for repairing severe or large-scale wounds. Our hydrogel adhesive provides sufficient adhesion for tissue sealing and promotes neovascularization at the first stage, and then gradually detaches from the tissue while a dynamic wrinkling pattern transition happens. The gel film can be progressively ejected out from the side margins after host-guest integration. Our findings provide insights into tunable bioadhesion by manipulating the wrinkling pattern transition.

Liquid Metal-Doped Conductive Hydrogel for Construction of Multifunctional Sensors

Interest in wearable and stretchable multifunctional sensors has grown rapidly in recent years. The sensing elements must accurately detect external stimuli to expand their applicability as sensors. However, the sensor's self-healing and adhesion to a target object have been major challenges in developing such practical and versatile devices. In this study, we prepared a hydrogel (LM-SA-PAA) composed of liquid metal (LM), sodium alginate (SA), and poly(acrylic acid) (PAA) with ultrastretchable, excellent self-healing, self-adhesive, and high-sensitivity sensing capabilities that enable the conformal contact between the sensor and skin even during dynamic movements. The excellent self-healing performance of the hydrogel stems from its double cross-linked networks, including physical and chemical cross-linked networks. The physical cross-link formed by the ionic interaction between the carboxyl groups of PAA and gallium ions provide the hydrogel with reversible autonomous repair properties, whereas the covalent bond provides the hydrogel with a stable and strong chemical network. Alginate forms a microgel shell around LM nanoparticles via the coordination of its carboxyl groups with Ga ions. In addition to offering exceptional colloidal stability, the alginate shell has sufficient polar groups, ensuring that the hydrogel adheres to diverse substrates. Based on the efficient electrical pathway provided by the LM, the hydrogel exhibited strain sensitivity and enabled the detection of various human motions and electrocardiographic monitoring. The preparation method is simple and versatile and can be used for the low-cost fabrication of multifunctional sensors, which have broad application prospects in human-machine interface compatibility and medical monitoring.

Self-Healing Mechanical Properties of Selected Roofing Felts

In this work, roof felts are considered. Special attention is paid to the mechanical properties and self-healing (SH) phenomena under elevated temperatures. The results of the heating and strength tests for the entire range of material work, from the first load to sample breaking, are shown with respect to the angle of reinforcement relative to the longitudinal axis of the sample and different ways of breaking the continuity of the material. The influence that the material thickness and modifiers used for the production of the base material have on the obtained results was also pointed out. The meaningful SH strength is reported-from 5% up to 20% of the strength of the undamaged material-which, in perspective, can provide comprehensive knowledge of the optimal use of roofing felts and its proper mathematical modeling.

Strong Anchoring of Hydrogels through Superwetting-Assisted High-Density Interfacial Grafting

Strong adhesion of hydrogels on solids plays an important role in stable working for various practical applications. However, current hydrogel adhesion suffers from poor interfacial bonding with solid surfaces. Here, we propose a general superwetting-assisted interfacial polymerization (SAIP) strategy to robustly anchor hydrogels onto solids by forming high-density interfacial covalent bonds. The key of our strategy is to make the initiator fully contact solid surfaces via a superwetting way for enhancing the interfacial grafting efficiency. The designed anchored hydrogels show strong bulk failure with a high breaking strength of ≈1.37 MPa, different from weak interfacial failure that occurs in traditional strategies. The strong interfacial adhesion greatly enhances the stability of hydrogels against swelling destruction. This work opens up new inspirations for designing strongly anchored hydrogels from an interfacial chemistry perspective.

Enhancement of Self-Healing Efficacy of Conductive Nanocomposite Hydrogels by Polysaccharide Modifiers

The proper design of a polysaccharide/hydrocolloid modifier significantly affects the conductivity, self-healing, and viscoelastic properties of nanocomposite hydrogels. Due to the presence of different functional groups, these hydrogels can participate in the covalent, hydrogen and dynamic bonding of a system. The improvement of interactions in this system can lead to the development of high-performance nanocomposite hydrogels. In this study, resilient, self-healing and self-adhesive conductive nanocomposite hydrogels were produced by multiple and diverse coordination connections between various polysaccharide-based modifiers (Arabic gum, sodium carboxymethyl cellulose, and xanthan), the poly(vinyl alcohol) (PVA) network and different graphene-based fillers. Graphene nanoplatelets (GNP), activated carbon black (ACB), and reduced graphene oxide (rGO) have distinct functionalized surfaces, which were analyzed by X-ray photoelectron spectroscopy (XPS). Furthermore, the introduction of fillers balanced the hydrogels' viscoelastic properties and electrical conductivity, providing the hydrogels with resilience, improved electrical conductivity, and extreme stretchability (5000%). The self-healing properties were analyzed using time-dependent measurements in a shear strain mode using an RSO Rheometer. The improvement in electrochemical and conductivity properties was confirmed by electrochemical impedance spectroscopy (EIS). The obtained conductive nanocomposite hydrogels design opens new possibilities for developing high-performance polysaccharide-based hydrogels with wearable electrical sensors and healthcare monitoring applications.

Flexible Stretchable, Dry-Resistant MXene Nanocomposite Conductive Hydrogel for Human Motion Monitoring

Conductive hydrogels with high electrical conductivity, ductility, and anti-dryness have promising applications in flexible wearable electronics. However, its potential applications in such a developing field are severely hampered by its extremely poor adaptability to cold or hot environmental conditions. In this research, an "organic solvent/water" composite conductive hydrogel is developed by introducing a binary organic solvent of EG/H₂O into the system using a simple one-pot free radical polymerization method to create Ti₃C₂T_X MXene nanosheet-reinforced polyvinyl alcohol/polyacrylamide covalently networked nanocomposite hydrogels (PAEM) with excellent flexibility and mechanical properties. The optimized PAEM contains 0.3 wt% MXene has excellent mechanical performance (tensile elongation of ~1033%) and an improved modulus of elasticity (0.14 MPa), a stable temperature tolerance from -50 to 40 °C, and a high gauge factor of 10.95 with a long storage period and response time of 110 ms. Additionally, it is worth noting that the elongation at break at -40 °C was maintained at around 50% of room temperature. This research will contribute to the development of flexible sensors for human-computer interaction, electronic skin, and human health monitoring.

High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines

Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semiseparated network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alcohol) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mechanical twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiological signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and technical platforms toward stretchable electronic skins and intelligent robotic systems.