Synergistic strengthening of PVA ionic conductive hydrogels using aramid nanofibers and tannic acid for mechanically robust, antifreezing, water-retaining and antibacterial flexible sensors

Hydrogel adhesives for tissue recovery

Hydrogel adhesives (HAs) are promising and rewarding tools for improving tissue therapy management. Such HAs had excellent properties and potential applications in biological tissues, such as suture replacement, long-term administration, and hemostatic sealing. In this review, the common designs and the latest progress of HAs based on various methodologies are systematically concluded. Thereafter, how to deal with interfacial water to form a robust wet adhesion and how to balance the adhesion and non-adhesion are underlined. This review also provides a brief description of gelation strategies and raw materials. Finally, the potentials of wound healing, hemostatic sealing, controlled drug delivery, and the current applications in dermal, dental, ocular, cardiac, stomach, and bone tissues are discussed. The comprehensive insight in this review will inspire more novel and practical HAs in the future.

Ultrafast fabrication of ε-polylysine/amide-modified chitin-based conductive hydrogel with self-healing, adhesive and antibacterial abilities as a wearable strain sensor

In the realm of health monitoring, wearable electronic devices that utilize conductive hydrogels have garnered considerable attention. Nonetheless, the formidable challenge persists in streamlining the intricate preparation process and formulating a conductive hydrogel that seamlessly integrates diverse functionalities. In this study, we propose a strategy to fabricate multifunctional wearable hydrogel-based strain sensors (ε-PL/AMC-Al) in an ultrafast manner. Amide-modified chitin (AMC) was synthesized homogeneously, thereafter, Al3+ ions and ε-Polylysine (ε-PL) were introduced to interact with AMC through physical cross-linking techniques to form a three-dimensional network. Favorable mechanical and self-healing properties were achieved through the presence of multiple noncovalent interactions. The incorporation of ε-PL imparted antibacterial properties to the hydrogel sensor, thereby safeguarding it against bacterial contamination. Importantly, the incorporation of Al3+ not only facilitated the gelation process but also imparted electrical conductivity to the hydrogel, enabling it to function as a strain sensor. Notably, the adhesive property of the ε-PL/AMC-Al hydrogel ensured intimate contact, thereby allowing it to accurately monitor and differentiate between both gross and subtle human body movements without compromising its long-term stability. Based on its straightforward manufacturing process and versatility, the as-prepared hydrogel sensor exhibits significant potential for a diverse array of large-scale applications, including wearable electronic devices.

Quasi-Solid-State Zn-Ion Batteries Based on Sol-Gel Transition and in Situ Electrodeposition of MnO2

Zinc-ion batteries (ZIBs) have received much attention recently, because they are low-cost and environmentally friendly. In addition, ZIBs can be safer than lithium-ion batteries (LIBs) if issues, such as dendrite formation, side reactions, and hydrogen evolution, are resolved. Overcoming these challenges is crucial for the large-scale commercialization of ZIBs. In this article, Kevlar derivatized Aramid nanofiber combined with poly(vinyl alcohol) hydrogel was used to prevent the Zn dendrite growth and side reactions and increase the cyclability of the cells. A fast and simple method that involves sol-gel transition and in situ electrodeposition of the MnO2 cathode was developed to fabricate quasi-solid-state ZIB cells with an initial capacity of 5.2 and 3.3 mA h after 900 cycles at 10C. In addition, we have optimized the assembling process of ZIBs pouch cells by taking advantage of sol-gel transition and eliminating the conventional complex slurry-casting-drying steps. This work paves the way to efficiently assemble ZIB pouch cells with excellent rate performance and cycling stability.

Strain redistribution for achieving wide-range and high-sensitivity monitoring of natural rubber-based sensors

Strain sensors with high sensitivity and wide detection range are essential for meeting diverse applications, such as precisely monitoring the movement of patients with bone defects during rehabilitation. However, extending the sensing range without compromising sensitivity, particularly for small strains, remains a significant challenge for flexible sensors. Here, a strain redistribution strategy was employed to achieve wide-range and high-sensitivity monitoring of natural rubber (NR)-based sensors. A rectangular NR-based sensor was initially developed using the swelling-infiltration method, demonstrating a broad strain range but low sensitivity. The introduction of V-notches on both sides of the sensor resulted in significant local strain enhancement, substantially improving sensitivity but significantly reducing the sensing range. For example, the gauge factor (GF) increased from 4.2 to 28.4 at 20 % strain, while the sensing range decreased from 400.5 % to 71.4 %. Furthermore, O-notches were integrated into the NR-based sensor to facilitate strain redistribution. A well-designed O-notch enhanced the sensing range by 40 % without sacrificing small-strain sensitivity. Additionally, the NR-based sensor with strain redistribution demonstrated a low detection limit (0.1 %), excellent cyclic stability, and biocompatibility, making it highly effective for detecting large and small deformations in the human body.

Super Tough Anti-freezing and Antibacterial Hydrogel With Multi-crosslinked Network for Flexible Strain Sensor

Addressing the diverse environmental demands for electronic material performance, the design of a multifunctional ionic conductive hydrogel with mechanical flexibility, anti-freezing capability, and antibacterial characteristics represents an optimal solution. Leveraging the Dead Sea effect and the strong hydrogen bonding, this study exploits the CaCl2 and the abundant hydroxyl groups in phytic acid (PA) to induce chain entanglements, thereby constructing a complex, multi-crosslinked network. Furthermore, PA and ternary solvent systems (CaCl2/Glycerol/H2O) synergistically impart excellent mechanical strength, toughness (with tensile strength of 8.93 MPa, elongation at break of 859.93%, and toughness of 39.92 MJ m-3), high electrical conductivity, antifreeze capability, antibacterial properties, and high strain sensitivity (gauge factor up to 2.10) to the hydrogels. Remarkably, the hydrogel structure maintains stability even after undergoing 6000 loading-unloading cycles, demonstrating its outstanding fatigue resistance. Upon receiving external stimuli, the hydrogel exhibits a response time of 126 ms, making it ideal for the dynamic monitoring of human motion signals. This study offers novel insight into the potential application of ionic conductive hydrogels as flexible sensors in challenging environments.

Spin-coating ANF based multilayer symmetric composite films for enhanced electromagnetic interference shielding and thermal management

The demand for flexible composite films with electromagnetic interference (EMI) shielding capabilities is rapidly increasing. Balancing high EMI performance with flexibility and portability has become a critical research focus in practical applications. In this study, an optimized strategy for aramid nanofibers (ANF) films was developed using spin-coating and sol-gel techniques. The resulting film features a smooth surface and excellent mechanical properties. ANF, initially an insulator, was transformed into a conductor through the in-situ polymerization of ion-doped polypyrrole (PPy). Leveraging a multilayer structural strategy, we prepared a symmetric composite film, ANF@PPy-(TA-MXene)-AgNWs-(TA-MXene)-ANF@PPy (PMA), using vacuum-assisted filtration and lamination hot pressing. This film, composed of ANF@PPy (PA) as the matrix, tannic acid (TA) modified MXene, and silver nanowires (AgNWs) as fillers, exhibited multiple shielding mechanisms as electromagnetic wave (EMW) passed through its various layers. This multilayer configuration provides significant flexibility in EMW shielding. Moreover, TA-modified MXene expands the lamellar spacing, enhancing the scattering efficiency of EMWs within the film, and serves as a medium connecting the upper and lower layers. This results in the efficient integration of the multilayer structure, synergistically improving both EMI shielding performance and mechanical properties. When the ratio of PA/MXene/AgNWs was 1:3:1, the film demonstrated optimal properties, including an EMI shielding effectiveness of 70.2 dB, thermal conductivity of 4.62 W/(m•K), and tensile strength of 50.2 MPa. Due to the exceptional EMI shielding and thermal properties of the PMA composite film, it holds great potential for applications in artificial intelligence, wearable heaters, and military equipment.

Bio-inspired e-skin with integrated antifouling and comfortable wearing for self-powered motion monitoring and ultra-long-range human-machine interaction

Electronic skin (e-skin) inspired by the sensory function of the skin demonstrates broad application prospects in health, medicine, and human-machine interaction. Herein, we developed a self-powered all-fiber bio-inspired e-skin (AFBI E-skin) that integrated functions of antifouling, antibacterial, biocompatibility and breathability. AFBI E-skin was composed of three layers of electrospun nanofibrous films. The superhydrophobic outer layer Poly(vinylidene fluoride)-silica nanofibrous films (PVDF-SiO2 NFs) possessed antifouling properties against common liquids in daily life and resisted bacterial adhesion. The polyaniline nanofibrous films (PANI NFs) were used as the electrode layer, and it had strong "static" antibacterial capability. Meanwhile, the inner layer Polylactic acid nanofibrous films (PLA NFs) served as a biocompatible substrate. Based on the triboelectric nanogenerator principle, AFBI E-skin not only enabled self-powered sensing but also utilized the generated electrical stimulation for "dynamic" antibacterial. The "dynamic-static" synergistic antibacterial strategy greatly enhanced the antibacterial effect. AFBI E-skin could be used for self-powered motion monitoring to obtain a stable signal output even when water was splashed on its surface. Finally, based on AFBI E-skin, we constructed an ultra-long-range human-machine interaction control system, enabling synchronized hand gestures between human hand and robotic hand in any internet-covered area worldwide theoretically. AFBI E-skin exhibited vast application potential in fields like smart wearable electronics and intelligent robotics.

PNIPAAm-based temperature responsive ionic conductive hydrogels for flexible strain and temperature sensing

Conductive hydrogels have received much attention in the field of flexible wearable sensors due to their outstanding flexibility, conductivity, sensitivity and excellent compatibility. However, most conductive hydrogels mainly focus on strain sensors to detect human motion and lack other features such as temperature response. Herein, we prepared a strain and temperature dual responsive ionic conductive hydrogel (PPPNV) with an interpenetrating network structure by introducing a covalent crosslinked network of N-isopropylacrylamide (NIPAAm) and 1-vinyl-3-butylimidazolium bromide (VBIMBr) into the skeleton of the hydrogel composed of polyvinylalcohol (PVA) and polyvinylpyrrolidone (PVP). The PPPNV hydrogel exhibited excellent anti-freezing properties (-37.34 °C) and water retention with high stretchability (∼930 %) and excellent adhesion. As a wearable strain sensor, the PPPNV hydrogel has good responsiveness and stability to a wide range of deformations and exhibits high strain sensitivity (GF=2.6) as well as fast response time. It can detect large and subtle body movements with good signal stability. As wearable temperature sensors, PPPNV hydrogels can detect human physiological signals and respond to temperature changes, and the volumetric phase transition temperature (VPTT) can be easily controlled by adjusting the molar ratio of NIPAAm to VBIMBr. In addition, a bilayer temperature-sensitive hydrogel was prepared with the temperature responsive hydrogel by two-step synthesis, which shows great promising applications in temperature actuators.

Hydrogels in wearable neural interfaces

The integration of wearable neural interfaces (WNIs) with the human nervous system has marked a significant progression, enabling progress in medical treatments and technology integration. Hydrogels, distinguished by their high-water content, low interfacial impedance, conductivity, adhesion, and mechanical compliance, effectively address the rigidity and biocompatibility issues common in traditional materials. This review highlights their important parameters-biocompatibility, interfacial impedance, conductivity, and adhesiveness-that are integral to their function in WNIs. The applications of hydrogels in wearable neural recording and neurostimulation are discussed in detail. Finally, the opportunities and challenges faced by hydrogels for WNIs are summarized and prospected. This review aims to offer a thorough examination of hydrogel technology's present landscape and to encourage continued exploration and innovation. As developments progress, hydrogels are poised to revolutionize wearable neural interfaces, offering significant enhancements in healthcare and technological applications.

Graphical Abstract

Lignin-modified cellulose nanofibers hydrogel under adjustable binary solvent systems with excellent adhesion, self-healing and anti-freeze properties

Hydrogels with remarkable flexibility have gained popularity as materials for current research. However, the unfavorable properties of short-term adhesion, susceptibility to damage, and freezing in low-temperature presented by conventional hydrogels have become bottlenecks for further applications. In this work, an anti-freezing hydrogel with excellent mechanical, adhesion, and self-healing properties were developed by constructing a persistent semiquinone/quinone-catechol redox equilibrium environment. The introduction of lignin-modified cellulose nanofibers (LCNFs) significantly improved the overall mechanical properties of the material, driven by strong hydrogen bond interactions. This enhancement was evident in the tensile properties (97.74 ± 1.72 kPa, 783 %) and compression properties (> 90 %). Within the internal network of the gel, the synergistic action of lignin and ammonium persulfate resulted in the production of catechol, which imparted the gel with excellent adhesion properties (28.26 ± 2.13 KPa) and broad adhesion applicability. In addition, the incorporation of ethylene glycol (EG) positively contributed to the strengthening of the gel while endowed with tunable anti-freezing properties. Given the exceptional advantages of the prepared hydrogels, they were used to assemble flexible strain sensors with outstanding sensitivity for monitoring human motions.

Poly(3,4-Ethylenedioxythiophene):Sulfamic Acid Modified Aramid Nanofibers: An Innovative Conductive Polymer With Enhanced Electromagnetic Interference Shielding and Thermoelectric Performance

The development of alternative conductive polymers for the well-known poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is of great significance for improving the stability in long-term using and high-temperature environments. Herein, an innovative PEDOT:S-ANF aqueous dispersion is successfully prepared by using sulfamic acid (SA) to modified aramid nanofibers (S-ANF) as an alternative dispersant for PSS and the subsequent in situ polymerization of PEDOT. Thanks to the excellent film forming ability and surface negative groups of S-ANF, the PEDOT:S-ANF films show comparable tensile strength and elongation to unmodified PEDOT:ANF. Meanwhile, PEDOT:S-ANF has a high conductivity of 27.87 S cm-1, which is more than 20 times higher than that of PEDOT:PSS. The film exhibits excellent electromagnetic interference (EMI) shielding and thermoelectric performance, with a shielding effectiveness (SE) of 31.14 dB and a power factor (PF) of 0.43 µW m-1K-2. As a substitute for PSS, S-ANF exhibits significant structural and physicochemical properties, resulting in excellent chemical and thermal stability. Even under harsh conditions such as immersing to 0.1 M HCl, 0.1 M NaOH, and 3.5% NaCl solution, or high temperature conditions, the PEDOT:S-ANF films still maintain exceptional EMI shielding performance. Therefore, this multifunctional conductive polymer exhibits enormous potential and even proves its reliability in extreme situations.

Wide-humidity, anti-freezing and stretchable multifunctional conductive carboxymethyl cellulose-based hydrogels for flexible wearable strain sensors and arrays

Hydrogels play an important role in the design and fabrication of wearable sensors with outstanding flexibility, high sensitivity and versatility. Since hydrogels lose and absorb water during changes in humidity and temperature, it is critical and challenging to obtain hydrogels that function properly under different environmental conditions. Herein, a dual network hydrogel based on tannic acid (TA) reinforced polyacrylamide (PAM) and sodium carboxymethylcellulose (CMC) was constructed, while the introduction of the green solvents Solketal and LiCl endowed the hydrogel with greater possibilities for further modification to improve the water content and consistency of the mechanical properties over 30-90 % RH. This composite hydrogel (PTSL) has long-term stability, excellent mechanical strength, and freezing resistance. As strain sensors, they are linear over the entire strain range (R2 = 0.994) and have a high sensitivity (GF = 2.52 over 0-680 % strain range). Furthermore, the hydrogel's exceptional electrical conductivity and freezing resistance are a result of the synergistic effect of Solketal and LiCl, which intensifies the contact between the water molecules and the colloidal phase. This research could address the suitability of hydrogels over a wide range of humidity and temperature, suggesting great applications for smart flexible wearable electronics in harsh environmental conditions.

A Highly Sensitive, Low Creep Hydrogel Sensor for Plant Growth Monitoring

Ion-conducting hydrogels show significant potential in plant growth monitoring. Nevertheless, traditional ionic hydrogel sensors experience substantial internal creep and inadequate sensitivity, hindering precise plant growth monitoring. In this study, we developed a flexible hydrogel sensor composed of polyvinyl alcohol and acrylamide. The hydrogel sensor exhibits low creep and high sensitivity. Polyvinyl alcohol, acrylamide, and glycerol are crosslinked to create a robust interpenetrating double network structure. The strong interactions, such as van der Waals forces, between the networks minimize hydrogel creep under external stress, reducing the drift ratio by 50% and the drift rate by more than 60%. Additionally, sodium chloride and AgNWs enrich the hydrogel with conductive ions and pathways, enhancing the sensor's conductivity and demonstrating excellent response time (0.4 s) and recovery time (0.3 s). When used as a sensor for plant growth monitoring, the sensor exhibits sensitivity to small strains and stability for long-term monitoring. This sensor establishes a foundation for developing plant health monitoring systems utilizing renewable biomass materials.

Ultrafine Aramid Nanofiber-Assisted Large-Area Dense Stacking of MXene Films for Electromagnetic Interference Shielding and Multisource Thermal Conversion

Polymers are often used as adhesives to improve the mechanical properties of flexible electromagnetic interference (EMI) shielding layered films, but the introduction of these insulating adhesives inevitably reduces the EMI performance. Herein, ultrafine aramid nanofibers (UANF) with a diameter of only 2.44 nm were used as the binder to effectively infiltrate and minimize the insulating gaps in MXene films, for balancing the EMI shielding and mechanical properties. Combining the evaporation-induced scalable assembly assisted by blade coating, flexible large-scale MXene/UANF films with highly aligned and compact MXene stacking are successfully fabricated. Compared with the conventional ANF with a larger diameter of 7.05 nm, the UANF-reinforced MXene film exhibits a "brick-mortar" structure with higher orientation and compacter stacking MXene nanosheets, thus showing the higher mechanical properties, electrical conductivity, and EMI shielding performance. By optimizing MXene content, the MXene/UANF film can achieve the optimal tensile strength of 156.9 MPa, a toughness of 2.9 MJ m-3, satisfactory EMI shielding effectiveness (EMI SE) of 40.7 dB, and specific EMI SE (SSE/t) of 22782.4 dB cm2/g). Moreover, the composite film exhibits multisource thermal conversion functions including Joule heating and photothermal conversion. Therefore, the multifunctional MXene/UANF EMI shielding film with flexibility, foldability, and robust mechanical properties shows the practical potential in complex application environments.

Nanocomposite hydrogel for skin motion sensing - An antifreezing, nanoreinforced hydrogel with decorated AuNP as a multicrosslinker

In this study, we present a nanocomposite hydrogel designed for skin motion sensing. The hydrogel is based on poly(acrylamide) crosslinked with gold nanoparticles covalently bound to the polymer matrix, yielding a robust, highly elastic and conductive material. The choice of amino acid derivative - N,N'-diacryloylcystine salt (BISS) - as a crosslinker allows for the introduction of gold nanoparticles, due to the presence of sulfide groups in its structure. During the nanoparticle modification process, covalent bonds between gold and sulfur atoms are formed as the disulfide bond is cleaved. In result of this self-assembly process, a multifunctional Au-BISS crosslinker is formed, enhancing the material's mechanical properties and introducing electrical conductivity. To confer anti-freezing properties and limit water evaporation, a binary mixture of water and glycerol was used. The resultant hydrogel exhibits high elasticity, strain sensitivity across a wide strain range and various types of deformation (elongation, bending, compression) with exceptional response time (120 ms) and recovery time (90 ms). The material's cold-resistance, resilience, and conductivity make it well-suited for real-time monitoring of joint movements and speech recognition, with potential applications in electronic skin and healthcare monitoring devices.

Frost-resistant nanocellulose-based organohydrogel with high mechanical strength and transparency

Improving mechanical strength and frost-resistance is an important research direction in the field of hydrogel materials. Herein, using bacterial nanocellulose (BC) as a reinforcing agent and polyvinyl alcohol (PVA) as a polymer matrix, a frost-resistant organohydrogel was constructed via the freezing-thawing method in a new binary solvent system of N, N-dimethylformamide and water (DMF-H2O), which was designed according to the Hansen Solubility Parameter. Owing to the solvent-induced crystallization effect that led to the enhanced 3D hydrogen bonding network during the freezing-thawing process, the optimal organohydrogel achieved excellent mechanical properties with the tensile strength of 2,974 kPa and the stretchability of 277 % at room temperature, respectively. In the visiblelight range, the organohydrogel demonstrated high transmittance. Moreover, the presence of a DMF-H2O binary solvent endows it with frost-resistance, retaining the tensile strength of 508 kPa and a stretchability of 190 % even at -70 °C, respectively. This kind of transparent, frost-resistant organohydrogel has potential uses in harsh settings due to its great mechanical strength.

Antibacterial conductive polyacrylamide/quaternary ammonium chitosan hydrogel for electromagnetic interference shielding and strain sensing

The utilization of biomass-based conductive polymer hydrogels in wearable electronics holds great promise for advancing performance and sustainability. An interpenetrating network of polyacrylamide/2-hydroxypropyltrimethyl ammonium chloride chitosan (PAM/HACC) was firstly obtained through thermal-initiation polymerization of AM monomers in the presence of HACC. The positively charged groups on HACC provide strong electrostatic interactions and hydrogen bonding with the PAM polymer chains, leading to improved mechanical strength and stability of the hydrogel network. Subsequently, the PAM/HACC networks served as the skeletons for the in-situ polymerization of polypyrrole (PPy), and then the resulting conductive hydrogel demonstrated stable electromagnetic shielding performance (40 dB), high sensitivity for strain sensing (gauge factor = 2.56). Moreover, the incorporation of quaternary ammonium chitosan into PAM hydrogels enhances their antimicrobial activity, making them more suitable for applications in bacterial contamination or low-temperature environments. This conductive hydrogel, with its versatility and excellent mechanical properties, shows great potential in applications such as electronic skin and flexible/wearable electronics.

Strong, tough, high-release, and antibacterial nanocellulose hydrogel for refrigerated chicken preservation

Enormous amounts of food resources are annually wasted because of microbial contamination, highlighting the critical role of effective food packaging in preventing such losses. However, traditional food packaging faces several limitations, such as low mechanical strength, poor fatigue resistance, and low water retention. In this study, we aimed to prepare nanocellulose hydrogels with enhanced stretchability, fatigue resistance, high water retention, and antibacterial properties using soy hull nanocellulose (SHNC), polyvinyl alcohol (PVA), sodium alginate (SA), and tannic acid (TA) as raw materials. These hydrogels were applied in food packaging to extend the shelf life of refrigerated chicken. The structure and properties (e.g., mechanical, antibacterial, and barrier properties) of these hydrogels were characterized using different techniques. Fourier-transform infrared spectroscopy revealed the presence of hydrogen and ester bonds in the hydrogels, whereas scanning electron microscopy revealed the three-dimensional network structure of the hydrogels. Mechanical testing demonstrated that the SHNC/PVA/SA/TA-2 hydrogel exhibited excellent tensile properties (elongation = 160 %), viscoelasticity (storage modulus of 1000 Pa), and mechanical strength (compressive strength = 10 kPa; tensile strength = 0.35 MPa). Moreover, under weak acidic and alkaline conditions, the ester bonds of the hydrogel broke down with an increase in pH, improving its swelling and release properties. The SHNC/PVA/SA/TA-2 hydrogel displayed an equilibrium swelling ratio exceeding 300 %, with a release rate of >80 % for the bioactive substance TA. Notably, antibacterial testing showed that the SHNC/PVA/SA/TA-2 hydrogel effectively deactivated Staphylococcus aureus and Escherichia coli, prolonging the shelf life of refrigerated chicken to 10 d. Therefore, the SHNC/PVA/SA/TA hydrogels can be used in food packaging to extend the shelf life of refrigerated meat products. Their cost-effectiveness and simple preparation make them suitable for various applications in the food industry.

Phytic acid/tannic acid reinforced hydrogels with ultra-high strength for human motion monitoring and arrays

Conductive hydrogels have been widely researched for their potential applications in soft electronic devices. Creating environmentally friendly and multifunctional high-strength hydrogels for high-performance devices remains a significant challenge. This study employs the biodegradable material polyvinyl alcohol (PVA) as the primary component, with phytic acid (PA) and tannic acid (TA) as reinforcing phases, to create a multifunctional, high-strength "green" hydrogel. Through the multiple complexations of two bio-enhancing phases with the PVA main chain, this hydrogel attains ultra-high tensile strength (9.341 MPa), substantial toughness (4.262 MJ m-3), and extensive fracture strain (> 1000%), making it a representative with both mechanical performance and antibacterial capabilities. Additionally, it exhibits a low strain sensing limit (0.5%) and excellent durability (500 cycles under 50% strain). This work introduces a novel strategy of combining biodegradable materials with biomass to fabricate multifunctional hydrogels suitable for human motion monitoring and 2D pressure distribution.