TEMPO bacterial cellulose and MXene nanosheets synergistically promote tough hydrogels for intelligent wearable human-machine interaction

Porous Carbon Nanoparticle Composite Paper Fiber with Laser-Induced Graphene Surface Microstructure for Pressure Sensing

In recent years, flexible pressure sensors have played an increasingly important role in human health monitoring. Inspired by traditional papermaking techniques, we have developed a highly flexible, low-cost, and ecofriendly flexible pressure sensor using shredded paper fibers as the substrate. By combining the properties of laser-induced graphene with the structure of paper fibers, we have improved the internal structure of pressure-sensitive paper and designed a conical surface microstructure, providing new insights into nanomaterial engineering. It features low resistance (424.44 Ω), low energy consumption of only 0.367 μW under a pressure of 1.96 kPa, high sensitivity (1.68 kPa-1), and a wide monitoring range (98 Pa-111.720 kPa). The pressure-sensitive paper with surface microstructure (MFTG) developed in this study has a total thickness comparable to A4 paper, is soft and bendable, can be cut into any shape like paper to fit the human body, and holds great potential for continuous monitoring of human activity status and physiological information.

High-Conductivity, Self-Healing, and Adhesive Ionic Hydrogels for Health Monitoring and Human-Machine Interactions Under Extreme Cold Conditions

Ionic conductive hydrogels (ICHs) are emerging as key materials for advanced human-machine interactions and health monitoring systems due to their unique combination of flexibility, biocompatibility, and electrical conductivity. However, a major challenge remains in developing ICHs that simultaneously exhibit high ionic conductivity, self-healing, and strong adhesion, particularly under extreme low-temperature conditions. In this study, a novel ICH composed of sulfobetaine methacrylate, methacrylic acid, TEMPO-oxidized cellulose nanofibers, sodium alginate, and lithium chloride is presented. The hydrogel is designed with a hydrogen-bonded and chemically crosslinked network, achieving excellent conductivity (0.49 ± 0.05 S m-1), adhesion (36.73 ± 2.28 kPa), and self-healing capacity even at -80 °C. Furthermore, the ICHs maintain functionality for over 45 days, showcasing outstanding anti-freezing properties. This material demonstrates significant potential for non-invasive, continuous health monitoring, adhering conformally to the skin without signal crosstalk, and enabling real-time, high-fidelity signal transmission in human-machine interactions under cryogenic conditions. These ICHs offer transformative potential for the next generation of multimodal sensors, broadening application possibilities in harsh environments, including extreme weather and outer space.

A robust and conductive hydrotalcite/nanocellulose/PVA hydrogel constructed based on the Hofmeister effect

Conductive hydrogel is one of the basic materials for constructing flexible sensors, and polyvinyl alcohol (PVA) hydrogel is commonly used. However, the current PVA hydrogels have apparent defects in strength and conductivity. The freeze-salting-out process based on the Hofmeister effect can effectively improve the strength of PVA. This study uses hydrotalcite and nanocellulose as additives to construct PVA composite hydrogel using the freeze-salting-out method. Due to the reconstruction of the hydrogen bond and the construction of a multi-level three-dimensional network structure, the tensile strength and elongation of PVA composite hydrogels are improved to 8.2 times and 8.1 times that of the pure PVA hydrogels, respectively. Meanwhile, the conductivity of PVA composite hydrogel is increased by 6.4 times with the significant development of ion content and the effective establishment of the transport path. Based on the characteristics of high ion concentration and stable network structure, the composite hydrogels show excellent elastic and strain recovery properties at -20 °C and room temperature. The prepared composite hydrogels have good biocompatibility. This work realizes the construction of PVA composite hydrogel material with high strength, high conductivity and wide temperature application range. It provides a new idea for the development of flexible biosafety sensors.

Advances in polysaccharide-based conductive hydrogel for flexible electronics

Polysaccharides, being the most abundant natural polymers, play a pivotal role in the development of hydrogel materials. Polysaccharide-based conductive hydrogels have found extensive applications in flexible electronics due to their excellent conductivity and biocompatibility. This review highlights recent advancements in this area, starting with an overview of polysaccharide materials such as chitosan, cellulose, starch, cyclodextrin, alginate, hyaluronic acid, and agarose. It then explores different classifications of conductive hydrogels: ionic conductive, electronic conductive, and ionic-electronic composite types. The review also covers key characteristics of these hydrogels, including mechanical properties, self-healing, adhesion, structural color, antibacterial, responsiveness, biocompatibility and anti-swelling. Representative applications, such as flexible sensors, triboelectric nanogenerators, supercapacitors, and flexible electronic wound dressings, are summarized. Finally, the review addresses current challenges and provides guidance for future research, aiming to advance the field of polysaccharide-based conductive hydrogels in flexible electronics.

MXene Hydrogels for Soft Multifunctional Sensing: A Synthesis-Centric Review

Intelligent wearable sensors based on MXenes hydrogels are rapidly advancing the frontier of personalized healthcare management. MXenes, a new class of transition metal carbon/nitride synthesized only a decade ago, have proved to be a promising candidate for soft sensors, advanced human-machine interfaces, and biomimicking systems due to their controllable and high electrical conductivity, as well as their unique mechanical properties as derived from their atomistically thin layered structure. In addition, MXenes' biocompatibility, hydrophilicity, and antifouling properties render them particularly suitable to synergize with hydrogels into a composite for mechanoelectrical functions. Nonetheless, while the use of MXene as a multifunctional surface or an electrical current collector such as an energy device electrode is prevalent, its incorporation into a gel system for the purpose of sensing is vastly less understood and formalized. This review provides a systematic exposition to the synthesis, property, and application of MXene hydrogels for intelligent wearable sensors. Specific challenges and opportunities on the synthesis of MXene hydrogels and their adoption in practical applications are explicitly analyzed and discussed to facilitate cross gemination across disciplines to advance the potential of MXene multifunctional sensing hydrogels.

Fishing net-inspired PVA-chitosan-CNT hydrogels with high stretchability, sensitivity, and environmentally stability for textile strain sensors

Soft electronic products are being extensively investigated in diverse applications including sensors and devices, due to their superior softness, responsiveness, and biocompatibility. One-dimensional (1-D) fiber electronic devices are recognized for their lightweight, wearable, and stretchable qualities, thus emerging as critical constituents for seamless integration with the human body and attire, exhibiting great potential in wearable applications. However, wearable conductive hydrogel fibers usually face challenges in combining stretchability and excellent stability, notably in high-temperature environment. Herein, a novel stretchable conductive hydrogel fiber, namely PVA-CS-CNT (Polyvinyl Alcohol-Chitosan-Carbon Nanotube) hydrogel fiber, was successfully prepared through a straightforward low-temperature process. This hydrogel fiber not only maintains stable signal transmission at high temperatures but also exhibits significant mechanical and sensing capabilities, ensuring signal stability during repetitive cyclic stretching. Inspired by fishing net, textile sensors were fabricated by weaving PVA-CS-CNT hydrogel fibers, which offered breathability, high stability (withstanding over 500 stretch cycles), high sensitivity (detecting strains as low as 1 %), and exceptional mechanical strength (exceeding 17 MPa). The wearable sensor could not only accurately monitor human movements like stretching and bending, but also adeptly captured delicate signals such as pulses and sounds. These characteristics demonstrated the potential applications of the hydrogel fibers encompassing human motion tracking, intelligent textiles, and soft robotics.

Alkali Ion-Accelerated Gelation of MXene-Based Conductive Hydrogel for Flexible Sensing and Machine Learning-Assisted Recognition

Conductive hydrogels are promising active materials for wearable flexible electronics, yet it is still challenging to fabricate conductive hydrogels with good environmental stability and electrical properties. In this work, a conductive MXene/LiCl/poly(sulfobetaine methacrylate) hydrogel system was successfully prepared with an impressive conductivity of 12.2 S/m. Interestingly, the synergistic effect of MXene and a lithium bond can significantly accelerate the polymerization process, forming the conductive hydrogel within 1 min. In addition, adding LiCl to the hydrogel not only significantly increases its water retention ability, but also enhances its conductivity, both of which are important for practical applications. The flexible strain sensors based on the as-prepared hydrogel have demonstrated excellent monitoring ability for human joint motion, pulse, and electromyographic signals. More importantly, based on machine learning image recognition technology, the handwritten letter recognition system displayed a high accuracy rate of 93.5%. This work demonstrates the excellent comprehensive performance of MXene-based hydrogels in health monitoring and image recognition and shows potential applications in human-machine interfaces and artificial intelligence.

Lignin sulfonate induced ultrafast fabrication of polypyrrole-based conductive organohydrogel for high performance flexible strain and temperature sensor

The ultrafast preparation of electrically conductive hydrogels to endow high sensing performance and temperature tolerance remains a critical challenge. Herein, lignosulfonate sodium-templated polypyrrole (LS-PPy) nanofillers were rapidly introduced into polyacrylic acid (PAA) hydrogel through ultrafast free radical polymerization in a glycerol/water binary solvent system. The resultant LS-PPy/PAA electrically conductive organohydrogel possesses satisfactory mechanical performance (strength of 56 kPa at a tensile strain of 800 %), strong adhesion, and a desirable low freezing point (-35 °C). Furthermore, this organohydrogel exhibits high strain sensitivity (gauge factor = 2.65), fast response time (~160 ms), low signal hysteresis, and excellent cyclic stability (over 1200 cycles). And the wearable LS-PPy/PAA organohydrogel sensor could accurately and real-time monitor various intense or subtle human movements, such as joint bending, facial expression and hand writing. Besides, the developed LS-PPy/PAA temperature sensor can respond to environmental temperature variations over a wide range of -20-100 °C. High resolution of 0.5 °C with remarkable sensitivity (-0.80 %/°C and linearity of R2 = 0.99) and repeatability were achieved within 36.5-40 °C, which makes it suitable for human body temperature monitoring. All these results demonstrate the substantial prospective value of the LS-PPy/PAA hydrogel in wearable sensors and other associated fields.

Highly stretchable, self-healing, antibacterial, conductive, and amylopectin-enhanced hydrogels with gallium droplets loading as strain sensors

In this study, we address the challenge of developing highly conductive hydrogels with enhanced stretchability for use in wearable sensors, which are critical for the precise detection of human motion and subtle physiological strains. Our novel approach utilizes amylopectin, a biopolymer, for the uniform integration of liquid metal gallium into the hydrogel matrix. This integration results in a conductive hydrogel characterized by remarkable elasticity (up to 7100 % extensibility) and superior electrical conductance (Gauge Factor = 31.4), coupled with a minimal detection limit of less than 0.1 % and exceptional durability over 5000 cycles. The hydrogel demonstrates significant antibacterial activity, inhibiting microbial growth in moist environments, thus enhancing its applicability in medical settings. Employing a synthesis process that involves ambient condition polymerization of acrylic acid, facilitated by a hydrophobic associative framework, this hydrogel stands out for its rapid gelation and robust mechanical properties. The potential applications of this hydrogel extend beyond wearable sensors, promising advancements in human-computer interaction through technologies like wireless actuation of robotic systems. This study not only introduces a viable material for current wearable technologies but also sets a foundation for future innovations in bio-compatible sensors and interactive devices.

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.

Multifunctional chitosan-based composite hydrogels engineered for sensing applications

Chitosan-based hydrogels, as natural high-molecular-weight flexible materials, are widely utilized due to their outstanding properties. In this research, we developed a one-pot method for synthesizing a novel PVA/CS@PPy-PDAx% conductive hydrogel and explored the internal bonding patterns through molecular dynamics simulations. By adding PPy-PDA nanoparticles into a hydrogel matrix, an interpenetrating conductive network established successfully. The uniform distribution of PPy-PDA nanoparticles endowed the hydrogel with good electrical conductivity (0.171 S/m), significantly enhanced mechanical properties, and strain sensing (S = 5.04), as well as near-infrared photothermal responsiveness (temperature increase of 41.9 °C within 30 s). Additionally, due to the hydrogel's significant photothermal conversion efficiency under near-infrared radiation, it exhibits rapid elimination of Escherichia coli with an antibacterial efficiency exceeding 90 %. The unique hydrogen-bonded crosslinked structure provides the hydrogel with excellent re-healing properties, allowing for restoration through a freeze-thaw process after damage. The conductivity remains nearly unchanged after re-healing, maintaining the material's integrity and functionality. The flexible sensor based on this hydrogel has a response time of 100 ms and can sensitively detect large-scale deformations (e.g., joint bending at various angles), different gravitational forces, and recognize human handwriting. These characteristics make this hydrogel a promising candidate for advancing intelligent wearable technologies and human-machine interaction systems.

Recent advances in nanocellulose based hydrogels: Preparation strategy, typical properties and food application

Nanocellulose has been favored as one of the most promising sustainable nanomaterials, due to its competitive advantages and superior performances such as hydrophilicity, renewability, biodegradability, biocompatibility, tunable surface features, excellent mechanical strength, and high specific surface area. Based on the above properties of nanocellulose and the advantages of hydrogels such as high water absorption, adsorption, porosity and structural adjustability, nanocellulose based hydrogels integrating the benefits of both have attracted extensive attention as promising materials in various fields. In this review, the main fabrication strategies of nanocellulose based hydrogels are initially discussed in terms of different crosslinking methods. Then, the typical properties of nanocellulose based hydrogels are comprehensively summarized, including porous structure, swelling ability, adsorption, mechanical, self-healing, smart response performances. Especially, relying on these properties, the general application of nanocellulose based hydrogels in food field is also discussed, mainly including food packaging, food detection, nutrient embedding delivery, 3D food printing, and enzyme immobilization. Finally, the safety of nanocellulose based hydrogel is summarized, and the current challenges and future perspectives of nanocellulose based hydrogels are put forward.

Conductive, sensitivity, flexibility, anti-freezing and anti-drying silica/carbon nanotubes/sodium ions modified sodium alginate hydrogels for wearable strain sensing applications

The biocompatibility and salient gelling feature of alginate via forming the interpenetrating network structure has received extensive interests for different applications. Traditional alginate hydrogels freeze at low temperature and evaporate easily at room temperature, leading to reduced performance. Consequently, it is crucial to develop methods to prevent alginate hydrogel from freezing at subzero temperature and dehydration at normal temperature to maintain the performance stability. Utilizing polyacrylic acid, sodium alginate, and acrylamide-hydroxyethyl methacrylate copolymers as flexible matrix materials, this study develops a wearable silica (SiO2)/carbon nanotubes (CNT)/sodium ions (SiO2/CNT/Na+) modified sodium alginate hydrogel strain sensor characterized by high sensitivity, flexibility, and anti-freezing and anti-drying properties. The hydrogel doped with NaCl (50 mg), CNT (10 mg) and M-SiO2 (200 mg) shows excellent mechanical and electrical properties, the tensile strength is 436 KPa, the break elongation is 426 %, the elastic modulus is 99 KPa, and the toughness is 897 kJ/m3. The modified sodium alginate hydrogel used as strain sensor shows fast response time (∼100 ms), high sensitivity factor and excellent stability. The strain sensor exhibits excellent flexibility, ductility, self-adhesion, anti-freezing and anti-drying properties, significantly enhancing its strain sensing application field.

Tough, self-healing, adhesive double network conductive hydrogel based on gelatin-polyacrylamide covalently bridged by oxidized sodium alginate for durable wearable sensors

Pursuing high-performance conductive hydrogels is still hot topic in development of advanced flexible wearable devices. Herein, a tough, self-healing, adhesive double network (DN) conductive hydrogel (named as OSA-(Gelatin/PAM)-Ca, O-(G/P)-Ca) was prepared by bridging gelatin and polyacrylamide network with functionalized polysaccharide (oxidized sodium alginate, OSA) through Schiff base reaction. Thanks to the presence of multiple interactions (Schiff base bond, hydrogen bond, and metal coordination) within the network, the prepared hydrogel showed outstanding mechanical properties (tensile strain of 2800 % and stress of 630 kPa), high conductivity (0.72 S/m), repeatable adhesion performance and excellent self-healing ability (83.6 %/79.0 % of the original tensile strain/stress after self-healing). Moreover, the hydrogel-based sensor exhibited high strain sensitivity (GF = 3.66) and fast response time (<0.5 s), which can be used to monitor a wide range of human physiological signals. Based on this, excellent compression sensitivity (GF = 0.41 kPa-1 in the range of 90-120 kPa), a three-dimensional (3D) array of flexible sensor was designed to monitor the intensity of pressure and spatial force distribution. In addition, a gel-based wearable sensor was accurately classified and recognized ten types of gestures, achieving an accuracy rate of >96.33 % both before and after self-healing under three machine learning models (the decision tree, SVM, and KNN). This paper provides a simple method to prepare tough and self-healing conductive hydrogel as flexible multifunctional sensor devices for versatile applications in fields such as healthcare monitoring, human-computer interaction, and artificial intelligence.

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.

From Fluorescence-Transfer-Lightening-Printing-Assisted Conductive Adhesive Nanocomposite Hydrogels toward Wearable Interactive Optical Information-Electronic Strain Sensors

The interactive flexible device, which monitors the human motion in optical and electrical synergistic modes, has attracted growing attention recently. The incorporation of information attribute within the optical signal is deemed advantageous for improving the interactive efficiency. Therefore, the development of wearable optical information-electronic strain sensors holds substantial promise, but integrating and synergizing various functions and realizing strain-mediated information transformation keep challenging. Herein, an amylopectin (AP) modified nanoclay/polyacrylamide-based nanocomposite (NC) hydrogel and an aggregation-induced-emission-active ink are fabricated. Through the fluorescence-transfer printing of the ink onto the hydrogel film in different strains with nested multiple symbolic information, a wearable interactive fluorescent information-electronic strain sensor is developed. In the sensor, the nanoclay plays a synergistic "one-stone-three-birds" role, contributing to "lightening" fluorescence (≈80 times emission intensity enhancement), ionic conductivity, and excellent stretchability (>1000%). The sensor has high biocompatibility, resilience (elastic recovery ratio: 97.8%), and strain sensitivity (gauge factor (GF): 10.9). Additionally, the AP endows the sensor with skin adhesiveness. The sensor can achieve electrical monitoring of human joint movements while displaying interactive fluorescent information transformation. This research poses an efficient strategy to develop multifunctional materials and provides a general platform for achieving next-generation interactive devices with prospective applications in wearable devices, human-machine interfaces, and artificial intelligence.

Multifunctional sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide ionic hydrogels composite as a high-performance wearable strain sensor

Recently, conductive hydrogels have shown great promise in flexible electronics and are ideal materials for the preparation of wearable strain sensors. However, developing a simple method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, transparency, anti-freezing, and UV resistance remains a significant challenge. A novel sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide/Zn2+/DMSO (SLS/XG/SA/PAM/Zn2+/DMSO) ionic conductive hydrogel was developed using a one-pot method. The resulting ionic conductive hydrogels have excellent mechanical properties (stress: 0.13 MPa, strain: 1629 %), high anti-fatigue properties, self-adhesion properties (iron: 7.37 kPa, pigskin: 4.74 kPa), anti-freezing (freezing point: -33.49 °C) and UV resistance by constructing a chemical and physical hybrid cross-linking network. In particular, the conductivity of G hydrogel reached 6.02 S/m at room temperature and 5.52 S/m at -20 °C. Thus, the hydrogel was assembled into a flexible sensor that could distinguish a variety of large and small scales human movements, such as joint bending, swallowing and speaking in real time with high stability and sensitivity. Moreover, the hydrogel could be used as electronic skin just like human skin and touch screen pen to write.