Approaching intrinsic dynamics of MXenes hybrid hydrogel for 3D printed multimodal intelligent devices with ultrahigh superelasticity and temperature sensitivity

Engineering functional electroconductive hydrogels for targeted therapy in myocardial infarction repair

Myocardial infarction (MI) is characterized by a paucity of cardiomyocyte regeneration, leading to significant morbidity and mortality. Contemporary therapeutic modalities, while mitigating ischemic effects, fail to reconstitute the impaired electromechanical coupling within the infracted myocardium. Emerging evidence supports the utility of electroconductive hydrogels (ECHs) in facilitating post-MI cardiac function recovery by restoring the conductive microenvironment of the infarcted tissue. This comprehensive review delineates the taxonomy of ECHs predicated on their constituent conductive materials. It also encapsulates prevailing research trends in ECH-mediated MI repair, encompassing innovative design paradigms and microenvironment-sensitive strategies. The review also provides a critical appraisal of various implantation techniques, underscored by a thorough examination of the attendant considerations. It elucidates the mechanistic underpinnings by which hydrogels exert salutary effects on myocardial repair, namely by augmenting mechanical and electrical integrity, exerting anti-inflammatory actions, fostering angiogenesis, and curtailing adverse remodeling processes. Furthermore, the review engages with the pressing challenge of optimizing ECH functionality to achieve superior reparative outcomes post-MI. The discourse concludes with an anticipatory perspective on the evolution of ECH scaffolds, advocating for a tailored approach that integrates multifaceted physicochemical properties to cater to the nuances of personalized medicine.

Eco-Friendly Multifunctional Hydrogel Sensors Enabled Sustainable and Accurate Human-Machine Interaction System

Wearable epidermic electronics assembled by conductive hydrogels exhibit great application potential for their seamless integration with the human body for human-machine interactions (HMI). However, most multifunctional hydrogel sensors are prone to water loss and become useless e-waste, resulting in a growing threat to the global environment and human health. Inspired by the resurrection plants, this paper introduces the reversible intermolecular forces and physical crosslinking method into the hydrogel system to obtain a fully recyclable multifunctional smart hydrogel sensor (RMSHS), which can be completely recycled in a simple step. Meanwhile, RMSHS possesses admirable biocompatibility, excellent antibacterial ability (S. aureus and E. coli bacterial inhibition rate of 99.8%), rapid self-healing ability, and outstanding sensing performances such as low detection limit, fast response/recovery time (160/200 ms). Intelligent medical rehabilitation and smart HMI systems are developed for medical diagnostics and real-time remote controlling. Moreover, the recyclable triboelectric nanogenerator (R-TENG) intelligent array, designed based on RMSHS, replaces the electrode consumables in traditional TENGs, realizing the generation of green energy and 100% recyclability of electrode materials. RMSHS can be generalized and applied to other remote-controlling platforms, paving the way for large-scale, multi-scenario applications.

Conductive Hydrogel Motion Sensor with Low-Temperature Stability for Winter Sports and Sensing Rescue

Hydrogels with conductive properties hold significant promise in the realm of flexible electronics, owing to their pliability, outstanding conductivity, and diverse functionalities. Nevertheless, the majority of conductive hydrogels are prone to being brittle and easily damaged; as such, they are not adapt to cold environments, which seriously hinders their practical applications. Therefore, hydrogels that possess both conductivity and anti-freezing, as well as moisturizing, capabilities have garnered considerable interest, and these hydrogels can work stably in harsh environments. Phytic acid (PA), which mainly exists in plant seeds, is a kind of natural compound widely existing in nature that can be recycled; it provides electrical conductivity and anti-freezing to hydrogels. Here, a highly conductive hydrogel with excellent anti-freezing and moisturizing capabilities was prepared by incorporating PA into a polyacrylamide/gelatin hydrogel. The incorporation of PA endowed the hydrogel with an excellent conductivity of 5.8 S·cm-1. In addition, robust hydrogen bonding was formed between water and phytic acid molecules, and the hydrogel demonstrated remarkable anti-freezing and water retention. On this basis, hydrogels can be used for human winter sports sensing and low-temperature environmental alarm devices to provide faster rescue. This study provides a novel method for the development of hydrogels with low-temperature stability, and provides a revelation for the application of anti-freezing hydrogels in icy and snowy environments.

Strong MXene Induced Conductive Silk Fibers

Conductive silk fibers (CSFs) are attractive in the field of flexible wearable electronics and textiles, but it still exists a great challenge to simultaneously enhance the electrical conductivity and mechanical properties. Inspired by the core-sheath structure of Bombyx mori silks, a continuous strategy is demonstrated for the fabrication the strong MXene induced conductive silk fibers (MCSFs). The sericin sheath of silk fibers (SFs) is replaced by MXene/sodium alginate (MSA) layer, and the ultrathin sheath layer is tightly bridged with the core through strong interfacial interactions, including hydrogen bonds and electrostatic interactions. Therefore, the MCSFs show extraordinary tensile strength of 1037.9 MPa and outstanding electrical conductivity of 6400 S m-1, which exhibits obvious advantages compared with the previous reported silk fibers modified by other methods. In addition, the MCSFs also have a high toughness of 194.9 MJ m-3 and an ultra-sensitive gauge factor of 2269.3, resulting in their ability to monitor human pulse, body movements, and changes of ambient humidity in real time. The proposed bioinspired strategy for continuously fabricating ultra-strong and tough MCSFs provides an avenue for implementing functionalized silk fibers in next-generation wearable technologies, intelligent textiles, and human-machine interaction etc.

Self-Powered Switchable Gas-Humidity Difunctional Flexible Chemosensors Based on Smart Adaptable Hydrogel

The development of self-powered, flexible, and multi-function sensors is highly anticipated in wearable electronics, however, it remains a daunting challenge to identify different signals based on a single device with singular sensing material without algorithmic support. Here, a smart adaptable hydrogel is developed by co-introducing two ions with vastly different hydrophilicity for the construction of an electrochemically self-powered, flexible, and reversibly switchable difunctional chemosensor with a metal-air battery structure. The prepared hydrogel can readily switch between water-rich and water-deficient states for crosstalk-free detection of oxygen and humidity respectively, since O2 gas and water molecules can directly participate in the oxygen reduction reaction in the device and act alone as limiting reactants and catalysts to affect the reaction rate under different hydrogel states. The resulting sensor demonstrates breakthrough O2 and humidity sensing performance with sensitivities as high as 4170.5%/% and 380.2%/% RH in water-rich and water-deficient states, respectively, and ultrawide detection ranges. Thanks to these, the devices can be applied for real-time and remote monitoring of ambient oxygen, transcutaneous oxygen pressure changes, respiration, and skin moisture by combining with wireless communication technology, and therefore have important application prospects in the fields of safety, health management, and non-contact human-machine interaction.

Thermally Conductive Ti3C2Tx Fibers with Superior Electrical Conductivity

High-performance Ti3C2Tx fibers have garnered significant potential for smart fibers enabled fabrics. Nonetheless, a major challenge hindering their widespread use is the lack of strong interlayer interactions between Ti3C2Tx nanosheets within fibers, which restricts their properties. Herein, a versatile strategy is proposed to construct wet-spun Ti3C2Tx fibers, in which trace amounts of borate form strong interlayer crosslinking between Ti3C2Tx nanosheets to significantly enhance interactions as supported by density functional theory calculations, thereby reducing interlayer spacing, diminishing microscopic voids and promoting orientation of the nanosheets. The resultant Ti3C2Tx fibers exhibit exceptional electrical conductivity of 7781 S cm-1 and mechanical properties, including tensile strength of 188.72 MPa and Young's modulus of 52.42 GPa. Notably, employing equilibrium molecular dynamics simulations, finite element analysis, and cross-wire geometry method, it is revealed that such crosslinking also effectively lowers interfacial thermal resistance and ultimately elevates thermal conductivity of Ti3C2Tx fibers to 13 W m-1 K-1, marking the first systematic study on thermal conductivity of Ti3C2Tx fibers. The simple and efficient interlayer crosslinking enhancement strategy not only enables the construction of thermal conductivity Ti3C2Tx fibers with high electrical conductivity for smart textiles, but also offers a scalable approach for assembling other nanomaterials into multifunctional fibers.

Recent progress in the synthesis of nanostructured Ti3C2T x MXene for energy storage and wastewater treatment: a review

MXene-based functional 2D materials hold significant potential for addressing global challenges related to energy and water crises. Since their discovery in 2011, Ti3C2T x MXenes have demonstrated promising applications due to their unique physicochemical properties and distinctive morphology. Recent advancements have explored innovative strategies to enhance Ti3C2T x into multifunctional materials, enabling applications in gas sensing, electromagnetic interference shielding, supercapacitors, batteries, water purification, and membrane technologies. Unlike previous reviews that primarily focused on the synthesis, properties, and individual applications of MXenes, this work provides a fundamental discussion of their role in wastewater treatment, recent advancements in energy harvesting, and their broader implications. Additionally, this review offers a comparative analysis of MXene-based systems with other state-of-the-art materials, providing new insights into their future development and potential applications.

Comparative electromagnetic shielding performance of Ti3C2Tx-PVA composites in various structural forms: compact films, hydrogels, and aerogels

The structural design of light-weight MXene-polymer composites has attracted significant interest for enhancing both electromagnetic interference (EMI) shielding performance and mechanical strength, which are critical for practical applications. However, a systematic understanding of how various structural configurations of MXene composites affect EMI shielding is lacking. In this study, light-weight Ti3C2Tx-PVA composites were fabricated in three structural forms, hydrogel, aerogel, and compact film, while varying the Ti3C2Tx areal density (14 to 20 mg cm-2) to elucidate the role of structural design in X-band EMI shielding and mechanical properties. The EMI shielding performance depends on the structural configuration and areal density of the MXene in Ti3C2Tx-PVA composites. The shielding effectiveness increases with increasing Ti3C2Tx content in each configuration. At a fixed Ti3C2Tx areal density of 0.02 g cm-2, the Ti3C2Tx-PVA hydrogel demonstrated the highest shielding effectiveness (SE = 70 dB at 10 GHz), attributed to strong dipole polarization and efficient ionic conduction behavior, followed by the compact film (40 dB) and then the aerogel (21 dB). Notably, the aerogel achieved the highest absorption coefficient (A = 0.89) due to the improved impedance matching and pronounced internal reflections, whereas the hydrogel and compact film exhibited reflection-dominated shielding. Furthermore, the incorporation of PVA polymer molecules into Ti3C2Tx MXenes significantly enhanced their mechanical properties across all configurations: the hydrogel achieved high stretchability (636%), the aerogel displayed superior compressive strength (0.215 MPa), and the compact film reached a tensile strength of 56 MPa, each surpassing the performance of its pristine Ti3C2Tx MXene counterpart. Overall, tailoring the structural configuration into a hydrogel, aerogel, or compact film offers versatile routes for optimizing both EMI attenuation and mechanical performance of MXene-polymer composites.

Multifunctional Hydrogel Electronics for Synergistic Therapy and Visual Monitoring in Wound Healing

To overcome the limitations of precise monitoring and inefficient wound exudate management in wound healing, an advanced multifunctional hydrogel electronics (MHE) platform based on MXene@MOF/Fe3O4@C photonic crystal hydrogel is developed. This platform combines optical/electrical sensing, synergistic therapy, and real-time visual monitoring into a single, efficient system, offering a comprehensive solution for wound healing. Under photothermal stimulation, the hydrogel releases metal ions that generate hydroxyl radicals, effectively eliminating antibiotic-resistant bacteria. Beyond its antibacterial efficacy, this system offers unprecedented real-time monitoring through temperature-responsive visualization, while structural color changes upon wound exudate absorption provide a clear indication for dressing replacement. By integrating these functionalities, MHE platform allows for precise control of the therapeutic process, significantly improving wound healing and treatment monitoring. The platform's optical/electrical sensing capabilities further broaden its potential applications across other biomedical fields. This breakthrough technology provides clinicians with a powerful tool to optimize therapeutic outcomes, marking a major advancement in wound care and biomedical applications.

Low-temperature, low-pressure Zn-ion hybrid supercapacitor in extreme near-space application

The development and utilization of airspace, especially near-space particularly rely on power units with superior tolerance in low-temperature and low-pressure environments to output a stable energy supply. Here we propose a strategy towards low-temperature, low-pressure Zn-ion hybrid supercapacitor based on a weakly hydrogen-bonded electrolyte and a hyacinth-shaped Ti2CTx MXene@CC cathode with hierarchical bridge-linked structure, which synergistically reduces the internal resistance of the device and enables the assembled supercapacitor showing a good low-temperature resistance while combining low-gas-voltage safety. The ACN additive weakens the hydrogen bond between water molecules and reshapes the solvation structure of Zn2+, thus reducing the ion transfer resistance and achieving a reversible Zn/Zn2+ chemical reaction. The bridge-linked hierarchical structure of the hyacinth-shaped Ti2CTx MXene@CC cathode provides a rich conductive network and optimizes the ion diffusion path, which reduces the ion diffusion resistance. At -40 °C, the assembled device can still achieve an area specific capacitance of 64.0 mF cm-2 at a scan rate of 500 mV s-1, and long-term stability after 20 000 cycles at a current density of 20 mA cm-2. An integrated temperature and pressure sensing system driven by the supercapacitor successfully realizes the monitoring of atmospheric indicators in extreme environments, providing new ideas for auxiliary power units in airspace and near-space.

Dual-Terminal Ion-Modulation Multiplier-Based Ion-Doped Stacked Semiconducting Nanosheets for Multifunctional Biomedical Applications

Stacked semiconducting nanosheets (SSNs), which feature strong in-plane covalent bonds but weak van der Waals (vdWs) interactions between adjacent layers, hold substantial promise in next-generation, printable, and flexible devices. Among them, SSN-based transistors with high current multiplication offer significant potential for large-area, high-integration electronics and biomedical applications. However, the three-terminal configuration of the transistor inevitably increases the process step and power unit. Here, we demonstrate a dual-terminal ion modulation multiplier (IMM) based on ion-doped SSNs, which was obtained through a solution-processed and cost-effective method. We observed an ion-induced self-multiplication effect occurring in the IMM, which significantly enhanced the sensing performance, particularly in thermal sensing. The IMM thermal sensor exhibited a high resolution of 0.02 K and ultrahigh sensitivity of ∼27%/K, more than 7 times higher than that of ion-type thermal sensors. By combining the enhanced operational stability of IMMs, we successfully developed a dual-channel stretchable respiratory sensor (dSRS) based on IMMs, capable of real-time monitoring of subnasal respiratory signals. The dSRS effectively distinguished normal, rapid, and deep breathing states while accurately detecting abnormal respiration, including apnea and hypopnea. Utilizing the unique properties of IMMs, we developed a monolithically integrated and high-performance IMM glucose sensor with temperature compensation. This IMM glucose sensor demonstrated a high sensitivity of 0.91%/μM, a low detection limit of 100 nM, and a high detection accuracy under temperature interference. Our results clearly demonstrate that IMM devices endow SSNs with promising electrical and sensing capabilities, paving the way for next-generation electronics in the post-Moore era.

Ink formulation of functional nanowires with hyperbranched stabilizers for versatile printing of flexible electronics

Functional nanowire ink formulations require elaborate control over their composition, rheological properties, and fluidic properties to optimize their printing processes. They also require harsh post-fabrication treatments to maximize the performance of the resulting printed flexible devices, making it challenging to uniformly deposit nanowire-based architectures and ensure device reproducibility and scalability. Here, we propose a strategy for developing silver nanowire (AgNW) ink formulations, where hyperbranched molecules (HPMs) are employed as both dispersant and stabilizer for nanowires. The three-dimensional architecture with functional groups on the periphery of HPMs enables the preparation of thixotropic HPMs-AgNW inks with solid contents of up to 20 wt.% in both aqueous and organic solvents using a low amount of HPMs (AgNW and HPMs weight ratio = 1:0.001). The HPMs-AgNW inks can be printed into patterns with a resolution of 20 μm on various flexible substrates without needing harsh post-treatments. We obtain bar-coated transparent electrodes (sheet resistance of 17.1 Ω sq-1 at 94.7% transmittance), slot-die-coated flexible conductive patterns, screen-printed conductive lines (conductivity exceeding 6.2 × 104 S cm-1), and 3D printed stretchable wires. Importantly, this HPMs-stabilized formulation strategy is general for various functional nanowires, enabling the integration of a diverse set of nanowire-based wearable electronic systems.

Thermal-Sensitive Artificial Ionic Skin with Environmental Stability and Self-Healing Property

Wearable temperature-sensitive electronic skin enables robots to rapidly detect environmental changes and respond intelligently, thereby reducing temperature-related mechanical failures. Additionally, this temperature-sensitive skin can measure and record the temperature of external objects, broadening its potential applications in the medical field. In this study, we designed a thermally sensitive artificial ionic skin using ionic liquids (ILs) as solvents and carbon nanotubes (CNTs) as thermally conductive fillers. The incorporation of ILs into the polymer network enhances thermal stability, while the CNTs establish dual thermal conduction pathways (CNTs-CNTs and CNTs-polymer chain segments), leading to rapid thermal response times of only 16 s. The initiation of IL dissociation at elevated temperatures boosts carrier density, resulting in a substantial improvement in thermal sensitivity (5%/°C). Furthermore, the skin displays remarkable self-healing properties (90%), thereby extending the lifespan of the skin in practical applications. This kind of skin can stably sense the wearer's body temperature and environmental temperature and provide an ideal temperature-sensitive and long-term stable new functional material for the development of human skin such as robots.

Dual-Mode Textile Sensor Based on PEDOT:PSS/SWCNTs Composites for Pressure-Temperature Detection

As an innovative branch of electronics, intelligent electronic textiles (e-textiles) have broad prospects in applications such as e-skin, human-computer interaction, and smart homes. However, it is still a challenge to distinguish multiple stimuli in the same e-textile. Herein, we propose a dual-parameter smart e-textile that can detect human pulse and body temperature in real time, with high performance and no signal interference. The doping of SWCNTs in PEDOT:PSS improves the electrical conductivity and Seebeck coefficient of the prepared composites, which results in excellent pressure and temperature-sensing properties of the PEDOT:PSS/SWCNTs/CS@PET-textile (PSCP) sensor. The dual-mode sensor has high sensitivity (32.4 kPa-1), fast response time (~21 ms), and excellent durability (>2000 times) in pressure detection. Concurrently, this sensor maintains a high Seebeck coefficient of 25 μV/K in the 0-120 K temperature range with a tremendous linear relationship. Based on impressive dual-mode sensing characteristics and independent temperature-difference- and pressure-sensing mechanisms, smart e-textile sensors realize the real-time simultaneous monitoring of weak pulse signals and human body temperature, showing great potential in medical healthcare. In addition, the potential energy is excited by the temperature gradient between the human skin and the environment, which provides a novel idea for wearable self-powered devices.

Self-Healing and Antifreezing/Antidrying Conductive Eutectohydrogel-Based Biosignal Monitoring Multisensors with Integrated Supercapacitor

A novel self-healing and antifreezing/antidrying conductive eutectohydrogel, ideal for wearable multifunctional sensors and supercapacitors, is reported. Conductive eutectohydrogel with self-healing and facilely tunable mechanical performance is obtained by incorporation of trehalose and phytic acid as reversible cross-linkers into a polyacrylamide network, forming the dynamic hydrogen bonding and electrostatic interactions. Furthermore, combined use of deep eutectic solvent with water ensures the air stability as well as the antifreezing/antidrying characteristics. The synthesized eutectohydrogel exhibits a self-healing efficiency of 90.7% after 24 h at room temperature, Young's modulus of 140.9 kPa, and strain at break of 352.8%. With the eutectohydrogel as a versatile platform, self-healing strain and temperature sensors, electrocardiogram electrodes, and supercapacitor are fabricated, recovering the device performance after self-healing from complete bisection and exhibiting stable performance over a wide temperature range from -20 to 50 °C. With a vertically integrated patch device of supercapacitor and strain sensor attached onto skin, various body movements are successfully detected using the energy stored in the supercapacitor, without performance degradation even after self-healing from complete bisection of the full patch device. This work demonstrates high potential application of the synthesized eutectohydrogel to flexible wearable devices featuring durability and longevity.

Functional Hydrogels for Aqueous Zinc-Based Batteries: Progress and Perspectives

Aqueous zinc batteries (AZBs) hold great potential for green grid-scale energy storage due to their affordability, resource abundance, safety, and environmental friendliness. However, their practical deployment is hindered by challenges related to the electrode, electrolyte, and interface. Functional hydrogels offer a promising solution to address such challenges owing to their broad electrochemical window, tunable structures, and pressure-responsive mechanical properties. In this review, the key properties that functional hydrogels must possess for advancing AZBs, including mechanical strength, ionic conductivity, swelling behavior, and degradability, from a perspective of the full life cycle of hydrogels in AZBs are summarized. Current modification strategies aimed at enhancing these properties and improving AZB performance are also explored. The challenges and design considerations for integrating functional hydrogels with electrodes and interface are discussed. In the end, the limitations and future directions for hydrogels to bridge the gap between academia and industries for the successful deployment of AZBs are discussed.

3D and 4D printing of MXene-based composites: from fundamentals to emerging applications

The advent of three-dimensional (3D) and four-dimensional (4D) printing technologies has significantly improved the fabrication of advanced materials, with MXene-based composites emerging as a particularly promising class due to their exceptional electrical, mechanical, and chemical properties. This review explores the fundamentals of MXenes and their composites, examining their unique characteristics and the underlying principles of their synthesis and processing. We highlight the transformative potential of 3D and 4D printing techniques in tailoring MXene-based materials for a wide array of applications. In the field of tissue regeneration, MXene composites offer enhanced biocompatibility and mechanical strength, making them ideal for scaffolds and implants. For drug delivery, the high surface area and tunable surface chemistry of MXenes enable precise control over drug release profiles. In energy storage, MXene-based electrodes exhibit superior conductivity and capacity, paving the way for next-generation batteries and supercapacitors. Additionally, the sensitivity and selectivity of MXene composites make them excellent candidates for various (bio)sensing applications, from environmental monitoring to biomedical diagnostics. By integrating the dynamic capabilities of 4D printing, which introduces time-dependent shape transformations, MXene-based composites can further adapt to complex and evolving functional requirements. This review provides a comprehensive overview of the current state of research, identifies key challenges, and discusses future directions for the development and application of 3D and 4D printed MXene-based composites. Through this exploration, we aim to underscore the significant impact of these advanced materials and technologies on diverse scientific and industrial fields.

Engineering robust and transparent dual-crosslinked hydrogels for multimodal sensing without conductive additives

Conductive hydrogels are pivotal for the advancement of flexible sensors, electronic skin, and healthcare monitoring systems, facilitating transformative innovations. However, issues such as inadequate intrinsic compatibility, mismatched mechanical properties, and limited stability curtail their potential, resulting in compromised device efficacy and performance degradation. In this research, we engineered functional hydrogels featuring a dual-crosslinked network composed of (PA/PVA)-P(AM-AA) to address these challenges. This design eliminates the need for conductive additives, thereby enhancing intrinsic compatibility. Notably, the hydrogels exhibit exceptional mechanical properties, with high tensile strength (∼700 %), Young's modulus (∼5.33 MPa), increased strength (∼2.46 MPa) and toughness (∼6.59 MJ m-3). They also achieve a compressive strength of ∼7.33 MPa at 80 % maximal compressive strain and maintain about 89 % transparency. Moreover, flexible sensors derived from these hydrogels demonstrate enhanced multimodal sensing capabilities, including temperature, strain, and pressure detection, enabling precise monitoring of human movements. The integration of multiple hydrogels into a three-dimensional sensor array facilitates detailed spatial pressure distribution mapping. By strategically applying dual-crosslinked network engineering and eliminating conductive additives, we have streamlined the design and manufacturing of hydrogels to meet the rising demand for high-performance wearable sensors.

Stretchable Unsymmetrical Piezoelectric BiO2-x Deposited-Hydrogel as Multimodal Triboelectric Nanogenerators for Biomechanical Motion Harvesting

Enhancing the output performance of triboelectric nanogenerators (TENGs) is essential for increasing their application in smart devices. Oxygen-vacancy-rich BiO2-x nanosheets (BiO2-x NSs) are advanced-engineered nanomaterials with excellent piezoelectric properties. Herein, a stretchable unsymmetrical BiO2-x NSs deposited-hydrogel made of polyacrylamide (PAM) as a multimodal TENG is rationally fabricated, and the performance of TENG can be tailored by controlling the BiO2-x NSs deposition amount and spatial distribution. The alteration of resistance caused by the Poisson effect of PAM/BiO2-x composite hydrogel (H-BiO2-x) can be used as a piezoresistive sensor, and the piezoelectricity of BiO2-x NSs can effectively enhance the density of transfer charge, thus improving the output performance of the H-BiO2-x-based TENG. In addition, the chemical cross-linking between the BiO2-x NSs and the PAM polymer chain allows the hydrogel electrode to have a higher tensile capacity (867%). Used for biomechanical motion signal detection, the sensors made of H-BiO2-x have high sensitivity (gauge factor = 6.93) and can discriminate a range of forces (0.1-5.0 N) at low frequencies (0.5-2.0 Hz). Finally, the prepared TENG can collect biological energy and convert it into electricity. Consequently, the improved TENG shows a good application prospect as multimodal biomechanical sensors by combining piezoresistive, piezoelectric, and triboelectric effects.

Bioinspired Ultrasensitive Flexible Strain Sensors for Real-Time Wireless Detection of Liquid Leakage

Liquid leakage of pipeline networks not only results in considerable resource wastage but also leads to environmental pollution and ecological imbalance. In response to this global issue, a bioinspired superhydrophobic thermoplastic polyurethane/carbon nanotubes/graphene nanosheets flexible strain sensor (TCGS) has been developed using a combination of micro-extrusion compression molding and surface modification for real-time wireless detection of liquid leakage. The TCGS utilizes the synergistic effects of Archimedean spiral crack arrays and micropores, which are inspired by the remarkable sensory capabilities of scorpions. This design achieves a sensitivity of 218.13 at a strain of 2%, which is an increase of 4300%. Additionally, it demonstrates exceptional durability by withstanding over 5000 usage cycles. The robust superhydrophobicity of the TCGS significantly enhances sensitivity and stability in detecting small-scale liquid leakage, enabling precise monitoring of liquid leakage across a wide range of sizes, velocities, and compositions while issuing prompt alerts. This provides critical early warnings for both industrial pipelines and potential liquid leakage scenarios in everyday life. The development and utilization of bioinspired ultrasensitive flexible strain sensors offer an innovative and effective solution for the early wireless detection of liquid leakage.

Overview of 3D Printing Multimodal Flexible Sensors

With the growing demand for flexible sensing systems and precision engineering, there is an increasing need for sensors that can accurately measure and analyze multimode signals. 3D printing technology has emerged as a crucial tool in the development of multimodal flexible sensors due to its advantages in design flexibility and manufacturing complex structures. This paper provides a review of recent advancements in 3D printing technology within the field of multimode flexible sensors, with particular emphasis on the relevant working mechanisms involved in decoupling complex signals. First, the research status of 3D printed multimodal flexible sensors is discussed, including their responsiveness to different modal stimuli such as mechanics, temperature, and gas. Furthermore, it explores methods for decoupling multimodal signals through structural and material design, artificial intelligence, and other technologies. Finally, this paper summarizes current challenges such as limited material selection, difficulties in miniaturization integration, and crosstalk between multisignal outputs. It also looks forward to future research directions in this area.

Flexible Temperature Sensor with High Reproducibility and Wireless Closed-Loop System for Decoupled Multimodal Health Monitoring and Personalized Thermoregulation

Temperature and pulse waves are two fundamental indicators of body health. Specifically, thermoresistive flexible temperature sensors are one of the most applied sensors. However, they suffer from poor reproducibility of resistivity; and decoupling temperature from pressure/strain is still challenging. Besides, autonomous thermoregulation by wearable sensory systems is in high demand, but conventional commercial apparatuses are cumbersome and not suitable for long-term portable use. Here, a material-design strategy is developed to overcome the problem of poor reproducibility of resistivity by tuning the thermal expansion coefficient to nearly zero, precluding the detriment caused by shape expansion/shrinkage with temperature variation and achieving high reproducibility. The strategy also obtains more reliable sensitivity and higher stability, and the designed thermoresistive fiber has strain-insensitive sensing performance and fast response/recovery time. A smart textile woven by the thermoresistive fiber can decouple temperature and pulse without crosstalk; and a flexible wireless closed-loop system comprising the smart textile, a heating textile, a flexible diminutive control patch, and a smartphone is designed and constructed to monitor health status in real-time and autonomously regulate body temperature. This work offers a new route to circumvent temperature-sensitive effects for flexible sensors and new insights for personalized thermoregulation.

Harnessing chemical functionality of xylan hemicellulose towards carbohydrate polymer-based pH/magnetic dual-responsive nanocomposite hydrogel for drug delivery

This study reports a pH/magnetic dual-responsive hemicellulose-based nanocomposite hydrogel with nearly 100 % carbohydrate polymer-based and biodegradable polymer compositions for drug delivery. We synthesized pure Fe3O4 magnetic nanoparticles (Fe3O4 MNPs) using a co-precipitation method, then engineering xylan hemicellulose (XH), acrylic acid, poly(ethylene glycol) diacrylate, and Fe3O4 to synthesize the pH/magnetic dual-responsive hydrogel (Fe3O4@XH-Gel), through graft polymerization on XH with in-situ doping Fe3O4 MNPs initiated by the ammonium persulfate/tetramethylethylenediamine redox system. Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (1H NMR), X-ray diffractometry (XRD), scanning electron microscopy and energy dispersive spectrometer (SEM-EDS), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET), swelling gravimetric analysis, vibrating sample magnetometer (VSM) were employed to analyze the hydrogel's chemical structures, morphologies, pH-responsive behaviors, and magnetic responsiveness characteristics, mechanical and rheological properties, as well as cytotoxicity and biodegradability. The results indicate that the Fe3O4@XH-Gel exhibited excellent dual responsiveness to pH and magnetism. Furthermore, an emphasis was placed on the in-depth analysis of the pH response mechanism. Finally, we utilized this cutting-edge hydrogel to investigate the controlled-release behavior of two model drugs, Acetylsalicylic acid and Theophylline. The hydrogel demonstrated exceptional controlled release attributes, positioning it as a potential carrier for targeted drug delivery, particularly to the gastrointestinal conditions.

PEDOT:PSS-Based Wearable Flexible Temperature Sensor and Integrated Sensing Matrix for Human Body Monitoring

Flexible temperature sensors have been widely used in electronic skins and health monitoring. Body temperature as one of the key physiological signals is crucial for detecting human body's abnormalities, which necessitates high sensitivity, quick responsiveness, and stable monitoring. In this paper, we reported a resistive temperature sensor designed as an ultrathin laminated structure with a serpentine pattern and a bioinspired adhesive layer, which was fabricated with a composite of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/single-wall carbon nanotubes/reduced graphene oxide (PEDOT:PSS/SWCNTs/rGO) and polydimethylsiloxane (PDMS). The temperature sensor exhibited a high temperature sensitivity of 0.63% °C-1, coupled with outstanding linearity of 0.98 within 25-45 °C. Furthermore, it showed fast response and recovery speeds of 4.8 and 5.8 s, respectively, between 25 and 36 °C. It also demonstrated exceptional stability when subjected to stress and bending disturbances with the maximum bending interference deviation of 0.03%. Additionally, it displayed good cyclic stability over a broad temperature range from 25 to 85 °C, and the standard deviation at 25 °C is 0.14%. A series of experiments including blowing detection, respiratory monitoring with or without a mask, and during rest or sleep were conducted to show the potential of the flexible temperature sensors in human body monitoring. Furthermore, a 4 × 4 flexible temperature sensor matrix was integrated to detect and map objects such as wrenches and blood vessels through human hand skin. The results were consistent with those of infrared measurements. The flexible temperature sensor is capable of real-time temperature monitoring and has the potential in tracking human respiration, assessing sleep quality, and mapping the temperature of various objects.

3D printed multi-coupled bioinspired skin-electronic interfaces with enhanced adhesion for monitoring and treatment

Skin-electronic interfaces have broad applications in fields such as diagnostics, therapy, health monitoring, and smart wearables. However, they face various challenges in practical use. For instance, in wet environments, the cohesion of the material may be compromised, and under dynamic conditions, maintaining conformal adhesion becomes difficult, leading to reduced sensitivity and fidelity of electrical signal transmission. The key scientific issue lies in forming a stable and tight mechanical-electronic coupling at the tissue-electronic interface. Here, inspired by octopus sucker structures and snail mucus, we propose a strategy for hydrogel skin-electronic interfaces based on multi-coupled bioinspired adhesion and introduce an ultrasound (US)-mediated interfacial toughness enhancement mechanism. Ultimately, using digital light processing micro-nano additive manufacturing technology (DLP 3D), we have developed a multifunctional, diagnostic-therapeutic integrated patch (PAMS). This patch exhibits moderate water swelling properties, a maximum deformation of up to 460%, high sensitivity (GF = 4.73), and tough and controllable bioadhesion (shear strength increased by 109.29%). Apart from outstanding mechanical and electronic properties, the patch also demonstrates good biocompatibility, anti-bacterial properties, photothermal properties, and resistance to freezing at -20 °C. Experimental results show that this skin-electronic interface can sensitively monitor temperature, motion, and electrocardiogram signals. Utilizing a rat frostbite model, we have demonstrated that this skin-electronic interface can effectively accelerate the wound healing process as a wound patch. This research offers a promising strategy for improving the performance of bioelectronic devices, sensor-based educational reforms and personalized diagnostics and therapeutics in the future. STATEMENT OF SIGNIFICANCE: Establishing stable and tight mechanical-electronic coupling at the tissue-electronic interface is essential for the diverse applications of bioelectronic devices. This study aims to develop a multifunctional, diagnostic-therapeutic integrated hydrogel skin-electronic interface patch with enhanced interfacial toughness. The patch is based on a multi-coupled bioinspired adhesive-enhanced mechanism, allowing for personalized 3D printing customization. It can be used as a high-performance diagnostic-therapeutic sensor and effectively promote frostbite wound healing. We anticipate that this research will provide new insights for constructing the next generation of multifunctional integrated high-performance bioelectronic interfaces.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Facile Formation of Multifunctional Biomimetic Hydrogel Fibers for Sensing Applications

To face the challenges in preparing hydrogel fibers with complex structures and functions, this study utilized a microfluidic coaxial co-extrusion technique to successfully form functional hydrogel fibers through rapid ionic crosslinking. Functional hydrogel fibers with complex structures, including linear fibers, core-shell structure fibers, embedded helical channels, hollow tubes, and necklaces, were generated by adjusting the composition of internal and external phases. The characteristic parameters of the hydrogel fibers (inner and outer diameter, helix generation position, pitch, etc.) were achieved by adjusting the flow rate of the internal and external phases. As biocompatible materials, hydrogel fibers were endowed with electrical conductivity, temperature sensitivity, mechanical enhancement, and freeze resistance, allowing for their use as temperature sensors for human respiratory monitoring and other biomimetic application developments. The hydrogel fibers had a conductivity of up to 22.71 S/m, a response time to respiration of 37 ms, a recovery time of 1.956 s, and could improve the strength of respiration; the tensile strength at break up to 8.081 MPa, elongation at break up to 159%, and temperature coefficient of resistance (TCR) up to -13.080% °C-1 were better than the existing related research.

High-strength, anti-fatigue, cellulose nanofiber reinforced polyvinyl alcohol based ionic conductive hydrogels for flexible strain/pressure sensors and triboelectric nanogenerators

Ionic conductive hydrogels (ICHs) have attracted great attention because of their excellent biocompatibility and structural similarity with biological tissues. However, it is still a huge challenge to prepare a high strength, conductivity and durability hydrogel-based flexible sensor with dual network structure through a simple and environmentally friendly method. In this work, a simple one-pot cycle freezing thawing method was proposed to prepare ICHs by dissolving polyvinyl alcohol (PVA) and ferric chloride (FeCl3) in cellulose nanofiber (CNF) aqueous dispersion. A dual cross-linked network was established in hydrogel through the hydrogen bonds and coordination bonds among PVA, CNF, and FeCl3. This structure endows the as-prepared hydrogel with high sensitivity (pressure sensitivity coefficient (S) = 5.326 in the pressure range of 0-5 kPa), wide response range (4511 kPa), excellent durability (over 3000 cycles), short response time (83 ms) and recovery time (117 ms), which can accurately detect various human activities in real time. Furthermore, the triboelectric nano-generator (TENG) made from PVA@CNF-FeCl3 hydrogel can not only supply power for commercial capacitors and LED lamps, but also be used as a self-powered sensor to detect human motion. This work provides a new approach for the development of the next generation of flexible wearable electronic devices.

Recent Advances in Flexible Temperature Sensors: Materials, Mechanism, Fabrication, and Applications

Flexible electronics is an emerging and cutting-edge technology which is considered as the building blocks of the next generation micro-nano electronics. Flexible electronics integrate both active and passive functions in devices, driving rapid developments in healthcare, the Internet of Things (IoT), and industrial fields. Among them, flexible temperature sensors, which can be directly attached to human skin or curved surfaces of objects for continuous and stable temperature measurement, have attracted much attention for applications in disease prediction, health monitoring, robotic signal sensing, and curved surface temperature measurement. Preparing flexible temperature sensors with high sensitivity, fast response, wide temperature measurement interval, high flexibility, stretchability, low cost, high reliability, and stability has become a research target. This article reviewed the latest development of flexible temperature sensors and mainly discusses the sensitive materials, working mechanism, preparation process, and the applications of flexible temperature sensors. Finally, conclusions based on the latest developments, and the challenges and prospects for research in this field are presented.

MXene-Based Pressure Sensor with a Self-Healing Property for Joule Heating and Friction Sliding

As a kind of flexible electronic device, flexible pressure sensor has attracted wide attention in medical monitoring and human-machine interaction. With the continuous deepening of research, high-sensitivity sensor is developing from single function to multi-function. However, Current multifunctional sensors lack the ability to integrate joule heating, detect sliding friction, and self-healing. Herein, a MXene/polyurethane (PU) flexible pressure sensor with a self-healing property for joule heating and friction sliding is fabricated. The MXene/PU sensitive layer with special spinosum structure is prepared by a simple spraying method. After face-to-face assembly of the sensitive layers, the MXene/PU flexible pressure sensor is obtained and showed excellent sensitivity (150.65 kPa-1), fast response/recovery speed (75.5/63.9 ms), and good stability (10 000 cycles). Based on the self-healing property of PU, the sensor also has the ability to heal after mechanical damage. In addition, the sensor realizes the joule heating function under low voltage, and has the real-time monitoring ability of sliding objects. Combined with low cost and simple manufacturing method, the multi-functional MXene/PU flexible sensor shows a wide range of application potential in human activity monitoring, thermal management, and slip recognition.

Neuroprosthetic contact lens enabled sensorimotor system for point-of-care monitoring and feedback of intraocular pressure

The wearable contact lens that continuously monitors intraocular pressure (IOP) facilitates prompt and early-state medical treatments of oculopathies such as glaucoma, postoperative myopia, etc. However, either taking drugs for pre-treatment or delaying the treatment process in the absence of a neural feedback component cannot realize accurate diagnosis or effective treatment. Herein, a neuroprosthetic contact lens enabled sensorimotor system is reported, which consists of a smart contact lens with Ti3C2Tx Wheatstone bridge structured IOP strain sensor, a Ti3C2Tx temperature sensor and an IOP point-of-care monitoring/display system. The point-of-care IOP monitoring and warning can be realized due to the high sensitivity of 12.52 mV mmHg-1 of the neuroprosthetic contact lens. In vivo experiments on rabbit eyes demonstrate the excellent wearability and biocompatibility of the neuroprosthetic contact lens. Further experiments on a living rate in vitro successfully mimic the biological sensorimotor loop. The leg twitching (larger or smaller angles) of the living rat was demonstrated under the command of motor cortex controlled by somatosensory cortex when the IOP is away from the normal range (higher or lower).

Microneedle Patches-Integrated Transdermal Bioelectronics for Minimally Invasive Disease Theranostics

Wearable epidermal electronics with non- or minimally-invasive characteristics can collect, transduce, communicate, and interact with accessible physicochemical health indicators on the skin. However, due to the stratum corneum layer, rich information about body health is buried under the skin stratum corneum layer, for example, in the skin interstitial fluid. Microneedle patches are typically designed with arrays of special microsized needles of length within 1000 µm. Such characteristics potentially enable the access and sample of biomolecules under the skin or give therapeutical treatment painlessly and transdermally. Integrating microneedle patches with various electronics allows highly efficient transdermal bioelectronics, showing their great promise for biomedical and healthcare applications. This comprehensive review summarizes and highlights the recent progress on integrated transdermal bioelectronics based on microneedle patches. The design criteria and state-of-the-art fabrication techniques for such devices are initially discussed. Next, devices with different functions, including but not limited to health monitoring, drug delivery, and therapeutical treatment, are highlighted in detail. Finally, key issues associated with current technologies and future opportunities are elaborated to sort out the state of recent research, point out potential bottlenecks, and provide future research directions.

MXene-Stabilized VS2 Nanostructures for High-Performance Aqueous Zinc Ion Storage

Aqueous zinc-ion batteries (AZIBs) based on vanadium oxides or sulfides are promising candidates for large-scale rechargeable energy storage due to their ease of fabrication, low cost, and high safety. However, the commercial application of vanadium-based electrode materials has been hindered by challenging problems such as poor cyclability and low-rate performance. To this regard, sophisticated nanostructure engineering technology is used to adeptly incorporate VS2 nanosheets into the MXene interlayers to create a stable 2D heterogeneous layered structure. The MXene nanosheets exhibit stable interactions with VS2 nanosheets, while intercalation between nanosheets effectively increases the interlayer spacing, further enhancing their stability in AZIBs. Benefiting from the heterogeneous layered structure with high conductivity, excellent electron/ion transport, and abundant reactive sites, the free-standing VS2/Ti3C2Tz composite film can be used as both the cathode and the anode of AZIBs. Specifically, the VS2/Ti3C2Tz cathode presents a high specific capacity of 285 mAh g-1 at 0.2 A g-1. Furthermore, the flexible Zn-metal free in-plane VS2/Ti3C2Tz//MnO2/CNT AZIBs deliver high operation voltage (2.0 V) and impressive long-term cycling stability (with a capacity retention of 97% after 5000 cycles) which outperforms almost all reported Vanadium-based electrodes for AZIBs. The effective modulation of the material structure through nanocomposite engineering effectively enhances the stability of VS2, which shows great potential in Zn2+ storage. This work will hasten and stimulate further development of such composite material in the direction of energy storage.

Ultrastretchable Segmented Sensors for Functional Human-Machine Interfaces

The key feature that enables soft sensors to shorten the performance gap between robots and biological structures is their deformability, coupled with their capability to measure physical changes. Robots equipped with these sensors can interact safely and proprioceptively with their environments. This has sparked interest in developing novel sensors with high stretchability for application in human-robot interactions. This study presents a novel ultrasoft optoelectronic segmented sensor design capable of measuring strains exceeding 500%. The sensor features an ultrastretchable segment physically joined with an asymmetrically configured soft proprioceptive segment. This configuration enables it to measure high strain and to detect both the magnitude and direction of bending. Although the sensor cannot decouple these types of deformations, it can sense prescribed motions that combine stretching and bending. The proposed sensor was applied to a highly deformable scissor mechanism and a human-robot interface (HRI) device for a robotic arm, capable of quantifying parameters in complex interactions. The results from the experiments also demonstrate the potential of the proposed segmented sensor concept when used in tandem with machine learning, affording new dimensions of proprioception to robots during multilayered interactions with humans.

Self-adhesive, conductive, and multifunctional hybrid hydrogel for flexible/wearable electronics based on triboelectric and piezoresistive sensor

Flexible electronics are highly developed nowadays in human-machine interfaces (HMI). However, challenges such as lack of flexibility, conductivity, and versatility always greatly hindered flexible electronics applications. In this work, a multifunctional hybrid hydrogel (H-hydrogel) was prepared by combining two kinds of 1D polymer chains (polyacrylamide and polydopamine) and two kinds of 2D nanosheets (Ti3C2Tx MXene and graphene oxide nanosheets) as quadruple crosslinkers. The introduced Ti3C2Tx MXene and graphene oxide nanosheets are bonded with the PAM and PDA polymer chains by hydrogen bonds. This unique crosslinking and stable structure endow the H-hydrogel with advantages such as good flexibility, electrical conductivity, self-adhesion, and mechanical robustness. The two kinds of nanosheets not only improved the mechanical strength and conductivity of the H-hydrogel, but also helped to form the double electric layers (DELs) between the nanosheets and the bulk-free water phase inside the H-hydrogel. When utilized as the electrode of a triboelectric nanogenerator (TENG), high electrical output performances were realized due to the dynamic balance of the DELs between the nanosheets and the H-hydrogel's inside water molecules. Moreover, flexible sensors, including triboelectric, and strain/pressure sensors, were achieved for human motion detection at low frequencies. This hydrogel is promising for HMI and e-skin.

Robust Fiber-Shaped Flexible Temperature Sensors for Safety Monitoring with Ultrahigh Sensitivity

Flexible temperature sensors capable of detecting and transmitting temperature data from the human body, environment, and electronic devices hold significant potential for applications in electronic skins, human-machine interactions, and disaster prevention systems. Nonetheless, fabricating flexible temperature sensors with exceptional sensing performance remains a formidable task, primarily due to the intricate process of constructing an intrinsically flexible sensing element with high sensitivity. In this study, a facile in situ two-step synthetic method is introduced for fabricating flexible fiber-shaped NiO/carbon nanotube fiber (CNTF) composites. The resulting NiO/CNTF flexible temperature sensors demonstrate outstanding deformability and temperature sensing characteristics, encompassing a broad working range (-15 to 60 °C) and high sensitivity (maximum TCR of -20.2% °C-1 and B value of 3332 K). Importantly, the mechanical and thermal behaviors of the sensor in various application conditions are thoroughly examined using finite element analysis simulations. Moreover, the temperature sensors can effectively capture diverse thermal signals in wearable applications. Notably, a temperature monitoring and warning system is developed to prevent fire accidents resulting from abnormal thermal runaway in electronic devices.

Multiple physical crosslinked highly adhesive and conductive hydrogels for human motion and electrophysiological signal monitoring

Hydrogel-based flexible electronic devices serve as a next-generation bridge for human-machine interaction and find extensive applications in clinical therapy, military equipment, and wearable devices. However, the mechanical mismatch between hydrogels and human tissues, coupled with the failure of conformal interfaces, hinders the transmission of information between living organisms and flexible devices, which resulted in the instability and low fidelity of signals, especially in the acquisition of electromyographic (EMG) and electrocardiographic (ECG) signals. In this study, we designed an ion-conductive hydrogel (ICHgel) utilizing multiple physical interactions, successfully applied for human motion monitoring and the collection of epidermal physiological signals. By incorporating fumed silica (F-SiO2) nanoparticles and calcium chloride into an interpenetrating network (IPN) composed of polyvinyl alcohol (PVA) and polyacrylamide (AAm)/acrylic acid (AA) chains, the ICHgel exhibited exceptional tunable stretchability (>1450% strain) and conductivity (10.58 ± 0.85 S m-1). Additionally, the outstanding adhesion of the ICHgel proved to be a critical factor for effective communication between epidermal tissues and flexible devices. Demonstrating its capability to acquire stable electromechanical signals, the ICHgel was attached to different parts of the human body. More importantly, as a flexible electrode, the ICHgel outperformed commercial Ag/AgCl electrodes in the collection of ECG and EMG signals. In summary, the synthesized ICHgel with its outstanding conformal interface capabilities and mechanical adaptability paves the way for enhanced human-machine interaction, fostering the development of flexible electronic devices.

Ultrafast universal fabrication of configurable porous silicone-based elastomers by Joule heating chemistry

Silicone-based elastomers (SEs) have been extensively applied in numerous cutting-edge areas, including flexible electronics, biomedicine, 5G smart devices, mechanics, optics, soft robotics, etc. However, traditional strategies for the synthesis of polymer elastomers, such as bulk polymerization, suspension polymerization, solution polymerization, and emulsion polymerization, are inevitably restricted by long-time usage, organic solvent additives, high energy consumption, and environmental pollution. Here, we propose a Joule heating chemistry method for ultrafast universal fabrication of SEs with configurable porous structures and tunable components (e.g., graphene, Ag, graphene oxide, TiO2, ZnO, Fe3O4, V2O5, MoS2, BN, g-C3N4, BaCO3, CuI, BaTiO3, polyvinylidene fluoride, cellulose, styrene-butadiene rubber, montmorillonite, and EuDySrAlSiOx) within seconds by only employing H2O as the solvent. The intrinsic dynamics of the in situ polymerization and porosity creation of these SEs have been widely investigated. Notably, a flexible capacitive sensor made from as-fabricated silicone-based elastomers exhibits a wide pressure range, fast responses, long-term durability, extreme operating temperatures, and outstanding applicability in various media, and a wireless human-machine interaction system used for rescue activities in extreme conditions is established, which paves the way for more polymer-based material synthesis and wider applications.

Three-Dimensional Printing of Hydrogels for Flexible Sensors: A Review

The remarkable flexibility and heightened sensitivity of flexible sensors have drawn significant attention, setting them apart from traditional sensor technology. Within this domain, hydrogels-3D crosslinked networks of hydrophilic polymers-emerge as a leading material for the new generation of flexible sensors, thanks to their unique material properties. These include structural versatility, which imparts traits like adhesiveness and self-healing capabilities. Traditional templating-based methods fall short of tailor-made applications in crafting flexible sensors. In contrast, 3D printing technology stands out with its superior fabrication precision, cost-effectiveness, and satisfactory production efficiency, making it a more suitable approach than templating-based strategies. This review spotlights the latest hydrogel-based flexible sensors developed through 3D printing. It begins by categorizing hydrogels and outlining various 3D-printing techniques. It then focuses on a range of flexible sensors-including those for strain, pressure, pH, temperature, and biosensors-detailing their fabrication methods and applications. Furthermore, it explores the sensing mechanisms and concludes with an analysis of existing challenges and prospects for future research breakthroughs in this field.

A Wearable Electrochemical Biosensor Utilizing Functionalized Ti3C2Tx MXene for the Real-Time Monitoring of Uric Acid Metabolite

Wearable, noninvasive sensors enable the continuous monitoring of metabolites in sweat and provide clinical information related to an individual's health and disease states. Uric acid (UA) is a key indicator highly associated with gout, hyperuricaemia, hypertension, kidney disease, and Lesch-Nyhan syndrome. However, the detection of UA levels typically relies on invasive blood tests. Therefore, developing a wearable device for noninvasive monitoring of UA concentrations in sweat could facilitate real-time personalized disease prevention. Here, we introduce 1,3,6,8-pyrene tetrasulfonic acid sodium salt (PyTS) as a bifunctional molecule functionalized with Ti3C2Tx via π-π conjugation to design nonenzymatic wearable sensors for sensitive and selective detection of UA concentration in human sweat. PyTS@Ti3C2Tx provides many oxidation-reduction active groups to enhance the electrocatalytic ability of the UA oxidation reaction. The PyTS@Ti3C2Tx-based electrochemical sensor demonstrates highly sensitive detection of UA in the concentration range of 5 μM-100 μM, exhibiting a lower detection limit of 0.48 μM compared to the uricase-based sensor (0.84 μM). In volunteers, the PyTS@Ti3C2Tx-based wearable sensor is integrated with flexible microfluidic sweat sampling and wireless electronics to enable real-time monitoring of UA levels during aerobic exercise. Simultaneously, it allows for comparison of blood UA levels via a commercial UA analyzer. Herein, this study provides a promising electrocatalyst strategy for nonenzymatic electrochemical UA sensor, enabling noninvasive real-time monitoring of UA levels in human sweat and personalized disease prevention.

High-Performance Hydrogel Sensors Enabled Multimodal and Accurate Human-Machine Interaction System for Active Rehabilitation

Human-machine interaction (HMI) technology shows an important application prospect in rehabilitation medicine, but it is greatly limited by the unsatisfactory recognition accuracy and wearing comfort. Here, this work develops a fully flexible, conformable, and functionalized multimodal HMI interface consisting of hydrogel-based sensors and a self-designed flexible printed circuit board. Thanks to the component regulation and structural design of the hydrogel, both electromyogram (EMG) and forcemyography (FMG) signals can be collected accurately and stably, so that they are later decoded with the assistance of artificial intelligence (AI). Compared with traditional multichannel EMG signals, the multimodal human-machine interaction method based on the combination of EMG and FMG signals significantly improves the efficiency of human-machine interaction by increasing the information entropy of the interaction signals. The decoding accuracy of the interaction signals from only two channels for different gestures reaches 91.28%. The resulting AI-powered active rehabilitation system can control a pneumatic robotic glove to assist stroke patients in completing movements according to the recognized human motion intention. Moreover, this HMI interface is further generalized and applied to other remote sensing platforms, such as manipulators, intelligent cars, and drones, paving the way for the design of future intelligent robot systems.

Naked-Eye Visual Thermometer Based on Glycerol─Nonclose-Packed Photonic Crystals for Real-Time Temperature Sensing and Monitoring

Real-time sensing and monitoring of temperature are of great significance for assessing human health. The sensitivity and stability are inevitable issues for thermometers. In this study, a thermometer with the cylindrical thermochromic hydrogel was prepared for real-time visual monitoring of temperature, which had excellent temperature sensitivity, angle-independence axially, and environmental stability. The customization of their initial optical properties depended on the PMMA concentrations and the content of the hydrogel monomer. The glycerol introduced with solvent displacement formed hydrogen bonds with the hydrogel network, which stabilized their mechanical properties, and the reflection peak blue-shifted from 653 to 499 nm when tensile strain was 57.85%. At the same time, the environmental stability originated from the moisturizing properties of the glycerol, which enabled the hydrogel to reliably transmit the information on temperature into the air without losing moisture. The reflection peak of the cylindrical thermochromic hydrogel shifted from 657 to 455 nm when the temperature increased from 22 to 45 °C, which realized temperature visual monitoring in the full-color range. The temperature sensitivity of the glycerol─nonclose-packed photonic crystals remained stable for 1 month, which provided an optimal option for continuous visual temperature monitoring.

Tough Hydrogels with Different Toughening Mechanisms and Applications

Load-bearing biological tissues, such as cartilage and muscles, exhibit several crucial properties, including high elasticity, strength, and recoverability. These characteristics enable these tissues to endure significant mechanical stresses and swiftly recover after deformation, contributing to their exceptional durability and functionality. In contrast, while hydrogels are highly biocompatible and hold promise as synthetic biomaterials, their inherent network structure often limits their ability to simultaneously possess a diverse range of superior mechanical properties. As a result, the applications of hydrogels are significantly constrained. This article delves into the design mechanisms and mechanical properties of various tough hydrogels and investigates their applications in tissue engineering, flexible electronics, and other fields. The objective is to provide insights into the fabrication and application of hydrogels with combined high strength, stretchability, toughness, and fast recovery as well as their future development directions and challenges.

Construction methods and biomedical applications of PVA-based hydrogels

Polyvinyl alcohol (PVA) hydrogel is favored by researchers due to its good biocompatibility, high mechanical strength, low friction coefficient, and suitable water content. The widely distributed hydroxyl side chains on the PVA molecule allow the hydrogels to be branched with various functional groups. By improving the synthesis method and changing the hydrogel structure, PVA-based hydrogels can obtain excellent cytocompatibility, flexibility, electrical conductivity, viscoelasticity, and antimicrobial properties, representing a good candidate for articular cartilage restoration, electronic skin, wound dressing, and other fields. This review introduces various preparation methods of PVA-based hydrogels and their wide applications in the biomedical field.

Self-Healable, Self-Adhesive and Degradable MXene-Based Multifunctional Hydrogel for Flexible Epidermal Sensors

Conductive hydrogels have garnered significant interest in the realm of wearable flexible sensors due to their close resemblance to human tissue, wearability, and precise signal acquisition capabilities. However, the concurrent attainment of an epidermal hydrogel sensor incorporating reliable self-healing capabilities, biodegradability, robust adhesiveness, and the ability to precisely capture subtle electrophysiological signals poses a daunting and intricate challenge. Herein, an innovative MXene-based composite hydrogel (PBM hydrogel) with exceptional self-healing, self-adhesive, and versatile functionality is engineered through the integration of conductive MXene nanosheets into a well-structured poly(vinyl alcohol) (PVA) and bacterial cellulose (BC) hydrogel three-dimensional (3D) network, utilizing multiple dynamic cross-linking synergistic repeated freeze-thaw strategy. The hydrogel harnesses the presence of dynamically reversible borax ester bonds and multiple hydrogen bonds between its constituents, endowing it with rapid self-healing efficiency (97.8%) and formidable self-adhesive capability. The assembled PBM hydrogel epidermal sensor possesses a rapid response time (10 ms) and exhibits versatility in detecting diverse external stimuli and human movements such as vocalization, handwriting, joint motion, Morse code signals, and even monitoring infusion status. Additionally, the PBM hydrogel sensor offers the added advantage of swift degradation in phosphate-buffered saline solution (within a span of 56 days) and H2O2 solution (in just 53 min), maintaining an eco-friendly profile devoid of any environmental pollution. This work lays the groundwork for possible uses in electronic skins, interactions between humans and machines, and the monitoring of individualized healthcare.

Packaged WGM MBR sensor for high-performance temperature measurement using CNN-based multimode barcode images

The whispering gallery mode (WGM) optical microresonator sensors are emerging as a promising platform for precise temperature measurements, driven by their excellent sensitivity, resolution and integration. Nevertheless, challenges endure regarding stability, single resonant mode tracking, and real-time monitoring. Here, we demonstrate a temperature measurement approach based on convolutional neural network (CNN), leveraging the recognition of multimode barcode images acquired from a WGM microbottle resonator (MBR) sensor with robust packaged microresonator-taper coupling structure (packaged-MTCS). Our work ensures not only a high sensitivity of -14.28 pm/℃ and remarkable resolution of 3.5 × 10-4 ℃ across a broad dynamic range of 96 ℃ but also fulfills the demands for real-time temperature measurement with an average detection accuracy of 96.85% and a speed of 0.68s per image. These results highlight the potential of high-performance WGM MBR sensors in various fields and lay the groundwork for stable soliton microcomb excitation through thermal tuning.

Highly stretchable polyvinyl alcohol composite conductive hydrogel sensors reinforced by cellulose nanofibrils and liquid metal for information transmission

Conductive hydrogels have received widespread attention in the field of flexible sensors. However, a single network structure inside the hydrogel sensor usually makes it difficult to bear larger mechanical loadings, greatly limiting practical applications. Developing a recoverable conductive hydrogel sensor with high toughness and adaptability is still challenging. Herein, a high-performance polyvinyl alcohol (PVA)-based conductive composite hydrogel was constructed, assisted by green cellulose nanofibrils (CNFs), magnesium chloride (MgCl2), ethylene glycol (EG), and liquid metal (LM). The synergistic effects between CNFs and LM enhanced the network structure inside the recoverable hydrogel. This resulted in an excellent tensile strength of 3.86 MPa with an elongation at break of as high as 918.4 % and compressive strength of 4.04 MPa at 80 % strain. In addition, the conductive network composed of MgCl2 and LM endowed the hydrogel good electrical conductivity. Moreover, it could be used as a flexible strain sensor for various application scenarios, e.g., micro-stress monitoring (water droplet falling) and information encryption transmission of Morse code. Such uniqueness will provide a design strategy for developing a new generation of hydrogel sensors.

A wearable electrostimulation-augmented ionic-gel photothermal patch doped with MXene for skin tumor treatment

A wearable biological patch capable of producing multiple responses to light and electricity without interfering with daily activities is highly desired for skin cancer treatment, but remains a key challenge. Herein, the skin-mountable electrostimulation-augmented photothermal patch (eT-patch) comprising transparent ionic gel with MXene (Ti3C2Tx) doping is developed and applied for the treatment of melanoma under photostimulation at 0.5 W/cm2. The eT-patch designed has superior photothermal and electrical characteristics owing to ionic gels doped with MXene which provides high photothermal conversion efficiency and electrical conductivity as a medium. Simultaneously, the ionic gel-based eT-patch having excellent optical transparency actualizes real-time observation of skin response and melanoma treatment process under photothermal and electrical stimulation (PES) co-therapy. Systematical cellular study on anti-tumor mechanism of the eT-patch under PES treatment revealed that eT-patch under PES treatment can synergically trigger cancer cell apoptosis and pyroptosis, which together lead to the death of melanoma cells. Due to the obvious advantages of relatively safe and less side effects in healthy organs, the developed eT-patch provides a promising cost-effective therapeutic strategy for skin tumors and will open a new avenue for biomedical applications of ionic gels.

The Need for Smart Materials in an Expanding Smart World: MXene-Based Wearable Electronics and Their Advantageous Applications

As a result of the transformation of inflexible electronic structures into flexible and stretchy devices, wearable electronics now provide great advantages in a variety of fields, including mobile healthcare sensing and monitoring, human-machine interfaces, portable energy storage and harvesting, and more. Because of their enriched surface functionalities, large surface area, and high electrical conductivity, transition metal nitrides and carbides (also known as MXenes) have recently come to be extensively considered as a group of functioning two-dimensional nanomaterials as well as exceptional fundamental elements for forming flexible electronics devices. This Review discusses the most recent advancements that have been made in the field of MXene-enabled flexible electronics for wearable electronics. The emphasis is placed on extensively established nonstructural features in order to highlight some MXene-enabled electrical devices that were constructed on a nanometric scale. These attributes include devices configured in three dimensions: printed materials, bioinspired structures, and textile and planar substrates. In addition, sample applications in electromagnetic interference (EMI) shielding, energy, healthcare, and humanoid control of machinery illustrate the exceptional development of these nanodevices. The increasing potential of MXene nanoparticles as a new area in next-generation wearable electronic technologies is projected in this Review. The design challenges associated with these electronic devices are also discussed, and possible solutions are presented.

Wide-range soft anisotropic thermistor with a direct wireless radio frequency interface

Temperature sensors are one of the most fundamental sensors and are found in industrial, environmental, and biomedical applications. The traditional approach of reading the resistive response of Positive Temperature Coefficient thermistors at DC hindered their adoption as wide-range temperature sensors. Here, we present a large-area thermistor, based on a flexible and stretchable short carbon fibre incorporated Polydimethylsiloxane composite, enabled by a radio frequency sensing interface. The radio frequency readout overcomes the decades-old sensing range limit of thermistors. The composite exhibits a resistance sensitivity over 1000 °C-1, while maintaining stability against bending (20,000 cycles) and stretching (1000 cycles). Leveraging its large-area processing, the anisotropic composite is used as a substrate for sub-6 GHz radio frequency components, where the thermistor-based microwave resonators achieve a wide temperature sensing range (30 to 205 °C) compared to reported flexible temperature sensors, and high sensitivity (3.2 MHz/°C) compared to radio frequency temperature sensors. Wireless sensing is demonstrated using a microstrip patch antenna based on a thermistor substrate, and a battery-less radio frequency identification tag. This radio frequency-based sensor readout technique could enable functional materials to be directly integrated in wireless sensing applications.