Muscle-Inspired Self-Healing Hydrogels for Strain and Temperature Sensor

Ultra-strong and tough cellulose-based conductive hydrogels via orientation inspired by noodles pre-stretching

Due to the unsatisfactory mechanical properties of natural polymer-based conductive hydrogels, their applications are limited. Shaanxi Biangbiang noodles can be toughened by applying external mechanical forces through stretching and beating movements; this process provides inspiration for the preparation of high-strength hydrogels. In this paper, we propose a strategy for the preparation of ultrastrong and ultratough conductive hydrogels by directional prestretching and solvent exchange. Neatly arranged fiber bundles containing many intermolecular hydrogen bonds and metal ion coordination bonds are successfully constructed inside the prepared hydrogels. The hydrogel has exceptional mechanical properties, with a fracture stress exceeding 50 MPa, fracture strain approaching 105 %, fracture toughness exceeding 30 MJ m-3, and high conductivity reaching 11.738 ± 0.06 mS m-1. Impressively, the hydrogel can maintain its high mechanical properties after being frozen at an ultralow temperature of -80 °C for 7 days. Compared with other tough hydrogels, natural tendons and synthetic rubbers, the hydrogel exhibits excellent mechanical properties. The cellulose-based conductive hydrogel prepared in this study can be applied to robotic soft tissues (such as the Achilles tendon) that require high strength and are operated in extreme environments.

A Conductive and Anti-impact Composite for Flexible Piezoresistive Sensors

Flexible piezoresistive sensors, which can convert specific mechanical information (such as compression, bending, tensile, and torsion) into a resistance value change signal through the piezoresistive effect, have attracted more and more attention. However, how to achieve the simple, low-cost fabrication of a piezoresistive sensor is still a challenge. Herein, we report a facile strategy that introduces conductive carbon black (CB) and shear thickening gel (SG) composite into a melamine sponge (MS) to generate an MS-SG-CB composite with a unique force-electric coupling effect. A flexible sensor derived from the MS-SG-CB composite can not only accurately identify deformation signals during static stretching and compression while monitoring human movement status in real time but also recognize electrical signals under dynamic impact in a very short time (6 ms). The 3 × 3 flexible array built on this basis can accurately identify the mass and position of heavy objects. Furthermore, based on the flame-retardant properties of MS, the flame-retardant ammonium polyphosphate (APP) is further introduced into MS-SG-CB to obtain MS-SG-CB-APP composite with excellent flame retardancy and stable temperature electrical response behavior, expanding its application in the field of high temperature trigger alarm.

A wearable conductive hydrogel with triple network reinforcement inspired by bio-fibrous scaffolds for real-time quantitatively sensing compression force exerted on fruit surface

INTRODUCTION

Mechanical stresses incurred during post-harvest fruit storage and transportation profoundly impact decay and losses. Currently, the monitoring of mechanical forces is primarily focused on vibrational forces experienced by containers and vehicles and impact forces affecting containers. However, the detection of compressive forces both among interior fruit and between fruit and packaging surfaces remains deficient. Hence, conformable materials capable of sensing compressive stresses are necessary.

OBJECTIVES

In the present study, a triple-network-reinforced PSA/LiCl/CCN@AgNP conductive hydrogel was synthesized for compression force detection on fruit surfaces based on changes in intrinsic impedance under mechanical loading.

METHODS

The conductive hydrogel was characterized in terms of its adhesion, mechanics, frost resistance, water retention, conductivity, mechanical force-sensing properties, and feasibility for monitoring mechanical forces. Then, a portable complex impedance recorder was developed to interface with the conductive hydrogel and its mechanical force sensing ability was evaluated.

RESULTS

Beyond its inherent conductivity, the hydrogel exhibited notable pressure sensitivity within the strain range of 1 % to 80 %. The conductive hydrogel also demonstrated a commendable adhesion property, favorable tensile property (580 % elongation at break), substantial compressive strength and durability, and a long-term water retention capability. After exposure to -20 °C for 96 h, the hydrogel maintained its mechanical strength, affirming its anti-freezing property. In addition, a portable complex impedance recorder with sustained signal measurement stability was developed to quantitatively acquire the hydrogel resistance changes in response to compression forces. Finally, the effectiveness of the conductive hydrogel for sensing compression force on the surface of apple fruits was validated.

CONCLUSION

The conductive hydrogel holds promise for applications in smart packaging, wherein it can detect crucial mechanical stress on fruit, convert it into electrical signals, and further transmit these signals to the cloud, thereby enabling the real-time sensing of mechanical forces experienced by fruits and enhancing post-harvest fruit loss management.

Microsecond-Scale Transient Thermal Sensing Enabled by Flexible Mo1-xWxS2 Alloys

Real-time thermal sensing through flexible temperature sensors in extreme environments is critically essential for precisely monitoring chemical reactions, propellant combustions, and metallurgy processes. However, despite their low response speed, most existing thermal sensors and related sensing materials will degrade or even lose their sensing performances at either high or low temperatures. Achieving a microsecond response time over an ultrawide temperature range remains challenging. Here, we design a flexible temperature sensor that employs ultrathin and consecutive Mo1-x W x S2 alloy films constructed via inkjet printing and a thermal annealing strategy. The sensing elements exhibit a broad work range (20 to 823 K on polyimide and 1,073 K on flexible mica) and a record-low response time (about 30 μs). These properties enable the sensors to detect instantaneous temperature variations induced by contact with liquid nitrogen, water droplets, and flames. Furthermore, a thermal sensing array offers the spatial mapping of arbitrary shapes, heat conduction, and cold traces even under bending deformation. This approach paves the way for designing unique sensitive materials and flexible sensors for transient sensing under harsh conditions.

What Exactly Can Bionic Strategies Achieve for Flexible Sensors?

Flexible sensors have attracted great attention in the field of wearable electronic devices due to their deformability, lightness, and versatility. However, property improvement remains a key challenge. Fortunately, natural organisms exhibit many unique response mechanisms to various stimuli, and the corresponding structures and compositions provide advanced design ideas for the development of flexible sensors. Therefore, this Review highlights recent advances in sensing performance and functional characteristics of flexible sensors from the perspective of bionics for the first time. First, the "twins" of bionics and flexible sensors are introduced. Second, the enhancements in electrical and mechanical performance through bionic strategies are summarized according to the prototypes of humans, plants, and animals. Third, the functional characteristics of bionic strategies for flexible sensors are discussed in detail, including self-healing, color-changing, tangential force, strain redistribution, and interfacial resistance. Finally, we summarize the challenges and development trends of bioinspired flexible sensors. This Review aims to deepen the understanding of bionic strategies and provide innovative ideas and references for the design and manufacture of next-generation flexible sensors.

Temperature-Tolerant Versatile Conductive Zwitterionic Nanocomposite Organohydrogel toward Multisensory Applications

Conductive hydrogels (CHs) are emerging materials for next generation sensing systems in flexible electronics. However, the fabrication of competent CHs with excellent stretchability, adhesion, self-healing, photothermal conversion, multisensing, and environmental stability remains a huge challenge. Herein, a nanocomposite organohydrogel with the above features is constructed by in situ copolymerization of zwitterionic monomer and acrylamide in the existence of carboxylic cellulose nanofiber-carrying reduced graphene oxide (rGO) plus a solvent displacement strategy. The synergy of abundant dipole-dipole interactions and intermolecular hydrogen bonds enables the organohydrogel to exhibit high stretchability, strong adhesion, and good self-healing. The presence of glycerol weakens the formation of hydrogen bonds between water molecules, endowing the organohydrogel with excellent environmental stability (-40 to 60 °C) to adapt to different application scenarios. Importantly, the multimodal organohydrogel presents excellent sensing behavior, including a high gauge factor of 16.3 at strains of 400-1440% and a reliable thermal coefficient of resistance (-4.2 °C-1) over a wide temperature widow (-40 to 60 °C). Moreover, the organohydrogel displays a highly efficient and reliable photothermal conversion ability due to the favorable optical absorbing behavior of rGO. Notably, the organohydrogel can detect accurate human activities at ambient temperature, demonstrating potential applications in flexible intelligent electronics.

Junction Piezotronic Transistor Arrays Based on Patterned ZnO Nanowires for High-Resolution Tactile and Photo Mapping

Recently, a great deal of interest has been focused on developing sensors that can measure both pressure and light. However, traditional sensors are difficult to integrate into silicon (Si)-based integrated circuits. Therefore, it is particularly important to design a sensor that operates on a new principle. In this paper, junction piezotronic transistor (JPT) arrays based on zinc oxide (ZnO) nanowire are demonstrated. And the JPT arrays show high spatial resolution pressure and light mapping with 195 dpi. Because ZnO nanowires are arranged vertically above the p-type Si channel's center of the transistor, the width of the heterojunction depletion region is constricted by the positive piezoelectric potential generated by strained ZnO. In addition, photogenerated charge carriers can be created in the Si channel when JPT is stimulated by light, which increases its electrical conductivity. Consequently, the external pressure and light distribution information can be obtained from the variation in the output current of the device. The prepared JPT arrays can be compatible with Si transistors, which make them highly competitive and make it possible to incorporate both pressure and light sensors into large integrated circuits. This work will contribute to many applications, such as intelligent clothing, human-computer interaction, and electronic skin.

Low-Temperature Rapid Polymerization of Intrinsic Conducting PAD/OC Hydrogels with a Self-Adhesive and Sensitive Sensor for Outdoor Damage Repair and Detection

Intrinsic conducting hydrogels fabricated in situ at low temperatures with self-adhesive properties and excellent flexibility hold significant promise for energy applications and outdoor damage repair. However, challenges such as low polymerization rate and self adhesion, insufficient ionic conductivity, inflexibility, and poor stability under extreme cold conditions have hindered their utilization as high-performance sensors. In this study, we designed an intrinsic conducting hydrogel (PADOC) composed of acrylic acid, acryloyloxyethyltrimethylammonium chloride, N,N'-methylenebis(2-propenamide), self-fabricated oxidized curdlan (OC), and a water/glycerol binary solvent. The novel hydrogel exhibited rapid gelation (30 s) at 0 °C facilitated by the promotion of OC, without the need for external energy input. Our findings from FT-IR, NMR, XPS, XRD, EPR spectra, MS, and DSC analyses revealed that OC underwent selective oxidation via the evolved Fenton reaction at 30 °C, serving as bioaccelerators for PAD polymerization. Due to OC's reductive structure and increased solubility, the reaction activation energy of the PAD polymerization reaction significantly reduced from 103.2 to 54.4 kJ/mol. PADOC ionic hydrogels demonstrated an electrical conductivity of 1.00 S/m, 0.7% low hysteresis, 39.6 kPa self-adhesive strength, and 923% strain-at-break and kept even at -20 °C owing to dense hydrogen and ionic bonds between PAD and OC chains. Furthermore, PADOC ionic hydrogels exhibited antifatigue properties for 10 cycles (0-100%) due to electrostatic interactions and hydrogen bonding. Remarkably, using a self-designed device, the rapid polymerization of PADOC effectively repaired copper pipeline leakage under 86 kPa pressure and detected 1% strain variation as a strain sensor. This study opens a new avenue for the rapid gelation of self-adhesive and flexible intrinsic conducting hydrogels with robust sensor performance.

Highly flexible yet strain-insensitive conjugated polymer

Imparting excellent electrical properties, mechanical robustness, suppleness, conduction stability during deformation, and self-healing to intrinsic conducting polymers is a challenging endeavor. The reversibly interlocked macromolecular networks (RILNs) approach is utilized to tackle this problem. Specifically, poly(3,4-ethylenedioxythiophene) (PEDOT) is mixed with flexible polysulfonic acid networks crosslinked by reversible Diels-Alder bonds, while rigid polyaniline networks crosslinked by reversible Schiff base bonds act as molecular staples. Owing to the joint actions of the doping effect of polyaniline on PEDOT, the specific interlocking architecture and synergy between the component materials, the electrical conductivity (59.3-980.5 S cm-1), tensile strength (8.4-81.6 MPa) and elongation at break (44.5-411.0%) of the resultant PEDOT/RILNs films is significantly tunable according to different usage scenarios by adjusting the PEDOT content from 1.48 to 22.24 wt%. More importantly, the electrical resistance of PEDOT/RILNs remains constant during not only a single large extension and deflection but also repeated stretching (up to 1500 cycles) and bending (up to 106 cycles). The built-in reversible covalent bonds enable the PEDOT/RILNs to autonomously restore damaged mechanical and electrical performance. These record-breaking results and the demonstration of self-powered sensor made of PEDOT/RILNs suggest that the proposed approach successfully satisfies various conflicting requirements of flexible electronics regarding the properties of conducting polymers.

Ureido-Ionic Liquid Mediated Conductive Hydrogel: Superior Integrated Properties for Advanced Biosensing Applications

Ionic conductive hydrogels (ICHs) have recently gained prominence in biosensing, indicating their potential to redefine future biomedical applications. However, the integration of these hydrogels into sensor technologies and their long-term efficacy in practical applications pose substantial challenges, including a synergy of features, such as mechanical adaptability, conductive sensitivity, self-adhesion, self-regeneration, and microbial resistance. To address these challenges, this study introduces a novel hydrogel system using an imidazolium salt with a ureido backbone (UL) as the primary monomer. Fabricated via a straightforward one-pot copolymerization process that includes betaine sulfonate methacrylate (SBMA) and acrylamide (AM), the hydrogel demonstrates multifunctional properties. The innovation of this hydrogel is attributed to its robust mechanical attributes, outstanding strain responsiveness, effective water retention, and advanced self-regenerative and healing capabilities, which collectively lead to its superior performance in various applications. Moreover, this hydrogel  exhibited broad-spectrum antibacterial activity. Its potential for biomechanical monitoring, especially in tandem with contact and noncontact electrocardiogram (ECG) devices, represents a noteworthy advancement in precise real-time cardiac monitoring in clinical environments. In addition, the conductive properties of the hydrogel make it an ideal substrate for electrophoretic patches aimed at treating infected wounds and consequently enhancing the healing process.

Unveiling Trends: Nanoscale Materials Shaping Emerging Biomedical Applications

The realm of biomedical materials continues to evolve rapidly, driven by innovative research across interdisciplinary domains. Leveraging big data from the CAS Content Collection, this study employs quantitative analysis through natural language processing (NLP) to identify six emerging areas within nanoscale materials for biomedical applications. These areas encompass self-healing, bioelectronic, programmable, lipid-based, protein-based, and antibacterial materials. Our Nano Focus delves into the multifaceted utilization of nanoscale materials in these domains, spanning from augmenting physical and electronic properties for interfacing with human tissue to facilitating intricate functionalities like programmable drug delivery.

Enhancing regenerative medicine with self-healing hydrogels: A solution for tissue repair and advanced cyborganic healthcare devices

Considering the global burden related to tissue and organ injuries or failures, self-healing hydrogels may be an attractive therapeutic alternative for the future. Self-healing hydrogels are highly hydrated 3D structures with the ability to self-heal after breaking, this property is attributable to a variety of dynamic non-covalent and covalent bonds that are able to re-linking within the matrix. Self-healing ability specially benefits minimal invasive medical treatments with cell-delivery support. Moreover, those tissue-engineered self-healing hydrogels network have demonstrated effectiveness for myriad purposes; for instance, they could act as delivery-platforms for different cargos (drugs, growth factors, cells, among others) in tissues such as bone, cartilage, nerve or skin. Besides, self-healing hydrogels have currently found their way into new and novel applications; for example, with the development of the self-healing adhesive hydrogels, by merely aiding surgical closing processes and by providing biomaterial-tissue adhesion. Furthermore, conductive hydrogels permit the stimuli and monitoring of natural electrical signals, which facilitated a better fitting of hydrogels in native tissue or the diagnosis of various health diseases. Lastly, self-healing hydrogels could be part of cyborganics - a merge between biology and machinery - which can pave the way to a finer healthcare devices for diagnostics and precision therapies.

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

A Soft Robot Tactile Finger Using Oxidation-Reduction Graphene-Polyurethane Conductive Sponge

Currently, intelligent robotics is supplanting traditional industrial applications. It extends to business, service and care industries, and other fields. Stable robot grasping is a necessary prerequisite for all kinds of complex application scenarios. Herein, we propose a method for preparing an elastic porous material with adjustable conductivity, hardness, and elastic modulus. Based on this, we design a soft robot tactile fingertip that is gentle, highly sensitive, and has an adjustable range. It has excellent sensitivity (~1.089 kpa-1), fast response time (~35 ms), and measures minimum pressures up to 0.02 N and stability over 500 cycles. The baseline capacitance of a sensor of the same size can be increased by a factor of 5-6, and graphene adheres better to polyurethane sponge and has good shock absorption. In addition, we demonstrated the application of the tactile fingertip to a two-finger manipulator to achieve stable grasping. In this paper, we demonstrate the great potential of the soft robot tactile finger in the field of adaptive grasping for intelligent robots.

A Solvent-Templated Porous Liquid Crystal Elastomer with Tactile Sensation beyond Reversible Actuation toward Versatile Artificial Muscles

Liquid crystal elastomers (LCEs), as a classical two-way shape-memory material, are good candidates for developing artificial muscles that mimic the contraction, expansion, or rotational behavior of natural muscles. However, biomimicry is currently focused more on the actuation functions of natural muscles dominated by muscle fibers, whereas the tactile sensing functions that are dominated by neuronal receptors and synapses have not been well captured. Very few studies have reported the sensing concept for LCEs, but the signals were still donated by macroscopic actuation, that is, variations in angle or length. Herein, we develop a conductive porous LCE (CPLCE) using a solvent (dimethyl sulfoxide (DMSO))-templated photo-cross-linking strategy, followed by carbon nanotube (CNT) incorporation. The CPLCE has excellent reversible contraction/elongation behavior in a manner similar to the actuation functions of skeletal muscles. Moreover, the CPLCE shows excellent pressure-sensing performance by providing real-time electrical signals and is capable of microtouch sensing, which is very similar to natural tactile sensing. Furthermore, macroscopic actuation and tactile sensation can be integrated into a single system. Proof-of-concept studies reveal that the CPLCE-based artificial muscle is sensitive to external touch while maintaining its excellent actuation performance. The CPLCE with tactile sensation beyond reversible actuation is expected to benefit the development of versatile artificial muscles and intelligent robots.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

Ultrasensitive Touch Sensor for Simultaneous Tactile and Slip Sensing

Touch is a general term to describe mechanical stimuli. It is extremely difficult to develop touch sensors that can detect different modes of contact forces due to their low sensitivity and data decoupling. Simultaneously conducting tactile and slip sensing presents significant challenges for the design, structure, and performance of sensors. In this work, a highly sensitive sandwich-structured sensor is achieved by exploiting the porosity and compressive modulus of the sensor's functional layer materials. The sensor shows an ultra-high sensitivity of 1167 kPa-1 and a low-pressure detection limit of 1.34 Pa due to its considerably low compression modulus of 23.8 Pa. Due to this ultra-high sensitivity, coupled with spectral analysis, it allows for dual-mode detection of both tactile and slip sensations simultaneously. This novel fabrication strategy and signal analysis method provides a new direction for the development of tactile/slip sensors.

Effects of charge state of nano-chitin on the properties of polyvinyl alcohol composite hydrogel

The present work investigates the effects of nano-chitin with different charge, obtained by acid hydrolysis and TEMPO oxidation, on the structure and properties of borax crosslinked polyvinyl alcohol (PVA) hydrogels. In detail, nano-chitin prepared by acid hydrolysis (ACh) is positively charged (+28.8 mV). The electrostatic attraction between ACh and borax ions leads to a maximum tensile stress of composite hydrogel (ACh/PB), 54.25 KPa, 17 times of the borax crosslinked PVA (PB). In contrast, nano-chitin prepared by TEMPO-oxidation (TCh) shows negative charge (-59.0 mV). Due to the electrostatic repulsion with borax ions, the maximum tensile stress of composite hydrogel (TCh/PB) is only 9.25 KPa, a very limit reinforcing effect. However, TCh/PB showed better self-healing efficiency (96.0 %) as well as ionic conductivity (1.25 × 10-5 S/m). The present work shows that the charge state of the nano-chitin exerts great influence on the interaction with the crosslinking agent borax, therefore, affects the structure and properties of the final PVA composite hydrogels. The results could provide important information about making full use of nano-chitin as a reinforcement by adjusting its surface charge state.

A High-Stretching, Rapid-Self-Healing, and Printable Composite Hydrogel Based on Poly(Vinyl Alcohol), Nanocellulose, and Sodium Alginate

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (<30 s) and achieves mechanical properties of 33.92 kPa. Additionally, it exhibits high tensile strain performance (4000%). The abundant internal ions and functional groups of SPBC hydrogels provide support for the good electrical conductivity (0.62 S/cm) and electrical response properties. In addition, the SPBC hydrogel can be attached to surfaces such as fingers and wrists to monitor human movements in real time, and its good rheological property supports three-dimensional (3D) printing molding methods. In summary, this study successfully prepared a self-healing, conductive, printable, and mechanically superior SPBC hydrogel. Its suitability for 3D-printing personalized fabrication and outstanding sensor properties makes it a useful reference for hydrogels in wearable devices and human motion monitoring.

Biomimetic Electronic Skin Based on a Stretchable Ionogel Mechanoreceptor Composed of Crumpled Conductive Rubber Electrodes for Synchronous Strain, Pressure, and Temperature Detection

Electronic skin (e-skin) is showing a huge potential in human-computer interaction, intelligent robots, human health, motion monitoring, etc. However, it is still challenging for e-skin to realize distinguishable detection of stretching strain, vertical pressure, and temperature through a simple noncoupling structure design. Here, a stretchable multimodal biomimetic e-skin was fabricated by integrating layer-by-layer self-assembled crumpled reduced graphene oxide/multiwalled carbon nanotubes film on natural rubber (RGO/MWCNTs@NR) as stretchable conductive electrodes and polyacrylamide/NaCl ionogel as a dielectric layer into an ionotropic capacitive mechanoreceptor. Unlike natural skin receptors, the sandwich-like stretchable ionogel mechanoreceptor possessed a distinct ionotropic capacitive behavior for strain and pressure detection. The results showed that the biomimetic e-skin displayed a negative capacitance change with superior stretchability (0-300%) and a high gauge factor of 0.27 in 180-300% strain, while exhibiting a normal positive piezo-capacitance behavior in vertical pressure range of 0-15 kPa with a maximal sensitivity of 1.759 kPa-1. Based on this feature, the biomimetic e-skin showed an excellent synchronous detection capability of planar strain and vertical pressure in practical wearable applications such as gesture recognition and grasping movement detection without a complicated mathematical or signal decoupling process. In addition, the biomimetic e-skin exhibited a quantifiable linear responsiveness to temperature from 20-90 °C with a temperature coefficient of 0.55%/°C. These intriguing properties gave the biomimetic e-skin the ability to perform a complete function similar to natural skin but beyond its performance for future wearable devices and artificial intelligence devices.

Antifreezing, Antidrying, and Conductive Hydrogels for Electronic Skin Applications at Ultralow Temperatures

Although conductive hydrogel-based flexible electronic devices have superb flexibility and high conductivities, they tend to malfunction in dry or frigid areas. Herein, an ultralow-temperature tolerant, antidrying, and conductive composite hydrogel is designed for electronic skin applications on the basis of the synergy of double-cross-linked polymer networks, Hofmeister effect, and electrostatic interaction and fabricated by in situ free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid and acrylic acid in the presence of poly(vinyl alcohol) and conductive MXene sheets, followed by impregnation with LiCl. Thanks to the synergy of LiCl and the charged polar terminal groups of the synthesized polymers, the composite hydrogel can not only bear an ultralow temperature of -80 °C without freezing but also maintain its original mass. Meanwhile, the resultant hydrogel possesses satisfactory self-regeneration ability benefiting from the moisturizing effect of LiCl. The conductive network of MXene sheets greatly improves the ionic conductivity of the hydrogel at low temperatures, exhibiting an ionic conductivity of 1.4 S m-1 at -80 °C. Furthermore, the electronic skin assembled by the multifunctional hydrogel is efficient in monitoring human motions at -80 °C. The antifreezing and antidrying features along with favorable ionic conductivity, high tensile strength, and outstanding flexibility make the composite hydrogel promising for applications in frigid and dry regions.

Unzipped carbon nanotubes assisted 3D printable functionalized chitosan hydrogels for strain sensing applications

Developing multifunctional hydrogels for wearable strain sensors has received significant attention due to their diverse applications, including human motion detection, personalized healthcare, soft robotics, and human-machine interfaces. However, integrating the required characteristics into one component remains challenging. To overcome these limitations, we synthesized multifunctional hydrogels using carboxymethyl chitosan (CMCS) and unzipped carbon nanotubes (f-CNTs) as strain sensor via a one-pot strategy. The polar groups in CMCS and f-CNTs enhance the properties of the hydrogels through different interactions. The hydrogels show superior printability with a uniformity factor (U) of 0.996 ± 0.049, close to 1. The f-CNTs-assisted hydrogels showed improved storage modulus (8.8 × 105 Pa) than the pure polymer hydrogel. The hydrogels adequately adhered to different surfaces, including human skin, plastic, plastic/glass interfaces, and printed polymers. The hydrogels demonstrated rapid self-healing and good conductivity. The biocompatibility of the hydrogels was assessed using human fibroblast cells. No adverse effects were observed with hydrogels, showing their biocompatibility. Furthermore, hydrogels exhibited antibacterial potential against Escherichia coli. The developed hydrogel exhibited unidirectional motion and complex letter recognition potential with a strain sensitivity of 2.4 at 210 % strain. The developed hydrogels could explore developing wearable electronic devices for detecting human motion.

Preparation of environmentally friendly, high strength, adhesion and stability hydrogel based on lignocellulose framework

Hydrogels are extensively utilized in the fields of electronic skin, environmental monitoring, biological dressings due to their excellent flexibility and conductivity. However, traditional hydrogel materials possess drawbacks such as environmental toxicity, low strength, poor stability, and water loss deactivation, which limited its frequent applications. Here, a flexible conductive hydrogel called wood-based DES hydrogel (WDH) with high strength, high adhesion, high stability, and high sensitivity was successfully synthesized by using environmentally friendly lignocellulose as skeleton and deep eutectic solvent as matrix. The strength of WDH prepared from lignocellulose framework is approximately 50 times higher than poly deep eutectic solvent hydrogel, and about 4.5 times higher than that prepared from cellulose skeleton. The WDH exhibits stable adhesion to most common materials and demonstrates exceptional dimensional stability. Its conductivity remains unaffected by water, even after prolonged exposure to air, maintaining a value of 0.0245 S/m. The anisotropy inherent in the system results in three distinct linear sensing intervals for WDH, exhibiting a maximum sensitivity of 5.45. This paper verified the advantages of lignocellulose framework in improving the strength and stability of hydrogels, which provided a new strategy for the development of sensor materials.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

Bioinspired Supramolecular Hydrogel from Design to Applications

Nature offers a wealth of opportunities to solve scientific and technological issues based on its unique structures and function. The dynamic non-covalent interaction is considered to be the main base of living functions of creatures including humans, animals, and plants. Supramolecular hydrogels formed by non-covalent bonding interactions has become a unique platform for constructing promising materials for medicine, energy, electronic, and biological substitute. In this review, the self-assemble principle of supramolecular hydrogels is summarized. Next, the stimulation of external environment that triggers the assembly or disassembly of supramolecular hydrogels are recapitulated, including temperature, mechanics, light, pH, ions, etc. The main applications of bioinspired supramolecular hydrogels in terms of bionic objects including humans, animals, and plants are also described. Although so many efforts are done for revealing the synergized mechanism of the function and non-covalent interactions on the supramolecular hydrogel, the complexity and variability between stimulus and non-covalent bonding in the supramolecular system still require impeccable theories. As an outlook, the bioinspired supramolecular hydrogel is just beginning to exhibit its great potential in human life, offering significant opportunities in drug delivery and screening, implantable devices and substitutions, tissue engineering, micro-fluidic devices, and biosensors.

Ultrasensitive Flexible Temperature Sensors Based on Thermal-Mediated Ions Migration Dynamics in Asymmetrical Polymer Bilayers

Accurately acquiring crucial data on the ambient surroundings and physiological processes delivered via subtle temperature fluctuation is vital for advancing artificial intelligence and personal healthcare techniques but is still challenging. Here, we introduce an electrically induced cation injection mechanism based on thermal-mediated ion migration dynamics in an asymmetrical polymer bilayer (APB) composed of nonionic polymer and polyelectrolyte layers, enabling the development of ultrasensitive flexible temperature sensors. The resulting optimized sensor achieves ultrahigh sensitivity, with a thermal index surpassing 10,000 K-1, which allows identifying temperature differences as small as 10 mK with a sensitivity that exceeds 1.5 mK. The mechanism also enables APB sensors to possess good insensitivity to various mechanical deformations─features essential for practical applications. As a proof of concept, we demonstrate the potential impact of APB sensors in various conceptual applications, such as mental tension evaluation, biomimetic thermal tactile, and thermal radiation detection.

Solid-state, liquid-free ion-conducting elastomers: rising-star platforms for flexible intelligent devices

Soft ionic conductors have emerged as a powerful toolkit to engineer transparent flexible intelligent devices that go beyond their conventional counterparts. Particularly, due to their superior capacities of eliminating the evaporation, freezing and leakage issues of the liquid phase encountered with hydrogels, organohydrogels and ionogels, the emerging solid-state, liquid-free ion-conducting elastomers have been largely recognized as ideal candidates for intelligent flexible devices. However, despite their extensive development, a comprehensive and timely review in this emerging field is lacking, particularly from the perspective of design principles, advanced manufacturing, and distinctive applications. Herein, we present (1) the design principles and intriguing merits of solid-state, liquid-free ion-conducting elastomers; (2) the methods to manufacture solid-state, liquid-free ion-conducting elastomers with preferential architectures and functions using advanced technologies such as 3D printing; (3) how to leverage solid-state, liquid-free ion-conducting elastomers in exploiting advanced applications, especially in the fields of flexible wearable sensors, bioelectronics and energy harvesting; (4) what are the unsolved scientific and technical challenges and future opportunities in this multidisciplinary field. We envision that this review will provide a paradigm shift to trigger insightful thinking and innovation in the development of intelligent flexible devices and beyond.

Cold-resistant, highly stretchable ionic conductive hydrogels for intelligent motion recognition in winter sports

Conductive hydrogels have attracted much attention for their wide application in the field of flexible wearable sensors due to their outstanding flexibility, conductivity and sensing properties. However, the weak mechanical properties, lack of frost resistance and susceptibility to microbial contamination of traditional conductive hydrogels greatly limit their practical application. In this work, multifunctional polyvinyl alcohol (PVA)/carboxymethyl cellulose (CMC)/poly(acrylamide-co-1-vinyl-3-butylimidazolium bromide) (P(AAm-co-VBIMBr)) (PCPAV) ionic conductive hydrogels with high strength and good conductive, transparent, anti-freezing and antibacterial properties were constructed by introducing a network of chemically crosslinked AAm and VBIMBr copolymers into the base material of PVA and CMC by in situ free radical polymerization. Owing to the multiple interactions between the polymers, including covalent crosslinking, multiple hydrogen bonding interactions, and electrostatic interactions, the obtained ionic conductive hydrogels exhibit a high tensile strength (360.6 kPa), a large elongation at break (810.6%), good toughness, and fatigue resistance properties. The introduction of VBIMBr endows the PCPAV hydrogels with excellent transparency (∼92%), a high ionic conductivity (15.2 mS cm-1), antimicrobial activity and good flexibility and conductivity at sub-zero temperatures. Notably, the PCPAV hydrogels exhibit a wide strain range (0-800%), high strain sensitivity (GF = 3.75), fast response, long-term stability, and fantastic durability, which enable them to detect both large joint movements and minute muscle movements. Based on these advantages, it is believed that the PCPAV-based hydrogel sensors would have potential applications in health monitoring, human motion detection, soft robotics, ionic skins, human-machine interfaces, and other flexible electronic devices.

Ultrasensitive stretchable bimodal sensor based on novel elastomer and ionic liquid for temperature and humidity detection

In this work, we present a novel stretchable bimodal sensor that can simultaneously detect temperature and humidity changes based on poly-hydroxyethyl acrylate (PHEA) elastomer infused with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) ionic liquid. The sensor exhibits high transparency, stability, and biocompatibility, as well as excellent mechanical and sensing properties. The sensor can achieve a maximum strain of 761%, a sensitivity of 4.5%/°C at room temperature, a detection range from -35 to 120 °C, and a response time of 10 ms. The sensor is able to provide acute response to movement of human hand at close range and can detect temperature changes as small as 0.004 °C in the range of 20-30 °C. The sensor also responds to humidity change, showing a high sensitivity to humidity change of 4.4%/RH% under the temperature of 30 °C. The sensor can be used for various applications in wearable electronics, human-machine interfaces, and soft robotics.

Gradiently Foaming Ultrasoft Hydrogel with Stop Holes for Highly Deformable, Crack-Resistant and Sensitive Conformal Human-Machine Interfaces

Hydrogels are considered as promising materials for human-machine interfaces (HMIs) owing to their merits of tailorable mechanical and electrical properties; nevertheless, it remains challenging to simultaneously achieve ultrasoftness, good mechanical robustness and high sensitivity, which are the pre-requisite requirements for wearable sensing applications. Herein, for the first time, this work proposes a universal phase-transition-induced bubbling strategy to fabricate ultrasoft gradient foam-shaped hydrogels (FSHs) with stop holes for high deformability, crack-resistance and sensitive conformal HMIs. As a typical system, the FSH based on polyacrylamide/sodium alginate system shows an ultralow Young's modulus (1.68 kPa), increased sustainable strain (1411%), enhanced fracture toughness (915.6 J m-2 ), improved tensile sensitivity (21.77), and compressive sensitivity (65.23 kPa-1 ). The FSHs are used for precisely acquiring and identifying gesture commands of the operator to remotely control a surgical robot for endoscopy and an electric ship in a first-person perspective for cruising, feeding crabs and monitoring the environmental change in real-time.

Constructing a novel humidity sensor using acrylic acid/bagasse cellulose porous hydrogel combining graphene oxide and citral for antibacterial and intelligent fruit preservation

A novel hydrogel humidity sensor was developed using acrylic acid/bagasse cellulose (AA/BC) porous hydrogel triggered by cold plasma (CP) combining graphene oxide (GO) and embedding citral for antibacterial and intelligent fruit preservation. Results showed that both GO and citral were loaded in AA/BC and had strong hydrogen bond interaction with hydrogel. Acrylic acid/bagasse cellulose/graphene oxide (AA/BC/GO) showed the highest humidity response when the compound concentration of GO was 1.0 mg/mL and the test frequency was 1 kHz, and exhibited high electrical conductivity (-2.6 mS/cm). In addition, in continuous and cyclic relative humidity (RH) tests, the response time of AA/BC/GO from 33.70 % RH to 75.30 % RH was about 177.4 s and the recovery time was about 150.6 s, with excellent sensitivity and durability. The sensors also revealed remarkable antibacterial properties against Escherichia coli and Staphylococcus aureus, among which acrylic acid/bagasse cellulose/graphene oxide-citral (AA/BC/GO-C) was the most prominent, and could extend the shelf life of mangoes for about 8 days. By intuitively judging the appearances and total color difference (TCD) of the hydrogel sensors, it could play the role of intelligent preservation by connecting their water absorption and the release of citral. Therefore, this work provided innovative strategies for the application of hydrogel sensors in food preservation.

High-Performance Flexible Temperature Sensors Based on Laser-Irradiated Ag-MWCNTs/PEDOT:PSS

Recently, flexible temperature sensors have attracted significant interest due to their wide-ranging applications in areas such as biomedical monitoring, environmental monitoring, electronic skin, and intelligent robots. However, a combination of high sensitivity and high resolution remains a critical challenge. These properties depend on the synthesis techniques of the sensitive materials. In this work, we use a laser irradiation method to prepare a silver nanoparticle-modified carbon nanotube (Ag-MWCNT) which is further mixed with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). The developed temperature sensor exhibited a high sensitivity of -0.45% °C-1 and linearity with an R2 value of 0.998 in the temperature range of 25-80 °C. Additionally, the sensor demonstrated remarkable repeatability, making it suitable for real-time temperature monitoring of the human body and environment. This temperature sensor is successfully demonstrated in practical applications such as monitoring the temperature of various parts of the human body and sensing the spatial temperature. These demonstrations highlight their significant potential in electronic skin and other related fields.

Multifunctional organohydrogel via the synergy of dialdehyde starch and glycerol for motion monitoring and sign language recognition

Conductive hydrogel which belongs to a type of soft materials has recently become promising candidate for flexible electronics application. However, it remains difficult for conductive hydrogel-based strain sensors to achieve the organic unity of large stretchability, high conductivity, self-healing, anti-freezing, anti-drying and transparency. Herein, a multifunctional conductive organohydrogel with all of the above superiorities is prepared by crosslinking polyacrylamide (PAM) with dialdehyde starch (DAS) in glycerol-water binary solvent. Attributing to the synergy of abundant hydrogen bonding and Schiff base interactions caused by introducing glycerol and dialdehyde starch, respectively, the organohydrogel achieved balanced mechanical and electrical properties. Besides, the addition of glycerol promoted the water-locking effects, making the organohydrogel retain the superior mechanical properties and conductivity even at extreme conditions. The resultant organohydrogel strain sensor exhibits desirable sensing performance with high sensitivity (GF = 6.07) over a wide strain range (0-697 %), enabling the accurate monitoring of subtle body motions even at -30 °C. On the basis, a hand gesture monitor system based on the organohydrogel sensors arrays is constructed using machine learning method, achieving a considerable sign language recognition rate of 100 %, and thus providing convenience for communications between the hearing or speaking-impaired and general person.

3D printed mechanical robust cellulose derived liquid-free ionic conductive elastomer for multifunctional electronic devices

Ionic gel-based wearable electronic devices with robust sensing performance have gained extensive attention. However, the development of mechanical robustness, high conductivity, and customizable bio-based ionic gel for multifunctional wearable sensors still is a challenge. Herein, we first report the preparation of 3D printed cellulose derived ionic conductive elastomers (ICEs) with high mechanical toughness, high conductivity, and excellent environment stability through one-step photo-polymerization of polymerizable deep eutectic solvents. In the ICEs, carboxylate cellulose nanocrystals (C-CNCs) were used as a bio-template for the in-situ polymerization of the aniline to avoid the aggregation of polyaniline and yield a high conductivity (58.7 mS/m). More importantly, the well-defined structural design combining multiple hydrogen bonds with strong coordination bonds endows the ICEs with extremely high mechanical strength (4.4 MPa), toughness (13.33 MJ*m-3), high elasticity and excellent environment stability. Given by these features, the ICE was utilized to assemble multifunctional strain, humidity, and temperature sensors for real-time and reliable detection the human motions, respiration, and body temperature. This work provides a promising strategy for designing the new generation of strong, tough bio-based ionic gel for multifunctional wearable electronic devices.

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

Ion-conductive hydrogels with multi-functionality have gained significant attraction as flexible sensors in various fields such as wearable health monitoring and human motion detection, owing to their high ion conductivity, excellent flexibility and stretchability, and easy availability. In this work, multifunctional ion-conductive hydrogel with excellent mechanical properties, antifreezing properties, water retention and antibacterial performance was fabricated by the freeze-thaw crosslinking between polyvinyl alcohol (PVA) and aramid nanofibers (ANF), and the subsequent solution immersion crosslinking in a mixture of tannic acid (TA) and CaCl2 solution (DMSO/H2O as co-solvent). The rational engineering of a multi-spatial distributed hydrogen bond and Ca2+ coordination bond networks within the hydrogel led to a significant improvement in mechanical properties. Furthermore, through the introduction of TA and binary solvents (DMSO/H2O), the hydrogel had witnessed a substantial enhancement in its antimicrobial properties and water retention capacity. The resultant PAT5/CaCl2-5% (DMSO/H2O) hydrogel exhibited outstanding elongation at break (754.73%), tensile strength (6.25 MPa), electrical conductivity (3.09 S/m), which can be employed in flexible sensors to monitor real-time functional motion for human under diverse conditions. As such, this innovation opens up a novel pathway for envisioning flexible sensor devices, particularly in the realm of human activity monitoring.

A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring

Hydrogel-enabled skin bioelectronics that can continuously monitor health for extended periods is crucial for early disease detection and treatment. However, it is challenging to engineer ultrathin gas-permeable hydrogel sensors that can self-adhere to the human skin for long-term daily use (>1 week). Here, we present a ~10-micrometer-thick polyurethane nanomesh-reinforced gas-permeable hydrogel sensor that can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 8 days under daily life conditions. This research involves two key steps: (i) material design by gelatin-based thermal-dependent phase change hydrogels and (ii) robust thinness geometry achieved through nanomesh reinforcement. The resulting ultrathin hydrogels exhibit a thickness of ~10 micrometers with superior mechanical robustness, high skin adhesion, gas permeability, and anti-drying performance. To highlight the potential applications in early disease detection and treatment that leverage the collective features, we demonstrate the use of ultrathin gas-permeable hydrogels for long-term, continuous high-precision electrophysiological monitoring under daily life conditions up to 8 days.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Low-temperature, ultra-fast, and recyclable self-healing nanocomposites reinforced with non-solvent silylated modified cellulose nanocrystals

Developing polymeric materials with remarkable mechanical properties and fast self-healing performance even at low temperatures is challenging. Herein, the polymeric nanocomposites containing silane-treated cellulose nanocrystals (SCNC) with ultrafast self-healing and exceptional mechanical characteristics were developed even at low temperatures. First, CNC is modified with a cyclic silane coupling agent using an eco-friendly chemical vapor deposition method. The nanocomposite was then fabricated by blending SCNC with matrix prepolymer, prepared from monomers that possess lower critical solution temperature, followed by the inclusion of dibutyltin dilaurate and hexamethylene diisocyanate. The self-healing capability of the novel SCNC/polymer nanocomposites was enhanced remarkably by increasing the content of SCNC (0-3 wt%) and reaching (≥99 %) at temperatures (5 & 25 °C) within <20 min. Moreover, SCNC-3 showed a toughness of (2498 MJ/m3) and SCNC-5 displayed a robust tensile strength of (22.94 ± 0.4 MPa) whereas SCNC-0 exhibited a lower tensile strength (7.4 ± 03 MPa) and toughness of (958 MJ/m3). Additionally, the nanocomposites retain their original mechanical properties after healing at temperatures (5 & 25 °C) owing to the formation of hydrogen bonds via incorporation of the SCNC. These novel SCNC-based self-healable nanocomposites with tunable mechanical properties offer novel insight into preparing damage and temperature-responsive flexible and wearable devices.

Salt-Adaptively Conductive Ionogel Sensor for Marine Sensing

Hydrophobic ionogel has attracted much attention in underwater sensing as the artificial electronic skins and wearable sensors. However, when the low conductive ionogel-based sensor works in the marine environment, the salty seawater weakens its sensing performance, which is difficult to recognize. Herein, a salt-adaptively conductive ionogel with high submarine strain sensitivity is reported. Based on the preliminary improvement via the proton conduction mechanism, the conductivity of the ionogel further increases with the surrounding salinity rising up since the salt-induced dissociation phenomenon, which is described as the environmental salt-adaptive feature. In seawater, the conductivity of the ionogel is as high as 2.90 × 10-1 S m-1 . Significantly, with its long-term underwater stability and adhesion, the resultant ionogel-based sensor features prominent strain sensing performance (gauge factor: 1.12) while combining with various soft actuators in the marine environment. The ionogel-based sensor is capable of monitoring human breath frequency, human actions, and the locomotion of soft actuators, demonstrating its great potential in diving detection and intelligent preceptive soft robotics for marine environmental protection and exploration.

Multimodal and flexible hydrogel-based sensors for respiratory monitoring and posture recognition

The accurate monitoring of respiratory events and human motion states holds paramount importance in the realm of health surveillance and disease prognostication. An exquisitely precise, multifaceted, portable, and environmentally resilient sensor designed for health monitoring would undeniably be of utmost desirability, despite its persisting as a formidable challenge. Here, we propose a breath monitoring and posture recognition system that utilizes hydrogel electrolytes based on polyvinyl alcohol, sodium alginate, and starch, suitable for supercapacitors and multimodal wearable sensors. The multimodal smart sensors can independently detect mechanical and thermal changes through the output signals of capacitance and resistance, respectively. Moreover, we have cultivated an artificial neural network to achieve a finger-pressing posture recognition accuracy of up to 99.259%. Our hydrogel sensors have also been successfully employed in the diagnosis of obstructive sleep apnea syndrome. The flexible electronic device derived from this study exhibit a plethora of functionalities, thereby affording a novel perspective for the design and fabrication of advanced flexible electronic contrivances that find applications across diverse domains such as medicine and virtual reality.

Multifunctional Conductive Double-Network Hydrogel Sensors for Multiscale Motion Detection and Temperature Monitoring

As typical soft materials, hydrogels have demonstrated great potential for the fabrication of flexible sensors due to their highly compatible elastic modulus with human skin, prominent flexibility, and biocompatible three-dimensional network structure. However, the practical application of wearable hydrogel sensors is significantly constrained because of weak adhesion, limited stretchability, and poor self-healing properties of traditional hydrogels. Herein, a multifunctional sodium hyaluronate (SH)/borax (B)/gelatin (G) double-cross-linked conductive hydrogel (SBG) was designed and constructed through a simple one-pot blending strategy with SH and gelatin as the gel matrix and borax as the dynamic cross-linker. The obtained SBG hydrogels exhibited a moderate tensile strength of 25.3 kPa at a large elongation of 760%, high interfacial toughness (106.5 kJ m-3), strong adhesion (28 kPa to paper), and satisfactory conductivity (224.5 mS/m). In particular, the dynamic cross-linking between SH, gelatin, and borax via borate ester bonds and hydrogen bonds between SH and gelatin chain endowed the SBG hydrogels with good fatigue resistance (>300 cycles), rapid self-healing performance (HE (healing efficiency) ∼97.03%), and excellent repeatable adhesion. The flexible wearable sensor assembled with SBG hydrogels demonstrated desirable strain sensing performance with a competitive gauge factor and exceptional stability, which enabled it to detect and distinguish various multiscale human motions and physiological signals. Furthermore, the flexible sensor is capable of precisely perceiving temperature variation with a high thermal sensitivity (1.685% °C-1). As a result, the wearable sensor displayed dual sensory performance for temperature and strain deformation. It is envisioned that the integration of strain sensors and thermal sensors provide a novel and convenient strategy for the next generation of multisensory wearable electronics and lay a solid foundation for their application in electronic skin and soft actuators.

A facile strategy to fabricate a skin-like hydrogel with adhesive and highly stretchable attributes through small molecule triggering toward flexible electronics

Polyacrylamide hydrogel is a promising matrix in biomedical applications due to its biocompatibility, transparency and flexibility. However, its implementation in skin-attachable applications is impeded by its inherent deficiency in surface-adaptive adhesion and inadequate mechanical conformity to skin tissues. Herein, tris, a biocompatible small molecule with a triple hydrogen bonding cluster in its molecule structure, is introduced for the first time into a polyacrylamide hydrogel. This incorporation is achieved via a facile one-pot strategy, resulting in a highly stretchable hydrogel with an impressive strain capacity (2574.75 ± 28.19%), a human dermis tissue-compatible Young's modulus (27.89 ± 2.05 kPa) and an intrinsically universal adhesion capacity (16.66 ± 0.32 N). These superior properties are attributed to the elevated hydrogen bonding density and the plasticizing effect induced by tris, without compromising the hydrogel's excellent transparency (>90% transmittance). Moreover, by incorporating calcium ions into the resulting soft adhesive hydrogel, we demonstrate its utility in skin-like sensors, leading to a substantial enhancement in strain sensitivity and electrical conductivity, in conjunction with the plasticizing influence exerted by tris. This work offers a facile and environmentally friendly solution to fabricate ultra-stretchable adhesive polyacrylamide hydrogel matrixes for dynamic surfaces, even under large deformation, which can broaden their potential applications in integrated bioelectronics.

An Antisweat Interference and Highly Sensitive Temperature Sensor Based on Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) Fiber Coated with Polyurethane/Graphene for Real-Time Monitoring of Body Temperature

Body temperature is an important indicator of human health. The traditional mercury and medical electronic thermometers have a slow response (≥1 min) and can not be worn for long to achieve continuous temperature monitoring due to their rigidity. In this work, we prepared a skin-core structure polyurethane (PU)/graphene encapsulated poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) temperature-sensitive fiber in one step by combining wet spinning technology with impregnation technology. The composite fiber has high sensitivity (-1.72%/°C), super-resolution (0.1 °C), fast time response (17 s), antisweat interference, and high linearity (R2 = 0.98) in the temperature sensing range of 30-50 °C. The fiber is strong enough to be braided into the temperature-sensitive fabric with commercial cotton yarns. The fabric with good comfort and durability can be arranged in the armpit position of the cloth to realize real-time body temperature monitoring without interruption during daily activities. Through Bluetooth wireless transmission, body temperature can be monitored in real-time and displayed on mobile phones to the parents or guardians. Overall, the fiber-based temperature sensor will significantly improve the practical applications of wearable temperature sensors in intelligent medical treatment due to its sensing stability, comfort, and durability.

A Skin-Inspired Multifunctional Conductive Hydrogel with High Stretchable, Adhesive, Healable, and Decomposable Properties for Highly Sensitive Dual-Sensing of Temperature and Strain

Developing smart hydrogels with excellent physicochemical properties and multi-sensing capabilities for various simulation of human skin's functions still remains a great challenge. Here, based on simple and convenient one-step covalent cross-linking method enhanced by dynamic RS-Ag interactions, a skin-inspired multifunctional conductive hydrogel with desirable physicochemical properties (including high stretchability, self-adhesion, self-healing, decomposition and removability) is developed for highly sensitive dual-sensing of temperature and strain. Benefiting from the synergistic action of multiple hydrogen bonds, RS-Ag bonds and S-S bonds, the gel exhibited a novel thermosensitive mechanism. The prepared hydrogels exhibited extremely high mechanical properties (maximum tensile strength of 0.35 MPa, elongation at break nearly 1800%, compressive stress over 4.43 MPa), excellent self-healing (96.82% (stress), 88.45% (temperature), 73.89% (mechanical property)), decomposition (the molecular weight after decomposition is below 700) and self-adhesion (enhanced contact with the material interface). In addition, this conductive hydrogel could also simultaneously achieve highly sensitive temperature-sensing (TCR: 10.89) and stress-sensing (GF: 1.469). As a proof-to-concept, the hydrogel displayed superior capability for simulation of human skin to perception of touch, pressure and ambient temperature simultaneously, indicating promising applications in the fields of wearable devices, personal health care, and human-machine interfaces.

Platelet-Rich Plasma Composite Organohydrogel with Water-Locking and Anti-Freezing to Accelerate Wound Healing

Hydrogels have gained impressive attention in biological medicine due to their excellent biosafety, softness, and varied functional components. However, conventional hydrogels have inherent defects, such as low tensile strength, weak water-locking, and poor anti-freezing. In tissue engineering, once the hydrogel loses water or freezes, it will harden the interaction interfaces and destroy the nascent granulation tissue. Herein, based on the design concept of "hard frame-soft penetration", a composite adhesive organohydrogel is fabricated by introducing bacterial cellulose and platelet-rich plasma (PRP) into a poly-N-(tris[hydroxymethyl]methyl)acrylamide (THMA)/N-acryloyl aspartic acid (AASP) hybrid gel network infiltrated with glycerol/water binary solvent. The resultant organohydrogels exhibit excellent antifreeze properties at low temperatures (-80 °C) and demonstrate stable long-term water retention (91%) in the open environment within 12 days and can adhere firmly to the tissues by the action of "hydrogen bond clusters". Additionally, the introduction of bacterial cellulose matrix endows the organohydrogel with high tensile strength similar to that of skin. In vivo, the PRP-loaded organohydrogel can release a variety of growth factors to accelerate the wound healing process through collagen deposition and angiogenesis. Altogether, this strategy will extend the life of the hydrogel in some harsh medical environments.

Tracing the Flu Symptom Progression via a Smart Face Mask

Respiration and body temperature are largely influenced by the highly contagious influenza virus, which poses persistent global public health challenges. Here, we present a wireless all-in-one sensory face mask (WISE mask) made of ultrasensitive fibrous temperature sensors. The WISE mask shows exceptional thermosensitivity, excellent breathability, and wearing comfort. It offers highly sensitive body temperature monitoring and respiratory detection capabilities. Capitalizing on the advances in the Internet of Things and artificial intelligence, the WISE mask is further demonstrated by customized flexible circuitry, deep learning algorithms, and a user-friendly interface to continuously recognize the abnormalities of both the respiration and body temperature. The WISE mask represents a compelling approach to tracing flu symptom progression in a cost-effective and convenient manner, serving as a powerful solution for personalized health monitoring and point-of-care systems in the face of ongoing influenza-related public health concerns.

Twisted Optical Micro/Nanofibers Enabled Detection of Subtle Temperature Variation

The detection of subtle temperature variation plays an important role in many applications, including proximity sensing in robotics, temperature measurements in microfluidics, and tumor monitoring in healthcare. Herein, a flexible miniaturized optical temperature sensor is fabricated by embedding twisted micro/nanofibers in a thin layer of polydimethylsiloxane. Enabled by the dramatic change of the coupling ratio under subtle temperature variation, the sensor exhibits an ultrahigh sensitivity (-30 nm/°C) and high resolution (0.0012 °C). As a proof-of-concept demonstration, a robotic arm equipped with our sensor can avoid undesired collisions by detecting the subtle temperature variation caused by the existence of a human. Moreover, benefiting from the miniaturized and engineerable sensing structure, real-time measurement of subtle temperature variation in microfluidic chips is realized. These initial results pave the way toward a category of optical sensing devices ranging from robotic skin to human-machine interfaces and implantable healthcare sensors.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

The Addition of an Ultra-Small Amount of Black Phosphorous Quantum Dots Endow Self-Healing Polyurethane with a Biomimetic Intelligent Response

This study explores new applications of black phosphorus quantum dots (BPQDs) by adding them to self-healing material systems for the first time. Self-healing polyurethane with an ultra-small amount of BPQDs has biomimetic intelligent responsiveness and achieves balance between its mechanical and self-healing properties. By adding 0.0001 wt% BPQDs to self-healing polyurethane, the fracture strength of the material increases from 3.0 to 12.3 MPa, and the elongation at break also increases from 750% to 860%. Meanwhile, the self-healing efficiency remains at 98%. The addition of BPQDs significantly improves the deformation recovery ability of the composite materials and transforms the surface of self-healing polyurethane from hydrophilic to hydrophobic, making it suitable for applications in fields such as electronic skin and flexible wearable devices. This study provides a simple and feasible strategy for endowing self-healing materials with biomimetic intelligent responsiveness using a small amount of BPQDs.