An ultrastretchable, high-performance, and crosstalk-free proximity and pressure bimodal sensor based on ionic hydrogel fibers for human-machine interfaces

Highly Sensitive Fiber Crossbar Sensors Enabled by Second-Order Synergistic Effect of Air Capacitance and Equipotential Body

Fiber crossbars, an emerging electronic device, have become the most promising basic unit for advanced smart textiles. The demand for highly sensitive fiber crossbar sensors (FCSs) in wearable electronics is increased. However, the unique structure of FCSs presents challenges in replicating existing sensitivity enhancement strategies. Aiming at the sensitivity of fiber crossbar sensors, a second-order synergistic strategy is proposed that combines air capacitance and equipotential bodies, resulting in a remarkable sensitivity enhancement of over 20 times for FCSs. This strategy offers a promising avenue for the design and fabrication of FCSs that do not depend on intricate microstructures. Furthermore, the integrative structure of core-sheath fibers ensures a robust interface, leading to a low hysteresis of only 2.33% and exceptional stability. The outstanding capacitive response performance of FCSs allows them to effectively capture weak signals such as pulses and sounds. This capability opens up possibilities for the application of FCSs in personalized health management, as demonstrated by wireless monitoring systems based on pulse signals.

In Situ Structural Densification of Hydrogel Network and Its Interface with Electrodes for High-Performance Multimodal Artificial Skin

The multisensory responsiveness of hydrogels positions them as promising candidates for artificial skin, whereas the mismatch of modulus between soft hydrogels and hard electrodes as well as the poor adhesion and conductance at the interface greatly impairs the stability of electronics devices. Herein, we propose an in situ postprocessing approach utilizing electrochemical reactions between metals (Zn, etc.) and hydrogels to synergistically achieve strong adhesion of the hydrogel-electrode interface, low interfacial impedance, and local strain isolation due to the structural densification of the hydrogel network. The mechanism is that Zn electrochemically oxidizes to Zn2+ and injects into the hydrogel, gradually forming a mechanically interlocked structure, Zn2+-polymer dual-helix structural nodes, and a high-modulus ZnO from the surface to the interior. Compared to untreated samples, the treated sample displays 8.7 times increased interfacial adhesion energy between the hydrogel and electrode (87 J/m2), 95% decreased interfacial impedance (218.8 Ω), and a high-strain isolation efficiency (εtotal/εisolation > 400). Akin to human skin, the prepared sensor demonstrates multimodal sensing capabilities, encompassing highly sensitive strain perception and simultaneous perception of temperature, humidity, and oxygen content unaffected by strain interference. This easy on-chip preparation of hydrogel-based multimodal sensor array shows great potential for health and environment monitoring as artificial skin.

Proximity Sensing Electronic Skin: Principles, Characteristics, and Applications

The research on proximity sensing electronic skin has garnered significant attention. This electronic skin technology enables detection without physical contact and holds vast application prospects in areas such as human-robot collaboration, human-machine interfaces, and remote monitoring. Especially in the context of the spread of infectious diseases like COVID-19, there is a pressing need for non-contact detection to ensure safe and hygienic operations. This article comprehensively reviews the significant advancements in the field of proximity sensing electronic skin technology in recent years. It covers the principles, as well as single-type proximity sensors with characteristics such as a large area, multifunctionality, strain, and self-healing capabilities. Additionally, it delves into the research progress of dual-type proximity sensors. Furthermore, the article places a special emphasis on the widespread applications of flexible proximity sensors in human-robot collaboration, human-machine interfaces, and remote monitoring, highlighting their importance and potential value across various domains. Finally, the paper provides insights into future advancements in flexible proximity sensor technology.

Transparent multifunctional cellulose-based conductive hydrogel for wearable strain sensors and arrays

Conductive hydrogels as promising candidate materials for flexible strain sensors have gained considerable attentions. However, it is still a great challenge to construct hydrogel with multifunctional performance via natural polymer. Herein, a novel multifunctional conductive hydrogel based on methylcellulose and cellulose nanocrystal was prepared via a facile and low-cost strategy. Methylcellulose (MC) was introduced to not only guarantee the stability of tannic acid coated cellulose nanocrystal (TA@CNCs) in LiCl solution, but also improve anti-freezing ability. The obtained hydrogel exhibited high transparency (98 % at 800 nm), good stretchability (663.1 %), low temperature tolerance (-23.9 °C), superior conductivity (2.89 S/m) and excellent UV shielding behavior. Flexible strain sensor assembled by the prepared hydrogels can be used to detect human body motions include subtle and large motions, and exhibited good sensitivity and stability over a wide temperature range. Multiple flexible hydrogels can also be assembled into a 3D sensor array to detect the distribution and magnitude of spatial pressure. Therefore, the hydrogels prepared via natural polymers will have broad application prospects in wearable devices, electronic skin and multifunctional sensor components.

A Microphase-Separated Design toward an All-Round Ionic Hydrogel with Discriminable and Anti-Disturbance Multisensory Functions

Stretchable ionic hydrogels with superior all-round properties that can detect multimodal sensations with excellent discriminability and robustness against external disturbances are highly required for artificial electronic skinapplications. However, some critical material parameters exhibit intrinsic tradeoffs with each other for most ionic hydrogels. Here, a microphase-separated hydrogel is demonstrated by combining three strategies: (1) using of a low crosslinker/monomer ratio to obtain highly entangled polymer chains as the first network; (2) the introduction of zwitterions into the first network; (3) the synthesis of an ultrasoft polyelectrolyte as the second network. This all-round elastic ionic hydrogel exhibits a low Young's modulus (< 60 kPa), large stretchability (> 900%), high resilience (> 95%), unique strain-stiffening behavior, excellent fatigue tolerance, high ionic conductivity (> 2.0 S m⁻1), and anti-freezing capability, which have not been achieved before. These properties allow the ionic hydrogel to operate as a stretchable multimodal sensor that can detect and decouple multiple stimuli (temperature, pressure, and proximity) with excellent discriminability, high sensitivity, and strong sensing-robustness against strains or temperature perturbations. The ionic hydrogel sensor exhibits great potential for intelligent electronic skin applications such as reliable health monitoring and accurate object identification.

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.

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Robust and sensitive conductive nanocomposite hydrogel with bridge cross-linking-dominated hierarchical structural design

Conductive hydrogels have a remarkable potential for applications in soft electronics and robotics, owing to their noteworthy attributes, including electrical conductivity, stretchability, biocompatibility, etc. However, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in nature, we successfully fabricate a robust and sensitive conductive nanocomposite hydrogel through self-assembly-induced bridge cross-linking of MgB2 nanosheets and polyvinyl alcohol hydrogels. By combining the hierarchical lamellar microstructure with robust molecular B─O─C covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength and toughness. Moreover, the hydrogel demonstrates exceptional sensitivity (response/relaxation time, 20 milliseconds; detection lower limit, ~1 Pascal) under external deformation. Such characteristics enable the conductive hydrogel to exhibit superior performance in soft sensing applications. This study introduces a high-performance conductive hydrogel and opens up exciting possibilities for the development of soft electronics.

Fiber-Based Noncontact Sensor with Stretchability for Underwater Wearable Sensing and VR Applications

The rapid development of artificial intelligent wearable devices has led to an increasing need for seamless information exchange between humans, machines, and virtual spaces, often relying on touch sensors as the primary interaction medium. Additionally, the demand for underwater detection technologies is on the rise owing to the prevalent wet and submerged environment. Here, a fiber-based capacitive sensor with superior stretchability and hydrophobicity is proposed, designed to cater to noncontact and underwater applications. The sensor is constructed using bacterial cellulose (BC)@BC/carbon nanotubes (CNTs) (BBT) helical fiber as the matrix and methyltrimethoxysilane (MTMS) as the hydrophobic modified agent, forming a hydrophobic silylated BC@BC/CNT (SBBT) helical fiber by the chemical vapor deposition (CVD) technique. These fibers exhibit an impressive contact angle of 132.8°. The SBBT helicalfiber-based capacitive sensor presents capabilities for both noncontact and underwater sensing, which exhibits a significant capacitance change of -0.27 (at a distance of 0.5 cm). We have achieved interactive control between real space and virtual space through intelligent data analysis technology with minimal interference from the presence of water. This work has laid a solid foundation of noncontact sensing with attributes such as degradability, stretchability, and hydrophobicity. Moreover, it offers promising solutions for barrier-free communication in virtual reality (VR) and underwater applications, providing avenues for smart human-machine interfaces for submerged use.

Highly Sensitive Capacitive Fiber Pressure Sensors Enabled by Electrode and Dielectric Layer Regulation

Capacitive pressure sensors play an important role in the field of flexible electronics. Despite significant advances in two-dimensional (2D) soft pressure sensors, one-dimensional (1D) fiber electronics are still struggling. Due to differences in structure, the theoretical research of 2D sensors has difficulty guiding the design of 1D sensors. The multiple response factors of 1D sensors and the capacitive response mechanism have not been explored. Fiber sensors urgently need a tailor-made theoretical research and development path. In this regard, we established a fiber pressure-sensing platform using a coaxial wet spinning process. Aiming at the two problems of the soft electrode modulus and dielectric layer thickness, the conclusions are drawn from three aspects: model analysis, experimental verification, and formula derivation. It makes up some theoretical blanks of capacitive fiber pressure sensors. Through the self-regulation of these two factors without a complex structural design, the sensitivity can be significantly improved. This provides a great reference for the design and development of fiber pressure sensors. Besides, taking advantage of the scalability and easy integration of 1D electronics, multipoint sensors prepared by fibers have verified their application potential in health monitoring, human-machine interface, and motion behavior analysis.

Touchless Artificial Perception beyond Fingertip Probing

Touchless perception technology allows us to acquire information beyond the contact interfaces, making it ideal for scenarios where physical engagements are not possible. Unlike tactile devices, which have so far achieved impressive results, touchless strategies are fascinating yet underdeveloped. We envisage that touchless technologies could be powerful supplements to current haptics. In this Perspective, we include emerging touchless electronics, aiming to provide a broader and comprehensive picture toward artificial perceptual realm. We overview popular touchless protocols, sketch what could be detected by touchless probing, and summarize their latest spectacular achievements. In addition, we present the promises and challenges posed by touchless technologies and discuss possible directions for their future deployments.

Wireless, Smart Hemostasis Device with All-Soft Sensing System for Quantitative and Real-Time Pressure Evaluation

The properly applied pressure between the skin and hemostasis devices is an essential parameter for preventing bleeding and postoperative complications after a transradial procedure. However, this parameter is usually controlled based on the subjective judgment of doctors, which might cause insufficient hemostatic effect or thrombosis. Here this study develops a compact and wireless sensing system for continuously monitoring the pressure applied on the radial artery and wrist skin in clinical practice. A liquid metal (LM)-based all-soft pressure sensor is fabricated to enable conformal attachment between the device and skin even under large deformation conditions. The linear sensitivity of 0.007 kPa-1 among the wide pressure range of 0-100 kPa is achieved and the real-time detection data can be wirelessly transmitted to mobile clients as a reference pressure value. With these devices, detailed pressure data can be collected, analyzed, and stored for medical assistance as well as to improve surgery quality.

Hierarchical Fermat helix-structured electrochemical sensing fibers enable sweat capture and multi-biomarker monitoring

Wearable electrochemical sensors have shown potential for personal health monitoring due to their ability to detect biofluids non-invasively at the molecular level. Smart fibers with high flexibility and comfort are currently ideal for fabricating electrochemical sensors, but little research has focused on fluid transport at the human-machine interface, which is of great significance for continuous and stable monitoring and skin comfort. Here, we report an electrochemical sensing fiber with a special core-sheath structure, whose outer layer is wound by nanofibers with a hierarchical Fermat helix structure which has excellent moisture conductivity, and the inner layer is based on CNT fibers covered by three-dimensional reduced graphene oxide folds which have good sensing properties after modification of active materials such as enzymes and selective membranes. This kind of fiber enables efficient sweat capture, and thus only 0.1 μL of sweat is required to activate the device, and it responds very quickly (1.5 s). The fibers were further integrated into a garment to build a wireless sweat detection system, enabling stable monitoring of six physiological markers in sweat (glucose, lactate, Na+, K+, Ca2+, and pH). This work provides a feasible proposal for future personalized medicine and the construction of "smart sensing garments".

An Ultrahighly Stretchable and Recyclable Starch-Based Gel with Multiple Functions

With the development of various gel-based flexible sensors, novel gels with multiple integrated and efficient properties, particularly recyclability, have been developed. Herein, a starch-based ADM (amylopectin (AP)-poly(3-[dimethyl-[2-(2-methylprop-2- enoyloxy)ethyl]azaniumyl]propane-1-sulfonate) (PDMAPS)-MXene) gel is prepared by a facile "cooking" strategy accompanying the gelatinization of AP and polymerization reaction of zwitterionic monomers. Reversible crosslinking in the gel occurs through hydrogen bonding and electrostatic interactions. The ADM gel exhibits high stretchability (≈2700%, after one month), swift self-healing performance, self-adhesive properties, favorable freezing resistance, and satisfactory moisturizing properties (≥30 days). Interestingly, the ADM gel can be recycled and reused by a "kneading" method and "dissolution-dialysis" process, respectively. Furthermore, the ADM gel can be assembled as a strain sensor with a broad working strain range (≈800%) and quick response time (response time 211 ms and recovery time 253 ms, under 10% strain) to detect various macro- and micro-human-motions, even under harsh conditions such as pronunciation and handwriting. The ADM gel can also be used as a humidity sensor to investigate humidity and human respiratory status, suggesting its practical application in personal health management. This study provides a novel strategy for the preparation of high-performance recycled gels and flexible sensors.

Bioinspired Luminescent HOF-Based Foam as Ultrafast and Ultrasensitive Pressure and Acoustic Bimodal Sensor for Human-Machine Interactive Object and Information Recognition

Bionic sensors have extensively served smart robots, medical equipment, and flexible wearable devices. The luminescent pressure-acoustic bimodal sensor can be treated as a remarkable, multifunctional, integrated bionic device. Here, a blue-emitting hydrogen-bonded organic framework (HOF-TTA) as luminogen combines with melamine foam (MF), generating the flexible and elastic HOF-TTA@MF (1 and 2) as a pressure-auditory bimodal sensor. In the luminescent pressure sensing process, 1 has excellent maximum sensitivity (132.02 kPa-1 ), low minimum detection limit (0.0 1333 Pa), fast response time (20 ms), high precision and great recyclability. 2 as a luminescent auditory sensor exhibits the highest response to the 520 Hz sound at 255-1453 Hz. In the process of sensing sound at 520 Hz, 2 possesses high sensitivity (1 648 441.3 cps Pa-1  cm-2 ), low detection limit (0.36 dB) and ultrafast response time (10 ms) within 11.47-91.77 dB. The sensing mechanisms toward pressure and auditory are analyzed in detail by finite element simulation. Furthermore, 1 and 2, as a human-machine interactive bimodal sensor, can recognize nine different objects and word information of "Health", "Phone", and "TongJi" with high accuracy and strong robustness. This work provides a facile fabricated method of luminescent HOF-based pressure-auditory bimodal sensors and endows them with new recognition functions and dimensions.

Stretchable and sensitive sodium alginate ionic hydrogel fibers for flexible strain sensors

Ionic conductive hydrogel fibers based on natural polymers provide an immense focus for a new generation of electronics due to their flexibility and knittability. The feasibility of utilizing pure natural polymer-based hydrogel fibers could be drastically improved if their mechanical and transparent performances satisfy the requirements of actual practice. Herein, we report a facile fabrication strategy for significantly stretchable and sensitive sodium alginate ionic hydrogel fibers (SAIFs), by glycerol initiating physical crosslinking and by CaCl2 inducing ionic crosslinking. The obtained ionic hydrogel fibers not only show significant stretchability (tensile strength of 1.55 MPa and fracture strain of ∼161 %), but also exhibit wide-range sensing, satisfactorily stable, rapidly responsive, and multiply sensitive abilities to external stimulus. In addition, the ionic hydrogel fibers have excellent transparency (over 90 % in a wide wavelength range), and good anti-evaporation and anti-freezing properties. Furthermore, the SAIFs have been easily knitted into a textile, and successfully applied as wearable sensors to recognize human motions, by observing the output electrical signals. Our methodology for fabrication intelligent SAIFs will shed light on artificial flexible electronics and other textile-based strain sensors.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

Fabric-like Electrospun PVAc-Graphene Nanofiber Webs as Wearable and Degradable Piezocapacitive Sensors

Flexible piezocapacitive sensors utilizing nanomaterial-polymer composite-based nanofibrous membranes offer an attractive alternative to more traditional piezoelectric and piezoresistive wearable sensors owing to their ultralow powered nature, fast response, low hysteresis, and insensitivity to temperature change. In this work, we propose a facile method of fabricating electrospun graphene-dispersed PVAc nanofibrous membrane-based piezocapacitive sensors for applications in IoT-enabled wearables and human physiological function monitoring. A series of electrical and material characterization experiments were conducted on both the pristine and graphene-dispersed PVAc nanofibers to understand the effect of graphene addition on nanofiber morphology, dielectric response, and pressure sensing performance. Dynamic uniaxial pressure sensing performance evaluation tests were conducted on the pristine and graphene-loaded PVAc nanofibrous membrane-based sensors for understanding the effect of two-dimensional (2D) nanofiller addition on pressure sensing performance. A marked increase in the dielectric constant and pressure sensing performance was observed for graphene-loaded spin coated membrane and nanofiber webs respectively, and subsequently the micro dipole formation model was invoked to explain the nanofiller-induced dielectric constant enhancement. The robustness and reliability of the sensor have been underscored by conducting accelerated lifetime assessment experiments entailing at least 3000 cycles of periodic tactile force loading. A series of tests involving human physiological parameter monitoring were conducted to underscore the applicability of the proposed sensor for IoT-enabled personalized health care, soft robotics, and next-generation prosthetic devices. Finally, the easy degradability of the sensing elements is demonstrated to emphasize their suitability for transient electronics applications.

Dynamic Cross-Linking Network Construction of Carboxymethyl Starch Enabling Temperature and Strain Bimodal Film Sensors

Building stimulus-responsive units in the hydrogel coatings remains challenging for film sensors consisting of alternated layers of inert substrates and hydrogel coatings. An interesting film sensor with a carboxymethyl starch-based hydrogel coating was developed here. The cross-linking networks of carboxymethyl starch play the roles of structure-constructing units and stimulus-controlling units simultaneously, endowing the coatings with thermal sensing and strain sensing capabilities. The dynamic cross-links formed via the boronic ester bonds are temperature-sensitive, releasing or consuming additional acid ions with temperature alteration, and also as primary networks give the hydrogel strength and stretchability with the assistance of semi-penetrated polyacrylamide chains. Therefore, as-prepared flexible film sensors can be used to detect the periodic changes of human temperature and small-scale motion with multiple working modes, discriminating the physical states related to human health. Moreover, this kind of starch-based coating is degradable in a strongly alkaline solution and the inert substrate layer can protect the skin from erosion caused by direct hydrogel-skin contact, and thereby the film sensor is human- and environmentally friendly. This work also proposes a strategy of building temperature-sensitive units in the film sensor via regulating the chemical networks, instead of tuning physical structures.

High strength, anti-freezing and conductive silkworm excrement cellulose-based ionic hydrogel with physical-chemical double cross-linked for pressure sensing

Recently, ionic conductive hydrogels have attracted extensive attention in the field of flexible pressure sensors due to their mechanical flexibility and high conductivity. However, the trade-off between the high electrical and mechanical properties of ionic conductive hydrogels and the loss of mechanical and electrical properties of traditional high water content hydrogels at low temperature are still the main hurdles in this area. Herein, a rigid Ca-rich silkworm excrement cellulose (SECCa) extracted from silkworm breeding waste was prepared. SEC-Ca was combined with the flexible hydroxypropyl methylcellulose (HPMC) molecules through hydrogen bonding and double ionic bonds of Zn²⁺ and Ca²⁺ to obtain the physical network SEC@HPMC-(Zn²⁺/Ca²⁺). Then, the covalently cross-linked network of polyacrylamide (PAAM) and the physical network were cross-linked by hydrogen bonding to obtain the physical-chemical double cross-linked hydrogel (SEC@HPMC-(Zn²⁺/Ca²⁺)/PAAM). The hydrogel showed excellent compression properties (95 %, 4.08 MPa), high ionic conductivity (4.63 S/m at 25 °C) and excellent frost resistance (possessing ionic conductivity of 1.20 S/m at -70 °C). Notably, the hydrogel can monitor pressure changes in a wide temperature range (-60-25 °C) with high sensitivity, stability and durability. This newly fabricated hydrogel-based pressure sensors can be deemed of great prospects for large-scale application of pressure detection at ultra-low temperatures.

Polymer Complex Fiber: Property, Functionality, and Applications

Polymer complex fibers (PCFs) are a novel kind of fiber material processed from polymer complexes that are assembled through noncovalent interactions. These can realize the synergy of functional components and miscibility on the molecular level. The dynamic character of noncovalent interactions endows PCFs with remarkable properties, such as reversibility, stimuli responsiveness, self-healing, and recyclability, enabling them to be applied in multidisciplinary fields. The objective of this article is to provide a review of recent progress in the field of PCFs. The classification based on chain interactions will be first introduced followed by highlights of the fabrication technologies and properties of PCFs. The effects of composition and preparation method on fiber properties are also discussed, with some emphasis on utilizing these for rational design. Finally, we carefully summarize recent advanced applications of PCFs in the fields of energy storage and sensors, water treatment, biomedical materials, artificial actuators, and biomimetic platforms. This review is expected to deepen the comprehension of PCF materials and open new avenues for developing PCFs with tailor-made properties for advanced application.

High-Performance Strain Sensors Based on Organohydrogel Microsphere Film for Wearable Human-Computer Interfacing

Stretchable hydrogel-based strain sensors suffer from limited sensitivity, which urgently requires further breakthroughs for precise and stable human-computer interaction. Here, an efficient microstructural engineering strategy is proposed to significantly enhance the sensitivity of hydrogel-based strain sensors by sandwiching an emulsion-polymerized polyacrylamide organohydrogel microsphere membrane between two Ecoflex films, which are accompanied by crack generation and propagation effects upon stretching. Consequently, the as-developed strain sensor exhibits ultrahigh sensitivity (gauge factor (GF) of 1275), wide detection range (100% strain), low hysteresis, ultralow detection limit (0.05% strain), good fatigue resistance, and low fabrication cost. In addition, the sensor features good water, dehydration, and frost resistance, enabling real-time strain monitoring in various complex conditions due to the encapsulation of Ecoflex film and the addition of glycerol and KCl. Through further structural manipulation, the device achieves superior response to tiny strains, with a GF value of 98.3 in the strain range of less than 1.5%. Owing to the high strain sensing performance, the sensor is able to detect various human activities from swallowing to finger bending even under water. On this basis, a wireless sensing system with apnea warning and single-channel gesture recognition capabilities is successfully demonstrated, demonstrating its great promise as wearable electronics.

3D Single-Layer-Dominated Graphene Foam for High-Resolution Strain Sensing and Self-Monitoring Shape Memory Composite

Flexible intelligent materials are desired to effectively regulate their own deformation and accurately sense their immediate morphology at the same time. Graphene foam is an attractive material for strain sensing and electrical/thermal performance control due to its outstanding mechanical, electrical, and thermal properties. However, graphene-foam-based materials with both strain sensing and deformation control capabilities are rarely reported. Here, a multiscale design of graphene foam with a single-layer-graphene-dominated microstructure and resilient 3D network architecture, which leads to exceptional strain sensing performance as well as modulation ability of the electrical and thermal conductivity for shape memory polymers, is reported. The graphene foams exhibit a strain detection limit of 0.033%, a rapid response of 53 ms, long-term stability over 10 000 cycles, significant thermoacoustic effect, and great heat-generation and heat-diffusion ability. By combining these advantages, an electro-activated shape-memory composite that is capable of monitoring its own shape state during its morphing process, is demonstrated.

Self-Powered Respiratory Monitoring Strategy Based on Adaptive Dual-Network Thermogalvanic Hydrogels

As a low-grade sustainable heat source, the breath waste heat exhaled by human bodies is always ignored, although producing a greater temperature than ambient. Converting this heat into electric energy for use as power sources or detecting signals is extremely important in cutting-edge wearable medicine. This heat-to-electricity conversion is possible with thermogalvanic hydrogels. However, challenges remain in their antifreezing and antidrying properties, significantly restricting the durability of thermogalvanic gels in practical applications. Herein, a dual-network poly(vinyl alcohol)/gelatin (PVA/GEL) gel thermogalvanic device with Fe(CN)₆^3-/4- as a redox pair is developed, with an outstanding low-temperature durability and antidrying capacity. These features result from the use of a binary H₂O/GL (glycerin) solvent to limit hydrogen bonding between water molecules. The prepared thermogalvanic gel patch is capable of easily converting physiological data into understandable electrical impulses using the temperature difference between the ambient environment and the heat produced by human breathing, realizing a simple self-powered respiratory monitoring strategy for the first time. Even below zero temperature, the gel patch-based mask can operate normally, implying it fits into low-temperature environments. This study sheds fresh light on the development of active wearable medical electronics that are powered by demic low-level heat.

Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Ultrasensitive, stretchable, and transparent humidity sensor based on ion-conductive double-network hydrogel thin films

Ion-conductive hydrogels with intrinsic biocompatibility, stretchability, and stimuli-responsive capability have attracted considerable attention because of their extensive application potential in wearable sensing devices. The miniaturization and integration of hydrogel-based devices are currently expected to achieve breakthroughs in device performance and promote their practical application. However, currently, hydrogel film is rarely reported because it can be easily wrinkled, torn, and dehydrated, which severely hinders its development in microelectronics. Herein, thin, stretchable, and transparent ion-conductive double-network hydrogel films with controllable thickness are integrated with stretchable elastomer substrates, which show good environmental stability and ultrahigh sensitivity to humidity (78,785.5%/% relative humidity (RH)). Benefiting from the ultrahigh surface-area-to-volume ratio, abundant active sites, and short diffusion distance, the hydrogel film humidity sensor exhibits 2 × 10⁵ times increased response to 98% RH, as well as 5.9 and 7.6 times accelerated response and recovery speeds compared with the bulk counterpart, indicating its remarkable thickness-dependent humidity-sensing properties. The humidity-sensing mechanism reveals that the adsorption of water improves the ion migration and dielectric constant, as well as establishes the electrical double layer. Furthermore, the noncontact human-machine interaction and real-time respiratory frequency detection are enabled by the sensors. This work provides an innovative strategy to achieve further breakthroughs in device performance and promote the development of hydrogel-based miniaturized and integrated electronics.

Hydrogel- and organohydrogel-based stretchable, ultrasensitive, transparent, room-temperature and real-time NO2 sensors and the mechanism

Highly stretchable, sensitive and room-temperature nitrogen dioxide (NO₂) sensors are fabricated by exploiting intrinsically stretchable, transparent and ion-conducting hydrogels and active metals as the novel transducing materials and electrodes, respectively. The NO₂ sensor exhibits high sensitivity (60.02% ppm^-1), ultralow theoretical limit of detection (6.8 ppb), excellent selectivity, linearity and reversibility at room temperature. Notably, the sensitivity can be maintained even under 50% tensile strain. For the first time, it's found that the metal electrodes significantly impact the sensing performance. Specifically, the sensitivity is boosted from 31.18 to 60.02% ppm^-1 by replacing the anodic silver with copper-tin alloy. Importantly, by applying specially designed sensing tests, and microscopic and composition analyses, we have obtained the inherent NO₂ sensing mechanism: the anodic metal tends to be oxidized and the NO₂ molecules tend to react in the cathode-gel interface. The introduction of glycerol converts the hydrogel into the organohydrogel with remarkably enhanced anti-drying and anti-freezing capacities and toughness, which effectively improved the long-time stability of the sensors. Importantly, we execute sound/light alarms and a wireless smartphone alarm by utilizing a designed circuit board and applet. This work gives an incisive investigation for the preparation, performance improvement, mechanism and application of hydrogel-based NO₂ sensors, promoting the evolution of hydrogel ionotronics.