Ultrasensitive, Stretchable, and Fast-Response Temperature Sensors Based on Hydrogel Films for Wearable Applications

Recent advances in smart wearable sensors for continuous human health monitoring

In recent years, the biochemical and biological research areas have shown great interest in a smart wearable sensor because of its increasing prevalence and high potential to monitor human health in a non-invasive manner by continuous screening of biomarkers dispersed throughout the biological analytes, as well as real-time diagnostic tools and time-sensitive information compared to conventional hospital-centered system. These smart wearable sensors offer an innovative option for evaluating and investigating human health by incorporating a portion of recent advances in technology and engineering that can enhance real-time point-of-care-testing capabilities. Smart wearable sensors have emerged progressively with a mixture of multiplexed biosensing, microfluidic sampling, and data acquisition systems incorporated with flexible substrate and bodily attachments for enhanced wearability, portability, and reliability. There is a good chance that smart wearable sensors will be relevant to the early detection and diagnosis of disease management and control. Therefore, pioneering smart wearable sensors into reality seems extremely promising despite possible challenges in this cutting-edge technology for a better future in the healthcare domain. This review presents critical viewpoints on recent developments in wearable sensors in the upcoming smart digital health monitoring in real-time scenarios. In addition, there have been proactive discussions in recent years on materials selection, design optimization, efficient fabrication tools, and data processing units, as well as their continuous monitoring and tracking strategy with system-level integration such as internet-of-things, cyber-physical systems, and machine learning algorithms.

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.

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.

Digital Light Processing 4D Printing of Poloxamer Micelles for Facile Fabrication of Multifunctional Biocompatible Hydrogels as Tailored Wearable Sensors

The lack of both digital light processing (DLP) compatible and biocompatible photopolymers, along with inappropriate material properties required for wearable sensor applications, substantially hinders the employment of DLP 3D printing in the fabrication of multifunctional hydrogels. Herein, we discovered and implemented a photoreactive poloxamer derivative, Pluronic F-127 diacrylate, which overcomes these limitations and is optimized to achieve DLP 3D printed micelle-based hydrogels with high structural complexity, resolution, and precision. In addition, the dehydrated hydrogels exhibit a shape-memory effect and are conformally attached to the geometry of the detection point after rehydration, which implies the 4D printing characteristic of the fabrication process and is beneficial for the storage and application of the device. The excellent cytocompatibility and in vivo biocompatibility further strengthen the potential application of the poloxamer micelle-based hydrogels as a platform for multifunctional wearable systems. After processing them with a lithium chloride (LiCl) solution, multifunctional conductive ionic hydrogels with antifreezing and antiswelling properties along with good transparency and water retention are easily prepared. As capacitive flexible sensors, the DLP 3D printed micelle-based hydrogel devices exhibit excellent sensitivity, cycling stability, and durability in detecting multimodal deformations. Moreover, the DLP 3D printed conductive hydrogels are successfully applied as real-time human motion and tactile sensors with satisfactory sensing performances even in a -20 °C low-temperature environment.

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.

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.

Flexible thermoelectric CMTs/KCl/gelatin composite for a wearable pressure and temperature sensor

Flexible sensors have promising applications in the fields of health monitoring and artificial intelligence, which have attracted much attention from researchers. However, the design and manufacture of sensors with multiple sensing functions (like simultaneously having both temperature and pressure sensing capabilities) still present a significant challenge. Here, an ionic thermoelectric sensor for synchronous temperature and pressure sensing was developed on the basis of a carbon microtubes (CMTs)/potassium chloride (KCl)/gelatin composite consisting of gelatin as the polymer matrix, CMTs as the conductive material and KCl as the ion source. The designed CMTs/KCl/gelatin composite with the good ductility (830%) and flexibility can achieve a Seebeck coefficient of 4 mV K-1 and a dual stimulus responsiveness to pressure and temperature. In addition, not only the movement of the human body (e.g., fingers, arms), but also the temperature difference between the human body and the environment, were able to be monitored by the designed CMTs/KCl/gelatin sensors. This study provides a novel strategy for the design and preparation of high-performance flexible sensors by utilizing ion-gel thermoelectric materials and promotes the research of temperature and pressure sensing technologies.

Mussel-inspired adhesive and anti-swelling hydrogels for underwater strain sensing

The application of hydrogels in an underwater environment is limited due to their swelling behavior and the existence of a hydration layer. In this study, a hydrogel based on poly(sulfobetaine methacrylate) (PSBMA), tannic acid (TA) and montmorillonite (MMT) was prepared with excellent anti-swelling properties and underwater self-adhesion properties. The PSBMA hydrogel has excellent anti-swelling properties due to the strong electrostatic interaction between charged groups of PSBMA chains. Inspired by marine mussels, tannic acid modified montmorillonite (TA@MMT) was introduced. Natural polyphenol tannic acid, as a catechol donor, provides a large number of catechol groups for hydrogels. Montmorillonite acts as the physical cross-linking point of PSBMA chains through electrostatic interaction to improve the cohesion of the hydrogel. By combining the adhesion mechanism of zwitterions and catechol, the hydrogel maintains adhesion in air and underwater environments. In addition, a strain sensor was prepared based on the PSBMA/TA@MMT hydrogel, which can closely fit the human skin and stably monitor different movements in air and in underwater environments. Through a Bluetooth communication system, long-distance information transmission can be achieved. Therefore, the PSBMA/TA@MMT hydrogel broadens the application prospect of wearable devices in the underwater environment.

High-Sensitivity and Wide-Range Flexible Ionic Piezocapacitive Pressure Sensors with Porous Hemisphere Array Electrodes

The development of high-performance flexible pressure sensors with porous hierarchical microstructures is limited by the complex and time-consuming preparation processes of porous hierarchical microstructures. In this study, a simple modified heat curing process was first proposed to achieve one-step preparation of porous hemispherical microstructures on a polydimethylsiloxane (PDMS) substrate. In this process, a laser-prepared template was used to form surface microstructures on PDMS film. Meanwhile, the thermal decomposition of glucose monohydrate additive during heat curing of PDMS led to the formation of porous structures within PDMS film. Further, based on the obtained PDMS/CNTs electrodes with porous hemisphere array and ionic polymer dielectric layers, high-performance ionic piezocapacitive sensors were realized. Under the synergistic effect of the low-stiffness porous hemisphere microstructure and the electric double layer of the ionic polymer film, the sensor based on an ionic polymer film with a 1:0.75 ratio of P(VDF-HFP):[EMIM][TFSI] not only achieves a sensitivity of up to 106.27 kPa-1 below 3 kPa, but also has a wide measurement range of over 400 kPa, which has obvious advantages in existing flexible piezocapacitive sensors. The rapid response time of 110 s and the good stability of 2300 cycles of the sensor further elucidate its practicality. The application of the sensor in pulse monitoring, speech recognition, and detection of multiple dynamic loads verifies its excellent sensing performance. In short, the proposed heat curing process can simultaneously form porous structures and surface microstructures on PDMS films, greatly simplifying the preparation process of porous hierarchical microstructures and providing a simple and feasible way to obtain high-performance flexible pressure sensors.

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.

Ultrasensitive Flexible Thermal Sensor Arrays based on High-Thermopower Ionic Thermoelectric Hydrogel

Ionic circuits using ions as charge carriers have demonstrated great potential for flexible and bioinspired electronics. The emerging ionic thermoelectric (iTE) materials can generate a potential difference by virtue of selective thermal diffusion of ions, which provide a new route for thermal sensing with the merits of high flexibility, low cost, and high thermopower. Here, ultrasensitive flexible thermal sensor arrays based on an iTE hydrogel consisting of polyquaternium-10 (PQ-10), a cellulose derivative, as the polymer matrix and sodium hydroxide (NaOH) as the ion source are reported. The developed PQ-10/NaOH iTE hydrogel achieves a thermopower of 24.17 mV K-1 , which is among the highest values reported for biopolymer-based iTE materials. The high p-type thermopower can be attributed to thermodiffusion of Na+ ions under a temperature gradient, while the movement of OH- ions is impeded by the strong electrostatic interaction with the positively charged quaternary amine groups of PQ-10. Flexible thermal sensor arrays are developed through patterning the PQ-10/NaOH iTE hydrogel on flexible printed circuit boards, which can perceive spatial thermal signals with high sensitivity. A smart glove integrated with multiple thermal sensor arrays is further demonstrated, which endows a prosthetic hand with thermal sensation for human-machine interaction.

Nanofiber Composite Reinforced Organohydrogels for Multifunctional and Wearable Electronics

Composite organohydrogels have been widely used in wearable electronics. However, it remains a great challenge to develop mechanically robust and multifunctional composite organohydrogels with good dispersion of nanofillers and strong interfacial interactions. Here, multifunctional nanofiber composite reinforced organohydrogels (NCROs) are prepared. The NCRO with a sandwich-like structure possesses excellent multi-level interfacial bonding. Simultaneously, the synergistic strengthening and toughening mechanism at three different length scales endow the NCRO with outstanding mechanical properties with a tensile strength (up to 7.38 ± 0.24 MPa), fracture strain (up to 941 ± 17%), toughness (up to 31.59 ± 1.53 MJ m-3) and fracture energy (up to 5.41 ± 0.63 kJ m-2). Moreover, the NCRO can be used for high performance electromagnetic interference shielding and strain sensing due to its high conductivity and excellent environmental tolerance such as anti-freezing performance. Remarkably, owing to the organohydrogel stabilized conductive network, the NCRO exhibits superior long-term sensing stability and durability compared to the nanofiber composite itself. This work provides new ideas for the design of high-strength, tough, stretchable, anti-freezing and conductive organohydrogels with potential applications in multifunctional and wearable electronics.

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.

A Flexible Sensor with Excellent Environmental Stability Using Well-Designed Encapsulation Structure

The hydrogel-based sensors suffer from poor stability and low sensitivity, severely limiting their further development. It is still "a black box" to understand the effect of the encapsulation as well as the electrode on the performance of the hydrogel-based sensors. To address these problems, we prepared an adhesive hydrogel that could robustly adhere to Ecoflex (adhesive strength is 4.7 kPa) as an encapsulation layer and proposed a rational encapsulation model that fully encapsulated the hydrogel within Ecoflex. Owing to the excellent barrier and resilience of Ecoflex, the encapsulated hydrogel-based sensor can still work normally after 30 days, displaying excellent long-term stability. In addition, we performed theoretical and simulation analyses on the contact state between the hydrogel and the electrode. It was surprising to find that the contact state significantly affects the sensitivity of the hydrogel sensors (the maximum difference in sensitivity was 333.6%), indicating that the reasonable design of the encapsulation and electrode are indispensable parts for fabricating successful hydrogel sensors. Therefore, we paved the way for a novel insight to optimize the properties of the hydrogel sensors, which is greatly favorable to developing hydrogel-based sensors to be applied in various fields.

Stimuli-bioresponsive hydrogels as new generation materials for implantable, wearable, and disposable biosensors for medical diagnostics: Principles, opportunities, and challenges

Hydrogels are excellent water-swollen polymeric materials for use in wearable, implantable, and disposable biosensors. Hydrogels have unique properties such as low cost, ease of preparation, transparency, rapid response to external conditions, biocompatibility and self-adhesion to the skin, flexibility, and strain sensitivity, making them ideal for use in biosensor platforms. This review provides a detailed overview of advanced applications of stimuli-responsive hydrogels in biosensor platforms, from hydrogel synthesis and functionalization for bioreceptor immobilization to several important diagnostic applications. Emphasis is placed on recent advances in the fabrication of ultrasensitive fluorescent and electrically conductive hydrogels and their applications in wearable, implantable, and disposable biosensors for quantitative measurements. Design, modification, and assembly techniques of fluorescent, ionically conductive, and electrically conductive hydrogels to improve performance will be addressed. The advantages and performance improvements of immobilizing bioreceptors (e.g., antibodies, enzymes, and aptamers), and incorporating fluorescent and electrically conductive nanomaterials are described, as are their limitations. Potential applications of hydrogels in implantable, wearable, disposable portable biosensors for quantitative detection of the various bioanalytes (ions, molecules, drugs, proteins, and biomarkers) are discussed. Finally, the global market for hydrogel-based biosensors and future challenges and prospects are discussed in detail.

Environmentally Stable, Robust, Adhesive, and Conductive Supramolecular Deep Eutectic Gels as Ultrasensitive Flexible Temperature Sensor

It is essential and of great significance to impart high mechanical performance, environmental stability, and high sensitivity to emerging flexible temperature sensors. In this work, polymerizable deep eutectic solvents are designed and prepared by simply mixing N-cyanomethyl acrylamide (NCMA) containing an amide group and a cyano group in the same side chain with lithium bis(trifluoromethane) sulfonimide (LiTFSI), and obtain supramolecular deep eutectic polyNCMA/LiTFSI gels after polymerization. These supramolecular gels exhibit excellent mechanical performance (tensile strength of 12.9 MPa and fracture energy of 45.3 kJ m^-2 ), strong adhesion force, high-temperature responsiveness, self-healing ability, and shape memory behavior due to the reversible reconstruction ability of amide hydrogen bonds and cyano-cyano dipole-dipole interactions in the gel network. In addition, the gels also demonstrate good environmental stability and 3D printability. To verify its application potential as a flexible temperature sensor, the polyNCMA/LiTFSI gel-based wireless temperature monitor is developed and displays outstanding thermal sensitivity (8.4%/K) over a wide detection range. The preliminary result also suggests the promising potential of PNCMA gel as a pressure sensor.

Multifunctional Wearable System for Mapping Body Temperature and Analyzing Sweat

Integrated wearable bioelectronic health monitoring systems have revealed new possibilities for collecting personalized physiological information. Wearable sweat sensors have the potential to noninvasively measure valuable biomarkers. Mapping sweat and skin-temperature throughout the body can provide detailed information on the human body. However, existing wearable systems cannot evaluate such data. Here, we report a multifunctional wearable platform that can wirelessly measure local sweat loss, sweat chloride concentration, and skin temperature. The approach combines a reusable electronics module to monitor skin temperature and a microfluidic module for monitoring sweat loss and sweat chloride concentration. The miniaturized electronic system takes temperature measurements from the skin and wirelessly transmits the obtained data to a user device using Bluetooth technology. In contrast, the microfluidic system provides accurate colorimetric analysis of the chloride concentration and sweat loss. Thus, this integrated wearable system has great application potential in individualized health management systems for sports researchers and competitors and can also be applied in clinical settings.

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

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.

Biocompatible Material-Based Flexible Biosensors: From Materials Design to Wearable/Implantable Devices and Integrated Sensing Systems

Human beings have a greater need to pursue life and manage personal or family health in the context of the rapid growth of artificial intelligence, big data, the Internet of Things, and 5G/6G technologies. The application of micro biosensing devices is crucial in connecting technology and personalized medicine. Here, the progress and current status from biocompatible inorganic materials to organic materials and composites are reviewed and the material-to-device processing is described. Next, the operating principles of pressure, chemical, optical, and temperature sensors are dissected and the application of these flexible biosensors in wearable/implantable devices is discussed. Different biosensing systems acting in vivo and in vitro, including signal communication and energy supply are then illustrated. The potential of in-sensor computing for applications in sensing systems is also discussed. Finally, some essential needs for commercial translation are highlighted and future opportunities for flexible biosensors are considered.

Multifunctional Iontronic Sensor Based on Liquid Metal-Filled Ho llow Ionogel Fibers in Detecting Pressure, Temperature, and Proximity

Fiber-based pressure/temperature sensors are highly desired in wearable electronics because of their natural advantages of good breathability and easy integrability. However, it is still a great challenge to fabricate reliable and highly sensitive fiber-based pressure/temperature sensors via a scalable and facile strategy. Herein, a novel fiber-based iontronic sensor with excellent pressure- and temperature-sensing capabilities is designed by assembling two crossed hollow and porous ionogel fibers filled with liquid metal. Serving as a pressure sensor, a high detection resolution (1.16 Pa), a high sensitivity of 13.30 kPa^-1 (0-2 kPa), and a wide detection range (∼207 kPa) are realized owing to its novel hierarchical structure and the selection of deformable liquid electrodes. As a temperature sensor, it exhibits a high temperature sensitivity of 25.99% °C^-1 (35-40 °C), high resolution of 0.02 °C, and good repeatability and reliability. On the basis of these excellent sensing capabilities, the as-prepared sensor can detect not only pressure signals varied from weak pulse to large joint movements but also the proximity of different objects. Furthermore, a large-area fiber array can be easily woven for acquiring the pressure mapping to intuitively distinguish the location, magnitude, and shape of the loaded object. This work provides a universal strategy to design fiber-shaped iontronic sensors for wearable electronics.

A Metal Nanoparticle Thermistor with the Beta Value of 10 000 K

The thermistor, typically made from metallic oxides, is a type of resistor whose electrical resistance is dependent on its temperature. Despite the wide usage, the limitations of ceramic thermistors become increasingly apparent as devices with improved performances are sought and as new applications emerge. Herein, a thermistor that is showed with a beta (B) value of 10 000 K can be made exclusively from metal nanoparticles functionalized with charged organic ligands. This B value is hard to achieve for ceramic devices, which is due to the increase of effective counterion concentration and its mobility upon thermal activation. Importantly, the performance of the nanoparticle thermistor is maintained when it is fabricated on a flexible substrate and experiences reversible bending. Demos of thermistor arrays for heat transfer, distribution, and comparison of their performance with commercial products are also demonstrated. Owing to the low temperature and simple casting process, conformably flexible characteristics, stable solid states, and ultra-high sensitivities, this device is expected to be practically used soon.

Ultrathin Hydrogel Films toward Breathable Skin-Integrated Electronics

On-skin electronics that offer revolutionary capabilities in personalized diagnosis, therapeutics, and human-machine interfaces require seamless integration between the skin and electronics. A common question remains whether an ideal interface can be introduced to directly bridge thin-film electronics with the soft skin, allowing the skin to breathe freely and the skin-integrated electronics to function stably. Here, an ever-thinnest hydrogel is reported that is compliant to the glyphic lines and subtle minutiae on the skin without forming air gaps, produced by a facile cold-lamination method. The hydrogels exhibit high water-vapor permeability, allowing nearly unimpeded transepidermal water loss and free breathing of the skin underneath. Hydrogel-interfaced flexible (opto)electronics without causing skin irritation or accelerated device performance deterioration are demonstrated. The long-term applicability is recorded for over one week. With combined features of extreme mechanical compliance, high permeability, and biocompatibility, the ultrathin hydrogel interface promotes the general applicability of skin-integrated electronics.

Ultrastretchable Composite Organohydrogels with Dual Cross-Links Enabling Multimodal Sensing

Building multiple cross-links or networks is a favorable way of diversifying applications of the hydrogels, which is also available for the organohydrogels prepared via the solvent replacement way. However, the situations become more complicated for organohydrogels due to the presence of replaced solvents. Therefore, the correlations between the multiple cross-links and final performance need to be better understood for the organohydrogels, which is vital for tailoring their inherent properties to expand final application scenarios. Polyacrylamide (PAM)/poly(vinyl alcohol) (PVA)/MXene composite organohydrogels with dual cross-links, namely, the covalently cross-linked PAM chains as the primary network and the physically cross-linked PVA/PAM chains with MXene particles as the secondary cross-links, were developed here for the study. The occurrence of the secondary cross-links plays multiple roles as sacrificial units endowing the system with ultrastretchability with an excellent strain-resistance effect and as temperature-sensitive units endowing the system with thermosensation ability with an outstanding temperature coefficient of resistance. Thus, the optimized sample can be used as a strain sensor with excellent environmental tolerance for detecting human motion as a pressure sensor to probe compression with weak deformation and as a thermal sensor to capture environmental temperature changes. This work provides valuable information on developing organohydrogels with superior performance for multimodal sensors.

Environmentally Stable, Stretchable, Adhesive, and Conductive Organohydrogels with Multiple Dynamic Interactions as High-Performance Strain and Temperature Sensors

Nowadays, with the rapid development of artificial intelligence, conductive hydrogel-based sensors play an increasingly vital role in health monitoring and temperature sensing. However, the perfect integration of the environmental stability and applied performance of the hydrogel has always been a challenging and significant problem. Herein, we report an environmentally tolerant, stretchable, adhesive, self-healing conductive gel through multiple dynamic interactions in the water/glycerol/ionic liquids medium, which can be used as a high-performance strain and temperature sensor. The random copolymer poly(acrylic acid-co-acetoacetoxyethyl methacrylate) interacts with the branched poly(ethylene imine) (PEI) and Zr⁴⁺ ions via the dynamic covalent enamine bonds, coordinations, and electrostatic interactions to improve stretchable (1300%), compressible, fatigue-resistant (1000 cycles at 50% strain), and self-healing performance (95%, 24 h). The combination of water/glycerol/ionic liquids imparts the resulting gel with excellent electrical conductivity, anti-drying, and anti-freezing performance. By means of the above excellent performance, the gel could be used as the flexible strain or pressure sensor with high sensitivity and stability for the detection of the movement, expression, handwriting, pronouncing, and electrocardiogram (ECG) signals in various models. Meanwhile, the resulting gel can be assembled as the temperature sensor to trace the change of temperature accurately and steadily, which has a wide operating window (0 to 100 °C), an ultralow detection limit (0.2 °C), and high sensitivity (2.1% °C^-1). It is believed that the strategy for the multifunction and high-performance gel will blaze a new trail for the smart device in health management, temperature detection, and information transmission under various environmental conditions.

Mechanically Strong, Wet Adhesive, and Self-Healing Polyurethane Ionogel Enhanced with a Semi-interpenetrating Network for Underwater Motion Detection

The gel-based sensors have developed rapidly in recent years toward multifunctionality. However, there are still some challenges that need to be solved, such as poor mechanical properties and inaccessibility to wet or water environments. To address these issues, we have developed an ionogel with a semi-interpenetrating network structure by adopting poly(vinylidene fluoride-co-hexafluoropropylene) as the linear non-cross-linked network, a double-bonded ionic liquid and double-bonded capped polyurethane as the cross-linked network, and an ionic liquid as the conductive media. The obtained ionogel exhibits tunable mechanical properties (3.67-8.76 MPa) and excellent sensing properties (IG-20, GF = 8.2). The superb environmental stability and self-healing properties of the ionogel were also demonstrated. Meanwhile, adhesion, self-healing, and sensing performance were guaranteed for underwater due to the presence of a large number of C-F bonds. We strongly believe that this ionogel with excellent mechanical properties and underwater communication is expected for monitoring the health of the human body and information transmission in the future.

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.

Wearable Ionogel-Based Fibers for Strain Sensors with Ultrawide Linear Response and Temperature Sensors Insensitive to Strain

Fiber-shaped stretchable strain and temperature sensors are highly desirable for wearable electronics due to their excellent flexibility, comfort, air permeability, and easiness to be weaved into fabric. Herein, we prepare a smart ionogel-based fiber composed of thermoplastic polyurethane (TPU) and ionic liquid (IL) by the facile and scalable wet-spinning technique, which can serve as a wearable strain sensor with good linearity (a correlation coefficient of 0.997) in an ultrawide sensing range (up to 700%), ultralow-detection limit (0.05%), fast response (173 ms) and recovery (120 ms), and high reproducibility. Attributed to these outstanding strain sensing performances, the designed TPU/IL ionogel fiber-shaped sensor is able to monitor both subtle physiological activities and large human motions. More interestingly, because of the fast response and high resolution to strain, the fiber-shaped sensor can be sewn into the fabric to secretly encrypt and wirelessly translate message according to the principle of Morse code. More importantly, a wearable strain-insensitive temperature sensor can be obtained from the ionogel fiber if it is designed into an "S" shape, which can effectively eliminate the interference of strain on temperature sense. It is found that the inaccuracy of temperature sense is within 0.15 °C when the sensor is subjected to 30% tensile strain simultaneously. Moreover, this strain-insensitive temperature sensor shows a monotonic temperature response over a wide temperature range (-15 to 100 °C) with an ultrahigh detecting accuracy of 0.1 °C and good reliability, owing to the fast and stable thermal response of IL. This temperature sensor can realize the detection of thermal radiation, proximity, and respiration, exhibiting enormous potential in smart skin, personal healthcare, and wearable electronics. This work proposes a simple but effective strategy to realize the essential strain and temperature sensing capabilities of wearable electronics and smart fabrics without mutual interference.

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.

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

The traditional human-machine interaction mode of communicating solely with pressure sensors needs modification, especially at a time when COVID-19 is circulating globally. Here, a transparent, stretchable, resilient, and high-performance hydrogel fiber-based bimodal sensor is fabricated by using a polyacrylamide-alginate double network hydrogel, which features high sensitivity (3.17% cm^-1), wide working range (18 cm), fast response/recovery speeds (90/90 ms) and good stability in proximity sensing, and impressive pressure sensing performance, including high sensitivity (0.91 kPa^-1), short response/recovery time (40/40 ms), low detection limit (63 Pa) and good linearity. Moreover, the response switch between proximity/pressure modes is measured and non-interfering dual-mode detection is achieved. Notably, the stretchable bimodal sensor is capable of working under 100% tensile strain without degrading the sensing performance. Specifically, the proximity sensor shows good immunity to the strain, while the pressure sensitivity is even promoted. Furthermore, the sensor is tough enough to work normally after punctures from a knife and strikes from a wrench. Notably, the sensor can be used for gesture recognition and subtle pressure detection, such as small water droplets (10 mg), wrist pulse, etc. A 3 × 3 array is further shown for accurate spatial sensing and location identification, verifying the feasibility of its practical application.

Improving disease prevention, diagnosis, and treatment using novel bionic technologies

Increased human life expectancy, due in part to improvements in infant and childhood survival, more active lifestyles, in combination with higher patient expectations for better health outcomes, is leading to an extensive change in the number, type and manner in which health conditions are treated. Over the next decades as the global population rapidly progresses toward a super-aging society, meeting the long-term quality of care needs is forecast to present a major healthcare challenge. The goal is to ensure longer periods of good health, a sustained sense of well-being, with extended periods of activity, social engagement, and productivity. To accomplish these goals, multifunctionalized interfaces are an indispensable component of next generation medical technologies. The development of more sophisticated materials and devices as well as an improved understanding of human disease is forecast to revolutionize the diagnosis and treatment of conditions ranging from osteoarthritis to Alzheimer's disease and will impact disease prevention. This review examines emerging cutting-edge bionic materials, devices and technologies developed to advance disease prevention, and medical care and treatment in our elderly population including developments in smart bandages, cochlear implants, and the increasing role of artificial intelligence and nanorobotics in medicine.

Stretchable solvent-free ionic conductor with self-wrinkling microstructures for ultrasensitive strain sensor

Stretchable solvent-free ionic conductors with outstanding physicochemical stability are attractive in emerging sensing devices. However, existing ionic conductors struggle to keep high strain sensitivity with a gauge factor (GF) smaller than 4 at a strain range of up to 100%, which seriously affects the reliability of the signal output. Herein, we prepared a novel solvent-free ionic conductor, consisting of a hybrid cross-linked polymer network and mobile ions. Exquisite reticular wrinkling microstructures with ion channels were formed spontaneously by utilizing modulus mismatch in shrinkage during polymerization. The ion channels promote the ionic transport in the polymer, and the reticular wrinkling microstructures dramatically reinforce the mechanical stability and enhance energy dissipation. After being carefully optimized, the as-prepared ionic conductor demonstrated many unique advantages including perfect physicochemical stability, wide operating temperature range, the high ionic conductivity of 1.17 mS cm^-1 at 15 °C and excellent sensitivity with GF as high as 7.03 at 100% strain. Moreover, benefiting from the abundant hydrogen bonds and ionic interactions in the polymer network, the ionic conductor also possesses self-healable and adhesion ability. The resulting devices can be used for quantitative evaluation of human joint motion and exhibit great potential in soft electronics or iontronics.

Temperature-Responsive Ionic Conductive Hydrogel for Strain and Temperature Sensors

Flexible wearable devices have achieved remarkable applications in health monitoring because of the advantages of multisignal collecting and real-time wireless transmission of information. However, the integration of bulky sensing elements and rigid metal circuit components in traditional wearable devices may lead to a mechanical and signal-conducting mismatch between wearable devices and biological tissues, thus restricting their wide applications in the human body. The excellent mechanical properties, conductivity, and high tissue resemblance of conductive hydrogel contribute to its application in flexible electronic sensors to monitor human health. In this work, a dual-network, temperature-responsive ionic conductive hydrogel with excellent stretchability, fast temperature responsiveness, and good conductivity was developed by introducing a polyvinylpyrrolidone (PVP)/ tannic acid (TA)/ Fe3+ cross-linked network into the N,N-methylene diacrylamide (MBAA) cross-linked poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AM)) network. Furthermore, the introduction of the PVP/TA/Fe3+ cross-linked network endowed the hydrogel with excellent stretchability and conductivity. By adjusting the molar ratio of TA and Fe3+ to 3:5, a hydrogel with a maximal stretching ratio of 720% and sensitive strain response (GF = 3.61) was achieved, showing a promising application in wearable strain sensors to monitor both large and fine human motions. Moreover, by introducing PNIPAAm with a lower critical solution temperature (LCST), the hydrogel may be used to monitor the environmental temperature through the temperature-conductivity responsiveness, which can be applied as a wearable temperature sensor to detect fever or tissue hyperthermia in the human body.

Ti2C3Tx/Polyurethane Constructed by Gas-Liquid Interface Self-Assembly for Underwater Sensing

Nowadays, multifunctional, easily prepared, and highly sensitive flexible sensors have attracted extensive attention and are gradually used in various scenarios. Here, we report the design of the Ti₂C₃T_x/polyurethane composites prepared by a facile gas-liquid interface self-assembly. The obtained flexible sensor has a wide detection range (∼900%), a low-stress detection limit (<1%), a high sensitivity (GF = 1.3, strain from 0 to 100%), and a fast response time (<140 ms). The multifunctional stress sensor can be applied to not only wearable motion monitoring and detection of various signals but also the detection of underwater human motion, as well as different motion states and swimming frequencies of toy fish in water, demonstrating its great prospects in a variety of applications, such as human movement monitoring and marine biological detection and research.

A button switch inspired duplex hydrogel sensor based on both triboelectric and piezoresistive effects for detecting dynamic and static pressure

The capability to sense complex pressure variations comprehensively is vital for wearable electronics and flexible human–machine interfaces. In this paper, inspired by button switches, a duplex tactile sensor based on the combination of triboelectric and piezoresistive effects is designed and fabricated. Because of its excellent mechanical strength and electrical stability, a double-networked ionic hydrogel is used as both the conductive electrode and elastic current regulator. In addition, micro-pyramidal patterned polydimethylsiloxane (PDMS) acts as both the friction layer and the encapsulation elastomer, thereby boosting the triboelectric output performance significantly. The duplex hydrogel sensor demonstrates comprehensive sensing ability in detecting the whole stimulation process including the dynamic and static pressures. The dynamic stress intensity (10–300 Pa), the action time, and the static variations (increase and decrease) of the pressure can be identified precisely from the dual-channel signals. Combined with a signal processing module, an intelligent visible door lamp is achieved for monitoring the entire “contact–hold–release–separation” state of the external stimulation, which shows great application potential for future smart robot e-skin and flexible electronics.

Environment tolerant, adaptable and stretchable organohydrogels: preparation, optimization, and applications

Multiple stretchable materials have been successively developed and applied to wearable devices, soft robotics, and tissue engineering. Organohydrogels are currently being widely studied and formed by dispersing immiscible hydrophilic/hydrophobic polymer networks or only hydrophilic polymer networks in an organic/water solvent system. In particular, they can not only inherit and carry forward the merits of hydrogels, but also have some unique advantageous features, such as anti-freezing and water retention abilities, solvent resistance, adjustable surface wettability, and shape memory effect, which are conducive to the wide environmental adaptability and intelligent applications. This review first summarizes the structure, preparation strategy, and unique advantages of the reported organohydrogels. Furthermore, organohydrogels can be optimized for electro-mechanical properties or endowed with various functionalities by adding or modifying various functional components owing to their modifiability. Correspondingly, different optimization strategies, mechanisms, and advanced developments are described in detail, mainly involving the mechanical properties, conductivity, adhesion, self-healing properties, and antibacterial properties of organohydrogels. Moreover, the applications of organohydrogels in flexible sensors, energy storage devices, nanogenerators, and biomedicine have been summarized, confirming their unlimited potential in future development. Finally, the existing challenges and future prospects of organohydrogels are provided.

Ultra-Sensitive, Deformable, and Transparent Triboelectric Tactile Sensor Based on Micro-Pyramid Patterned Ionic Hydrogel for Interactive Human-Machine Interfaces

Rapid advances in wearable electronics and mechno-sensational human-machine interfaces impose great challenges in developing flexible and deformable tactile sensors with high efficiency, ultra-sensitivity, environment-tolerance, and self-sustainability. Herein, a tactile hydrogel sensor (THS) based on micro-pyramid-patterned double-network (DN) ionic organohydrogels to detect subtle pressure changes by measuring the variations of triboelectric output signal without an external power supply is reported. By the first time of pyramidal-patterned hydrogel fabrication method and laminated polydimethylsiloxane (PDMS) encapsulation process, the self-powered THS shows the advantages of remarkable flexibility, good transparency (≈85%), and excellent sensing performance, including extraordinary sensitivity (45.97 mV Pa^-1 ), fast response (≈20 ms), very low limit of detection (50 Pa) as well as good stability (36 000 cycles). Moreover, with the LiBr immersion treatment method, the THS possesses excellent long-term hyper anti-freezing and anti-dehydrating properties, broad environmental tolerance (-20 to 60 °C), and instantaneous peak power density of 20 µW cm^-2 , providing reliable contact outputs with different materials and detecting very slight human motions. By integrating the signal acquisition/process circuit, the THS with excellent self-power sensing ability is utilized as a switching button to control electric appliances and robotic hands by simulating human finger gestures, offering its great potentials for wearable and multi-functional electronic applications.

A Degradable-Renewable Ionic Skin Based on Edible Glutinous Rice Gel

Traditional wearable devices are commonly nonrecyclable and nondegradable, resulting in energy waste and environmental pollution. Here, a household degradable and renewable ionic skin based on edible glutinous rice gel is developed for a strain, temperature and salivary enzyme activity sensor. This gel depends on intermolecular and intramolecular H-bonds among amylopectin and amylose, and this presents excellent skin-like properties, including stretchability, self-healing property, and adhesion to various substrates. The glutinous rice gel-based skin sensor can be used to monitor vital signs and physiological parameters such as body temperature and heart rate. The sensor also achieves specific speech recognition and detects temperature and body micromovements, which provides the potential to reconstruct language or sensory/motor functions. More importantly, because of the excellent biocompatibility and degradability, the sensor can directly detect the activity of human salivary amylase, which is useful for diagnosing pancreas-, kidney-, and spleen-related diseases in the elderly. Finally, the raw material of ionic skin that originates from traditional grains is degradable and renewable as well as it can be used to prepare household wearable devices. Hence, this work not only extends the application of wearable electronics in daily life but also facilitates health monitoring in the elderly and improves their quality of life.

Self-Healing, Self-Adhesive and Stable Organohydrogel-Based Stretchable Oxygen Sensor with High Performance at Room Temperature

With the advent of the 5G era and the rise of the Internet of Things, various sensors have received unprecedented attention, especially wearable and stretchable sensors in the healthcare field. Here, a stretchable, self-healable, self-adhesive, and room-temperature oxygen sensor with excellent repeatability, a full concentration detection range (0-100%), low theoretical limit of detection (5.7 ppm), high sensitivity (0.2%/ppm), good linearity, excellent temperature, and humidity tolerances is fabricated by using polyacrylamide-chitosan (PAM-CS) double network (DN) organohydrogel as a novel transducing material. The PAM-CS DN organohydrogel is transformed from the PAM-CS composite hydrogel using a facile soaking and solvent replacement strategy. Compared with the pristine hydrogel, the DN organohydrogel displays greatly enhanced mechanical strength, moisture retention, freezing resistance, and sensitivity to oxygen. Notably, applying the tensile strain improves both the sensitivity and response speed of the organohydrogel-based oxygen sensor. Furthermore, the response to the same concentration of oxygen before and after self-healing is basically the same. Importantly, we propose an electrochemical reaction mechanism to explain the positive current shift of the oxygen sensor and corroborate this sensing mechanism through rationally designed experiments. The organohydrogel oxygen sensor is used to monitor human respiration in real-time, verifying the feasibility of its practical application. This work provides ideas for fabricating more stretchable, self-healable, self-adhesive, and high-performance gas sensors using ion-conducting organohydrogels.

Temperature sensing using junctions between mobile ions and mobile electrons

We develop temperature sensors on the basis of charges accumulated at the electrolyte/dielectric interface and dielectric/electrode interface. The accumulated charges make the temperature sensors self-powered, which simplifies circuit design and enables portable sensing. The sensors are stretchable, but deformation does not affect temperature sensing. The sensors have high sensitivity and fast response. They can be made small and transparent. Such temperature sensors open new possibilities to create human–machine interfaces and soft robots in healthcare and engineering.

Sensing technology is under intense development to enable the Internet of everything and everyone in new and useful ways. Here we demonstrate a method of stretchable and self-powered temperature sensing. The basic sensing element consists of three layers: an electrolyte, a dielectric, and an electrode. The electrolyte/dielectric interface accumulates ions, and the dielectric/electrode interface accumulates electrons (in either excess or deficiency). The ions and electrons at the two interfaces are usually not charge-neutral, and this charge imbalance sets up an ionic cloud in the electrolyte. The design functions as a charged temperature-sensitive capacitor. When temperature changes, the ionic cloud changes thickness, and the electrode changes open-circuit voltage. We demonstrate high sensitivity (∼1 mV/K) and fast response (∼10 ms). Such temperature sensors can be made small, stable, and transparent. Depending on the arrangement of the electrolyte, dielectric, and electrode, we develop four designs for the temperature sensor. In addition, the temperature sensor has good linearity in the range of tens of Kelvin. We further show that the temperature sensors can be integrated into stretchable electronics and soft robots.

Porous Polydimethylsiloxane Elastomer Hybrid with Zinc Oxide Nanowire for Wearable, Wide-Range, and Low Detection Limit Capacitive Pressure Sensor

We propose a flexible capacitive pressure sensor that utilizes porous polydimethylsiloxane elastomer with zinc oxide nanowire as nanocomposite dielectric layer via a simple porogen-assisted process. With the incorporation of nanowires into the porous elastomer, our capacitive pressure sensor is not only highly responsive to subtle stimuli but vigorously so to gentle touch and verbal stimulation from 0 to 50 kPa. The fabricated zinc oxide nanowire-porous polydimethylsiloxane sensor exhibits superior sensitivity of 0.717 kPa-1, 0.360 kPa-1, and 0.200 kPa-1 at the pressure regimes of 0-50 Pa, 50-1000 Pa, and 1000-3000 Pa, respectively, presenting an approximate enhancement by 21-100 times when compared to that of a flat polydimethylsiloxane device. The nanocomposite dielectric layer also reveals an ultralow detection limit of 1.0 Pa, good stability, and durability after 4000 loading-unloading cycles, making it capable of perception of various human motions, such as finger bending, calligraphy writing, throat vibration, and airflow blowing. A proof-of-concept trial in hydrostatic water pressure sensing has been demonstrated with the proposed sensors, which can detect tiny changes in water pressure and may be helpful for underwater sensing research. This work brings out the efficacy of constructing wearable capacitive pressure sensors based on a porous dielectric hybrid with stress-sensitive nanostructures, providing wide prospective applications in wearable electronics, health monitoring, and smart artificial robotics/prosthetics.

Thermoresponsive Lignin-Reinforced Poly(Ionic Liquid) Hydrogel Wireless Strain Sensor

To meet critical requirements on flexible electronic devices, multifunctionalized flexible sensors with excellent electromechanical performance and temperature perception are required. Herein, lignin-reinforced thermoresponsive poly(ionic liquid) hydrogel is prepared through an ultrasound-assisted synthesized method. Benefitting from the electrostatic interaction between lignin and ionic liquid, the hydrogel displays high stretchability (over 1425%), excellent toughness (over 132 kPa), and impressive stress loading-unloading cyclic stability. The hydrogel strain sensor presents excellent electromechanical performance with a high gauge factor (1.37) and rapid response rate (198 ms), which lays the foundation for human body movement detection and smart input. Moreover, owing to the thermal-sensitive feature of poly(ionic liquid), the as-prepared hydrogel displays remarkable thermal response sensitivity (0.217°C⁻¹) in body temperature range and low limit of detection, which can be applied as a body shell temperature indicator. Particularly, the hydrogel can detect dual stimuli of strain and temperature and identify each signal individually, showing the specific application in human-machine interaction and artificial intelligence. By integrating the hydrogel strain sensor into a wireless sensation system, remote motion capture and gesture identification is realized in real-time.

Fabrication of Multiresponsive Magnetic Nanocomposite Double-Network Hydrogels for Controlled Release Applications

Nanocomposite double-network hydrogels (ncDN hydrogels) have been demonstrated as promising biomaterials to present several desired properties (e.g., high mechanical strength, stimuli-responsiveness, and local therapy) for biomedicine. Here, a new type of ncDN hydrogels featuring definable microstructures and properties as well as multistimuli responsiveness for controlled release applications is developed. Amine-functionalized iron oxide nanoparticles (IOPs_NH₂ ) are used as nanoparticle cross-linkers to simultaneously connect the dual networks of gelatin (Gel) and polydextran aldehyde (PDA) through hydrogen bonding, electrostatic interactions, and dynamic imine bonds. The pH- and temperature-responsive Gel/PDA/IOP_NH₂ ncDN hydrogels present a fast release profile of proteins at acidic pH and high temperature. Besides, IOP_NH₂ also contributes the magnetic-responsiveness to the ncDN hydrogels, allowing the use of magnetic field to generate heat to facilitate the structural change of hydrogels and the subsequent applications. Taken together, a versatile ncDN hydrogel platform capable of multistimuli responsiveness and local heating for controlled release is developed for advanced biomedical applications.

Ion-Conductive Hydrogel-Based Stretchable, Self-Healing, and Transparent NO2 Sensor with High Sensitivity and Selectivity at Room Temperature

Here stretchable, self-healable, and transparent gas sensors based on salt-infiltrated hydrogels for high-performance NO₂ sensing in both anaerobic environment and air at room temperature, are reported. The salt-infiltrated hydrogel displays high sensitivity to NO₂ (119.9%/ppm), short response and recovery time (29.8 and 41.0 s, respectively), good linearity, low theoretical limit of detection (LOD) of 86 ppt, high selectivity, stability, and conductivity. A new gas sensing mechanism based on redox reactions occurring at the electrode-hydrogel interface is proposed to understand the sensing behaviors. The gas sensing performance of hydrogel is greatly improved by incorporating calcium chloride (CaCl₂ ) in the hydrogel via a facile salt-infiltration strategy, leading to a higher sensitivity (2.32 times) and much lower LOD (0.06 times). Notably, both the gas sensing ability, conductivity, and mechanical deformability of hydrogels are readily self-healable after cutting off and reconnection. Such large deformations as 100% strain do not deprive the gas sensing capability, but rather shorten the response and recovery time significantly. The CaCl₂ -infiltrated hydrogel shows excellent selectivity of NO₂ , with good immunity to the interference gases. These results indicate that the salt-infiltrated hydrogel has great potential for wearable electronics equipped with gas sensing capability in both anaerobic and aerobic environments.

Design of ultra-stretchable, highly adhesive and self-healable hydrogels via tannic acid-enabled dynamic interactions

Hydrogels have emerged as a landmark soft material for a wide range of applications such as in biomedical devices, soft robotics, artificial electronic skins, and the Internet of Things (IoT). To date, engineering hydrogels that simultaneously possess high stretchability (>3000%) and strong on-skin adhesion (>30 kPa) has not been an easy task. Generally, good stretchability is mainly dominated by the bulk interactions of hydrogels, whereas robust adhesion relies on the interfacial interactions of hydrogels with their surroundings. Here, we report a facile strategy to engineer an ultra-stretchable, highly adhesive and self-healable hydrogel, by virtue of tannic-acid-enabled dynamic interactions (TEDI) to fully substitute conventional covalent crosslinking. The TEDI strategy allows us to synchronously regulate both bulk and interfacial interactions to obtain exciting properties that outperform conventional hydrogels, including an extraordinary stretchability of over 7300%, remarkable self-healing abilities, and a robust on-skin adhesion of 50 kPa. With these intriguing merits, TEDI hydrogels are demonstrated to be a wearable strain sensor that accurately detect the motion of the human body. Moreover, our TEDI strategy unlocks new opportunities to design next-generation ionic hydrogels that may be valuable for applications in wearable electronic devices and healthcare monitoring.