Ultrastretchable Multilayered Fiber with a Hollow-Monolith Structure for High-Performance Strain Sensor

Temperature-Tolerant Versatile Conductive Zwitterionic Nanocomposite Organohydrogel toward Multisensory Applications

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

Multilayer Core-Shell Fiber Device for Improved Strain Sensing and Supercapacitor Applications

1D fiber devices, known for their exceptional flexibility and seamless integration capabilities, often face trade-offs between desired wearable application characteristics and actual performance. In this study, a multilayer device composed of carbon nanotube (CNT), transition metal carbides/nitrides (MXenes), and cotton fibers, fabricated using a dry spinning method is presented, which significantly enhances both strain sensing and supercapacitor functionality. This core-shell fiber design achieves a record-high sensitivity (GF ≈ 4500) and maintains robust durability under various environmental conditions. Furthermore, the design approach markedly influences capacitance, correlating with the percentage of active material used. Through systematic optimization, the fiber device exhibited a capacitance 26-fold greater than that of a standard neat CNT fiber, emphasizing the crucial role of innovative design and high active material loading in improving device performance.

In-situ forming ultra-mechanically sensitive materials for high-sensitivity stretchable fiber strain sensors

Fiber electronics with flexible and weavable features can be easily integrated into textiles for wearable applications. However, due to small sizes and curved surfaces of fiber materials, it remains challenging to load robust active layers, thus hindering production of high-sensitivity fiber strain sensors. Herein, functional sensing materials are firmly anchored on the fiber surface in-situ through a hydrolytic condensation process. The anchoring sensing layer with robust interfacial adhesion is ultra-mechanically sensitive, which significantly improves the sensitivity of strain sensors due to the easy generation of microcracks during stretching. The resulting stretchable fiber sensors simultaneously possess an ultra-low strain detection limit of 0.05%, a high stretchability of 100%, and a high gauge factor of 433.6, giving 254-folds enhancement in sensitivity. Additionally, these fiber sensors are soft and lightweight, enabling them to be attached onto skin or woven into clothes for recording physiological signals, e.g. pulse wave velocity has been effectively obtained by them. As a demonstration, a fiber sensor-based wearable smart healthcare system is designed to monitor and transmit health status for timely intervention. This work presents an effective strategy for developing high-performance fiber strain sensors as well as other stretchable electronic devices.

Prototyping a wearable and stretchable graphene-on-PDMS sensor for strain detection on human body physiological and joint movements

In the era of wearable electronic devices, which are quite popular nowadays, our research is focused on flexible as well as stretchable strain sensors, which are gaining humongous popularity because of recent advances in nanocomposites and their microstructures. Sensors that are stretchable and flexible based on graphene can be a prospective 'gateway' over the considerable biomedical speciality. The scientific community still faces a great problem in developing versatile and user-friendly graphene-based wearable strain sensors that satisfy the prerequisites of susceptible, ample range of sensing, and recoverable structural deformations. In this paper, we report the fabrication, development, detailed experimental analysis and electronic interfacing of a robust but simple PDMS/graphene/PDMS (PGP) multilayer strain sensor by drop casting conductive graphene ink as the sensing material onto a PDMS substrate. Electrochemical exfoliation of graphite leads to the production of abundant, fast and economical graphene. The PGP sensor selective to strain has a broad strain range of ⁓60%, with a maximum gauge factor of 850, detection of human physiological motion and personalized health monitoring, and the versatility to detect stretching with great sensitivity, recovery and repeatability. Additionally, recoverable structural deformation is demonstrated by the PGP strain sensors, and the sensor response is quite rapid for various ranges of frequency disturbances. The structural designation of graphene's overlap and crack structure is responsible for the resistance variations that give rise to the remarkable strain detection properties of this sensor. The comprehensive detection of resistance change resulting from different human body joints and physiological movements demonstrates that the PGP strain sensor is an effective choice for advanced biomedical and therapeutic electronic device utility.

Flexible Multimodal Magnetoresistive Sensors Based on Alginate/Poly(vinyl alcohol) Foam with Stimulus Discriminability for Soft Electronics Using Machine Learning

Flexible foam-based sensors have attracted substantial interest due to their high specific surface area, light weight, superior deformability, and ease of manufacture. However, it is still a challenge to integrate multimodal stimuli-responsiveness, high sensitivity, reliable stability, and good biocompatibility into a single foam sensor. To achieve this, a magnetoresistive foam sensor was fabricated by an in situ freezing-polymerization strategy based on the interpenetrating networks of sodium alginate, poly(vinyl alcohol) in conjunction with glycerol, and physical reinforcement of core-shell bidisperse magnetic particles. The assembled sensor exhibited preferable magnetic/strain-sensing capability (GF ≈ 0.41 T-1 for magnetic field, 4.305 for tension, -0.735 for bending, and -1.345 for pressing), quick response time, and reliable durability up to 6000 cycles under external stimuli. Importantly, a machine learning algorithm was developed to identify the encryption information, enabling high recognition accuracies of 99.22% and 99.34%. Moreover, they could be employed as health systems to detect human physiological motion and integrated as smart sensor arrays to perceive external pressure/magnetic field distributions. This work provides a simple and ecofriendly strategy to fabricate biocompatible foam-based multimodal sensors with potential applications in next-generation soft electronics.

Multifunctional cross-sensitive magnetic alginate-chitosan-polyethylene oxide nanofiber sensor for human-machine interaction

Flexible nanofiber membranes are compelling materials for the development of functional multi-mode sensors; however, their essential features such as high cross-sensitivity, reliable stability and signal discrimination capability have rarely been realized simultaneously in one sensor. Here, a novel multi-mode sensor with a nanofiber membrane structure based on multiple interpenetrating networks of bidisperse magnetic particles, sodium alginate (SA), chitosan (CHI) in conjunction with polyethylene oxide hydrogels was prepared in a controllable electrospinning technology. Specifically, the morphology distributions of nanofibers could be regulated by the crosslinking degree of the interpenetrating networks and the spinning process parameters. The incorporation of SA and CHI endowed the sensor with desirable flexibility, ideal biocompatibility and skin-friendly property. Besides, the assembled sensors not only displayed preferable magnetic sensitivity of 0.34 T-1 and reliable stability, but also exhibited favorable cross-sensitivity, quick response time, and long-term durability for over 5000 cycles under various mechanical stimuli. Importantly, the multi-mode stimuli could be discriminated via producing opposite electrical signals. Furthermore, based on the signal distinguishability of the sensor, a wearable Morse code translation system assisted by the machine learning algorithm was demonstrated, enabling a high recognizing accuracy (>99.1 %) for input letters and numbers information. Due to the excellent multifunctional sensing characteristics, we believe that the sensor will have a high potential in wearable soft electronics and human-machine interactions.

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.

Piezoresistive Free-standing Microfiber Strain Sensor for High-resolution Battery Thickness Monitoring

Highly sensitive microfiber strain sensors are promising for the detection of mechanical deformations in applications where limited space is available. In particular for in situ battery thickness monitoring where high resolution and low detection limit are key requirements. Herein, the realization of a highly sensitive strain sensor for in situ lithium-ion (Li-ion) battery thickness monitoring is presented. The compliant fiber-shaped sensor is fabricated by an upscalable wet-spinning method employing a composite of microspherical core-shell conductive particles embedded in an elastomer. The electrical resistance of the sensor changes under applied strain, exhibiting a high strain sensitivity and extremely low strain detection limit of 0.00005 with high durability of 10 000 cycles. To demonstrate the accuracy and ease of applicability of this sensor, the real-time thickness change of a Li-ion battery pouch cell is monitored during the charge and discharge cycles. This work introduces a promising approach with the least material complexity for soft microfiber strain gauges.

A highly stretchable and ultra-sensitive strain sensing fiber based on a porous core-network sheath configuration for wearable human motion detection

Functional fibers have attracted much research attention due to their potential application in developing advanced electronic textiles for wearable devices. However, challenges still exist in preparing high-performance fiber-shaped sensors with superior flexibility and stretchability while achieving a high sensitivity and a wide detection range. Herein, we propose the design and fabrication of an ultra-flexible and super-elastic fiber-shaped strain sensor via a facile combining approach of wet-spinning and dip-coating. The sensor adopts a core-sheath configuration of liquid metal droplets dispersed in porous thermoplastic polyurethane as a substrate core and a carbon nanotube intertwined network embedding silver nanowires as a strain sensitive sheath. By taking advantage of both the composition of multiple functional materials and the design of a microstructured device configuration, the developed fiber-shaped sensor exhibits an ultrahigh sensitivity (maximum gauge factor of 7336.1), an extremely large workable strain range (500%), a low strain detection limit (0.5%), a fast response time (200 ms) and good stability (10 000 cycles). In addition, the sensor is temperature insensitive, inert under harsh solution conditions, degradable and recyclable. Intriguingly, the fiber-shaped sensor can be used to detect various human motions and gestures by directly attaching to skins or elaborately weaving into textiles, demonstrating its great potential in human healthcare monitoring and human-machine interactions.

A resilient and lightweight cellulose/graphene oxide/polymer-derived multifunctional carbon aerogel generated from Pickering emulsion toward a wearable pressure sensor

In the new era of competitive smart electronics, the development of compressible multifunctional carbon aerogels is highly needed, but still faces enormous challenges. Here we demonstrate a robust strategy to fabricate multifunctional carbon aerogel via freeze-drying of cellulose nanofibers (CNF) and graphene oxide (GO) co-stabilized Pickering emulsion gel followed by high-temperature annealing. The resulting carbon aerogel exhibits tunable mechanical, hydrophilic and hydrophobic properties due to varying the elemental composition and the pyrolysis of introduced polymers. The carbon aerogel is resilient against high compression strain up to 99 % and has ultralow density (1.82 mg/cm³). The CNF/GO/acrylonitrile butadiene styrene-derived carbon aerogel (CRA)-based sensor has shown desirable sensitivity (17.65 kPa^-1), ultralow detection limit of pressure (60 Pa), and fast responsive time (130 ms), which is capable of detecting human activity, identifying spatial pressure distribution, and communicating with smartphones via Wi-Fi. Moreover, the carbon aerogel reveals effective thermal insulation and photothermal conversion performance. These results suggest the great potentials for developing lightweight and compressible carbon aerogels with multiple functions to meet various applications.

Vertical Graphene Canal Mesh for Strain Sensing with a Supereminent Resolution

The development of microstrain sensors offers significant prospects in diverse applications, such as microrobots, intelligent human-computer interaction, health monitoring, and medical rehabilitation. Among strain sensor materials, vertical graphene (VG) has demonstrated considerable potential as a resistive material; however, VG-based strain sensors with high resolution are yet to be developed. In addition, the detection mechanism of VG has not been extensively investigated. Herein, we developed a VG canal mesh (VGCM) to fabricate a flexible strain sensor for ultralow strain sensing, achieving an accurate response to strains as low as 0.1‰ within a total strain range of 0%-4%. The detection of such low strains is due to the rigorous structural design and strain concentration effect of the three-dimensional micronano structure of the VGCM. Through experimental results and theoretical simulation, the evolution of microcracks in VG and the sensing mechanism of VG and VGCM are elaborated, and the unique advantages of VGCM are revealed. Finally, the VGCM-based strain sensors are proposed as portable breathing test equipment for rapid breathing detection.

Crack-Based Core-Sheath Fiber Strain Sensors with an Ultralow Detection Limit and an Ultrawide Working Range

With the booming development of flexible wearable sensing devices, flexible stretchable strain sensors with crack structure and high sensitivity have been widely concerned. However, the narrow sensing range has been hindering the development of crack-based strain sensors. In addition, the existence of the crack structure may reduce the interface compatibility between the elastic matrix and the sensing material. Herein, to overcome these problems, integrated core-sheath fibers were prepared by coaxial wet spinning with partially added carbon nanotube sensing materials in thermoplastic polyurethane elastic materials. Due to the superior interface compatibility and the change in the conductive path during stretching, the fiber strain sensor exhibits excellent durability (5000 tensile cycles), high sensitivity (>10⁴), large stretchability (500%), a low detection limit (0.01%), and a fast response time of ∼60 ms. Based on these outstanding strain sensing performances, the fiber sensor is demonstrated to detect subtle strain changes (e.g., pulse wave and swallowing) and large strain changes (e.g., finger joint and wrist movement) in real time. Moreover, the fabric sensor woven with the core-sheath fibers has an excellent performance in wrist bending angle detection, and the smart gloves based on the fabric sensors also show exceptional recognition ability as a wireless sign language translation device. This integrated strategy may provide prospective opportunities to develop highly sensitive strain sensors with durable deformation and a wide detection range.

Textile Strain Sensor Enhancement by Coating Metal Yarns with Carbon-Filled Silicone

Flexible and stretchable strain sensors are an important development for measuring various movements and forces and are increasingly used in a wide range of smart textiles. For example, strain sensors can be used to measure the movements of arms, legs or individual joints. Thereby, most strain sensors are capable of detecting large movements with a high sensitivity. Very few are able to measure small movements, i.e., strains of less than 5%, with a high sensitivity, which is necessary to carry out important health measurements, such as breathing, bending, heartbeat, and vibrations. This research deals with the development of strain sensors capable of detecting strain of 1% with a high sensitivity. For this purpose, a total of six commercially available metallic yarns were coated with a carbon-containing silicone coating. The process is based on a vertical dip-coating technology with a self-printed 3D coating bath. Afterwards, the finished yarns were interlooped and stretched by 1% while electrical resistance measurements were carried out. It was shown that, although the coating reduced the overall conductivity of the yarns, it also improved their sensitivity to stress. Conclusively, highly sensitive strain sensors, designed specially for small loads, were produced by a simple coating set-up and interlooping structure of the sensory yarns, which could easily be embedded in greater textile structures for wearable electronics.

Phase Change Energy Storage Elastic Fiber: A Simple Route to Personal Thermal Management

A novel thermoplastic polyurethane (TPU) PCFs possessing a high loaded ratio and high elasticity was simply prepared by vacuum absorption following wet spinning, then coated by waterborne polyurethane (WPU). Octadecane (OCC), hexadecanol (HEO), and stearic acid (SA), which have different tendencies to form hydrogen bonds with TPU, were selected as PCMs, and their thermal behavior, thermal storge properties, and elasticity were systematically studied, respectively. The hierarchical pore structure though from the sheath to the core part of TPU filaments weakened the influence of the nonfreezing layer and hydrogen bond on the crystallization behavior of PCMs. The resulting HEO/TPU fiber has the highest enthalpy of 208.1 J/g compared with OCC and SA. Moreover, the HEO/TPU fiber has an elongation at break of 354.8% when the phase change enthalpy is as high as 177.8 J/g and the phase change enthalpy is still 174.5 J/g after fifty cycles. After ten tensile recovery cycles, the elastic recovery rate of HEO/TPU fiber was only 71.3%. When the HEO in the fiber was liquid state, the elastic recovery rate of HEO/TPU fiber promoted to 91.6%. This elastic PCFs have excellent thermal cycle stability, elastic recovery, and temperature sensitivity. It has great application potential in the fields of flexible wearable devices, intelligent fabrics, and temperature sensors.

Continuously Reinforced Carbon Nanotube Film Sea-Cucumber-like Polyaniline Nanocomposites for Flexible Self-Supporting Energy-Storage Electrode Materials

The charge storage mechanism and capacity of supercapacitors completely depend on the electrochemical and mechanical properties of electrode materials. Herein, continuously reinforced carbon nanotube film (CNTF), as the flexible support layer and the conductive skeleton, was prepared via the floating catalytic chemical vapor deposition (FCCVD) method. Furthermore, a series of novel flexible self-supporting CNTF/polyaniline (PANI) nanocomposite electrode materials were prepared by cyclic voltammetry electrochemical polymerization (CVEP), with aniline and mixed-acid-treated CNTF film. By controlling the different polymerization cycles, it was found that the growth model, morphology, apparent color, and loading amount of the PANI on the CNTF surface were different. The CNTF/PANI-15C composite electrode, prepared by 15 cycles of electrochemical polymerization, has a unique surface, with a "sea-cucumber-like" 3D nanoprotrusion structure and microporous channels formed via the stacking of the PANI nanowires. A CNTF/PANI-15C flexible electrode exhibited the highest specific capacitance, 903.6 F/g, and the highest energy density, 45.2 Wh/kg, at the current density of 1 A/g and the voltage window of 0 to 0.6 V. It could maintain 73.9% of the initial value at a high current density of 10 A/g. The excellent electrochemical cycle and structural stabilities were confirmed on the condition of the higher capacitance retention of 95.1% after 2000 cycles of galvanostatic charge/discharge, and on the almost unchanged electrochemical performances after 500 cycles of bending. The tensile strength of the composite electrode was 124.5 MPa, and the elongation at break was 18.9%.

In Situ Formation of Ag Nanoparticles for Fiber Strain Sensors: Toward Textile-Based Wearable Applications

Wearable electronic devices have attracted significant attention as important components in several applications. Among various wearable electronic devices, interest in textile electronic devices is increasing because of their high deformability and portability in daily life. To develop textile electronic devices, fiber-based electronic devices should be fundamentally studied. Here, we report a stretchable and sensitive fiber strain sensor fabricated using only harmless materials during an in situ formation process. Despite using a mild and harmless reducing agent instead of typical strong and hazardous reducing agents, the developed fiber strain sensors feature a low initial electrical resistance of 0.9 Ω/cm, a wide strain sensing range (220%), high sensitivity (∼5.8 × 10⁴), negligible hysteresis, and high stability against repeated stretching-releasing deformation (5000 cycles). By applying the fiber sensors to various textiles, we demonstrate that the smart textile system can monitor various gestures in real-time and help users maintain accurate posture during exercise. These results will provide meaningful insights into the development of next-generation wearable applications.

Tunable and Nacre-Mimetic Multifunctional Electronic Skins for Highly Stretchable Contact-Noncontact Sensing

Electronic skins (e-skins) have attracted great attention for their applications in disease diagnostics, soft robots, and human-machine interaction. The integration of high sensitivity, low detection limit, large stretchability, and multiple stimulus response capacity into a single e-skin remains an enormous challenge. Herein, inspired by the structure of nacre, an ultra-stretchable and multifunctional e-skin with tunable strain detection range based on nacre-mimetic multi-layered silver nanowires /reduced graphene oxide /thermoplastic polyurethane mats is fabricated. The e-skin possesses extraordinary strain response performance with a tunable detection range (50 to 200% strain), an ultralow response limit (0.1% strain), a high sensitivity (gauge factor up to 1902.5), a fast response time (20 ms), and an excellent stability (stretching/releasing test of 11 000 cycles). These excellent response behaviors enable the e-skin to accurately monitor full-range human body motions. Additionally, the e-skin can detect relative humidity quickly and sensitively through a reversible physical adsorption/desorption of water vapor, and the assembled e-skin array exhibits excellent performance in noncontact sensing. The tunable and multifunctional e-skins show promising applications in motion monitoring and contact-noncontact human machine interaction.

Wearable Strain Sensors with Aligned Macro Carbon Cracks Using a Two-Dimensional Triaxial-Braided Fabric Structure for Monitoring Human Health

Recently, wearable sensors, due to their ability to exhibit characteristics, have been appealing for health monitoring through detection of human motions and vital signals. The development of strain sensors with high sensing performance and wearability has been a great challenge to date. In this study, a textile-based strain sensor with good skin affinity was fabricated through a simple fabrication process of dip-coating 2D triaxial-braided fabrics using carbon ink and then drying. The macro crack aligned on the 2D triaxial-braided fabric with a high-density structure and good recovery force. The sensitivity of textile-based strain sensor can be enhanced due to aligned macro crack formed by prestrained fabricating process and characteristic of the 2D triaxial braided fabric with high dense structure. The optimized sensor exhibits high sensitivity (gauge factor: 128) in a strain range of 0-30%, durability (5000 cycles), washability, low hysteresis, and fast response time (90 ms). Therefore, it can be applied as a wearable sensor that can monitor human motions (large strain) and biosignals (subtle strain).

A Simple Model Relating Gauge Factor to Filler Loading in Nanocomposite Strain Sensors

Conductive nanocomposites are often piezoresistive, displaying significant changes in resistance upon deformation, making them ideal for use as strain and pressure sensors. Such composites typically consist of ductile polymers filled with conductive nanomaterials, such as graphene nanosheets or carbon nanotubes, and can display sensitivities, or gauge factors, which are much higher than those of traditional metal strain gauges. However, their development has been hampered by the absence of physical models that could be used to fit data or to optimize sensor performance. Here we develop a simple model which results in equations for nanocomposite gauge factors as a function of both filler volume fraction and composite conductivity. These equations can be used to fit experimental data, outputting figures of merit, or predict experimental data once certain physical parameters are known. We have found these equations to match experimental data, both measured here and extracted from the literature, extremely well. Importantly, the model shows the response of composite strain sensors to be more complex than previously thought and shows factors other than the effect of strain on the interparticle resistance to be performance limiting.

Hollow-porous fibers for intrinsically thermally insulating textiles and wearable electronics with ultrahigh working sensitivity

Wearable smart devices should be flexible and functional to imitate the warmth and sensing functions of human skin or animal fur. Despite the recent great progress in wearable smart devices, it is still challenging to achieve the required multi-functionality. Here, stretchable hollow-porous fibers with self-warming ability are designed, and the properties of electrical heating, strain sensing, temperature sensing and pressure sensing are achieved. The hollow-porous TPU fiber possesses an ultra-high stretchability (1468%), and the textiles woven from the fibers present a splendid thermal insulation property (the absolute value difference in temperature |ΔT| = 68.5 and 44 °C at extreme temperatures of 115 and -40.0 °C). Importantly, after conductive filler decoration, the fiber-based strain sensor exhibits one of the highest reported gauge factor (2.3 × 10⁶) towards 100% strain in 7200 working stretch-release cycles. A low detection limit of 0.5% strain is also achieved. Besides, the fibers can be heated to 40 °C in 18 s at a small voltage of 2 V as an electrical heater. The assembled thermal sensors can monitor the temperature from 30 to 90 °C in real time, and the fiber-based capacitive type pressure sensor exhibits good sensing performance under force from 1 to 25 N. The hollow-porous fiber based all-in-one integrated wearable systems illustrate promising prospects for next generation electronic skins to detect human motions and body temperature with thermal therapy and inherent self-warming ability.

Flexible, Robust, and Durable Aramid Fiber/CNT Composite Paper as a Multifunctional Sensor for Wearable Applications

Flexible paper-based sensors may be applied in numerous fields, but this requires addressing their limitations related to poor thermal and water resistance, which results in low service life. Herein, we report a paper-based composite sensor composed of carboxylic carbon nanotubes (CCNTs) and poly-m-phenyleneisophthalamide (PMIA), fabricated by a facile papermaking process. The CCNT/PMIA composite sensor exhibits an ability to detect pressures generated by various human movements, attributed to the sensor's conductive network and the characteristic "mud-brick" microstructure. The sensor exhibits the capability to monitor human motions, such as bending of finger joints and elbow joints, speaking, blinking, and smiling, as well as temperature variations in the range of 30-90 °C. Such a capability to sensitively detect pressure can be realized at different applied frequencies, gradient sagittas, and multiple twists with a short response time (104 ms) even after being soaked in water, acid, and alkali solutions. Moreover, the sensor demonstrates excellent mechanical properties and hence can be folded up to 6000 times without failure, can bear 5 kg of load without breaking, and can be cycled 2000 times without energy loss, providing a great possibility for a long sensing life. Additionally, the composite sensor shows exceptional Joule heating performance, which can reach 242 °C in less than 15 s even when powered by a low input voltage (25 V). From the perspective of industrialization, low-cost and large-scale roll-to-roll production of the paper-based sensor can be achieved, with a formed length of thousands of meters, showing great potential for future industrial applications as a wearable smart sensor for detecting pressure and temperature, with the capability of electric heating.

Multifunctional and Ultrasensitive-Reduced Graphene Oxide and Pen Ink/Polyvinyl Alcohol-Decorated Modal/Spandex Fabric for High-Performance Wearable Sensors

Sensitive and flexible sensors capable of monitoring physiological signals of human body for healthcare have been developed in recent years. It is still a challenge to fabricate a wearable sensor-integrated multifunctional performances and a good fit to human body. Here, an rGO and pen ink/PVA-layered strain-humidity sensor based on MS fabric is prepared through a cost-effective and scalable solution process. The conductive fabric as a strain sensor has a workable strain range (∼300%), ultrahigh sensitivity (maximum gauge factor of 492.8), great comfort, and long-term stability. Notably, a step increase in relative resistance variation will be achieved by controlling the coverage of an ink layer. Moreover, the reliable linear humidity-dependent resistance void of hysteresis and excellent repeatability renders conductive fabrics an opportunity as humidity sensors. Based on these superior multifunctions, the resultant conductive fabric can be applied to detect both human motions and skin humidity, showing potential in applications of wearable electronics for professional athletes.

Self-Repairing, Large Linear Working Range Shape Memory Carbon Nanotubes/Ethylene Vinyl Acetate Fiber Strain Sensor for Human Movement Monitoring

Flexible strain sensors have shown great application value in wearable devices. In the past decades, researchers have spent numerous efforts on developing high-stretchability, excellent dynamic durability, and large linear working range flexible strain sensors and shaped a series of important research results. However, the viscoelasticity of the elastic polymer is always a big challenge to develop a flexible sensor. Here, to overcome this challenge, we developed a novel self-repairing carbon nanotubes/ethylene vinyl acetate (CNTs/EVA) fiber strain sensor prepared by embedding the CNTs on the surface of the swollen shape memory EVA fiber via the ultrasonic method. The CNTs/EVA fiber strain sensors responded with significant results, with high stretchability (190% strain), large linear working range (up to 88% strain), excellent dynamic durability (5000 cycles), and fast response speed (312 ms). In addition, the permanently damaged conductive network of the strain sensors, caused by the viscoelasticity of elastic polymer, can restore above the transforming temperature of the shape memory CNTs/EVA fiber. Moreover, the performance of the restored strain sensors was almost as same as that of the original strain sensors. Furthermore, human health monitoring tests show that the CNTs/EVA fiber has a broad application prospect for human health monitoring in wearable electronic devices.

Flexible conductive Ag nanowire/cellulose nanofibril hybrid nanopaper for strain and temperature sensing applications

With the rapid development of smart wearable devices, flexible and biodegradable sensors are in urgent needs. In this study, "green" electrically conductive Ag nanowire (AgNW)/cellulose nanofiber (CNF) hybrid nanopaper was fabricated to prepare flexible sensors using the facial solution blending and vacuum filtration technique. The amphiphilic property of cellulose is beneficial for the homogeneous dispersion of AgNW to construct effective electrically conductive networks. Two different types of strain sensors were designed to study their applications in strain sensing. One was the tensile strain sensor where the hybrid nanopaper was sandwiched between two thermoplastic polyurethane (TPU) films through hot compression, and special micro-crack structure was constructed through the pre-strain process to enhance the sensitivity. Interestingly, typical pre-strain dependent strain sensing behavior was observed due to different crack densities constructed under different pre-strains. As a result, it exhibited an ultralow detection limit as low as 0.2%, good reproducibility under different strains and excellent stability and durability during 500 cycles (1% strain, 0.5 mm/min). The other was the bending strain sensor where the hybrid nanopaper was adhered onto TPU film, showing stable and recoverable linearly sensing behavior towards two different bending modes (tension and compression). Importantly, the bending sensor displayed great potential for human motion and physiological signal detection. Furthermore, the hybrid nanopaper also exhibited stable and reproducible negative temperature sensing behavior when it was served as a temperature sensor. This study provides a guideline for fabricating flexible and biodegradable sensors.

Naturally Derived Wearable Strain Sensors with Enhanced Mechanical Properties and High Sensitivity

Flexible strain sensors are of great interest for future applications in the next-generation wearable electronic devices. However, most of the existing flexible sensors are based on synthetic polymer materials with limitations in biocompatibility and biodegradability, which may lead to potential environmental pollution. Here, we propose a naturally derived wearable strain sensor based on natural-sourced materials including milk protein fabric, natural rubber, tannic, and vitamin C. The obtained sensors exhibit remarkably enhanced mechanical properties and high sensitivity contrast to currently reported natural resource-based sensors, owing to the metal-ligand interface design and the construction of an organized three-dimensional conductive network, which well fit the requirements of electronic skin. This work represents an important advance toward the fabrication of naturally derived high-performance strain sensors and expanding possibilities in the design of environmental-friendly soft actuators, artificial muscle, and other wearable electronic devices.

Self-Derived Superhydrophobic and Multifunctional Polymer Sponge Composite with Excellent Joule Heating and Photothermal Performance for Strain/Pressure Sensors

Flexible strain or pressure sensors have potential applications in electronic skin, healthcare, etc. It remains a challenge to explore multifunctional strain or pressure sensors that possess excellent water repellent and heating performance and hence can be used in harsh environments such as high moisture and low-temperature conditions. Here, a self-derived superhydrophobic and multifunctional polymer composite foam is prepared by adsorption of the Ag precursor in tetrahydrofuran (THF) onto the rubber sponge followed by reduction of Ag⁺ to Ag nanoparticles (AgNPs). During the Ag⁺ reduction in hydrazine solution, the swollen rubber sponge by THF is partially precipitated based on the nonsolvent-induced phase separation (NIPS). The NIPS creates a porous structure on the sponge surface and thus a high surface roughness, contributing to the material superhydrophobicity. The precipitated polymer wrapping the AgNPs could enhance the interaction between the individual AgNPs. The obtained conductive sponge composite possesses excellent Joule heating and photothermal performance and can be used as both a strain and pressure sensor. The conductive sponge composite sensor possesses good reliability and durability and can be applied to real-time monitoring of human body movements.

Highly Stretchable and Sensitive Strain Sensor with Porous Segregated Conductive Network

Flexible strain sensors based on elastomeric conductive polymer composites (ECPCs) play an important role in wearable sensing electronics. However, the achievement of good conjunction between broad detection range and high sensitivity is still challenging. Herein, a highly stretchable and sensitive strain sensor was developed with the formation of porous segregated conductive network in the carbon nanotube/thermoplastic polyurethane composite via a facile and nontoxic compression-molding plus salt-leaching method. The strain sensor with porous segregated conductive network exhibited perfect combination of ultrawide sensing range (800% strain), large sensitivity (gauge factor of 356.4), short response time (180 ms) and recovery time (180 ms), as well as superior stability and durability. The integrated porous structure intensifies the deformation of segregated conductive network when tension strain is applied, which benefits enhancement of the sensitivity. Our sensor could monitor not only subtle oscillation and physiological signals but also energetic human motions efficiently, revealing promising potential applications in wearable motion monitoring systems. This work provides a unique and effective strategy for realizing ECPCs based strain sensors with excellent comprehensive sensing performances.

Superhydrophobic Electrically Conductive Paper for Ultrasensitive Strain Sensor with Excellent Anticorrosion and Self-Cleaning Property

Recently, a paper-based (PB) strain sensor has turned out to be an ideal substitute for the polymer-based one because of the merits of renewability, biodegradability, and low cost. However, the hygroexpansion and degradation of the paper after absorbing water are the great challenges for the practical applications of the PB strain sensor. Herein, the superhydrophobic electrically conductive paper was fabricated by simply dip-coating the printing paper into the carbon black (CB)/carbon nanotube (CNT)/methyl cellulose suspension and hydrophobic fumed silica (Hf-SiO₂) suspension successively to settle the problem. Because of the existence of ultrasensitive microcrack structures in the electrically conductive CB/CNT layer, the sensor was capable of detecting an ultralow strain as low as 0.1%. During the tension strain range of 0-0.7%, the sensor exhibited a gauge factor of 7.5, almost 3 times higher than that of the conventional metallic-based sensors. In addition, the sensor displayed frequency-independent and excellent durability and reproductivity over 1000 tension cycles. Meanwhile, the superhydrophobic Hf-SiO₂ layer with a micro-nano structure and low surface energy endowed the sensor with outstanding waterproof and self-cleaning properties, as well as great sustainability toward cyclic strain and harsh corrosive environment. Finally, the PB strain sensor could effectively monitor human bodily motions such as finger/elbow joint/throat movement and pulse in real time, especially for the wet or rainy conditions. All these pave way for the fabrication of a high-performance PB strain sensor.

Facile Fabrication of Superhydrophobic and Eco-Friendly Poly(lactic acid) Foam for Oil-Water Separation via Skin Peeling

Superhydrophobic polymer foams are a good candidate for oil absorption because of their lightweight and tunable porosity and have promising applications in the long-term application of oil-water separation. However, developing a facile and green strategy to fabricate pure polymer foams with superhydrophobicity and eco-friendliness for large-scale oil-water separation remains a challenge. Here, a facile template-free water-assisted thermally impacted phase separation approach combined with skin peeling for the fabrication of superhydrophobic and eco-friendly pure poly(lactic acid) (PLA) foam for oil-water separation is proposed for the first time. The PLA foam with special micro- and nanostructures possesses a water contact angle of 151°, and the maximum saturated adsorption capacity is 31.5 g/g. More importantly, during the continuous oil-water pumping experiment, the foam has an efficiency of 98% and could maintain for more than 15 h, showing a promising prospect for cleaning large-scale oil pollution.

Ultrasensitive and Highly Compressible Piezoresistive Sensor Based on Polyurethane Sponge Coated with a Cracked Cellulose Nanofibril/Silver Nanowire Layer

With the rapid development of flexible wearable electronics, a piezoresistive sensor with low detection limit and wide strain sensing range turns out to be a great challenge for its application in this field. Here, a cracked cellulose nanofibril/silver nanowire (CA) layer-coated polyurethane (PU) sponge was acquired through a simple dip-coating process followed by precompression treatment. The electrical conductivity and mechanical property of the conductive CA@PU sponge could be effectively tuned through changing the dip-coating number. As a piezoresistive sensor, the sponge exhibited the capability of detecting both small and large motions over a wide compression strain range of 0-80%. Based on the "crack effect", the sensor possessed a detection limit as low as 0.2% and the gauge factor [GF, GF = (Δ R/ R₀)/ε, where Δ R, R₀, and ε represent the instantaneous resistance change, original resistance, and strain applied, respectively] was as high as 26.07 in the strain range of 0-0.6%. Moreover, the "contact effect" enabled the sensor to be applicable for larger strain, and the GF decreased first and then became stable with increasing compression strain. In addition, frequency- and strain-dependent sensing performances were observed, demonstrating that the sensor can respond reliably to different applied frequencies and strains. Furthermore, the sensor displayed exceptional stability, repeatability, and durability over 500 cycles. Finally, the sensor could be applicable for the detection of various human bodily motions, such as phonation, stamping, knee bending, and wrist bending. Most importantly, the sponge also exhibited great potential for the fabrication of artificial electronic skin. Herein, the conductive CA@PU sponge will undoubtedly promote the development of high-performance flexible wearable electronics.