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Flexible electronic devices based on reversible bonds for self-curing capabilities arouses extensive interest. However, most of the composite conductive elastomers have the problems of poor mechanical properties and slow recovery of mechanical properties during multiple stretching, which hinder their stability in continuous operation. In this study, hyperbranched-MWCNTs/hyperbranched-PDMS self-healable conductive elastomers inspired by cephalopods were successfully developed. The prepared conductive elastomer exhibited good self-healing ability (91%) at room temperature excited by multiple reversible interactions. The prepared elastomer had outstanding mechanical properties and anti-fatigue ability, so that it can cope with more arduous tasks. Moreover, the elastomer was sensitive to the change of stress states and can be used as a stable strain sensor. Therefore, the self-repairing conductive elastomer has potential practicability in the fields of human-computer interaction, motion monitoring, soft robot and so on.

Wearable sensors have gained considerable popularity due to the potential feasibility in flexible electronics. In this work, a facile fabrication strategy for a wearable strain sensor is presented based on a hierarchical-gap response mechanism. A double-helical-structured elastic string was employed as the stretchable substrate, and carbon nanotubes were chosen as the resistance-sensitive nanostructures. A CNT-modified elastic string (CES) was obtained by directly coating CNTs onto the substrate. After polydimethylsiloxane encapsulation, a fiber-shaped resistive-type strain sensor was fabricated. It was attached to different joints for human body motion monitoring. The double-helical structure could produce hierarchical gaps as the applied strain increased, which served as spatial complements to allow a well-connected structure to be formed. The CES sensor could accurately capture different levels of strain caused by physiological signals and joint motions. This facile fabrication strategy is expected to be used to produce large-scale strain sensors with ultra-sensitivity.

We propose a novel flexible mechano-optical sensor by modulating the transmittance of Ecoflex/molybdenum disulfide (MoS2). This device is successfully used in wearable and robotic applications.

The flexible hydrogel sensors in the field of artificial intelligence have been widely concerned, which could be applied in medical monitoring, human motion detection, and intelligent robots. However, the integration of the synergistic properties of excellent mechanical properties, temperature sensitivity, adhesion ability and self-healing ability for preparation of hydrogel-type strain sensor is still a challenge. Moreover, how can we recover cumulated sensors without affecting the environment? Herein, a self-healing hydrogel was prepared based on deep eutectic solvent (DES) combined with polyvinyl alcohol (PVA), and cellulose nanocrystals (CNCs), which has the peotential application as wearable strain sensors. The DES network crosslinks PVA/CMC-Na/CNCs through ionic bonds, and the overall network is further linked through physical entanglement and hydrogen bonding interactions. Interestingly, the hydrogel could sensitively detect large or subtle movements (such as finger bending, wrist bending, knee bending, pulse and pronounce), indicating its potential applications in human–computer interaction and personal health monitoring.

Currently, the exploration of wearable strain sensors that can work under subzero temperatures while simultaneously possessing anti-interference capability toward temperature is still a grand challenge. Herein, we present a low-temperature wearable strain sensor that is constructed via the incorporation of a Ag nanowires/graphene (Ag NWs/G) composite into the polydimethylsiloxane (PDMS) polymer. The Ag NWs/G/PDMS strain sensor exhibits promising flexibility at a very low temperature (-40 °C), outstanding fatigue resistance with low hysteresis energy, and near-zero temperature coefficient of resistance (TCR). The Ag NWs/G/PDMS strain sensor shows excellent sensing performance under subzero temperatures with a very high gauge factor of 9156 under a strain of36%, accompanied by a noninterference characteristic to temperature (-40 to 20 °C). The Ag NWs/G/PDMS strain sensor also demonstrates the feasibility of monitoring various human movements such as finger bending, arm waving, wrist rotation, and knee bending under both room temperature and low-temperature conditions. This work initiates a new promising strategy to construct next-generation wearable strain sensors that can work stably and effectively under very low temperatures.

This paper proposes the use of sensor-integrated silicon cantilever arrays to measure drug-induced cardiac toxicity in real time. The proposed cantilever sensors, unlike the conventional electrophysiological methods, aim to evaluate cardiac toxicity by measuring the contraction force of the cardiomyocytes corresponding to the target drugs. The surface of the silicon cantilever consists of microgrooves to maximize the alignment and the contraction force of the cardiomyocytes. This type of surface pattern also helps in the maturation of the cardiomyocytes by increasing the sarcomere length. The preliminary characterization of the cantilever sensors was performed on the cantilever surface, with the cardiomyocytes seeded with a density of 1000 cells/mm2, and the cardiac contractility was measured as a function of the culture days. The change in the contraction force of the cardiomyocytes due to the drug concentration was successfully measured through the integrated strain sensor in the culture media. The reliability of the sensor-integrated cantilevers and the feasibility of their mass production ensure that they meet the practical requirements in the medical applications.

Ultrathin gold nanowires (AuNWs) have shown great potential toward applications in flexible electronics. However, they suffer from high surface instability that consequently limits their applicability for solution-processed composite materials. To overcome this limitation, we show for the first time how a synthetic thiolated small-molecule ligand such as ethyl 11-(4-mercaptobenzamido) undecanoate can stabilize the AuNW surface enabling their manipulation in solution-processed composite manufacturing techniques. As a proof of concept, we show how such stabilized NWs can be formulated into a PDMS matrix and spin-coated to afford stretchable elastomeric strips capable of sensing mechanical deformation with loading as low as 0.03 wt %.

Flexible strain sensors with high performance are actively and widely investigated for wearable electronic devices. However, the conventional sensors often suffer from a lack of detection of complex multidimensional strain, which severely limits their wide applications. To overcome this critical challenge, we propose a pattern design by screen printing to construct an asymmetrical cross-conductive network in the piezoresistive strain sensor, which can enhance the response to external stimuli in different directions. The unique network endows the prepared sensors with the excellent ability of instantaneous detection and accurate identification of multidimensional strains. Moreover, the sensor also demonstrates high sensitivity, fast response, an ultra-wide sensing range, and excellent stability and durability. Benefiting from the outstanding comprehensive performance of the prepared sensor, a full range of human actions (wink, smile, swallowing, and joint bending) and subtle bio-signals (pulse and breathing) are easily and accurately monitored. A wireless wearable device assembled by the sensor shows great potential applications in practical real-time physiological monitoring and intelligent mobile diagnosis for humans. This work provides an innovative and effective strategy for manufacturing flexible and multifunctional strain sensors to fully satisfy versatile applications of new-generation wearable electronic devices.

Intrinsically conducting polymers (ICPs), such as polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(3,4‐ethylenedioxythiophene) (PEDOT), can have important application in flexible electronics owing to their unique merits including high conductivity, high mechanical flexibility, low cost, and good biocompatibility. The requirements for their application in flexible electronics include high conductivity and appropriate mechanical properties. The conductivity of some ICPs can be enhanced through a postpolymerization treatment, the so‐called “secondary doping.” A conducting polymer film with high conductivity can be used as flexible electrode and even as flexible transparent electrode of optoelectronic devices. The application of ICPs as stretchable electrode requires high mechanical stretchability. The mechanical stretchability of ICPs can be improved through blending with a soft polymer or plasticization. Because of their good biocompatibility, ICPs can be modified as dry electrode for biopotential monitoring and neural interface. In addition, ICPs can be used as the active material of strain sensors for healthcare monitoring, and they can be adopted to monitor food processing, such as the fermentation, steaming, storage, and refreshing of starch‐based food because of the resistance variation caused by the food volume change. All these applications of ICPs are covered in this review article.

Nowadays, great effort has been devoted to fabricate flexible wearable sensor with high stretchability, moderate modulus, favorable durability, excellent transparency, and satisfactory sensitivity. In this work, we report the preparation of a hybrid double-network (DN) hydrogel by a simple one-pot method. First, chitosan was added into an AlCl

Stretchable strain sensor detects a wide range of strain variation and is therefore a key component in various applications. Unlike traditional ones made of elastomers doped with conductive components or fabricated with liquid conductors, ionically conductive hydrogel-based strain sensors remain conductive under large deformations and are biocompatible. However, dehydration is a challenging issue for the latter. Researchers have developed hydrogel-elastomer-based strain sensors where an elastomer matrix encapsulates a hydrogel circuit to prevent its dehydration. However, the reported multi-step approaches are generally time-consuming. Our group recently reported a multimaterial 3D printing approach that enables fast fabrication of such sensors, yet requires a self-built digital-light-processing-based multimaterial 3D printer. Here, we report a simple projection lithography method to fabricate hydrogel-elastomer-based stretchable strain sensors within 5 minutes. This method only requires a UV projector/lamp with photomasks; the chemicals are commercially available; the protocols for preparing the polymer precursors are friendly to users without chemistry background. Moreover, the manufacturing flexibility allows users to readily pattern the sensor circuit and attach the sensor to a 3D printed soft pneumatic actuator to enable strain sensing on the latter. The proposed approach paves a simple and versatile way to fabricate hydrogel-elastomer-based stretchable strain sensors and flexible electronic devices.

Aim of this work is to improve the bond between a strain sensor and a device on which the strain shall be determined. As strain sensor, a CMOS-integrated chip featuring piezoresistive sensor elements was used which is capable of wireless energy and data transmission. The sensor chip was mounted on a standardized tensile test specimen of stainless steel by a bonding process using reactive multilayer systems (RMS). RMS provide a well-defined amount of heat within a very short reaction time of a few milliseconds and are placed in-between two bonding partners. RMS were combined with layers of solder which melt during the bonding process. Epoxy adhesive films were used as a reference bonding process. Under mechanical tensile loading, the sensor bonded with RMS shows a linear strain sensitivity in the whole range of tested forces whereas the adhesive-bonded sensor has slightly nonlinear behavior for low forces. Compared to the adhesive-bonded chips, the sensitivity of the reactively bonded chips is increased by a factor of about 2.5. This indicates a stronger mechanical coupling by reactive bonding as compared to adhesive bonding.

In this work, a highly stretchable and sensitive strain sensor using silver nanowires (Ag NWs)/multi-walled carbon nanotubes (MWCNTs) on hair band is proposed. The fabrication process of such sensors enabled scalable and facile production with low cost by dipping hair band into Ag NWs/MWCNTs composites suspension. The sensing performance of the sensors was optimized by tuning the Ag NWs/MWCNTs mass ratio. The hair band based strain sensor with Ag NWs/MWCNTs mass ratio of 30 exhibited the best performance with high sensitivity of 416.0 and good stretchability up to 70%. Moreover, great repeatability and long-term stability could be achieved over 7500 cycles of stretch-release. This Ag NWs/MWCNTs-hair band sensor can be used for monitoring human motions and swallowing action.

The flourishing development in flexible electronics has provoked intensive research in flexible strain sensors to realize accurate perception acquisition under different external stimuli. However, building hydrogel-based strain sensors with high stretchability and sensitivity remains a great challenge. Herein, MXene nanosheets were composited into polyacrylamide-sodium alginate matrix to construct mechanical robust and sensitive double networked hydrogel strain sensor. The hydrophilic MXene nanosheets formed strong interactions with the polymer matrix and endowed the hydrogel with excellent tensile properties (3150%), compliant mechanical strength (2.03 kPa−1 in Young’s Module) and long-lasting stability and fatigue resistance (1000 dynamic cycles under 1,600% strain). Due to the highly oriented MXene-based three dimensional conductive networks, the hydrogel sensor achieved extremely high tensile sensitivity (18.15 in gauge factor) and compression sensitivity (0.38 kPa−1 below 3 kPa). MXene hydrogel-based strain sensors also displayed negligible hysteresis in electromechanical performance, typical frequent-independent feature and rapid response time to external stimuli. Moreover, the sensor exhibited accurate response to different scales of human movements, providing potential application in speech recognition, expression recognition and handwriting verification.

Artificial skins with sensing ability have great potential in applications of wearable devices and soft robotics. Inspired by the functions of human skins including sensing stimuli via electrical signal and bruising for injury indication, an ionic conductive and mechanochromic organohydrogel is synthesized and demonstrated as ionic skin (I-skin). The gel consisting of mechanochromophore cross-linked micelles is mechanically robust, stretchable, and deformation durable with minor hysteresis, and it also displays good solvent retention. The change of relative resistance during elongation and compression suggests a high sensitivity. An optical change from pale yellow to bruise-like blue-purple color is observed under a large deformation. The ionic conductive organohydrogel as I-skin is attached to different parts of the human body with movements mimicking various body-bruising scenarios, demonstrating successful perception and visualization of mechanical stimuli. The work vividly presents a strain sensor with the functions of injury visualization and damage warning for mechanical impacts. The I-skin can be potentially used in the applications including prosthetic devices, wearable electronics, and intelligent robots.

Optical fiber sensor network has attracted considerable research interests for geoscience applications. However, the sensor capacity and ultra-low frequency noise limits the sensing performance for geoscience data acquisition. To achieve a high-resolution and lager sensing capacity, a strain sensor network is proposed based on phase-sensitive optical time domain reflectometer (φ-OTDR) technology and special packaged fiber with scatter enhanced points (SEPs) array. Specifically, an extra identical fiber with SEPs array which is free of strain is used as the reference fiber, for compensating the ultra-low frequency noise in the φ-OTDR system induced by laser source frequency shift and environment temperature change. Moreover, a hysteresis operator based least square support vector machine (LS-SVM) model is introduced to reduce the compensation residual error generated from the thermal hysteresis nonlinearity between the sensing fiber and reference fiber. In the experiment, the strain sensor network possesses a sensing capacity with 55 sensor elements. The phase bias drift with frequency below 0.1 Hz is effectively compensated by LS-SVM based hysteresis model, and the signal to noise ratio (SNR) of a strain vibration at 0.01 Hz greatly increases by 24 dB compared to that of the sensing fiber for direct compensation. The proposed strain sensor network proves a high dynamic resolution of 10.5 pε·Hz-1/2 above 10 Hz, and ultra-low frequency sensing resolution of 166 pε at 0.001 Hz. It is the first reported a large sensing capacity strain sensor network with sub-nε sensing resolution in mHz frequency range, to the best of our knowledge.

This paper presents a textile-based stretchable microstrip patch antenna with intrinsic strain for e-textiles with seamlessly integrated multifunctional devices. Several microstrip antennas have been developed with the patch alone (stretchable up to 40%) or both the patch and the ground plane (stretchable up to 100%) meshed by using rectangular serpentine units. The changes in the resonant frequency of the meshed antennas, as a result of stretching, have been exploited to demonstrate the intrinsic uniaxial strain sensing. The obtained results indicate that resonant frequency decreases linearly with increasing applied strain, suggesting that the designed antennas can also be used as strain sensors with stretchability up to 100% and a sensitivity of 0.25. The results were validated through full-wave electromagnetic simulations and a two-dimensional digital image correlation (DIC) technique to model the antenna deformations during the tensile tests. In terms of stretchability, the meshed textile patch antenna on a solid ground plane showed more than a 2-fold improvement compared to a meshed gold patch antenna, showing a linear frequency shift. As potential applications, we demonstrate the use of a highly deformable fully meshed textile antenna as a strain sensor capable of measuring joint angles of a human limb. To do that, a microwave readout circuit based on RF to DC rectifier was realized. The rectifier obtained a peak conversion efficiency of 71% at 10 dBm input power overload resistor of 3 kΩ.

Flexible electronics intensely pursues the development of new-generation electronic skin and human–machine interfaces. However, monitoring human motions with both large dynamic range and high sensi...

Implantable sensors can be used to monitor biomechanical strain continuously. However, three key challenges need to be addressed before they can be of use in clinical practice: the structural mismatch between the sensors and tissue or organs should be eliminated; a practical suturing attachment process should be developed; and the sensors should be equipped with wireless readout. Here, we report a wireless and suturable fibre strain-sensing system created by combining a capacitive fibre strain sensor with an inductive coil for wireless readout. The sensor is composed of two stretchable conductive fibres organized in a double helical structure with an empty core, and has a sensitivity of around 12. Mathematical analysis and simulation of the sensor can effectively predict its capacitive response and can be used to modulate performance according to the intended application. To illustrate the capabilities of the system, we use it to perform strain measurements on the Achilles tendon and knee ligament in an ex vivo and in vivo porcine leg. A capacitive, fibre-like stretchable strain sensor, formed of two conductors in a double helical structure, can be combined with an inductive coil to create a wireless strain-sensing system for biomedical applications.

Wearable strain sensor can be mounted on textiles and attached onto human skins to detect various mechanical motions and health conditions. Here, we report a sensitive and response-stable strain sensor based on the polymers consisting of the super acid-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polydimethylsiloxane. The polymer-based strain sensor exhibits a high sensitivity of 26.6–30.6 and a stable response with a large sensing strain of up to 30% for a potential application.

Two‐dimensional (2D) materials have attracted extensive research interests due to their excellent properties related to unique structure. Strain engineering, as an important strategy for tuning the lattice and electronic structure of 2D materials, has been widely used in the modulation of physical properties, which broadens their applications in flexible nanoelectronic and optoelectronic devices. In this review, we first summarize the methods of inducing strain to 2D materials and discuss the advantages and problems of various methods. We then introduce the strain‐induced effects on optical, electrical, and magnetic properties, together with the phase transition of 2D materials. Finally, we illustrate the potential applications of strained 2D materials and further look forward to their opportunities and challenges in practical applications in the future.

A strain sensor based on the fibre ring cavity laser with a structure that consists of a few-mode fibre (FMF) sandwiched between two single-mode fibres (SMF), which is used as an optical filter and...

Different from conductive hydrogels, ion gels, another kind of ionic conductor, possess excellent nonvolatility and anti-freezing capability, which have attracted increasing research interest in el...

Freezing-tolerant and conductive hydrogels have attracted tremendous interest as promising materials for flexible sensors under the gelid condition. In this work, a freezing-tolerant, dual-responsi...

Fiber-shaped sensors have great potential for real-time monitoring of human physiological signals thanks to the merging of electronic and textile technologies. This work reports on the fabrication of a core–sheath structured strain-sensing fiber based on the wet-spinning method. The sensing fiber is composed of a core of non-conducting polyurethane and a conducting sheath of carbon black in a polyurethane matrix. Microscopic observation reveals the irregular shape or scattered appearance of the core as well as the porous structure of the fiber, the diameter of which is in the range 200–500 μm. The electro-mechanical properties and their dependence on carbon black concentration in the sheath and draw ratio between the spinneret and first drafting roller are experimentally investigated. It has been found that the percolation threshold of the fiber is in the range 15–16 wt%. The resistance of the fiber rises stably as the fiber is stretched up to a strain of 120% and increases with the increase of draw ratio between the spinneret and first drafting roller. In the cyclic tensile tests, the resistance of the fiber exhibits good repeatability in subsequent loading–unloading cycles after pre-stretching, despite partial recovery of the resistance in the first few cycles. The integration of the strain-sensing fiber into textiles is demonstrated by the core-spun yarn fabricated based on a modified vortex spinning method. The results of this study indicate the fiber could be a promising candidate for a sensor for smart textiles.

A high sensitivity, simple fabrication all‐fibre strain sensor is proposed and demonstrated experimentally. The sensor consists of a section of Panda‐type polarization maintaining fibre (PMF) built into a Sagnac interferometer. The Panda‐type PMF is intentionally spliced to the single‐mode fibre through enhanced fusion splicing to excite high‐order polarized modes. The enhanced fusion splicing characterizes a large magnitude of overlap. The measured strain sensitivity reaches up to 32 pm/με and the strain resolution of 0.63 με has been achieved. The proposed fibre strain sensor characterizing easy fabrication and high sensitivity can be a good candidate in the field of structural health monitoring.

Wearable sensors with conductive polymer as the sensing material exhibit the combined advantages including comfortable attachment above epidermis, regulable configurations, and cost‐effective fabrication. It is highly desired to fabricate the conductive polymer‐based strain sensor that can precisely detect all the human physiological information. In this paper, we report a high‐performance strain sensor with the H2SO4 doped poly (3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) sensing network. This sensor exhibits comprehensive properties integrating high sensitivity (gauge factor up to 32), excellent stability (> 2000 cycles), and wide strain range detectability (0.1% to 70% strain). This sensor is attractive and promising for the practical full‐range detection of human physiological information from subtle vital signs to large human motions.

The paper proposes a mathematical model for estimating changes in the strain fields caused by the installation of a substrate with an optical fiber sensor on the surface of the controlled structure. It also analyzes the information on changes in the strain tensor components in the zone of sensor location at different dimensions of the structure. It has been shown that these changes depend on the ratio of mechanical characteristics of the substrate and the material of the structure with the fiber optic strain sensor placed on its surface. A comparative analysis of strain changes is made under different loading conditions of the structure with the substrate-bonded optical fiber sensor placed on its surface.

A knitted textile-type strain sensor with resistive random access memory (RRAM) characteristics is developed. This study focuses on the change in resistance at the contact between carbon and aluminum fibers along the strain to develop a strain sensor that can be used in smart clothing. Unlike conventional textile sensors that use only one type of conducting yarn, this study proposes a new configuration that uses two different materials to obtain a large change in resistance value. The all-in-one yarn, comprising carbon fibers, aluminum-coated yarns, and normal yarns, was used to construct sensor devices by knitting. In this knitted textile, the resistance switching characteristics of the RRAM between aluminum and carbon were observed as confirmed in previous studies for flat devices and woven textiles. Along the strain applied to the knitted fabric sensor, the change in the resistance along the stretching was detected and verified through a physical mechanism of the change in the contact area. The sensitivity of the developed knitted strain sensor is more than 10 times higher than previously demonstrated textile strain sensors. Also, cyclic testing of the sensor between the stretched and the recovered state on a machine and on a knee was demonstrated to prove that it can be used as a motion sensor by applying it to the joint part of clothing. In addition, there are advantages of low-cost and large-scale production that are possible since this yarn can be used in existing fabric production industries.

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.

In recent years, graphene based flexible strain sensors have attracted great attention. Although various designed micro/nanostructure morphologies have been introduced to optimize the performance parameters such as sensitivity and working range, there are relatively limited studies on the effective patterning of graphene on various substrates on curved surfaces to manufacture functional devices. In this work, we report a general route to realize the patterning of graphene into predesigned structure on a variety of complex curved substrates. The methodology involves a combination of polydopamine surface modification of the substrate and the spraying process of graphene. The polydopamine modification method preserves the intrinsic electrical properties and stability of graphene, and the modification process by self‐polymerization of dopamine is available on many types of materials of complex shape. This general approach of interface modification to achieve free patterning has capability of large‐scale fabrication. By this interface‐enhancement patterning method, sensing structure of cut‐through channel crack has been obtained, resulting in flexible strain sensors with adjusted high sensitivity (gauge factor of 100–2000 at 1% strain), little hysteresis and conformality of complex surfaces. In addition, the strain sensor array has also been fabricated to detect the deformation spatial distribution of complex curved structural components, thereby expanding their potential applications in wearable electronics.

Yarn-based strain sensor is an emerging candidate for the fabrication of wearable electronic devices. The intrinsic properties of yarn, such as excellent lightweight, flexibility, stitchability, and especially its highly stretchable performance, stand out the yarn-based strain sensor from conventional rigid sensors in detection of human body motions. Recent advances in conductive materials and fabrication methods of yarn-based strain sensors are well reviewed and discussed in this work. Coating techniques including dip-coating, layer by layer assemble, and chemical deposition for deposition of conductive layer on elastic filament were first introduced, and fabrication technology to incorporate conductive components into elastic matrix via melt extrusion or wet spinning was reviewed afterwards. Especially, the recent advances of core–sheath/wrapping yarn strain sensor as-fabricated by traditional spinning technique were well summarized. Finally, promising perspectives and challenges together with key points in the development of yarn strain sensors were presented for future endeavor.

Flexible fiber batteries with self-powering functionality will offer new opportunities for next-generation wearable electronic devices. Here, we present a high-capacity aqueous Zn-ion battery fiber that can directly harvest energy from ambient air to recharge without using additional power supply. The air-recharging capability stems from the freestanding cathode fiber consisting of nano-structured V6O13/aligned carbon nanotubes, which can induce a spontaneous redox reaction with ambient air at its discharged state for capacity recovery. This air-recharging process is reversible and compatible with the routine galvanostatic charge–discharge of aqueous Zn-ion batteries. The resultant battery fiber shows a high specific capacity (371 mA h g−1 at 200 mA g−1), stable cyclability (91% capacity retention after 5000 cycles at 5 A g−1), and can be efficiently recharged to ∼60% upon exposure to air. Finally, we demonstrate a self-charging battery fiber to effectively power a strain sensor in an integrated, wearable fingertip.

Strain sensors based on silver nanowires (AgNWs) have attracted much attention due to their wide application of electronic skin, smart textiles, and health monitoring. In this work, we report a flexible strain sensor based on an Ecoflex flexible substrate. AgNWs and Multi-walled carbon nanotubes (MWCNTs) doped as a conductive material. The excellent ductility and elongation at break in Ecoflex, as well as excellent heat resistance and impact resistance, can increase the test range and increase the sensing range of the sensor. AgNWs-based sensors enhance the sensitivity of the sensor to 118.19 while improving linearity and stability and reducing hysteresis. In addition, strain sensors can be utilized to monitor body movements such as finger flexion, leg movements, and so on. In this work, smart gloves are used to monitor finger movements using flexible strain sensors, which show its great potential for wearable electronics applications.

Conductive hydrogels have attracted tremendous attention due to their excellent softness and stretchability as wearable strain sensing devices. However, most of hydrogel-based strain sensors suffered from poor self-recoverability and fatigue resistance, resulting in significant decrease of strain sensitivity after recycling. Here, a soft and flexible wearable strain sensor is prepared by using an ionic conductive hydrogel with latex particles as physical cross-linking centers. The dynamic physical cross-linking structure can effectively dissipate energy through disruption and reconstruction of molecular segments, thereby imparting excellent stretchability, self-recoverability and fatigue resistance. In addition, the hydrogel exhibits excellent strain-sensitive resistance changes, which enables it to be assembled as a wearable sensor to monitor human motions. As a result, the hydrogel strain sensor can provide precise feedback for a wide range of human activities, including large-scale joint bending and tiny phonating. Therefore, the tough ionic conductive hydrogel would be widely applied in electronic skin, medical monitoring and artificial intelligence.