Wearable Strain Sensors Research Articles

Ruptured liquid metal microcapsules enabling hybridized silver nanowire networks towards high-performance deformable transparent conductors.

Extensive studies have been carried out on silver nanowires (AgNWs) in view of their impressive conductivity and highly flexible one-dimensional structure. They are seen as a promising choice for producing deformable transparent conductors. Nonetheless, the widespread adoption of AgNW-based transparent conductors is hindered by critical challenges represented by the significant contact resistance at the nanowire junctions and inadequate interfacial adhesion between the nanowires and the substrate. This study presents a novel solution to tackle the aforementioned challenges by capitalizing on liquid metal microcapsules (LMMs). Upon exposure to acid vapor, the encapsulated LMMs rupture, releasing the fluid LM which then forms a metallic overlay and hybridizes with the underlying Ag network. As a result, a transparent conductive film with greatly enhanced electrical and mechanical properties was obtained. The transparent conductor displays negligible resistance variation even after undergoing chemical stability, adhesion, and bending tests, and ultrasonic treatment. This indicates its outstanding adhesion strength to the substrate and mechanical flexibility. The exceptional electrical properties and robust mechanical stability of the transparent conductor position it as an ideal choice for direct integration into flexible touch panels and wearable strain sensors, as evidenced in this study. By resolving the critical challenges in this field, the proposed strategy establishes a compelling roadmap to navigate the development of high-performance AgNW-based transparent conductors, setting a solid foundation for further advancement in the field of deformable electronics.

A highly conductive and ultra-stretchable polyaniline/cellulose nanocrystal/polyacrylamide hydrogel with hydrophobic associations for wearable strain sensors

A highly stretchable hydrogel with extraordinary conductivity forms by polymerizing polyaniline within polyacrylamide. The latter contains hydrophobic associations and cellulose nanocrystals to provide excellent mechanical properties.

Low hysteresis zwitterionic supramolecular polymer ion-conductive elastomers with anti-freezing properties, high stretchability, and self-adhesion for flexible electronic devices.

The fabrication of stretchable ionic conductors with low hysteresis and anti-freezing properties to enhance the durability and reliability of flexible electronics even at low temperatures remains an unmet challenge. Here, we report a facile strategy to fabricate low hysteresis, high stretchability, self-adhesion and anti-freezing zwitterionic supramolecular polymer ion-conductive elastomers (ICEs) by photoinitiated polymerization of aqueous precursor solutions containing a newly designed zwitterionic monomer carboxybetaine ureido acrylate (CBUIA) followed by solvent evaporation. The resultant poly(carboxybetaine ureido acrylate) (PCBUIA) ICEs are highly stretchable and self-adhesive owing to the presence of strong hydrogen bonds between ureido groups and dipole-dipole interactions of zwitterions. The zwitterion groups on the polymer side chains and loaded-lithium chloride endow PCBUIA ICEs with excellent anti-freezing properties, demonstrating mechanical flexibility and ionic transport properties even at a low temperature (-20 °C). Remarkably, the PCBUIA ICEs demonstrate a low hysteresis (≈10%) during cyclic mechanical loading-unloading (≤500%), and are successfully applied as wearable strain sensors and triboelectric nanogenerators (TENGs) for energy harvesting and human motion monitoring. In addition, the PCBUIA ICE-based TENG was used as a wireless sensing terminal for Internet of Things smart devices to enable wireless sensing of finger motion state detection.

Facile and fast preparation of stretchable, self-adhesive, moisturizing, antifreezing and conductive tough hydrogel for wearable strain sensors

Stretchable, self-adhesive, moisturizing, antifreezing and conductive tough hydrogels for wearable strain sensors were conveniently and quickly prepared based on the catalytic mechanism of metal ions.

Hydrodynamic control of silicone elastomers on between porous media

Silicone elastomers, for example, polydimethylsiloxane (PDMS), have been widely used as cross-linkers for fabrication of flexible strain sensors. They not only lend strong adhesion to adjacent materials, for example, porous fabrics, but also tune their elastic property. Silicone elastomer precursors, which are typical non-Newtonian fluids, can easily penetrate into porous fabrics, driven by the capillary effects of fibers. Unfortunately, such a penetration has negative effects on both adhesion strength and elastic property of PDMS, thus limiting their applications. Here we report a facile method for preparing uniform silicone elastomer films, that is, PDMS, on between porous media via controlling the hydrodynamics of elastomer precursors. Our experiments show that the hydrodynamics of elastomer precursors can be easily controlled by modulating the pre-curing time of PDMS precursors to prevent them from penetration into porous media but keep their high adhesion. Based on this hydrodynamic modulation of PDMS precursors, we firmly adhere conductive silver nanowires (AgNWs) onto knitted fabrics, and further combine composites with common clothing from the point of view of ergonomics, showing the possibility of applying such a modulation to the fabrication of wearable strain sensors. Our findings not only present an understanding of liquid transport in porous media, but also provide a novel method of controlling the hydrodynamics of elastomer precursors in porous media for achieving the effective wearable sensors.

Tough and adhesive conductive hydrogels with fast gelation from a polyphenol–aluminium ion dual self-catalysis system for wearable strain sensors and triboelectric nanogenerators

A tough and adhesive conductive double network hydrogel (PVA/PHEAA–TA–Al3+ gel) was prepared via rapid in situ room temperature gelation processes (25 °C, 215 s) in a tannic acid–aluminium ion (TA–Al3+) dual self-catalysis system.

A double-crack structure for bionic wearable strain sensors with ultra-high sensitivity and a wide sensing range.

Flexible strain sensors are crucial in fully monitoring human motion, and they should have a wide sensing range and ultra-high sensitivity. Herein, inspired by lyriform organs, a flexible strain sensor based on the double-crack structure is designed. An MXene layer and an Au layer with cracks are constructed on both sides of the insulated polydimethylsiloxane (PDMS) film, forming an equivalent parallel circuit that guarantees the integrity of the conductive path under a large strain. The rapid disconnection of the crack junctions causes a significant change in the resistance value. Due to the effect of cracks on the conductive path, the sensitivity of the sensor is largely improved. Benefiting from the double-crack structure, the as-obtained sensor shows ultra-high sensitivity (maximum gauge factor of up to 14 373.6), a wide working range (up to 21%), a fast response time (183 ms) and excellent dynamical stability (almost no performance loss after 1000 stretching cycles and different frequency cycles). In practical applications, the sensor is applied to different parts of the human body to sense the deformation of the skin, demonstrating its great potential application value in human physiological detection and the human-machine interaction. This study can provide new ideas for preparing high-performance flexible strain sensors.

Multi-role conductive hydrogels for flexible transducers regulated by MOFs for monitoring human activities and electronic skin functions.

Metal organic frameworks (MOFs) have garnered significant attention in the development of stretchable and wearable conductive hydrogels for flexible transducers. However, MOFs used in hydrogel networks have been hampered by low mechanical performance and poor dispersibility in aqueous solutions, which affect the performance of hydrogels, including low toughness, limited self-recovery, short working ranges, low conductivity, and prolonged response-recovery times. To address these shortcomings, a novel approach was adopted in which micelle co-polymerization was used for the ex situ synthesis of Zn-MOF-based hydrogels with exceptional stretchability, robust toughness, anti-fatigue properties, and commendable conductivity. This breakthrough involved the ex situ integration of Zn-MOFs into hydrophobically cross-linked polymer chains. Here the micelles of EHDDAB had two functions, first they uniformly dispersed the Zn-MOFs and secondly they dynamically cross-linked the polymer chains, profoundly influencing the mechanical characteristics of the hydrogels. The non-covalent synergistic interactions introduced by Zn-MOFs endowed the hydrogels with the capacity for high stretchability, high stress, rapid self-recovery, anti-fatigue properties, and conductivity, all achieved without external stimuli. Furthermore, hydrogels based on Zn-MOFs can serve as durable and highly sensitive flexible transducers, adept at detecting diverse mechanical deformations with swift response-recovery times and high gauge factor values. Consequently, these hydrogels can be tailored to function as wearable strain sensors capable of sensing significant human joint movements, such as wrist bending, and motions involving the wrist, fingers, and elbows. Similarly, they excel at monitoring subtle human motions, such as speech pronunciation, distinguishing between different words, as well as detecting swallowing and larynx vibrations during various activities. Beyond these applications, the hydrogels exhibit proficiency in distinguishing and reproducing various written words with reliability. The Zn-MOF-based hydrogels hold promising potential for development in electronic skin, medical monitoring, soft robotics, and flexible touch panels.

A conductive hydrogel with excellent self-adhesion, sensitivity, and stability for wearable strain sensors to monitor human motion

A high adhesion and stable hydrogel was synthesized using CS-DOPA and AA for monitoring human motion.

Polyvinyl alcohol/chitosan based nanocomposite organohydrogel flexible wearable strain sensors for sports monitoring and underwater communication rescue

Hydrogel-based flexible wearable sensors have garnered significant attention in recent years. However, the use of hydrogel, a biomaterial known for its high toughness, environmental friendliness, and frost resistance, poses a considerable challenge. In this study, we propose a stepwise construction and multiple non-covalent interaction matching strategy to successfully prepare dynamically physically crosslinked multifunctional conductive hydrogels. These hydrogels self-assembled to form a rigid crosslinked network through intermolecular hydrogen bonding and metal ion coordination chelation. Furthermore, the freeze-thawing process promoted the formation of poly(vinyl alcohol) microcrystalline domains within the amorphous hydrogel network system, resulting in exceptional mechanical properties, including a tensile strength (2.09 ± 0.01 MPa) and elongation at break of 562 ± 12 %. It can lift 10,000 times its own weight. Additionally, these hydrogels exhibit excellent resistance to swelling and maintain good toughness even at temperatures as low as −60 °C. As a wearable strain sensor with remarkable sensing ability (GF = 1.46), it can be effectively utilized in water and underwater environments. Moreover, it demonstrates excellent antimicrobial properties against Escherichia coli (Gram-negative bacteria). Leveraging its impressive sensing ability, we combine signal recognition with a deep learning model by incorporating Morse code for encryption and decryption, enabling information transmission.

Antibacterial zwitterionic hydrogel for flexible and wearable ultrafast-response strain sensors with low hysteresis

Zwitterionic hydrogels, with their unique antibacterial and tissue-adhesive nature, show great potential in the fields of wearable sensors, healthcare monitoring, and e-skins. However, it remains a huge challenge to integrate high elasticity and low hysteresis in such a hydrogel via simple methods. Herein, a novel tissue-adhesive zwitterionic hydrogel sensor was fabricated by a simple one-pot method using copolymerization of [2-(Methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), N-[Tris (hydroxymethyl)methyl] acrylamide (THMA), and acrylic acid (AA), which was crosslinked by polyethylene glycol diglycidyl ether (PEGDE), a crosslinker famous for its low-toxicity and biocompatibility. The obtained SAT hydrogel shows robust elasticity with low hysteresis and fatigue resistance, which is essential for the long-term use. The introduction of THMA and PAA enhances the adhesive strength and reversible resilience of the hydrogel by interfacial hydrogen bonding, resulting in superior adhesion to skin without causing harm. The chemical crosslinker existing in SAT hydrogel enables it to withstand large deformations while maintaining stability. Besides the well-known antibacterial properties, the addition of SBMA enables the hydrogel to exhibit excellent electrical properties and a rapid response (19.5 ms) within a wide strain-sensing range, which facilitates accurate detection of various human body motions, such as joint bending, swallowing, and heartbeats. Moreover, the hydrogel demonstrates biocompatibility and excellent antibacterial activity, promising a safer and more comfortable experience for users.

Ionic conductive konjac glucomannan/liquid crystal cellulose composite hydrogels with dual sensing of photo- and electro-signals capacities as wearable strain sensors

The ionic conductive hydrogel-based sensor exhibits wide applications in wearable electronic devices. However, the strength and ductility trade-off, multimodal requirements, and water-soluble polymer alternatives are significant challenges for the hydrogel-based sensor. Herein, a stretchable and conductive hydrogel is developed with a double network formed by incorporating polyacrylamide and ionic liquid into the konjac glucomannan network. The hydrogel displays significantly enhanced mechanical properties, and good tear/puncture resistance owing to the existence of covalent and non-covalent interactions. In addition, by the introduction of nematic liquid crystal hydroxypropyl cellulose, the hydrogel/cellulose-based strain sensor demonstrates excellent sensing performance in monitoring human motions and writing recognition ability with optical and electrical bimodal sensing response. This work provides new insights to further expand the options of hydrogel-based sensor matrix and to construct bimodal sensors.

Highly stretchable, robust, sensitive and wearable strain sensors based on mesh-structured conductive hydrogels

With the rapid development of wearable electronic devices, flexible sensors, as the core components of wearable devices, have received extensive attention. Manufacturing of the strain sensors with excellent mechanical properties, wide working range, and high sensitivity is crucial for the development of wearable devices. Herein, for the first time we proposed a strategy based on mesh-structured conductive hydrogel to sensitively detect the body movement in a large workable strain ranges, and demonstrated the facile fabrication procedure of the sensors by using a direct ink writing 3D printing technique. Low cost conductive hydrogels were prepared by a simple one-pot method of combining dispersant-free carbon nanocolloids with easily synthesized PVA/TA/PAM. The printing of complex mesh-structured hydrogels was optimized with high fidelity. The strain sensor based on the mesh-structured hydrogel exhibited excellent mechanical properties, of which the maximum failure stress of the honeycomb was 1.28 MPa and the maximum failure tensile strain was 704 %, amazing sensitivity (GF = 32.95 for the tensile strain range of 3.5 %-5%, GF = 21.5 for the tensile strain range of 100 %-120 %), wide detection range, good stability and durability (for 500 tensile cycles, the ΔR/R0 value remained unchanged). In addition, the working mechanism during the stretching process of mesh-structured hydrogels was revealed by combining finite element simulation with microscopic morphology observation. At last, the applications of the mesh-structured hydrogel-based sensors to accurately and reliably detect both subtle physiological signals and large joint movements were soundly demonstrated, endowing the ability of achieving all-round detection of human motion. Our work could provide a universe strategy for fabricating highly ventilated, stretchable, robust, sensitive and wearable strain sensors, which would greatly expand the applications of strain sensors in wearable devices.

A self-healing polypyrrole/oxidized natural rubber conductive composite as a building block for highly sensitive strain sensors

Wearable flexible strain sensors have attracted significant attention in human movements due to their high sensitivity, broad strain range and long-term stability. However, the fragility of the sensors limits their widespread applications. Here, a self-healing rubber-based conductive composite with excellent electrical conductivity and high sensitivity are prepared by in-situ polymerization of pyrrole in oxidized deproteinized natural rubber latex. Based on the hydrogen bonds between oxidized deproteinized natural rubber, polypyrrole and polyethylene glycol monolauryl ether, the composites show efficient self-healing behavior (η = 91.1%) under 50 °C for 24 h. The conductive composites containing 5 wt% polypyrrole perform excellent conductivity (3.5 S/m), fast response time (180 ms), low strain detection limit (1%), and long-term reliability (over 1500 cycles at 40% strain). The gauge factors are 3.2 at low strain (<60%) and up to 477.6 at large strain (>430%). Furthermore, the rubber-based conductive composites are fabricated as sensors to accurately monitor a variety of human joint movements (finger, wrist, elbow, and knee bending) and facial expressions (pouting, and winking).

Preparation of Tough and Adhesive PVA/P(AM-AMPS)/Glycerol/Laponite/Na2SO4 Organohydrogels for All-Solid-State Supercapacitors and Self-Powered Wearable Strain Sensors.

Hydrogel electrolytes are ideal for flexible wearable electronic devices because of their high ionic conductivity, flexibility, and biocompatibility. However, some problems, such as poor mechanical properties, low conductivity, and lack of adhesivity, are encountered in the process of hydrogel preparation and application, which restrict the further development of hydrogel electrolytes. In this study, PVA was used as the first network, and P(AM-co-AMPS) as the second network to prepare a double-network hydrogel electrolyte. Laponite and Na2SO4 were introduced into the hydrogel during hydrogel formation as the nanofiller and salt with the salting-out effect to enhance its mechanical properties. The hydrogel electrolyte with high toughness (1663 kJ·m-3), adhesivity (77 kPa), and ionic conductivity (1.7 S·m-1) was obtained. In addition, the hydrogel electrolyte also has excellent antifatigue performance. In the 10 consecutive tensile cycles, the tensile strength does not decay. Due to the high adhesivity of the hydrogel electrolyte, a symmetrical all-solid-state supercapacitor was assembled with a tight interface between the hydrogel electrolyte and the AC/CNT composite electrode. The supercapacitor has a high specific capacitance of 186.1 mF·cm-2 at the current density of 1 mA·cm-2. In addition, the capacitor has good flexibility and can withstand bending at various angles. The hydrogel electrolyte also has excellent strain sensing performance, with an ultrafast tensile response time (0.17 s) and high sensitivity factor (GF = 10.01). Finally, the self-powered sensor system composed of a supercapacitor as the power supply device and hydrogel electrolyte as the sensing part was obtained and applied to human motion monitoring, which provides a potential application in the integrated flexible electronic system.

A chitosan-based conductive double network hydrogel doped by tannic acid-reduced graphene oxide with excellent stretchability and high sensitivity for wearable strain sensors

Conductive hydrogels usually suffer from weak mechanical properties and are easily destroyed, resulting in limited applications in flexible electronics. Concurrently, adding conductive additives to the hydrogel solution increases the probability of agglomeration and uneven dispersion issues. In this study, the biocompatible natural polymer chitosan was used as the network substrate. The rigid network employed was the Cit3−ion crosslinked chitosan (CS) network, and the MBA chemically crosslinked polyacrylamide (PAM) network was used as the flexible network. Tannic acid-reduced graphene oxide (TA-rGO), which has excellent conductivity and dispersibility, is used as a conductive filler. Thus, a CS/TA-rGO/PAM double network conductive hydrogel with excellent performance, high toughness, high conductivity, and superior sensing sensitivity was prepared. The prepared CS/TA-rGO/PAM double network conductive hydrogels have strong tensile properties (strain and toughness as high as 2009 % and 1045 kJ/cm3), excellent sensing sensitivity (GF value was 4.01), a wider strain detection range, high cycling stability and durability, good biocompatibility, and antimicrobial properties. The hydrogel can be assembled into flexible wearable devices that can not only dynamically detect human movements, such as joint bending, facial expression changes, swallowing, and saying, but also recognize handwriting and enable human-computer interaction.

Mechanically toughed and self-adhesive organohydrogel with anti-freezing and non-drying properties for human motion monitoring and information transmission

With the rise of artificial intelligence, hydrogel-based wearable flexible strain sensors have acquired increasing attention in recent years. Herein, a mechanically toughed versatile organohydrogel with anti-freezing and non-drying properties is constructed by simultaneously introducing methacrylic acid (MAA), acrylamide (AAm) and ZrCl4 into the glycerol/water binary solvent system. Based on the carboxyl-Zr4+ strong metal-coordination interaction, the fracture strain and tensile strength of as-prepared organohydrogel can reach 1015% and 180 kPa, respectively. At the same time, the glycerol/water binary solvent endows the organohydrogel with excellent low-temperature tolerance (−35 °C) and long-term moisture-retention capability (80% at 30 °C for 10 days). Furthermore, the organohydrogel-based strain sensor exhibits wide detection range (5–400%) and remarkable cycling stability (500 cycles at 200% strain), which can not only real-timely monitor various human movements but also precisely distinguish the direction of human motion. Impressively, the as-fabricated organohydrogel sensor also demonstrates promising potential in the application of rehabilitation training of a patient and information transmission. Benefiting from the remarkable environmental stability, the organohydrogel sensor after being stored at − 35 °C and 30 °C for 10 days, respectively, the resistance signals corresponding to different stretching deformations and complex human movements have no obvious decay. It is anticipated that the obtained organohydrogel can as a promising candidate of the next-generation flexible electronic devices for their substantial applications in motion sensing, human-machine interaction, artificial intelligence and biomedicine.

Integrated wearable anti-wetting strain sensor with the ability of achieving human joints bending recognition and electricity generation simultaneously

Wearable strain sensor has attracted much attention in human health monitoring, medical rehabilitation, and human-computer interaction. However, it is still a huge challenge for strain sensors to achieve long-term and stable operation in harsh conditions such as exposing to wet environments, directly contacting aqueous solutions or power shortage. Herein, we integrated a wearable, anti-wetting, and self-powered combined-type (WASC) strain sensor consists of three parts: a superhydrophobic and conductive Ti3C2TX (MXene) based composite coating, Ecoflex substrate and a triboelectric nanogenerator (TENG) composed of polyamide and polydimethylsiloxane films. The hierarchical micro/nano mastoid wrinkle structure of the MXene composite coating endowed WASC strain sensor with a wide sensing range (strain over 150%), superior sensitivity (gauge factor of 5678.2), and a stable superhydrophobic surface. Most importantly, the WASC sensor could efficiently monitor real-time human joints bending movements (such as finger bending) and generate electricity simultaneously, without being affected by water, acid, alkali, and salt solution. This described integration strategy of combined-type strain sensor provided a promising method for wearable strain sensors to deal with emergencies such as power shortage, wet environment, or corrosive conditions.

A temperature and pressure dual-responsive, stretchable, healable, adhesive, and biocompatible carboxymethyl cellulose-based conductive hydrogels for flexible wearable strain sensor

The study aimed to develop a novel temperature and pressure dual-responsive conductive hydrogel with self-healing, self-adhesive, biocompatible, and stretchable properties, for the development of multifunctional anti-counterfeiting and wearable flexible electronic materials. A conductive hydrogel based on carboxymethyl cellulose (CMC) was synthesized by simple “one pot” free radical polymerization of CMC, acrylamide (AAm) and acrylic acid (AAc). The hydrogel displayed temperature responsiveness and possessed an upper critical solution temperature (UCST) value. In addition, hydrogels also had surprising pressure responsiveness. The synthesized hydrogels were characterized by FTIR, TGA, DSC, and XRD analysis. Importantly, the obtained hydrogels exhibited exceptional mechanical properties (stress: 730 kPa, strain: 880%), fatigue resistance, stretchability, self-healing capability, self-adhesive properties, and conductivity. In addition, valuable insights were obtained into the synthesis and application of flexible anti-counterfeiting and camouflage materials by the temperature and pressure dual-responsive hydrogels. Moreover, the prepared hydrogel, with an electrically sensitive perception of external strain (GF = 2.61, response time: 80 ms), can be utilized for monitoring human movement, emotional changes, physiological signals, language, and more, rendering it suitable for novel flexible anti-counterfeiting materials and versatile wearable iontronics. Overall, this study provided novel insights into the simple and efficient synthesis and sustainable manufacturing of environmentally friendly multifunctional anti-counterfeiting materials and flexible electronic skin sensors.

Extremely Tough and Stretchable Hydrophobic Deep Eutectic Solvent-Based Gels with Strong Adhesion and Moisture Resistance for Wearable Strain Sensors

Extremely Tough and Stretchable Hydrophobic Deep Eutectic Solvent-Based Gels with Strong Adhesion and Moisture Resistance for Wearable Strain Sensors