Sensitive Strain Sensor Research Articles

Highly sensitive and superhydrophobic fabric sensor based on AgNPs/Polypyrrole composite conductive networks for body movement monitoring

Fabric sensors due to favorable flexibility and comfortability have remarkable progress in wearable sensing electronics recently, but it is enormous challenge to achieve broad sensing range, high sensitivity, stable sensing capability and great hydrophobicity for their practical applications. Herein, a highly sensitive and superhydrophobic strain sensor based on conductive polydimethylsiloxane/Ag nanoparticles/polypyrrole/nylon strip (PDMS/AgNPs/PPy/NS) is successfully fabricated via constructing AgNPs/PPy composite conductive networks on elastic NS, and then coating a thin protective layer (PDMS). The AgNPs/PPy composite conductive networks render the PDMS/AgNPs/PPy/NS large ΔR/R0 scale, and the assembled PDMS/AgNPs/PPy/NS strain sensor presents broad sensing range (0.1%–70%), high Gauge Factor value of 1.61 × 103 (with the strain range from 60% to 70%), ultralow detection limit (0.1% strain), fast response time (70 ms) and satisfying sensing stability (5000 cycles). The protective PDMS layer not only endows PDMS/AgNPs/PPy/NS strain sensor with excellent superhydrophobicity (WCA = 156°), but also improves the stability of sensor for daily use. Owing to the high sensitivity and excellent superhydrophobicity, PDMS/AgNPs/PPy/NS strain sensor can monitor various joint motions (throat, neck, elbow, wrist, knee and finger) and even work in harsh conditions (moisture, acid, alkaline and salt environment). In addition, PDMS/AgNPs/PPy/NS with low resistance (1.09 Ω cm−1) displays an outstanding electrothermal temperature (87.4 °C at 2.0 V). These remarkable performances demonstrate great potential of PDMS/AgNPs/PPy/NS in wearable electronics such as sensors and heaters.

Electrospun nanofibrous yarn based piezoresistive flexible strain sensor for human motion detection and speech recognition

This work reports the development of highly sensitive Piezoresistive flexible strain sensor for human motion detection and speech recognition. Initially, a conductive polymer composite (CPC) solution comprising of thermoplastic polyurethane and Carbon Nanoparticles was prepared using Dimethylformamide as solvent and Chloroform as dispersant with the composition of 50% v/v. The solution was heated to a temperature of 60°C for evaporation of the solvent until it contained 13.5% w/v solvent for steady electrospinning. In this way, the CPC solution was used to develop Electrospun Nanofibrous Yarns (ENFYs) by applying a potential difference of 40 KV between the electrospinning needle and Aluminum collector. A cotton fabric was wrapped on the Aluminum collector to allow twisting of the deposited electrospun nanofibers. This novel collector configuration resulted in the formation of nanofibrous yarns due to the whirling action of the advancing jet of CPC solution. The cotton fabric on the collector facilitated twisting of fibers by allowing them to roll over the fabric. The fabricated ENFY sensors showed remarkable stretchability up to 102% strain while achieving a gauge factor of 70 at 100% strain. Long-term usage necessitates repeatability, which was demonstrated by cyclic loading at a crosshead speed of 40 mm/min for up to 1000 cycles using a custom-developed linear actuator, with no signs of fracture. ENFY strain sensor was attached to different parts of human body such as finger, fist, elbow, knee and ankle and was found capable of measuring and observing angle, position and frequency of motion. Owing to its ultrasensitive behavior, the developed sensor was able to measure heart rate as well. When the developed sensor was attached to Adam’s apple for speech recognition it showed remarkable response towards different utterances and breathing and gulping actions with clearly distinguishable signals. These results demonstrate that the developed novel ENFY flexible strain sensor can be employed for proprioceptive sensing and speech recognition for human-machine interaction, soft robotics and wearable devices etc.

Mxene Reinforced Supramolecular Hydrogels with High Strength, Stretchability, and Reliable Conductivity for Sensitive Strain Sensors.

Conductive hydrogels used as electronics have received much attention due to their great flexibility and stretchability. However, the fabrication of ideal conductive hydrogels fulfilling the excellent mechanical properties and outstanding sensitivity remains a great challenge until now. Moreover, high sensitivity and broad linearity range are pivotal for the feasibility and accuracy of hydrogel sensors. In this study, a conductive supramolecular hydrogel is engineered by directly mixing the aqueous dispersion of MXene with the precursor of N-acryloyl glycinamide (NAGA) monomer and then rapidly photo cross-linked by UV irradiation. The resultant PNAGA/MXene hydrogel-sensors exhibit high mechanical strength (4.8MPa), great stretchability (630%), and excellent durability. The conductive hydrogel-based sensor presents excellent conductivity (17.3Sm-1 ) and a wide scope of linear dependence of sensitivity on strain (0%-125%, gauge factor = 2.05). It displays reliable detection of various motions, including repeated subtle movements and large strain. It also shows good degradation in vitro and antifouling capability. This work may provide a simple and promising platform for engineering conductive supramolecular hydrogels with integrated high performance aiming for smart wearable electronics, electronic skin, soft robots, and human-machine interfacing.

Dynamic metal–ligand cross‐link promoted mechanically robust and pH responsive hydrogels for shape memory, programmable actuation and resistive sensing application

AbstractEmploying Fe3+‐dicarboxylate dynamic metal–ligand cross‐links in combination with low density chemical cross‐links, herein we have developed a mechanically robust and stretchable poly(acrylamide‐co‐maleic acid) hydrogel. The mechanical properties of these hydrogels can be controlled by altering the concentrations of maleic acid and Fe3+ ions. The hydrogels showed high tensile strength (~1–2 MPa), toughness (~3.3 MJ m−3) and stretchability (up to ~5 times of their original length) along with high compressive strength (~13.5 MPa). Because of the dynamic nature of the metal–ligand cross‐links, the hydrogels exhibited good self‐recovery, anti‐fatigue properties and fast self‐healing capacity. Since the Fe3+‐dicarboxylate cross‐links can be dissociated at low pH, the mechanical properties could further be tuned in response to the pH of the medium. This property is exploited to demonstrate shape memory behavior of these hydrogel materials. A temperature responsive bilayer actuator is reported, with poly(N‐isopropylacrylamide) as the thermoresponsive layer and the Fe3+ cross‐linked poly(acrylamide‐co‐maleic acid) hydrogel as the memorizing layer, demonstrating programmable and reversible shape transition. The application of these ionically conducting hydrogels as highly sensitive and flexible resistive strain sensor is also demonstrated.

Poly(vinyl alcohol)/Gelatin-Based Eutectogels for the Sensitive Strain Sensor with Recyclability and Multienvironmental Suitability

Poly(vinyl alcohol)/Gelatin-Based Eutectogels for the Sensitive Strain Sensor with Recyclability and Multienvironmental Suitability

Tannic acid modified antifreezing gelatin organohydrogel for low modulus, high toughness, and sensitive flexible strain sensor

Current hydrogel strain sensors have met assorted essential requirements of wearing comfort, mechanical toughness, and strain sensitivity. However, an increment in the toughness of a hydrogel usually leads to an increase in elastic moduli that could be unfavorable for wearing comfort. In addition, traits of biofriendly and sustainability require synthesis of the hydrogels from natural polymer-based networks. We propose a novel strategy to fabricate an ionic conductive organohydrogel from natural biological macromolecule “gelatin” and polyacid “tannic acid” to resolve these challenges. Tannic acid modified the structure of the gelatin network in the ionic conductive organohydrogels, that not only led to an increase in toughness accompanying a decrease in elastic moduli but also headed to higher strain sensitivity and tunability. The proposed methodology exhibited tunable tensile modulus from 27 to 13 kPa, tensile strength from 287 to 325 kPa, elongation at fracture from 510 to 620%, toughness from 500 to 550 kJ/m3, conductivity from 0.29 to 0.8 S/m, and strain sensitivity (GF = 1.4–6.5). Moreover, the proposed organohydrogel exhibited excellent freezing tolerance. This study provides a facile yet powerful strategy to tune the mechanical and electrical properties of organohydrogels which can be adapted to various wearable sensors.

High Sensitivity Strain Sensor Based on Micro-Helix Micro Taper Long Period Fiber Grating

A high sensitivity strain sensor based on micro-helix micro taper long period fiber grating (MH-MTLPFG) is proposed and experimentally investigated. The MH-MTLPFG is fabricated by periodically tapering and micro helix structure on a standard single-mode fiber with arc discharge technology. The arrangement of micro helix taper makes the sensor have high strain sensing sensitivity. The experiment results show that the sensitivity of strain sensing is up to −3.478 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . Compared with the conventional MTLPFG sensor, the strain sensitivity is improved by about 10 times. The proposed high sensitivity strain sensor has a good potential for accurate strain measurement, due to its low cost, simple fabrication and good performance.

Water‐resistant conductive organogels with sensation and actuation functions for artificial neuro‐sensory muscular systems

AbstractThe development of functional flexible conductive materials can significantly contribute to the improvement of intelligent human–computer integration. However, it is a challenge to endow human–machine interface with perception and response actuation simultaneously. Herein, a customizable and multifunctional electronic conductive organogel is proposed by combining conductive carbon nanotube (CNT) clusters and flexible adhesive organogels. The conductive CNT cluster layers generated on the surface of organogels equip the resulting organogel‐based conductors with considerable quasi‐superhydrophobicity and increase their potential applicability as highly sensitive stress and strain sensors. In particular, this quasi‐superhydrophobicity is insensitive to tensile strain. Based on customizable conductive networks and entropy‐driven organogel actuation, the conductive organogels can sense various strain and stress signals and imitate natural organisms with muscle actuation and neurofeedback. This strategy for preparing electronic conductors can enhance the rational design of soft robotics and artificial intelligence devices, facilitating further progress of human‐like intelligent systems.

Revealing the AC electromechanically coupled effects and stable sensitivity on the dielectric loss in CNT-based nanocomposite sensors

This work is concerned with the characterization of load-dependent dielectric loss and sensitivity analysis for CNT-based nanocomposite sensors (CNCSs) under AC loading. To this end, an electromechanically coupled microstructural theory is developed from the bottom up to quantitatively predict their overall dielectric loss change ratio and strain-sensitivity factor. In the theory, various categories of load-dependent functional interface effects, such as strain- and filler-dependent electron hopping and dielectric relaxation, are incorporated into it. The electric damage process under mechanical load is characterized through the principle of irreversible thermodynamics. The outcome is a microstructure-based coupled theory whose predictions of AC dielectric loss change ratio can be directly calibrated with the experimental data of MWCNT/PVDF nanocomposite sensor over a broad strain loading range from 0 to 10% and a wide frequency spectrum from 5 kHz to 500 kHz, where PVDF is a shape memory polymer. The theory further demonstrates the advantage of demarcating CNCSs via the dielectric loss over the traditional electric resistance. It can be used to rapidly determine the macroscopic dielectric loss change ratio by choosing a specific CNT volume concentration and AC working frequency, and further simplify the design procedure of highly sensitive strain sensors.

High Sensitivity Strain Sensors Using Four-Core Fibers through a Corner-Core Excitation.

A weakly-coupled multicore fiber can generate supermodes when the multi-cores are closer to enter the evanescent power coupling region. The high sensitivity strain sensors using tapered four-core fibers (FCFs) were demonstrated. The fan-in and fan-out couplers were used to carry out light coupling between singlemode fibers and the individual core of the FCFs. A broadband lightsource from superlumminescent diodes (SLDs) was launched into one of the four cores arranged in a rectangular configuration. When the FCF was substantially tapered, the asymmetric supermodes were produced to generate interferences through this corner-core excitation scheme. During tapering, the supermodes were excited based on a tri-core structure initially and then transited to a rectangular quadruple-core structure gradually to reach the sensitivity of 185.18 pm/μԑ under a tapered diameter of 3 μm. The asymmetric evanescent wave distribution due to the corner-core excitation scheme is helpful to increase the optical path difference (OPD) between supermodes for improving the strain sensitivity.

Highly stretchable, sensitive, and flexible strain sensors based on Ag@Cu/PDMS composites

Highly stretchable, sensitive, and flexible strain sensors based on Ag@Cu/PDMS composites

A flexible and sensitive strain sensor with three-dimensional reticular structure using biomass Juncus effusus for monitoring human motions

Flexible strain sensors have received extensive attention owing to their booming development. However, achieving high sensitivity and stretchability simultaneously for strain sensors is still a challenge. Herein, we report a fiber-based stretchable strain sensor with a unique three-dimensional (3D) reticular structure for human motion monitoring. The biomass Juncus effusus (JE) fiber was functionalized with carbon nanotubes (CNTs) and Ecoflex (EF) using a facile dip-coating-drying approach, and a syringe extraction method, respectively. The resulted JE/CNTs/EF fiber strain sensors exhibited advanced performance of strong flexibility, high sensitivity, and multifunctionality, which displayed a superior sensitivity (gauge factor of 24.95–76.79) and high stretchability (600%). Moreover, good repellency to chemicals and stable responses to various stimuli frequencies over 270 cycles were obtained for the as-prepared sensors. The microfibrils of the JE fiber formed a complicated 3D reticular structure acting as the conductive bridges when stretching. Consequently, the fiber-based JE/CNTs/EF strain sensor can not only accurately detect a full range of body motions and subtle signals, but also can be a promising candidate as a Morse code generator for cryptographic communication.

Highly stretchable and sensitive flexible resistive strain sensor based on waterborne polyurethane polymer for wearable electronics

In recent years, flexible resistive sensors have attracted wide attention as one of the important components of wearable devices. However, it is difficult for most flexible sensors to have high stretchability and high sensitivity at the same time. In this study, the modified multi-walled carbon nanotubes as conductive fillers were filled into waterborne polyurethane to form conductive composites, leading to high stretchability of the sensor (120%). The regular crack structure was designed into the sensor to achieve a high gauge factor (∼2000), and the consistency of the sensor was realized by combining the screen-printing process. In addition, the flexible sensor has a short response time (90 ms), good long-term stability (>1000 cycles), and hysteresis characteristics. Furthermore, the excellent characteristics of the flexible sensor enabled it to have a wide working range for human motion and physiological detection, and the sensor was also successfully applied to Morse Code communication and human motion posture recognition for elder fall alarm. This provides a solution for applications in the field of medical health and human-computer interaction.

Highly Stretchable and Sensitive Strain Sensor based on Ionogel/Ag Synergistic Conductive Network

AbstractRecently, flexible strain sensors have been intensively studied in order to meet the growing demands of detecting multiscale mechanical deformations. However, it remains a big challenge to integrate high stretchability and high sensitivity in a strain sensor simultaneously. In this study, a highly stretchable and sensitive strain sensor is successfully fabricated by constructing ionogel/silver synergistic conductive network. The transmetalation reaction is utilized to transform the copper atoms in the copper/ionogel nanocomposite into more silver atoms with larger radius, which eliminates the interspace among the nanoparticles, and therefore a silver/ionogel nanocomposite with high conductivity is obtained. Owing to the fragility of the conductive network, the obtained nanocomposite possesses high sensitivity. In addition, as the polymer substrate, the conductive ionogel can connect the crack of the silver conductive network during stretching, endowing the nanocomposite with high stretchability. Hence, the obtained strain sensor exhibits a high sensitivity in a wide sensing range and can detect various human activities. Therefore, this strategy can be envisioned as promising strategy for fabricating high‐performance strain sensors and other flexible electronics.

Highly sensitive strain sensors based on piezotronic tunneling junction

Piezotronics with capacity of constructing adaptive and seamless interactions between electronics/machines and human/ambient are of value in Internet of Things, artificial intelligence and biomedical engineering. Here, we report a kind of highly sensitive strain sensor based on piezotronic tunneling junction (Ag/HfO2/n-ZnO), which utilizes the strain-induced piezoelectric potential to control the tunneling barrier height and width in parallel, and hence to synergistically modulate the electrical transport process. The piezotronic tunneling strain sensor has a high on/off ratio of 478.4 and high gauge factor of 4.8 × 105 at the strain of 0.10%, which is more than 17.8 times larger than that of a conventional Schottky-barrier based strain sensor in control group as well as some existing ZnO nanowire or nanobelt based sensors. This work provides in-depth understanding for the basic mechanism of piezotronic modulation on tunneling junction, and realizes the highly sensitive strain sensor of piezotronic tunneling junction on device scale, which has great potential in advanced micro/nano-electromechanical devices and systems.

Highly sensitive and stretchable piezoelectric strain sensor enabled wearable devices for real-time monitoring of respiratory and heartbeat simultaneously

The World Health Organization has declared COVID-19 a pandemic. The demand for devices or systems to diagnose and track COVID-19 infections noninvasively not only in hospitals but also in home settings has led to increased interest in consumer-grade wearables. A common symptom of COVID-19 is dyspnea, which may manifest as an increase in respiratory and heart rates. In this paper, a novel piezoelectric strain sensor is presented for real-time monitoring of respiratory and heartbeat signals. A highly sensitive and stretchable piezoelectric strain sensor is fabricated using a piezoelectric film with a serpentine layout. The thickness of the patterned PVDF flexible piezoelectric strain sensor is only 168 μm, and the voltage sensitivity reaches 0.97 mV/μɛ. The effective modulus is 13.5 MPa, which allows the device to fit to the skin and detect the small strain exhibited by the human body. Chest vibrations are captured by the piezoelectric sensor, which produces an electrical output voltage signal conformally mapped with respiratory–cardiac activities. The separate heart activity and respiratory signals are extracted from the mixed respiratory–cardiac signal by an empirical mode decomposition data processing algorithm. By detecting vital signals such as respiratory and heart rates, the proposed device can aid early diagnosis and monitoring of respiratory diseases such as COVID-19.

Highly sensitive, direction-aware, and transparent strain sensor based on oriented electrospun nanofibers for wearable electronic applications

Wearable strain sensors have made great progress in sensing performance, stretchability and durability. However, practical applications of these sensors are still quite challenging because they are incapable of detecting multi-degree-of-freedom strains due to the interference of multidirectional strains. Herein, a high-sensing-performance, direction-aware and transparent strain sensor is reported based on antimony-doped tin oxide oriented nanofiber (ATO-ONF) films prepared by electrospinning. The monolayer ATO-ONF strain sensor shows remarkable anisotropic sensing performance, namely GFs of 250 and 1.2 for the nanofiber orientation and its transverse directions, suggesting the realization of the unidirectional sensing capability of the strain sensor, i.e., only responding to strains along the nanofiber direction. In addition, this strain sensor also exhibits high transparency with a light transmittance of ∼ 80%, and excellent sensing performance including high sensitivity, high linearity, low hysteresis, good repeatability and durability (>2000 cycles). Based on these superior sensing properties, the direction-aware biaxial strain sensor is designed by orthogonally stacking ATO-ONF films, by which the predicted magnitude and direction of the tensile strains agree well with those of the actual strains. Furthermore, the multi-degree-of-freedom applications of direction-aware strain sensors in human motion monitoring and human–machine interaction are demonstrated, showing a great application potential in next-generation wearable electronics.

Helical Fiber Strain Sensors Based on Triboelectric Nanogenerators for Self-Powered Human Respiratory Monitoring.

Respiration is a major vital sign, which can be used for early illness diagnosis and physiological monitoring. Wearable respiratory sensors present an exciting opportunity to monitor human respiratory behaviors in a real-time, noninvasive, and comfortable way. Among them, fiber-shaped triboelectric nanogenerators (FS-TENGs) are attractive for their comfort and high degree of freedom. However, the single-electrode FS-TENGs cannot respond to their own tensile strains, and the coaxial double-electrode FS-TENGs show low sensitivity to strain due to structural limitations. Here, a type of helical fiber strain sensor (HFSS) is developed, which can respond to tiny tensile strains. In addition, a smart wearable real-time respiratory monitoring system is developed based on the HFSSs, which can measure some key breathing parameters for disease prevention and medical diagnosis. An intelligent alarm can automatically call a preset mobile phone for help in response to respiratory behavior changes. This work provides an effective helical structure for fabricating highly sensitive strain sensors based on FS-TENGs and develops wearable self-powered real-time respiratory monitoring systems.

Development of High-Sensitivity Electrically Conductive Composite Elements by Press Molding of Polymer and Carbon Nanofibers.

Carbon nanofibers (CNFs) have various excellent properties, such as high tensile strength, electric conductivity and current density resistance, and thus have great application potential in electrical sensor development. In this research, electrically conductive composite elements using CNFs sandwiched by thermoplastic olefin (TPO) substrates were developed by press molding. The metal mold used for press molding was processed by a femtosecond laser to generate laser-induced periodic surface structures (LIPSS) on the mold surface. The aggregate of CNFs was then flexibly fixed by the LIPSSs imprinted on the TPO substrate surface to produce a wavy conductive path of CNFs. The developed composite elements exhibited a sharp increase in electrical resistance as strain increased. A high gauge factor of over 47 was achieved, which demonstrates high sensitivity against strain when the composite element is used as a strain gauge. Scanning electron microscope observation revealed that the TPO filled the spaces in the aggregate of CNFs after press molding, and the conductive path was extended by the tensile strain. The strain-induced dynamic changes of contact states of CNFs and CNFs networks are discussed based on the electrical performance measurement and cross-sectional observation of the elements. This research provides a new approach to the production of flexible and high sensitivity strain sensors.

Healable Strain Sensor Based on Tough and Eco-Friendly Biomimetic Supramolecular Waterborne Polyurethane.

Stretchable sensors are essential for flexible electronics, which can be made with polymer elastomers as the matrix. The main challenge in producing practical devices is to obtain polymers with mechanical stability, eco-friendliness, and self-healing properties. Herein, we introduce urea bonds and 2-ureido-4[1H]-pyrimidinone (UPy) to synthesize tailored waterborne polyurethanes (WPU-UPy-x) with a hierarchical hydrogen bond (H-bond). Accordingly, sound tensile performance (strength: 53.33 MPa, toughness: 128.97 MJ m-3), satisfying deformation recovery, and good self-healing capability of the WPU-UPy-x film are demonstrated. With atomic force microscope characterization, we find that UPy groups contribute to the highly improved microphase separation of WPU-UPy-x, responsible for good mechanical properties. As a proof of concept, a strain sensor is successfully configured, thanks to the good interfacial interactions between the polyurethane matrix and the Ti3C2Tx MXene conductive filler, which features sensitive and stable performance for monitoring diverse human and mechanical motions. Intriguingly, this sensor is capable of self-healing after cutting and displays well-retained sensitivity to detect the stretched signal. The as-proposed design concept for healable and sensitive strain sensors can shed light on future wearable electronics.