Sensitive Strain Sensor Research Articles

A Highly Sensitive Strain Sensor with Self-Assembled MXene/Multi-Walled Carbon Nanotube Sliding Networks for Gesture Recognition

Flexible electronics is pursuing a new generation of electronic skin and human–computer interaction. However, effectively detecting large dynamic ranges and highly sensitive human movements remains a challenge. In this study, flexible strain sensors with a self-assembled PDMS/MXene/MWCNT structure are fabricated, in which MXene particles are wrapped and bridged by dense MWCNTs, forming complex sliding conductive networks. Therefore, the strain sensor possesses an impressive sensitivity (gauge factor = 646) and 40% response range. Moreover, a fast response time of 280 ms and detection limit of 0.05% are achieved. The high performance enables good prospects in human detection, like human movement and pulse signals for healthcare. It is also applied to wearable smart data gloves, in which the CNN algorithm is utilized to identify 15 gestures, and the final recognition rate is up to 95%. This comprehensive performance strain sensor is designed for a wide array of human body detection applications and wearable intelligent systems.

Synthesis of Ag Nanowires with High Aspect Ratio for Highly Sensitive Flexible Strain Sensor

AbstractIn recent years, the research and development of various flexible wearable devices have rapidly advanced due to the continuous introduction of flexible electronic product. These devices have found applications in health monitoring, cardiovascularcare, internal and external workload, and more.Flexible sensors, being a vital component of flexible devices, determine the functionalities and performance capabilities of the devices. Metal nanomaterials, especially silver nanowires, are widely used in flexible sensors in past research due to their great advantages in flexibility and sensitivity. In this work, Silver nanowires with aspect ratios of 600, 1000, and 1400 were synthesized by adjusting the synergy of Br‐ and Cl‐, along with other process parameters, leading to an improved production efficiency of silver nanowires.The stability of the conductive network is enhanced when the aspect ratio of silver nanowires is 1000 and 1400.The sensor demonstrated high sensitivity at high strain, along with an extended strain range.Moreover, it was observed that increasing the aspect ratio of silver nanowires led to a more stable conductive network, thus enhancing the sensor's stability with over 10000 stretching cycles. under the appropriate deposition density, silver nanowires with aspect ratio of 600 have high sensitivity to low strain, and silver nanowires with aspect ratio of 1400 have high sensitivity to high strain, up to 247.3. Introducing microstructures on the surface of PDMS resulted in an increased maximum sensitivity of the sensor with decreasing microstructure size, reaching a maximum sensitivity of 322.2.

Highly conductive and sensitive alginate hydrogel strain sensors fabricated using near-field electrohydrodynamic direct-writing process

Hydrogel flexible sensors have attracted considerable attention because of their wearability, biocompatibility, and precision signal transmission capability. However, the hydrogel strain sensors fabricated by conventional printing or hand-injection methods have difficulty balancing their mechanical strength and sensing characteristics, limiting the application of hydrogel strain sensors. Herein, polyvinyl alcohol and polyacrylamide were loosely crosslinked with sodium alginate through chemical cross-linking. Subsequently, MXene nanosheets were introduced for doping, the crosslinked hydrogel conductive network was constructed, and the hydrogel strain sensors were fabricated using the electrohydrodynamic (EHD) printing method. The ions in the EHD-printed hydrogel undergo directional movement under an externally enhanced electric field, causing the formation of more uniform and dense porous conductive networks inside the hydrogel, and high electrical conductivity (0.49 S m−1) is obtained. These hydrogel strain sensors have excellent mechanical properties (tensile strength: 0.17 MPa at 787 % strain), high sensitivity (gauge factor: 1.54 at 0–100 % strain), and low detection limits (1 % strain). Furthermore, demonstrations of real-time Morse code tapping information transmission, handwriting recognition during writing, and human physiological behavior monitoring demonstrations using the fabricated sensors indicate that the EHD-printed hydrogel strain sensor method has significant potential for wearable devices and human-computer interaction applications.

Supramolecular Luminescent Copper-Nanocluster-Based Dough with Excellent Electrical Conductivity Sensing Properties.

In recent years, the rapid advancement of flexible conductive materials has significantly increased the demand for dough materials that offer high flexibility and conductivity for diverse applications. Here, we developed a flexible, stretchable, and self-healing dough utilizing hydrogen-bonding interactions between glutathione-stabilized copper nanoclusters (GSH-Cu NCs) and poly(acrylic acid) (PAA). The dough materials can be kneaded, readily reshaped, and further processed to create bulk materials of arbitrary form factors. The incorporation of PAA not only preserved the vibrant blue emission of GSH-Cu NCs but also enhanced their electrical conductivity and stretchability. The dough can be stretched up to 25 times its initial length and achieves complete self-healing in a short time. Moreover, the dough can automatically repair physical damage and return to its initial conductivity levels after healing. Surprisingly, the electrical conductivity of the dough can reach as high as 2.97 S/m, which is relatively superior compared to that of conventional conductive materials. This study presents a dough that serves as a highly sensitive strain sensor, capable of effectively monitoring human movement across a broad range of strains.

Ultrasensitive strain sensor of dual parallel FPIs utilizing femtosecond laser processing and Vernier effect

This study proposes and develops a highly sensitive fiber optic strain sensor utilizing femtosecond laser processing and the Vernier effect. The sensor features two parallel Fabry-Pérot interferometers (FPI), with each FPI consisting of an air bubble and a tapered fiber optic cascade. FPI 2 serves as the sensing cavity, while FPI 1 acts as the reference cavity. The strain sensing capability of a single FPI 2 is measured at 6.67 p.m./με, while the parallel configuration of FPI 2 with FPI 1 achieves a sensitivity of 68.3 p.m./με, resulting in an amplification of 10.24. Furthermore, the proposed sensor demonstrates excellent repeatability, low-temperature cross-sensitivity, simple fabrication, and cost-effectiveness, making it a promising tool for strain measurement applications.

Mechanically Driven Self-Healing MXene Strain Gauges for Overstrain-Tolerant Operation.

Compliant strain gauges are well-suited to monitor tiny movements and processes in the body. However, they are easily damaged by unexpected impacts in practical applications, limiting their utility in controlled laboratory environments. This study introduces elastic microcracked MXene films for mechanically driven self-healing strain gauges. MXene films are deposited on soft silicone substrates and intentionally stretched to create saturated microcracks. The resulting device not only has high sensitivity but also can recover its original sensing capability even after experiencing failure-level overstrains. This electrical self-healing ability is achieved through the elastic rebound of the substrate, which autonomously restores the microcracked morphology of the MXene film. The MXene strain gauge can withstand overextension, twisting, impact forces, and even car rolling. The device is also resilient to touch-induced damage during monitoring of physiological motions. The mechanically driven self-healing strategy may effectively improve the durability of highly sensitive strain sensors.

Development of a Highly Sensitive and Stretchable Charge-Transfer Fiber Strain Sensor for Wearable Applications.

Wearable electronics have significantly advanced the development of highly stretchable strain sensors, which are essential for applications such as health monitoring, human-machine interfaces, and energy harvesting. Fiber-based sensors and polymeric materials are promising due to their flexibility and tunable properties, although balancing sensitivity and stretchability remains a challenge. This study introduces a novel composite strain sensor that combines poly(3-hexylthiophene) and tetrafluoro-tetracyanoquinodimethane to form a charge-transfer complex (CTC) with carbon nanotubes (CNTs) on a styrene-butadiene-styrene substrate. The CTC improves conductivity through effective charge transfer, while CNTs provide mechanical reinforcement and maintain conductive paths, preventing cracks under large strains. Purposefully introduced wrinkles in the structure enhance the detection of small strains. The sensor demonstrated a broad strain-sensing range from 0.01 to 200%, exhibiting high sensitivity to both minor and major deformations. Mechanical tests confirmed strong stress-strain performance, and electrical tests indicated significant conductivity improvements with CNT integration. These results highlight the potential of the sensor for applications in health monitoring, human-machine interfaces, and energy harvesting, effectively mimicking the tactile sensing abilities of human skin.

Motion Sensing by a Highly Sensitive Nanogold Strain Sensor in a Biomimetic 3D Environment.

Recent advancements in flexible electronics have highlighted their potential in biomedical applications, primarily due to their human-friendly nature. This study introduces a new flexible electronic system designed for motion sensing in a biomimetic three-dimensional (3D) environment. The system features a self-healing gel matrix (chitosan-based hydrogel) that effectively mimics the dynamics of the extracellular matrix (ECM), and is integrated with a highly sensitive thin-film resistive strain sensor, which is fabricated by incorporating a cross-linked gold nanoparticle (GNP) thin film as the active conductive layer onto a biocompatible microphase-separated polyurethane (PU) substrate through a clean, rapid, and high-precision contact printing method. The GNP-PU strain sensor demonstrates high sensitivity (a gauge factor of ∼50), good stability, and waterproofing properties. The feasibility of detecting small motion was evaluated by sensing the beating of human induced pluripotent stem cell (hiPSC)-derived cardiomyocyte spheroids embedded in the gel matrix. The integration of these components exemplifies a proof-of-concept for using flexible electronics comprising self-healing hydrogel and thin-film nanogold in cardiac sensing and offers promising insights into the development of next-generation biomimetic flexible electronic devices.

Recyclable and Stable Strain Sensors Based on Semi‐Wrapped Structure of Silver Nanowires in Polyvinyl Alcohol for Human Motion Monitoring

AbstractHighly sensitive strain sensors have been widely used in human motion monitoring, medical treatment, soft robots, and human–computer interaction, and the recycling of functional materials is in a huge demand for eco‐friendly and sustainable electronics. However, the manufacturing of recyclable strain sensors still remains challenging. Here, a semi‐wrapped structure based on silver nanowires and polyvinyl alcohol is proposed to realize a recyclable and stable strain sensor. It has shown excellent sensitivity, fast response, high stretchability and good environmental stability, and is successfully applied for human motion monitoring. In addition, a simple strategy is developed to effectively recycle silver nanowires in an eco‐friendly manner. The recyclable and stable strain sensor demonstrates potential applications in wearable and stretchable electronics, and the recycling strategy can be extended to other noble metal nanomaterials.

Laser-Induced Graphene Strain Sensors for Body Movement Monitoring.

To enable the development of artificial intelligence of things, the improvement of the strain sensing mechanisms and optimization of the interconnections are needed. Direct laser writing to obtain laser-induced graphene (LIG) is being studied as a promising technique for producing wearable, lightweight, highly sensitive, and reliable strain sensors. These devices show a higher degree of flexibility and stretchability when transferred to an elastomeric substrate. In this article, we manufactured polydimethylsiloxane (PDMS)-encapsulated LIG piezoresistive strain sensors with a quasi-linear behavior and a gauge factor of 111. The produced LIG was morphologically characterized via Raman spectroscopy and scanning electron microscopy before and after the electromechanical characterization and before and after the LIG transfer to PDMS. The results from these analyses revealed that the integrity of the material after the test was not affected and that the LIG volume in contact with the substrate increased after transfer and encapsulation in PDMS, leading to the improvement of the sensor performance. The sensors' capability for measuring bend angles accurately was demonstrated experimentally, making them useable in a wide range of applications for human body movement monitoring as well as for structural health monitoring. Regarding body monitoring, a PDMS-encapsulated LIG sensor for knee bending angle detection was proposed. This device showed unaffected performance of 1500 cycles under 8% uniaxial deformation and with response times in the range of 1-2 s.

An Antibacterial, Highly Sensitive Strain Sensor Based on an Anionic Copolymer Interpenetrating with κ-Carrageenan.

Polysaccharide-based hydrogels are suitable for use in the field of flexible bioelectronics due to their benign mechanical properties and biocompatibility. However, the preparation of hydrogel sensors with high performance without affecting their physicochemical properties (e.g., flexibility, toughness, self-healing, and antibacterial activity) remains a challenge and needs to be solved. Herein, a metal ion cross-linking reinforced, double network hydrogel was formed from a 2-acrylamide-2-methylpropanesulfonic acid (AMPS) copolymer interpenetrating κ-carrageenan (CAR), followed by immersing the gel in a Cu2+ ion solution to obtain an antibacterial CAR/P(AM-co-AMPS)-Cu2+ conductive hydrogel. LiCl was added as the electrolyte. The presence of the LiCl electrolyte and sulfonated molecular chain units not only gives the hydrogel good electrical conductivity (conductivity up to 2.68 S/m) but also improves the sensitivity of the hydrogel as a stress-strain sensor, with a hydrogel sensitivity GF of up to 3.76 in the 20%-100% strain range and response time of up to 280 ms. The CAR double-helical structure and sol-gel properties and the interaction of multiple noncovalent bonds between polymers provide the hydrogel with excellent self-healing, with a self-healing efficiency of 68%. In addition, the electrostatic interaction of Cu2+ with Escherichia coli cells can inhibit their growth, exhibiting good antibacterial properties with an inhibition circle diameter of 20.5 mm. This work could provide an effective strategy for antibacterial multifunctional CAR-based bionic sensors.

A highly responsive and sensitive flexible strain sensor with conductive cross-linked network by laser direct writing

As a critical component of wearable electronic devices, flexible strain sensors have made significant contributions in the fields of smart healthcare, motion monitoring and communication, garnering escalating research interests. However, the pursuit of ideal materials synergizing high sensitivity with facile and cost-effective manufacturing methodologies remains a persistent challenge, significantly hindering the full realization of the potential of flexible strain sensors. Herein, an innovative hybrid conductive network architecture constituted by multi-walled carbon nanotubes and laser-induced graphene (MWCNTs/LIG) has been explored via a facile, cost-effective, controllable, and environmentally friendly laser direct writing (LDW) technique. Subsequently, the resultant MWCNTs/LIG composite with a cross-linked microstructure has been harnessed for the fabrication of a high-performance strain sensor. Notably, the proposed sensor exhibits remarkable sensitivity with a gauge factor (GF) reaching 192, rapid response and recovery times of 27/20 ms, and favorable durability exceeding 2300 cycles. Furthermore, the developed sensor demonstrates the capability to accurately capture full-range motions in real-time, as well as the ability to transmit information as a portable communication device. This promising discovery offers a new insight for the flourishing development of high-performance flexible strain sensors in the realm of wearable sensing.

Fabrication of PHFPO Surface-Modified Conductive AgNWs/PNAGA Hydrogels with Enhanced Water Retention Capacity toward Highly Sensitive Strain Sensors.

Conductive hydrogels, characterized by their unique features of flexibility, biocompatibility, electrical conductivity, and responsiveness to environmental stimuli, have emerged as promising materials for sensitive strain sensors. In this study, a facile strategy to prepare highly conductive hydrogels is reported. Through rational structural and synthetic design, silver nanowires (AgNWs) are incorporated into poly(N-acryloyl glycinamide) (PNAGA) hydrogels, achieving high electrical conductivity (up to 0.88 S m-1), significantly enhanced mechanical properties, and elevated deformative sensitivity. Furthermore, surface modification with polyhexafluoropropylene oxide (PHFPO) has substantially improved the water retention capacity and dressing comfort of this hydrogel material. Based on the above merits, these hydrogels are employed to fabricate highly sensitive wearable strain sensors which can detect and interpret subtle hand and finger movements and enable precise control of machine interfaces. The AgNWs/PNAGA based strain sensors can effectively sense finger motion, enabling the control of robotic fingers to replicate the human hand's gestures. In addition, the high deformative sensitivity and elevated water retention performance of the hydrogels makes them suitable for flow sensing. These conceptual applications demonstrate the potential of this conductive hydrogel in high-performance strain sensors in the future.

Highly sensitive and stretchable fiber strain sensors based on ITO NPs/CNTs composite

Flexible and multifunctional sensors are in demand in various fields, but achieving excellent sensitivity and an extensive operational range poses a challenge. Here, we developed a flexible strain sensor using a cost-effective dip-coating method, incorporating microcracked synergistic indium tin oxide nanoparticles (ITO NPs) and carbon nanotubes (CNTs) in a conductive layer. Utilizing an elastic rubber core string as the stretchable substrate and Thermoplastic polyurethane (TPU) as an interfacial modifier, we established connectivity between the substrate and conductive layer. After encapsulation with polydimethylsiloxane, the ITO NPs/CNTs/rubber fiber (ICRF) strain sensor was fabricated. This sensor demonstrates a broad sensing range (0 ∼ 300 %), remarkable sensitivity (GF=1033.57), swift response/recovery times (174.55 ms/180.27 ms), and exceptional durability and stability (>2000 cycles). Moreover, it proves effective for human motion detection, underscoring its potential in flexible and wearable devices. Finally, precise responses of the mechanical claw’s posture and grasping demonstrate its potential applications across multiple fields.

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor.

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

High stretchable and self-adhesive multifunctional hydrogel for wearable and flexible sensors

Ionic conductive hydrogel has recently garnered significant research attention due to its potential applications in the field of wearable and flexible electronics. Nonetheless, the integration of multifunctional and synergistic advantages, including reliable electronic properties, high swelling capacity, exceptional mechanical characteristics, and self-adhesive properties, presents an ongoing challenge. In this study, we have developed an ionic conductive hydrogel through the co-polymerization of 4-Acryloylmorpholine (ACMO) and sodium acrylate using UV curing technology. The hydrogel exhibits excellent mechanical properties, high conductivity, superior swelling capacity, and remarkable self-adhesive attributes. The hydrogel serves as a highly sensitive strain sensor, enabling precise monitoring of both substantial and subtle human motions. Furthermore, the hydrogel demonstrates the capability to adhere to human skin, functioning as a human-machine interface for the detection of physiological signals, including electromyogram (EMG) signals, with low interfacial impedance. This work is anticipated to yield a new class of stretchable and conductive materials with diverse potential applications, ranging from flexible sensors and wearable bio-electronics to contributions in the field of artificial intelligence.

Fiber Laser Writing of Highly Sensitive Nickel Nanoparticle-Incorporated Graphene Strain Sensors.

Unlocking new dimensions in wearable sensor technology, this research highlights ultrasensitive stretchable strain sensors fabricated with the customized laser-induced graphene (LIG) decorated with uniformly distributed nickel nanoparticles with a fiber laser writing process. The nickel nanoparticle-incorporated LIG (Ni-NPs@LIG) strain sensors fabricated by a simple all-laser-based method utilize a commercial fiber laser. The Ni-NPs@LIG sensors showcase an impressive gauge factor, reaching up to 248 for strain values below 5%, demonstrating a sensitivity increase of up to 430% compared to the pure LIG sensors. Moreover, these sensors offer adjustable strain sensitivity based on laser fluence. The key advancement of this study lies in the direct laser writing of highly porous nickel-graphene nanostructures with adjustable properties, making them applicable across a broad range of applications. As an application demonstration, the strain sensors were employed to assess the small deformation of a pouch battery or track the large deformation of a balloon surface.

Sensitive strain sensors with wide-range detection and high stability for intelligent human motion monitoring

Miniaturized wearable electronic devices with strain sensors as core sensing elements hold significant applications in health monitoring and human-machine interaction. However, it remains a challenge to develop strain sensors with optimal response linearity, a wide detection range, and stability. In this study, to satisfy the above requirements, we construct high-performance wearable strain sensors (rGO-CNTOH@PSE) by a strategy of hybridizing conductive fillers with stretchable substrates. In detail, a synergistic conductive network is developed by adopting OH-containing carbon nanotube (CNTOH) and reduced graphene oxide (rGO) fillers with an appropriate mass ratio. After the network is integrated into a porous silicone elastomer substrate formed by the salt templating method, the developed strain sensor exhibits distinct resolution, consistent repeatability, and superior response linearity across a wide strain detection range of 0.3–303.03 %. Furthermore, the sensor also maintains good response stability over 15000 cycles of strain. We also demonstrate that this strain sensor showing outstanding exceptional detection performance has the capability for full-range human motion detection. A wireless smart electronic glove is developed based on the sensor, which can accurately recognize gesture movements and control a bionic robotic hand to replicate human hand actions, offering the potential for replacing manual tasks (such as, grasping in hazardous environments) in the future.

Thermal and light-driven soft actuators based on a conductive polypyrrole nanofibers integrated poly(N-isopropylacrylamide) hydrogel with intelligent response

The development of soft hydrogel actuators with outstanding mechanical properties, fast actuation speed, and available quantification of self-sensing actuation remains a challenging endeavor. In this work, dopamine-decorated polypyrrole nanofibers (DAPPy) were introduced into the polyethylene glycol diacrylate (PEGDA)-crosslinked poly(N-isopropyl acrylamide) network to generate a stretchable, NIR-responsive, and strain sensitive DAPPy/PNIPAM hydrogel layer. Besides, this active layer was combined with the passive ligninsulfonate sodium/polyacrylamide (LS/PAAM) to give DAPPy/PNIPAM//LS/PAAM bilayer hydrogel actuator, which exhibits ultrafast thermo-responsive actuation (19°/s) and underwater grasping and lifting performance. Moreover, the DAPPy/PNIPAM layer has excellent electrical conductivity (0.29 S/m) and thermal conversion ability (10.8 °C/min), which enable such a conductive hydrogel to act as a highly sensitive strain and temperature sensor with real-time resistance change in response to tensile strain (gauge factor up to 3.4), applied pressure, temperature, and remote NIR light irradiation. More importantly, the bilayer hydrogel actuator can integrate both actuation and self-sensing functions through the bending angle-surface temperature-relative resistance change relationship of the photothermal process. With excellent mechanical actuation and self-sensing ability, the resulting bilayer hydrogel showed a promising application potential as soft biomimetic actuating materials and soft intelligent actuators.

Highly Sensitive Flexible Resistive Strain Sensor Based on Swelling-Coating Method for Wearable Electronics

Highly Sensitive Flexible Resistive Strain Sensor Based on Swelling-Coating Method for Wearable Electronics