Strain Sensors Research Articles

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.

Balancing stretchability and conductivity: Carbon nanotube layer-enhanced non-ionic conductive hydrogels with a sandwich structure

In non-ionic conductive hydrogels, conductivity often conflicts with mechanical properties, posing a challenge in creating hydrogels that strike a balance between conductivity and mechanical properties. Here, we addressed this issue by integrating carboxylated carbon nanotube (CNT) layers in a sandwich structure onto the surface of the cationic hydrogel (P(AM-co-DMDAAC), PAD), followed by dehydration for densification. This approach simultaneously improved the conductivity of the hydrogel while maintaining its stretchability. The stability of the composite structure was ensured through electrostatic interactions, hydrogen bonding, and physical entanglement between the PAD hydrogel and the CNT layer. Additionally, adjusting the water–solid ratio of the hydrogel allows fine-tuning of its conductivity and stretchability. Remarkably, at a 500 % water–solid ratio, the hydrogel achieved a conductivity of 2.3 S/m. By further reducing the water content, the conductivity could be increased to 25 S/m, significantly higher than similar hydrogels. The PAD-CNT hydrogel found applications in swelling response, tunable resistance wires, and as strain sensors. Notably, the sandwich structure enables the PAD-CNT hydrogel to output capacitive signals, exhibiting a more sensitive response in monitoring human movement. Overall, this study introduces a novel composite structure for conductive hydrogels, holding tremendous potential in flexible conductivity and smart response fields.

Data-Driven Sparse Sensor Placement Optimization on Wings for Flight-By-Feel: Bioinspired Approach and Application.

Flight-by-feel (FBF) is an approach to flight control that uses dispersed sensors on the wings of aircraft to detect flight state. While biological FBF systems, such as the wings of insects, often contain hundreds of strain and flow sensors, artificial systems are highly constrained by size, weight, and power (SWaP) considerations, especially for small aircraft. An optimization approach is needed to determine how many sensors are required and where they should be placed on the wing. Airflow fields can be highly nonlinear, and many local minima exist for sensor placement, meaning conventional optimization techniques are unreliable for this application. The Sparse Sensor Placement Optimization for Prediction (SSPOP) algorithm extracts information from a dense array of flow data using singular value decomposition and linear discriminant analysis, thereby identifying the most information-rich sparse subset of sensor locations. In this research, the SSPOP algorithm is evaluated for the placement of artificial hair sensors on a 3D delta wing model with a 45° sweep angle and a blunt leading edge. The sensor placement solution, or design point (DP), is shown to rank within the top one percent of all possible solutions by root mean square error in angle of attack prediction. This research is the first to evaluate SSPOP on a 3D model and the first to include variable length hairs for variable velocity sensitivity. A comparison of SSPOP against conventional greedy search and gradient-based optimization shows that SSPOP DP ranks nearest to optimal in over 90 percent of models and is far more robust to model variation. The successful application of SSPOP in complex 3D flows paves the way for experimental sensor placement optimization for artificial hair-cell airflow sensors and is a major step toward biomimetic flight-by-feel.

Fabrication of TPU-supported Ti3C2Tx/Ag2S/TiO2 electrospun mat: Dual-functional materials for human motion and gas detection

In our work, a dual-functional sensor for detecting ammonia and human motion was realized by fabrication of TPU-Supported Ti3C2Tx/Ag2S/TiO2 electrospun mat. The synthesis of Ag2S/TiO2–doped Ti3C2Tx nanocomposites was achieved through a simple hydrothermal approach, which served as the sensing component. The findings from the microscopic morphology analysis indicate that the nanocomposites were evenly dispersed across the flexible TPU. The performance of the fabricated flexible gas-sensitive and strain sensors was evaluated, the results showed a 25.2 % response to 100 ppm ammonia at room temperature (25 ℃), which was approximately five times higher than that of the pure Ti3C2Tx gas-sensitive materials. When subjected to a strain range of 40 %–60 %, the response was measured as 378.597, exhibiting a notably high level of linearity (linear fitting value of 0.9849). The stability and repeatability of the flexible sensor make it play a significant role in room temperature gas sensing and motion monitoring.

Composite Janus film based on the synergistic interactions of π-π stacking and dynamic covalent bond toward direction recognition sensing

Although flexible strain sensors have made important advancements recently, most of them are unable to recognize the direction of motion, which greatly limits their application in fields such as human-machine interaction. This paper presents the fabrication of a bilayer asymmetric composite film that exhibits Janus dual-sided characteristics and interfacial properties. Specifically, the two sides possess different chemical compositions and surface features. The strong π-π stacking interaction between carbon nanotubes (CNTs) and pyrene enables a tight coating on the surface of poly(glycidyl propyl urethane) (PGPU), resulting in excellent sensing capabilities and electromagnetic shielding properties for the composite material. This composite film can effectively monitor the amplitude and direction of motion. Firstly, pyrene-grafted polyurethane (PGPU) was synthesized including on dynamic covalent bonds. The tensile strength of different samples can reach up to 19.69 MPa, and the strain at break is up to 501.95 %. Furthermore, PGPU/CNTs conductive composite films were fabricated by spray-coating carbon nanotubes (CNTs) onto PGPU, and the pyrene units in PGPU can effectively interact with CNTs via π-π stacking, ensuring that stable adhesion of CNTs layer during long-term usage. Due to the dynamic covalent bonds and hydrogen bonds inside PGPU, PGPU and PGPU/CNTs both exhibit well-performed self-healing capability. Notably, the Janus structure of PGPU/CNTs can adjust the positive and negative values of relative resistance based on stretchable and compressive status of CNTs layer. Thus, PGPU/CNTs are directionally sensitive and self-healing flexible wearable sensor, which might apply in human-machine interaction field.

Mechanism and Performance Evaluation of a Strain Sensor Made from a Composite Hydrogel Containing Conductive Fibers of Thermoplastic Polyurethane and Polyvinyl Alcohol

Mechanism and Performance Evaluation of a Strain Sensor Made from a Composite Hydrogel Containing Conductive Fibers of Thermoplastic Polyurethane and Polyvinyl Alcohol

Ultra-Stretchable, Adhesive, Conductive, and Antifreezing Multinetwork Borate Ester-Based Hydrogel for Wearable Strain Sensor and VOC Absorption.

Hydrogels based on borate ester bonds exhibit remarkable tensile strength and self-healing ability, which make them a promising material for various biological research and strain sensor applications. However, in order to meet the practical application of hydrogel strain sensors, they must also show high conductivity, frost resistance, and proper adhesion, which is still a continuous challenge. Herein, a triple network hydrogel was prepared using poly(vinyl alcohol) (PVA) as the first network, ethylene imine polymer (PEI) as the second network, and poly(acrylamide-co-acrylic acid) copolymer (denoted as P(AM-Co-AA)) as the third network. 3-Carboxy-4-fluorophenylboronic acid (CFBS) was used as the cross-linking agent, glycerol (GL) was added to improve low-temperature resistance, and sodium chloride (NaCl) was incorporated to enhance electrical conductivity. The resulting PVA-CFBS@PEI@P(AM-Co-AA) triple network hydrogel exhibited impressive mechanical properties, including ultra tensile strength (4100%, 266.8 kPa), high toughness (6.5 MJ/m3), and low-temperature resistance (-60 °C). Additionally, it demonstrated high conductivity (σ = 1.83 mS/cm). The incorporation of CFBS endowed the hydrogel with excellent self-healing ability, while GL improved low-temperature resistance and strain sensing sensitivity (gauge factor (GF) = 2.8 (0-300%), GF = 5.6 (300-600%), GF = 8.7 (600-1000%)). The prepared hydrogel sensor can repetitively detect and differentiate between a wide range of human activities such as joint movements, frowning, and smiling. Additionally, the hydrogel demonstrated favorable mechanical properties at -20 °C (good adhesion, tensile strength: 1169.8%, 1.2 MPa; conductivity: 0.71 mS/cm, and strain sensing coefficient: GF = 1.3), making it suitable for applications in low-temperature environments. Furthermore, it also functions as an exceptional adsorbent, capable of selectively absorbing volatile organic compounds at high capacity (e.g., methanol: 1.80 g/g; acetone: 1.50 g/g).

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.

Preparation and characterization of functional dual network hydrogel for motion monitoring in smart bra

The smart bra is a kind of sports underwear with providing health monitoring and comfort protection for women breast in sports. Developing smart sensing materials that combine comfort and functionality for sensing breast condition remains a challenge. Here, natural polysaccharide sodium alginate (SA), acrylamide (AM) monomer and chitosan (CS) and polypyrrole (PPy) in-situ polymerized from pyrole (Py) monomer were used as raw materials to successfully synthesize double network conductive hydrogel by two-step method. The results show that in the hydrogels, the entanglement and interpenetration of macromolecules produce toughness (tensile/compressive strength is ∼0.248 MPa/∼2.836 MPa), elasticity (tensile/compression elastic recovery rate is∼94.06 %/∼98.41 %) and durability (after 30 tensile/compression cycles conducting at 25 %, 50 % and 100 % constant strain keep at high recovery rate). The rich moisture content provides its adhesion (adhesion strength ∼4.7557 kPa) and flexibility (bending stiffness 1.0843 cN cm∼), presenting sufficient adhesion reliability and robustness to different substrates. The addition of PPy enables the hydrogel to have conductivity (6.77 × 10−2 S cm−1) and sensing performance (tensile/compression sensitivity is ∼1.94/∼1.43). The conductive hydrogel, as a flexible strain sensor, shows good adhesion, flexibility, elasticity and sensitivity in application of sports smart bra, can detect both the tensile behavior of skin and the compression state of tissue through its mechano-electricity, and accurately sense the changes of physiological parameters during movement. The conductive hydrogel with high elasticity and strength has broad application prospects in smart bra.

Real‐Time Force Monitoring of Electrically Stimulated 3D‐Bioengineered Muscle Bioactuators Using Organic Sensors with Tunable Sensitivity

The contractile nature of skeletal muscle tissue makes it especially attractive for powering biohybrid actuators. Significant efforts have been dedicated to the improvement and control of contraction force, going one step forward toward the automation of these biohybrid platforms. Herein, 3D‐bioengineered skeletal muscle tissues are integrated with organic transistor‐based sensors to define a soft bioactuator with real‐time force monitoring capabilities. The muscle tissue is electrically stimulated while the organic sensor ensures transduction of the exerted force into an electrical signal that allows direct monitoring of the bioactuator performance. Sensor calibration is carried out to define its sensitivity at different biasing conditions: as opposed to standard, two‐terminal piezoresistive devices, transistor‐based strain sensors show tunable sensitivity by acting on the voltage applied to a third terminal—the gate. A complete evaluation of sensing performances is provided, demonstrating that real‐time monitoring is effective under different conditions, including stimulation signal frequency and chemical modulation of the bioactuator contraction, demonstrating its potential use as a drug testing platform. In the reported results, the way is paved for a complete exploitation of organic devices in soft robotic applications and to the development of novel biohybrid machines in bioengineering and biomedicine.

Robust biomimetic strain sensor based on butterfly wing-derived skeleton structure

A biomimetic strain sensor was designed and constructed based on Ir nanoparticles-modified multi-wall carbon nanotubes (Ir NPs@MWCNTs) and parallel Pt layer/dragon skin with carbonized butterfly wing patterns. This sensor exhibits high gauge factor (∼515.4), extensive tensile range (0%–96%), and swift response (∼300 ms), especially remarkable stability up to 60 000 cycles. The work mechanism has been proposed based on the experimental test and finite-element method. Some important applications such as human motion and micro-expression recognition have been confirmed using 3 × 3 flexible biomimetic sensor array.

Ultrasensitive Ultrasoft Buckled Crack‐Based Sensor for Respiration Measurement and Enhanced Human–Machine Interface

Wearable strain sensors have transformed the real‐time monitoring of health conditions and human–machine interactions. However, recently developed wearable strain sensors exhibit several limitations. For example, when a sensor is designed with high sensitivity to detect strain, it struggles to accurately measuring the deformation of low‐stiffness materials like skin. Additionally, finding the optimal balance between sensitivity, durability, hysteresis, and strain range in sensor design is challenging. To address these challenges, a Buckled, Ultrasoft, Crack‐based, Large strain, EpiDermal (BUCKLED) sensor is developed. This sensor integrates the benefits of soft structure engineering with high sensitivity of crack‐based sensing mechanisms to ensure optimal skin deformation measurements. The BUCKLED sensor exhibits significant improvements in compliance (18 500 mm N−1), stretchability (100%), hysteresis (2%), durability (10 000 cycles with 100% strain), and force sensitivity () owing to its buckled shape, confirming its ability to detect subtle movements with enhanced accuracy. The sensor's high compliance allows it to accurately measure low‐stiffness objects, ensuring reliable performance. Furthermore, the sensor's tunability is demonstrating its effectiveness in applications such as respiratory monitoring, facial expression recognition, and silent speech interfaces. Consequently, the proposed sensor is versatile and holds great potential for a wide range of sensing applications.

Hydrophobic association hydrogel with toughness, high stretch, and sensitivity for flexible sensing

AbstractPolypyrrole (PPy), as a highly conductive polymer, is limited in application due to the difficulty of uniform dispersion in hydrogels. To improve the compatibility of PPy with hydrogels, xanthan gum (XG) is employed as an emulsifier to homogeneously disperse pyrrole (Py) in water. XG is used as a template for in situ polymerization, and PPy is coated on XG to form nanoparticles (PX) with a core‐shell structure, enabling the nanoparticles to be dispersed uniformly in water for a long time. PX nanoparticles are combined with pure hydrophobic association hydrogel (HA) to form a HA/PX nanocomposite hydrogel. The HA/PX2% nanocomposite hydrogels exhibiting high toughness (equivalent to 5.1 MJ/m3) and high sensitivity (GF = 11.07 for 600%–1400% strain range) are prepared by combining dynamic hydrophobic cross‐linking sites, as well as hydrogen bonding between PX nanoparticles and the cross‐linked network. The test results show that the HA/PX nanocomposite hydrogel strain sensor has excellent strain sensing durability (800 cycles of 100% strain) and has the ability to accurately detect human joint movements for voice recognition and handwriting recognition. The nanocomposite hydrogel is a new method for the preparation of flexible electronic materials, which has great promise for application in the field of strain sensors.

A Review: Non-Contact and Full-Field Strain Mapping Methods for Experimental Mechanics and Structural Health Monitoring.

Non-contact and full-field strain mapping captures strain across an entire surface, providing a complete two-dimensional (2D) strain distribution without attachment to sensors. It is an essential technique with wide-ranging applications across various industries, significantly contributing to experimental mechanics and structural health monitoring. Although there have been reviews that focus on specific methods, such as interferometric techniques or carbon nanotube-based strain sensors, a comprehensive comparison that evaluates these diverse methods together is lacking. This paper addresses this gap by focusing on strain mapping techniques specifically used in experimental mechanics and structural health monitoring. The fundamental principles of each method are illustrated with specific applications. Their performance characteristics are compared and analyzed to highlight strengths and limitations. The review concludes by discussing future challenges in strain mapping, providing insights into potential advancements and developments in this critical field.

PDMS-assisted Mach–Zehnder interferometer for simultaneous sensing of strain and temperature based on hollow-core fibers

A PDMS-assisted Mach-Zehnder interferometric (MZI) fiber-optical sensor based on hollow-core fibers (HCF) is proposed and experimentally demonstrated for the simultaneous detection of strain and temperature. The transmission spectral evolution in response to the applied strain and temperature sensor are investigated. Experimental results suggest that our proposed sensor exhibits strain and temperature sensitivities up to −40.95 dB/N and 2.5 nm/°C for the sensing ranges of 0.01 N to 0.08 N and 35 °C to 42 °C, respectively. Moreover, crosstalk analyses were performed to discriminate the strain and temperature measurement. The proposed sensor possesses desirable advantages such as high sensitivity, easy-fabrication, and low-crosstalk, which paves new way toward developing photonic devices for strain/temperature dual-parameter sensing and related applications.

Super-Tough, Non-Swelling Zwitterionic Hydrogel Sensor Based on the Hofmeister Effect for Potential Motion Monitoring of Marine Animals.

Hydrogel-based electronic devices in aquatic environments have sparked widespread research interest. Nevertheless, the challenge of developing hydrogel electronics underwater has not been profoundly surmounted because of the fragility and swelling of hydrogels in aquatic environments. In this work, a zwitterionic double network hydrogel comprisedof polyvinyl alcohol (PVA), poly(sulfobetaine methacrylate) (PSBMA), and sulfuric acid (H2SO4) demonstrates super-tough and non-swelling performance. The Hofmeister effect of H2SO4 and PSBMA induces the PVA chains to form numerous nanocrystalline domains, which serve as the primary physical crosslinking points and provide effective energy dissipation. H2SO4 induces a strong salting-out effect to facilitate PVA crystallization and the formation of a dense and stable network structure that inhibits swelling. The resulting hydrogel exhibits an ultra-high toughness of 4.61MJm-3, non-swelling, and long-term stability for up to a month in pure water and seawater. Based on this, a hydrogel-based seawater strain sensor has been developed to monitor the underwater movements of marine animal models. Reliable and stable sensing performance ensures real-time collection of underwater motion signals, despite the impacts of water flow and the interference of ions. This study provides a facile approach to designing super-tough and non-swelling hydrogels and further expands the application of underwater electronic devices.

Unique framework effect induced by uniform silk fibroin dynamic nanospheres enables multiscale hydrogel with outstanding elastic resilience and strain sensing performance

It is a significant challenge to obtain hydrogels simultaneously with low tensile energy dissipation, high compressive resilience and long durability. Herein, the uniform dynamic nanospheres (Sil-4H0.75) derived from 4-Hydroxybutyl acrylate glycidyl ether grafted silk fibroin is designed to overcome this issue. Due to its uniform and dynamic characteristic, Sil-4H0.75 could endow hydrogel with homogeneous multiscale structure and produce unique framework effect. Thus, transparent Sil-4H0.75 crosslinked acrylamide hydrogel doped with Ag nanowires APS3.75%/AgNW0.1 exhibits a high stretchability (1260 %) and outstanding elastic resilience. The tensile energy dissipation ratio maintains a low value of 9 % across a wide 800 % strain range. A high compression resilience ratio of 92.2 % is kept after ten compression cycles under 90 % compressive strain. The orderly AgNWs motion guided by framework effect also make it be used as both tensile and compressive sensors and exhibits high gauge factor of 7.35, outstanding compression sensitivity of 30.379 kPa−1 and excellent durability (up to 2000 cycles). The detection or other applications based on both two sensing modes are also demonstrated. In a word, this work affords a general strategy to achieve high-performance hydrogel based on uniform dynamic nanospheres which exhibits great potential in the applications of flexible wearable strain sensors.

Graphene-Based Tunneling Strain Sensors: A Mechanism Study

Graphene-Based Tunneling Strain Sensors: A Mechanism Study

Preparation and application of organic hydrogels incorporating polyacrylamide/sodium alginate/DMSO for enhanced anti-drying, anti-freezing, and self-healing properties

The double-network hydrogel comprises two asymmetrically structured cross-linked networks, demonstrating exceptional mechanical properties and possessing significant potential for application in the field of flexible sensors. However, conventional hydrogels primarily disperse in water as a medium and continuously lose moisture in natural environments, rendering them incapable of retaining hydration over extended periods and unsuitable for utilization under low-temperature conditions. In this study, we devised an organic hydrogel based on polyacrylamide (PAM), sodium alginate (SA), and dimethyl sulfoxide (DMSO). Through hydrogen bonding interactions among DMSO molecules, polyacrylamide, and sodium alginate, this organic hydrogel not only exhibits anti-drying and anti-freezing properties but also demonstrates self-healing characteristics. Moreover, the organic hydrogel showcases remarkable mechanical properties and conductivity capabilities that render it suitable for constructing flexible strain sensors to monitor human movements such as finger bending, elbow flexion, knee joint bending, and ankle extension. Additionally, the sensor exhibits excellent stability and durability.

Enhancing piezoresistive sensitivity of PEDOT:PSS-based strain sensors through nickel microparticles incorporation

Enhancing piezoresistive sensitivity of PEDOT:PSS-based strain sensors through nickel microparticles incorporation