High-performance Strain Sensors Research Articles

Flexible Tactile Sensing Microfibers Based On Liquid Metals.

High-performance and intelligent fibers are indispensable parts of wearable electronics in the future. This work mainly demonstrates the preparation of flexible intelligent liquid metal (LM) fibers with three core–sheath structures. An ultra-thin (10–50 μm), conductive, and highly flexible LM was deposited on the fiber core [carbon/polyethylene terephthalate (C/PET)––150–500 μm] along the fiber direction and then deposited on a polymer-protective layer [polyvinyl alcohol/epoxy resin (PVA/EP)––10 μm]. Four kinds of LM intelligent fibers were manufactured, including the C–LM–PVA fiber, C–LM–EP fiber, PET–LM–PVA fiber, and PET–LM–EP fiber. These LM intelligent fibers (diameter, 150–600 μm) were demonstrated with a high conductivity of 7.839 × 104 S·m–1. The changes in resistance in different torsion directions were measured, and these smart LM fibers could also be used as electrical heaters or thermoelectric generators, which released heat (36–36.9 °C/1–1.5 V) into the environment. Then, these multifunctional LM fibers were applied as high-performance strain sensors and bending sensors. These flexible LM conductive fibers could be successfully utilized in intelligent wearable fabrics and were expected to be widely utilized in artificial muscle and sensor fields.

Magnetically induced robust anisotropic structure of multi-walled carbon nanotubes/Ni for high-performance flexible strain sensor

Multi-walled carbon nanotubes (MWCNTs) are widely used as functional nanomaterials in flexible strain sensing polymer composites. They are expected to exhibit high anisotropy when aligned along a certain direction due to their unique one-dimensional structure, which has a positive effect on enhancing the sensing performance of the original polymer composites. Here, we used a weak curing magnetic field (less than 0.7 T) to induce the self-assembly of MWCNTs in polymer composites to obtain a novel flexible strain sensor with anisotropic structure (MAPC). To further optimize the comprehensive sensing performance, the synergistic effect between MWCNTs and conductive particles was utilized to enhance the sensitivity of MAPC to tensile strain by filling a small amount of nickel nanoparticles (MAPCN). The sensor can measure the tensile strain range up to 720%, with high sensitivity (GF=1768.7, 90%–100% strain), fast response (loading delay time τ1 = 92.2 ms and unloading delay time τ2 = 143.6 ms, 0–10% strain), and high durability (1050 cycles at 10% strain). These important properties allowed MAPCN to accurately monitor human physiological activities, and a gesture recognition system was designed to demonstrate its practical value and potential application prospect in wearable devices.

Flame-Retardant PEDOT:PSS/LDHs/Leather Flexible Strain Sensor for Human Motion Detection.

Flexible piezoresistive sensors have demonstrated great potential in human-motion-detection applications. However, it still remains a challenge to fabricate strain sensors with high sensitivity, broad sensing range, and good linear response to strain. In this report, a simple and scalable fabrication strategy is developed to construct high performance strain sensors by using leather as the substrates to filtrate poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) modified layered double hydroxides (LDHs) suspensions. The naturally aligned collagen fibers in leather enable size selection for the 2D conductive materials and as such dual-conductive pathways are effectively formed on the surface and in the matrix of leather. Due to the unique design of conductive networks, the prepared sensor possesses high gauge factor (maximum value of 2326.84), tunable strain range (0-70%), fast tensile response time (160ms), and good stability in 1000 stretching-relaxing/compression-relaxing cycles, making it suitable for various human motion detections including coughing and large-scale motions of joint bending. In addition, the incorporated LDHs is a non-toxic flame retardant, which is helpful to reduce electronic fire risk and can bring added value to the sensor.

Ultra-stretchable, super-hydrophobic and high-conductive composite for wearable strain sensors with high sensitivity

With continuous development of artificial intelligence technology, strain sensors have attracted widespread attention. In this work, a novel high-performance wearable strain sensor is prepared by using a kind of ultra-stretchable, super-hydrophobic and high-conductive composite. The preparation process is as follows, i.e., using common elastic band (EB) as the polymer matrix, nano carbon black (CB) and carbon nanotubes (CNTs) as mixed conductive filler, and then modified by polydimethylsiloxane (PDMS) to obtain the PDMS/(CB + CNTs)/EB composite for assembling assemble flexible wearable strain sensors. Experimental results reveal the following excellent properties: 1) The composite exhibits excellent mechanical properties and super-hydrophobicity, i.e., the tensile strength is up to 996.5%, and the elastic modulus and tensile strength increase 49.2% and 59.2%, compared with pristine EB; 2) The composite strain sensor exhibits high sensitivity (the gauge factor reaches up to 648.83 under strain range of 979.9–996.5%), and it still shows stable performance after 3000 cycles tests (100% strain); 3) It is a well candidate to be used for monitoring human body motions including large and subtle body movements; 4) The composite sensor also has advantages of easy access of raw materials, simple preparation, easy mass production and relatively low production cost, showing a broad application prospect in wearable electronic products.

High-performance fiber strain sensor of carbon nanotube/thermoplastic polyurethane@styrene butadiene styrene with a double percolated structure

High-performance fiber strain sensor of carbon nanotube/thermoplastic polyurethane@styrene butadiene styrene with a double percolated structure

Highly Stretchable and Self-Adhesive Elastomers Based on Polymer Chain Rearrangement for High-Performance Strain Sensors.

Polydimethylsiloxane (PDMS) has been widely used in many fields. However, the polymerization process of the siloxane chain is highly complex, and it is challenging to enhance the mechanical properties of PDMS elastomers significantly. We found that adding a small amount of polyoxyethylene lauryl ether (Brij-35) into siloxane polymers can result in B-PDMS elastomers with high tensile properties and strong adhesion. It is worth noting that this is the first study to improve the mechanical properties of PDMS using Brij-35. Here, we intensely studied a variety of process conditions that influence the cross-linking of PDMS, emphasizing the modification mechanism of the polymer chain. The hydroxyl groups in Brij-35 and the platinum catalyst in PDMS form a complex, which inhibits the cross-linking process of PDMS, not only forming a heterogeneous cross-linking network in the B-PDMS but also disentangling the strongly wound siloxane polymer chain, thereby rearranging the PDMS polymer chains. Furthermore, in order to prepare a strain sensor based on the B-PDMS elastomer under safe and convenient conditions, we prepared laser-scribed graphene powder (LSGP) by laser-scribing of graphene oxide (GO) films, and the LSGP and carbon nanotubes (CNTs) endowed the B-PDMS elastomers with excellent electrical properties. The sensor could firmly adhere to the skin and generate a high-quality response to a variety of human motions, and it could drive the robotic hand to grasp and lift objects accurately. The high-performance strain sensors based on B-PDMS have broad applications in medical sensing and biopotential measurement.

High performance flexible and wearable strain sensor based on rGO and PANI modified Lycra cotton e-textile

Intelligent flexible strain sensors based on fabric have caused tremendous attention in the application of wearable devices because of their lightweight and excellent air permeability. It is still a great challenge to achieve excellent stability, high sensitivity, and wide working range. In this work, a high-performance strain sensor with reduced graphene oxide (rGO) reinforced polyaniline (PANI) based on polydopamine (PDA) modified Lycra cotton (LC) electronic textile (e-textile) was fabricated. The strain sensor of PANI/rGO/PDA/LC exhibits high sensitivity with a gauge factor (GF) of 24, a wide strain range of 0.2–50%, and excellent mechanical durability and stability for 1500 stretching/releasing cycles test. Such e-textile is promising to be integrated with other wearable devices to track human motion signals in real-time for human-machine and medical monitoring.

Rational structural design of vanillin based vitrimers in one-pot to facilely fabricate highly stretchable and sensitive composite strain sensor

The preparation of high performance and multifunctional strain sensors by a facile fabrication process is highly desirable but still challenging. In this study, vanillin based vitrimers were simply synthesized in one-pot by the combination of thiol-epoxy and amine-aldehyde reactions, which was used for strain sensor fabrication. The reaction kinetics were extensively investigated by 1H NMR analysis to demonstrate the feasibility to prepare vitrimers in one-pot. The physicochemical properties and dynamic natures of vanillin based vitrimers were then analyzed by various techniques, such as FTIR, DSC and rheometer. The mechanical properties of vitrimers were tunable via controlling the structure of crosslinkers. Moreover, it exhibited a strong UV-adsorption ability to potentially protect the human skins. Due to the enhanced interfacial interaction between vitrimers and silver nanowire layers, and the dynamic feature of Schiff base, the fabrication of strain sensors with sandwich-structures could be facilely realized. The strain sensors show a wide linear response range (0–100%) and a high gauge factor (15.8), which was comparable to previous sensors prepared by complicated steps. This study has demonstrated the potential to combine dynamic covalent chemistry and sustainability to overcome the general challenges in the fabrication of high-performance strain sensors.

In situ laser synthesis of Pt nanoparticles embedded in graphene films for wearable strain sensors with ultra-high sensitivity and stability

Carbon-based materials have recently attracted tremendous attention for their applications in highly sensitive wearable devices for human motion monitoring. Embedding rare metal nanoparticles such as Au, Ag, and Pt has been proved to significantly improve the sensitivity of the carbon-based strain sensors. However, the existing method to prepare nanoparticles usually involves multi-step, time-consuming procedures. Here, we report a facile one-step laser process to synthesize porous 3D graphene structures embedded with Pt nanoparticles. This is achieved by irradiating an ultraviolet (355 nm) picosecond laser on a film formed with a mixture of polybenzimidazole (PBI) and Pt(acac)2 solution to simultaneously induce the formation of graphene and reduction of the Pt precursor. The assembled strain sensor with loaded Pt nanoparticles in laser-induced graphene (Pt/LIG) demonstrates ultra-high sensitivity (a gauge factor, GFmax ≈ 489.3) and a wide strain range up to 20%. Additionally, the strain sensor with an optimized concentration of Pt(acac)2 has low hysteresis, minor lag errors, and a highly stable response (>5000 cycles). Various human motion signals are detected, from subtle physical activities (including chewing muscles movement, pulse beating) to large-scale limb track (finger bending, knee movement, and waist extending). This one-step laser process is facile, rapid, and potentially scalable for the mass production of high-performance wearable strain sensors.

Stretchable, Sensitive Strain Sensors with a Wide Workable Range and Low Detection Limit for Wearable Electronic Skins.

With the rapid development of wearable electronics, a multifunctional and flexible strain sensor is urgently required. Even though enormous progress has been achieved in designing high-performance strain sensors, the conflict between high sensitivity and a large workable range still restricts their further advance. Herein, a "point to point" conductive network is proposed to design and fabricate a carbon black/polyaniline nanoparticles/thermoplastic polyurethane film (CPUF). The designed structure renders CPUF composites with a wide sensitive range (up to 680% strain), highly sensitive response with a low detection limit of 0.03% strain, and high gauge factor (GF) of 3030.8, together with good sensing stability, fast response/recovery time (80 ms/95 ms), and good durability even after 10000 stretching/releasing cycles. CPUF composites are assembled as wearable strain sensors with the ability of precisely detecting full-range human motions and organic solvents, showing a potential application in human-machine interaction and environmental monitoring.

Ultrafast dynamic response of waterproof stretchable strain sensors based on wrinkle-templated microcracking

A wrinkle-templated microcracking mechanism involved in a strain-sensing bilayer configuration provides a universal strategy to fabricate high-performance waterproof strain sensors with ultrafast dynamic response ability.

One-step synthesis of carbon-doped PPy nanoparticles interspersed in 3D porous melamine foam as a high-performance piezoresistive pressure, strain, and breath sensor

Flexible and compressible piezoresistive pressure, strain, and breath sensor based on carbon-doped PPy nanoparticles interspersed in 3D porous melamine foam.

Flexible and high-performance piezoresistive strain sensors based on multi-walled carbon nanotubes@polyurethane foam.

Flexible wearable pressure sensors have attracted special attention in the last 10 years due to their great potential in health monitoring, activity detection and as electronic skin. However, it is still a great challenge to develop high sensitivity, fast response, and good reliable stability through a simple and reproducible large-scale fabrication process. Here, we develop a simple and efficient method to fabricate three-dimensional (3D) light-weight piezoresistive sensing materials by coating multi-walled carbon nanotubes (MWCNTs) on the surface of polyurethane (PU) foam using a dip-spin coating process. The PU foam prepared with SEBS-g-MAH and polyether polyols has high elasticity and good stability in MWCNTs/DMF solution. Subsequently, a piezoresistive sensor was assembled with the prepared MWCNTs/PU composite foam and copper foil electrodes. The assembled pressure sensor has high sensitivity (62.37 kPa−1), a wide working range (0–172.6 kPa, 80% strain), a fast response time (less than 0.6 s), and reliable repeatability (≥2000 cycles). It has shown potential application in real-time human motion detection (e.g., arm bending, knee bending), and monitoring the brightness of LED lights.

High-performance strain sensor for detection of human motion and subtle strain by facile fabrication

Flexible strain sensors are significant components for smart wearable electronic devices because of wide applications. However, the design and construction of high-performance strain sensors through facile fabrication remains a challenge. In this study, we designed a simple and high-performance flexible strain sensor based on gold nanoparticles and a polyimide substrate. The fabrication included drop-casting and high-temperature annealing, enhancing the connections between the nanoparticles. The results reveal that the sensor can sensitively detect subtle strains (GF = 19.94). It exhibits excellent linearity (R2 = 0.99) when sensing outward bending strain and low power consumption (<2.5 mW). Furthermore, it can be adhered to human skin to detect human physiological parameters. The strain sensor can detect the angles of the cervical vertebra in different bending states. It can show the distinctions of artery pulse waveforms at different conditions (before and after exercise) and in people (pregnant and unpregnant) with a high precision.

Design of a Superhydrophobic Strain Sensor with a Multilayer Structure for Human Motion Monitoring.

A flexible strain sensor is of significant importance in wearable electronics since it can help monitor the physical signals from the human body. Among various strain sensors, the polyurethane (PU)-based ones have received widespread attention owing to their excellent toughness, large working range, and nice gas permeability. However, the hydrophobicity of these sensors is not good enough, which may affect their use life and sensitivity. In this work, a high-performance strain sensor composed of PU, reduced graphene oxide (rGO), polydopamine (PDA), and 1H,1H,2H,2H-perfluorodecane-thiol (PFDT) was designed and prepared. The results revealed that this PU/rGO/PDA/PFDT device possessed good superhydrophobicity with a water contact angle of 153.3°, a wide working strain range of 590%, and an outstanding gauge factor as high as 221 simultaneously. Because of these above advantages, the sensor worked effectively in detecting both subtle and large human movements (such as joint motion, finger motion, and vocal cord vibration) even in a high humidity environment. This strain sensor with high sensitivity, wide working range, and suitable modulus may have great potential in the field of flexible and wearable electronics in the near future.

Strain Sensing Behavior of 3D Printable and Wearable Conductive Polymer Composites Filled with Silane-Modified MWCNTs.

3D printing of conductive polymers is an attractive technique for achieving high flexibility, wearability, and sensing characteristics without geometrical limitations. However, there is an urgent need to integrate printability, conductivity, and sensing capability. Herein, a conductive polymer ink for 3D printing that combines the desirable features of high electrical conductivity, flexible stretchability, and strain-sensing monitoring is prepared. The ink matrix is polydimethylsiloxane and synergistically enhanced by acetylene carbon black (ACB) and multi-walled carbon nanotubes (MWCNTs) (silane or un-silane-modified). The inks are screened step-by-step to explore their printability, rheology, mechanical properties, and electrical performance upon loading. The formation of an electrically conductive network, electrical properties upon tensile load, and strain sensing stability under cyclic stretching are investigated intensively. It is demonstrated that conductive polymers filled by ACB and silane-modified, MWCNTs (MWCNTs-MTES) possess superior printability, stretchability, conductivity, and strain sensing behaviors. Finally, a flexible wearable strain-sensing skin patch is printed, and it successfully records joint motion signals on human fingers, wrists, and elbows with good stability and repeatability. Those results show the extent of potential applications in healthcare and motion monitoring fields. This work provides an efficient and simple route to achieve comfortably wearable and high-performance strain sensors.

Highly stretchable and durable fibrous strain sensor with growth ring-like spiral structure for wearable electronics

Wearable strain sensors promote various emerging intelligent fields. Compared with the planar strain sensors, fibrous strain sensors are highly desirable for practical applications because they are favour of conformably attaching onto uneven surfaces and integrating into regular fabrics. Herein, inspired by the growth ring of tree, a highly stretchable silver nanowire/polydimethylsiloxane based fibrous strain sensor with spiral structure was innovatively prepared by cost-efficient coating plus rolling up technique. This unique spiral structure endowed the fibrous strain sensor with wide sensing range of 100% strain, outstanding linearity, gauge factor of 3.0, negligible hysteresis, and excellent reliability. Thanks to these superior characteristics, our sensor can not only accurately detect human movements when being attached to joints directly or woven in a cotton glove but also control the brightness of bulb when being connected into a circuit, showing great potential for wearable sensing electronics and next-generation circuit control elements. This work paves a promising way for the development of high-performance fibrous strain sensor.

High-performance flexible strain sensors based on biaxially stretched conductive polymer composites with carbon nanotubes immobilized on reduced graphene oxide

In this paper, reduced graphene oxide (rGO), decorated with immobilized carbon nanotubes (CNTs), was introduced into a thermoplastic polyurethane (TPU) matrix to obtain a conductive nanocomposite with 2 wt% nanofillers content. Hot-pressed composite sheets were then biaxially stretched into films to efficiently fabricate flexible strain sensors. The biaxial stretching process was shown to promote secondary dispersion and parallel alignment of the nanofillers, resulting in the sensors exhibited significantly higher sensitivity (GF = 150 for the rGO/TPU4×4 sensor with a stretching ratio of 4 × 4 relative to GF = 3.5 for the rGO/TPU1×1 sensor at 30% strain). Furthermore, due to the synergy of the CNTs and rGO nanoparticles, the rGO/CNT/TPU4×4 sensor had a lower resistivity (5.62 × 104 Ω·m), wider monitoring range (0.3 ∼ 400% strain), and higher stability compared with the rGO/TPU4×4 sensor. The ability of the sensor to recognize body movements and physiological activities was also shown.

3D printed high-performance spider web-like flexible strain sensors with directional strain recognition based on conductive polymer composites

Inspired by the unique structure of spider web, we designed and prepared a spider web-like strain sensor based on multi-walled carbon nanotubes (MWCNT)/graphene nanoplatelets (GNP)/thermoplastic polyurethane (TPU) conductive polymer composites. Fused filament fabrication (FFF) 3D printing was used to meet the growing demand for flexible electronic devices and overcome limitations with the directional sensitivity of traditional strain sensors. The sensor response exhibited high linearity (R2 = 0.98 at 50% strain), a large strain response range (0–300%), high sensitivity (GF > 10000 at 300% strain), short response time (40 ms), low hysteresis, and good stability (1000 cycles). In addition, due to the special structural design, the sensor identified external forces in different directions. This further expands the application range of the sensor. This work provides valuable guidance for the customized fabrication of flexible strain sensors.

A stretchable array of high-performance electrochromic devices for displaying skin-attached multi-sensor signals

We report on the fabrication of an interactive display system consisting of a stretchable array of electrochromic devices (ECDs) and temperature and strain sensors for visual display of skin temperature and wrist movement. The electrochromic device is prepared by coating poly(3-hexylthiophene) and tungsten trioxide as electrochromic materials on a flexible and transparent electrode of indium-tin-oxide-coated polyethylene terephthalate, and the subsequent assembly of the prepared electrodes using a gel-type electrolyte of Li-mixed 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(methyl methacrylate). The fabricated ECD exhibits three different colors due to the redox reactions of the electrochromic materials at the corresponding potentials: magenta, violet, and blue at −1.5 V, 0.5 V, and 1.5 V, respectively. By using patterned liquid metal Galinstan interconnections, a stretchable array of the ECDs is fabricated based on a strain relieving mechanism with an island-bridge design, resulting in mechanical stability to repeated stretching of up to 30%. A high-performance strain sensor is fabricated by embedding fragmentized graphene foams inside poly(demithylsiloxane) thin film, and a temperature sensor is prepared by embedding multi-walled carbon nanotubes/polyaniline composite in poly(vinyl alcohol) film. With the integrated system, the strain due to wrist bending and the skin temperature measured by the sensors are displayed as changes in the color pattern of the ECD array aided by an Arduino circuit. This work demonstrates the high potential of our system for application to wearable healthcare devices providing real-time visual display of varied bio-signals.