An Ultra-Stretchable and Highly Sensitive Photoelectric Effect-Based Strain Sensor: Implementation and Applications

The fabrication of strain sensors with high sensitivity, large sensing range and excellent stability is highly desirable because of their promising applications in human motion detection, human-machine interface and electric skin, etc. Herein, by introducing a highly conductive silver nanowire (AgNW) layer between two serried silver nanoparticle (AgNP) layers, forming a sandwich structure, a strain sensor with high sensitivity (a large gauge factor of 2.8 × 105), large sensing range (up to 80% strain) and excellent stability (over 1000 cycles) can be achieved. A combination of experimental and mechanism studies shows that the high performance of the obtained strain sensor is ascribed to the synergy of the highly conductive AgNW layer, astatic AgNP layers and the presence of large cracks in stretching. As a proof-of-concept application, the obtained strain sensor can be used for highly effective human motion detection ranging from large scale motions, i.e. kneel bending and wrist flexion, to subtle scale motions, i.e. pulse and swallowing.

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

Stretchable strain sensors suffer the trade-off between sensitivity and linear sensing range. Developing sensors with both high sensitivity and wide linear range remains a formidable challenge. Different from conventional methods that rely on the structure design of sensing nanomaterial or substrate, here a heterogeneous-surface strategy for silver nanowires (AgNWs) and MXene is proposed to construct a hierarchical microcrack (HMC) strain sensor. The heterogeneous surface with distinct differences in cracks and adhesion strengths divides the sensor into two regions. One region contributes to high sensitivity through penetrating microcracks of the AgNW/MXene composite film during stretching. The other region maintains conductive percolation pathways to provide a wide linear sensing range through network microcracks. As a result, the HMC sensor exhibits ultrahigh sensitivity (gauge factor ≈ 244), broad linear range (ɛ = 60%, R2 ≈ 99.25%), and fast response time (<30ms). These merits are confirmed in the detection of large and subtle human motions and digital joint movement for Morse coding. The manipulation of cracks on the heterogeneous surface provides a new paradigm for designing high-performance stretchable strain sensors.

Nanomaterials with low-dimensional morphology have been explored for enhancing the performance of strain sensors, but it remains difficult to achieve high stretchability and sensitivity simultaneously. In this work, a composite structure strain sensor based on nanomaterials and conductive liquid is designed, demonstrated, and engineered. The nanowire-microfluidic hybrid (NMH) strain sensor responds to multiscale strains from 4% to over 400%, with a high sensitivity and durability under small strain. Metal nanowires and carbon nanotubes are used to fabricate the NMH strain sensors, which simultaneously exhibit record-high average gauge factors and stretchability, far better than the conventional nanowire devices. Quantitative modeling of the electrical characteristics reveals that the effective conductivity percolation through the hybrid structures is the key to achieving high gauge factors for multiscale sensing. The sensors can operate at low voltages and are capable of responding to various mechanical deformations. When fixed on human skin, the sensors can monitor large-scale deformations (skeleton motion) and small-scale deformations (facial expressions and pulses). The sensors are also employed in multichannel, interactive electronic system for wireless control of robotics. Such demonstrations indicate the potential of the sensors as wearable detectors for human motion or as bionic ligaments in soft robotics.

With the development of information technology, high-performance wearable strain sensors with high sensitivity and stretchability have played a significant role in motion detection. However, many high-sensitivity and outstanding-stretchability strain sensors possess a limited linear sensing range, which limits the enhancement of the flexible strain sensors' performance. Herein, we develop a hybrid-structured carbon nanotube (CNT)/Ecoflex strain sensor with laser-engraved grooves along with punched circular holes in a composite CNT/Ecoflex film by vacuum filtration and permeation. By optimizing the distribution of grooves and circular holes, the strain in the sensing layer can be locally regulated, which alters the morphology of cracks under strain and allows the hybrid-structured CNT/Ecoflex strain sensor to simultaneously exhibit high sensitivity (GF = 43.8) as well as a wide linear sensing range (200%). On the basis of excellent performance, the hybrid-structured CNT/Ecoflex strain sensor is capable of detecting movements in various parts of the human body, including movements of larynx and joint bending.

Fiber-based flexible sensors have promising application potential in human motion and healthcare monitoring, owing to their merits of being lightweight, flexible, and easy to process. Now, high-performance elastic fiber-based strain sensors with high sensitivity, a large working range, and excellent durability are in great demand. Herein, we have easily and quickly prepared a highly sensitive and durable fiber-based strain sensor by dip coating a highly stretchable polyurethane (PU) elastic fiber in an MXene/waterborne polyurethane (WPU) dispersion solution. Benefiting from the electrostatic repulsion force between the negatively charged WPU and MXene sheets in the mixed solution, very homogeneous and stable MXene/WPU dispersion was successfully obtained, and the interconnected conducting networks were correspondingly formed in a coated MXene/WPU shell layer, which makes the as-prepared strain sensor exhibit a gauge factor of over 960, a large sensing range of over 90%, and a detection limit as low as 0.5% strain. As elastic fiber and mixed solution have the same polymer constitute, and tight bonding of the MXene/WPU conductive composite on PU fibers was achieved, enabling the as-prepared strain sensor to endure over 2500 stretching-releasing cycles and thus show good durability. Full-scale human motion detection was also performed by the strain sensor, and a body posture monitoring, analysis, and correction prototype system were developed via embedding the fiber-based strain sensors into sweaters, strongly indicating great application prospects in exercise, sports, and healthcare.

Wearable sensors have been developed for various healthcare applications with human motion detection. For strain sensors to be adopted in smart wearable devices for human joint motion detection, it is necessary that they have excellent mechanical properties such as stretchability, flexibility, softness, and they should be non-toxic to the human body. In this paper, we report on a possible candidate for such a sensor: the polymer fiber composited with polydimethylsiloxane (PDMS) and Zinc Oxide (ZnO) nanoparticles. This polymer fiber exhibits softness, flexibility, stretchability, and reproducibility. Moreover, it has a tensile property of over 100% and excellent resilience. The strain evaluation of the ZnO-PDMS polymer fiber with silica fibers on the ends was carried out with a light emitting diode (LED) and photodetector. The strain sensor with polymer fiber has good properties of strain measurement; stable signal recovery, as well as rapid, immediate, and repeatable response in real-time. The polymer strain sensor was evaluated on the human finger and wrist joints for detection of the bending motion. Moreover, because this polymer strain sensor is waterproof, it can be used for measuring finger bending and releasing even in wet conditions. The results demonstrate that the ZnO-PDMS optical fiber strain sensor can be employed in wearable devices for healthcare applications such as sports and rehabilitation.

Hollow-porous fiber-shaped strain sensor with multiple wrinkle-crack microstructure for strain visualization and wind monitoring

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.

Conductive polymer composites used to develop stretchable strain sensors have great potential for a wide range of applications, but engineering such a sensor with high sensitivity and durability remains very challenging. In this study, we propose a hydrothermal approach coupled with a freeze-drying technique to fabricate durable and stretchable strain sensors based on polydopamine-functionalized halloysite nanotube/reduced graphene oxide/polydimethylsiloxane (PDA@HNT/rGO/PDMS) aerogel composites. These sensors exhibit exceptional sensing performance, enhanced stretchability, linearity range, and stability. A comparative analysis of graphene oxide at different concentrations demonstrates that the flexible PDA@HNT/rGO/PDMS composites exhibit a significantly broader sensing range when the graphene oxide concentration is reduced to 2.5 mg/mL, in contrast to the higher concentration of 5.0 mg/mL. Specifically, the synergistic effect of both PDA and natural fiber HNTs results in aerogel composite strain sensors with a desirable gauge factor and a linearity sensing range as evidenced by the theoretical analysis, which demonstrates great potential for wearable electronics and human motion detection. The synergy between PDA and HNTs enhances the properties of aerogel composite strain sensors by improving interfacial adhesion, uniformly dispersing reinforcing agents, and maintaining conductive pathways, resulting in a highly sensitive, broad-range, and durable device. The development of conductive PDA@HNT/rGO/PDMS aerogel composites for flexible strain sensors represents an important advancement in the field of wearable technology and has the potential to revolutionize the way we monitor and respond to mechanical stress in various applications.

Tunable porous fiber-shaped strain sensor with synergistic conductive network for human motion recognition and tactile sensing

Facile fabrication of flexible strain sensors with AgNPs-decorated CNTs based on nylon/PU fabrics through polydopamine templates

The flexible cellulose nanocrystal (CNC) strain sensors have gained attention owing to their great promising in human motion detection, electronic skin, and soft robotics. However, the effect of bending on the mechanical performance of the sensor altered by the variation of temperature is a key limitation to their potential. Here, the temperature‐dependent bending fatigue of the CNC‐graphene oxide‐Ag nanoparticles (NPs) strain sensor is systematically investigated, in which the sensitivity of the sensor is attenuated to 20% with the decreasing temperature from 30 to −30 °C after bending over 10 000 times. The finite element studies and theoretical calculations indicate that the interfacial crack can be caused by the stiffness mismatch between the Ag NPs and CNC at different temperatures. Under room temperature, the destruction and recombination of the hydrogen bonds network at the interfacial crack can effectively increase the dissipation of energy and hinder the development of cracks with repetitive bending stress. However, under low temperature, such recombination processes are broken by the formation of ice crystals in the CNC/Ag NPs interfacial cracks. The ice crystals accelerate the crack propagation and eventually make the CNC sensor suffer from deterioration.

&lt;p&gt;Laminated elastomeric bearings are broadly used in large bridges for adjusting the deformation and ensuring the safety of the bridge structure. Interestingly, the bridge bearing’s deformation contains rich information that reflects the complex load conditions of the bridge structure. Nevertheless, the structural health monitoring (SHM) of the bridge bearings has not been carried out effectively due to the special characteristics of bridge bearings’ deformation, including the large range and the existence of varied forms. Along these lines, a novel conductive polymer composite-based flexible strain sensor was fabricated by employing a facile solution-blending method. More specifically, the proposed strain sensor was composed of thermoplastic polyurethane (TPU), multiwall carbon nanotubes (MCNTs), and silane coupling agent KH550. The utilization of the KH550 not only promoted the dispersion of MCNTs within the TPU matrix but also enhanced the interfacial bonding between the MCNTs and the TPU matrix, resulting in the large strain range (~150%) of the silanized MCNT/TPU strain sensor. More specifically, the silanized strain sensor with 3.0 wt% MCNTs demonstrated extraordinary linearity, promising gauge factor (~ 8.12), and great durability under the enforcement of a tensile strain up to the value of 150% through the application of consecutive stretch–release tests. On top of that, the compressive bending sensing tests proved the stable and repeatable bending sensing capability of the proposed silanized strain sensor. Moreover, the silanized strain sensor could effectively monitor the shear and the compressive deformation of the laminated elastomeric bridge bearings. This work provides a new way for fabricating flexible strain sensors with enhanced strain range, which validates the feasibility of the strain sensor for monitoring the bearing’s deformation and provides an experimental basis for developing the SHM of the laminated elastomeric bridge bearings.&lt;/p&gt;

An adhesive, highly stretchable and low-hysteresis alginate-based conductive hydrogel strain sensing system for motion capture