Flexible and Anisotropic Strain Sensors with the Asymmetrical Cross-Conducting Network for Versatile Bio-Mechanical Signal Recognition

Plasma-Induced Wrinkle-Crack Dual Structure for Robust Directional Strain Sensing in Dynamic Motion Perception

Flexible multi-directional sensors hold vast potential for complex application scenarios, yet creating sensitive, stable, and linear strain sensors capable of flexibly detecting multi-directional strain remains a significant challenge. Here, a plasma-induced wrinkle-crack dual structure is introduced for multi-directional force detection of high-reliability flexible strain sensors. By combining pre-stretching and oxidative plasma bombardment, a multi-layer structure exhibits film stress engineering with varying elastic moduli established on the surface of the elastomer. The rigid silicon oxide hardened layer induces shear film stress at the pre-stretched interface. Periodic wrinkles are generated upon release from pre-stretching, which not only suppresses crack propagation but also significantly enhances the linearity between the signal and applied force, thanks to the conductive network's wrinkles. By adjusting the direction of pre-stretching under plasma treatment, the morphology and orientation of the wrinkles on the conductive sensitive layer can be effectively controlled. The coexisting parallel wrinkle and perpendicular crack structures impart anisotropic properties, significantly improving the sensor's directional detection capabilities. With a high gauge factor (GF = 454), excellent cyclic durability (over 100 000 cycles), and multi-directional force detection capability, this sensor demonstrates promising applications in wearable electronics and robot motion detection, positioning it as a next-generation flexible strain sensor.

Research Progress of Flexible Electronic Devices Based on Electrospun Nanofibers

Electrospun nanofibers have become an important component in fabricating flexible electronic devices because of their permeability, flexibility, stretchability, and conformability to three-dimensional curved surfaces. This review delves into the advancements in adaptable and flexible electronic devices using electrospun nanofibers as the substrates and explores their diverse and innovative applications. The primary development of key substrates for flexible devices is summarized. After briefly discussing the principle of electrospinning, process parameters that affect electrospinning, and two major electrospinning techniques (i.e., single-fluid electrospinning and multifluid electrospinning), the review shines a spotlight on the recent breakthroughs in multifunctional and stretchable electronic devices that are based on electrospun substrates. These advancements include flexible sensors, flexible energy harvesting and storage devices, flexible accessories for electronic devices, and flexible environmental monitoring devices. In particular, the review outlines the challenges and potential solutions of developing electrospun nanofibers for flexible electronic devices, including overcoming the incompatibility of multiple interfaces, developing 3D microstructure sensor arrays with gradient geometry for various imperceptible on-skin devices, etc. This review may provide a comprehensive understanding of the rational design of application-oriented flexible electronic devices based on electrospun nanofibers.

Deep learning neural network-assisted badminton movement recognition and physical fitness training optimization

This work aims to solve the problem of low accuracy in recognizing the trajectory of badminton movement. This work focuses on the visual system in badminton robots and conducts side detection and tracking of flying badminton in two-dimensional image plane video streams. Then, the cropped video images are input into a convolutional neural network frame by frame. By adding an attention mechanism, it helps identify the badminton movement trajectory. Finally, to address the detection challenge of flying badminton as a small target in video streams, the deep learning one-stage detection network, Tiny YOLOv2, is improved from both the loss function and network structure perspectives. Moreover, it is combined with the Unscented Kalman Filter algorithm to predict the trajectory of badminton movement. Simulation results show that the improved algorithm performs excellently in tracking and predicting badminton trajectories compared with the existing algorithms. The average accuracy of the proposed method for tracking badminton trajectories is 91.40 %, and the recall rate is 84.60 %. The average precision, recall, and frame rate of the measured trajectories in four simple and complex scenarios of badminton flight video streams are 96.7 %, 95.7 %, and 29.2 frames/second, respectively. They are all superior to other classic algorithms. It is evident that the proposed method can provide powerful support for badminton trajectory recognition and help improve the accuracy of badminton movement recognition.

Temperature-Switching Flexible Strain Sensors Based on Vanadium Dioxide for Intelligent Packaging Applications

Flexible sensors are promising for intelligent packaging and artificial intelligence, but the required multistimulus response is still a challenge in external environments. A candidate material for such multistimulus response is VO2 due to its unique semiconducting properties. Herein, W-doped VO2(M) with a tunable phase transition temperature was prepared by the hydrothermal method, and then, VO2(M)-based flexible sensors were fabricated employing a direct-write strategy, where conductive inks with VO2(M) powders were patterned onto various substrates. These sensors achieve dual responses to temperature and strain and exhibit high stability (over 2000 stretch-release cycles) to accurately monitor various statuses (opening and closing, temperature changes, etc.) of intelligent packaging. The spatial pressure distribution of different objects was discerned by the prepared VO2(M)/poly(dimethylsiloxane) (PDMS) sponge flexible pressure sensor arrays, and the information was successfully edited using the Morse code. The sensing signals from the intelligent packaging were collected and remotely transmitted to intelligent terminals via a wireless local-area network to achieve real-time monitoring of the packaged contents. Therefore, in this work, we not only designed new flexible sensors with multiple stimulus responses but also demonstrated the potential applications of W-doped VO2(M)-based flexible sensors in intelligent packaging.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

Highly Stretchable and Fluorescent Visualizable Thermoplastic Polyurethane/Tetraphenylethylene Plied Yarn Strain Sensor with Heterogeneous and Cracked Structure for Human Health Monitoring

Smart wearable technology has been more and more widely used in monitoring and prewarning of human health and safety, while flexible yarn-based strain sensors have attracted extensive research interest due to their ability to withstand greater external strain and their significant application potential in real-time monitoring of human motion and health signals. Although several strain sensors based on yarn structures have been reported, it remains challenging to strike a balance between high sensitivity and wide strain ranges. At the same time, visual signal sensing is expected to be used in strain sensors thanks to its intuitiveness. In this work, thermoplastic polyurethane (TPU) and tetraphenylethylene (TPE) were wet-spun to fabricate flexible fluorescent fibers used as the substrate of the sensor, followed by the drop addition of polydimethylsiloxane (PDMS) beads and curing to produce a heterogeneous structure, which were further twisted into a plied yarn. Finally, a visualizable flexible yarn strain sensor based on solidified liquid beads and crack structure was obtained by loading polydopamine (PDA) and polypyrrole (PPy) in situ. The sensor exhibited high sensitivity (the GF value was 58.9 at the strain range of 143-184%), a wide working strain range (0-184%), a low monitoring limit (<0.1%), a fast response (58.82 ms), reliable responses at different frequencies, and excellent cycle durability (over 2000 cycles). At the same time, the yarn strain sensor also had excellent photothermal characteristics and a fluorescence crack visualization effect. These attractive advantages enabled yarn strain sensors to accurately monitor various human activities, showing great application potential in health monitoring, personalized medical diagnosis, and other aspects.

A Selective-Response Hypersensitive Bio-Inspired Strain Sensor Enabled by Hysteresis Effect and Parallel Through-Slits Structures

Flexible strain sensors are promising in sensing minuscule mechanical signals, and thereby widely used in various advanced fields. However, the effective integration of hypersensitivity and highly selective response into one flexible strain sensor remains a huge challenge. Herein, inspired by the hysteresis strategy of the scorpion slit receptor, a bio-inspired flexible strain sensor (BFSS) with parallel through-slit arrays is designed and fabricated. Specifically, BFSS consists of conductive monolayer graphene and viscoelastic styrene-isoprene-styrene block copolymer. Under the synergistic effect of the bio-inspired slit structures and flexible viscoelastic materials, BFSS can achieve both hypersensitivity and highly selective frequency response. Remarkably, the BFSS exhibits a high gage factor of 657.36, and a precise identification of vibration frequencies at a resolution of 0.2 Hz through undergoing different morphological changes to high-frequency vibration and low-frequency vibration. Moreover, the BFSS possesses a wide frequency detection range (103 Hz) and stable durability (1000 cycles). It can sense and recognize vibration signals with different characteristics, including the frequency, amplitude, and waveform. This work, which turns the hysteresis effect into a "treasure," can provide new design ideas for sensors for potential applications including human-computer interaction and health monitoring of mechanical equipment.

Anisotropic and Highly Sensitive Flexible Strain Sensors Based on Carbon Nanotubes and Iron Nanowires for Human-Computer Interaction Systems

Flexible strain sensors for multi-directional strain detection are crucial in complicated hman-computer interaction (HCI) applications. However, enhancing the anisotropy and sensitivity of the sensors for multi-directional detection in a simple and effective method remains a significant issue. Therefore, this study proposes a flexible strain sensor with anisotropy and high sensitivity based on a high-aspect-ratio V-groove array and a hybrid conductive network of iron nanowires and carbon nanotubes (Fe NWs/CNTs). The sensor exhibits significant anisotropy, with a difference in strain detection sensitivity of up to 35.92 times between two mutually perpendicular directions. Furthermore, the dynamic performance of the sensor shows a good response rate, ranging from 223 ms to 333 ms. The sensor maintains stability and consistent performance even after undergoing 1000 testing cycles. Additionally, the constructed flexible strain sensor is tested using the remote control application of a trolley, demonstrating its high potential for usage in practical HCI systems. This research offers a significant competitive advantage in the development of flexible strain sensors in the field of HCI.

Ultra-sensitive, Multi-directional Flexible Strain Sensors Based on an MXene Film with Periodic Wrinkles

The fast-growing motion capturing/monitoring technique has raised a great demand for flexible strain sensors. For capturing complex motions (e.g., facial motion), both the strain amplitude and direction should be accurately detected. Although some reported sensors based on anisotropic conductive networks are proved to show accurate localization of strain directions, it is still a great challenge to achieve both high sensitivity and a high sensing range in these designs. Here, a self-assembled Ti₃C₂T_x MXene film with parallel and periodic wrinkles is fabricated on a stretchable poly(dimethylsiloxane) substrate for constructing multi-directional strain sensors. During stretching, relative slip and crack will occur between the stacked MXene nanosheets, which contribute to high structural sensitivity in the MXene film. Meanwhile, the wrinkled structure contributes to high stretchability. As a result, the sensor based on the film with one-dimensional periodic wrinkles shows a large sensing range (>50%) and a gauge factor of 45. Furthermore, the sensor can accurately detect both the strain amplitude and direction by using the MXene film with two-dimensional wrinkles. It shows distinguishable electrical responses when detecting different-amplitude human/robot motions such as joint bending and walking. Additionally, the directions in complex human motions (e.g., facial motion) can also be well-tracked. This work provides an effective strategy to detect multi-directional motions.

From Piezoelectric Nanogenerator to Non-Invasive Medical Sensor: A Review

Piezoelectric nanogenerators (PENGs) not only are able to harvest mechanical energy from the ambient environment or body and convert mechanical signals into electricity but can also inform us about pathophysiological changes and communicate this information using electrical signals, thus acting as medical sensors to provide personalized medical solutions to patients. In this review, we aim to present the latest advances in PENG-based non-invasive sensors for clinical diagnosis and medical treatment. While we begin with the basic principles of PENGs and their applications in energy harvesting, this review focuses on the medical sensing applications of PENGs, including detection mechanisms, material selection, and adaptive design, which are oriented toward disease diagnosis. Considering the non-invasive in vitro application scenario, discussions about the individualized designs that are intended to balance a high performance, durability, comfortability, and skin-friendliness are mainly divided into two types: mechanical sensors and biosensors, according to the key role of piezoelectric effects in disease diagnosis. The shortcomings, challenges, and possible corresponding solutions of PENG-based medical sensing devices are also highlighted, promoting the development of robust, reliable, scalable, and cost-effective medical systems that are helpful for the public.

Adaptive Triboelectric Nanogenerators for Long-Term Self-Treatment: A Review

Triboelectric nanogenerators (TENGs) were initially invented as an innovative energy-harvesting technology for scavenging mechanical energy from our bodies or the ambient environment. Through adaptive customization design, TENGs have also become a promising player in the self-powered wearable medical market for improving physical fitness and sustaining a healthy lifestyle. In addition to simultaneously harvesting our body's mechanical energy and actively detecting our physiological parameters and metabolic status, TENGs can also provide personalized medical treatment solutions in a self-powered modality. This review aims to cover the recent advances in TENG-based electronics in clinical applications, beginning from the basic working principles of TENGs and their general operation modes, continuing to the harvesting of bioenergy from the human body, and arriving at their adaptive design toward applications in chronic disease diagnosis and long-term clinical treatment. Considering the highly personalized usage scenarios, special attention is paid to customized modules that are based on TENGs and support complex medical treatments, where sustainability, biodegradability, compliance, and bio-friendliness may be critical for the operation of clinical systems. While this review provides a comprehensive understanding of TENG-based clinical devices that aims to reach a high level of technological readiness, the challenges and shortcomings of TENG-based clinical devices are also highlighted, with the expectation of providing a useful reference for the further development of such customized healthcare systems and the transfer of their technical capabilities into real-life patient care.

Structural Flexibility in Triboelectric Nanogenerators: A Review on the Adaptive Design for Self-Powered Systems

There is an increasing need for structural flexibility in self-powered wearable electronics and other Internet of Things (IoT), where adaptable triboelectric nanogenerators (TENGs) play a key role in realizing the true potential of IoT by endowing the latter with self-sustainability. Thus, in this review, the topic was restricted to the adaptive design of TENGs with structural flexibility that aims to promote the sustainable operation of various smart electronics. This review begins with an emphatical discussion of the concept of flexible electronics and TENGs, and continues with the introduction of TENG-based self-powered intelligent systems while placing the emphasis on self-powered flexible intelligent devices. Self-powered healthcare sensors, e-skins, and other intelligent wearable electronics with enhanced intelligence and efficiency in practical applications due to the integration with TENGs are illustrated, along with an emphasis on the design strategy of structural flexibility of TENGs and the associated integration schemes. This review aims to cover recent achievements in the field of self-powered systems, and provides information on how flexibility or adaptability in TENGs can be adopted, their types, and why they are required in promoting advanced IoT applications with sustainability and intelligence algorithms.

Serpentine-inspired Strain Sensor with Predictable Cracks for Remote Bio-Mechanical Signal Monitoring

The flexible strain sensors have attracted intense interests due to their application as intelligent wearable electronic devices. However, it is still a huge challenge to achieve the flexible sensor with simultaneous high sensitivity, excellent durability and wide sensing region. In this work, a crack-based strain sensor with paired-serpentine conductive network is fabricated onto flexible film by screen printing. The innovative conductive network exhibits a controlled crack morphology during stretching, which endows the prepared sensor with outstanding sensing characteristics, including the high sensitivity (gauge factor up to 2391.5), wide detection (rang up to 132%), low strain detection limit, fast response time (about 40 ms), as well as excellent durability (more than 2000 stretching/releasing cycles). Benefiting from these excellent performances, full-range human body motions including subtle physiological signals and large motions are accurately detected by the prepared sensor. Besides, wearable electronic equipment integrated with wireless transmitter and the prepared strain sensor shows great potential for remote motion monitoring and intelligent mobile diagnosis for humans. This work provides an effective strategy for the fabrication of the novel strain sensors with highly comprehensive performance. This article is protected by copyright. All rights reserved.

Wearable RGO/MXene Piezoresistive Pressure Sensors with Hierarchical Microspines for Detecting Human Motion

Flexible piezoresistive pressure sensors may exhibit excellent sensing performances to be applied in wearable electronics, medical diagnosis, and electronic skin. Herein, we report a multi-layer and phased-responsive reduced graphene oxide/MXene-based piezoresistive pressure sensor with hierarchical microspines constructed by sandpaper as the template. Thanks to the multi-level and multi-layer structure, the obtained sensor realized phased response and showed wide detection range (up to 70 kPa), fast response (response/recovery time of 40/80 ms), and excellent working stability (1000 fatigue cycles). Furthermore, the sensor was successfully applied for detecting various human motions including pulse beats, cheek bulging, nodding, finger bending, speech recognition, handwriting, and other pressure signals. Besides, a 6 × 6 sensing matrix integrated by the sensors was able to sensitively perceive the distribution of plane pressure. The findings in this work conceivably stand out as a new strategy to fabricate high-performance piezoresistive pressure sensors in the fields of intelligent healthcare and medical diagnosis, wearable electronic devices, electronic skin, and human-machine interaction.

Ultrasensitive, Highly Stable, and Flexible Strain Sensor Inspired by Nature

For advanced flexible strain sensors, it is not difficult to achieve high sensitivity only. However, integrating high sensitivity, high stability, and high durability into one sensor still remains a great challenge. Fortunately, natural creatures with diversified excellent performances have given us a lot of ready-made solutions. Here, scorpion and spiderweb are selected as coupling bionic prototypes, which are famous for their ultrasensitive sensing capacity and excellent structural durability, respectively. Based on that, a bioinspired strain sensor is successfully fabricated. The results demonstrate that the bioinspired strain sensor has a sensitivity of 940.5 in the strain range of 0-1.5% and a sensitivity of 2742.3 between 1.5 and 2.5%. Meantime, this sensor with a spiderweb-like reticular structure has a great improvement in stability and durability. Specifically, the sensor exhibits excellent stability during bending and stretching cycles over 80,000 times. Moreover, the response time and recovery time of the sensor are 169 and 195 ms, respectively. Besides, the sensor also has functions such as vibrating frequency identification due to its low hysteresis. Based on the excellent performance, the sensor can be applied to monitor human body motions serving as wearable electronics.