Sensitivity Improvement of Stretchable Strain Sensors by the Internal and External Structural Designs for Strain Redistribution

3D Printed MXene-Based Wire Strain Sensors with Enhanced Sensitivity and Anisotropy

Stretchable strain sensors play a crucial role in intelligent wearable systems, serving as the interface between humans and environment by translating mechanical strains into electrical signals. Traditional fiber strain sensors with intrinsic uniform axial strain distribution face challenges in achieving high sensitivity and anisotropy. Moreover, existing micro/nano-structure designs often compromise stretchability and durability. To address these challenges, a novel approach of using 3D printing to fabricate MXene-based flexible sensors with tunable micro and macrostructures.  Poly(tetrafluoroethylene) (PTFE) as a pore-inducing agent is added into 3D printable inks to achieve controllable microstructural modifications. In addition to microstructure tuning, 3D printing is employed for macrostructural design modifications, guided by finite element modeling (FEM) simulations. As a result, the 3D printed sensors exhibit heightened sensitivity and anisotropy, making them suitable for tracking static and dynamic displacement changes. The proposed approach presents an efficient and economically viable solution for standardized large-scale production of advanced wire strain sensors.

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.

Flexible Sensors with a Multilayer Interlaced Tunnel Architecture for Distinguishing Different Strains

The diversity of body joints and the complexity of joint motions cause flexible strain sensors to undergo complex strains such as stretching, compression, bending, and extrusion, which results in sensors that do not recognize different strains, facing great challenges in detecting the true motion characteristics of joints. Here, the monitoring of body joints' real motion characteristics has been realized by the sensor that can output response signals with different resistance trends for different strains. The sensor prepared by the sacrificial template method is characterized by a multilayered interlaced tunnel architecture and carbon black embedded in the inner wall of the tunnel. Stretching, compressive, and bending strains result in increasing, decreasing, and increasing resistance, followed by a decrease in resistance of the sensor, respectively. The sensor can still output distinguishable response signals, even in the presence of complex strains induced by squeezing. Low strain detection limits (0.03%) and wide detection ranges (>600%) are achieved due to the localized strain enhancement caused by the unique structure. The sensor can detect the motion characteristics of different joints in flexion-extension, abduction-adduction, and internal-external rotation, which, in turn, can be used for real-time monitoring of complex joint motions involved in limb rehabilitation. In addition, the sensor recognizes the 26 letters of the alphabet represented by sign language gestures. The above studies demonstrate the potential application of our prepared sensors in flexible, wearable devices.

Fabrication and characterization of highly sensitive flexible strain sensor based on biodegradable gelatin nanocomposites and double strain layered structures with crack for gesture recognition

Flexible sensors have attracted extensive attention in the field of human-computer interaction. However, it is still a challenging task to realize accuracy gesture recognition with flexible sensor, which requires sensor not only have high sensitivity, but also have appropriate strain detection range. Here, a high gauge factor flexible sensor (gauge factor ∼ 1296 under 12-20 % strain) based on crack structure is reported. The sensor is made of a biodegradable and stretchable gelatin composite combined with fabric bases, with good repeatability (6000 cycles) and a fast response (60 ms). Because of the double-layer structure, it has a suitable detection range (20 % strain). The sensor is manufactured by a screen-printing process, and it has been used to make data gloves and has realized 9 gestures recognition with machine learning algorithm (99.6 % accuracy). In general, this study offers a wearable gestures recognition scheme through the proposed sensor.

From Triboelectric Nanogenerator to Multifunctional Triboelectric Sensors: A Chemical Perspective toward the Interface Optimization and Device Integration

Triboelectric nanogenerators (TENGs) have intrigued scientists for their potential to alleviate the energy shortage crisis and facilitate self-powered sensors. Triboelectric interfaces containing triboelectric functionalized molecular groups and tunable surface charge densities are important for improving the electrical output capability of TENGs and the versatility of future electronics. In this review, following an introduction to the fundamental progress of TENG systems for mechanic energy harvesting, surface modifications that aim to increase the surface charge density and functionality are highlighted, with an emphasis on interfacial chemical modification and triboelectric energetics/dynamics optimization for efficient electrostatic induction and charge transfer. Recent advances in assemblies of multifunctional triboelectric sensing are briefly introduced, and future challenges and chemical perspectives in the field of TENG-based electronics are concisely reviewed. This review presents and advances the understanding of the state-of-the-art chemical strategies toward rational triboelectric interface engineering and system assembly and is expected to guide the rational design of highly efficient and versatile triboelectric sensing.

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.

A Mini-Review on Preparation of Functional Composite Fibers and Their Based Devices

Composite fibers are composed of two or more different components by functionating, coating or direct spinning, enabling unique characteristics, such as design ability, high strength, and high- and low-temperature resistance. Due to their ability to be directly woven into or stitched onto textiles to prepare flexible electronic devices, stretchable composite fibers have drawn great attention, enabling better wearability and integrality to wearable devices. Fiber or fiber-based electronic film or textiles represent a significant component in wearable technology, providing the possibility for portable and wearable electronics in the future. Herein, we introduce the composite fiber together with its preparation and devices. With the advancement of preparation technology, the as-prepared composite fibers exhibit good performance in various applications closely related to human life. Moreover, a simple discussion will be provided based on recent basic and advanced progress on composite fibers used in various devices.

Programmable Sensitivity Screening of Strain Sensors by Local Electrical and Mechanical Properties Coupling

Owing to the canonical trade-off between the gauge factor and the working range, there is an emergent need for strain sensors with customizable sensitivity for various applications of different deformation ranges. However, current optimization strategies typically allow possessing either, not both, high-sensing performance or customizable sensing performance. Here, a laser-programmed heterogeneous strain sensor featured locally coupled electrical and mechanical properties (named an LCoup sensor) is developed to access customized sensor performance. Coupled electromechanical properties enable the applied strain to be mainly experienced by the higher sensitivity regions when stretched. By optimizing the parameters of laser processes, the gauge factor can systematically screen within 2 orders of magnitude (from 7.8 to 266.6) while maintaining good stretchability (50%). To prove the potential in human-machine interaction, the real-time monitoring and recognition of set hand gestures (left-click, right-click, and double-click) are demonstrated, representing the traditional input patterns of the computer mouse. Multiscale programming of material properties can further achieve excellent and tailored device performances, offering more opportunities for the design of a broad range of flexible electronics.