High-performance Flexible Sensors Research Articles

Flexible micro-strain graphene sensors enhanced by laser-induced cracks for health monitoring

In-situ fabrication of patterned laser-induced graphene (LIG) has unique advantages, holding broad application prospects in the preparation of flexible electronic devices. However, a facile strategy is still demanded to improve the sensitivity of LIG-based sensors in detecting micro-strains. Herein, we propose a flexible graphene strain sensor enhanced by laser-induced cracks, which is capable of detecting small deformations. When the scanning spacing was expanded from 100 μm to 140 μm, abundant micro-cracks formed between adjacent LIG stripes, providing more sensing units. The open and close of cracks lead to significant change of resistance, enhancing the sensitivity of LIG-based sensors. The average gauge factors of sensors were improved to 98 (0–2 % strain), increasing nearly 20 times. The strain sensors exhibit high performance response to subtle signals of the human body. High-quality seismocardiography (SCG), pulse wave and respiration curves were achieved by our sensors, providing a lot of useful information for screening chronic diseases. Overall, this research highlights an effective method for fabricating high performance flexible strain sensors for long-term health monitoring.

Synergistic Etching and Hydrogen Bonding-Induced Self-Assembly of MXene/MOF Hybrid Aerogel for Flexible Room-Temperature Gas Sensing.

Limited by insufficient active sites and restricted mechanical strength, designing reliable and wearable gas sensors with high activity and ductility remains a challenge for detecting hazardous gases. In this work, a thermally induced and solvent-assisted oxyanion etching strategy was implemented for selective pore opening in a rigid microporous Cu-based metal-organic framework (referred to as CuM). A conductive CuM/MXene aerogel was then self-assembled through cooperative hydrogen bonding interactions between the carbonyl oxygen atom in PVP grafted on the surface of defect-rich Cu-BTC and the surface functional hydroxyl group on MXene. A flexible NO2 sensing performance using the CuM/MXene aerogel hybridized sodium alginate hydrogel is finally achieved, demonstrating extraordinary sensitivity (S = 52.47 toward 50 ppm of NO2), good selectivity, and rapid response/recovery time (0.9/4.5 s) at room temperature. Compared with commercial sensors, the relative error is less than 7.7%, thereby exhibiting significant potential for application in monitoring toxic and harmful gases. This work not only provides insights for guiding rational synthesis of ideal structure models from MOF composites but also inspires the development of high-performance flexible gas sensors for potential multiscenario applications.

MXene, protein, and KCl-assisted ionic conductive hydrogels with excellent anti-freezing capabilities, self-adhesive, ultra-stretchability, and remarkable mechanical properties for a high-performance wearable flexible sensor.

Developing a hydrogel with switchable features and freeze tolerance is remarkably significant for designing flexible electronics to adjust various application needs. Herein, MXenes, AFPs (antifreeze proteins), and potassium chloride (KCl) were introduced to a polyacrylamide (PAM) polymer network to design an anti-freezing hydrogel. The ionic hydrogels are characterized by excellent ionic conductivity, presenting adjustable properties of remarkable mechanical strength and self-adhesion to meet individualized application demands. The capability of KCl and AFPs to inhibit ice crystals gives hydrogels with anti-icing properties under a low-temperature atmosphere. The PAM/MXene15/AFP30/KCl15 hydrogel demonstrated negligible hysteresis behavior, quick electromechanical response (0.10 s), and excellent sensitivity (gauge factor (GF) = 13.1 within the strain range of 1200-2000%). The resulting hydrogel could be immobilized on the animal or human skin to detect different body movements and physiological motions, offering reproducibility and precise accuracy as primary advantages. The approach of developing materials with tunable features, along with inorganic salt and the fish-inspired freeze-tolerance method, offers a new prospect for wearable gadgets.

Flexible micro-strain graphene sensors enhanced by laser-induced cracks for health monitoring

In-situ fabrication of patterned laser-induced graphene (LIG) has unique advantages, holding broad application prospects in the preparation of flexible electronic devices. However, a facile strategy is still demanded to improve the sensitivity of LIG-based sensors in detecting micro-strains. Herein, we propose a flexible graphene strain sensor enhanced by laser-induced cracks, which is capable of detecting small deformations. When the scanning spacing was expanded from 100 μm to 140 μm, abundant micro-cracks formed between adjacent LIG stripes, providing more sensing units. The open and close of cracks lead to significant change of resistance, enhancing the sensitivity of LIG-based sensors. The average gauge factors of sensors were improved to 98 (0–2 % strain), increasing nearly 20 times. Furthermore, the sensor exhibits excellent cyclic stability, remaining stable throughout nearly 5000 successive stretch-release cycles. The strain sensors exhibit high performance response to subtle signals of the human body. High-quality seismocardiography (SCG), pulse wave and respiration curves were achieved by our sensors, providing a lot of useful information for screening chronic diseases. Overall, this research highlights an effective method for fabricating high performance flexible strain sensors for long-term health monitoring.

Dual network hydrogel sensors based on borate ester bond: Multifunctional performance in high conductivity, stretchability, mechanical strength, and anti-freezing properties

Conductive polymer hydrogels have attracted great attention due to their application prospects in the fields of health care monitoring, wearable electronics and soft robots. Developing hydrogel sensors with robust mechanical characteristics, outstanding sensing abilities, the ability to self-heal and adhere effectively is critical, yet it remains a major challenge. Here, we present a simple one-pot method to prepare a dual-network flexible hydrogel based on polyvinyl alcohol (PVA), acrylamide (AM), pyridine-4-boronic acid derivatives (PBAD), and lithium chloride (LiCl). The cross-linked network is established through reversible and dynamic borate ester bonds, hydrogen bonds, electrostatic interactions and coordination bonds, imparting outstanding mechanical and self-healing characteristics to the material. The incorporation of PBAD not only imparts high conductivity (9.84 mS·cm−1) and sensitive sensing performance (GF = 13.89) to the hydrogel, but also synergistically improves its elongation (1689 %), tensile strength (0.6 MPa), and toughness (5.74 MJ/m3). In addition, based on the participation of LiCl, the hydrogel shows outstanding anti-freezing properties. This work is expected to provide new ideas for the assembly of high performance flexible sensor and to expand their application in electronic skin, wearable devices, and soft actuators.

An Ionic Assisted Enhancement Strategy Enabled High Performance Flexible Pressure-Temperature Dual Sensor.

Flexible pressure sensors with a broad range and high sensitivity are greatly desired yet challenging to build. Herein, we have successfully fabricated a pressure-temperature dual sensor via an ionic assisted charge enhancement strategy. Benefiting from the immobilization effect for [EMIM+] [TFSI-] ion pairs and charge transfer between ionic liquid (IL) and HFMO (H10Fe3Mo21O51), the formed IL-HFMO-TPU pressure sensor shows a high sensitivity of 25.35 kPa-1 and broad sensing range (∼10 MPa), respectively. Furthermore, the sensor device exhibits high durability and stability (5000 cycles@1 MPa). The IL-HFMO-TPU sensor also shows the merit of good temperature sensing properties. Attributed to these superior properties, the proposed sensor device could detect pressure in an ultrawide sensing range (from Pa to MPa), including breathe and biophysical signal monitoring etc. The proposed ionic assisted enhancement approach is a generic strategy for constructing high performance flexible pressure-temperature dual sensor.

Nature-inspired wood-like TPU/CB aerogels for high performance flexible strain sensors

Strain sensors based on porous conductive polymers (CPCs) have garnered growing research interest for their potential applications in motion detection, healthcare, human–computer interaction, and artificial intelligence. However, the complexity of CPC processing makes it difficult to achieve the controlled design of microscopic porous structures, leading to simple and random porous structures, thus limiting their further use in the field of pressure sensing. This paper presents a strain sensor with a high-performance, wood-like structure composed of flexible conductive carbon black/plastic polyurethane foam (BWCT) using a bidirectional freeze casting process. The results show that, compared with conventional random freezing and unidirectional freezing, the bidirectional freeze casting process can effectively realize multiscale control of the composite structure, which results in a good laminar porous structure of the prepared BWCT. This parallel laminar structure not only contributes to the layered transfer of stresses but also avoids the local concentration of stresses. At the same time, it significantly increases the directional electrical conduction ability, which results in high sensing stability performance. In particular, the BWCT sensors had a wide detection range (80%), a lower limit of detection (0.2%), rapid response and relaxation times (200 ms), as well as exceptional durability (>2000 cycles). Furthermore, the BWCT was integrated into a wearable sensor to monitor various human motions, including arm bending, squatting, and walking, demonstrating reliable detection performance. Altogether, the BWCT sensors are promising in expanding the application but also offer guidance for designing high-performance wearable strain sensors.

High-performance flexible piezoelectric sensors based on honeycomb graphene macro-film exploiting auxetic mechanical property

Flexible piezoelectric sensors have received widespread attention in wearable electronics for their ability to convert mechanical energy into electrical energy, while the insufficient output power and complex preparation process have limited their application. In this work, several honeycomb structures based on graphene macro-film (GAMF) were created via laser engraving, which can utilize negative Poisson’s ratio (NPR) properties to improve the output electric performance of flexible piezoelectric sensors. The effect of different electrode materials on the piezoelectric output was investigated in the experiments and compared with the theoretical equations, indicating that the output power of the sensor could be enhanced up to 142 % by improving the materials auxetic mechanical properties. The stability tests have proven that the sensors can operate effectively for a long time under different operating conditions. Moreover, the optimized sensor shows powerful potential for monitoring human movements such as finger tapping, joint bending, and foot stepping. This work opens up a new development path for the preparation of convenient and high-performance wearable electronic devices.

Solution-Processed Flexible Temperature Sensor Array for Highly Resolved Spatial Temperature and Tactile Mapping Using ESN-Based Data Interpolation.

High-performance flexible temperature sensors are crucial in various technological applications, such as monitoring environmental conditions and human healthcare. The ideal characteristics of these sensors for stable temperature monitoring include scalability, mechanical flexibility, and high sensitivity. Moreover, simplicity and low power consumption will be essential for temperature sensor arrays in future integrated systems. This study introduces a solution-based approach for creating a V2O5 nanowire network temperature sensor on a flexible film. Through optimization of the fabrication conditions, the sensor exhibits remarkable performance, sustaining long-term stability (>110 h) with minimal hysteresis and excellent sensitivity (∼-1.5%/°C). In addition, this study employs machine learning techniques for data interpolation among sensors, thereby enhancing the spatial resolution of temperature measurements and adding tactile mapping without increasing the sensor count. Introducing this methodology results in an improved understanding of temperature variations, advancing the capabilities of flexible-sensor arrays for various applications.

Sandwich-Like Flexible Breathable Strain Sensor with Tunable Thermal Regulation Capability for Human Motion Monitoring.

High-performance flexible strain sensors with synergistic and outstanding thermal regulation function are poised to make a significant impact on next-generation multifunctional sensors. However, it has long been intractable to optimize the sensing performance and high thermal conductivity simultaneously. Herein, a novel flexible sandwich-like strain sensor with advanced thermal regulation capability was prepared by assembling electrospun thermoplastic polyurethane (TPU) fibrous membrane, MXene layer, and TPU/boron nitride nanosheet (BNNS) composite films. The as-prepared sensor demonstrates a wide strain working range (∼100% strain), an ultrahigh gauge factor (2080.9), and a satisfactory reliability. Meanwhile, benefiting from the uniform dispersion and promising orientation of BNNSs in TPU composites, the sensor possesses a high thermal conductivity of 1.5 W·m-1·K-1, guaranteeing wearer comfort. Additionally, the unique structure endows the sensor with high stretchability, breathability, biocompatibility, and tunable electromagnetic interference shielding performances. Furthermore, an integrated wireless motion monitoring device based on this sensor is rationally designed. It exhibits a fast response time, a wide recognition range, and the ability to maintain skin temperature during prolonged physical activity. These encouraging findings provide a new and feasible approach to designing high-performance and versatile flexible strain sensors with broad applications in advanced wearable technology.

Two-Stage Micropyramids Enhanced Flexible Piezoresistive Sensor for Health Monitoring and Human-Computer Interaction.

High-performance flexible piezoresistive sensors are becoming increasingly essential in various novel applications such as health monitoring, soft robotics, and human-computer interaction. The evolution of the interfacial contact morphology determines the sensing properties of piezoresistive devices. The introduction of microstructures enriches the interfacial contact morphology and effectively boosts the sensitivity; however, the limited compressibility of conventional microstructures leads to rapid saturation of the sensitivity in the low-pressure range, which hinders their application. Herein, we present a flexible piezoresistive sensor featuring a two-stage micropyramid array structure, which effectively enhances the sensitivity while widening the sensing range. Owing to the synergistic enhancement effect resulting from the sequential contact of micropyramids of various heights, the devices demonstrate remarkable performance, including boosting sensitivity (30.8 kPa-1) over a wide sensing range (up to 200 kPa), a fast response/recovery time (75/50 ms), and an ultralong durability of 15,000 loading-unloading cycles. As a proof of concept, the sensor is applied to detect human physiological and motion signals, further demonstrating a real-time spatial pressure distribution sensing system and a game control system, showing great potential for applications in health monitoring and human-computer interaction.

Large-scale fabrication of flexible macroscopic SERS substrates with high sensitivity and long-term stability by wet-spinning technique: Uniform encapsulation of plasmon in graphene oxide fibers

Graphene oxide has attracted wide interest for theoretical research and application exploration in surface enhancement Raman scattering (SERS). However, how to break the geometric limitations of the nanoscale and extend its properties to the macroscopic level remains an urgent problem. Here, we prepared flexible graphene oxide/Au nanoparticles (GO/AuNP) fibers with high sensitivity and reproducibility for SERS sensing using wet spinning method. Benefitting from the plasmonic coupling of GO/AuNP fibers in vertical direction and uniform adsorption of the target molecules, the limit of detection for fiber was as low as 5 × 10-11 M for rhodamine 6G (R6G) and crystal violet (CV) molecules. In addition, due to the optimal distribution of AuNP in the macroscopic assembly of GO/AuNP hybrid, the fiber substrates provided uniform and stable signals for both temporal (6 months) and spatial (RSD = 8.18 %) scales, exhibited excellent reliability and durability. Moreover, an ultra-low concentrations of the pesticide thiram residues could be rapidly traced by direct sampling from the apple peel with a sensitivity of 5 × 10-9 M, which is much lower than the residue limit set by the US environmental protection agency. We envision that this work could provide new insights into the large-scale and economical fabrication of high-performance flexible SERS sensors for real world applications.

High-Performance Flexible Temperature Sensors Based on Laser-Irradiated Ag-MWCNTs/PEDOT:PSS.

Recently, flexible temperature sensors have attracted significant interest due to their wide-ranging applications in areas such as biomedical monitoring, environmental monitoring, electronic skin, and intelligent robots. However, a combination of high sensitivity and high resolution remains a critical challenge. These properties depend on the synthesis techniques of the sensitive materials. In this work, we use a laser irradiation method to prepare a silver nanoparticle-modified carbon nanotube (Ag-MWCNT) which is further mixed with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). The developed temperature sensor exhibited a high sensitivity of -0.45% °C-1 and linearity with an R2 value of 0.998 in the temperature range of 25-80 °C. Additionally, the sensor demonstrated remarkable repeatability, making it suitable for real-time temperature monitoring of the human body and environment. This temperature sensor is successfully demonstrated in practical applications such as monitoring the temperature of various parts of the human body and sensing the spatial temperature. These demonstrations highlight their significant potential in electronic skin and other related fields.

A High-Performance Strain Sensor for the Detection of Human Motion and Subtle Strain Based on Liquid Metal Microwire.

Flexible strain sensors have a wide range of applications, such as human motion monitoring, wearable electronic devices, and human-computer interactions, due to their good conformability and sensitive deformation detection. To overcome the internal stress problem of solid sensing materials during deformation and prepare small-sized flexible strain sensors, it is necessary to choose a more suitable sensing material and preparation technology. We report a simple and high-performance flexible strain sensor based on liquid metal nanoparticles (LMNPs) on a polyimide substrate. The LMNPs were assembled using the femtosecond laser direct writing technology to form liquid metal microwires. A wearable strain sensor from the liquid metal microwire was fabricated with an excellent gauge factor of up to 76.18, a good linearity in a wide sensing range, and a fast response/recovery time of 159 ms/120 ms. Due to these extraordinary strain sensing performances, the strain sensor can monitor facial expressions in real time and detect vocal cord vibrations for speech recognition.

Recycling of Flyash: Route toward high-performance, eco-friendly, and cost-effective flexible strain sensor via synergizing multi-walled carbon nanotubes

Along with the widespread utilization of wearable and portable electronics, the rise of sensitive flexible strain sensors with wide detection range, excellent stability and affordable costs emerges as a substantial crux of this domain. Herein, we present a convenient and scalable approach for constructing high-performance flexible piezoresistive strain sensor by rationally assembling Flyash/Multi-carbon nanotubes (MWCNTs) active sensing hybrids. Owing to the unique heterostructure of Flyash/MWCNTs wherein Flyash is anchored with MWCNTs via polydopamine, the active sensing hybrids accommodated by porous polyurethane enables a sophisticated conductive network and large tensile deformability, permitting outstanding gauge factor of 1379, wide strain detection range of 0–100 %, remarkable operation stability and durability. Furthermore, the feasibility of sensor in human motions monitoring is validated. For the first time, the synergy of Flyash and MWCNTs demonstrates effective in advancing the sensing performance of piezoresistive strain sensor, providing a new sight towards high-performance, eco-friendly, and cost-effective flexible strain sensor.

Highly Sensitive and Wide Detection Range Thermoplastic Polyurethane/Graphene Nanoplatelets Multifunctional Strain Sensor with a Porous and Crimped Network Structure.

High-performance flexible strain sensors have tremendous potential applications in wearable devices and health monitoring. However, developing a flexible strain sensor with high sensitivity over a wide strain range remains a significant challenge. In this study, a fibrous membrane with a porous and crimped structure was designed as the substrate material for TPU/GNPs flexible strain sensors. This structural design effectively balances sensitivity with the strain range. The TPU-PEO fibrous membrane prepared using electrospinning with water washing, resulted in a porous fibrous membrane with a TPU framework. Subsequently, the fibrous membrane was subjected to anhydrous ethanol stimulation to obtain a porous and crimped network structure. GNPs were modified on the TPU fibrous membrane through ultrasonic treatment. The produced flexible strain sensor exhibited high sensitivity (GF = 4047.5) within a large strain range (350%) and demonstrated excellent sensing performance, stability, and durability (>10,000 cycles). It not only captured basic movements but also efficiently recognized and measured bending angles, enabling a more sophisticated human-machine interaction experience. This advancement opens up possibilities for future intelligent wearable technology and human-machine interaction, contributing to the evolution of these fields.

High performance and cost-effective flexible humidity sensor based on eco-friendly and biodegradable nanocellulose

High performance and cost-effective flexible humidity sensor based on eco-friendly and biodegradable nanocellulose

A double-crack structure for bionic wearable strain sensors with ultra-high sensitivity and a wide sensing range.

Flexible strain sensors are crucial in fully monitoring human motion, and they should have a wide sensing range and ultra-high sensitivity. Herein, inspired by lyriform organs, a flexible strain sensor based on the double-crack structure is designed. An MXene layer and an Au layer with cracks are constructed on both sides of the insulated polydimethylsiloxane (PDMS) film, forming an equivalent parallel circuit that guarantees the integrity of the conductive path under a large strain. The rapid disconnection of the crack junctions causes a significant change in the resistance value. Due to the effect of cracks on the conductive path, the sensitivity of the sensor is largely improved. Benefiting from the double-crack structure, the as-obtained sensor shows ultra-high sensitivity (maximum gauge factor of up to 14 373.6), a wide working range (up to 21%), a fast response time (183 ms) and excellent dynamical stability (almost no performance loss after 1000 stretching cycles and different frequency cycles). In practical applications, the sensor is applied to different parts of the human body to sense the deformation of the skin, demonstrating its great potential application value in human physiological detection and the human-machine interaction. This study can provide new ideas for preparing high-performance flexible strain sensors.

MXene, protein, and KCl-assisted ionic conductive hydrogels with excellent anti-freezing capabilities, self-adhesive, ultra-stretchability, and remarkable mechanical properties for a high-performance wearable flexible sensor.

Developing a hydrogel with switchable features and freeze tolerance is remarkably significant for designing flexible electronics to adjust various application needs. Herein, MXenes, AFPs (antifreeze proteins), and potassium chloride (KCl) were introduced to a polyacrylamide (PAM) polymer network to design an anti-freezing hydrogel. The ionic hydrogels are characterized by excellent ionic conductivity, presenting adjustable properties of remarkable mechanical strength and self-adhesion to meet individualized application demands. The capability of KCl and AFPs to inhibit ice crystals gives hydrogels with anti-icing properties under a low-temperature atmosphere. The PAM/MXene15/AFP30/KCl15 hydrogel demonstrated negligible hysteresis behavior, quick electromechanical response (0.10 s), and excellent sensitivity (gauge factor (GF) = 13.1 within the strain range of 1200-2000%). The resulting hydrogel could be immobilized on the animal or human skin to detect different body movements and physiological motions, offering reproducibility and precise accuracy as primary advantages. The approach of developing materials with tunable features, along with inorganic salt and the fish-inspired freeze-tolerance method, offers a new prospect for wearable gadgets.

Fully printed minimum port flexible interdigital electrode sensor arrays.

Screen-printed interdigital electrode-based flexible pressure sensor arrays play a crucial role in human-computer interaction and health monitoring due to their simplicity of fabrication. However, the long-standing challenge of how to reduce the number of electrical output ports of interdigital electrodes to facilitate integration with back-end circuits is still commonly ignored. Here, we propose a screen-printing strategy to avoid wire cross-planes for rapid fabrication of flexible pressure sensor arrays. By innovatively introducing an insulating ink to realize electrical insulation and three-dimensional interconnection of wire crossings, the improved sensor array (4 × 4) successfully reduces the number of output ports from 17 to 8. In addition, we further constructed microstructures on the laser-etched electrode surfaces and the sensitive layer, which enabled the sensor to achieve a sensitivity as high as 17 567.5 kPa-1 in the range of 0-50 kPa. Moreover, we integrated the sensors with back-end circuits for the precise detection of tactile and physiological information. This provides a reliable method for preparing high-performance flexible sensor arrays and large-scale integration of microsensors.