Flexible Sensor Research Articles

Data-Driven Strain Sensor Design Based on a Knowledge Graph Framework.

Wearable flexible strain sensors require different performance depending on the application scenario. However, developing strain sensors based solely on experiments is time-consuming and often produces suboptimal results. This study utilized sensor knowledge to reduce knowledge redundancy and explore designs. A framework combining knowledge graphs and graph representational learning methods was proposed to identify targeted performance, decipher hidden information, and discover new designs. Unlike process-parameter-based machine learning methods, it used the relationship as semantic features to improve prediction precision (up to 0.81). Based on the proposed framework, a strain sensor was designed and tested, demonstrating a wide strain range (300%) and closely matching predicted performance. This predicted sensor performance outperforms similar materials. Overall, the present work is favorable to design constraints and paves the way for the long-awaited implementation of text-mining-based knowledge management for sensor systems, which will facilitate the intelligent sensor design process.

Recyclable Organic Ionic Conductive Hydrogels for Flexible Wearable Sensors with Excellent Environmental Resistance

Recyclable Organic Ionic Conductive Hydrogels for Flexible Wearable Sensors with Excellent Environmental Resistance

Fabrication of PU-supported PPy/Ti3C2Tx yarns for flexible and multi-functional sensors

In the wake of rapid advancements in the realm of smart textiles, flexible ammonia sensors have garnered significant interest because of its potential implications in environmental monitoring, health diagnosis, and human-machine interaction. Despite their promise, challenges such as limited flexibility, insufficient gas detection sensitivity, and complex production processes have hindered their pragmatic deployment. This study introduces a flexible and multi-functional sensors based on PU-Supported PPy/Ti3C2Tx yarns fabricated through a straightforward in-situ polymerization technique. The gas sensitivity and stretch characteristics of NH3 gas at ambient temperature were assessed, revealing that the flexible and multi-functional sensors has exceptional sensitivity, stretch range, stability, and durability, which can be employed for detecting human physiological activity and recognising gestures. Its remarkable selectivity and strong reaction to ammonia gas also demonstrate its multi-functionality. This study presents a successful approach for integrating high-performance multifunctional flexible electronic products, with wide-ranging application potential and promising possibilities in future smart textiles and wearable devices.

Cutting-Edge Perovskite-Based Flexible Pressure Sensors Made Possible by Piezoelectric Innovation.

In the area of flexible electronics, pressure sensors are a widely utilized variety of flexible electronics that are both indispensable and prevalent. The importance of pressure sensors in various fields is currently increasing, leading to the exploration of materials with unique structural and piezoelectric properties. Perovskite-based materials are ideal for use as flexible pressure sensors (FPSs) due to their flexibility, chemical composition, strain tolerance, high piezoelectric and piezoresistive properties, and potential integration with other technologies. This article presents a comprehensive study of perovskite-based materials used in FPSs and discusses their components, performance, and applications in detecting human movement, electronic skin, and wireless monitoring. This work also discusses challenges like material instability, durability, and toxicity, the limited widespread application due to environmental factors and toxicity concerns, and complex fabrication and future directions for perovskite-based FPSs, providing valuable insights for researchers in structural health monitoring, physical health monitoring, and industrial applications.

Nanoengineering Ultrathin Flexible Pressure Sensors with Superior Sensitivity and Wide Range via Nanocomposite Structures.

Flexible pressure sensors have attracted great interest due to their bendable, stretchable, and lightweight characteristics compared to rigid pressure sensors. However, the contradictions among sensitivity, detection limit, thickness, and detection range restrict the performance of flexible pressure sensors and the scope of their applications, especially for scenarios requiring conformal fitting, such as rough surfaces such as the human skin. This paper proposes a novel flexible pressure sensor by combining the nanoengineering strategy and nanocomposite structures. The nanoengineering strategy utilizes the bending deformation of nanofilm instead of the compression of the active layer to achieve super high sensitivity and low detection limit; meanwhile, the nanocomposite structures introduce distributed microbumps that delay the adhesion of nanofilm to enlarge the detection range. As a result, this device not only ensures an ultrathin thickness of 1.6 μm and a high sensitivity of 84.29 kPa-1 but also offers a large detection range of 20 kPa and an ultralow detection limit of 0.07 Pa. Owing to the ultrathin thickness as well as high performance, this device promotes applications in detecting fingertip pressure, flexible mechanical gripping, and so on, and demonstrates significant potential in wearable electronics, human-machine interaction, health monitoring, and tactile perception. This device offers a strategy to resolve the conflicts among thickness, sensitivity, detection limit, and detection range; therefore, it will advance the development of flexible pressure sensors and contribute to the community and other related research fields.

Selective and Sensitive Detection of Ammonia at Room Temperature by the WS2-PANI Nanocomposite on a Flexible Paper-Based Sensor with Cost-Effective Chemically Expanded Graphite Ink Electrodes

Selective and Sensitive Detection of Ammonia at Room Temperature by the WS₂-PANI Nanocomposite on a Flexible Paper-Based Sensor with Cost-Effective Chemically Expanded Graphite Ink Electrodes

Construction of Fully Integrated and Energy Self-Sufficient NO2 Gas Sensors Utilizing Zinc-Air Batteries.

Exploration of novel self-powered gas sensors free of external energy supply restrictions, such as light illumination and mechanical vibration, for flexible and wearable applications is in urgent need. Herein, this work constructs a flexible and self-powered NO2 gas sensor based on zinc-air batteries (ZABs) with the cathode of the ZABs also acting as the gas-sensitive layer. Furthermore, the SiO2 coating film, serving as a hydrophobic layer, increases the three-phase interfaces for the NO2 reduction reaction. The constructed sensors exhibit a high sensing response (0.3 V @ 5 ppm), an ultralow detection limit (61 ppb), a fast sensing process (129 and 103 s), and excellent selectivity. Moreover, the sensors also possess a wide working temperature range and a low working temperature tolerance (0.34 V at -15 °C). Simulations indicate that the hydrophobic surface at the cathode-hydrogel interface will accommodate more NO2 gas molecules at the reaction sites and prevent the influence of inner water evaporation and direct dissolution of NO2 in the electrolyte, which is beneficial to the enhanced gas sensing abilities. Finally, the self-powered sensing device is incorporated into a smart sensing system for practical applications. This work will pave a new insight into the construction of integrated and energy self-sufficient smart gas sensing systems.

Development of MXene-based flexible piezoresistive sensors

Abstract The flexibility and sensitivity of traditional sensors is hard to achieve unless wearable technology develops. Flexible piezoresistive sensor (FPS) is one of the solutions in the nondestructive health monitoring of living body. In the application of sensing devices for physiological or biochemical signals, fast feedback speed and accurate signal feedback are essential requirements for obtaining sensitive response signals. Additionally, the development of FPS has promoted the research of conductive materials that could be used in wearable devices. However, improving the performance of functional materials is an important way of effort for researchers. Recently, MXene as a new kind of 2D materials and their composites have made a tremendous impact in the field of sensors for wearable health sensors. Numerous conductive materials based 2D MXene could expedite their practical application in FPS by overcoming the present limitations of FPS such as poor responsivity, signal accuracy, and the narrower corresponding range. There has been plenty of breakthrough in the MXene-based FPS in the past several years. The main purpose of this paper is reviewing the recent development of MXene-based FPS and providing an outlook on the future development of it.

Porous reduced graphene oxide@multi-walled carbon nanotubes/polydimethylsiloxane piezoresistive pressure sensor for human motion detection

With the rapid development of wearable electronic, smart robot and health monitoring, there is a growing focus on flexible piezoresistive pressure sensors. Herein, a new strategy is proposed to prepare flexible piezoresistive pressure sensor with porous polydimethylsiloxane (PDMS) sponge as matrix and reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs) for the construction of dual-conductive network. The sensor exhibited excellent overall sensing performances (pressure detection range of 0–200 kPa, sensitivity of 1.62 kPa−1 in 0–29 kPa and 0.41 kPa−1 in 29–65 kPa, response/recovery time of 61/40 ms and 22,000 loading-unloading cycles at 0–15 % compressive strain). Meanwhile, the sensor was able to normally work in the range of 30–100 °C and affected little by temperature. In addition, the sensor was successfully applied for detecting various human motions as well as music recognition and identification. The piezoresistive pressure sensor has great application prospect in wearable devices, health monitoring, and human-machine interaction.

Dual-mode flexible sensor based on magnetic film for wearable smart finger sleeve

Dual-mode flexible sensor based on magnetic film for wearable smart finger sleeve

Multifunctional Flexible Sensor Based on Cracked Laser-Induced Graphene for Tactile Perception and Self-Healable Applications

Multifunctional Flexible Sensor Based on Cracked Laser-Induced Graphene for Tactile Perception and Self-Healable Applications

High-performance flexible pressure sensors with bionic dome-shaped fold structures inspired by crocodile skin

Flexible pressure sensors have drawn substantial community interest as a result of the advancement of contemporary technology. Inspired by the unique sensory organs of crocodiles, an ordered bionic dome-shaped fold structure array was prepared using a facile process in this paper, and electrical experiments confirmed the enhancement of the capabilities of piezoresistive flexible pressure sensors. The results revealed that this type of flexible pressure sensor presented a sensitivity of 8.87 kPa−1, could withstand a maximum pressure of 60 kPa, a minimum detection pressure of 10 Pa, a response time of 70 ms, and maintained stable performance under 5000 repeated presses. In addition, the flexible pressure sensors based on dome-shaped fold structure arrays have been successfully applied for human signal detection and plantar pressure monitoring system. The current research has significant implications for the manufacturing and practical applications of highly effective flexible pressure sensors.

Development and characterization of flexible carbon temperature sensors: a study on different inks and sensor designs

Several industries and applications rely on the accurate monitoring of temperature. Consequently, the development and integration of reliable and flexible temperature sensors that are able to conform to different surfaces is desirable. Despite being a known fact that the used materials, chosen technologies, sensor size, and design tend to impact the performance of said sensors, not many studies focus on the analysis of the influence of these factors. As a result, this work intended to explore how the performance of carbon-based screen-printed temperature sensors could be tuned or optimized by changing the ink and the design of the printed sensors. Therefore, two commercial carbon inks and three different designs were used to create the herein-studied temperature sensors. Firstly, to explore potential differences between the used inks, their morphologic, chemical, and electrical properties were evaluated, allowing further insights to be gathered regarding the filler type, composition, and bulk resistivity. After printing the sensors, they were also submitted to electrical impedance spectroscopy, which allowed for a circuit equivalent model to be proposed for each sensor type, unraveling how current flew across the printed materials. Then, a full factorial statistical analysis was outlined and conducted to uncover which combination of factors allowed to optimize the sensors’ performance, i.e. lower initial resistance, higher thermal coefficient of resistance (TCR), and higher correlation coefficient (R2) of the responses. To gather the experimental data, the electrical response of the sensors (resistance variation) to increasing temperature was collected. As a result of these experiments, it was revealed that ink 1 (based upon carbon nanoparticles) allowed to obtain less resistive sensors and, even though ink 2 (based upon carbon nanotubes) presented higher sensitivity, ink 1 resulted in higher linearity of the sensors. Overall, the factor design had less influence on the results than the factor ink. Notwithstanding, design 1 could be used to obtain sensors with lower native resistance and higher linearity. To sum up, using ink 1 combined with design 1 (fewer hoops and thicker line width) allowed to obtain temperature sensors with average resistance at room temperature of 7.8 kΩ, a TCR of 1.13 %.ºC−1, and R2 of 0.99, which means the herein studied sensors present a very promising behavior when compared to the sensors in the preexisting literature. Thus, the proposed optimized sensors presented attractive characteristics as flexible printed temperature sensors.

Electromechanically stable and flexible poro-hyperelastic composites for multimodal robotic tactile sensing

The expansion of automated production places increased demands on intelligent sensory networks for collecting and processing various input signals, which necessitate higher standards for multimodal detection, sensitivity, and sensing response speed. Here, this work developed a poro-hyperelastic composites-based multimodal tactile sensor capable of triboelectric, piezoresistive, and capacitive modes across multiple strains and frequency domains. Such the multilayered flexible multimodal mechano-electronic sensor (MMES) is constructed from a carbon nanotubes porous sponge (CNPS), facilitating switching between trimodal sensing modes integrated within a single sensor structure, effectively enhancing the electromechanical robustness of the sensory system. The present MMES sensor exhibits excellent pressure sensing performances, with a wide range (0–20 kPa), fast response (less than 102 ms in piezoresistive mode and less than 62 ms in capacitive mode), high sensitivity (up to 0.12 V/kPa⁻1 in triboelectric mode), and long-term durability (exceeding 10,000 cycles). Moreover, a micro-CT generated 3D model was established using finite element analysis (FEA) to investigate porous hyperelastic behaviours of the CNPS composites, in agreement with experimental data. The results demonstrated that the MMES sensor is particularly suitable for dynamic feedback in robotic arms, allowing production lines to make accurate distinguishments and operations by sensing distinct materials and resistances encountered during assembly in real time. The outcomes offer promising applications of multimodal robotic tactile sensing for human-machine interaction.

Photo- and thermo-responsive poly(ionic liquid) hydrogel regulated by hierarchical host–guest interactions

Supramolecular hydrogels constructed through various host–guest interactions have attracted tremendous attention in view of their excellent dynamic and multi-stimuli responsive performances. Herein, benzyl (Bn) and azobenzene (Azo) grafted poly(ionic liquid)s (PILs) were prepared via a one-pot post-synthesis strategy. By virtue of the difference in binding affinity of Azo and Bn groups with the host compound cucurbituril [8] urea (CB[8]), the newly-developed supramolecular hydrogels exhibited distinctive photo- and thermo-responsive behaviors. Under UV light irradiation, PIL-Azo-Bn/CB[8] hydrogel got weaker, while a gel-to-sol transition was observed when treated by heating. Moreover, the above tunable behaviors were highly reversible. Variable-temperature measurements such as proton nuclear magnetic resonance and ultraviolet–visible spectroscopy illustrated that the photo- and thermo-regulated performances of PIL hydrogel were ascribed to the hierarchical host–guest interactions. As a prototype application in smart circuits, PIL hydrogel exhibited a photo- and thermo-tuned conductivity. Thus, our finding presented a novel and versatile platform to fabricate responsive functional hydrogels via host–guest interactions, which has great promising applications in smart materials and flexible sensors.

Preparation and sensing properties of multiscale conductive filler hybrid CNTs@Ag-MXene-TPU/TPU double-layer strain sensing materials

The performance of sensors is often limited by the structural characteristics of the single form of conductive filler, making it difficult for sensors to simultaneously consider the broad sensing range and high sensitivity. How to achieve high sensitivity and broad working range of sensors through innovative sensing materials and sensor structure design remains a challenge. Here, a flexible sensor with multidimensional sensing materials and a double-layer structure is constructed. It consists of a high-performance CNTs@Ag-MXene-TPU sensing layer and a TPU flexible layer with significant deformability. Thanks to the synergistic effect of the double-layer structure and multidimensional sensing materials, this sensor has a high sensitivity of up to maximum gauge factor of 12216.3 and a wide sensing range of 300 %. Its response time and recovery time can reach 200 and 250 ms, respectively, and it also exhibits excellent cycling stability.

Development of wearable textile-based moisture sensor for smart diaper applications and examination of its comfort properties

Urinary incontinence stands as a prevalent challenge encountered among elderly and sick individuals, necessitating the use of diapers for management. Despite existing systems that offer indicators for timely diaper changes upon wetness detection, there remains a demand for the development of wearable, facilely manufacturable, and unobtrusive solutions. Thus, this study presents the fabrication of wearable and flexible moisture sensors utilizing a pad printing technique employing silver and silver/nickel inks. Varied sintering processes with differing pass numbers were employed in sensor fabrication, subsequently subjecting them to evaluation for conductivity and surface morphology. Further, controlled applications of distilled water and artificial urine solutions were administered onto sensors integrated diapers to simulate urinary incontinence scenarios. When the sensors were exposed to the artificial urine solution, sensors’ resistance changes were more noticeable according to the water. Comparative analyses revealed that 5 pass silver sensor exhibited superior performance relative to other sensor configurations investigated. Ultimately, through physical testing, a comparative assessment between sensorless and sensor-integrated diapers was conducted, with findings indicating the promising potential of sensor-integrated diapers in enhancing user comfort and usability.

Nacre-inspired hierarchical bionic substrate for enhanced thermal and mechanical stability in flexible applications

Flexible substrates, essential for flexible electronics by providing support and flexibility while influencing sensor stability, still face challenges in achieving mechanical and thermal stability, particularly in large-scale production and cost control. Inspired by the nacre's “brick-and-mortar” hierarchical microstructure, this study introduces PPSN (paper/polydimethylsiloxane (PDMS)/Si₃N₄ nanoparticles), a novel flexible substrate that mimics the “brick-and-mortar” structure. PPSN, with its topological interlocking and assembly technique, provides excellent thermal and mechanical stability, including high-temperature resistance and stress dissipation. An optimized Si₃N₄ content of 1.0 wt% yields a peak elastic modulus of 1245.27 MPa while maintaining hydrophobicity, with a contact angle of 127.1° at 3.0 wt%. The substrate shows excellent fatigue durability, retaining its morphology after 10,000 bending cycles. Thermal analysis indicates a stable CTE below 20 ppm/°C up to 300°C, rising to 200 ppm/°C between 350 and 400°C. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) reveal minimal thermal decomposition with mass loss below 0.5 % from 25 to 300°C and under 3 % at 400°C. Additionally, electrodes were printed on the substrate using screen printing and ion beam deposition (IBD), demonstrating good material adhesion. A flexible temperature sensor was fabricated and exhibited good sensitivity and stability with a TCR of −0.262 % °C⁻¹. PPSN advances traditional materials with superior mechanical strength, thermal stability, and durability, offering significant scientific and practical value for flexible electronics and low-cost biomimetic processes.

Enhanced multifunctional sensing in human motion with speed-controlled coaxial wet-spun hollow MWCNT-TPU/TPU smart fibers

In the field of personal health management and athletic performance optimization, the demand for multi-parameter human signal monitoring devices is rapidly increasing. Currently, flexible sensors produced through wet spinning techniques face challenges of simplistic structure and functionality, as well as difficulty in balancing strength and sensitivity. In-depth research is needed to develop high-performance wet spinning techniques, which will lay the foundation for commercial smart garments capable of unobtrusively monitoring human motion in the future. This study innovatively employs specialized slurry formulations and spinning speed controls to utilize the timing differences in fiber precipitation and the affinity among similar materials. Through the coaxial wet spinning process, it enables the continuous, large-scale manufacturing of hollow core–shell fibers, MT/T-S5, which feature a dense inner layer and a porous outer conductive layer. This special structure endows the fibers with high strength, high signal stability, and capabilities for both active and passive heating. These fibers achieve a maximum gauge factor (GF) 65.2 in stretch sensing applications and can detect pressures as low as 0.1 N. The MT/T-S5 smart fibers can sense human joint movements, and through connection with an Arduino microcontroller, they can control a robotic hand and perform independent and real-time motion detection of five fingers to distinguish gestures. This research not only provides a new type of fiber material for the development of future smart textiles but also reveals a formation mechanism for hollow core–shell fibers in the field of wet spinning. This discovery can offer valuable inspiration for fiber structural design in future research.

Multifunctional chitosan-based composite hydrogels engineered for sensing applications

Chitosan-based hydrogels, as natural high-molecular-weight flexible materials, are widely utilized due to their outstanding properties. In this research, we developed a one-pot method for synthesizing a novel PVA/CS@PPy-PDAx% conductive hydrogel and explored the internal bonding patterns through molecular dynamics simulations. By adding PPy-PDA nanoparticles into a hydrogel matrix, an interpenetrating conductive network established successfully. The uniform distribution of PPy-PDA nanoparticles endowed the hydrogel with good electrical conductivity (0.171 S/m), significantly enhanced mechanical properties, and strain sensing (S = 5.04), as well as near-infrared photothermal responsiveness (temperature increase of 41.9 °C within 30 s). Additionally, due to the hydrogel's significant photothermal conversion efficiency under near-infrared radiation, it exhibits rapid elimination of Escherichia coli with an antibacterial efficiency exceeding 90 %. The unique hydrogen-bonded crosslinked structure provides the hydrogel with excellent re-healing properties, allowing for restoration through a freeze-thaw process after damage. The conductivity remains nearly unchanged after re-healing, maintaining the material's integrity and functionality. The flexible sensor based on this hydrogel has a response time of 100 ms and can sensitively detect large-scale deformations (e.g., joint bending at various angles), different gravitational forces, and recognize human handwriting. These characteristics make this hydrogel a promising candidate for advancing intelligent wearable technologies and human-machine interaction systems.