Flexible Piezoresistive Pressure Sensor Research Articles

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.

Design Rules for a Wearable Micro-Fabricated Piezo-Resistive Pressure Sensor.

Wearable flexible piezo-resistive pressure sensors hold a wide-ranging potential in human health monitoring, electronic skin, robotic limbs, and other human–machine interfaces. Out of the most successful recent efforts for arterial pulse monitoring are sensors with micro-patterned conductive elastomers. However, a low-current output signal (typically in the range of nano-amperes) and bulky and expensive measurement equipment for useful signal acquisition inhibits their wearability. Herein, through a finite element analysis we establish the design rules for a highly sensitive piezo-resistive pressure sensor with an output that is high enough to be detectable by simple and inexpensive circuits and therefore ensure wearability. We also show that, out of four frequently reported micro-feature shapes in micro-patterned piezo-resistive sensors, the micro-dome and micro-pyramid yield the highest sensitivity. Furthermore, investigations of different conductivity values of micro-patterned elastomers found that coating the elastomer with a conductive material (usually metallic) leads to higher current response when compared to composited conductive elastomers. Finally, the geometric parameters and spatial configurations of micro-pyramid design of piezo-resistive sensors were optimized. The results show that an enhanced sensitivity and higher current output is achieved by the lower spatial density configuration of three micro-features per millimeter length, a smaller feature size of around 100 μm, and a 60–50 degrees pyramid angle.

All-Nanofiber Network Structure for Ultrasensitive Piezoresistive Pressure Sensors.

Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa-1 at 0-1.4 kPa), fast response time (53 ms), and wide detection range (0-60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

Stiffness Engineering of Ti3C2TX MXene‐Based Skin‐Inspired Pressure Sensor with Broad‐Range Ultrasensitivity, Low Detection Limit, and Gas Permeability

AbstractHigh sensitivity and broad sensing range are considered as two crucial parameters toward flexible and wearable piezoresistive pressure sensors for human healthcare, human–machine interfaces, robots, and so on. Though various strategies are developed to improve the sensor performance, the trade‐off between sensitivity and sensing range remains a bottleneck for device design. Besides, conventional pressure sensors without gas permeability normally cause discomfort and inflammation to the human skin. Herein, enlightened by the functions and structure of the human skin, this article proposes a stiffness engineering strategy for constructing Ti3C2TX MXene‐based porous spinosum structure to boost sensitivity over a broad detection range. Consequently, the as‐fabricated pressure sensor exhibits superior performance, including ultrahigh sensitivity of 367 kPa−1 over a widened detection range, a low detection limit of 1 Pa, good gas permeability, and exceptional mechanical stability over 5000 compression cycles at high pressure. Benefiting from such excellent performances, a variety of fancy applications in detecting static pressure, including physiological signals, stealth transmission, the magnitude and spatial distribution of external tactile stimuli, as well as differentiating spatiotemporal tactile stimuli, are demonstrated. It is believed that stiffness engineering is a universal strategy for tailoring the performance of other types of stress/strain sensors.

Facile fabrication of flexible piezo-resistive pressure sensor array using reduced graphene oxide foam and silicone elastomer

Electronic skins have attracted significant interest due to their broad applications in our daily lives. Flexible pressure sensors form one of the essential parts of any electronic skin. Achieving high-performance electronic skins requires ways to make pressure sensor arrays with high sensitivity and good flexibility. We report on the facile fabrication of flexible piezo-resistive pressure sensor arrays with excellent mechanical stability, high sensitivity, and a wide working range (0–30 kPa) of pressure using reduced graphene oxide (rGO) on flexible PCB (FPCB). It shows a remarkable sensitivity of 0.13 kPa−1 in the pressure range < 10 kPa and 0.05 kPa−1 in the range of > 10 kPa, covering a broad spectrum of testing conditions. We demonstrated that it could provide a pressure map and intensity, giving tactile information of the pressure exerted on our prototype with a 4 × 4 array.

Skin-inspired flexible and high-performance MXene@polydimethylsiloxane piezoresistive pressure sensor for human motion detection

In recent years, flexible high-performance piezoresistive pressure sensors have attracted considerable attention for the important application potential in the emerging fields of smart robots, wearable electronics and electronic skin. Herein, inspired by human skin, a new strategy was proposed for the fabrication of a double-layer piezoresistive pressure sensor with wide sensing range and high sensitivity. It was based on the utilization of sandpaper as template and MXene for the constructions of micro-protrusion rough surface on polydimethylsiloxane film and electrically conductive pathways, respectively. The prepared sensor demonstrated high sensitivity of 2.6 kPa−1 in wide linear range of 0–30 kPa, fast response/recovery time of 40/40 ms and excellent repeatability. Importantly, the sensor was successfully applied for the real-time detection of radial artery heart rate, limb movement, handwriting and vocal cord vocalization. Moreover, the integrated device by the sensors had the capability of identifying and visualizing spatial pressure distribution. The findings conceivably stand out a new methodology to prepare flexible high-performance piezoresistive pressure sensors for wearable electronics, human-computer interaction, intelligent robots and health monitoring.

Flexible pressure sensor with a wide pressure measurement range and an agile response based on multiscale carbon fibers/carbon nanotubes composite

As an important branch of wearable electronics, flexible pressure sensors have aroused extensive concern owing to their wide range of applications, such as human health monitoring, human–machine interfaces, and artificial intelligence. Therefore, higher requirements are put forward for the dominating performances of sensitivity, pressure measurement range and response time about flexible pressure sensors. Here, we present a flexible piezoresistive pressure sensor (FPPS) based on carbon fibers (CFs)/carbon nanotubes (CNTs)-polydimethylsiloxane (PDMS) composite and interdigital electrode, which displays a wide measurement range of stress and an agile response at loading and unloading forces. Due to the cooperating advantages of the multiscale carbon fillers, the microstructures on the surface of the piezoresistive film and the interdigital electrode, the FPPS reveals a wide measurement range (from 0 kPa to 185.9 kPa) and a high sensitivity (2.02 kPa‐1 at pressures between 36 Pa and 50.2 kPa). Meanwhile, owing to the carefully designed trilateral double-sided adhesive structure, the FPPS exhibits an agile response (response time of 43 ms) and a stable output (test time of 9000 s). Moreover, the FPPS is capable of identifying clapping and trampling motions, demonstrating its effectiveness as a flexible sensing unit. The novel pressure sensor was integrated with additional software and hardware to realize a real-time pressure acquisition and display system, providing a potential use of the device in an intelligent industry application.

Flexible Piezoresistive Pressure Sensor Based on Electrospun Rough Polyurethane Nanofibers Film for Human Motion Monitoring.

Flexible piezoresistive pressure sensors have been attracted a lot of attention due to their simple mechanism, easy fabrication, and convenient signal acquisition and analysis. Herein, a new flexible piezoresistive sensor based on microstructured electrospun rough polyurethane (PU) nanofibers film is assembled. The microstructured PU film with tiny bumps is prepared in one step via electrospinning technology, which imparts a microstructured rough upper surface and a smooth lower surface. With this unique microstructure, we have made it possible for PU/Ag films to serve as sensing layers for piezoresistive sensors by introducing a silver conductive layer on the surface of electrospun PU film. The fabricated piezoresistive pressure sensor delivers high sensitivity (10.53 kPa−1 in the range of 0–5 kPa and 0.97 kPa−1 in the range of 6–15 kPa), fast response time (60 ms), fast recovery time (30 ms), and long-time stability (over 10,000 cycles). This study presents a fabrication strategy to prepare the microstructured PU nanofiber film using electrospinning technology directly, and the as-developed sensor shows promise in applications such as wrist pulse measurement, finger movement monitoring, etc., proving its great potential for monitoring human activities.

Flexible Pressure Sensor Based on a Thermally Induced Wrinkled Graphene Sandwich Structure

Flexible piezoresistive pressure sensors based on a graphene-elastomer composite have gained popularity as electronic wearable sensors due to their simplicity and promising practical application in health monitoring and wearable devices. There is an urgent need to develop pressure sensors that are simplistic in design with good sensitivity and equally capable of production in large volumes. Studies have shown that implementing micro/nanostructures improves the sensitivity of graphene-elastomer pressure sensors. Therefore, in this paper, we fabricated thick nano wrinkles on a graphene-elastomer sandwich structure through a rapid thermal annealing procedure. The wrinkled structure was fabricated by rapid thermal annealing of spin-coated PEDOT:PSS (PH1000) on graphene-PDMS composite to facilitate the generation of wrinkles and optimize the sensitivity of our pressure sensor. We performed analyses on our pressure sensor under various compressive loading ranges. The pressure sensor detected pressure applied at these various ranges within a reasonable response time of 160ms and demonstrated excellent repeatability without hysteresis. In addition, we observed satisfactory sensitivity under tensile strain exhibiting high stretchability. The sensor displayed good sensitivity of 2.83 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> from compressive loading in the low-pressure range 0 Pa −490 Pa, and from 490 Pa-10 kPa the sensitivity was 0.12k Pa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The sensor demonstrated stretchability of up to 50% with a gauge factor of 20.1. We observed that it is highly responsive in the lower pressure range and capable of detecting subtle pressure. Furthermore, we demonstrated pressure detection in human motion monitoring. Overall, the pressure sensor displayed potential applications in human monitoring devices and wearable electronics.

MXene‐Sponge Based High‐Performance Piezoresistive Sensor for Wearable Biomonitoring and Real‐Time Tactile Sensing (Small Methods 2/2022)

Flexible Piezoresistive Pressure Sensor In article number 2101051, Ye, Xie, Su, Chen and co-workers were inspired by Helium embrittlement during nuclear fusion processes, to utilize Helium plasma irradiation to yield a nano-fuzz-shaped molybdenum electrode for a flexible piezoresistive pressure sensor, which was demonstrated to measure human radial pulse, detect the tapping, stamping, and perform object identification, revealing great feasibility in wearable biomonitoring and healthcare assessment.

Wide-Range Motion Recognition Through Insole Sensor Using Multi-Walled Carbon Nanotubes and Polydimethylsiloxane Composites.

High linearity/sensitivity and a wide dynamic sensing range are the most desirable features for pressure sensors to accurately detect and respond to external pressure stimuli. Even though a number of recent studies have demonstrated a low-cost pressure sensing device for a smart insole system by using scalable and deformable conductive materials, they still lack stretchability and desirable properties such as high sensitivity, hysteresis, linearity, and fast response time to obtain accurate and reliable data. To resolve this issue, a flexible and stretchable piezoresistive pressure sensor with high linear response over a wide pressure range is developed and integrated in a wearable insole system. The sensor uses multi-walled carbon nanotubes and polydimethylsiloxane (MWCNT/PDMS) composites with gradient density double-stacked configuration as well as randomly distributed surface microstructure (RDSM). The randomly distributed surface of the MWCNT/PDMS composite is easily and non-artificially generated by the evaporation of residual IPA solvent during a composite curing process. Due to two functional features consisting of the double-stacked composite configuration with different gradient MWCNT density and RDSM, the pressure sensor shows high linear sensitivity (∼82.5 kPa) and a pressure range of 0-1 MPa, providing extensive potential applications in monitoring human motions. Moreover, for a practical wearable application detecting the user's real-time motions, a custom-designed output signal acquisition system has been developed and integrated with the insole pressure sensor. As a result, the insole sensor can successfully detect walking, running, and jumping movements and can be used in daily life to monitor gait patterns by virtue of its long-term stability.

Fabrication and performance of full textile-based flexible piezoresistive pressure sensor

Fabrication and performance of full textile-based flexible piezoresistive pressure sensor

The Use of Parylene C as a Flexible Piezoelectric, Piezoresistive, and Capacitive Pressure Sensor in Low-Pressure Range (&lt;4 Kpa)

The Use of Parylene C as a Flexible Piezoelectric, Piezoresistive, and Capacitive Pressure Sensor in Low-Pressure Range (&lt;4 Kpa)

One-step synthesis of carbon-doped PPy nanoparticles interspersed in 3D porous melamine foam as a high-performance piezoresistive pressure, strain, and breath sensor

Flexible and compressible piezoresistive pressure, strain, and breath sensor based on carbon-doped PPy nanoparticles interspersed in 3D porous melamine foam.

Bioinspired High-Performance Multilayer Flexible Piezoresistive Pressure Sensor with Broader Linearity Range and Higher Sensitivity

Bioinspired High-Performance Multilayer Flexible Piezoresistive Pressure Sensor with Broader Linearity Range and Higher Sensitivity

Ultra-sensitive flexible piezoresistive pressure sensor prepared by laser-assisted copper template for health monitoring

The flexible sensors with microstructures have attracted much attention due to their ultrahigh sensitivity. To develop a controllable method for large-scale preparation of microstructures is the key to promoting the application of the flexible sensors. Herein, combining the laser processing technology with a copper template commonly used in roll-to-roll process, ultra-sensitive flexible piezoresistive pressure sensors with carbon nanotubes (CNTs) conductive layers on the ridge-shaped microstructures were fabricated and the effects of structure parameters on the performance of the sensors were systematically studied. Experimental results showed that the sensitivity of the sensors is positively correlated with the height h and spacing d of the ridge-shaped microstructure which are determined by laser processing parameters. The sensitivity of the sensor with the ridge-shaped microstructure (h=150 µm, d=200 µm) is as high as 1150.9 kPa−1 at an applied pressure< 50 Pa. Meanwhile, the sensor shows excellent stability (>2000 cycles), rapid response time (43 ms) and recovery time (15 ms). Besides, the sensor was used to detect human physiological signals such as speaking, cheek bugling and artery pulse. These results indicate that combining the laser processing with copper templates has great potential in the controllable and large-scale fabrication of flexible sensors with microstructures for health monitoring.

Tunable Wide Range and High Sensitivity Flexible Pressure Sensors with Ordered Multilevel Microstructures

AbstractWith the rapid development of healthcare and human‐machine interactions, there is a growing demand for flexible electronic skin to have a high sensitivity over a wide range. While current multilevel and hierarchical structures inspired by nature broadened the sensing range, the structural design still lacks systematic study. Hereby, an ordered multilevel microstructure is proposed, and the design strategy for performance regulation of piezoresistive pressure sensors is studied. Both microstructure height and spatial distribution are investigated systematically through simulation and experiments. The flexible piezoresistive pressure sensor is fabricated by combining a fast, cost‐effective laser marking technology, molding, and pneumatic spray. The fabricated sensors show high sensitivity (1.5 – 8.3 kPa−1) over a wide range (0.01 – 200 kPa), and have a detection limit of 10 Pa, a response time of below 70 ms, and a mechanical stability over 10 000 cycles. This design and fabrication strategy can be further optimized by combining advanced materials and fabrication systems, and is expected to be applicable for a wide range of flexible materials.

3D Printed Skin‐Inspired Flexible Pressure Sensor with Gradient Porous Structure for Tunable High Sensitivity and Wide Linearity Range

AbstractAdvanced nanomaterials and novel microstructures have been developed to achieve flexible piezoresistive pressure sensors with high sensitivity and wide linearity range. However, the theoretical relationships between the sensitivity and the structure of the flexible pressure sensor have rarely been established. Consequently, the sensitivities of the sensors cannot be controlled and tailored for practical application. In this work, a skin‐inspired flexible pressure sensor with gradient porous structure is proposed. Based on the stress distribution of finite element analysis (FEA), the quantitative descriptions between the sensitivity and sensor structure parameters are derived. Then, carbon black (CB)/polydimethylsiloxane (PDMS) composites are successfully fabricated with desired gradient porous structures by 3D printing technology. Encouragingly, the skin‐inspired gradient structure sensor shows good sensitivity (0.0048 kPa−1) over the ultrawide linear range of 0–500 kPa. Meanwhile, the sensor shows excellent stability and durability during over 500 compression cycles. Taking advantage of these excellent sensing properties, the skin‐inspired gradient porous sensor is demonstrated in the application of voice recognition, grasp sensing, and intermittent pneumatic compression (IPC) interface pressure monitoring.

MXene-Sponge Based High-Performance Piezoresistive Sensor for Wearable Biomonitoring and Real-Time Tactile Sensing.

Electrode microfabrication technologies such as lithography and deposition have been widely applied in wearable electronics to boost interfacial coupling efficiency and device performance. However, a majority of these approaches are restricted by expensive and complicated processing techniques, as well as waste discharge. Here, helium plasma irradiation is employed to yield a molybdenum microstructured electrode, which is constructed into a flexible piezoresistive pressure sensor based on a Ti3 C2 Tx nanosheet-immersed polyurethane sponge. This electrode engineering strategy enables the smooth transition between sponge deformation and MXene interlamellar displacement, giving rise to high sensitivity (1.52kPa-1 ) and good linearity (r2 =0.9985) in a wide sensing range (0-100kPa) with a response time of 226ms for pressure detection. In addition, both the experimental characterization and finite element simulation confirm that the hierarchical structures modulated by pore size, plasma bias, and MXene concentration play a crucial role in improving the sensing performance. Furthermore, the as-developed flexible pressure sensor is demonstrated to measure human radial pulse, detect finger tapping, foot stomping, and perform object identification, revealing great feasibility in wearable biomonitoring and health assessment.

Conductive and room-temperature self-healable polydimethylsiloxane-based elastomer film with ridge-like microstructure for piezoresistive pressure sensor

The fabrication of piezoresistive pressure sensor with self-healing function has attracted worldwide attention for its great prospects in intelligent robots, remote health monitoring and electronic skin. In this work, a conductive and self-healable elastomer (CSE) film with ridge-like microstructure was designed for flexible piezoresistive pressure sensor. Firstly, self-healable polydimethylsiloxane (PDMS)-based elastomer with ureido and imine groups was synthesized via a simple one-pot two-step polycondensation reaction with aminopropyl-terminated PDMS, isophorone diisocyanate and 1, 3, 5-triformylbenzene as monomers. Secondly, the synthesized PDMS-based elastomer solution was casted onto a sandpaper pre-sprayed with ureido pyrimidinone grafted carbon nanotubes/polyurea mixed solution, and the CSE film was obtained after drying and peeling off from the sandpaper. The piezoresistive pressure sensor fabricated with two single-electrode CSE films exhibited high sensitivity of 8.7 kPa−1 (0–6.1 kPa), low detection limit (50 Pa), fast response capability (response/relaxation time of 40/117 ms) and repeatability for 10,000 loading–unloading tests. Moreover, the sensor was able to fully recover to normal work after healing for 36 h at room temperature or even 200 s under the irradiation of near infrared (NIR) light. Importantly, the sensor was successfully applied for not only detecting human motions including radial pulse, speech recognitions and joint movements but also monitoring health status wirelessly through Bluetooth transmission. Our findings put forward a novel method to fabricate a high-performance self-healable piezoresistive sensor for wearable electronics, healthcare monitoring and electronic skin.