Flexible Piezoresistive Pressure Sensor Research Articles

Rational design of a laminate-structured flexible sensor for human dynamic plantar pressure monitoring

Flexible sensors are essential components in emerging fields such as epidermal electronics, biomedicine, and human-computer interactions, and creating high-performance sensors through simple structural design for practical applications is increasingly needed. Presently, challenges still exist in establishing efficient models of flexible piezoresistive pressure sensors to predict the design required for achieving target performance. This work establishes a theoretical model of a flexible pressure sensor with a simple laminated and enclosed structure. In the modeling, the electrical constriction effect is innovatively introduced to explain the sensitization mechanism of the laminated structure to a broad range of pressures and to predict the sensor performance. The experimental results confirmed the effectiveness of the theoretical model. The sensor exhibited excellent stability for up to three million cycles and superior durability when exposed to salt solution owing to its simple laminated and enclosed structural design. Finally, a wearable sensing system for real-time collection and analysis of plantar pressure is constructed for exercise and rehabilitation monitoring applications. This work aims to provide theoretical guidance for the rapid design and construction of flexible pressure sensors with target performance for practical applications.

Highly compressive aerogel-elastomer hybrid porous material with redistributed strain for high-sensitive soft pressure sensors

Owing to the good sensitivity and simple fabrication process, flexible piezoresistive pressure sensors based on aerogels shows great applications in soft robotics, bioelectronics and so on. However, the poor compressibility and stability of the aerogel is still challenging the development of aerogel based flexible piezoresistive pressure sensors, simultaneously, it is urgent to pave a way to enhance the sensitivity of these sensors. Here, for the first time, we propose a universal strategy to prepare aerogel skeleton-elastomer shell hybrid porous material with enhanced compressibility for high-sensitive and stable flexible pressure sensors. We found that the elastomer shell disperses the strain in the aerogel skeleton and enhances the compressibility of the aerogel, which improve the stability of the sensor (10000 cycles of compression). More importantly, there are elastomer micro-blocks formed in the hybrid material, these micro-blocks redistribute the strain in the material which contributes to the high sensitivity (10.25 kPa−1) of the sensors. Furthermore, the sensors are successful in monitoring both of the large joint bending and subtle skin vibration, as well as remote controlling toy car and artificial hand. This work provides an effective and universal strategy to enhance the sensitivity and stability of the aerogel based flexible pressure sensors and promotes their development towards practical applications.

Low-cost, reliable and flexible piezoresistive pressure sensors coated with single layer graphene and silver nanowires on three-dimensional polyurethane sponge

Flexible wearable sensors perform a significant role in practical applications such as medical monitoring, rehabilitation exercises, and artificial skin, as they can quantify and visualize external stress and strain. Polyurethane (PU) sponge stands out as a highly elastic and reproducible material frequently employed as a substrate in flexible electronic devices. Sponge materials with different pore sizes and densities exhibit varying degrees of deformation resistance under long-term cyclic pressure, thereby affecting the performance of sensors. Herein, piezoresistive pressure sensors based on PU with different pore diameters were successfully fabricated by dip-coating technology. By incorporating silver nanowires (AgNWs) and single layer graphene (SLG) into the sponge, both the low-density and high-density composite sponges can withstand over 70 % compressive strain, with stresses of 62.57 kPa and 31.79 kPa, respectively. Moreover, high-density composite sponge demonstrates a more stable conductive structure, more consistent relative resistance and better flame-retardant performance. The response and recovery time between 10 % and 40 % strains of the resulting pressure sensor based on high-density composite sponge (PU@AgNWs@SLG) are 110 ms and 106 ms and its gauge factor reaches 2.39. The pressure sensor exhibits a satisfactory reproducibility after over 3000 compression-release cycles, and performs during various applications, such as controlling brightness of LEDs, monitoring human activities, sensing finger movements.

Silver nanowires/waterborne polyurethane composite film based piezoresistive pressure sensor for ultrasensitive human motion monitoring

Flexible piezoresistive pressure sensors are gaining significant attention, particularly in the realm of flexible wearable electronic skin. Here, a flexible piezoresistive pressure sensor was developed with a broad sensing range and high sensitivity. We achieved this by curing polydimethylsiloxane (PDMS) on sandpaper, creating a PDMS film as the template with a micro-protrusion structure. The core sensing layer was formed using a composite of silver nanowires (AgNWs) and waterborne polyurethane (WPU) with a similar micro-protrusion structure. The sensor stands out with its exceptional sensitivity, showing a value of 1.04 × 106 kPa−1 with a wide linear range from 0 to 27 kPa. It also boasts a swift response and recovery time of 160 ms, coupled with a low detection threshold of 17 Pa. Even after undergoing more than 1000 cycles, the sensor continues to deliver stable performance. The flexible piezoresistive pressure sensor based on AgNWs/WPU composite film (AWCF) can detect small pressure changes such as pulse, swallowing, etc, which indicates that the sensor has great application potential in monitoring human movement and flexible wearable electronic skin.

Flexible Nanofiber Pressure Sensors with Hydrophobic Properties for Wearable Electronics

In recent years, flexible pressure sensors have received considerable attention for their potential applications in health monitoring and human–machine interfaces. However, the development of flexible pressure sensors with excellent sensitivity performance and a variety of advantageous characteristics remains a significant challenge. In this paper, a high-performance flexible piezoresistive pressure sensor, BC/ZnO, is developed with a sensitive element consisting of bacterial cellulose (BC) nanofibrous aerogel modified by ZnO nanorods. The BC/ZnO pressure sensor exhibits excellent mechanical and hydrophobic properties, as well as a high sensitivity of −15.93 kPa−1 and a wide range of detection pressure (0.3–20 kPa), fast response (300 ms), and good cyclic durability (>1000). Furthermore, the sensor exhibits excellent sensing performance in real-time monitoring of a wide range of human behaviors, including mass movements and subtle physiological signals.

Novel Polyurethane Based, Fully Flexible, High-Performance Piezoresistive Sensor for Real-Time Pressure Monitoring.

Flexible piezoresistive pressure sensors are garnering substantial attention, in line with advancements in biointegrated and wearable electronics. However, a significant portion of piezoresistive pressure sensors suffer from the trade-off between sensitivity and pressure range. Moreover, the current piezoresistive sensors generally rely on a rigid metallic electrode, severely deteriorating their long-term durability. Herein, a fully flexible piezoresistive sensor coupling polyurethane (PU) based electrode and active sensing element is proposed to circumvent the aforementioned problems. By rationally regulating the double-permeable conductive networks within the PU matrix, an elastomeric electrode and sensing element are implemented, respectively. The assembled heterostructured configurations enable impressive sensitivity up to 7.023 kPa-1, broad pressure detection (up to 420 kPa), an ultralow pressure sensing limit (0.1 Pa), and extraordinary operation stability over 80000 cyclic pressings along with fast response/relaxation times (60 ms/80 ms). Additionally, the fully flexible sensor is capable of both real-time detection of physiological signals and mimicking keyboards, implying its viability as a high-performance pressure sensor.

Flexible piezoresistive pressure sensor based on a graphene-carbon nanotube-polydimethylsiloxane composite

Flexible sensors are used widely in wearable devices, specifically flexible piezoresistive sensors, which are common and easy to manipulate. However, fabricating such sensors is expensive and complex, so proposed here is a simple fabrication approach involving a sensor containing microstructures replicated from a sandpaper template onto which polydimethylsiloxane containing a mixture of graphene and carbon nanotubes is spin coated. The surface morphologies of three versions of the sensor made using different grades of sandpaper are observed, and the corresponding pressure sensitivities and linearity and hysteresis characteristics are assessed and analyzed. The results show that the sensor made using 80-mesh sandpaper has the best sensing performance. Its sensitivity is 0.341 kPa−1 in the loading range of 0–1.6 kPa, it responds to small external loading of 100 Pa with a resistance change of 10%, its loading and unloading response times are 0.126 and 0.2 s, respectively, and its hysteresis characteristic is ∼7%, indicating that the sensor has high sensitivity, fast response, and good stability. Thus, the presented piezoresistive sensor is promising for practical applications in flexible wearable electronics.

High Sensitivity and Wide Linear Range Flexible Piezoresistive Pressure Sensor with Microspheres as Spacers for Pronunciation Recognition.

Flexible piezoresistive pressure sensors have received great popularity in flexible electronics due to their simple structure and promising applications in health monitoring and artificial intelligence. However, the contradiction between sensitivity and detection range limits the application of the sensors in the medium-pressure regime. Here, a flexible piezoresistive pressure sensor is fabricated by combining a hierarchical spinous microstructure sensitive layer and a periodic microsphere array spacer. The sensor achieves high sensitivity (39.1 kPa-1) and outstanding linearity (0.99, R2 coefficient) in a medium-pressure regime, as well as a wide range of detection (100 Pa-160.0 kPa), high detection precision (<0.63‰ full scale), and excellent durability (>5000 cycles). The mechanism of the microsphere array spacer in improving sensitivity and detection range was revealed through finite element analysis. Furthermore, the sensors have been utilized to detect muscle and joint movements, spatial pressure distributions, and throat movements during pronouncing words. By means of a full-connect artificial neural network for machine learning, the sensor's output of different pronounced words can be precisely distinguished and classified with an overall accuracy of 96.0%. Overall, the high-performance flexible pressure sensor based on a microsphere array spacer has great potential in health monitoring, human-machine interface, and artificial intelligence of medium-pressure regime.

Highly Sensitive and Breathability Flexible Piezoresistive Pressure Sensor Based on Xylon

AbstractIn recent years, the prevalence of cervical spondylosis has been increasing, which is associated with unhealthy posture in daily life, flexible pressure sensors are beneficial in preventing cervical spine disorders. This work is dedicated to developing a flexible, high‐performance, and breathable device to monitor the health of the neck. Here, a flexible pressure sensor with a “sandwich” structure is presented, which incorporates patterned xylon fabric as the substrate and MXene as the sensitive layer. Furthermore, the application of silver nanowires (AgNWs) as the electrode material improves the piezoresistive performance. Combined with an internal elastic encapsulation method (IEEM), the sensor exhibits high sensitivity (1417.9 kPa−1 under pressure below 100 kPa) and a fast response time (30.77 ms). Compared with traditional flexible pressure sensors, the breathability of devices with an all‐fiber structure is nearly seven times higher, which better satisfies the monitoring requirements of long‐term wear. Finally, a wireless system is established by integrating a Bluetooth module and a terminal app. It enables monitoring and sensing of various physiological signals of the neck, as well as identification and alerting of long‐term poor posture such as prolonged neck flexion, showing a potential candidate for the applications of the next‐generation wearable device.

High-sensitivity, ultrawide linear range, antibacterial textile pressure sensor based on chitosan/MXene hierarchical architecture

It is still a great challenge for the flexible piezoresistive pressure sensors to simultaneously achieve wide linearity and high sensitivity. Herein, we propose a high-performance textile pressure sensor based on chitosan (CTS)/MXene fiber. The hierarchical "point to line" architecture enables the pressure sensor with high sensitivity of 1.16kPa-1 over an ultrawide linear range of 1.5 MPa. Furthermore, the CTS/MXene pressure sensor possesses a low fatigue over 1000 loading/unloading cycles under 1.5 MPa pressure load, attributed to the strong chemical bonding between CTS fiber and MXene and excellent mechanical stability. Besides, the proposed sensor shows good antibacterial effect benefiting from the strong interaction between polycationic structure of CTS/MXene and the predominantly anionic components of bacteria surface. The sensor is also applied to detect real-time human action, an overall classification accuracy of 98.61% based on deep neural network-convolutional neural network (CNN) for six human actions is realized.

Heater-Integrated Flexible Piezoresistive Pressure Sensor Array for Smart-Car Seats

Heater-Integrated Flexible Piezoresistive Pressure Sensor Array for Smart-Car Seats

Flexible pressure sensors based on reduced graphene oxide @ porous silicone elastomers with good resolution and wide working range

Current flexible piezoresistive pressure sensors still face challenges, such as poor stability and a limited detection range, making it difficult to accurately discern small-scale pressures in the Pa or MPa range when meeting sudden pressure events. To tackle the above issues, this study presents the development of a competent sensor composed of porous conductive elastomer and laser-induced graphene electrodes. The substrate of the composite elastomer is a porous silicone elastomer obtained by the additive reaction using template methods, and the conductive filler consisting of reduced graphene oxide is synthesized through an eco-friendly approach. We demonstrated that the sensors fabricated here have the ability to precisely resolve pressures from 542 Pa to 1.5 MPa with long-term stability, as well as the detection of subtle signals, such as pulse pulsation and throat vibrations. Above all, such sensors have great potential in the application of human-machine interaction.

Flexible Switching Pressure Sensors with Fast Response and Less Bending-Sensitive Performance Applied to Pain-Perception-Mimetic Gloves.

A strategy is proposed herein for preparing a flexible switching piezoresistive pressure sensor, which has a bridge-like structure and inverted micropyramids (IMPs) on its lower conductive substrate. The sensor substrates were prepared by injection compression molding (an industrial manufacturing process) using thermoplastic polyurethane (TPU; an industrial grade polymer). The designed bridge-like structure enables the sensor to obtain a pressure threshold. The flexibility of upper and lower TPU substrates allows them to contact quickly when pressed, and so the sensor exhibits a fast response (as short as 2 ms) and can respond to both static force and dynamic force (up to 50 Hz frequency), which are prominent for the sensor made from TPU. The sensor exhibits less bending-sensitive performance, which is attributed to the conformality of the upper and lower substrates and lower strain on the lower substrate with the IMP under bending. The sensor can amplify signal response at the monitoring limit (the relative resistance change is up to 46%). It can achieve a higher sensitivity in different low-pressure ranges by changing the gap of the bridge-like structure. Moreover, the sensor can obviously and steadily respond to an additional very low pressure under preloading and exhibits good durability performance. As the sensor has a pressure threshold similar to the human pain perception process, a pain-perception-mimetic glove that can identify the external mechanical stimuli but reduces the interference of finger bending is prepared, displaying potential applications of the flexible switching sensor in intelligent wearable protectors.

Temperature-Immune, Wide-Range Flexible Robust Pressure Sensors for Harsh Environments.

Flexible pressure sensors possess vast potential for various applications such as new energy batteries, aerospace engines, and rescue robots owing to their exceptional flexibility and adaptability. However, the existing sensors face significant challenges in maintaining long-term reliability and environmental resilience when operating in harsh environments with variable temperatures and high pressures (∼MPa), mainly due to possible mechanical mismatch and structural instability. Here, we propose a composite scheme for a flexible piezoresistive pressure sensor to improve its robustness by utilizing material design of near-zero temperature coefficient of resistance (TCR), radial gradient pressure-dividing microstructure, and flexible interface bonding process. The sensing layer comprising multiwalled carbon nanotubes (MWCNTs), graphite (GP), and thermoplastic polyurethane (TPU) was optimized to achieve a near-zero temperature coefficient of resistance over a temperature range of 25-70 °C, while the radial gradient microstructure layout based on pressure division increases the range of pressure up to 2 MPa. Furthermore, a flexible interface bonding process introduces a self-soluble transition layer by direct-writing TPU bonding solution at the bonding interface, which enables the sensor to achieve signal fluctuations as low as 0.6% and a high interface strength of up to 1200 kPa. Moreover, it has been further validated for its capability of monitoring the physiological signals of athletes as well as the long-term reliable environmental resilience of the expansion pressure of the power cell. This work demonstrates that the proposed scheme sheds new light on the design of robust pressure sensors for harsh environments.

Ti3C2Tx/MXene Composite Aerogel Flexible Biosensor for Measuring Occlusal Force In Situ.

Occlusal force is an important parameter for evaluating the function of the occlusal system. Traditional occlusal forces can only be measured qualitatively. Here, we report a flexible piezoresistive pressure sensor with high sensitivity and a wide measurement range for in situ occlusal force measurements through the articulating paper. The sensing layer of the flexible piezoresistive sensor is a 3D porous MXene composite aerogel, which is fabricated by vacuum freezing. The MXene piezoresistive sensor is composed of the interdigital electrodes, the sensing layer, the PI encapsulation layer, and an articulating paper encapsulation layer. The sensor shows perfect performance with high sensitivity (210.21 kPa-1), wide measurement range (∼420 kPa), and a fast response time (123 ms response time, 163 ms recovery time). The amplitude of the occlusal force, which varied with time, can be observed on the mobile phone through the wireless system with the Bluetooth module. This technique has broad application prospects in oral health. In this work, we propose a simple method and a new idea for manufacturing high-performance wearable bioelectronic sensors.

Self-Assembly of Multiwalled Carbon Nanotubes on a Silicone Rubber Foam Skeleton for Durable Piezoresistive Sensors.

Conductive nanomaterial/flexible polymer composite foams are of great interest in the field of flexible and wearable piezoresistive pressure sensors. However, the existing composite foam sensors are faced with stability issues from conductive nanomaterials, which tends to decrease their long-term durability. Herein, we developed a solvent evaporation-induced self-assembly strategy, which could significantly improve the stability of multiwalled carbon nanotubes (MWCNTs) on a silicone rubber foam skeleton. The process for self-assembly of MWCNTs was straightforward. Aqueous MWCNT dispersion droplets were first hierarchically enclosed in silicone rubber via water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs). Then, the high pressure generated by fast evaporation of the solvent from the droplets could break the thinnest pore walls to form interconnected pores. As a result, very dense and firm MWCNT layers were self-assembled on the pore wall surface. Due to the excellent stability of MWCNTs and tetramodal interconnected porosity, our MWCNTs/silicone rubber composite foam showed the following "super" properties: low density of 0.26 g/mL, high porosity of 76%, and excellent mechanical strength (the maximum stress loss of 8.3% at 80% strain after 100 compression cycles). In addition, excellent piezoresistive performance, including superior discernibility for different amplitudes of compressive strain (up to 80%), rapid response time (150 ms), and high sensitivity (gauge factor of 1.44), was demonstrated for such foams, together with prominent durability (39,000 compression cycles at 60% strain in air) and excellent stability of resistance response in water and organic solvents (5000 compression cycles at 30% strain in water and ethanol). Regarding its application, a wearable piezoresistive sensor, which was assembled from the as-prepared conductive silicone rubber composite foam, could capture various movements from tiptoeing and finger bending to small deformations resulting from human pulse.

Flexible capacitive and piezoresistive pressure sensors based on screen-printed parylene C/polyurethane composites in low-pressure range

The use of polymers to fabricate flexible pressure sensors as an alternative to conventional pressure sensors has led to the development of physiological monitoring of human body and the electronic skin. In particular, the fabrication of flexible capacitive and piezoresistive sensors using a variety of materials and the investigation of their electromechanical properties are further developments in these fields. Herein, parylene C is synthesized via chemical vapor deposition method. Pressure-sensitive inks are prepared with a composite of parylene C, polyurethane, polymethylmethacrylate, and activated carbon at certain weight ratios. Flexible capacitive and piezoresistive pressure sensors are fabricated by the screen printing method. The sensitivity, detection limit, linearity range, and response/relaxation time, which define the capacitive and piezoresistive properties are investigated and presented in this paper. The sensitivities of the flexible capacitive and piezoresistive pressure sensors are 0.124 kPa−1 and 0.074 kPa−1 in the pressure range of 0.07–1.39 kPa. This study enables parylene C to be used in the composite structure and shows that it can be used not only as a protective layer but also in flexible pressure sensor applications. It also ensures that the design of the flexible capacitance pressure sensor can measure low pressure with high sensitivity compared to the flexible piezoresistive pressure sensor.

Flexible Piezoresistive Pressure and Temperature Sensor Module for Continuous Monitoring of Cardiac Health

Flexible and disposable sensors are highly coveted for their light and thin nature, robustness, pliability, and cost advantage in the manufacturing process. They are widely used for affordable health monitoring. This work presents a PEDOT: PSS+GO temperature sensor and porous PDMS+PEDOT:PSS pressure sensor assisted health band for continuous monitoring of cardiac health. A novel single-module sensor has been developed that can detect heart rate with a high sensitivity of 2.67 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (low to medium pressure range 0-5 kPa) and a temperature range of ~25-45 °C with average sensitivity 0.0022 °C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Further, the flexible and disposable sensors used demonstrated a longevity which is considerably high for a disposable sensor set-up. The sensors have been tested in rest and active conditions and promising results are obtained. The setup has been integrated with a smartphone application to wirelessly transmit the data obtained over a Bluetooth connection. The device has an accuracy of 95.81% for temperature and 96.03% for pressure sensing, which is comparable to conventional sensors of the same nature. The sensing patch requires considerably less power (5 nW) than the current optical-based measurement devices and is capable of detecting minute physiological signal fluctuations, making it a potential option for low-power, real-time, and home-based cardiac health monitoring.

Large area matrix of MXene/MoSe2 nanohybrid-based flexible piezoresistive pressure sensors for artificial e-skin application

In this study, a MoSe2/Ti3C2Tx nanohybrid-based fabricated flexible physical sensor demonstrates an elevated pressure sensitivity of 14.70 kPa−1 and a highly robust nature withstanding up to ∼2500 cycles. The fabricated pressure sensor's underlying transduction mechanism is elucidated by exploiting the inherent piezoresistive effect and the variation of the Schottky barrier height unveiled by the MoSe2/Ti3C2Tx interface with the assistance of comprehensive band structures that are appreciated by ultraviolet photoelectron spectroscopy. Also, sensors were employed as a 7 × 7 sensor matrix with a large area for tactile sensing and incorporated onto the volunteer hand to determine the artificial skin application.

Washable stability and sensitive piezoresistive sponge sensor

A washable and sensitivity flexible piezoresistive pressure sensor is necessary to realize the practical application in intelligent wearable field. However, it remains a great challenge to prepare piezoresistive pressure sensor with high sensitivity and superior washable stability. Herein, a 3D porous structure carbon nanotube (CNT)/carbon black (CB) sponge piezoresistive pressure sensor was constructed by salt-template and physical absorption method. The CNT/CB sponge with porous structure exhibited excellent compression cycle stability (372 ± 6 kPa), and lower hysteresis (≤9.38 % residual strain). Impressively, the assembled CNT/CB sponge pressure sensor showed high sensitivity of 164 kPa−1 (0–2.1 kPa), 8000 times long-term cycles stability and superior washable stability. The pressure sensor was used for human motion monitoring, such as: pulse, joint bending, micro-expression, foot type recognition and gesture detection. Furthermore, the pressure sensor can be connected with bluetooth module to monitor the gait characteristics of human movement in real time to prevent foot membrane inflammation.