Flexible Piezoresistive Pressure Sensor Research Articles

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.

Graphene-Based Smart Insole Sensor for Pedobarometry and Gait Analysis

Pedobarography and gait analysis are crucial components of healthcare, given their close association with chronic illnesses. Wearable electronics, such as smart insole systems, have emerged as a promising technology for remote and long-term gait monitoring, providing more accurate data in real time. To meet the demands of wearable electronics, components with both flexibility and sensitivity are crucial. This study employed low-cost and rapid prototyping techniques to develop a flexible piezoresistive pressure sensor with a graphene nanoplatelet/ polycaprolactone composite spray-deposited on a fiber-based commercial insole. The sensor demonstrated a pressure range of up to 400 kPa and high linear sensitivity (∼ 0.376 kPa-1), making it suitable for pedobarography. Additionally, to test its practical application, the sensor was linked to a low-power programmable data logging and transmitting device for data acquisition over custom designed android application. The sensor can differentiate various motion states, such as walking, jumping, and running, while also demonstrating potential for long-term stability.

Compressible, reliable, and flexible pressure sensor based on carbonized melamine foam capped by Ti3C2Tx MXene for versatile applications

Flexible pressure sensors with excellent sensing properties have caused widespread concern due to their various potential applications in personalized healthcare monitoring, disease diagnosis and human-machine interaction. However, the challenge remains to simultaneously achieve high sensitivity and low fabrication cost. Herein, a compressible, reliable and flexible piezoresistive pressure sensor of Ti3C2Tx MXene/carbonized melamine foam (MXene@CMF) was fabricated by coating two-dimensional (2D) Ti3C2Tx MXene on the skeleton of carbonized melamine foam. Owing to the synergic effect of carbonized melamine foam and Ti3C2Tx MXene, the as-prepared MXene@CMF shows excellent performance in terms of high sensitivity (similar to 0.166 kPa−1), fast response time (similar to 120 ms), subtle pressure detection limit of 9.6 Pa, and excellent cyclic stability. Benefiting from these excellent sensing capabilities, the applications of the sensor for artery pulse detection and bending joint monitoring are demonstrated. Moreover, with the assistance of the convolutional neural network, the proposed sensor can realize the exquisite identification of letter strokes with an average recognition accuracy of 96.3%. Based on the merit of high sensing performance and versatile application prospects, our proposed sensor may have great potential in medical and healthcare, human motion detection, handwriting verification, and anticounterfeiting applications.

Elucidating the Conducting Mechanisms in a Flexible Piezoresistive Pressure Sensor Using Reduced Graphene Oxide Film in Silicone Elastomer

Sensors as a composite film made from reduced graphene oxide (rGO) structures filled with a silicone elastomer are soft and flexible, making them suitable for wearable applications. The sensors exhibit three distinct conducting regions, denoting different conducting mechanisms when pressure is applied. This article aims to elucidate the conduction mechanisms in these sensors made from this composite film. It was deduced that the conducting mechanisms are dominated by Schottky/thermionic emission and Ohmic conduction.

Resonant printing flexible piezoresistive pressure sensor with spherical microstructures

Flexible pressure sensors have attracted much attention in academia owing to their wide-ranging applications in wearable electronics, medical electronics and digital health. However, practical engineering applications have been restricted because of limitations in efficiency, manufacturing costs and sensitivity. In this work, we propose an innovative method for high-efficiency printing of microstructures that replaces traditional inverted mold methods. We developed a high-sensitivity flexible piezoresistive pressure (FPP) sensor with a high manufacturing efficiency and low manufacturing cost. The sensor was encapsulated by connecting a polydimethylsiloxane film with microstructures prepared using the sandpaper-molding method, and then integrated with an interdigital electrode and spherical micro-structures fabricated via resonant printing. In this way, the manufacturing process was simplified by breaking it down into two steps. The performance of the sensor was assessed by conducting experiments under different pressure regimes. The results demonstrated ultra-high sensitivity (0.0058–0.024 kPa−1) and a wide pressure detection range (1–100 kPa), spanning the entire range of pressure monitoring typically observed for vital and health signals. The response time of the sensor was less than 72 ms. Furthermore, the performance of the fabricated sensor was highly stable after 1000 bending cycle. The potential applications of the FPP sensor are discussed in area such as the human body and mouse.

Application of flexible pressure sensor based on AgNWs in human motion detection

PurposeIn this paper, a new flexible piezoresistive pressure sensor which uses non-woven fabric as the flexible substrate and sliver nanowires (AgNWs) as the conductive materials was reported.Design/methodology/approachThe compression test of the pressure sensors was carried out at different compression frequencies and found that the sensors had more than 5,000 times reusability at high frequency.FindingsWhen pressure sensors were applied to different parts of the human body, such as fingers, elbows, knees and throat, the sensors respond differently to different degrees of movement.Originality/valueThe proposed pressure sensor has broad application prospects in the human motion detection.

A flexible piezoresistive pressure sensor comprising a microstructure printed with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) copolymers@graphene hybrid ink

The sensitivity of a sensor prepared by printing with the ink of PEDOT:PSS regulated by adding graphene is improved tremendously. Furthermore, human signals can be monitored effectively.

Synergistic control of engineered nanostructures toward sensitivity and reliability of a flexible piezoresistive pressure sensor

Nanostructures for a piezoresistive pressure sensor should be considered in terms of the interdigitated contacts formed through face-to-face assembly of the two electrodes to improve pressure-detecting performances.

Carbon Black/Multi-Walled Carbon Nanotube-Based, Highly Sensitive, Flexible Pressure Sensor.

Flexible piezoresistive pressure sensors have promising applications in wearable devices, artificial intelligence, and other fields. However, developing low-cost and high-performance pressure sensors still poses a great challenge. Herein, we utilize low-cost carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) mixed in porous polydimethylsiloxane to assemble a flexible piezoresistive pressure sensor combined with interdigitated electrodes. Simultaneously, the COMSOL Multiphysics simulation analysis was performed to predict the sensing behavior of the pressure sensor, which was verified by experiments; the preparation of the pressure sensor was guided according to the prediction. Additionally, we studied the effects of the mixed conductive filler's weight ratio, the shape of the interdigital electrode, and the line width and spacing of the interdigital electrode on the performance of the sensor. Based on the interaction of the 3D porous structure and the synergistic conductive network of CB/MWCNTs, the prepared pressure sensor exhibits a high sensitivity of 3.57 kPa-1 (∼21 kPa), a wide detection range of 0-275 kPa, fast response time (96 ms), fast recovery time (198 ms), good durability (about 3000 cycles), and good flexibility. Moreover, the fabricated sensor can monitor and recognize human activities (such as finger bending and mouse clicking), indicating that it has great potential in flexible wearable devices and other fields. It is worth noting that the preparation process of the entire pressure sensor was simple, low cost, and environmentally friendly, which provides a certain basis for industrial and commercial applications.

A low-cost 3D flexible pressure sensor with a wide working range prepared from articles for daily use for monitoring human motion

Flexible piezoresistive pressure sensors have attracted extensive attention due to their portability, flexibility, and high sensitivity. The development of low-cost, high-performance flexible piezoresistive sensors is a key issue. We have prepared a three-dimensional (3D) Carbon@MF flexible piezoresistive pressure sensor by dipping melamine foam (MF) in inexpensive ink with amorphous carbon materials. The material cost per unit volume (cm3) is only $0.006. The sensor has excellent sensing performance: wide working range of 150 kPa, high sensitivity of 0.62 kPa−1, superior stability with 2500 cycles of loading, fast response ability (30 ms). The sensor can monitor various human physiological activities such as different breathing modes, pulse, limb bending, and walking speed etc.

Flexible piezoresistive pressure sensors based on nanocellulose aerogels for human motion monitoring: A review

Recently, flexible piezoresistive pressure sensors have become a research hotspot owing to their broad applications in the fields of electronic skin, smart wearability, and human motion monitoring. The three-dimensional (3D) porous nanocellulose aerogel has attracted more attention as an ideal pressure sensor candidate due to the excellent characteristics of low density, unique pore structure, superior mechanical properties, and good biocompatibility and biodegradability. Herein, a comprehensive review of the recent advances in flexible piezoresistive pressure sensors based on nanocellulose aerogels is summarized, including the piezoresistive sensing working principles, different design strategies of nanocellulose aerogel sensors, various practical applications of nanocellulose aerogels in human motion monitoring, and future perspectives and challenges, which is expected to provide help for the development of flexible piezoresistive pressure sensors.

Flexible Pressure Sensor Decorated with MXene and Reduced Graphene Oxide Composites for Motion Detection, Information Transmission, and Pressure Sensing Performance.

Although fiber-based flexible piezoresistive pressure sensors have received extensive attention because of their simple fabrication and easy integration, the common practice of using a single material as the sensing layer often leads to unsatisfactory sensitivity and a limited sensing range. Herein, we exploit the combination of reduced graphene oxide (rGO) and two-dimensional transition-metal carbides and nitrides (MXene), use a polyester filament (PET) as the fiber matrix, and fabricate an MX/rGO PET-based flexible pressure sensor using the "dipping-drying" method. A systematic study is conducted concerning the effect of the dip-coating sequence and material combination on the sensor's resistance and sensitivity, which reveals that MX/rGO PET has the smallest resistance and the highest sensitivity (1.24 kPa-1). A series of tests are conducted to evaluate the pressure sensing characteristics of the MX/rGO PET-based pressure sensor, confirming its good linearity, fast response speed, low detection limit, and stable performance. In addition, the sensor has been successfully used to monitor various human joint activities and physiological signals such as breathing, demonstrating great application potential in the field of personal health care. To further enhance the practical utility, an APP has been designed to analyze and display the collected signals, and the constructed sensor network also provides an ingenious method for information encryption and transmission via pressure sensing. In all, the MX/rGO PET-based pressure sensor proposed in this work is expected to provide a competitive scheme for wearable flexible electronic devices in information transmission and human-computer interaction in the future.

High-Performance Flexible Piezoresistive Pressure Sensor Printed with 3D Microstructures.

Flexible pressure sensors have been widely used in health detection, robot sensing, and shape recognition. The micro-engineered design of the intermediate dielectric layer (IDL) has proven to be an effective way to optimize the performance of flexible pressure sensors. Nevertheless, the performance development of flexible pressure sensors is limited due to cost and process difficulty, prepared by inverted mold lithography. In this work, microstructured arrays printed by aerosol printing act as the IDL of the sensor. It is a facile way to prepare flexible pressure sensors with high performance, simplified processes, and reduced cost. Simultaneously, the effects of microstructure size, PDMS/MWCNTs film, microstructure height, and distance between the microstructures on the sensitivity and response time of the sensor are studied. When the microstructure size, height, and distance are 250 µm, 50 µm, and 400 µm, respectively, the sensor shows a sensitivity of 0.172 kPa−1 with a response time of 98.2 ms and a relaxation time of 111.4 ms. Studies have proven that the microstructured dielectric layer printed by aerosol printing could replace the inverted mold technology. Additionally, applications of the designed sensor are tested, such as the finger pressing test, elbow bending test, and human squatting test, which show good performance.

Silver nanoparticles decorated meta-aramid nanofibrous membrane with advantageous properties for high-performance flexible pressure sensor

Flexible pressure sensors have received tremendous attention for various wearable applications. However, it remains a critical challenge to develop a flexible pressure sensor with excellent sensitivity performances and multiple advantageous properties. Herein, a high-performance flexible piezoresistive pressure sensor PMIA@PDA@Ag was developed, which sensitive component is consisted of Ag nanoparticles decorated polydopamine (PDA)-modified meta-aramid (poly(m-phenylene isophthalamide), PMIA) nanofibrous membrane. The PMIA@PDA@Ag pressure sensor shows excellent mechanical, thermal insulation, antibacterial and breathable properties, as well as remarkable sensing performances including high sensitivity, wide detectable pressure range, rapid response speed and good cyclic durability. In addition, it also shows great sensing performances in monitoring various human behaviors in real-time, including large-scale motions and subtle physiological signals.

An Intelligent Tactile Sensor Based on Interlocked Carbon Nanotube Array for Ultrasensitive Physiological Signal Detection and Real‐Time Monitoring

AbstractHuman–robot interaction has become increasingly important due to technological advances in robotics. The tactile sensation range of humans ranges from detecting the touch of a hair (≈0.6 Pa) to the touch of a finger (&lt;10 kPa). Therefore, flexible pressure sensors with high sensitivity and a suitable pressure response range are vital to human–robot interaction. However, it is difficult to attain both sensitivity and a suitable pressure response in one device. This study proposes and prepares a flexible piezoresistive pressure sensor based on carbon nanotubes (CNTs)/carbon cloth (CC). The high performance of the sensor stems from the interlocked CNTs structure and the multistage contact at different pressures. The interlocked CNT structure endows the sensor with a low detection limit (≈0.15 Pa) and high sensitivity (120 kPa−1) in the pressure range (0–0.5 kPa). The flexible pressure sensor with an interlocked structure has significant potential as a wearable biosensor for healthcare applications. Furthermore, the sensor facilitates grasping soft objects due to high tactile sensitivity. In addition, the CNTs/CC‐based sensor can also be used as a bending sensor to develop a hand grasping model. Therefore, the proposed CNTs/CC‐based sensor is well suited for human–machine interaction and improving the tactile perception of robots.

High sensitivity and broad linearity range pressure sensor based on hierarchical in-situ filling porous structure

Flexible piezoresistive pressure sensor with high sensitivity over a broad linearity range have been attracting tremendous attention for its applications in health monitoring, artificial intelligence, and human-machine interfaces. Herein, we report a hierarchical in-situ filling porous piezoresistive sensor (HPPS) by direct ink writing (DIW) printing and curing of carbon nanofibers (CNFs)/polydimethylsiloxane (PDMS) emulsion. Hierarchical geometry significantly increases the contact area, distributes stress to multilayered lattice and internal porous structure, resulting in a broad sensing range. Moreover, unlike conventional hollow porous structure, the CNFs networks in-situ filling porous structure generates more contact sites and conductive pathways during compression, thereby achieving high sensitivity and linearity over entire sensing range. Therefore, the optimized HPPS achieves high sensitivity (4.7 kPa−1) and linearity (coefficient of determination, R2 = 0.998) over a broad range (0.03–1000 kPa), together with remarkable response time and repeatability. Furthermore, the applications in diverse pressure scenarios and healthcare monitoring are demonstrated.