High-performance Flexible Pressure Sensors Research Articles

Controllable Graphene Wrinkle for a High-Performance Flexible Pressure Sensor.

Flexible pressure sensors have aroused tremendous attention, owing to their broad applications in healthcare, robotics, and prosthetics. So far, it remains a critical challenge to develop low-cost and controllable microstructures for flexible pressure sensors. Herein, a high-sensitivity and low-cost flexible piezoresistive sensor was developed by combining a controllable graphene-nanowalls (GNWs) wrinkle and a polydimethylsiloxane (PDMS) elastomer. For the GNWs-PDMS bilayer, the vertically grown GNWs film can effectively improve the interface strength and form delamination-free conformal wrinkles. More importantly, a controllable microstructure can be easily tuned through the thermal wrinkling method. The wrinkled graphene-nanowalls (WG) piezoresistive sensor has a high sensitivity (S = 59.0 kPa-1 for the 0-2 kPa region and S = 4.8 kPa-1 for the 2-20 kPa region), a fast response speed (<6.9 ms), and a low limit of detection (LOD) of 2 mg (∼0.2 Pa). The finite element method was used to analyze the working mechanism of the sensor, which revealed that the periods of the wrinkles play a dominant role in the performances of the sensors. These prominent merits enable wrinkled graphene sensors to successfully detect various signals from a weak stimulus to large pressures, for example, the detection of weak gas and plantar pressure. Furthermore, object manipulation, tactile imaging, and braille recognition applications have been demonstrated, showing their great potential in prosthetics limbs and intelligent robotics.

Wearable pressure sensor based on MXene/single-wall carbon nanotube film with crumpled structure for broad-range measurements

High-performance flexible pressure sensors are attracting great interest owing to their potential applications for electronic skins, human–machine interfaces, and biomedical diagnostics. However, there remain significant challenges for the fabrication of low-cost and high-sensitivity sensors. Here, we report the preparation of Ti3C2T x MXene/single-wall carbon nanotube (SWNT) composite films through vacuum-assisted filtration followed by thermal shrinkage. SWNTs can effectively prevent MXenes from stacking and improve the electrical performance of the films. The films are used as a flexible piezoresistive sensor for pressures ranging from 33 Pa to 130 kPa. And experimental test results indicate that the fabricated pressure sensors have high sensitivity (116.15 kPa−1 below 40 kPa and 12.7 kPa−1 at 40–130 kPa), a fast response time of 13 ms, and long-term stability over 6000 periods. The sensor can be used to monitor human physiological signals, such as finger movements, voice detection, and wrist pulse in real-time. Moreover, a 4 × 4 sensor array was successfully applied in the pressure distribution mapping of different objects, indicating that the pressure sensor can be applied in electronic skin, medical devices, and other wearable devices.

Study on the forming and sensing properties of laser-sintered TPU/CNT composites for plantar pressure sensors

Conductive polymer composites (CPCs) combining with specific microstructures (micropores, microcracks, etc.) can exhibit unique resistance response changes, which can be widely regarded as an effective way to improve sensing performance. This study takes advantage of the characteristics of the formation of tiny pores between crystal grains during selective laser sintering (SLS) processing to introduce a microporous structure into the thermoplastic polyurethane (TPU)/carbon nanotube (CNT) sensing element to prepare a three-dimensional porous conductive structure. The effect of the SLS process on sensing sensitivity, accuracy, and density was studied, and its sensing and forming mechanism were discussed. By adjusting SLS process parameters to control the performance of porous structure sensor elements, a final TPU/CNT sensor element with a wide pressure detection range, high sensitivity, a fast response time, and good stability and durability was developed. Finally, the optimal performance of the developed flexible pressure sensor was successfully used to detect the pressure distribution of the human foot. This study provided a simple and effective research method to develop high-performance flexible pressure sensors.

Tunable piezoresistivity of low percolation threshold micro-nickel wires/PDMS conductive composite regulated by magnetic field

High-performance flexible pressure sensors with percolative composites using aligned μNi wires as conductive fillers for clinical diagnosis.

Wearable pressure sensor for athletes’ full-range motion signal monitoring

In order to real-time grasp of various physiological signals of athletes during sports, a high-performance flexible pressure sensor that can monitor various physiological signals and human motion was designed. Porous polydimethylsiloxane (PDMS) foam prepared by the sacrificial template method and graphene as raw materials were used to prepare a flexible pressure sensor with wide working range (0–100 kPa), ultra-high sensitivity (the average sensitivity in the range of 0–30 kPa is 17.9 kPa−1, the sensitivity in the range of 30–100 kPa reaches 79 kPa−1), fast response ability (response time is 20 ms) and long-term work stability (more than 10 000 cycles). The excellent performance of this pressure sensor depends on the use of PDMS foam with a high elastic modulus and the graphene loading level is controlled to an appropriate ratio. Finally, we used the conductive porous PDMS foam based flexible pressure sensor to demonstrate accurate and real-time monitoring of athletes’ tiny physiological signals (including pulse and electrocardiograph signals), vocalization and facial emotions, as well as violent joint and limb movements (including joint bending, walking, squats, jogging, and jumping), showing the potential in coaching athletes.

A High-Performance Flexible Pressure Sensor Realized by Overhanging Cobweb-like Structure on a Micropost Array.

Recent years have seen a rapid development of electronic skin for wearable devices, autonomous robotics, and human-machine interaction. As a result, the demand for flexible pressure sensors as the critical sensing element in electronic skin is also increasing. These sensors need to feature high sensitivity, short response time, low detection limit, and so on. In this paper, inspired from the cobweb in nature, we propose a piezoresistive pressure sensor by forming a cobweb-like network made of a zinc octaethylphorphyrin (ZnOEP)/carbon nanotube (CNT) hybrid on an array of polydimethylsiloxane (PDMS) microposts. The hybrid material exhibits excellent adhesion to PDMS, benefitting from ZnOEP's low Young's modulus and the nonpolar bonding between ZnOEP and PDMS such that no delamination and resistance variation are found after thousands of cycles of bending and twisting. With the overhanging morphology of the ZnOEP/CNT network on the micropost array, we realized a pressure sensor with an ultrahigh sensitivity of 39.4 kPa-1, a super-fast response time of 3 ms, a low detection limit of 10 Pa, and a reproducible response without degradation after 5000 cycles of pressure loading/unloading. The sensor can be employed for a variety of applications, including wrist pulse measurement, sound level detection, mechanical vibration monitoring, etc., proving its great potential for use in electronic skin systems.

Processing Natural Wood into a High-Performance Flexible Pressure Sensor

Flexible pressure sensors have received wide attention because of their potential applications in wearable electronics and electronic skins (e-skins). However, the high performance of the pressure sensors relies principally on the introduction of complex surface microstructures, which often involves either complicated procedures or costly microfabrication methods. Moreover, these devices predominantly use synthetic polymers as flexible substrates, which are generally nonbiodegradable or not ecofriendly. Here, we report a facile and scalable processing strategy to convert naturally rigid wood into reduced graphene oxide (rGO)-modified flexible wood (FW/rGO) via saw cutting, chemical treatment, and rGO coating, resulting in high-performance wood-based flexible piezoresistive pressure sensors. Benefiting from the largely deformable ribbon-like surface microstructures, the obtained wood-based pressure sensor displayed a high sensitivity of 1.85 kPa-1 over a broad linear range up to 60 kPa and showed high stability over 10 000 cyclic pressings. The favorable sensing performance of the pressure sensor allows for accurate recognition of finger movements, acoustic vibrations, and real-time pulse waves. Moreover, a large-area pressure sensor array has been successfully assembled on one piece of flexible wood for spatial pressure mapping. The proposed strategy of directly using natural wood for high-performance flexible pressure sensors is simple, low-cost, sustainable, and scalable, opening up a new avenue for the development of next-generation wearable electronics and e-skins.

CNT-coated magnetic self-assembled elastomer micropillar arrays for sensing broad-range pressures

Cost-effective and broad-range flexible pressure sensors are highly desirable for use in wearable electronic devices for a wide range of sensitive measurements, from pulse to dynamic human motion. Herein, we present a high-performance flexible pressure sensor based on carbon nanotube-coated elastomer micropillar arrays (EMPAs). The EMPAs are prepared using a facile magnetic-field-induced self-assembly technique, which provides a simple and low-cost strategy to fabricate microstructured elastomers for constructing high-performance pressure sensors. Since high-ratio microstructured elastomers are employed as the sensing materials, our pressure sensors show a superior pressure-sensing performance, with a high sensitivity of 0.497 kPa−1 in the low-pressure regime of <100 Pa, a broad sensing range from a few pascals up to 80 kPa, and a fast response time of <0.2 s. Finally, we demonstrate that the sensor can be used to not only detect human arthrosis movements, but also to monitor subtle human physiological activities.

Quantum effect-based flexible and transparent pressure sensors with ultrahigh sensitivity and sensing density

Although high-performance flexible pressure sensors have been extensively investigated in recent years owing to their diverse applications in biomedical and information technologies, fabricating ultrasensitive sensors with high pixel density based on current transduction mechanisms still remains great challenging. Herein, we demonstrate a design idea based on Fowler-Nordheim tunnelling effect for fabrication of pressure sensors with ultrahigh sensitivity and sensing density by spin-coating extremely low urchin-like hollow carbon spheres (less than 1.5 wt.%) dispersed in polydimethylsiloxane, which is distinct from the current transduction mechanisms. This sensor exhibits an ultrahigh sensitivity of 260.3 kPa−1 at 1 Pa, a proof-of-concept demonstration of a high sensing density of 400 cm−2, high transparency and temperature noninterference. In addition, it can be fabricated by an industrially viable and scalable spin-coating method, providing an efficient avenue for realizing large-scale production and application of ultrahigh sensitivity flexible pressure sensors on various surfaces and in in vivo environments.

A facile and cost-effective approach to fabrication of high performance pressure sensor based on graphene-textile network structure

High performance flexible pressure sensors have received tremendous attention due to the potential applications in wearable electronics and humanoid robotics. Herein, a low-cost, time-saving fabrication strategy is reported to efficiently construct highly sensitive and scalable pressure sensors based on graphene-textile. The flexible pressure sensor was prepared through a dip coating approach where the graphene ink was used as the active material and textile with ordered network structure, serves as the flexible support matrix. By increasing the number of layers of the textile, the pressure sensor could achieve a high linear sensitivity value of 0.23 kPa−1, a broad pressure range from 0 to 140 kPa, and a durability over 800 cycles. It was found that the network structure of the graphene-textile contributed to the enhancement of the sensing performance due to the piezoresistive effect resulting from the deformation of conductive graphene-textile. Moreover, by virtue of its exceptional properties, it is demonstrate that the high performance flexible pressure sensor could be further used to detect human physiological signals, such as wrist pulse, finger press detection and walking state monitoring, indicating its promising potential to flexible and wearable electronics.

A flexible pressure sensor based on melamine foam capped by copper nanowires and reduced graphene oxide

High-performance flexible pressure sensors with high sensitivity and large sensing range are vital components of the system for physiological signals collection, human-machine interaction and artificial intelligence. Herein, a flexible pressure sensor of copper nanowires/reduced graphene oxide/melamine foam (CuRGOMF) was fabricated by coating reduced graphene oxide (RGO) and in-situ growing of copper nanowires (Cu NWs) on the skeleton of melamine foam (MF). The as-prepared sensor features high sensitivity at wide working range (0.011 kPa−1, 0–1.5 kPa; 0.088 kPa−1, 1.5–10 kPa and 0.024kPa−1, 10–18 kPa) and cyclic stability (5000 cycles). The human motions of finger bending, grasping, clicking the mouse and picking up/dropping objects can be successfully detected in real-time based on the CuRGOMF sensor. The CuRGOMF sensor illustrates a promising candidate for healthcare monitoring and wearable device in artificial intelligence.

Anodized Aluminum Oxide-Assisted Low-Cost Flexible Capacitive Pressure Sensors Based on Double-Sided Nanopillars by a Facile Fabrication Method.

Flexible pressure sensors have garnered enormous attention in recent years as they hold great promise in wearable electronic devices. However, the realization of a high-performance flexible pressure sensor via a facile and cost-effective approach still remains a challenge. In this work, a capacitive pressure sensor based on a poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] dielectric film that incorporates nanopillars into both sides is demonstrated. Unlike the previous complicated and expensive methods, large-scale regular and uniform nanopillars are easily and economically achieved by the pattern transfer of anodized aluminum oxide templates. The double-sided nanopillars constituting the P(VDF-TrFE) dielectric layer enable the pressure sensor with high sensitivity (∼0.35 kPa-1), wide working range (4 Pa to 25 kPa), short response time (∼48 ms), and excellent durability. In addition to these salient features, our sensor also exhibits superior performances under bending states, ensuring that it can be used for detecting diverse practical stimuli as experimentally validated by perceiving real-time and in-site human physiological signals and body motions that, respectively, correspond to the low- and high-pressure range. A sensor array is finally constructed and is shown to be capable of perceiving the spatial pressure distribution of either a contact or noncontact object. These demonstrations show a promising future of our sensor in healthcare monitoring, smart robot skin, and human-machine interfaces.

Flexible Piezoresistive Sensor with the Microarray Structure Based on Self-Assembly of Multi-Walled Carbon Nanotubes.

High-performance flexible pressure sensors have great application prospects in numerous fields, including the robot skin, intelligent prosthetic hands and wearable devices. In the present study, a novel type of flexible piezoresistive sensor is presented. The proposed sensor has remarkable superiorities, including high sensitivity, high repeatability, a simple manufacturing procedure and low initial cost. In this sensor, multi-walled carbon nanotubes were assembled onto a polydimethylsiloxane film with a pyramidal microarray structure through a layer-by-layer self-assembly system. It was found that when the applied external pressure deformed the pyramid microarray structure on the surface of the polydimethylsiloxane film, the resistance of the sensor varied linearly as the pressure changed. Tests that were performed on sensor samples with different self-assembled layers showed that the pressure sensitivity of the sensor could reach , which ensured the high dynamic response ability and the high stability of the sensor. Moreover, it was proven that the sensor could be applied as a strain sensor under the tensile force to reflect the stretching extent or the bending object. Finally, a flexible pressure sensor was installed on five fingers and the back of the middle finger of a glove. The obtained results from grabbing different weights and different shapes of objects showed that the flexible pressure sensor not only reflected the change in the finger tactility during the grasping process, but also reflected the bending degree of fingers, which had a significant practical prospect.

Highly Sensitive Flexible Piezoresistive Pressure Sensor Developed Using Biomimetically Textured Porous Materials.

In recent times, high-performance flexible pressure sensors that can be fabricated in an environmentally friendly and low-cost manner have received considerable attention owing to their potential applications in wearable health monitors and intelligent soft robotics. This paper proposes a highly sensitive flexible piezoresistive pressure sensor based on hybrid porous microstructures that can be designed and fabricated using a bio-inspired and low-cost approach employing the Epipremnum aureum leaf and sugar as the template. The sensitivity and detection limit of the obtained pressure sensor can be as high and low as 83.9 kPa-1 (<140 Pa) and 0.5 Pa, respectively. According to the mechanism and simulation analyses, the hybrid porous microstructures lower the effective elastic modulus of the sensor and introduce an additional pore resistance, which increases the contact area and conductive path with loads, thereby contributing to the high sensitivity that exceeds that of traditional microstructured pressure sensors. Real-time monitoring of human physiological signals such as finger pressing, voice vibration, swallowing activity, and wrist pulse is demonstrated for the proposed device. The high performance and easy fabrication of the hybrid porous microstructured sensor can encourage the development of a novel approach for the design and fabrication of future pressure sensors.

Polyaniline Nanofiber Wrapped Fabric for High Performance Flexible Pressure Sensors.

The rational design of high-performance flexible pressure sensors with both high sensitivity and wide linear range attracts great attention because of their potential applications in wearable electronics and human-machine interfaces. Here, polyaniline nanofiber wrapped nonwoven fabric was used as the active material to construct high performance, flexible, all fabric pressure sensors with a bottom interdigitated textile electrode. Due to the unique hierarchical structures, large surface roughness of the polyaniline coated fabric and high conductivity of the interdigitated textile electrodes, the obtained pressure sensor shows superior performance, including ultrahigh sensitivity of 46.48 kPa−1 in a wide linear range (<4.5 kPa), rapid response/relaxation time (7/16 ms) and low detection limit (0.46 Pa). Based on these merits, the practical applications in monitoring human physiological signals and detecting spatial distribution of subtle pressure are demonstrated, showing its potential for health monitoring as wearable electronics.

Flexible, Tunable, and Ultrasensitive Capacitive Pressure Sensor with Microconformal Graphene Electrodes.

High-performance flexible pressure sensors are highly desirable in health monitoring, robotic tactile, and artificial intelligence. Construction of microstructures in dielectrics and electrodes is the dominating approach to improving the performance of capacitive pressure sensors. Herein, we have demonstrated a novel three-dimensional microconformal graphene electrode for ultrasensitive and tunable flexible capacitive pressure sensors. Because the fabrication process is controllable, the morphologies of the graphene that is perfectly conformal with the electrode are controllable consequently. Multiscale morphologies ranging from a few nanometers to hundreds of nanometers, even to tens of micrometers, have been systematically investigated, and the high-performance capacitive pressure sensor with high sensitivity (3.19 kPa-1), fast response (30 ms), ultralow detection limit (1 mg), tunable-sensitivity, high flexibility, and high stability was obtained. Furthermore, an ultrasensitivity of 7.68 kPa-1 was successfully achieved via symmetric double microconformal graphene electrodes. The finite element analysis indicates that the microstructured graphene electrode can enhance large deformation and thus effectively improve the sensitivity. Additionally, the proposed pressure sensors are demonstrated with practical applications including insect crawling detection, wearable health monitoring, and force feedback of robot tactile sensing with a sensor array. The microconformal graphene may provide a new approach to fabricating controllable microstructured electrodes to enhance the performance of capacitive pressure sensors and has great potential for innovative applications in wearable health-monitoring devices, robot tactile systems, and human-machine interface systems.

Capacitive Pressure Sensor with Wide-Range, Bendable, and High Sensitivity Based on the Bionic Komochi Konbu Structure and Cu/Ni Nanofiber Network.

High-performance flexible pressure sensors have an essential application in many fields such as human detection and human-computer interaction. Herein, on the basis of the dielectric layer of a bionic komochi konbu structure, we propose a low-cost and novel capacitive sensor that achieves high sensitivity and stability over a broad range of tactile pressures. Further, the flexible and durable electrode layer of the transparent junctionless copper/nickel-nanonetwork was prepared based on electrospinning and electroless deposition techniques, which ensured high bending stability and high cycle stability of our sensor. More importantly, because of the sizeable protruding structure and internal micropores in the elastomer structure we designed, the inward curling of the protruding structure and the effectual closing of the micropores increase the effective dielectric constant under the action of the compressive force, improving the sensitivity of the sensor. Measured response and relaxation time (162 ms) are 250 times faster than those of a conventional flatpolydimethylsiloxanecapacitive sensor. In addition, the fabricated capacitive pressure sensor demonstrates the ability to be used on wearable applications, not only to quickly recognize the tapping and bending of a finger but also to show that the pressure of the finger can be sensed when the finger grabs the object. The sensors we have developed have shown great promise in practical applications, such as human rehabilitation and exercise monitoring, as well as human-computer interaction control.

Recent Advances in Flexible and Wearable Pressure Sensors Based on Piezoresistive 3D Monolithic Conductive Sponges.

High-performance flexible strain and pressure sensors are important components of the systems for human motion detection, human-machine interaction, soft robotics, electronic skin, etc., which are envisioned as the key technologies for applications in future human healthcare monitoring and artificial intelligence. In recent years, highly flexible and wearable strain/pressure sensors have been developed based on various materials/structures and transduction mechanisms. Piezoresistive three-dimensional (3D) monolithic conductive sponge, the resistance of which changes upon external pressure or stimuli, has emerged as a forefront material for flexible and wearable pressure sensor due to its excellent sensor performance, facile fabrication, and simple circuit integration. This review focuses on the rapid development of the piezoresistive pressure sensors based on 3D conductive sponges. Various piezoresistive conductive sponges are categorized into four different types and their material and structural characteristics are summarized. Methods for preparation of the 3D conductive sponges are reviewed, followed by examples of device performance and selected applications. The review concludes with a critical reflection of the current status and challenges. Prospects of the 3D conductive sponge for flexible and wearable pressure sensor are discussed.

Flexible pressure sensor using carbon nanotube-wrapped polydimethylsiloxane microspheres for tactile sensing

High-performance flexible pressure sensors are highly demanded in wearable electronics for various applications. Herein, a flexible pressure sensor was designed using carbon nanotube (CNT)-wrapped polydimethylsiloxane (PDMS) microspheres (MSs). Compared to the device with flat configuration, the sensor with CNT-wrapped PDMS MSs has an enhanced sensitivity of −0.111 kPa−1, with a lowest detectable pressure of 0.02 kPa, response time approximately 0.1 s, as well as a long-term dynamic stability more than 10,000 cycles. The sensors with CNT-wrapped PDMS MSs show the potential to be integrated into wearable electronics for real-timely monitoring the physiological signals.

Tunable wrinkled graphene foams for highly reliable piezoresistive sensor

The rapid development of flexible electronics and artificial intelligence brings an urgent demand for high-performance flexible pressure sensors. In this paper, a method is proposed to prepare wrinkled graphene foam by freeze-drying and post-annealing method, where zinc chloride is used to tune structure. A piezoresistive pressure sensor based on the wrinkled graphene foam is assembled. Benefiting from the unique contact interface of wrinkled microstructures, as well as the mechanical strength and the superior resilience of the foam structure, the piezoresistive pressure sensor exhibits a reliable stability (>105 cycles), short response time(150 ms)and low relaxation time (120 ms). And it also demonstrates good performances in various applications, such as pulse detection, voice recognition as well as the finger joints movement. It is suggested that the pressure sensor based on wrinkled graphene foam has great potential in flexible electronics to achieve health monitoring and motion detection. It also paves the way for other wrinkled materials to be involved in the fabrication of pressure sensors.