Flexible Capacitive Pressure Sensor Research Articles

Transparent and Flexible Graphene Pressure Sensor with Self-Assembled Topological Crystalline Ionic Gel

This study demonstrates transparent and flexible capacitive pressure sensors using a high-k ionic gel composed of an insulating polymer (poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene), P(VDF-TrFE-CFE)) blended with an ionic liquid (IL; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide, [EMI][TFSA]). The thermal melt recrystallization of the P(VDF-TrFE-CFE):[EMI][TFSA] blend films develops the characteristic topological semicrystalline surface of the films, making them highly sensitive to pressure. Using optically transparent and mechanically flexible graphene electrodes, a novel pressure sensor is realized with the topological ionic gel. The sensor exhibits a sufficiently large air dielectric gap between graphene and the topological ionic gel, resulting in a large variation in capacitance before and after the application of various pressures owing to the pressure-sensitive reduction of the air gap. The developed graphene pressure sensor exhibits a high sensitivity of 10.14 kPa-1 at 20 kPa, rapid response times of <30 ms, and durable device operation with 4000 repeated ON/OFF cycles. Furthermore, broad-range detections from lightweight objects to human motion are successfully achieved, demonstrating that the developed pressure sensor with a self-assembled crystalline topology is potentially suitable for a variety of cost-effective wearable applications.

A highly sensitive and wide-range pressure sensor based on orientated and strengthened TPU nanofiber membranes fabricated by a conjugated electrospinning technology

Flexible capacitive pressure sensors as one of the crucial sensing components have aroused widespread concern because of their vital role in development of wearable artificial devices, healthcare biomonitoring and human-machine interface. However, there has been a considerable challenge to successfully fabricate them with a tradeoff between high sensitivity and wide sensing range by low-cost and convenient manufacturing methods. Herein, the orientated thermoplastic polyurethane elastomer rubber nanofiber membranes (TPU-O NMs) were prepared via conjugated electrospinning to build a dielectric layer for the flexible capacitive wide-range pressure sensors with high sensitivity. In addition, polyaniline (PANI) was in situ polymerized on the TPU-O NMs as flexible electrodes to maintain the conductive pathway, flexibility, and breathability of the sensors. The TPU@PANI NMs form a bamboo-raft-like microstructure between the aligned fibers under pressure, resulting in a significant increase in sensitivity and working range. Furthermore, the conductive TPU@PANI NMs can be stretched to 1600% without fracture. The assembled sensor has a high sensitivity of 31.73 kPa−1 and a fast response/recovery time of 96 ms. Meantime, it also exhibits a minimum detection limit of 1 Pa, an excellent stability of 10,000 cycles, and a good linearity in a broad working age (1 Pa∼122.5 kPa). Finally, the capacitive pressure sensor was successfully applied for full-range detection of human motions, such as finger bending, joint bending, and even boxing movements. Hence this highly sensitive and wide-range pressure sensor can be a promising candidate for healthcare monitoring, motion recognition, and wearable circuitry in artificial intelligence.

A Sensitive and Flexible Capacitive Pressure Sensor Based on a Porous Hollow Hemisphere Dielectric Layer

Capacitive pressure sensors based on porous structures have been widely researched and applied to a variety of practical applications. To date, it remains a big challenge to develop a capacitive pressure sensor with a high sensitivity and good linearity over a wide pressure range. In this paper, a sensitive, flexible, porous capacitive pressure sensor was designed and manufactured by means of the "salt template method" and man-made grooves. To this aim, the size of the salt particles used for forming pores/air voids, time taken for thorough dissolution of salt particles, and the depth of the man-made groove by a pin were taken into consideration to achieve a better effect. With pores and the groove, the sensor is more liable be compressed, which will result in a dramatic decrease in distance between the two electrodes and a conspicuous increase of the effective dielectric constant. The optimize-designed sensor represents a sensitivity 6-8 times more than the sensor without the groove in the pressure range of 0-10 kPa, not to mention the sensor without pores or the groove, and it can keep good linearity within the measurement range (0-50 kPa). Besides, the sensor shows a low detection limit of 3.5 Pa and a fast response speed (≈50 ms), which makes it possible to detect a tiny applied pressure immediately. The fabricated sensor can be applied to wearable devices to monitor finger and wrist bending, and it can be used in the object identification of mechanical claws and object cutting of mechanical arms, and so on.

Splashing-Assisted Femtosecond Laser-Activated Metal Deposition for Mold- and Mask-Free Fabrication of Robust Microstructured Electrodes for Flexible Pressure Sensors.

Flexible pressure sensors play an indispensable role in flexible electronics. Microstructures on flexible electrodes have been proven to be effective in improving the sensitivity of pressure sensors. However, it remains a challenge to develop such microstructured flexible electrodes in a convenient way. Inspired by splashed particles from laser processing, herein, a method for customizing microstructured flexible electrodes by femtosecond laser-activated metal deposition is proposed. It takes advantage of the catalyzing particles scattered during femtosecond laser ablation and is particularly suitable for moldless, maskless, and low-cost fabrication of microstructured metal layers on polydimethylsiloxane (PDMS). Robust bonding at the PDMS/Cu interface is evidenced by the scotch tape test and the duration test over 10 000 bending cycles. Benefiting from the firm interface, the developed flexible capacitive pressure sensor with microstructured electrodes presents several conspicuous features, including a sensitivity (0.22 kPa-1 ) 73 times higher than the one using flat Cu electrodes, ultralow detection limit (<1Pa), rapid response/recovery time (4.2/5.3ms), and excellent stability. Moreover, the proposed method, inheriting the merits of laser direct writing, is capable of fabricating a pressure sensor array in a maskless manner for spatial pressure mapping.

A flexible ultra-highly sensitive capacitive pressure sensor for basketball motion monitoring

Recently, flexible sensors with high sensitivity have been applied in wearable sports sensing field. Here, we reported a flexible and sensitive capacitive pressure sensor based on nylon textile and polyvinylidene fluoride (PVDF) dielectric film. From the experimental results, the sensor has an extremely high sensitivity of 33.5 kPa−1, a low detection limit of 0.84 Pa, a quick response time of 27 ms. Moreover, the pressure sensor shows excellent reliability under over 100,000 working cycles. With their superior overall performance, capacitive sensors have effectively proved their enormous potential for basketball motion monitoring. This research will promote the development of wearable sports sensors.

Highly Sensitive and Flexible Capacitive Pressure Sensors Based on Vertical Graphene and Micro-Pyramidal Dielectric Layer.

Many practical applications require flexible high-sensitivity pressure sensors. However, such sensors are difficult to achieve using conventional materials. Engineering the morphology of the electrodes and the topography of the dielectrics has been demonstrated to be effective in boosting the sensing performance of capacitive pressure sensors. In this study, a flexible capacitive pressure sensor with high sensitivity was fabricated by using three-dimensional vertical graphene (VG) as the electrode and micro-pyramidal polydimethylsiloxane (PDMS) as the dielectric layer. The engineering of the VG morphology, size, and interval of the micro-pyramids in the PDMS dielectric layer significantly boosted the sensor sensitivity. As a result, the sensors demonstrated an exceptional sensitivity of up to 6.04 kPa-1 in the pressure range of 0-1 kPa, and 0.69 kPa-1 under 1-10 kPa. Finite element analysis revealed that the micro-pyramid structure in the dielectric layer generated a significant deformation effect under pressure, thereby ameliorating the sensing properties. Finally, the sensor was used to monitor finger joint movement, knee motion, facial expression, and pressure distribution. The results indicate that the sensor exhibits great potential in various applications, including human motion detection and human-machine interaction.

Evaluation of blood pressure using a flexible and wearable capacitive pressure sensor.

In recent times, the high demand for flexible and wearable pressure sensors to monitor human health, particularly for patients afflicted with hypertension or high blood pressure (BP), has captured the keen interest of researchers. Capacitance-based flexible sensing devices offer real-time metrics regarding vital physiological parameters of the human body, such as BP and pulse rate (PR), thereby enabling the identification of cardiovascular complications. In this regard, we have developed a capacitive pressure sensor using polydimethylsiloxane (PDMS) and deionized water (DIW) and improved its key parameters by adding baking powder to PDMS-DIW. The sensor demonstrated excellent performance in static pressure measurements with a sensitivity of 0.021 Pa-1, detection limit of 1 Pa, and response time of 100 ms. We further investigated its application in human BP monitoring. The sensor successfully captured the oscillometric waveform (OMW) for all 160 participants and demonstrated excellent performance in accurately measuring BP, meeting all criteria outlined as the universal standard when compared with the reference devices: OMRON BP device and the gold-standard mercury-based sphygmomanometer. Furthermore, the sensor accurately provided the PR and agreed well with the reference BP device. Therefore, the developed BP sensor can be a viable alternative to replace the pressure sensors in existing BP devices.

Flexible Capacitive Pressure Sensor Based on a Double-Sided Microstructure Porous Dielectric Layer.

In the era of intelligent sensing, there is a huge demand for flexible pressure sensors. High sensitivity is the primary requirement for flexible pressure sensors, whereas pressure response range and resolution, which are also key parameters of sensors, are often ignored, resulting in limited applications of flexible pressure sensors. This paper reports a flexible capacitive pressure sensor based on a double-sided microstructure porous dielectric layer. First, a porous structure was developed in the polymer dielectric layer consisting of silicon rubber (SR)/NaCl/carbon black (CB) using the dissolution method, and then hemisphere microstructures were developed on both sides of the layer by adopting the template method. The synergistic effect of the hemispheric surface microstructure and porous internal structure improves the deformability of the dielectric layer, thus achieving high sensitivity (3.15 kPa-1), wide response range (0-200 kPa), and high resolution (i.e., the minimum pressure detected was 27 Pa). The proposed sensing unit and its array have been demonstrated to be effective in large-area pressure sensing and object recognition. The flexible capacitive pressure sensor developed in this paper is highly promising in applications of robot skin and intelligent prosthetic hands.

Sensitivity Improvements of Flexible Capacitive Pressure Sensors

In recent years, wearable electronic devices have developed rapidly and flexible sensors have been increasingly used in medical, aerospace, automotive, industrial and commercial applications. Flexible pressure sensors are critical in human-robot interaction, physiological signal monitoring and robotic haptics. Among them, capacitive type flexible pressure sensors are widely used due to their unsophisticated structure, good stability, small temperature deviation and environmentally friendly. Currently, most of capacitive sensors adopt the design with an electrical membrane inserted between two electrodes. This paper presents a comprehensive review of the development of flexible capacitive pressure sensor from the perspective of technological and engineering developments. This paper firstly introduces the capacitive pressure sensor by using the working mechanism as an entry point. Secondly, this paper discusses three general methods to increase the sensitivity of pressure sensing, including changing the dielectric microstructure, the dielectric constant, and different dielectric materials. Finally, the advantages and disadvantages of three methods for sensitivity improvement were discussed.

A Paper-Based Iontronic Capacitive Pressure Sensor for Human Muscle Motion Monitoring

In this study, a flexible capacitive pressure sensor based on filter paper substrate is proposed. The electric double layer (EDL) structure was constructed through the introduction of ionic liquid (IL), and the pressure-sensing was realized by using the change of ion-electron interface contact. The paper-based iontronic pressure sensor possesses a high sensitivity of 221.9 kPa−1, which is hundreds of times higher than that of the capacitive pressure sensors with conventional parallel plate structure, as well as an ultra-low detection limit of 2 Pa. The sensor exhibits an enormous application potential in healthcare wearable electronics and human-machine interface equipment.

Flexible capacitive pressure sensor array using acoustic-assisted fabrication of microstructures as surface and dielectric layers

Flexible pressure sensors have attracted considerable interests in the fields of intelligent robotics, human-machine interfaces and health monitoring for their portability, flexibility and real-time sensing performance. The microstructures of dielectric layer significantly affect the sensing performance of capacitive pressure sensors, but the common methods to fabricate microstructures always need physical and/or chemical molds, thus may not suit for low-cost fabrication of sensors. The wavy-shaped microstructures on the surface of fluid can be generated with the excitation of standing surface acoustic waves (SSAWs), which is suitable to be used as dielectric layers. Here, we developed a 6 × 6 flexible capacitive pressure sensor array with fingerprint-like surface cover layer and latticed dielectric layer. Both two layers were fabricated through the novel and rapid acoustic-assisted process. The fabricated capacitive sensor array exhibits high sensitivity of 28.8%/N for normal force loading, with spatial resolution of 1.5 mm. It has been successfully used to monitor the vibration of the Adam’s apple during swallowing and detecting the wrist activities in cycles. These studies demonstrate the great potential of the pressure sensor array with acoustically fabricated microstructures in wearable electronics for health monitoring. • A novel flexible pressure sensor array with spatial resolution of 1.5 mm for three-axis distributed force sensing is developed. • A novel and rapid acoustic-assisted process based on SSAWs is used to fabricate the surface layer and dielectric layer with microstructures. • The tactile sensor array features high flexibility and can be easily worn on neck or wrist for health monitoring. • Both Adam’s apple vibration sensing and wrist activity detection can be achieved by the developed tactile sensor array.

Biologically imitated capacitive flexible sensor with ultrahigh sensitivity and ultralow detection limit based on frog leg structure composites via 3D printing

Inspired by the geometry of the frog leg, flexible capacitive pressure sensors utilizing a three-dimensional (3D) array of biomimetic frog-leg structure composites for use as dielectric layer is proposed. We demonstrate a low-cost and facile strategy to fabricate a dielectric layer with 3D features via 3D printing technique. The feasibility of using bionic frog-leg structure as a dielectric layer in capacitive pressure sensors is evaluated via finite element analysis (FEA). Several dimensional parameters of the dielectric layer structure were optimized via an experimental study of the pressure-sensing characteristics. The 3D dielectric structure reduces the Young's modulus of the dielectric layer and retains the stability of the sensor. The flexible capacitive pressure sensor exhibits improved sensing performance, including a broad pressure detection range (0–200 kPa), ultralow detection limit (0.5 Pa), ultrahigh sensitivity (0.583 kPa−1 in the range of 0–1.2 kPa), rapid response and recovery (40 ms and 45 ms at 1 kPa), and reliable long-term stability. This work is expected to open up possible applications in intelligent robots, wearable electronic devices, information perception, and human–machine interaction.

Porous conductive electrode for highly sensitive flexible capacitive pressure sensor over a wide range

Flexible capacitive pressure sensors have aroused widespread concern due to their promising application in wearable electronics. However, the trade-off between sensitivity and response range is yet to be resolved. Herein, flexible capacitive pressure sensors are developed based on high concentration carbon nanotube(CNT)-doped porous electrode and an ultrathin porous polyvinylidene difluoride (PVDF) dielectric layer. Benefit from the continuous variations of contacting area between the porous electrode and dielectric layer, as well as the larger relative distance, the sensor shows high sensitivity over a wide range (1.033 kPa−1 within 0–1 kPa, 0.72 kPa−1 within 1–5 kPa, 0.34 kPa−1 within 5–10 kPa and 0.23 kPa−1 within 10–30 kPa). The sensor is differentiated from other conventional capacitive pressure sensor, which usually relies on air gap or dielectric constants to tune sensor’s performance. The effect of CNT doping ratio, the thickness of dielectric layer and the thickness of porous electrode on the capacitance performance are explored. Fundamental understanding of the sensing mechanism is achieved through finite-element analysis and simplified analytical models. The capacitive sensor demonstrates its application in monitoring physiological signals such as the artery pulse, and muscle activation, showing potential in wearable healthcare monitor.

Dielectrostriction Effect in Flexible Dielectric Realized by Ordered Doping of Iron Nanoparticles for Wide-Range Pressure Detection

Wide-range pressure sensors address diverse applications and have great research value. Nonetheless, the sensitivity of most of the pressure sensors reported in the literature decreases dramatically in the high-pressure region due to the limitation of their sensing mechanism. To solve this problem, the “dielectrostriction effect” was taken into consideration in the design of a flexible capacitive pressure sensor, in which the dielectric constant varies with the applied pressure and is boosted by controlling the distribution of the doped nanosized iron particles in the dielectrics with magnets. This method obviously improved the performance of the fabricated pressure sensor, which had a wide detection range (0–250 kPa) and exhibited a high sensitivity of 10.2% kPa–1 over the pressure range of 0–125 kPa and of 2% kPa–1 over the pressure range of 125–250 kPa. Consequently, the method of enhancing the dielectrostriction effect proposed in this work demonstrates great potential in wide-range pressure measurements.

Monitoring the delicate operations of surgical robots via ultra-sensitive ionic electronic skin.

The arrival of surgical robots in high-end medical equipment is a landmark, and the realization of tactile sensation a major challenge in this important cutting-edge research field. Aiming to address this issue, we present ultra-sensitive ionic electronic skin in the form of flexible capacitive pressure sensors, which incorporate multistage bionic microstructures in ion gels for the purpose of monitoring the delicate operations of surgical robots. Significantly, the ionic skin exhibits an ultra-high sensitivity of 9484.3kPa-1 (<15kPa), and the sensitivity remains higher than 235kPa-1 in the wide range of 15-155kPa. The device has also achieved a detection limit as low as 0.12Pa or, equivalently, 0.31mg, fast response within 24ms, and high robustness (loading/unloading for 5000 cycles without fatigue). The sensor facilitates the challenging task of tele-operated robotic threading, which exceeds the human tactile perception limit when threading a needle. We have also confirmed that ionic skin can be used in robot-assisted invasive surgery, such as incision/resection of tissues and suturing of wounds, providing tactile information to surgeons to improve operation success rates. The flexible ionic skin is capable of conforming to the various shapes of robotic manipulators, thus has great promise for applications in robotic dexterous manipulation, prosthetics and human-machine interfaces.

High-Performance Flexible Capacitive Proximity and Pressure Sensors with Spiral Electrodes for Continuous Human–Machine Interaction

High-Performance Flexible Capacitive Proximity and Pressure Sensors with Spiral Electrodes for Continuous Human–Machine Interaction

Sacrificial Template Method to Fabricate Highly Sensitive Porous Capacitive Pressure Sensor for Full Pulse Waveform Detection

Due to the recent advancements in sensing technology, skin mountable wearable devices have gained much attention due to their non-invasive usage and wearer comfort for health care monitoring. For instance, monitoring pulse waveform from the wrist arterial sites bears tremendous information about cardiovascular health and can show abnormalities such as hypertension or arterial fibrosis. Acquiring a high-quality pulse waveform from the wrist artery requires high-pressure sensitivity and fast response time. However, the need for building highly sensitive and low-cost fabrication of the sensors for subtle physiological monitoring is still challenging. Targeting these issues, this study presents a facile and cost-effective approach to realizing a flexible capacitive pressure sensor by sandwiching porous polydimethylsiloxane (PDMS) polymer functionalized by multi-walled carbon nanotubes (MWCNTs) fabricated by sacrificial template method for physiological monitoring. The fabrication steps for the dielectric layer involve mixing sucrose as the porogens and MWCNTs as the functionalizing material with PDMS followed by casting the mixture in a mold and curing of the PDMS. Subsequently, the sucrose particles are dissolved in water. The sucrose particles are easily dissolved in water leaving a highly uniform and porous structure behind. The dielectric layer is then sandwiched between two conductive textile based electrodes for physiological monitoring. The electromechanical tests were performed for different concentration of MWCNTs content in porous PDMS to obtain the best sensing performance. The champion sensor exhibits ultrahigh-pressure sensitivity of 2.41kPa-1 and a minimum detection limit of 1.4Pa in contrast to the sensor exhibiting only 0.31kPa-1 without addition of any MWCNTs. In addition, the pressure sensor shows a fast response time of 0.18s and excellent cyclic stability over 6000 cycles. The ultrahigh-pressure sensitivity comes from the addition of conductive MWCNTs filler that increases the dielectric permittivity. The highly porous structure of the polymer reduces the typical viscoelastic effects of polymer resulting in fast response time. The pressure sensor could detect arterial pulse waveform from the wrist artery with all the key characteristic points showing its promises for low-cost physiological monitoring. The detailed results will be discussed during the presentation.

Flexible conductivity-temperature-depth-strain (CTDS) sensor based on a CNT/PDMS bottom electrode for underwater sensing

Marine hydrological information has a significant impact on human development and the utilization of the oceans, which can be monitored with underwater sensors. In the past, ocean research has relied on the use of bulky underwater recorders and sensory telemetry networks. In this study, an integrated flexible sensor is developed for underwater conductivity, temperature, depth, and strain detection. Platinum resistance sensors were used for temperature and strain measurements, conductivity sensors with interdigitated electrodes were used for salinity measurements, and capacitive pressure sensors for depth measurements. Two kinds of flexible capacitive pressure sensors were fabricated with a carbon nanotube/polydimethylsiloxane (CNT/PDMS) bottom electrode and copper/polyimide (Cu/PI) bottom electrode. The sensor with the CNT/PDMS bottom electrode outperformed the sensor with the Cu/PI bottom electrode over a wide pressure range (<5 MPa) and showed stable capacitance up to 1000 cycles. COMSOL simulations also support our experimental results with high sensitivity of the sensor with a CNT/PDMS bottom electrode. The integrated flexible sensor is durable and lightweight, making it ideal for use as a stationary monitoring sensor or for attachment to a variety of marine animals.

A dual-response flexible capacitive pressure sensor based on ZnS:Cu2+

A dual-response flexible capacitive pressure sensor based on ZnS:Cu2+

A Highly Sensitive and Flexible Capacitive Pressure Sensor Based on Alignment Airgap Dielectric.

Flexible capacitive pressure sensors with a simple structure and low power consumption are attracting attention, owing to their wide range of applications in wearable electronic devices. However, it is difficult to manufacture pressure sensors with high sensitivity, wide detection range, and low detection limits. We developed a highly sensitive and flexible capacitive pressure sensor based on the porous Ecoflex, which has an aligned airgap structure and can be manufactured by simply using a mold and a micro-needle. The existence of precisely aligned airgap structures significantly improved the sensor sensitivity compared to other dielectric structures without airgaps. The proposed capacitive pressure sensor with an alignment airgap structure supports a wide range of working pressures (20–100 kPa), quick response time (≈100 ms), high operational stability, and low-pressure detection limit (20 Pa). Moreover, we also studied the application of pulse wave monitoring in wearable sensors, exhibiting excellent performance in wearable devices that detect pulse waves before and after exercise. The proposed pressure sensor is applicable in electronic skin and wearable medical assistive devices owing to its excellent functional features.