Piezocapacitive Sensors Research Articles

Smartphone-based and non-invasive sleep stage identification system with piezo-capacitive sensors

A non-invasive, wireless, smartphone-based electronic measurement system for sleep stage identification is presented in this work. Ballistocardiograph signals are collected by two piezo-capacitive thin film strips located on the mattress base. Suitable analog conditioning circuits and digital pre-processing techniques are applied to obtain the heart and breathing rates (HR, BR), and an activity index (ACT) related to the body movements during the sleep. An initial calibration stage is proposed where analog signal amplification is fitted to each subject, from which activity index is derived. Features considered for machine learning classifications were the mentioned data and the time variabilities of HR and BR represented by the features R(k) and B(k), respectively. Support Vector Machine (SVM) and K-Nearest-Neighbour (KNN) classifiers are employed in both flat and hierarchical classification scenarios for Wake – Non Rapid Eye Movement – Rapid Eye Movement (WAKE/NREM/REM) sleep stage identification. Twelve healthy subjects were recorded with the developed system using a polysomnograph (PSG) as reference data. When compared with PSG, the presented system achieved an average accuracy of 69 % using only three features: R(k), B(k), and ACT, highlighting an 88.2 % recall for NREM stage identification. These findings suggest that accounting only for time variability features and activity, satisfactory results can be provided as a complementary alternative for sleep stage identification, with a smartphone-based electronic system designed as an affordable, versatile, and simple tool for household applications.

High-Sensitivity and Wide-Range Flexible Ionic Piezocapacitive Pressure Sensors with Porous Hemisphere Array Electrodes.

The development of high-performance flexible pressure sensors with porous hierarchical microstructures is limited by the complex and time-consuming preparation processes of porous hierarchical microstructures. In this study, a simple modified heat curing process was first proposed to achieve one-step preparation of porous hemispherical microstructures on a polydimethylsiloxane (PDMS) substrate. In this process, a laser-prepared template was used to form surface microstructures on PDMS film. Meanwhile, the thermal decomposition of glucose monohydrate additive during heat curing of PDMS led to the formation of porous structures within PDMS film. Further, based on the obtained PDMS/CNTs electrodes with porous hemisphere array and ionic polymer dielectric layers, high-performance ionic piezocapacitive sensors were realized. Under the synergistic effect of the low-stiffness porous hemisphere microstructure and the electric double layer of the ionic polymer film, the sensor based on an ionic polymer film with a 1:0.75 ratio of P(VDF-HFP):[EMIM][TFSI] not only achieves a sensitivity of up to 106.27 kPa-1 below 3 kPa, but also has a wide measurement range of over 400 kPa, which has obvious advantages in existing flexible piezocapacitive sensors. The rapid response time of 110 s and the good stability of 2300 cycles of the sensor further elucidate its practicality. The application of the sensor in pulse monitoring, speech recognition, and detection of multiple dynamic loads verifies its excellent sensing performance. In short, the proposed heat curing process can simultaneously form porous structures and surface microstructures on PDMS films, greatly simplifying the preparation process of porous hierarchical microstructures and providing a simple and feasible way to obtain high-performance flexible pressure sensors.

Highly Sensitive Flexible Capacitive Pressure Sensor with Porous Hierarchical Structure Realized by a Microwave Curing Method

The introduction of porous hierarchical microstructures effectively improves the sensitivity of flexible pressure sensors. However, it is difficult to achieve porous hierarchical microstructures for pressure sensors through inexpensive, efficient, and simple preparation methods. Herein, a hemispherical array with porous hierarchical microstructure is prepared on polydimethylsiloxane substrate using a simple one‐step microwave curing process with glucose as porogen. Furthermore, the flexible electrode based on the polydimethylsiloxane substrate is combined with an ionic liquid polymeric gel membrane to obtain a flexible capacitive pressure sensor. Thanks to the deformability of porous hierarchical microstructure and the double‐dielectric layer effect of ionic liquid polymeric gel membrane, the sensor displays an ultrahigh sensitivity of 131.21 kPa−1 within 0–1 kPa. Meanwhile, the sensor has short response time, excellent dynamic loading stability, as well as excellent long‐term stability (>3000 cycles). In application testing, the sensor effectively monitors various physiological activities like pulse, breathe, speech, and swallowing, demonstrating its good prospects in the field of health electronics. Most importantly, the proposed microwave curing method can realize the preparation of porous hierarchical structure on flexible substrate through a one‐step process, which offers a fresh way for the economical, green, and efficient preparation of high‐sensitivity capacitive pressure sensors.

Conformable packaging of a soft pressure sensor for tactile perception

Humans can perceive surface properties of an unfamiliar object without relying solely on vision. One way to achieve it is by physically touching the object. This human-inspired tactile perception is a complementary skill for robotic tactile perception. Robot perception depends on the informational quality of the tactile sensor; thus, packaging sensors and integrating them with robots plays a crucial role. In this work, we investigate the influence of conformable packaging designs on soft polydimethylsiloxane-based flexible pressure sensors that work in a variety of surface conditions and load levels. Four different 3D printed packaging designs capable of maintaining sensor trends have been developed. The low detection limits of 0.7 kPa and 0.1 kPa in the piezoresistive and piezocapacitive sensors, respectively, remain unaffected, and a performance variation as low as 30% is observed. Coefficient of variation and sensitivity studies have also been performed. Limit tests show that the designs can handle large forces ranging from 500 N to more than a 1000 N. Lastly, a qualitative study was performed, which covered prospective use-case scenarios as well as the advantages and downsides of each sensor casing design. Overall, the findings indicate that each sensor casing is distinct and best suited for tactile perception when interacting with objects, depending on surface properties.

Fabric-like Electrospun PVAc-Graphene Nanofiber Webs as Wearable and Degradable Piezocapacitive Sensors.

Flexible piezocapacitive sensors utilizing nanomaterial-polymer composite-based nanofibrous membranes offer an attractive alternative to more traditional piezoelectric and piezoresistive wearable sensors owing to their ultralow powered nature, fast response, low hysteresis, and insensitivity to temperature change. In this work, we propose a facile method of fabricating electrospun graphene-dispersed PVAc nanofibrous membrane-based piezocapacitive sensors for applications in IoT-enabled wearables and human physiological function monitoring. A series of electrical and material characterization experiments were conducted on both the pristine and graphene-dispersed PVAc nanofibers to understand the effect of graphene addition on nanofiber morphology, dielectric response, and pressure sensing performance. Dynamic uniaxial pressure sensing performance evaluation tests were conducted on the pristine and graphene-loaded PVAc nanofibrous membrane-based sensors for understanding the effect of two-dimensional (2D) nanofiller addition on pressure sensing performance. A marked increase in the dielectric constant and pressure sensing performance was observed for graphene-loaded spin coated membrane and nanofiber webs respectively, and subsequently the micro dipole formation model was invoked to explain the nanofiller-induced dielectric constant enhancement. The robustness and reliability of the sensor have been underscored by conducting accelerated lifetime assessment experiments entailing at least 3000 cycles of periodic tactile force loading. A series of tests involving human physiological parameter monitoring were conducted to underscore the applicability of the proposed sensor for IoT-enabled personalized health care, soft robotics, and next-generation prosthetic devices. Finally, the easy degradability of the sensing elements is demonstrated to emphasize their suitability for transient electronics applications.

One-Step Laser Direct-Printing Process of a Hybrid Microstructure for Highly Sensitive Flexible Piezocapacitive Sensors

Microstructures can effectively improve the sensing performance of flexible piezocapacitive sensors. Simple, low-cost fabrication methods for microstructures are key to facilitating the practical application of piezocapacitive sensors. Herein, based on the laser thermal effect and the thermal decomposition of glucose, a rapid, simple, and low-cost laser direct-printing process is proposed for the preparation of a polydimethylsiloxane (PDMS)-based electrode with a hybrid microstructure. Combining the PDMS-based electrode with an ionic gel film, highly sensitive piezocapacitive sensors with different hybrid microstructures are realized. Due to the good mechanical properties brought about by the hybrid microstructure and the double electric layer induced by the ionic gel film, the sensor with a porous X-type microstructure exhibits an ultrahigh sensitivity of 92.87 kPa-1 in the pressure range of 0-1000 Pa, a wide measurement range of 100 kPa, excellent stability (>3000 cycles), fast response time (100 ms) and recovery time (101 ms), and good reversibility. Furthermore, the sensor is used to monitor human physiological signals such as throat vibration, pulse, and facial muscle movement, demonstrating the application potential of the sensor in human health monitoring. Most importantly, the laser direct-printing process provides a new strategy for the one-step preparation of hybrid microstructures on thermal curing polymers.

Stretchable Iontronic Pressure Sensor Array With Low Crosstalk and High Sensitivity for Epidermal Monitoring

Soft piezocapacitive sensor, owing to its simple assembly and low power consumption, draws intensive interests for physiological monitoring. However, crosstalk between proximity sensing units and limited mechanically stretching suppressed its practical application for conventional capacitive sensor array. Here we report a stretchable iontronic pressure sensor (SIPS) array with low proximate crosstalk for epidermal monitoring. Benefiting from electric double layer (EDL) capacitance effect between robust aramid nanofiber (ANF) /MXene (Ti3C2Tx) composite electrode and ionic film, the SIPS exhibits a high sensitivity up to 521.69 kPa−1, low limit of detection (0.22 Pa), broad linear sensing range (200 kPa) and rapid response time (17 ms) as well as long-term stable working durability for 10,000 cycles. The serpentine island bridge structure enables the SIPS array to stretch up to 40%, allowing pulse and grip monitoring of a bent wrist and finger, respectively, through processing circuits. This demonstrates the promising application prospects in vital physiological monitoring.

Low-cost dielectric sheets for large-area floor sensing applications

Sensitivity response is a critical parameter that decides the domain of dielectric materials to be implemented as piezocapacitive sensors for low- or high-pressure sensing applications. Here, we have clarified the sensitivity response behavior of three low-cost dielectric materials, two biodegradable paperboards, and one acoustic polymeric foam. The devices are fabricated in the form of a metal–insulator–metal structure, and the capacitive response of the devices is measured using the charge extraction by linearly increasing voltage technique. The sensitivity response curve (ΔC/C o vs. pressure) reveals that the paperboard materials are sensitive enough to detect low-pressure regimes (45 kPa), whereas the acoustic foam is quite promising for high-pressure monitoring (above 150 kPa). Using a multiplexer circuit, we demonstrated the sensitivity response via 2 by 2 matrix structure both as a steady-state and transient response. Our results show that the passive matrix structure interference between different pixels can be minimized after increasing the spacing between electrodes strip. Finally, a full-scale demonstrator (dimension 120 cm × 400 cm) with a 2 × 8 matrix structure laminated under floor tiling has been demonstrated. We show how such a floor sensor utilizing the low-cost substrates can be used to recognize single-stepping, walking, and falling.

Transparent, elastomer-free capacitive pressure sensors using CuNWs and ultrathin UV-cured polymer

• Transparent, elastomer-free capacitive sensors are obtained based on CuNWs. • The sensor can resist 1000 bending cycles at a curvature radius of 0.5 mm. • The senor possesses a high optical transmittance of 81.1%. • A suggested mechanism of capacitance change for the sensor is proposed. • The senor possesses a high sensitivity. Highly flexible transparent piezo-capacitive sensors have been prepared via using a combination of Cu nanowire (NW) conductive network and UV-curable resin. The CuNWs are interconnected and randomly distributed on the substrate. The percolating network structure of CuNW electrode and the ultrathin thickness (4 μm) of polymer film, make the senor possess a high optical transmittance of 81.1 %. The strong affinity of resin to the CuNW networks stabilizes the sensor mechanically that it can resist 1000 bending cycles at a curvature radius of 0.5 mm. The capacitance is generated via the CuNW interdigital electrode pattern by the fringing effect, and increased with increasing the pressure acted on the surface of the sensor. The sensitivity is seven times larger than that of an elastomeric piezo-capacitive sensor with the same sensor design.

Monitoring Flexions and Torsions of the Trunk via Gyroscope-Calibrated Capacitive Elastomeric Wearable Sensors.

Reliable, easy-to-use, and cost-effective wearable sensors are desirable for continuous measurements of flexions and torsions of the trunk, in order to assess risks and prevent injuries related to body movements in various contexts. Piezo-capacitive stretch sensors, made of dielectric elastomer membranes coated with compliant electrodes, have recently been described as a wearable, lightweight and low-cost technology to monitor body kinematics. An increase of their capacitance upon stretching can be used to sense angular movements. Here, we report on a wearable wireless system that, using two sensing stripes arranged on shoulder straps, can detect flexions and torsions of the trunk, following a simple and fast calibration with a conventional tri-axial gyroscope on board. The piezo-capacitive sensors avoid the errors that would be introduced by continuous sensing with a gyroscope, due to its typical drift. Relative to stereophotogrammetry (non-wearable standard system for motion capture), pure flexions and pure torsions could be detected by the piezo-capacitive sensors with a root mean square error of ~8° and ~12°, respectively, whilst for flexion and torsion components in compound movements, the error was ~13° and ~15°, respectively.

Microconformal electrode-dielectric integration for flexible ultrasensitive robotic tactile sensing

Flexible pressure sensors have attracted a lot of interest because of their widespread applications in healthcare, robotics, wearable smart devices, and human-machine interfaces. While microstructuring both the electrodes and dielectrics has been proven to have a significant improvement in the sensitivity and response speed of piezocapacitive sensors, the synergetic influence of microstructured electrodes and dielectrics has not been discussed yet. Herein, a flexible piezocapacitive sensor has been demonstrated with a microstructured graphene nanowalls (GNWs) electrode and a conformally microstructured dielectric layer that consists of polydimethylsiloxane (PDMS) and piezoelectric enhancer of zinc oxide (ZnO). Such microstructured assembly with piezoelectric film constructs a microconformal GNWs/PDMS/ZnO electrode-dielectric integration (MEDI), which can effectively enhance the sensitivity and the pressure-response range. The piezocapacitive sensor exhibits an ultra-high sensitivity (22.3 kPa−1), fast response speed (25 ms), and broad pressure range (22 kPa). The finite element analysis indicates that the polarized electric field caused by the ZnO film’s piezoelectric effect greatly enhances the capacitance of the sensor. Moreover, the integration of the electrode and dielectric layer can eliminate the slippage between contiguous layers, which effectively increases the mechanical stability. Benefitting from the outstanding comprehensive performance, the potential application in robotic tactile perception has been successfully demonstrated, including object grabbing, braille recognition, and roughness detection. The MEDI in structure capacitive sensors provides a new approach to achieve high-performance E-skin, which delivers great potential applications in next-generation robotic tactile sensing.

Piezocapacitive Flexible E‐Skin Pressure Sensors Having Magnetically Grown Microstructures

AbstractFlexible pressure sensors are highly desirable in artificial intelligence, health monitoring, and soft robotics. Microstructuring of dielectrics is the common strategy employed to improve the performance of capacitive type pressure sensors. Herein, a novel, low‐cost, large‐area compatible, and mold‐free technique is reported in which magnetically grown microneedles are self‐assembled from a film of curable magnetorheological fluid (CMRF) under the influence of a vertical curing magnetic field (Bcuring). After optimizing the microneedles' fabrication parameters, i.e., magnetic particles' (MPs') concentration and Bcuring intensity, piezocapacitive sensors capable of wide range pressure sensing (0–145 kPa) with ultrafast response time (50 ms), high cyclic stability (>9000 cycles), as well as very low detection limit (1.9 Pa) are obtained. Sensor properties are found dependent on microneedles' fabrication parameters that are controllable, produce variable‐sized microneedles, and allow to govern sensing properties according to desired applications. Finally, the sensor is employed in holding a bottle with different weights, human breath, and motion monitoring, which demonstrate its great potential for the applications of human–machine interaction, human health monitoring, and intelligent soft robotics.

Electrospun Spandex Nanofiber Webs with Ionic Liquid for Highly Sensitive, Low Hysteresis Piezocapacitive Sensor

Electrospun Spandex nanofiber webs having very high amount of nano-sized open cell can be used as a piezocapacitive sensor for monitoring both static and dynamic pressures due to excellent electrospinnability and very good elastic properties. Compared to our previously reported SBS and TPU electrospun nanoweb, Spandex showed relatively linear increase of capacitance with applied pressure and restored its initial thickness when the pressure was released due to the improved resilience and elasticity. Moreover, small amount of ionic liquid (IL) was added in Spandex dope solution to increase the sensitivity of sensor to pressure, which induced very large amount of capacitance change with pressure, as well as reducing the capacitance-pressure hysteresis. In this work, hysteresis of the sensor was assessed through measuring the capacitance values during 20 cyclic loading and unloading and was improved significantly from 7.5 % to 1.8 %. Creep and stress relaxation behaviors were also tested through measuring capacitance under the constant loading and loading under the constant capacitance as a function of time respectively, using a dynamic pressure tester and an LCR meter. The results are reported in detail.

Low cost sponge based piezocapacitive sensors using a single step leavening agent mediated autolysis process

A single step, low cost, large area and shape scalable method of obtaining elastomer sponge is achieved through leavening agent autolysis with exceptional sensitivity tunability for real time sensing applications.

Enhanced Piezocapacitive Effect in CaCu3Ti4O12–Polydimethylsiloxane Composited Sponge for Ultrasensitive Flexible Capacitive Sensor

Highly sensitive flexible piezocapacitive (PC) pressure sensor demonstrates wide applications in wearable electronics. In this paper, we first theoretically proposed an effective strategy to improve the sensitivity of the PC pressure sensor, by constructing a porous dielectric layer composted of inorganics with high dielectric constant (eH) and organics with low dielectric constant (eL). By using CaCu3Ti4O12 (CCTO) nanocrystals with giant e as the dopant and polydimethylsiloxane (PDMS) with a low e as the matrix, an ultrasoft CCTO-PDMS dielectric sponge was fabricated, via a simple porogen-assisted process. The CCTO-PDMS composited sponge exhibits an ultralow compression modulus of 6.3 kPa, a highly enhanced sensitivity with the gauge factor of up to 1.66 kPa–1 in a range of 0–640 Pa, and a response time of 33 ms, and the sensitivity outperforms that of pure PDMS and other PC sensors reported recently. This sensitivity enhancement is attributed to the hybridization of two phases of eH/eL in the composites...

Highly Sensitive Piezocapacitive Sensor for Detecting Static and Dynamic Pressure Using Ion-Gel Thin Films and Conductive Elastomeric Composites.

A new class of simple and highly sensitive piezocapacitive sensors that are capable of detecting static and dynamic pressure changes is reported. The pressure sensor structure is formed by vertically sandwiching a sandpaper-molded carbon nanotube/poly(dimethylsiloxane) composite (CPC) dielectric layer between two ion-gel thin film electrodes. Such a capacitive sensor system enables the distinguishable detection of directional movement of applied pressure as well as static pressure variation by modulating ion distribution in the ion-gel thin films. The resulting capacitive pressure sensors exhibit high sensitivity (9.55 kPa-1), high durability, and low operating voltage (0.1 V). Our proposed pressure sensors are successfully applied as potential platforms for monitoring human physiological signals and finger sliding motions in order to demonstrate their capability for practical usage. The outstanding sensor performance of the pressure sensors can permit applications in wearable electronic devices for human-machine connecting platforms, health care monitoring systems, and artificial skin.

Capacitive pressure-sensitive composites using nickel–silicone rubber: experiments and modeling

Capacitive pressure (i.e., piezo-capacitive) sensors have manifested their superiority as a potential electronic skin. The mechanism of the traditional piezo-capacitive sensors is mainly to change the relative permittivity of the flexible composites by compressing the specially fabricated microstructures in the polymer matrix under pressure. Instead, we study the piezo-capacitive effect for a newly reported isotropic flexible composite consisting of silicone rubber (SR) and uniformly dispersed micron-sized conductive nickel particles experimentally and theoretically. The Young’s modulus of the nickel-SR composites (NSRCs) is designed to meet that of human skin. Experimental results show that the NSRCs exhibit remarkable particle concentration dependent capacitance response under uniaxial pressure, and the NSRCs present a good repeatability. We propose a mathematical model at particle level to provide deep insights into the piezo-capacitive mechanism, by considering the adjacent particles in the axial direction as micro capacitors connected in series and in parallel on the horizontal plane. The piezo-capacitive effect is determined by the relative permittivity induced by the particles rearrangement, longitudinal interparticle gap, and deflection angle of micro particle capacitors under pressure. Specifically, the relative capacitance of NSRC capacitor is deduced to be product of two factors: the degree of particle rearrangement, and the relative capacitance of a micro capacitor with the average longitudinal gap. The proposed model well matches and interprets the experimental results.

Flexible piezocapacitive sensors based on wrinkled microstructures: toward low-cost fabrication of pressure sensors over large areas

Wrinkled elastomeric templates prepared by stretching and releasing are utilized for demonstrating highly sensitive, simple, and low-cost piezocapacitive pressure sensors over large area.

Comparative study of pressure distribution at the user-cushion interface with different cushions in a population with spinal cord injury

Background Few studies have offered comparative information on the mechanical characteristics of different wheelchair seat cushions. The objective of the present study was to compare the benefits of the wheelchair seat cushions most frequently used in a population of patients with spinal cord injury in terms of pressure distribution and contact surface at the user-cushion interface. Methods Each one of 48 patients with spinal cord injury was seated in his or her own wheelchair on the four models of cushions analyzed (low-profile air, high-profile air, dual-compartment air, and gel and firm foam), which were presented in randomized order. The pressure distribution readings and support surface area of the user-cushion interface were obtained with a matrix of piezocapacitive sensors. Findings The dual-compartment air cushion yielded lower readings for all pressure parameters analyzed ( P max, P mean, P sd, and P isch) than the other three cushion models ( P < 0.05). The best surface parameter results ( S tot, S > 60 and % S > 60) also were obtained with the dual-compartment air cushion ( P < 0.05). Interpretation In the sample analyzed, the dual-compartment air cushion was the cushion with the best pressure distribution and largest contact surface of the user-cushion interface compared to the other three cushions studied.

Development of a film sensor for static and dynamic force measurement

In this work an innovative double-layer film sensor for the measurement of forces is presented. The sensor is a thin film (thickness below 1 mm) based on a “sandwich” structure composed of two sensing elements glued together: one layer is a capacitive film and the other is a piezoelectric film. Both the layers are sensitive to compression loads, but they are suitable for working in different frequency ranges. In fact, while the capacitive element is capable of measuring from dc up to about 400 Hz, on the contrary the piezoelectric film works in the high frequency range. The output signals of both the sensors are acquired and then filtered and processed in order to achieve a single output signal. The piezocapacitive sensor has been developed in order to synthesize, in a small and cheap device, the capability to measure compression forces in a wide range of frequencies. The sensor is very small and has many potential applications, such as in the field of modal analysis. In particular, the very small thickness allows to insert it into a composite material to measure actual loads and excitations, as well as on the surface or between different components of a more complex system in order to obtain a smart structure. This article describes the realization of the sensor and the adopted signal processing strategies. The metrological characterization procedure is discussed and results are shown for both static and dynamic calibration of the film sensor. Finally, a simple application, that highlights the benefits of the sensor, is presented.