Flexible Piezoelectric Sensor Research Articles

A High-Reliability Piezoelectric Tile Transducer for Converting Bridge Vibration to Electrical Energy for Smart Transportation.

Piezoelectric energy transducers offer great potential for converting the vibrations of pedestrian footsteps or cars moving on a bridge or road into electricity. However, existing piezoelectric energy-harvesting transducers are limited by their poor durability. In this paper, to enhance this durability, a piezoelectric energy transducer with a flexible piezoelectric sensor is fabricated in a tile protype with indirect touch points and a protective spring. The electrical output of the proposed transducer is examined as a function of pressure, frequency, displacement, and load resistance. The maximum output voltage and maximum output power obtained were 6.8 V and 4.5 mW, respectively, at a pressure of 70 kPa, a displacement of 2.5 mm, and a load resistance of 15 kΩ. The designed structure limits the risk of destroying the piezoelectric sensor during operation. The harvesting tile transducer can work properly even after 1000 cycles. Furthermore, to demonstrate its practical applications, the tile was placed on the floor of an overpass and a walking tunnel. Consequently, it was observed that the electrical energy harvested from the pedestrian footsteps could power an LED light fixture. The findings suggest that the proposed tile offers promise with respect to harvesting energy produced during transportation.

An energy-efficient classification system for peach ripeness using YOLOv4 and flexible piezoelectric sensor

Peach is a kind of popular fruit with significant economic value, but it is commonly sorted through manual labor on sorting lines, which can negatively affect the economic efficiency. To address this issue, this paper proposes a system for classifying different ripeness of peaches based on their visual and tactile characteristics. This system consists of two stages. The first stage utilizes YOLOv4 as the core model for preliminary visual-based classification of fruit ripeness. The second stage proposes a method centered on flexible piezoelectric sensors to more deeply classify the fruit ripeness based on tactile characteristics. The system classifies peaches into five categories (A1, A2, B1, B2, and R), which can be applied to the peach sorting line. The mAP and meanIoU are used as evaluation parameters for the visual part and TPA test is used to validate the accuracy of the tactile part. The results of the experiments show that mAP value reaches 0.9304, meanIoU reaches 0.9454, and the accuracy of the tactile part reaches 92.22%. The results indicate that visual and tactile characteristics can accurately classify peach ripeness using YOLOv4 and flexible piezoelectric sensor. The proposed system completes the entire classification with lower cost and less power consumption, providing potential ways to increase productivity and automate the sorting line.

Near‐field electrospinning fabrication of piezoelectric polymer microfiber sensors for detection of weak mechanical excitation

AbstractCollection and conversion of widespread mechanical energy is one promising way to alleviate environmental pollution and energy crisis. Piezoelectric materials can effectively realise this conversion between mechanical and electrical energies. Here, via near‐field electrospinning, piezoelectric poly(vinylidene fluoride) microfibers were fabricated on flexible polyethylene terephthalate substrate. Bending measurement indicated that open‐circuit voltage response from piezoelectric microfibers was strain dependent but insensitive to bending frequency. The microfiber sensor could detect acoustic signals with sound pressure level between 70 and 120 dB and the recorded acoustic frequency was well consistent with the nominal frequency. Light wind from a low‐power hand fan was also detected by this microfiber sensor. This simply structured and highly flexible piezoelectric microfiber sensor provided a promising and low‐cost fabrication measure for weak mechanical excitation sensing.

Fabrication, Characterization, and Signal Processing Optimization of Flexible and Wearable Piezoelectric Tactile Sensors

Piezoelectric microelectromechanical systems (MEMS) meet the growing demand for sensors with small sizes and low power consumption. For tactile applications and pressure measurements, they are applied in various fields from robotics to healthcare. In this context, flexible devices are very important for their high responsivity and ability to conform to the analyzed surface. This work reports on miniaturized flexible and compliant piezoelectric devices to increase the number of integrable sensors for detecting and discriminating localized pressures and contacts per unit area with minimal crosstalk. For this purpose, a series of Aluminium Nitride-based (AlN) piezoelectric sensors with different diameters (from 5 μm to 500 μm) was realized and the generated electrical signal by sensor deformation was amplified by a differential voltage amplifier circuit. By the analysis of the shape of the piezoelectric signal as a function of the applied pressure, oscillations due to piezoelectric deformations, superimposed on the initial peak signal corresponding to the touch event, have been observed. The integral of the electrical signal was calculated for the most accurate representation to describe the sensor’s response. The best responsivity was obtained in samples with diameters of 200 μm and 500 μm. Furthermore, it was also found that the minimum distance between the edges of closed sensors, observing crosstalk below -20 dB, was around 500 μm.

Clinical Validation of Wearable Piezoelectric Blood Pressure Sensor for Continuous Health Monitoring.

Wearable blood pressure sensors have recently attracted attention as healthcare devices for continuous non-invasive arterial pressure (CNAP) monitoring. However, the accuracy of wearable blood pressure (BP) monitoring devices has been controversial due to the low signal quality of sensors, the absence of an accurate transfer function to convert the sensor signals into BP values, and the lack of clinical validation regarding measurement precision. Here, we report a wearable piezoelectric blood pressure sensor (WPBPS), which achieved a high normalized sensitivity (0.062 kPa-1 ), fast response time (23ms) for CNAP monitoring. The transfer function of a linear regression model was designed, offering a simple solution to convert the flexible piezoelectric sensor signals into BP values. In order to verify the measurement accuracy of WPBPS, clinical trials were performed on 35 subjects aged from 20s to 80s after screening. The mean difference between the WPBPS and a commercial sphygmomanometer of 175 BP data pairs was -0.89 ± 6.19mmHg and -0.32 ± 5.28mmHg for systolic blood pressure (SBP) and diastolic blood pressure (DBP), respectively. By building a WPBPS-embedded wristwatch, we demonstrated the potentially promising use of a convenient, portable continuous BP monitoring system for cardiovascular disease diagnosis in this study. This article is protected by copyright. All rights reserved.

Flexible piezoelectric sensor based on ATO/BNT multi-layers with high sensitivity, thermal healing and driving performance

Flexible piezoelectric sensors are widely used in energy harvesting and sensing, illustrating the potential applications in self-driving wearable devices. However, the self-healing sensors by light and heat can effectively solve the problem of damage in practical applications. Therefore, the thermally healing piezoelectric sensors with different multilayers integrating the ATO electrode film and BNT piezoelectric film are proposed and investigated, which can use natural resources (under sunlight) to repair its defects. It also has a high sensitivity of 2.09 V·kPa−1, a fast response time of 66 ms, a maximum output voltage of 61 V and an output power of 30 μW. The results show that the PES has great potential in future applications.

Flexible, monolithic piezoelectric sensors for large-area structural impact monitoring via MUSIC-assisted machine learning

Aircraft smart skin requires the integration of large-scale, ultrathin, and high-sensitive sensor network on the surface for structural health monitoring (SHM). However, it is fairly difficult to fabricate such large-area flexible sensor networks with low cost and high reliability. Although flexible and monolithic sensors are easy to be fabricated, their applications in large-area impact monitoring have yet to be developed and revealed. In this study, an impact monitoring and localization technology based on monolithic small-area sensors is proposed for large-area structures. The pivotal principle lies in the combination of the flexible piezoelectric sensor array and the Multiple Signal Classification (MUSIC)-assisted Artificial Intelligence Network (ANN) algorithm, where the key sorted feature matrix from the output signals to ANN can be captured by the MUSIC algorithm to achieve the spatial location estimation. The results show that the flexible piezoelectric sensors demonstrate excellent performance to monitor structural impacts in a region of more than 7500% of its area. Secondly, excellent conformation between the sensors and complex surfaces is achieved by the ultrathin thickness almost without affecting the surficial flow field. The overall recognition accuracy and robustness of impact location of machine learning is greatly increased by the integration of MUSIC algorithm, which is immune to the shape, material, thickness, or opening holes of the structure. Except for the impact location, impact energy, impact frequency, impact hardness and other structural health parameters can also be monitored. Therefore, a great prospect in the integrated monitoring of the aircraft’s structural and environmental parameters is expected.

Sensing Mechanisms of Flexible Pressure Sensors

In the first segment of this project, the merits of the flexible capacitive pressure sensor will be described. These benefits encompass lower power consumption, less signal drift, greater response repeatability, and others. However, as ductile materials are incompressible, the device's sensitivity won't be very good without adopting structures. The purpose and advantages of the resistance flexible pressure sensor will then be discussed, as well as its high nonlinearity and the metal-based strain effect of the resistance strain gauge, which results in mechanical deformation when an external force is applied and results in a corresponding change in resistance value. However, the output signal is weak because of the environment and time zone variations. The design and operation of the sensor will change. It is not appropriate for long-term monitoring due to its significant time and temperature drift and the potential that correct data may not be acquired after extended measurement. Additionally, it is sensitive to electromagnetic fields, vibration, radiation, air pressure, sound pressure, and air velocity. In the last part, the benefits of the flexible piezoelectric pressure sensor will be discussed, including its broad frequency range, high sensitivity, high signal-to-noise ratio, simple structure, dependable operation, and light weight. The disadvantage of piezoelectric sensors is that some of these materials have poor output direct current responsiveness and call for moisture protection, which calls for the usage of charge amplifiers or high input impedance circuits to address this shortcoming.

An Intelligent Glove of Synergistically Enhanced ZnO/PAN-Based Piezoelectric Sensors for Diversified Human–Machine Interaction Applications

Human–machine interaction is now deeply integrated into our daily lives. However, the rigidity and high-power supply of traditional devices limit their further development. Herein, a high-performance flexible piezoelectric sensor (HFPS) based on a novel zinc oxide/polyacrylonitrile/Ecoflex (ZnO/PAN/Ecoflex) composite membrane is proposed. Due to the synergistic piezoelectricity of ZnO and PAN, the output voltage/current of the HFPS is increased by 140%/100% compared to the pure Zno/Ecoflex composite membrane. Furthermore, the fabricated HFPSs also have excellent sensitivity, linearity, stability and flexibility under periodic pressure. On this basis, due to its flexibility, stretchability and battery-free characteristics, a self-powered HFPS-based intelligent glove is proposed to wirelessly control diverse electronic systems through human hand gestures. In the meanwhile, the intelligent glove has been successfully applied to car two-dimensional motion, light bulb control and fan control. With the advantages of simple operation, portability and low power consumption, the glove is expected to provide new application prospects for human–machine interaction systems.

Highly sensitive, ultra-reliable flexible piezoelectret sensor for non-contact sitting motion tracking and physiological signal monitoring

Non-contact, cost-effective, multifunctional and robust monitoring platform without mechanical wearing burden is highly desired to enhance the immersive experience for users during the long-term health management. However, most reported intelligent electronics in human life need to be adhered conformably to key human body parts for accurate signal detection, giving rising to activity interference and wearing burden in the long-term usage. Herein, a highly sensitive flexible film pressure sensor with a folded structure configuration assembled from irradiation-cross linked polypropylene (IXPP) piezoelectret film featured with piezoelectric-like dynamic electrical output attributes are reported. Such simple folded structure incorporated specially into the device design endows the piezoelectric film sensor with merits such as enhanced sensitivity, high signal-to-noise ratio and excellent anti-interference capacity. Consequently, the sensor can achieve a linear pressure sensitivity of 92 pC/kPa over a wide pressure range of 0.6–62.4 kPa, a pressure resolution of 0.98%, and fast response/recovery time of 40 ms. The flexible piezoelectric film sensors are embedded into cushions and attached to the back of chairs for application demonstrations of non-contact sensing. Remarkably, low-frequency human motion and weak physiological signal detection are achieved in a non-contact mode by a highly sensitive piezoelectric sensing principle in an all-in-one configuration, simplifying the signal processing and monitoring system design. A vital sign monitoring system based on the intelligent cushion is further developed to unlock the health status of sedentary crowd. This is the first work that ever use commercially available irradiated cross-linked polypropylene (IXPP) film to design pressure sensor and such sensors are successfully connected to the commercial “C-life” smart home ecosystem, presenting strong practicability and application prospects of such films in smart households and remote healthcare beyond the laboratory.

Optimization of PVDF nanocomposite based flexible piezoelectric tactile sensors: A comparative investigation

Optimization of PVDF nanocomposite based flexible piezoelectric tactile sensors: A comparative investigation

텍스처 인지를 위한 PZT/Epoxy 나노 복합소재 기반 유연 압전 촉각센서

텍스처 인지를 위한 PZT/Epoxy 나노 복합소재 기반 유연 압전 촉각센서

Design and Fabrication of a Hierarchical Structured Pressure Sensor Based on BaTiO3/PVDF Nanofibers via Near‐Field Electrospinning

Herein, a simple and low‐cost fabrication method for flexible piezoelectric pressure sensors with hierarchical structures over large areas is presented. First, polyvinylidene fluoride (PVDF) and barium titanate (BaTiO3, BTO) composite membranes are fabricated through near‐field electrospinning, and the hierarchical structured membrane (HSM) is obtained by spreading the flat membrane on the designed mold and drying. Then, the effect of encapsulation schemes on the piezoelectric properties of the sensor is investigated by combining different membranes with different PDMS substrates. The output voltage under periodic loads shows that the encapsulation scheme of “the hierarchical structured membrane with a hierarchical structured substrate” (HHS) can provide the highest peak voltage attributed to its largest contact area, which increases the active area (deformation area) for piezoelectricity generation. Also, the HHS with a bulge of 2 mm can produce a maximum output voltage of 2.32 V under a periodic load of 2 N, 2.5 Hz. Finally, the HHS pressure sensors are used for monitoring human motions, including joint bending, walking, and jumping, and the results show that the presented hierarchically microstructured flexible piezoelectric pressure sensor has great potential for applications of for human‐activity monitoring and wearable bioelectronic devices.

Flexible piezoelectric sensor for pregnant recognition based on the pulse‐taking procedure in traditional Chinese medicine

AbstractTraditional Chinese medicine (TCM) is an experienced‐based discipline and plays an important role in the current medical system. Digitalizing key features is the best way to con‐duct an in‐depth analysis of TCM. However, due to its complex generation mechanism, the crucial pulse‐taking procedure is hard to be digitalized and analyzed. This article fabricated a flexible piezoelectric sensor to gather data for the pulse‐taking procedure. The sensitivity of the proposed flexible sensor reached 0.34 V/kPa, so we adopted peripheral circuits to make its output signals stay in a reasonable range (± 0.5 V) for convenient data collection. We used the sensor to recognize whether a woman is pregnant since it has solid golden standards. The pulse signal was fused via mapping the signal spectrogram into a 3‐dimension tensor. The fused matrix was then processed by GoogleNet and reached a 90.48% correction rate. This result shows that the pulse condition has statistical differences, which solidified the objectivity of pulse diagnosis and escalated the TCM's digitalization level.

Enhanced the dielectric and piezoelectric properties of polyacrylonitrile piezoelectric composite fibers filled with ionic liquids

AbstractHere, we demonstrate the design and application of a multifunctional piezoelectric composite nanofiber membrane. In this study, the most widely used electrospinning method, by adding a small amount of 1‐allyl‐3‐butylimidazolium tetrafluoroborate ionic liquids and ferric chloride hexahydrate (FeCl3·6H2O) to regulate the structure and properties of polyacrylonitrile (PAN), the mechanical properties, piezoelectric properties, dielectric properties, and ferroelectric properties of composite nanofibers were improved. The results show that the piezoelectric properties of PAN composite fibers are greatly improved under the action of dual fillers. At 103 Hz, the dielectric constant of the PAN composite fiber is as high as 8.24, which is three times that of the pure PAN fiber. The sensor made from the composite nanofibers exhibits excellent sensitivity and high saturation polarization. At low pressure (<1 kPa), the sensitivity of the composite fiber is as high as 0.51 kPa. When we tap the sensor with our finger, the PAN composite fiber produces high piezoelectric output (5.7 V). The results show that the composite nanofibers have great potential in energy storage devices and flexible piezoelectric sensors, among other fields.

Piezoelectric Sensors Operating at Very High Temperatures and in Extreme Environments Made of Flexible Ultrawide‐Bandgap Single‐Crystalline AlN Thin Films (Adv. Funct. Mater. 10/2023)

Piezoelectric Sensor In article number 2212538, Jae-Hyun Ryou and co-workers develop a highly sensitive and mechanically flexible piezoelectric sensor using an ultrawide-bandgap single-crystalline AlN semiconductor thin film that can operate at higher than 900 °C. The operation temperature is believed to be amongst the highest for pressure sensors, and the inherent properties of AlN indicate that this approach offers a new solution for sensing in extreme environments often faced in current and next-generation energy, transportation, and defense applications.

High-Density Flexible Piezoelectric Sensor Array With Double Working Modes

Flexible and self-powered sensing device has aroused considerable interest in the field of the Internet of Things (IoT) and artificial intelligence (AI) in recent years. For more detailed and sensitive perception of the surroundings and external stimuli, sensors with high density and sensitivity are highly desirable. Here, this article reported a simple and low-cost strategy to realize a high-performance flexible piezoelectric sensor array composed of 30 sensing points with a high density of 11.11 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{2}}$ </tex-math></inline-formula> . The piezoelectric sensor array was analyzed by a finite-element model and characterized through sound pressure measurement and wrist bending for motion monitoring under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{{33}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{{31}}$ </tex-math></inline-formula> modes, respectively. The flexible sensor array with the total thickness of 125 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> exhibits a high sensitivity of 447.82 mV <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> kPa <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> with an ultralow detection limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 3.60$ </tex-math></inline-formula> Pa. Our work provides a simple strategy for the application potential in smart wearable electronics and human–machine interaction (HMI) areas.

Multilayered Electrospun/Electrosprayed Polyvinylidene Fluoride+Zinc Oxide Nanofiber Mats with Enhanced Piezoelectricity

AbstractElectrospun polyvinylidene fluoride (PVDF) nanofibers have been widely used in the fabrication of flexible piezoelectric sensors and nanogenerators, due to their excellent mechanical properties. However, their relatively low piezoelectricity is still a critical issue. Herein, a new and effective route to enhance the piezoelectricity of PVDF nanofiber mats by electrospraying zinc oxide (ZnO) nanoparticles between layers of PVDF nanofibers is demonstrated. As compared to the conventional way of dispersing ZnO nanoparticles into PVDF solution for electrospinning nanofiber mats, this approach results in multilayered PVDF+ZnO nanofiber mats with significantly increased piezoelectricity. For example, 6.2 times higher output is achieved when 100% of ZnO (relative to PVDF quantity) is electrosprayed between PVDF nanofibers. Moreover, this new method enables higher loading of ZnO without having processing challenges and the maximum peak voltage of ≈3 V is achieved, when ZnO content increases up to 150%. Additionally, it is shown that the samples with equal amount of material but consisting of different number of layers have no significant difference. This work demonstrates that the proposed multilayer design provides an alternative strategy to enhance the piezoelectricity of PVDF nanofibers, which can be readily scaled up for mass production.

Engineered kirigami design of PVDF-Pt core–shell nanofiber network for flexible transparent electrode

Nanofiber networks comprising polymer-metal core–shell structures exhibit several advantages, such as high uniformities and considerable flexibilities. Additionally, the flexibility of the nanofiber network may be further enhanced by engineering the network topology. Therefore, in this study, the topologies of polyvinylidene fluoride (PVDF)-Pt core–shell nanofiber (CS NF) networks were engineered, and their performances as flexible transparent electrodes were comprehensively evaluated. Three distinct topologies of nanofiber networks were induced using circular, square, and rectangular electrode collectors. A highly uniform nanofiber network was obtained using the square electrode collector, which generated a high density of nanofiber junctions (nodes). Consequently, this nanofiber network exhibited the smallest sheet resistance left({R}_{mathrm{s}}right) and lowest optical transmittance left(Tright) among the three CS NF networks. In contrast, nanofiber bundles were frequently formed in the randomly aligned CS NF network prepared using the circular electrode collector, reducing the node density. As a result, it simultaneously exhibited a very small {R}_{mathrm{s}} and high T, generating the largest percolation figure of merit left(Pi =330.5right). Under certain strain directions, the CS NF network with the engineered topology exhibited a significantly enhanced mechanical durability. Finally, a flexible piezoelectric pressure sensor with CS NF network electrodes was fabricated and its sensing performance was excellent.

Fabrication of Hydroxy-Terminated Polybutadiene with Piezoelectric Property by Functionalized Branch Chain Modification

Hydroxyl-terminated polybutadiene (HTPB)-based piezoelectric polymer (m-HTPB) is prepared for the first time by functionalized branch chain modification strategy. In the presence of HTPB with >98.8% cis-1,4 content, the C=C bond partly breaks down, and functionalized acetylferrocene groups are introduced to the cis-1,4 polybutadiene branch chain, retaining the high cis-1,4 content of HTPB. The whole process is conducted under mild conditions, without complicated manipulations. The microstructure and molecular weight of m-HTPB are characterized by Fourier-transform infrared (FTIR) spectra, 1H or 13C nuclear magnetic resonance spectrum (NMR), and gel permeation chromatography (GPC). The thermal properties of HTPB and m-HTPB are determined by differential scanning calorimetry (DSC). Electrochemical investigations reveal that m-HTPB exhibits higher conductance compared with HTPB. The m-HTPB flexible piezoelectric polymer is further used for in situ and real-time pressure monitoring. This simple and effective strategy provides a promising polymeric material for flexible piezoelectric sensors.