Flexible Pressure Research Articles

Flexible Pressure Sensor With High Sensitivity and Fast Response Based on Bionic Honeycomb-Structured Polydimethylsiloxane/Aluminum Oxide Composites Dielectric via 3-D Printing

Flexible Pressure Sensor With High Sensitivity and Fast Response Based on Bionic Honeycomb-Structured Polydimethylsiloxane/Aluminum Oxide Composites Dielectric via 3-D Printing

Flexible pressure sensor decorated with MXene and multi-walled carbon nanotube composites for motion monitoring and electronic skin

Flexible pressure sensor decorated with MXene and multi-walled carbon nanotube composites for motion monitoring and electronic skin

On the Development of a New Flexible Pressure Sensor.

The rapid advancement of the Internet of Things (IoT) serves as a significant driving force behind the development of innovative sensors and actuators. This technological progression has created a substantial demand for new flexible pressure sensors, essential for a variety of applications ranging from wearable devices to smart home systems. In response to this growing need, our laboratory has developed a novel flexible pressure sensor, designed to offer an improved performance and adaptability. This study aims to present our newly developed sensor, detailing the comprehensive investigations we conducted to understand how different parameters affect its behaviour. Specifically, we examined the influence of the resistive layer thickness and the elastomeric substrate on the sensor's performance. The resistive layer, a critical component of the sensor, directly impacts its sensitivity and accuracy. By experimenting with varying thicknesses, we aimed to identify the optimal configuration that maximizes sensor efficiency. Similarly, the elastomeric substrate, which provides the sensor's flexibility, was scrutinized to determine how its properties affect the sensor's overall functionality. Our findings highlight the delicate balance required between the resistive layer and the elastomeric substrate to achieve a sensor that is both highly sensitive and durable. This research contributes valuable insights into the design and optimization of flexible pressure sensors, paving the way for more advanced IoT applications.

Effect of nano-filler/matrix interfacial friction on temperature-dependent fatigue property of flexible pressure sensor

Flexible pressure sensors based on polymeric nanocomposites have attracted widespread attention owing to exceptional low-temperature fatigue tolerance, yet the damage evolution of the nano-filler/matrix interface is still unclear. Herein, we prepare Cu/MXene/multi-wall carbon nanotubes (MWCNTs)/thermoplastic polyurethane (TPU)/Cu and Cu/MXene/MWCNTs/carbon black (CB)/TPU/Cu pressure sensors using solution blending-casting method, and systematically investigate the influence of the nano-fillers/matrix interfacial friction damage on low-temperature bending fatigue of the sensor under cyclic bending. It is found that the sensing performance of the device with binary [MXene/MWCNTs] synergistic network decreases by 65 % with decreasing temperature from 20 down to − 40 °C under 20,000 bending cycles, while that of the sensor with ternary [MXene/MWCNTs/CB] synergistic network drops by only 14.9 %. For the ternary composites, owing to the introduction of CB particles, the synergistic lubrication effect among the nano-fillers and the nano-mechanical interlocking interface between the nano-fillers and the polymer can cooperatively enhance the anchorage of the nano-fillers to the TPU chains, and hence alleviating friction energy accumulation at the nano-fillers/polymer interface. Our work shows that the synergistic reinforcing strategy may be valuable for the development of ultra-flexible sensors with low-temperature fatigue resistant.

Graphene Sterically-Wrapped textile piezoresistive Sensors: A spray coating path for synergistically advancing sensitivity and response range

Trade-off between a high sensitivity and a broad sensing range remains a persistent challenge for flexible pressure sensors. Not to suffer from this dilemma, a flexible textile piezoresistive sensor sterically-wrapped with graphene by spray coating is reported, which constructs enriched three-dimensional conductive networks from leather fabric’s inherent fibers and pore structures, synergistically advancing the sensitivity and response range. Such a sensor renders bulky spatial deformation and augmented pressure-induced conducting paths, and hence affords an ultra-high sensitivity of 259.6 kPa−1(0–100 kPa) and an exceptional response range of up to 1 MPa. Besides, the graphene/textile sensor demonstrates outstanding durability (>12000 cycles). With these advantages, real-time monitoring of human physiological activities such as wrist pulses and throat swallowing is successfully achieved. Benefitting from the scalability of the spraying methodology, a large-size 45 × 45 cm2 sensing array with a 32 × 32 matrix is facilely fabricated to map the spatial pressure of human interactive behaviors for ergonomics. And smart insoles incorporating graphene/textile sensors can capture plantar pressure distribution and provide effective analysis for diverse static/dynamic movements of feet. Such spray-coated textile sensors could not only find potential applications in wearable electronics and intelligent robotics, but also open the possibility of monitoring complex heavy machinery and large-scale infrastructures.

Improving performance of AgNPs/PDA/TPU electrospun composite film based flexible pressure sensor by hot imprinted microstructure array

Highly sensitive sensors have attracted considerable attention in areas such as electronic skin, health monitoring and artificial intelligence. Herein, a simple and efficient approach was proposed to fabricate hierarchically structured flexible sensors. A common metal mesh was used as a template and its pattern was transferred to the surface of a composite film of silver nanoparticles (AgNPs) decorated polydopamine (PDA) and polyurethane (TPU) electrospun fibers by hot imprinting. The resulting film with a uniform convex microstructure array on the surface was used as the conductive sensing layer of the AgNPs/PDA/TPU sensor. The sensor exhibited a linear detection range increased from 15 kPa to 20 kPa compared to the control sensor without the microstructure. Furthermore, the sensitivity was improved from 8.13 kPa−1 to 10.44 kPa−1. The response time and recovery time of the sensor were determined to be 110 ms and 90 ms, respectively, and it maintained good stability at 10 kPa even after 1000 compression cycles. In addition, the sensor enabled real-time pressure signal detection for touch, joint movement and voice recognition, showing promising applications in health monitoring, pressure detection and human-machine interaction.

Laser patterned graphene pressure sensor with adjustable sensitivity in an ultrawide response range

Flexible pressure sensors have attracted wide attention because of their applications in wearable electronic, human-computer interface, and healthcare. However, it is still a challenge to design a pressure sensor with adjustable sensitivity in an ultrawide response range to satisfy the requirements of different application scenarios. Here, a laser patterned graphene pressure sensor (LPGPS) is proposed with adjustable sensitivity in an ultrawide response range based on the pre-stretched kirigami structure. Due to the out-of-plane deformation of the pre-stretched kirigami structure, the sensitivity can be easily tuned by simply modifying the pre-stretched level. As a result, it exhibits a maximum sensitivity of 0.243 kPa−1, an ultrawide range up to 1600 kPa, a low detection limit (6 Pa), a short response time (42 ms), and excellent stability with high pressure of 1200 kPa over 500 cycles. Benefiting from its high sensitivity and ultrawide response range, the proposed sensor can be applied to detect physiological and kinematic signals under different pressure intensities. Additionally, taking advantage of laser programmable patterning, it can be easily configured into an array to determine the pressure distribution. Therefore, LPGPS with adjustable sensitivity in an ultrawide response range has potential application in wearable electronic devices.

Highly Sensitive Flexible Capacitive Pressure Sensor Based on a Multicross-Linked Dual-Network Ionic Hydrogel for Blood Pressure Monitoring Applications.

Flexible capacitive pressure sensors based on ionic hydrogels (IHs) have garnered significant attention in the field of wearable technology. However, the vulnerability of traditional single-network hydrogels to mechanical damage and the complexity associated with preparing double-network hydrogels present challenges in developing a highly sensitive, easily prepared, and durable IH-based flexible capacitive pressure sensor. This study introduces a novel multicross-linked dual-network IH achieved through the physical and chemical cross-linking of polymers polyvinyl alcohol (PVA) and chitosan (CS), ionic solution H3PO4, and cross-linking agent gum arabic. Flexible capacitive pressure sensors, characterized by high sensitivity and a broad pressure range, are fabricated by employing mesh as templates to design cut-corner cube microstructures with high uniformity and controllability on the IHs. The sensor exhibits high sensitivity across a wide pressure range (0-290 kPa) and with excellent features such as high resolution (∼1.3 Pa), fast response-recovery time (∼11 ms), and repeatable compression stability at 25 kPa (>2000 cycles). The IHs as a dielectric layer demonstrate long-term water retention properties, enabling exposure to air for up to 100 days. Additionally, the developed sensor shows the ability to accurately measure the pulse wave within the small pressure range. By combining the pulse wave acquired by the sensor with a trained neural network model, we achieve successful blood pressure (BP) prediction, meeting the standards set by the Association for the Advancement of Medical Instrumentation and the British Hypertension Society. Ultimately, the sensor proposed in this study holds promising prospects for broad applications in high-precision wearable medical electronic devices.

A graphene flexible pressure sensor based on a fabric-like groove structure for high-resolution tactile imaging

Flexible pressure sensors hold significant promise for applications in fields such as prosthetics, electronic skin, robotics, and healthcare. Nevertheless, the challenge lies in preserving the linear sensitivity range of these sensors across a broad pressure range. In this study, we introduce a structure featuring Graphene Nano-walls (GNWs) arranged in a fabric-like groove pattern for flexible piezoresistive sensors. This design effectively accommodates material deformations, enabling the sensor to maintain exceptional sensitivity across a wide pressure range. The GNWs groove piezoresistive sensor exhibits ultra-high sensitivity (S = 2297.47 kPa−1) and outstanding linearity (R2 = 0.986) over an extensive pressure range (0.2 Pa − 425 kPa), while maintaining remarkable mechanical stability over 10,000 cycles. It boasts rapid response times of 9 ms and a recovery time of 7 ms. Owing to the fast response and high sensitivity of this sensor, we constructed a high-resolution tactile imaging system, with a surface height resolution of 20 μm and a elastic film thickness resolution of 0.1 mm. Noteworthy, we reconstructed the 3D texture of a leaf and reconstructed the shape of letters under soft elastomer. These applications underscore the potential of GNWs groove pressure sensors in various fields, including artificial intelligence, robotics, prosthetics, and so on.

Bioinspired, highly sensitive interlocked flexible textile pressure sensor based on multilayer SWCNTs/PVP/rGO dendritic for gesture recognition

The development of wearable gesture recognition systems, capable of converting intricate human gestures into electric signals and graphical representations, poses significant challenges for future advancements in health monitoring and human-machine interfaces. In response to this demand, we present an approach centered on a cell-inspired, multiscale hierarchical structural piezoresistive pressure sensor designed for gesture recognition applications. The breathable, hierarchically interlocked textile pressure sensor (PFPA), featuring a SWCNTs/polyvinylpyrrolidone (PVP)/reduced graphene oxide (rGO) dendritic structure combined with a PANI/PVA cylindrical array, exhibits remarkable properties. The dendritic electrodes, composed of SWCNTs and PVP in a 1:2 ratio, maintain optimal regularity. Meanwhile, the cylindrical PANI/PVA array is meticulously engineered with a radius of 200 μm, a height of 200 μm, and an inter-cylindrical spacing of 150 μm. The PFPA demonstrates an unprecedented ultrahigh sensitivity of 80.8 kPa−1 across a pressure range of 0–5 kPa, coupled with a rapid response time of 150 ms and minimal hysteresis of 16.1 %. The PFPA also demonstrates outstanding repeatability, with no significant performance degradation observed after 4000 cycles. By correlating the bending angles of finger joints with resistance changes, we successfully integrated these sensors into a glove, enabling precise gesture recognition. The multilayer SWCNTs/PVP/rGO dendritic and PANI/PVA cylindrical array pressure sensor exhibits superior sensing performance and accurate posture capture. This demonstrates its significant potential for applications in wearable electronics and intelligent devices, marking a substantial advancement in the field.

Design of Athlete Plantar Pressure Collection System based on Flexible Thin Film Pressure Sensor

According to the needs of athletes and coaches, a plantar pressure collection system for athletes based on flexible thin film pressure sensors is designed. The system integrates flexible thin film pressure sensors, STM32 microcontrollers, and HC-06 Bluetooth modules, and develops a supporting mobile app. Through system production and experimental verification, the system can real-time collect plantar pressure data of athletes and display it through computer serial port assistants and mobile apps. This system provides a convenient tool for analyzing plantar pressure of athletes, which helps improve the scientificity and specificity of athlete training.

Harmonizing Elastic Modulus and Dielectric Constant of Elastomers for Improved Pressure Sensing Performance.

Enhancing the sensitivity of capacitive pressure sensors through microstructure design may compromise the reliability of the device and rely on intricate manufacturing processes. It is an effective way to solve this issue by balancing the intrinsic properties (elastic modulus and dielectric constant) of the dielectric layer materials. Here, we introduce a liquid metal (LM) hybrid elastomer prepared by a chain-extension-free polyurethane (PU) and LM. The synergistic strategies of extender-free and LM doping effectively reduce the elastic modulus (7.6 ± 0.2-2.1 ± 0.3 MPa) and enhance the dielectric constant (5.12-8.17 @1 kHz) of LM hybrid elastomers. Interestingly, the LM hybrid elastomer combines reprocessability, recyclability, and photothermal conversion. The obtained flexible pressure sensor can be used for detecting hand and throat muscle movements, and high-precision speech recognition of seven words has been using a convolutional neural network (CNN) in deep learning. This work provides an idea for designing and manufacturing wearable, recyclable, and intelligent control pressure sensors.

Multispecies-coadsorption-induced rapid preparation of graphene glass fiber fabric and applications in flexible pressure sensor

Direct chemical vapor deposition (CVD) growth of graphene on dielectric/insulating materials is a promising strategy for subsequent transfer-free applications of graphene. However, graphene growth on noncatalytic substrates is faced with thorny issues, especially the limited growth rate, which severely hinders mass production and practical applications. Herein, graphene glass fiber fabric (GGFF) is developed by graphene CVD growth on glass fiber fabric. Dichloromethane is applied as a carbon precursor to accelerate graphene growth, which has a low decomposition energy barrier, and more importantly, the produced high-electronegativity Cl radical can enhance adsorption of active carbon species by Cl–CH2 coadsorption and facilitate H detachment from graphene edges. Consequently, the growth rate is increased by ~3 orders of magnitude and carbon utilization by ~960-fold, compared with conventional methane precursor. The advantageous hierarchical conductive configuration of lightweight, flexible GGFF makes it an ultrasensitive pressure sensor for human motion and physiological monitoring, such as pulse and vocal signals.

Wearable multichannel-active pressurized pulse sensing platform

With the modernization of traditional Chinese medicine (TCM), creating devices to digitalize aspects of pulse diagnosis has proved to be challenging. The currently available pulse detection devices usually rely on external pressure devices, which are either bulky or poorly integrated, hindering their practical application. In this work, we propose an innovative wearable active pressure three-channel pulse monitoring device based on TCM pulse diagnosis methods. It combines a flexible pressure sensor array, flexible airbag array, active pressure control unit, advanced machine learning approach, and a companion mobile application for human–computer interaction. Due to the high sensitivity (460.1 kPa−1), high linearity (R2 > 0.999) and flexibility of the flexible pressure sensors, the device can accurately simulate finger pressure to collect pulse waves (Cun, Guan, and Chi) at different external pressures on the wrist. In addition, by measuring the change in pulse wave amplitude at different pressures, an individual’s blood pressure status can be successfully predicted. This enables truly wearable, actively pressurized, continuous wireless dynamic monitoring of wrist pulse health. The innovative and integrated design of this pulse monitoring platform could provide a new paradigm for digitizing aspects of TCM and other smart healthcare systems.

Efficient Fabrication of Bioinspired Flexible Pressure Sensors via Electrohydrodynamic Jet Printing Method.

Bioinspired microdevices have made significant strides in various applications including human motion and health detection. However, facile and highly efficient fabrication approach of flexible pressure sensors remains a great challenge. Herein, inspired by the gecko's foot structure, a flexible pressure sensor with microdomes structure is fabricated by tip-assisted on-demand electrohydrodynamic jet (EHD-jet) printing method. Ascribed to the interlocking electrodes with microdome structure, 3Ddeformation rates are substantially enlarged. When the microdromes structure is under pressure, the resistivity of carbon nanotubes film coated on the surface of microdomes structure will change remarkably. By using the combined effect of assisted tip and ring focusing electrode, the influence and constraints on microstructure fabrication caused by substrate material and morphology are minimized. The desired uniform structures can be adjusted rapidly by changing the printing parameters and liquid properties. High length-height ratio (0.64) of microdomes enhances sensitivity, with minimum detection limit is 2Pa and response time is 40ms. Finally, the bionic flexible sensor indicated excellent performance in capable of detecting pressure, sound vibrations and human motion. This work presents a new method for high-efficiency fabrication micro-nano patterns for flexible sensors inspired, which could be used in wearable tech and health monitoring.

Flexible pressure sensor based on Pt/PI network with high sensitivity and high thermal resistance

The increasing development of wearable electronics necessitates the urgent need for highly sensitive flexible pressure sensors suitable for harsh environments at high/low temperatures, thus meeting the specific requirements of wilderness survival, lunar exploration, mine exploration, and so on. Herein, a flexible piezoresistive sensor based on 3D Pt/PI fibrous network is prepared through the electrospinning and ion-sputtering techniques, demonstrating excellent sensing performance over a wide temperature range from liquid nitrogen temperature to 250 °C. The 3D Pt/PI pressure sensor achieves high sensitivity of 158.23 kPa−1 at room temperature, and remains remarkable sensitivity of 71.47 kPa−1 and 7.63 kPa−1 at 150 °C and 250 °C, respectively. Furthermore, benefiting from the hydrophobicity of the Pt/PI fibrous network, the sensitivity of the pressure sensor stabilizes in about 148 ± 10 kPa−1 under relative humidity from 30 % to 90 %, indicating humidity-independent sensing performance. As a proof of concept, the fabricated piezoresistive sensor can be not only monitor the human physiological information in real-time, but also detect external mechanical stimuli at high/low temperatures, demonstrating great potential for application in harsh environments.

Flexible Mechanical Sensors Fabricated with Graphene Oxide-Coated Commercial Silk.

Many studies on flexible strain and pressure sensors have been reported due to growing interest in wearable devices for healthcare purposes. Here, we present flexible pressure and strain (motion) sensors prepared with only graphene oxide (GO) and commercial silk fabrics and yarns. The pressure sensors were fabricated by simply dipping the silk fabric into GO solution followed by applying a thermal treatment at 400 °C to obtain reduced GO (rGO). The pressure sensors were made from rGO-coated fabrics, which were stacked in three, five, and seven layers. A super-sensitivity of 2.58 × 103 kPa-1 at low pressure was observed in the seven-layer pressure sensor. The strain sensors were obtained from rGO-coated twisted silk yarns whose gauge factor was 0.307. Although this value is small or comparable to the values for other sensors, it is appropriate for motion sensing. The results of this study show a cost-effective and simple method for the fabrication of pressure and motion sensors with commercial silk and GO.

Direct-write printed controllable concave surface for fabricating flexible metal electrode

Structured surface of flexible substrate could generate a positive impact on flexibility of metal electrode due to the surface free energy relaxation, which could make the flexibility of metal electrode meet the performance requirement of flexible electronics. However, the preparation of controllable structured surface of flexible substrate is still a tough and challenge process. In this research work, controllable concave surface of flexible substrate was prepared by direct-write print with a viscoelastic substrate. When the controllable concave surface of flexible substrate was evaporated with a nanoscale metal layer, various metal electrodes could be obtained with a controllable morphology of concave surface. The direct-write printed concave surface efficiently enhanced the flexibility of metal electrodes. When the prepared concave surface generated a continuous connected structure with a 0.1 mm printing space, an excellently enhanced flexibility of metal electrode was achieved with an internal bending flexibility of 93.72%, external bending flexibility of 93.67%, stretchability of 85% and number of stable cyclic bending times over 12,000. Meanwhile, the direct-write printed concave surface generated a reversed convex surface, which could be used for pressure sensing as a flexible pressure sensor. Therefore, this research work provides an efficient method for fabricating flexible electrode, which has important research and application value in the area of high-performance flexible electronics.

Integrated surface microstructure and enhanced dielectric constant for constructing simple, low-cost, and high-performance flexible capacitive pressure sensor

Designing microstructure surface or enhancing the dielectric constant of functional layer is a common useful strategy to improve the pressure sensing performances of flexible capacitive pressure sensors, but it usually involves complex preparation processes (such as artificial microstructure and preparation of composite materials). Considering the limitations of a single strategy in improving the pressure sensing performance of capacitive pressure sensor, herein, we propose a simple strategy that integrates surface microstructure and enhanced dielectric constant of functional layer for constructing a high-performance flexible capacitive pressure sensor. Specifically, the polyester fiber conductive tape and filter paper with intrinsic rough surfaces are selected as microstructure electrodes and dielectric layer, respectively. Meanwhile, by introducing the daily carbon ink into filter paper, the moderate conductivity of the carbon ink is utilized to enhance the dielectric constant of the filter paper dielectric layer. The results show that the proposed the pressure sensor has wide pressure detection range of 0.1−100 kPa, high sensitivity (1.47 kPa−1, 0.1−10 kPa) and good durability (4000 cycles at 20 kPa). Taking the pressure range of 10−100 kPa as an example, the sensitivity of carbon ink paper-based sensor is 60 times higher than that of sensor without carbon ink. The enhanced working mechanism of the sensor is clarified by combining testing and characterization. In addition, the proposed pressure sensor has potential applications in wrist pulse, respiratory rate, walking record, morse code transmission，and stoma bag detection. This work provides a useful reference for constructing simple, low-cost and high-performance flexible capacitive pressure sensor.

The Design and numerical simulation of a 3D-printed flexible capacitive pressure sensor with a composite structure

Abstract Flexible pressure sensors are crucial for many areas, such as electronic skins, human motion, robot monitoring, and medical treatment. However, conventional manufacturing methods limit the speed and accuracy of manufacturing sensor dielectric layers. In this paper, a unique capacitive pressure sensor with a composite structure is introduced that can be fabricated by 3D printing technology. The structure, which has 3D hierarchical pyramids microstructures and porous substrate, can be directly fabricated by a DLP 3D printer. This allows flexible dielectric layers to be fabricated quickly while maintaining their superior performance. The dimension of the produced sensor is 10×10×4.5 mm3. The sensor’s design process, numerical simulation, and performance are provided and elucidated. The performance of the sensor is measured between 0 and 1 MPa. It has low hysteresis and high sensitivity (6.83 MPa−1 within the scope of 0 to 0.5 MPa and 2.56 MPa−1 within the scope of 0.5 to 1 MPa).