Flexible Piezoresistive Pressure Sensor Research Articles

Conductive MXene/cotton fabric based pressure sensor with both high sensitivity and wide sensing range for human motion detection and E-skin

• Conductive MXene coated cotton fabric is prepared for wearable pressure sensor. • The pressure sensor shows both high sensitivity and broad sensing range. • The porous and sandwiched architecture accounts for the good sensing property. • The pressure sensor has great potential for human motion detection and E-skin. Flexible wearable pressure sensors have attracted tremendous interest for applications in human health monitoring, electronic skin, artificial intelligence and so on. However, it is still a critical challenge for pressure sensors to simultaneously achieve high sensitivity and wide sensing range while performing well in response time, stability, reliability and wearing comfortability. Here, conductive MXene/cotton fabric was fabricated using the simple dip-coating technique and then sandwiched between polydimethylsiloxane film and an interdigitated electrode for flexible wearable piezoresistive pressure sensor. The abundant hydroxyl groups of cotton fabric and functional groups of MXene were beneficial for the good adhesion of conductive MXene nanosheets onto the entangled fiber networks, constructing effective conductive network. Taking advantage of the excellent flexibility and three-dimensional porous structure of cotton fabric and the special sandwich architecture of the sensor, the MXene/cotton fabric (MCF) based pressure sensor exhibited high sensitivity (5.30 kPa −1 in the pressure range of 0–1.30 kPa), broad sensing range (0–160 kPa), rapid response/recovery time (50 ms/20 ms), excellent stability and long-term durability. Moreover, the MCF based pressure sensor can be employed to detect and distinguish various human health signals, including finger motion, early stage Parkinson’s static tremor and wrist pulse. Importantly, E-skin was also designed based on the MCF based pressure sensor for the recognition of different tactile stimuli, exhibiting promising potential in next generation wearable electronics.

Integrated flexible piezoresistive pressure sensor based on CB/CNTs/SR composite with SR buffer layer for wide sensing range

In the present study, an integrated flexible piezoresistive pressure sensor with a silicon rubber (SR) buffer layer is proposed to optimize the workable pressure range of flexible pressure sensor. The sensitive unit of the integrated flexible pressure sensor is made of composite elastic dielectric SR filled with carbon black (CB) and carbon nanotubes (CNTs). This paper introduces the preparation process, working principle and structure design of the CB/CNTs/SR composite with an SR buffer layer. In addition, the influence of electrode structure and buffer layer is studied. A dispersible interdigital electrode structure is selected to prevent the decline of pressure sensing sensitivity in a small range. The proposed flexible pressure sensor achieves a wide pressure range of 0–120 N. Facile fabrication, high repeatability, high stability and low cost are also excellent characteristics of the sensors investigated in this paper. In this study, a design proposal for wearable equipment and wearable bionic skin study is provided.

Study of Three Interface Pressure Measurement Systems Used in the Treatment of Venous Disease

The aim of the publication is to report the accuracy, repeatability and the linearity of three commercially available interface pressure measurement systems employed in the treatment of venous disease. The advances in the treatment and management of chronic venous disease by compression therapy have led to considerable research interest in interface pressure measurement systems capable of measuring low-pressure ranges (10–60 mmHg). The application of a graduated pressure profile is key for the treatment of chronic venous disease which is achieved by using compression bandages or stockings; the required pressure profiles are defined in standards (BSI, RAL-GZ, or AFNOR) for different conditions. However, achieving the recommended pressure levels and its accuracy is still deemed to be a challenge. Thus, it is vital to choose a suitable pressure measurement system with high accuracy of interface pressure. The authors investigated the sensing performance of three commercially available different pressure sensors: two pneumatic based (AMI and PicoPress®) and one piezoresistive (FlexiForce®) pressure sensors, with extensive experimental work on their performance in terms of linearity, repeatability, and accuracy. Both pneumatic based pressure measurement systems have shown higher accuracy in comparison to the flexible piezoresistive pressure sensors.

Bioinspired, Self-Powered, and Highly Sensitive Electronic Skin for Sensing Static and Dynamic Pressures.

Flexible piezoresistive pressure sensors obtain global research interest owing to their potential applications in healthcare, human-robot interaction, and artificial nerves. However, an additional power supply is usually required to drive the sensors, which results in increased complexity of the pressure sensing system. Despite the great efforts in pursuing self-powered pressure sensors, most of the self-powered devices can merely detect the dynamic pressure and the reliable static pressure detection is still challenging. With the help of redox-induced electricity, a bioinspired graphite/polydimethylsiloxane piezoresistive composite film acting both as the cathode and pressure sensing layer, a neoteric electronic skin sensor is presented here to detect not only the dynamic forces but also the static forces without an external power supply. Additionally, the sensor exhibits a fascinating pressure sensitivity of ∼103 kPa-1 over a broad sensing range from 0.02 to 30 kPa. Benefiting from the advanced performance of the device, various potential applications including arterial pulse monitoring, human motion detecting, and Morse code generation are successfully demonstrated. This new strategy could pave a way for the development of next-generation self-powered wearable devices.

Flexible Waterproof Piezoresistive Pressure Sensors with Wide Linear Working Range Based on Conductive Fabrics

HighlightsThe laser-engraved method was introduced to fabricate the electrode for the sensor.The sensor showed a wide linear working range, superior sensitivity, and fast response time and also exhibited excellent viability in a wet situation.Wireless integrated network sensors successfully monitored the health states.Developing flexible sensors with high working performance holds intense interest for diverse applications in leveraging the Internet-of-things (IoT) infrastructures. For flexible piezoresistive sensors, traditionally most efforts are focused on tailoring the sensing materials to enhance the contact resistance variation for improving the sensitivity and working range, and it, however, remains challenging to simultaneously achieve flexible sensor with a linear working range over a high-pressure region (> 100 kPa) and keep a reliable sensitivity. Herein, we devised a laser-engraved silver-coated fabric as “soft” sensor electrode material to markedly advance the flexible sensor’s linear working range to a level of 800 kPa with a high sensitivity of 6.4 kPa−1 yet a fast response time of only 4 ms as well as long-time durability, which was rarely reported before. The integrated sensor successfully routed the wireless signal of pulse rate to the portable smartphone, further demonstrating its potential as a reliable electronic. Along with the rationally building the electrode instead of merely focusing on sensing materials capable of significantly improving the sensor’s performance, we expect that this design concept and sensor system could potentially pave the way for developing more advanced wearable electronics in the future.

Surface Engineering of a 3D Topological Network for Ultrasensitive Piezoresistive Pressure Sensors.

Polypyrrole (PPy) is a good candidate material for piezoresistive pressure sensors owing to its excellent electrical conductivity and good biocompatibility. However, it remains challenging to fabricate PPy-based flexible piezoresistive pressure sensors with high sensitivity because of the intrinsic rigidity and brittleness of the film composed of dense PPy particles. Here, a rational structure, that is, 3D-conductive and elastic topological film composed of coaxial nanofiber networks, is reported to dramatically improve the sensitivity of flexible PPy-based sensors. The film is prepared through surface modification of electrospun polyvinylidene fluoride (PVDF) nanofibers by polydopamine (PDA), in order to homogeneously deposit PPy particles on the nanofiber networks with strong interfacial adhesion (PVDF/PDA/PPy, PPP). This unique structure has a high surface area and abundant contact sites, leading to superb sensitivity against a subtle pressure. The as-developed piezoresistive pressure sensor delivers a low limit of detection (0.9 Pa), high sensitivity (139.9 kPa-1), fast response (22 ms), good cycling stability (over 10,000 cycles), and reliability, thereby showing a promising value for applications in the fields of health monitoring and artificial intelligence.

Highly sensitive, reliable and flexible piezoresistive pressure sensors based on graphene-PDMS @ sponge

In the research of flexible pressure sensors supported by three-dimensional skeleton structure, the combination of nanomaterials and skeleton structure is a difficult problem to be further solved. Herein, a piezoresistive sensor based on the graphene-PDMS @ sponge is prepared by fixing graphene on a sponge skeleton using PDMS. The as-prepared piezoresistive sensor exhibits high elasticity (strain up to 85%), high sensitivity (0.075 K Pa−1), a wide responding range (0–50 KPa) and high stability (2000 cycles pressure test). The piezoresistive sensor is used to detect blood pressure, heartbeat and human movements including finger bending, elbow movement and knee squatting, which shows good consistency and stability of the as-prepared sensor through the synergy of sponge, PDMS and graphene. The graphene-PDMS @ sponge sensor shows potential applications in medical testing and electronic skin.

Highly Sensitive and Wide Linear-Response Pressure Sensors Featuring Zero Standby Power Consumption under Bending Conditions.

The ability of a flexible pressure sensor to possess zero power consumption in standby mode, high sensitivity, and wide linear-response range is critical in real flexible matrix-based scenes. However, when the conventional flexible pressure sensors are attached on a curved surface, a pseudosignal response is generated because of the normal stress, resulting in a short linear-response range. Here, a flexible piezoresistive pressure sensor with high performance, zero standby power consumption is demonstrated. The flexible pressure sensor is fabricated from polydimethylsiloxane (PDMS)/carbon black (CB), patterned polyimide (PI) spacer layer, and laser-induced graphene (LIG) interdigital electrodes. Benefiting from the hierarchical structure and sufficient roughness of PDMS/CB and LIG interdigital electrodes, the proposed pressure sensors (PDMS/CB/PI/LIG) exhibit high sensitivity (43 kPa-1), large linear-response range (0.4-13.6 kPa), fast response (<40 ms), and long-term cycle stability (>1800 cycles). The resulting pressure sensor also features zero standby power consumption merit under certain bending conditions (bending angle: 0-5o). Furthermore, the effect of the hole diameter of the PI spacer layer on the performance of the pressure sensors is experimentally and theoretically investigated. As a proof of concept, a bioinspired artificial haptic neuron system has been successfully equipped to modulate the number of lit LED lights. The proposed high-performance pressure sensor has promising potential to be used in flexible and wearable electronics, especially for the applications in actual flexible matrix-based scenes.

Scalable fabrication of flexible piezoresistive pressure sensors based on occluded microstructures for subtle pressure and force waveform detection

Scalable fabrication of flexible PDMS/CNS pressure sensors with occluded microstructures were achieved by a simple, low-cost and eco-friendly manufacturing process.

Miniaturized Flexible Piezoresistive Pressure Sensors: Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Copolymers Blended with Graphene Oxide for Biomedical Applications.

Piezoresistive pressure sensors have garnered significant attention because of their wide applications in automobiles, intelligent buildings, and biomedicine. For in vivo testing, the size of pressure sensors is a vital factor to monitor the pressure of specific portions of a human body. Therefore, the primary focus of this study is to miniaturize piezoresistive pressure sensors with graphene oxide (GO)-incorporated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite films on a flexible substrate for biomedical applications. Prior to the fabrication of pressure sensors, a comprehensive material analysis was applied to identify the horizontal placement of GO flakes within the PEDOT:PSS copolymers, revealing a reduction in variable range hopping distance and an enhancement in carrier mobility. For devices scaled to 0.2 cm, the sensitivity of PEDOT:PSS pressure sensors was conspicuously decreased owing to the late response, which can be effectively solved by GO incorporation. Using technology computer-aided design simulations, the current crowded at the PEDOT:PSS film surface and in the vicinity of an indium-tin-oxide electrode corner was found to be responsible for the changes in piezoresistive behaviors of the scaled devices. The miniaturized flexible piezoresistive pressure sensors with PEDOT:PSS/GO composite films are capable of monitoring the brain pressure of intracranial surgery of a rat and discerning different styles of music for a potential application in hearing aids.

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

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

Novel flexible pressure sensor combining with dynamic-time-warping algorithm for handwriting identification

A novel handwriting identification technique, consisting of a flexible piezo-resistive pressure sensor with micro-pyramid structures and a Dynamic-Time-Warping algorithm, was presented in this paper. Real-time dynamic pressure was recorded by the piezo-resistive pressure sensors and similarities of characters written by different volunteers were evaluated using the Dynamic-Time-Warping algorithm. The piezo-resistive sensors were fabricated on polyimide thin films with designed micro-pyramid structures and the single-wall carbon nanotubes coated on the polyimide substrates were used the sensitive materials. It is found that the size of the micro-pyramid on polyimide substrates and film thickness have a significant influence on device performance. A device with a wider sensitive range (0–20 kPa) was fabricated with the optimized fabrication conditions and it is able to distinguish signatures of different volunteers correctly. Therefore, the device presented in this work shows an essential potential in the applications of the handwriting identification system.

Flexible and High Performance Piezoresistive Pressure Sensors Based on Hierarchical Flower-Shaped SnSe2 Nanoplates

Flexible piezoresistive pressure sensors featuring high sensitivity, wide operating pressure range, and short response time are required urgently due to the rapid development of smart devices and a...

Highly sensitive, reliable and flexible piezoresistive pressure sensors featuring polyurethane sponge coated with MXene sheets

Flexible pressure sensors still face a great challenge to combine fast frequency response, wide pressure range, multiple detection modes, satisfactory mechanical and environmental stability, and simple fabrication process into a sensor. Herein, flexible piezoresistive pressure sensors are fabricated by treating the backbone of polyurethane (PU) sponge with chitosan (CS) to obtain positively charged CS@PU sponge, followed by dip-coating of negatively charged Ti3C2Tx MXene sheets. The resulting MXene@CS@PU sponge-based sensor provides a versatile sensing platform with potentials for detecting both small and large pressure signals. Due to the highly compressive resilience of the PU sponge and its polar interaction with the MXene sheets, the MXene@CS@PU sensor has high compressibility and stable piezoresistive response for compressive strains of up to 85% with a stress of 245.7 kPa, and it also exhibits a satisfactory reproducibility for 5000 compression-release cycles. Even after washing in water for 1 h, the sensor still shows good performances. With a rapid response time of only 19 ms and a low detection limit of 30 μN corresponding to a pressure of 9 Pa, the MXene sponge sensor is promising for detecting human physiological signals and insect movements. In addition to the contact mode detection, the sponge sensor could detect voices and human breaths by a non-contact detection mode.

Flexible piezoresistive pressure sensors for smart textiles

The development of smart textiles relies in many applications on of textile-integrated sensors. Flexible piezoresistive pressure sensors have many potential applications, for instance in sports science. In this study, flexible pressure sensors are built using piezoresistive polymer film and conductive fabric. Tests using a universal testing machine show that the sensors are functional, accurate, although showing some hysteresis. However, methods for joining the electrode and piezoresistive layers are necessary to assure mechanical stability of the sensor, without affecting electrical contact between layers. Several methods were tested and results are reported here. The use of thermofusible bonding nets and webs has been found to be an interesting solution.

Cohesive thermoplastic-assisted patterning and assembly of a textile-supported piezoresistive sensor for monitoring human vital signs

Flexible piezoresistive pressure sensors have attracted great attentions due to their critical roles in wearable electronics. It is highly desirable to develop a low-cost and facile method for fabricating textile-supported piezoresistive pressure sensors with designed patterns. Herein, we use a pre-cut thermoplastic as both mask and separator to deposit silver nanowires (AgNWs) and to seal two conductive textiles, respectively, for the construction of a textile-supported piezoresistive pressure sensor. The typical sheet resistance of the AgNWs-coated textile is as low as ∼50 Ω/square. After sealing at 100 °C, the sensor could display a significant resistance change in response to a certain pressure. The as-prepared devices possess high sensitivity, fast response and excellent durability (more than 500 cycles of pressure loading/unloading). Human motions (bending/unbending of arm and thumb) and vital signs (respiration rate/intensity and neck pulse) could be real-time monitored by those devices. By simply cutting the thermoplastic with a paper cutter, the sensors with various patterns could be obtained, demonstrating that the thermoplastic-assisted approach could combine both functionality and aesthetics in building textile-supported piezoresistive pressure sensors. This work may provide a simple and cost-effective strategy to produce textile-based piezoresistive pressure sensors for wearable electronic applications.

Multiscale Hierarchical Design of a Flexible Piezoresistive Pressure Sensor with High Sensitivity and Wide Linearity Range.

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human-machine interfaces because of their simple device structure and easy-readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low-cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leaf's surface and spray-coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa-1 and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.

Impedance Matching Effect between a Triboelectric Nanogenerator and a Piezoresistive Pressure Sensor Induced Self‐Powered Weighing

AbstractHow to develop a simple and stable self‐powered force/pressure monitoring system has attracted extensive interests, yet it still faces intractable problems and challenges. As a mechanical force‐driven generator, the most straightforward application for triboelectric nanogenerator (TENG)‐based sensor is to directly monitor the external pressure. In this work, a self‐powered pressure monitor system is proposed by the conjunction of triboelectric and piezoresistive effects, in which a free‐standing disk‐shaped TENG (FD‐TENG) serves as an energy harvester, and a multiwalled carbon nanotube‐based flexible piezoresistive pressure sensor (PPS) works as a real‐time pressure monitor. The induced output voltage of the FD‐TENG is tuned by the working states of the PPS, which is responded to the external weight stimuli. Based on the impedance match between FD‐TENG and PPS, the self‐powered weighing system renders a voltage varying from 46.2 to 168.6 V in a range from 1 to 20 kg with a fast response/recovery kinetics (1.2 and 2.3 s) and good stability, which enable to directly reflect on the on/off status of appropriate amount LEDs powered by FD‐TENG. Finally, it is demonstrated to equip into a successive pedrail transportation system for monitoring the qualified/unqualified weight products.

Enhancing the sensitivity of graphene/polyurethane nanocomposite flexible piezo-resistive pressure sensors with magnetite nano-spacers

A robust piezo-resistive nanocomposite flexible sensor has been developed using an innovative combination of materials, i.e., magnetite-decorated graphene/poly(ionic liquid) compatibilized poly(urethane) (TPU/Fe3O4@RGO-PIL). The transducer has been fabricated by spray layer by layer (sLbL) to structure an original three-dimensional architecture in which: reduced graphene oxide (RGO) platelets are used to build the conducting network, PIL stabilizes this percolated assembly, Fe3O4 NP act as nano-spacers between platelets to prevent their stacking and facilitate their switching, and TPU matrix brings cohesion and flexibility to the whole assembly. This strategy results in highly sensitive TPU/Fe3O4@RGO-PIL flexible nanocomposite sensors (high gauge factor in pure compression up to 6.2, exceptionally low threshold pressure 0.5 kPa, Negative Pressure Coefficient), yielding a linear response in a large range of pressures (10–200 kPa), with robust and reproducible piezo-resistive properties (weak hysteresis upon loading unloading cycling, stability of signals up to 100 000 cycles, with 2 ms dephasing). These performances are promising for a variety of applications in flexible and wearable electronics, such as tactile sensors, sensing skins and compressive strain sensors.

A Flexible and Highly Sensitive Piezoresistive Pressure Sensor Based on Micropatterned Films Coated with Carbon Nanotubes

Excellent flexibility, high sensitivity, and low consumption are essential characteristics in flexible microtube pressure sensing occasion, for example, implantable medical devices, industrial pipeline, and microfluidic chip. This paper reports a flexible, highly sensitive, and ultrathin piezoresistive pressure sensor for fluid pressure sensing, whose sensing element is micropatterned films with conductive carbon nanotube layer. The flexible pressure sensor, the thickness of which is 40 ± 10 μm, could be economically fabricated by using biocompatible polydimethylsiloxane (PDMS). Experimental results show that the flexible pressure sensor has high sensitivity (0.047 kPa−1in gas sensing and 5.6 × 10−3 kPa−1in liquid sensing) and low consumption (&lt;180 μW), and the sensor could be used to measure the pressure in curved microtubes.