Flexible Piezoresistive Sensor Research Articles

MXene‐Sponge Based High‐Performance Piezoresistive Sensor for Wearable Biomonitoring and Real‐Time Tactile Sensing (Small Methods 2/2022)

Flexible Piezoresistive Pressure Sensor In article number 2101051, Ye, Xie, Su, Chen and co-workers were inspired by Helium embrittlement during nuclear fusion processes, to utilize Helium plasma irradiation to yield a nano-fuzz-shaped molybdenum electrode for a flexible piezoresistive pressure sensor, which was demonstrated to measure human radial pulse, detect the tapping, stamping, and perform object identification, revealing great feasibility in wearable biomonitoring and healthcare assessment.

Wide-Range Motion Recognition Through Insole Sensor Using Multi-Walled Carbon Nanotubes and Polydimethylsiloxane Composites.

High linearity/sensitivity and a wide dynamic sensing range are the most desirable features for pressure sensors to accurately detect and respond to external pressure stimuli. Even though a number of recent studies have demonstrated a low-cost pressure sensing device for a smart insole system by using scalable and deformable conductive materials, they still lack stretchability and desirable properties such as high sensitivity, hysteresis, linearity, and fast response time to obtain accurate and reliable data. To resolve this issue, a flexible and stretchable piezoresistive pressure sensor with high linear response over a wide pressure range is developed and integrated in a wearable insole system. The sensor uses multi-walled carbon nanotubes and polydimethylsiloxane (MWCNT/PDMS) composites with gradient density double-stacked configuration as well as randomly distributed surface microstructure (RDSM). The randomly distributed surface of the MWCNT/PDMS composite is easily and non-artificially generated by the evaporation of residual IPA solvent during a composite curing process. Due to two functional features consisting of the double-stacked composite configuration with different gradient MWCNT density and RDSM, the pressure sensor shows high linear sensitivity (∼82.5 kPa) and a pressure range of 0-1 MPa, providing extensive potential applications in monitoring human motions. Moreover, for a practical wearable application detecting the user's real-time motions, a custom-designed output signal acquisition system has been developed and integrated with the insole pressure sensor. As a result, the insole sensor can successfully detect walking, running, and jumping movements and can be used in daily life to monitor gait patterns by virtue of its long-term stability.

Fabrication and performance of full textile-based flexible piezoresistive pressure sensor

Fabrication and performance of full textile-based flexible piezoresistive pressure sensor

Secondary embossing method for the capsulation of high-sensitive flexible piezoresistive sensors

Due to the mismatch between thermal expansion and hygroscopic expansion coefficients of polymer packaging materials, the stress and reliability of packaging structure present significant influences on the performance of flexible sensors. In this work, a new capsulation method for polymer-based flexible piezoresistive sensing systems was proposed based on differential-temperature hot embossing. The capsulate mold of the flexible sensing system is designed and the flexible sensor is packaged into a polydimethylsiloxane (PDMS) film to form a “sandwich” structure. The influences of the first embossing pressure and the second embossing time on capsulation result were investigated systematically. The results showed that the optimal parameters for the first embossing pressure and second embossing time were 1.4 MPa and 180 s, respectively. The interlayer peel force of obtained samples under this condition could reach a maximum of 5.7 N. This PDMS bulk bonding method can improve the cracking problem of the sample and provide more effective protection for the flexible sensors, compared with the traditional capsulation methods, this packaging method has little effect on sensor sensitivity (decreased by 0.64 kPa−1). After the packaging is completed, the polymer-based flexible piezoresistive sensor presents a high sensitivity (−5.64 kPa−1), and at the same time, a fast response (92 ms).

Soft and Flexible Large-Strain Piezoresistive Sensor Mounted on an Under-Actuated Robotic Hand for Object Shape Classification

Soft and Flexible Large-Strain Piezoresistive Sensor Mounted on an Under-Actuated Robotic Hand for Object Shape Classification

Bioinspired High-Performance Multilayer Flexible Piezoresistive Pressure Sensor with Broader Linearity Range and Higher Sensitivity

Bioinspired High-Performance Multilayer Flexible Piezoresistive Pressure Sensor with Broader Linearity Range and Higher Sensitivity

The Use of Parylene C as a Flexible Piezoelectric, Piezoresistive, and Capacitive Pressure Sensor in Low-Pressure Range (<4 Kpa)

The Use of Parylene C as a Flexible Piezoelectric, Piezoresistive, and Capacitive Pressure Sensor in Low-Pressure Range (<4 Kpa)

Ultra-sensitive flexible piezoresistive pressure sensor prepared by laser-assisted copper template for health monitoring

The flexible sensors with microstructures have attracted much attention due to their ultrahigh sensitivity. To develop a controllable method for large-scale preparation of microstructures is the key to promoting the application of the flexible sensors. Herein, combining the laser processing technology with a copper template commonly used in roll-to-roll process, ultra-sensitive flexible piezoresistive pressure sensors with carbon nanotubes (CNTs) conductive layers on the ridge-shaped microstructures were fabricated and the effects of structure parameters on the performance of the sensors were systematically studied. Experimental results showed that the sensitivity of the sensors is positively correlated with the height h and spacing d of the ridge-shaped microstructure which are determined by laser processing parameters. The sensitivity of the sensor with the ridge-shaped microstructure (h=150 µm, d=200 µm) is as high as 1150.9 kPa−1 at an applied pressure< 50 Pa. Meanwhile, the sensor shows excellent stability (>2000 cycles), rapid response time (43 ms) and recovery time (15 ms). Besides, the sensor was used to detect human physiological signals such as speaking, cheek bugling and artery pulse. These results indicate that combining the laser processing with copper templates has great potential in the controllable and large-scale fabrication of flexible sensors with microstructures for health monitoring.

Tunable Wide Range and High Sensitivity Flexible Pressure Sensors with Ordered Multilevel Microstructures

AbstractWith the rapid development of healthcare and human‐machine interactions, there is a growing demand for flexible electronic skin to have a high sensitivity over a wide range. While current multilevel and hierarchical structures inspired by nature broadened the sensing range, the structural design still lacks systematic study. Hereby, an ordered multilevel microstructure is proposed, and the design strategy for performance regulation of piezoresistive pressure sensors is studied. Both microstructure height and spatial distribution are investigated systematically through simulation and experiments. The flexible piezoresistive pressure sensor is fabricated by combining a fast, cost‐effective laser marking technology, molding, and pneumatic spray. The fabricated sensors show high sensitivity (1.5 – 8.3 kPa−1) over a wide range (0.01 – 200 kPa), and have a detection limit of 10 Pa, a response time of below 70 ms, and a mechanical stability over 10 000 cycles. This design and fabrication strategy can be further optimized by combining advanced materials and fabrication systems, and is expected to be applicable for a wide range of flexible materials.

Laser Direct Writing of Highly Ordered Two‐Level Hierarchical Microstructures for Flexible Piezoresistive Sensor with Enhanced Sensitivity

AbstractWith the development of micro‐nano manufacturing technology, various hierarchical microstructures (HMs) are used to improve the sensitivity and measurement range of flexible pressure sensors. However, the fabrication of highly ordered HMs in simple, fast, and low‐cost ways remains a great challenge. In this work, laser direct writing technology is used to fabricate highly ordered HMs to enhance the sensitivity of flexible piezoresistive sensors. The HMs show a lateral expansion with the increasing pressure due to good flexibility and small force bearing areas, resulting in more significant change in contact area than the single‐level microstructures, which leads to an enhanced sensitivity. Two case studies are conducted to verify the performance of the sensor with laser processed HMs. Experimental results show that the pulsating blood pressure signal of radial artery can be detected by attaching the sensor on the wrist. When the sensor is attached on the neck, it can also detect the vibration signal of vocal cord when speaking. These results successfully demonstrate the potential of laser processing in fabricating highly ordered HMs to achieve highly sensitive flexible piezoresistive sensors for various applications, such as wearable health monitoring and human–computer interactions.

MXene-Sponge Based High-Performance Piezoresistive Sensor for Wearable Biomonitoring and Real-Time Tactile Sensing.

Electrode microfabrication technologies such as lithography and deposition have been widely applied in wearable electronics to boost interfacial coupling efficiency and device performance. However, a majority of these approaches are restricted by expensive and complicated processing techniques, as well as waste discharge. Here, helium plasma irradiation is employed to yield a molybdenum microstructured electrode, which is constructed into a flexible piezoresistive pressure sensor based on a Ti3 C2 Tx nanosheet-immersed polyurethane sponge. This electrode engineering strategy enables the smooth transition between sponge deformation and MXene interlamellar displacement, giving rise to high sensitivity (1.52kPa-1 ) and good linearity (r2 =0.9985) in a wide sensing range (0-100kPa) with a response time of 226ms for pressure detection. In addition, both the experimental characterization and finite element simulation confirm that the hierarchical structures modulated by pore size, plasma bias, and MXene concentration play a crucial role in improving the sensing performance. Furthermore, the as-developed flexible pressure sensor is demonstrated to measure human radial pulse, detect finger tapping, foot stomping, and perform object identification, revealing great feasibility in wearable biomonitoring and health assessment.

Conductive and room-temperature self-healable polydimethylsiloxane-based elastomer film with ridge-like microstructure for piezoresistive pressure sensor

The fabrication of piezoresistive pressure sensor with self-healing function has attracted worldwide attention for its great prospects in intelligent robots, remote health monitoring and electronic skin. In this work, a conductive and self-healable elastomer (CSE) film with ridge-like microstructure was designed for flexible piezoresistive pressure sensor. Firstly, self-healable polydimethylsiloxane (PDMS)-based elastomer with ureido and imine groups was synthesized via a simple one-pot two-step polycondensation reaction with aminopropyl-terminated PDMS, isophorone diisocyanate and 1, 3, 5-triformylbenzene as monomers. Secondly, the synthesized PDMS-based elastomer solution was casted onto a sandpaper pre-sprayed with ureido pyrimidinone grafted carbon nanotubes/polyurea mixed solution, and the CSE film was obtained after drying and peeling off from the sandpaper. The piezoresistive pressure sensor fabricated with two single-electrode CSE films exhibited high sensitivity of 8.7 kPa−1 (0–6.1 kPa), low detection limit (50 Pa), fast response capability (response/relaxation time of 40/117 ms) and repeatability for 10,000 loading–unloading tests. Moreover, the sensor was able to fully recover to normal work after healing for 36 h at room temperature or even 200 s under the irradiation of near infrared (NIR) light. Importantly, the sensor was successfully applied for not only detecting human motions including radial pulse, speech recognitions and joint movements but also monitoring health status wirelessly through Bluetooth transmission. Our findings put forward a novel method to fabricate a high-performance self-healable piezoresistive sensor for wearable electronics, healthcare monitoring and electronic skin.

High-Sensitivity Flexible Pressure Sensor-Based 3D CNTs Sponge for Human-Computer Interaction.

Researchers are showing an increasing interest in high-performance flexible pressure sensors owing to their potential uses in wearable electronics, bionic skin, and human–machine interactions, etc. However, the vast majority of these flexible pressure sensors require extensive nano-architectural design, which both complicates their manufacturing and is time-consuming. Thus, a low-cost technology which can be applied on a large scale is highly desirable for the manufacture of flexible pressure-sensitive materials that have a high sensitivity over a wide range of pressures. This work is based on the use of a three-dimensional elastic porous carbon nanotubes (CNTs) sponge as the conductive layer to fabricate a novel flexible piezoresistive sensor. The synthesis of a CNTs sponge was achieved by chemical vapor deposition, the basic underlying principle governing the sensing behavior of the CNTs sponge-based pressure sensor and was illustrated by employing in situ scanning electron microscopy. The CNTs sponge-based sensor has a quick response time of ~105 ms, a high sensitivity extending across a broad pressure range (less than 10 kPa for 809 kPa−1) and possesses an outstanding permanence over 4000 cycles. Furthermore, a 16-pixel wireless sensor system was designed and a series of applications have been demonstrated. Its potential applications in the visualizing pressure distribution and an example of human–machine communication were also demonstrated.

A 3D flexible piezoresistive sensor based on surface-filled graphene nanosheets conductive layer

The three-dimensional (3D) flexible piezoresistive sensors have promising potentials in wearable electronics because of their abundant structure forms and sensing functions. It is desired to develop 3D flexible piezoresistive sensors with high sensitivity and good stability. Herein, highly sensitive, stable and 3D flexible piezoresistive sensor with novel surface-filled (SF) graphene nanosheets (GNs) conductive layer was fabricated by fused deposition molding (FDM) 3D printing. Due to the surface-filled GNs conductive layer, the flexible piezoresistive sensor shows high stretchability of 1960% strain, good sensitivity of 175.57 kPa−1 at an applied pressure <300 Pa, high gauge factors of 18.95 (0–50% strain) and 66.29 (50–100% strain), excellent stability (>2000 cycles), the short mechanical response time (105 ms) and recovery time (66 ms). Compared with the SF-CNTs@styrene-ethylene-butylene-styrene (SEBS) sensor, the sensitivity of the SF-GNs@SEBS sensor is increased by 15.6%, and the GF values are increased by 167% (0–50%) and 974% (50–100%) respectively, indicating that GNs is more suitable for the sensor with the SF structure than CNTs. After 2000 compression/release cycles, the response signal of the GNs surface-filled sensor is basically not degraded, while the response signal of the GNs surface-coated sensor is degraded by 50%, indicating that the GNs surface-filled sensor has a far better stability than the GNs surface-coated sensor. Besides, the piezoresistive sensor was demonstrated the applications in detecting human activities, such as speaking, finger tapping, cheek bulging, and neck bending, exhibiting its great potential for application in health monitoring.

Accurate recognition of object contour based on flexible piezoelectric and piezoresistive dual mode strain sensors

The application of flexible wearable sensors in the grasping process of robot hand can recognize the contour, soft and hard, material, surface temperature and other information of the grasping object, which can effectively improve the intelligent level of the robot. In this work, a method of object contour recognition is proposed by combining the flexible PVDF polymer piezoelectric sensor and high conductivity hydrogel piezoresistive sensor aiming at the problem of profile recognition for objects of the same or similar material. The response of flexible piezoresistive sensor to the static strain is used to sense the angular displacement of robot fingers, and then the shape and size of the object is recognized indirectly. At the same time, the flexible piezoelectric sensor is used as the fingertip tactile sensor to reflect the surface morphology of the object through the dynamic strain information when touching the object. In the whole process of grasping the object, the dual-mode strain information is fully used to realize the recognition of the shape, size and surface morphology of the object. Combining these information, the accurate recognition of the object contour can be further realized. In the experiments, six objects with different shape and four objects with different surface morphology are recognized to verify the feasibility of piezoresistive sensors and piezoelectric sensors respectively. In a comprehensive experiment, eight objects made of the same rubber material with different shape, size and surface morphology are recognized, and the average recognition rate is about 84%, which shows good classification advantages for the objects with similar shape, size and material.

Fingerprint‐Inspired High Conductive PEDOT‐Coated Nanofiber Film for Ultra‐Sensitive, Stretchable, and Flexible Piezoresistive Sensor

AbstractFlexible piezoresistive sensors are extensively applied in smart health monitoring systems due to their excellent shape adaptability, portability, and comfortableness. However, constructing piezoresistive sensors with both high sensitivities and wide pressure range remains a huge challenge. In this paper, inspired by the fingerprint structure, a 3D poly (3,4‐ethylenedioxythiophene) coated wrinkled nanofiber film based flexible piezoresistive sensor is developed through a simple method. Benefiting from the undulating structure of the conductive nanofiber film, there are abundant micro‐ridges on the surface of the rough nanofiber network, which make a significant contribution to the construction of new conductive paths under different pressures. This assembled sensor exhibits an ultra‐high sensitivity of 397.54 kPa−1 in a small pressure range (0–3 kPa) as well as an ultra‐wide pressure range of 0–25.48 kPa. In addition, it can maintain stable signal output even after 16 500 compression cycles. Furthermore, this high‐performance piezoresistive sensor is demonstrated to accurately identify the wrist pulse and joint bending of human body, which enables an extensive application prospect in the field of wearable electronics.

Highly sensitive piezoresistive pressure sensors based on laser-induced graphene with molybdenum disulfide nanoparticles

Wearable pressure sensors have drawn significant attention because of their extensive applications in motion detection, tactile sensing, and health monitoring. However, the complex manufacturing process and high cost of active materials make low-cost, large-scale production elusive. In this work, we report a flexible piezoresistive pressure sensor assembled with two 3D laser-induced graphene (LIG) foam electrodes on a polyimide thin film from a simple laser scribing process in the ambient environment. The design of the air gap between the two foam electrodes allows the sensor to showcase a low limit of detection of 0.274 Pa, which provides favorable sensing performance in motion detection and wrist pulse monitoring. The addition of spherical MoS2 nanoparticles between the two foam electrodes further enhances the sensitivity to 88 kPa−1 and increases the sensing range to significantly outperform the previous literature reports. The demonstrated LIG pressure sensors also exhibit fast response/recovery rates and excellent durability/repeatability.

A novel flexible piezoresistive pressure sensor based on PVDF/PVA-CNTs electrospun composite film

A novel flexible piezoresistive pressure sensor based on PVDF/PVA-CNTs electrospun composite film

Ultralight polypyrrole crosslinked nanofiber aerogel for highly sensitive piezoresistive sensor

Nanofiber aerogel is an ideal carrier for constructing high-sensitivity flexible piezoresistive sensors due to its abundant pore structure and excellent mechanical property. However, the reported nanofiber-based piezoresistive sensors generally need tedious high-temperature carbonization process to fabricate the 3D conductive network. Here, a simple, low-cost, and effective method is developed for constructing high-sensitivity all nanofiber-based piezoresistive sensor. The conductive nanofiber aerogel is prepared through freeze-drying method by using in-situ polymerized conductive polymer polypyrrole and electrospinning cellulose acetate nanofiber. Benefiting from the nanofiber skeleton and multistage pore structure, the obtained aerogel possesses an ultra-low volume density of 14.67 mg cm−3. Specifically, the unique nanofiber-based conductive pathways endow the aerogel with an ultra-high sensitivity up to 60.28 kPa−1 in a wide pressure range of 0 ~ 24 kPa. Meanwhile, the flexible nanofiber-based piezoresistive sensor also exhibit excellent cycling stability, which can withstand more than 13,000 cycles of compression. Furthermore, it shows great application potential as a flexible wearable sensor in human motion and health monitoring.

A High–Performance Flexible Piezoresistive Pressure Sensor Features an Integrated Design of Conductive Fabric Electrode and Polyurethane Sponge

AbstractDue to its porous structure and good elasticity, conductive polyurethane (PU) sponge is used as the main substrate of the flexible piezoresistive pressure sensor. The effective combination of conductive PU sponge and electrode material is the foundation for the pressure sensor, but it needs to be bonded by expensive conductive silver paste or copper paste. In addition, the common electrode materials weaken the flexibility of the PU sponge pressure sensors because of their rigidity. Herein, PU sponge and polyester (PET) fabric are first bonded to produce (PET‐PU) composite, which is then impregnated with graphene oxide (GO). The obtained reduced graphene oxide(rGO)@PET fabric and rGO@PU are used as electrode and piezoresistive material, respectively. Then rGO@(PET‐PU) composite is assembled into a pressure sensor only by using wire connections in the rGO@PET fabric. Benefiting from excellent piezoresistive behavior, rGO@(PET‐TPU) pressure sensor displays high sensitivity (0.255 kPa−1 at below 2.6 kPa), wide detection limit (≈0–85.0%), and long durability (over 1800 cycles). Besides, the pressure sensor demonstrates good performance in monitoring human activities, including finger bending, clicking keyboard, breathing, elbow bending, and walking posture, thus providing a promising material for human activity monitoring.