Flexible Pressure Sensor With High Sensitivity and Low Hysteresis Based on a Hierarchically Microstructured Electrode

Skin mountable flexible pressure sensors are crucial for the next generation technologies. Capacitive pressure sensors bring several advantages and hence, they have been preferred over piezoelectric and piezoresistive alternatives. Flexible electrode and dielectric material are the fundamental components of the capacitive flexible pressure sensors. Hence, solution processable silver nanowires (AgNWs) are highly promising as electrode material or filler material in dielectric layer, owing to their superior inherent conductivity and flexibility. This article reviews recent progress on fabrication of flexible capacitive pressure sensors with AgNWs used as electrode and dielectric material. Performance of the sensors based on AgNWs is comprehensively discussed and emerging applications, as well as future outlook, are presented in detail.

Large scale production of highly sensitive and flexible pressure sensor has aroused widely interest for their high potential use in real-wearable electronics and e-textiles. In this work, a flexible and wearable capacitive pressure sensor has been fabricated based on the unique honeycomb weave architecture, conferring the sensor with an extreme synchronization between output capacitance and input pressure, as well as the high sensitivity of 0.045 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> under 10 kPa, large workable pressure range of 40 kPa, good durability and stability over 1000 cycles, fast response time of 135 ms and comparable stability under diverse pressures. The sensor can be used for various pressure detections such as finger pressing, tape loading and handshaking. Moreover, the self-made sensor has good ability in monitoring spatial distribution, showing its promising application in flexible and wearable devices.

Human–robot interaction has become increasingly important due to technological advances in robotics. The tactile sensation range of humans ranges from detecting the touch of a hair (≈0.6 Pa) to the touch of a finger (&lt;10 kPa). Therefore, flexible pressure sensors with high sensitivity and a suitable pressure response range are vital to human–robot interaction. However, it is difficult to attain both sensitivity and a suitable pressure response in one device. This study proposes and prepares a flexible piezoresistive pressure sensor based on carbon nanotubes (CNTs)/carbon cloth (CC). The high performance of the sensor stems from the interlocked CNTs structure and the multistage contact at different pressures. The interlocked CNT structure endows the sensor with a low detection limit (≈0.15 Pa) and high sensitivity (120 kPa−1) in the pressure range (0–0.5 kPa). The flexible pressure sensor with an interlocked structure has significant potential as a wearable biosensor for healthcare applications. Furthermore, the sensor facilitates grasping soft objects due to high tactile sensitivity. In addition, the CNTs/CC‐based sensor can also be used as a bending sensor to develop a hand grasping model. Therefore, the proposed CNTs/CC‐based sensor is well suited for human–machine interaction and improving the tactile perception of robots.

Flexible pressure sensors have attracted significant attention in both academic and industrial fields. However, how to design and fabricate a flexible pressure sensor having high sensitivity and good linearity in a wide detection range is still a challenge. To address this challenge, this article proposes a flexible piezoresistive pressure sensor with a multilayer structure consisting of microstructured composite layers (MCLs), which are sandwiched by indium tin oxide-polyethylene terephthalate electrodes and encapsulated by an ultrasoft elastomer frame. The preparation of the conductive composite and the fabrication procedure of the pressure sensor are presented. Due to the point-to-point hemisphere microstructures of the multilayer composite layers, the pressure sensor can perceive pressure with a sensitivity of 7.66 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> in a range of 0–583 kPa. Besides, the proposed sensor features good linearity of 5.86% and an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R^{2}$ </tex-math></inline-formula> of 0.99. The developed pressure sensor has excellent repeatability of 500 cycles and a fast response time of 80 ms. The proposed sensor is verified by the experiments with primary applications, and the obtained results indicate it could be potentially utilized as wearable detectors.

With the rapid emergence of flexible electronics, flexible pressure sensors are of importance in various fields. In this study, a dopamine-modified melamine sponge (MS) was used to prepare a honeycomb structure of carbon black (CB)/MXene-silicone rubber (SR)@MS flexible pressure sensor (CMSM) through layer-by-layer self-assembly technology. Using SR as a binder to construct the honeycomb structure not only improves the mechanical properties of the sensor but also provides more attachment sites for CB/MXene, enhancing the stability of the conductive network. The honeycomb structure CMSM flexible pressure sensor exhibits high sensitivity (7.44 kPa-1), a wide detection range (0-240 kPa), short response/recovery times (150 ms/180 ms), and exhibits excellent stability. In addition, a flexible smart insole has been developed based on a 6-unit CMSM sensor array, achieving plantar pressure detection. By combination of the ResNet-50 neural network algorithm with plantar pressure data under different postures, the recognition of 16 types of human motion postures has been achieved, with an accuracy rate of 90.63%. This study proposes a flexible sponge pressure sensor with excellent mechanical performance and sensing capabilities, providing new ideas and references for the design of flexible wearable sensor devices.

Flexible pressure sensors are emerging as a pivotal technology for applications such as robotics, prosthetic devices, electronic skin (e-skin), and human-machine interactions. Among various transduction mechanisms for flexible pressure sensors, the capacitance pressure sensors (CPS) have garnered significant attention due to their simplicity in architecture, high sensitivity, and potential for flexible and wearable designs. A typical CPS in parallel-plate geometry consists of a dielectric layer sandwiched between two conductive electrodes. However, achieving high sensitivity and a wide linear operating range remains a challenge in the development of wearable CPS in terms of the material section and fabrication procedures. In this study, a highly sensitive CPS was developed using a polymer nanocomposite as the dielectric layer. The nanocomposite incorporates high-dielectric zinc oxide (ZnO) nanoparticles and conductive carbon black (CB) fillers, which are uniformly dispersed within the polymer matrix to enhance its dielectric properties. This dielectric layer is integrated between silver conductive electrodes that are screen-printed onto flexible polyimide substrates. Among the PDMS-x wt% CB-y wt% ZnO (x-0&amp;3,y-0, 1&amp;, 2) composite sandwiched layers studied, the PDMS-CB (3 wt%)-ZnO (2 wt%) composite sandwiched layer based fCPS shows high sensitivity of 6.6083 pF kPa−1 (0.109 kPa−1) in 0–240 kPa pressure range with fast response and recovery times of 0.6 s and 0.48 s respectively, a low hysteresis value of 5.7% and long durability. These exceptional attributes demonstrate the sensor’s suitability for applications in e-skin and wearable devices, where precise pressure detection and reliable performance are critical.

Flexible pressure sensors have been studied as wearable voice-recognition devices to be utilized in human-machine interaction. However, the development of highly sensitive, skin-attachable, and comfortable sensing devices to achieve clear voice detection remains a considerable challenge. Herein, we present a wearable and flexible pressure and temperature sensor with a sensitive response to vibration, which can accurately recognize the human voice by combing with the artificial neural network. The device consists of a polyethylene terephthalate (PET) printed with a silver electrode, a filament-microstructured polydimethylsiloxane (PDMS) film embedded with single-walled carbon nanotubes and a polyimide (PI) film sputtered with a patterned Ti/Pt thermistor strip. The developed pressure sensor exhibited a pressure sensitivity of 0.398 kPa in the low-pressure regime, and the fabricated temperature sensor shows a desirable temperature coefficient of resistance of 0.13% C in the range of 25 C to 105 C. Through training and testing the neural network model with the waveform data of the sensor obtained from human pronunciation, the vocal fold vibrations of different words can be successfully recognized, and the total recognition accuracy rate can reach 93.4%. Our results suggest that the fabricated sensor has substantial potential for application in the human-computer interface fields, such as voice control, vocal healthcare monitoring, and voice authentication.

Flexible pressure sensors have drawn considerable attention for their potential applications as electronic skins with both sensitivity and pressure response range. Although the introduction of surface microstructures effectively enhances sensitivity, the confined volume of their compressible structures results in a limited pressure response range. To address this issue, a biomimetic kapok structure is proposed and implemented for constructing the dielectric layer of flexible capacitive pressure sensors employing 3D printing technology. The structure is designed with easily deformable concave and rotational structures, enabling continuous deformation under pressure. This design results in a significant expansion of the pressure response range and improvement in sensitivity. Further, the study purposively analyses crucial parameters of the devised structure that affect its compressibility and stability. These include the concave angle θ, height ratio d1/d2, rotation angle α, and width k. As a result, the ultimate pressure sensors demonstrate remarkable features such as high sensitivity (≈2.38kPa-1 in the range of 0-10kPa), broad detection range (734kPa), fast response time (23ms), and outstanding pressure resolution (0.4% at 500kPa). This study confirms the viability of bionic structures for flexible sensors, and their potential to expand the scope of wearable electronic devices.

Flexible pressure sensors have attracted great attention due to their extensive applications in human–computer interaction and health monitoring. So far, the development of flexible pressure sensors that balance high sensitivity and a wide measurement range remains a challenge. Herein, a double-layer dielectric structure with a surface convex structure is reported for the preparation of flexible capacitive pressure sensors. The double-layer dielectric structure, which is composed of a silicone rubber-based conductive elastomer with a surface micro-convex structure and a PVA-H-based conductive elastomer, balances the advantages and disadvantages of the two conductive elastomer dielectrics. It can form a complete micro-capacitive network under relatively large pressures, enabling the sensor to have high sensitivity at different stages (1.7 kPa−1, 0–104 kPa; 19.14 kPa−1, 104–140 kPa), thus achieving a dual enhancement of sensitivity and sensing range. Additionally, the sensor has been successfully applied to scenarios such as monitoring of human breathing, speaking, and movement, as well as mouse clicks, demonstrating its great potential in the fields of health monitoring and human–computer interaction applications.

High-performance flexible pressure sensors are highly desirable in health monitoring, robotic tactile, and artificial intelligence. Construction of microstructures in dielectrics and electrodes is the dominating approach to improving the performance of capacitive pressure sensors. Herein, we have demonstrated a novel three-dimensional microconformal graphene electrode for ultrasensitive and tunable flexible capacitive pressure sensors. Because the fabrication process is controllable, the morphologies of the graphene that is perfectly conformal with the electrode are controllable consequently. Multiscale morphologies ranging from a few nanometers to hundreds of nanometers, even to tens of micrometers, have been systematically investigated, and the high-performance capacitive pressure sensor with high sensitivity (3.19 kPa-1), fast response (30 ms), ultralow detection limit (1 mg), tunable-sensitivity, high flexibility, and high stability was obtained. Furthermore, an ultrasensitivity of 7.68 kPa-1 was successfully achieved via symmetric double microconformal graphene electrodes. The finite element analysis indicates that the microstructured graphene electrode can enhance large deformation and thus effectively improve the sensitivity. Additionally, the proposed pressure sensors are demonstrated with practical applications including insect crawling detection, wearable health monitoring, and force feedback of robot tactile sensing with a sensor array. The microconformal graphene may provide a new approach to fabricating controllable microstructured electrodes to enhance the performance of capacitive pressure sensors and has great potential for innovative applications in wearable health-monitoring devices, robot tactile systems, and human-machine interface systems.

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