3D Dielectric Layer Enabled Highly Sensitive Capacitive Pressure Sensors for Wearable Electronics

Interference-free nanogap pressure sensor array with high spatial resolution for wireless human-machine interfaces applications

Pressure sensors provide intuitive and easy-to-use information, making them essential for Human-Machine Interface (HMI) applications. However, capacitive pressure sensors, although prevalent in industrial applications, suffer from critical vulnerabilities to external interferences like water droplets and proximity. Without addressing these issues, pressure sensors can never be effectively utilized in various real-world applications. This paper addresses this critical issue by identifying fringe fields as the primary cause of interference and proposing a nanogap structured capacitive pressure sensor. By reducing the electrode gaps to hundreds of nanometers, our sensor effectively mitigates fringe field effects, ensuring reliable pressure detection and high spatial resolution. We demonstrate the sensor's capabilities through wireless applications such as an artificial pressure sensing system and a force touch pad system, both showcasing exceptional resilience and accuracy. This innovation paves the way for robust, interference-free pressure sensors, significantly advancing HMI technology.

An intelligent hybrid-fabric wristband system enabled by thermal encapsulation for ergonomic human-machine interaction

Human-machine interaction has emerged as a revolutionary and transformative technology, bridging the gap between human and machine. Gesture recognition, capitalizing on the inherent dexterity of human hands, plays a crucial role in human-machine interaction. However, existing systems often struggle to meet user expectations in terms of comfort, wearability, and seamless daily integration. Here, we propose a handwriting recognition technology utilizing an intelligent hybrid-fabric wristband system. This system integrates spun-film sensors into textiles to form the smart fabric, enabling intelligent functionalities. A thermal encapsulation process is proposed to bond multiple spun-films without additional materials, ensuring the lightweight, breathability, and stretchability of the spun-film sensors. Furthermore, recognition algorithms facilitate precise accurate handwriting recognition of letters, with an accuracy of 96.63%. This system represents a significant step forward in the development of ergonomic and user-friendly wearable devices for enhanced human-machine interaction, particularly in the virtual world.

Research Progress of Flexible Electronic Devices Based on Electrospun Nanofibers

Electrospun nanofibers have become an important component in fabricating flexible electronic devices because of their permeability, flexibility, stretchability, and conformability to three-dimensional curved surfaces. This review delves into the advancements in adaptable and flexible electronic devices using electrospun nanofibers as the substrates and explores their diverse and innovative applications. The primary development of key substrates for flexible devices is summarized. After briefly discussing the principle of electrospinning, process parameters that affect electrospinning, and two major electrospinning techniques (i.e., single-fluid electrospinning and multifluid electrospinning), the review shines a spotlight on the recent breakthroughs in multifunctional and stretchable electronic devices that are based on electrospun substrates. These advancements include flexible sensors, flexible energy harvesting and storage devices, flexible accessories for electronic devices, and flexible environmental monitoring devices. In particular, the review outlines the challenges and potential solutions of developing electrospun nanofibers for flexible electronic devices, including overcoming the incompatibility of multiple interfaces, developing 3D microstructure sensor arrays with gradient geometry for various imperceptible on-skin devices, etc. This review may provide a comprehensive understanding of the rational design of application-oriented flexible electronic devices based on electrospun nanofibers.

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.

M. V, Kini U. A, Naik N, Hiremath P. Optimizing Capacitive Pressure Sensor Geometry: A Design of Experiments Approach with a Computer-Generated Model

This study presents a comprehensive investigation into the design and optimization of capacitive pressure sensors (CPSs) for their integration into capacitive touch buttons in electronic applications. Using the Finite Element Method (FEM), various geometries of dielectric layers were meticulously modeled and analyzed for their capacitive and sensitivity parameters. The flexible elastomer polydimethylsiloxane (PDMS) is used as a diaphragm, and polyvinylidene fluoride (PVDF) is a flexible material that acts as a dielectric medium. The Design of Experiment (DoE) techniques, aided by statistical analysis, were employed to identify the optimal geometric shapes of the CPS model. From the prediction using the DoE approach, it is observed that the cylindrical-shaped dielectric medium has better sensitivity. Using this optimal configuration, the CPS was further examined across a range of dielectric layer thicknesses to determine the capacitance, stored electrical energy, displacement, and stress levels at uniform pressures ranging from 0 to 200 kPa. Employing a 0.1 mm dielectric layer thickness yields heightened sensitivity and capacitance values, which is consistent with theoretical efforts. At a pressure of 200 kPa, the sensor achieves a maximum capacitance of 33.3 pF, with a total stored electric energy of 15.9 × 10-12 J and 0.468 pF/Pa of sensitivity for 0.1 dielectric thickness. These findings underscore the efficacy of the proposed CPS model for integration into capacitive touch buttons in electronic devices and e-skin applications, thereby offering promising advancements in sensor technology.

Fabrication of a Capacitive 3D Spacer Fabric Pressure Sensor with a Dielectric Constant Change for High Sensitivity

Smart wearable sensors are increasingly integrated into everyday life, interfacing with the human body to enable real-time monitoring of biological signals. This study focuses on creating high-sensitivity capacitive-type sensors by impregnating polyester-based 3D spacer fabric with a Carbon Nanotube (CNT) dispersion. The unique properties of conductive particles lead to nonlinear variations in the dielectric constant when pressure is applied, consequently affecting the gauge factor. The results reveal that while the fabric without CNT particles had a gauge factor of 1.967, the inclusion of 0.04 wt% CNT increased it significantly to 5.210. As sensor sensitivity requirements vary according to the application, identifying the necessary CNT wt% is crucial. Artificial intelligence, particularly the Multilayer Perception (MLP) model, enables nonlinear regression analysis for this purpose. The MLP model created and validated in this research showed a high correlation coefficient of 0.99564 between the model predictions and actual target values, indicating its effectiveness and reliability.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Applications of flexible electronics related to cardiocerebral vascular system

Ensuring accessible and high-quality healthcare worldwide requires field-deployable and affordable clinical diagnostic tools with high performance. In recent years, flexible electronics with wearable and implantable capabilities have garnered significant attention from researchers, which functioned as vital clinical diagnostic-assisted tools by real-time signal transmission from interested targets in vivo. As the most crucial and complex system of human body, cardiocerebral vascular system together with heart-brain network attracts researchers inputting profuse and indefatigable efforts on proper flexible electronics design and materials selection, trying to overcome the impassable gulf between vivid organisms and rigid inorganic units. This article reviews recent breakthroughs in flexible electronics specifically applied to cardiocerebral vascular system and heart-brain network. Relevant sensor types and working principles, electronics materials selection and treatment methods are expounded. Applications of flexible electronics related to these interested organs and systems are specially highlighted. Through precedent great working studies, we conclude their merits and point out some limitations in this emerging field, thus will help to pave the way for revolutionary flexible electronics and diagnosis assisted tools development.

Recent advances in smart wearable sensors as electronic skin

Flexible and multifunctional electronic devices and soft robots inspired by human organs, such as skin, have many applications. However, the emergence of electronic skins (e-skins) or textiles in biomedical engineering has made a great revolution in a myriad of people's lives who suffer from different types of diseases and problems in which their skin and muscles lose their appropriate functions. In this review, recent advances in the sensory function of the e-skins are described. Furthermore, we have categorized them from the sensory function perspective and highlighted their advantages and limitations. The categories are tactile sensors (including capacitive, piezoresistive, piezoelectric, triboelectric, and optical), temperature, and multi-sensors. In addition, we summarized the most recent advancements in sensors and their particular features. The role of material selection and structure in sensory function and other features of the e-skins are also discussed. Finally, current challenges and future prospects of these systems towards advanced biomedical applications are elaborated.

Paper-Based Supercapacitive Pressure Sensor for Wrist Arterial Pulse Waveform Monitoring

Recent developments in wearable pressure sensors have led to the need for high sensitivity and a broad sensing range to accurately detect various physiological states. However, high sensitivity does not always translate to a wide sensing range, and manufacturing sensors with such high sensitivity is a complex and expensive process. In this study, we present a capacitive pressure sensor based on tissue paper that is simple to produce and cost-effective yet still exhibits high linear sensitivity of 2.9 kPa-1 in the 0-16 kPa range. The linear sensitivity of 1.5 kPa-1 was achieved from 16 to 90 kPa. The sensor also demonstrated a fast response time of 0.2 s, excellent pressure resolution at both low and high pressures, and a sufficient signal-to-noise ratio, making it ideal for detecting wrist arterial pulse waveforms. We were also able to demonstrate the sensor's practicality in real-world applications by cycling it 5000 times and showing its capability to capture pulse waveforms from different arterial locations. These low-cost sensors possess all the intrinsic features necessary for efficient measurement of pulse waveforms, which may facilitate the diagnosis of cardiovascular diseases.

All-Fabric Capacitive Pressure Sensors with Piezoelectric Nanofibers for Wearable Electronics and Robotic Sensing

Flexible pressure sensors are increasingly sought after for applications ranging from physiological signal monitoring to robotic sensing; however, the challenges associated with fabricating highly sensitive, comfortable, and cost-effective sensors remain formidable. This study presents a high-performance, all-fabric capacitive pressure sensor (AFCPS) that incorporates piezoelectric nanofibers. Through the meticulous optimization of conductive fiber electrodes and P(VDF-TrFE) nanofiber dielectric layers, the AFCPS exhibits exceptional attributes such as high sensitivity (4.05 kPa-1), an ultralow detection limit (0.6 Pa), an extensive detection range (∼100 kPa), rapid response time (<26 ms), and robust stability (>14,000 cycles). The sensor's porous structure enhances its compressibility, while its piezoelectric properties expedite charge separation, thereby increasing the interface capacitance and augmenting overall performance. These features are elucidated further through multiphysical field-coupling simulations and experimental testing. Owing to its comprehensive superior performance, the AFCPS has demonstrated its efficacy in monitoring human activity and physiological signals, as well as in discerning soft robotic grasping movements. Additionally, we have successfully implemented multiple AFCPS units as pressure sensor arrays to ascertain spatial pressure distribution and enable intelligent robotic gripping. Our research underscores the promising potential of the AFCPS device in wearable electronics and robotic sensing, thereby contributing significantly to the advancement of high-performance fabric-based sensors.

Manipulator Control System Based on Flexible Sensor Technology

The research on the remote control of manipulators based on flexible sensor technology is gradually extensive. In order to achieve stable, accurate, and efficient control of the manipulator, it is necessary to reasonably design the structure of the sensor with excellent tensile strength and flexibility. The acquisition of manual information by high-performance sensors is the basis of manipulator control. This paper starts with the manufacturing of materials of the flexible sensor for the manipulator, introduces the substrate, sensor, and flexible electrode materials, respectively, and summarizes the performance of different flexible sensors. From the perspective of manufacturing, it introduces their basic principles and compares their advantages and disadvantages. Then, according to the different ways of wearing, the two control methods of data glove control and surface EMG control are respectively introduced, the principle, control process, and detection accuracy are summarized, and the problems of material microstructure, reducing the cost, optimizing the circuit design and so on are emphasized in this field. Finally, the commercial application in this field is explained and the future research direction is proposed from two aspects: how to ensure real-time control and better receive the feedback signal from the manipulator.

Integration of Capacitive Pressure Sensor-on-Chip with Lead-Free Perovskite Solar Cells for Continuous Health Monitoring

The increasing prevalence of hypertension necessitates continuous blood pressure monitoring. This can be safely and painlessly achieved using non-invasive wearable electronic devices. However, the integration of analog, digital, and power electronics into a single system poses significant challenges. Therefore, we demonstrated a comprehensive multi-scale simulation of a sensor-on-chip that was based on a capacitive pressure sensor. Two analog interfacing circuits were proposed for a full-scale operation ranging from 0 V to 5 V, enabling efficient digital data processing. We also demonstrated the integration of lead-free perovskite solar cells as a mechanism for self-powering the sensor. The proposed system exhibits varying sensitivity from 1.4 × 10-3 to 0.095 (kPa)-1, depending on the pressure range of measurement. In the most optimal configuration, the system consumed 50.5 mW, encompassing a 6.487 mm2 area for the perovskite cell and a CMOS layout area of 1.78 × 1.232 mm2. These results underline the potential for such sensor-on-chip designs in future wearable health-monitoring technologies. Overall, this paper contributes to the field of wearable health-monitoring technologies by presenting a novel approach to self-powered blood pressure monitoring through the integration of capacitive pressure sensors, analog interfacing circuits, and lead-free perovskite solar cells.

Recent progress in flexible micro-pressure sensors for wearable health monitoring

In recent years, flexible micro-pressure sensors have been used widely in wearable health monitoring applications due to their excellent flexibility, stretchability, non-invasiveness, comfort wearing and real-time detection. According to the working mechanism of the flexible micro-pressure sensor, it can be classified as piezoresistive, piezoelectric, capacitive and triboelectric types. Herein, an overview of flexible micro-pressure sensors for wearable health monitoring is presented. The physiological signaling and body motions contain a lot of health status information. Thus, this review focuses on the applications of flexible micro-pressure sensors in these fields. Additionally, the contents of sensing mechanism, sensing materials and performance of flexible micro-pressure sensors are introduced in detail. Finally, we predict the future research directions of the flexible micro-pressure sensors, and discuss the challenges in practical applications.

Recent Progress in MXene Hydrogel for Wearable Electronics

Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human-machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface's negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.

Quantitative comparison of EGFR expression levels of optically trapped individual cells using a capacitance biosensor

Cellular endocytosis is an essential phenomenon which induces cellular reactions, such as waste removal, nutrient absorption, and drug delivery, in the process of cell growth, division, and proliferation. To observe capacitance responses upon endocytosis on a single-cell scale, this study combined an optical tweezer that can optically place a single cell on a desired location with a capacitance sensor and a cell incubation chamber. Single HeLa cancer cell was captured and moved to a desired location through optical trapping, and the single-cell capacitance change generated during the epidermal growth factor (EGF) molecule endocytosis was measured in real time. It was found that single HeLa cells showed a larger increase in capacitance values compared to that of the single NIH3T3 cells when exposed to varying EGF concentrations. In addition, the capacitance change was in proportion to the cell's EGF receptor (EGFR) level when cells of different levels of EGFR expression were tested. An equation derived from these results was able to estimate the EGFR expression level of a blind-tested cell. The biosensor developed in this research can not only quickly move a single cell to a desired location in a non-invasive manner but also distinguish specific responses between cancer and normal cells by continuous measurement of real-time interactions of a single cell in culture to the external ligands.

Micro-/Nano-Structured Biodegradable Pressure Sensors for Biomedical Applications

The interest in biodegradable pressure sensors in the biomedical field is growing because of their temporary existence in wearable and implantable applications without any biocompatibility issues. In contrast to the limited sensing performance and biocompatibility of initially developed biodegradable pressure sensors, device performances and functionalities have drastically improved owing to the recent developments in micro-/nano-technologies including device structures and materials. Thus, there is greater possibility of their use in diagnosis and healthcare applications. This review article summarizes the recent advances in micro-/nano-structured biodegradable pressure sensor devices. In particular, we focus on the considerable improvement in performance and functionality at the device-level that has been achieved by adapting the geometrical design parameters in the micro- and nano-meter range. First, the material choices and sensing mechanisms available for fabricating micro-/nano-structured biodegradable pressure sensor devices are discussed. Then, this is followed by a historical development in the biodegradable pressure sensors. In particular, we highlight not only the fabrication methods and performances of the sensor device, but also their biocompatibility. Finally, we intoduce the recent examples of the micro/nano-structured biodegradable pressure sensor for biomedical applications.

Comparative Study of Carbon Nanotube Composites as Capacitive and Piezoresistive Pressure Sensors under Varying Conditions

Conducting polymer composites consisting of carbon nanotubes (CNTs) as a conductive filler and polydimethylsiloxane (PDMS) as a polymer matrix were fabricated to investigate their capacitive and piezoresistive effects as pressure sensors. The pressure-sensing behavior and mechanism of the composites were compared in terms of basic configuration with a parallel plate structure. Various sensing experiments, such as sensitivity, repeatability, hysteresis, and temperature dependence according to the working principle, were conducted with varying filler contents. The hysteresis and repeatability of the pressure-sensing properties were investigated using cyclic tensile tests. In addition, a temperature test was performed at selected temperatures to monitor the change in the resistance/capacitance.

Polymer Conductive Membrane-Based Circular Capacitive Pressure Sensors from Non-Touch Mode of Operation to Touch Mode of Operation: An Analytical Solution-Based Method for Design and Numerical Calibration

Polymer-based conductive membranes play an important role in the development of elastic deflection-based pressure sensors. In this paper, an analytical solution-based method is presented for the design and numerical calibration of polymer conductive membrane-based circular capacitive pressure sensors from non-touch mode of operation to touch mode of operation. The contact problem of a circular membrane in frictionless contact with a rigid flat plate under pressure is analytically solved, and its analytical solution is used for the design of touch mode circular capacitive pressure sensors for the first time. The analytical relationship with input pressure as independent variable and output capacitance as dependent variable is precisely derived and is used for the numerical calibrations of the analytical relationships with input capacitance as the independent variable and output pressure as the dependent variable in order to meet the capacitive pressure sensor mechanism of detecting pressure by measuring capacitance. For the first time, an example showing the design and numerical calibration of a given (given design parameters) polymer conductive membrane-based circular capacitive pressure sensor from non-touch mode of operation to touch mode of operation is provided. Then, the influence of changing several important design parameters on input capacitance–output pressure relationships is comprehensively investigated in order to clarify the desired input–output relationships when changing design parameters.

A high-performance wearable pressure sensor based on an MXene/PVP composite nanofiber membrane for health monitoring

Flexible and wearable pressure sensors have attracted extensive attention in domains, such as electronic skin, medical monitoring and human-machine interaction. However, developing a pressure sensor with high sensitivity, mechanical stability and a wide detection range remains a huge challenge. In this work, a flexible capacitive pressure sensor, based on a Ti₃C₂T _x (MXene)/polyvinyl pyrrolidone (PVP) composite nanofiber membrane (CNM), prepared via an efficient electrospinning process, is presented. The experimental results show that even a small mass fraction of MXene can effectively decrease the compression modulus of the PVP nanofiber membrane, thus enhancing the sensing performance. Specifically, the sensor based on (0.1 wt% MXene)/PVP CNM has a high sensitivity (0.5 kPa^-1 at 0-1.5 kPa), a fast response/recovery time (45/45 ms), a wide pressure detection range (0-200 kPa), a low detection limit (∼9 Pa) and an excellent mechanical stability (8000 cycles). Due to its superior performance, the sensor can monitor subtle changes in human physiology and other signals, such as pulse, respiration, human joint motions and airflow. In addition, a 4 × 4 sensor array is fabricated that can accurately map the shape and position of objects with good resolution. The high-performance flexible pressure sensor, as developed in this work, shows good application prospects in advanced human-computer interface systems.

A flexible tactile sensor that uses polyimide/graphene oxide nanofiber as dielectric membrane for vertical and lateral force detection

Flexible force sensors are of great interest in the fields of healthcare, physiological signals, and aircraft smart skin applications because of their compatibility with curved surfaces. However, the simultaneous detection of multidirectional forces remains an engineering challenge, despite the great progress made in recent years. Herein, we present the development of a flexible capacitive force sensor capable of efficiently distinguishing normal and sliding shear forces. A two-layer electrospun polyimide/graphene oxide (PI/GO) nanofiber membrane is used as the dielectric layer, which is sandwiched between one top electrode and four symmetrically distributed bottom electrodes. This composite membrane has an improved dielectric constant, a reduced friction coefficient, and good compressibility, leading to superior performance that includes high sensitivity over a wide operational range with measured results of 3 MPa^-1for 0-242 kPa (0-2.2 N) and 0.92 MPa^-1for 242-550 kPa (2.2-5 N) in the normal direction; and better than 1 N^-1for 0-3 N in thex- andy-axis directions. The system also has a low detection limit of 10 Pa, fast response and recovery times of 39 ms and 13 ms, respectively, a good cyclic stability of 10,000 cycles at a pressure of 176 kPa, and promising potential for use in high-temperature environments (200 °C). Moreover, a prototype 4 × 4 sensor array has been fabricated and successfully used in a robotic system to grasp objects and operate a wireless toy car. As such, the proposed system could offer superior capabilities in simultaneous multidirectional force sensing for applications such as intelligent robots, human-machine interaction, and smart skin.

Textile-Based Flexible Capacitive Pressure Sensors: A Review

Flexible capacitive pressure sensors have been widely used in electronic skin, human movement and health monitoring, and human-machine interactions. Recently, electronic textiles afford a valuable alternative to traditional capacitive pressure sensors due to their merits of flexibility, light weight, air permeability, low cost, and feasibility to fit various surfaces. The textile-based functional layers can serve as electrodes, dielectrics, and substrates, and various devices with semi-textile or all-textile structures have been well developed. This paper provides a comprehensive review of recent developments in textile-based flexible capacitive pressure sensors. The latest research progresses on textile devices with sandwich structures, yarn structures, and in-plane structures are introduced, and the influences of different device structures on performance are discussed. The applications of textile-based sensors in human wearable devices, robotic sensing, and human-machine interaction are then summarized. Finally, evolutionary trends, future directions, and challenges are highlighted.

Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors

MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Chip-scale optical airflow sensor

Airflow sensors are an essential component in a wide range of industrial, biomedical, and environmental applications. The development of compact devices with a fast response and wide measurement range capable of in situ airflow monitoring is highly desirable. Herein, we report a miniaturized optical airflow sensor based on a GaN chip with a flexible PDMS membrane. The compact GaN chip is responsible for light emission and photodetection. The PDMS membrane fabricated using a droplet-based molding process can effectively transform the airflow stimuli into optical reflectance changes that can be monitored by an on-chip photodetector. Without the use of external components for light coupling, the proposed sensor adopting the novel integration scheme is capable of detecting airflow rates of up to 53.5 ms^-1 and exhibits a fast response time of 12 ms, holding great promise for diverse practical applications. The potential use in monitoring human breathing is also demonstrated.

A new strategy for the fabrication of a flexible and highly sensitive capacitive pressure sensor

The development of flexible capacitive pressure sensors has wide application prospects in the fields of electronic skin and intelligent wearable electronic devices, but it is still a great challenge to fabricate capacitive sensors with high sensitivity. Few reports have considered the use of interdigital electrode structures to improve the sensitivity of capacitive pressure sensors. In this work, a new strategy for the fabrication of a high-performance capacitive flexible pressure sensor based on MXene/polyvinylpyrrolidone (PVP) by an interdigital electrode is reported. By increasing the number of interdigital electrodes and selecting the appropriate dielectric layer, the sensitivity of the capacitive sensor can be improved. The capacitive sensor based on MXene/PVP here has a high sensitivity (~1.25 kPa^-1), low detection limit (~0.6 Pa), wide sensing range (up to 294 kPa), fast response and recovery times (~30/15 ms) and mechanical stability of 10000 cycles. The presented sensor here can be used for various pressure detection applications, such as finger pressing, wrist pulse measuring, breathing, swallowing and speech recognition. This work provides a new method of using interdigital electrodes to fabricate a highly sensitive capacitive sensor with very promising application prospects in flexible sensors and wearable electronics.

A high-resolution, ultrabroad-range and sensitive capacitive tactile sensor based on a CNT/PDMS composite for robotic hands

Tactile sensors are of great significance for robotic perception improvement to realize stable object manipulation and accurate object identification. To date, developing a broad-range tactile sensor array with high sensitivity economically remains a critical challenge. In this study, a flexible capacitive tactile sensor array, consisting of a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) film, parylene films, and two polyimide (PI) films patterned with electrodes, is facilely prepared. The CNT/PDMS film, acting as a giant dielectric permittivity material, is utilized to improve the sensitivity, while the parylene film serves as the scaffold architecture to extend the working range of the tactile sensor array. Also, it is promising to realize mass production for this sensor array due to the scalable fabrication procedure. The as-prepared sensor exhibits excellent sensing performance with a high sensitivity of 1.61% kPa^-1 (<1 MPa), an ultra-broad pressure working range of 0.9 kPa-2.55 MPa, an outstanding durability, a stability up to 5000 cycles, and a fast response time. By integrating our tactile sensor array with a robotic gripper, we show that robots can successfully differentiate object shapes and manipulate light and heavy objects with a closed-loop pressure feedback, demonstrating its great potential in robotic perception and wearable applications.

Flexible Wearable Sensors for Cardiovascular Health Monitoring

Cardiovascular diseases account for the highest mortality globally, but recent advances in wearable technologies may potentially change how these illnesses are diagnosed and managed. In particular, continuous monitoring of cardiovascular vital signs for early intervention is highly desired. To this end, flexible wearable sensors that can be comfortably worn over long durations are gaining significant attention. In this review, advanced flexible wearable sensors for monitoring cardiovascular vital signals are outlined and discussed. Specifically, the functional materials, configurations, mechanisms, and recent advances of these flexible sensors for heart rate, blood pressure, blood oxygen saturation, and blood glucose monitoring are highlighted. Different mechanisms in bioelectric, mechano-electric, optoelectric, and ultrasonic wearable sensors are presented to monitor cardiovascular vital signs from different body locations. Present challenges, possible strategies, and future directions of these wearable sensors are also discussed. With rapid development, these flexible wearable sensors will potentially be applicable for both medical diagnosis and daily healthcare use in tackling cardiovascular diseases.

Flexible Wireless Passive LC Pressure Sensor with Design Methodology and Cost-Effective Preparation

Continuous monitoring of physical motion, which can be successfully achieved via a wireless flexible wearable electronic device, is essential for people to ensure the appropriate level of exercise. Currently, most of the flexible LC pressure sensors have low sensitivity because of the high Young’s modulus of the dielectric properties (such as PDMS) and the inflexible polymer films (as the substrate of the sensors), which don’t have excellent stretchability to conform to arbitrarily curved and moving surfaces such as joints. In the LC sensing system, the metal rings, as the traditional readout device, are difficult to meet the needs of the portable readout device for the integrated and planar readout antenna. In order to improve the pressure sensitivity of the sensor, the Ecoflex microcolumn used as the dielectric of the capacitive pressure sensor was prepared by using a metal mold copying method. The Ecoflex elastomer substrates enhanced the levels of conformability, which offered improved capabilities to establish intimate contact with the curved and moving surfaces of the skin. The pressure was applied to the sensor by weights, and the resonance frequency curves of the sensor under different pressures were obtained by the readout device connected to the vector network analyzer. The experimental results show that resonant frequency decreases linearly with the increase of applied pressure in a range of 0–23,760 Pa with a high sensitivity of −2.2 MHz/KPa. We designed a coplanar waveguide-fed monopole antenna used to read the information of the LC sensor, which has the potential to be integrated with RF signal processing circuits as a portable readout device and a higher vertical readout distance (up to 4 cm) than the copper ring. The flexible LC pressure sensor can be attached to the skin conformally and is sensitive to limb bending and facial muscle movements. Therefore, it has the potential to be integrated as a body sensor network that can be used to monitor physical motion.

Flexible and Stretchable Capacitive Sensors with Different Microstructures

Recently, sensors that can imitate human skin have received extensive attention. Capacitive sensors have a simple structure, low loss, no temperature drift, and other excellent properties, and can be applied in the fields of robotics, human-machine interactions, medical care, and health monitoring. Polymer matrices are commonly employed in flexible capacitive sensors because of their high flexibility. However, their volume is almost unchanged when pressure is applied, and they are inherently viscoelastic. These shortcomings severely lead to high hysteresis and limit the improvement in sensitivity. Therefore, considerable efforts have been applied to improve the sensing performance by designing different microstructures of materials. Herein, two types of sensors based on the applied forces are discussed, including pressure sensors and strain sensors. Currently, five types of microstructures are commonly used in pressure sensors, while four are used in strain sensors. The advantages, disadvantages, and practical values of the different structures are systematically elaborated. Finally, future perspectives of microstructures for capacitive sensors are discussed, with the aim of providing a guide for designing advanced flexible and stretchable capacitive sensors via ingenious human-made microstructures.

Microstructures in All-Inkjet-Printed Textile Capacitors with Bilayer Interfaces of Polymer Dielectrics and Metal-Organic Decomposition Silver Electrodes

Soft printed electronics exhibit unique structures and flexibilities suited for a plethora of wearable applications. However, forming scalable, reliable multilayered electronic devices with heterogeneous material interfaces on soft substrates, especially on porous and anisotropic structures, is highly challenging. In this study, we demonstrate an all-inkjet-printed textile capacitor using a multilayered structure of bilayer polymer dielectrics and particle-free metal-organic decomposition (MOD) silver electrodes. Understanding the inherent porous/anisotropic microstructure of textiles and their surface energy relationship was an important process step for successful planarization. The MOD silver ink formed a foundational conductive layer through the uniform encapsulation of individual fibers without blocking fiber interstices. Urethane-acrylate and poly(4-vinylphenol)-based bilayers were able to form a planarized dielectric layer on polyethylene terephthalate textiles. A unique chemical interaction at the interfaces of bilayer dielectrics performed a significant role in insulating porous textile substrates resulting in high chemical and mechanical durability. In this work, we demonstrate how textiles' unique microstructures and bilayer dielectric layer designs benefit reliability and scalability in the inkjet process as well as the use in wearable electronics with electromechanical performance.

Supercapacitive brophene-graphene aerogel as elastic-electrochemical dielectric layer for sensitive pressure sensors

A sensitive pressure sensor based on ultralight and superelastic supercapacitive borophene-graphene aerogel as dielectric layer is reported. The borophene-graphene aerogel not only combines large specific surface area of reduced graphene oxide and high conductivity of borophene, but also exhibits rich porous structure. The strong synergy and intercalation between two different two-dimensional materials benefit electron transfer and electrolyte ion diffusion. On the one hand, the aerogel exhibits greater mass specific capacitance of 330 F g^-1 than pure graphene aerogel. More importantly, serving as dielectric layer for pressure sensors with a symmetrical structure, the sensor represents ultra-high sensitivity (0.90 KPa^-1) in the pressure range (<3 KPa), ultra-rapid response time (~110 ms), ultra-low detection limit as 8.7 Pa and excellent working stability after 1000 cycles. In practical application, the sensor demonstrates great performance in monitoring human physiological signals, and agricultural applications.

Natural bamboo leaves as dielectric layers for flexible capacitive pressure sensors with adjustable sensitivity and a broad detection range

Developing a high-performance capacitive sensor for diverse application scenarios has posed requirements for the sensor to have high sensitivity, broad detection range, and cost-effectiveness. In this experiment, a flexible pressure sensor with a high sensitivity of 2.08 kPa⁻¹ at pressure lower than 1 kPa, as well as a wide working range of 0–600 kPa and remarkable stability (for at least 4000 cycles), was designed. In the device structure, silver nanowires (Ag NWs)/MXene-composite-coated polydimethylsiloxane (PDMS) and natural bamboo leaves at different growth stages were used as the electrode and the micro-structured dielectric layers, respectively. The rough surface of the composite conductive materials and the hierarchical microstructure of the bamboo leaves ensured a high sensitivity and broad pressure range of the sandwich-structured sensor, and the different sizes of the microstructures yielded adjustable sensitivity of the sensor. Furthermore, the outstanding performance of the proposed device made it possible to detect the actual object load, human physical stimuli, and proximity distance, demonstrating applications of flexible and wearable devices in various fields, such as weight/force tapping, breath/wrist pulse/speech, joint bending, and approach distance.

Capacitive pressure sensors based on bamboo leaves endow adjustable sensitivity, wide working range and remarkable stability, indicating promising applications in diverse application scenarios.

Strain-Discriminable Pressure/Proximity Sensing of Transparent Stretchable Electronic Skin Based on PEDOT:PSS/SWCNT Electrodes

Pressure/proximity sensing as the essential function of electronic skin (e-skin) has become an emerging technological goal for new-generation electronic devices in a wide variety of application fields, for example, smart electronics, human-machine interaction, and prosthetics. However, the current research lacks pressure/proximity detection of the stretched e-skin, which ignores the key elastic characteristic of skin and hinders the development of e-skin. Here, the pressure/proximity detection of the transparent e-skin in the stretching state is demonstrated based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/single-walled carbon nanotube (SWCNT). The high transparency of the e-skin realizes the visual imperception for wearable electronic systems. The perfect combination of stretchable SWCNT and highly conductive PEDOT:PSS endows the sensors with high stretchability and high discrimination capability toward strain, providing an effective way to overcome the interference of strain to realize accurate pressure/proximity detection of stretched e-skin at different strains.