A graphene-based resistive pressure sensor with record-high sensitivity in a wide pressure range

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring

Respiration signals are a vital sign of life. Monitoring human breath provides critical information for health assessment, diagnosis, and treatment for respiratory diseases such as asthma, chronic bronchitis, and emphysema. Stretchable and wearable respiration sensors have recently attracted considerable interest toward monitoring physiological signals in the era of real time and portable healthcare systems. This review provides a snapshot on the recent development of stretchable sensors and wearable technologies for respiration monitoring. The article offers the fundamental guideline on the sensing mechanisms and design concepts of stretchable sensors for detecting vital breath signals such as temperature, humidity, airflow, stress and strain. A highlight on the recent progress in the integration of variable sensing components outlines feasible pathways towards multifunctional and multimodal sensor platforms. Structural designs of nanomaterials and platforms for stretchable respiration sensors are reviewed.

Unsymmetrical Alveolate PMMA/MWCNT Film as a Piezoresistive E-Skin with Four-Dimensional Resolution and Application for Detecting Motion Direction and Airflow Rate

Flexible and piezoresistive electronic skins (E-skins) with high spatial resolution are highly desired in artificial intelligence and human-machine interactions. In this study, a simple method is developed to pattern a piezoresistive layer using lithography, which can realize real-time tactile sensing and spatial resolution. The piezoresistive layer with a honeycomb hole array based on polymethyl methacrylate (PMMA)/multiwalled carbon nanotubes (MWCNTs) was fabricated using a reverse mold with a ZnO nanorod array. The device exhibits an ultrahigh sensitivity of 88 kPa^-1 in the low-pressure regime (<10 kPa) and a fast response time of 110 ms owing to the conductive honeycomb structure. The E-skin-based PMMA/MWCNT honeycomb array film can be applied to monitor bending and vibration by changing the contact area of the hole walls. A 4 × 4 piezoresistive matrix was fabricated by lithography for a 16-pixel tactile-sensing E-skin, which realizes a four-dimensional resolution including the space and time resolutions of pressure points. In addition, by using the unsymmetrical structure of an alveolate PMMA/MWCNT film, the detection of direction and velocity for the movement and gas flow were realized. The obtained piezoresistive and unsymmetrical tactile sensor realized a four-dimensional resolution, including a three-dimensional space and a fourth dimension of timeline, which enables future applications of human-machine interactions.

Flexible Piezoresistive Sensors with Wide-Range Pressure Measurements Based on a Graded Nest-like Architecture

Flexible pressure sensors present great potential in the application of human health monitoring, tactile function of prosthesis, and electronic skin for robotics. These applications require different trade-off between the sensitivity and sensing range, therefore, it is imperative to develop range-specific sensitivities in a single sensor. In this paper, a bioinspired strategy for a resistive pressure sensor using a graded porous material is proposed to measure pressures from several pascals to megapascals. Its fabrication is based on an easily accessible template method. The nest-architecture-based wide-range pressure sensor exhibits adequate sensitivity under an extensive pressure regime (20 Pa to 1.2 MPa). In addition, with rational structural design and subtle engineering of the material properties, the sensor achieves remarkable mechanical stability. To prove the concept, sensors were attached on a bicycle wheel to monitor the tire-pavement pressure and on human skin to detect biosignals such as venous and arterial blood pressure pulses.

Fabrication and Piezoresistive/Piezoelectric Sensing Characteristics of Carbon Nanotube/PVA/Nano-ZnO Flexible Composite

Flexible sensors with a high sensitivity and wide-frequency response are essential for structural health monitoring (SHM) while they are attached. Here, carbon nanotube (CNT) films doped with various PVA fractions (CNT/PVA) and ZnO nanowires (nano-ZnO) on zinc sheets were first fabricated by functionalized self-assembly and hydrothermal synthesis processes. A CNT/PVA/nano-ZnO flexible composite (CNT/PVA/ZnO) sandwiched with a zinc wafer was then prepared by the spin-coating method. The piezoresistive and/or piezoelectric capabilities of the CNT/PVA/ZnO composite were comprehensively investigated under cyclic bending and impact loading after it was firmly adhered to a substrate (polypropylene sheet or mortar plate). The results show that the piezoresistive sensitivity and linear stability of the CNT/PVA films doped with 20%, 50%, and 100% PVA during bending are 5.47%/mm, 11.082%/mm, and 11.95%/mm and 2.3%, 3.42%, and 4.78%, respectively. The piezoelectric sensitivity, linear stability, and response accuracy of the CNT/PVA/ZnO composite under impulse loading are 4.87 mV/lbf, 3.42%, and 1.496 ms, respectively. These merits support the use of CNT/PVA/ZnO as a piezoresistive and/or piezoelectric compound sensor to monitor the static/dynamic loads on concrete structures while it is attached.

Applications of Graphene and Its Derivatives in the Upstream Oil and Gas Industry: A Systematic Review

Graphene and its derivatives, with their unique two-dimensional structures and excellent physical and chemical properties, have been an international research hotspot both in the research community and industry. However, in application-oriented research in the oil and gas industry they have only drawn attention in the past several years. Their excellent optical, electrical, thermal and mechanical performance make them great candidates for use in oil and gas exploration, drilling, production, and transportation. Combined with the actual requirements for well working fluids, chemical enhanced oil recovery, heavy oil recovery, profile control and water shutoff, tracers, oily wastewater treatment, pipeline corrosion prevention treatment, and tools and apparatus, etc., this paper introduces the behavior in water and toxicity to organisms of graphene and its derivatives in detail, and comprehensively reviews the research progress of graphene materials in the upstream oil and gas industry. Based on this, suggestions were put forward for the future research. This work is useful to the in-depth mechanism research and application scope broadening research in the upstream oil and gas industry.

Linear piezoresistive strain sensor based on graphene/g-C3N4/PDMS heterostructure

Here we report a novel hybrid material consists of 2D graphitic carbon nitride (g-C₃N₄) and graphene heterostructure that exhibits piezoresistivity superior to graphene and potentially being used as a strain sensor. The g-C₃N₄ that contains periodically spaced triangular nanopores is used for improving the piezoresistivity of the sensor imparting change in the polarization upon application of strain. In this work, we have investigated graphene/g-C₃N₄ interfaced materials and quantified its piezoresistive effects through experimental analysis and density functional theory (DFT) based computational studies provide insights into the electronic structures of the hybrid interfaces. We have observed a linear response in electrical resistance for a wide range of uniaxial strains up to ∼25%. The observed increase in resistance upon application of strain corroborates with our computational finding of strain-dependent band gap opening. Further, it has been realized that band-gap opening occurs exclusively in the graphitic layer of the composite materials under strain. However, the g-C₃N₄ bands remain intact at the interface. The linearity and a considerably small gauge factor (1.89) make graphene/g-C₃N₄ a promising heterostructure material unlike conventional metal gauge sensor in wide strain pressure sensor devices.

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: 0-5^o). 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.

Transparent Pressure Sensor with High Linearity over a Wide Pressure Range for 3D Touch Screen Applications

The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10-100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R² = 0.995) over 5-100 kPa of pressure. To render the sensor transparent, a microstructured "hard" elastomer layer was filled in with a refractive index matching a "soft" elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor's unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

Nano Carbon Black-Based High Performance Wearable Pressure Sensors

The reasonable design pattern of flexible pressure sensors with excellent performance and prominent features including high sensitivity and a relatively wide workable linear range has attracted significant attention owing to their potential application in the advanced wearable electronics and artificial intelligence fields. Herein, nano carbon black from kerosene soot, an atmospheric pollutant generated during the insufficient burning of hydrocarbon fuels, was utilized as the conductive material with a bottom interdigitated textile electrode screen printed using silver paste to construct a piezoresistive pressure sensor with prominent performance. Owing to the distinct loose porous structure, the lumpy surface roughness of the fabric electrodes, and the softness of polydimethylsiloxane, the piezoresistive pressure sensor exhibited superior detection performance, including high sensitivity (31.63 kPa⁻¹ within the range of 0–2 kPa), a relatively large feasible range (0–15 kPa), a low detection limit (2.26 pa), and a rapid response time (15 ms). Thus, these sensors act as outstanding candidates for detecting the human physiological signal and large-scale limb movement, showing their broad range of application prospects in the advanced wearable electronics field.

Electric Spark Induced Instantaneous and Selective Reduction of Graphene Oxide on Textile for Wearable Electronics

Reduced graphene oxide (rGO) attracts great popularity as an alternative to pristine graphene because of the facile synthesis process of its precursor, graphene oxide (GO). Electrical conduction of GO is tunable, subject to the extent of reduction of oxygen functional groups in it. This work for the first time demonstrates rapid reduction of GO using spark at ambient conditions. A stream of spark generated by applying high electric potential across two electrodes, when passed through a film of GO deposited on a porous substrate, reduces it into rGO. Upon sparking, the electrical resistance of the GO film drops down by an order of six within a second, making the reduction process instantaneous. X-ray photoelectron spectroscopy and Raman spectra of spark-reduced graphene oxide (SrGO) films revealed a high C/O ratio with an increase in the domain of sp²-hybridized carbon. The electromechanical properties of SrGO were practically examined by testing it as a flex sensor by incorporating its films with commercially available gloves. It showed high sensitivity for bending and good repeatability while offering an easy route for textile integration, making an impactful statement about the potential of sparking as a cost-effective method to reduce GO on a large scale.

Laser Fabrication of Graphene-Based Flexible Electronics

Recent years have witnessed the rise of graphene and its applications in various electronic devices. Specifically, featuring excellent flexibility, transparency, conductivity, and mechanical robustness, graphene has emerged as a versatile material for flexible electronics. In the past decade, facilitated by various laser processing technologies, including the laser-treatment-induced photoreduction of graphene oxides, flexible patterning, hierarchical structuring, heteroatom doping, controllable thinning, etching, and shock of graphene, along with laser-induced graphene on polyimide, graphene has found broad applications in a wide range of electronic devices, such as power generators, supercapacitors, optoelectronic devices, sensors, and actuators. Here, the recent advancements in the laser fabrication of graphene-based flexible electronic devices are comprehensively summarized. The various laser fabrication technologies that have been employed for the preparation, processing, and modification of graphene and its derivatives are reviewed. A thorough overview of typical laser-enabled flexible electronic devices that are based on various graphene sources is presented. With the rapid progress that has been made in the research on graphene preparation methodologies and laser micronanofabrication technologies, graphene-based electronics may soon undergo fast development.

Fabrication of Two-Dimensional and Three-Dimensional High-Resolution Binder-Free Graphene Circuits Using a Microfluidic Approach for Sensor Applications

In this study, a simple microfluidic method, which can be universally applied to different rigid or flexible substrates, was developed to fabricate high-resolution, conductive, two-dimensional and three-dimensional microstructured graphene-based electronic circuits. The method involves controlled and selective filling of microchannels on substrate surfaces with a conductive binder-free graphene nanoplatelet (GNP) solution. The ethanol-thermal reaction of GNP solution at low temperatures (∼75 °C) prior to microchannel filling (preheating) can further reduce the GNP andprovide a homogeneous GNP solution, which in turn enhances conductivity, reduces sheet resistance (∼0.05 kΩ sq^-1), enables room-temperature fabrication, and eliminates harsh postprocessing, which makes this fabrication technique compatible with degradable substrates. This method can also be used in combination with 3D printing to fabricate 3D circuits. The feature sizes of the graphene patterns can range from a few micrometers (down to ∼15 μm in width and ∼5 μm in depth) to a few millimeters and use very small amounts of GNP solution (∼2.5 mg of graphene to obtain ∼0.1 kΩ sq^-1 of sheet resistance for 1 cm²). This microfluidic approach can also be implemented using other conductive liquids, such as conductive graphene-silver solutions. This technology has the potential to pave the way for low-cost, disposable, and biodegradable circuits for a range of electronic applications including near-field communication antennas and pressure or strain sensors.

A new approach for an ultrasensitive tactile sensor covering an ultrawide pressure range based on the hierarchical pressure-peak effect

Flexible tactile sensors that imitate the skin tactile system have attracted extensive research interest due to their potential applications in medical diagnosis, intelligent robots and so on. However, it is still a great challenge to date to fabricate tactile sensors with both high sensitivity and wide detection range due to the difficulties in modulating the resistance variation in the sensing materials in a wide pressure range. Here, a tactile sensor with a novel design based on the hierarchical pressure-peak effect (HPPE) consisting of PVP nanowires and electroless deposition (ELD) silver PDMS micro-pyramids is reported. The HPPE can effectively modulate the resistance change rate by adjusting the change of contact area during compression deformation, and the HPPE tactile sensor was demonstrated to have both ultrahigh sensitivity (11.60-1108.75 kPa-1) and ultrawide pressure range (0.04-600 kPa). The designed HPPE tactile sensor is successfully utilized in detecting multi-level pressures including respiration, finger heart rate, pulse and foot pressures. Moreover, it is used to sense a subtle clamping force in the Leonardo Da Vinci surgical robot demonstrating the potential of the sensor in surgical robot applications. In all these cases, the sensor exhibits enough capability to respond quickly to ultrawide-range pressures with high accuracy and stability.

Ultrathin, Biocompatible, and Flexible Pressure Sensor with a Wide Pressure Range and Its Biomedical Application

In this research, an ultrathin, biocompatible, and flexible pressure sensor with a wide pressure range has been developed and applied in biomedical applications. The pressure sensing mechanism is based on the variation of contact resistance between an electrode and a three-dimensional microstructured polyimide/carbon nanotube composite film. The sensor has a thickness of about 31.3 μm, a maximum sensitivity of 41.0 MPa^-1, and a sensing range of 10-500 kPa. Moreover, in situ temperature measurement by an integrated resistive temperature detector enables data correction for varying temperature conditions. In order to show the advantages of the fabricated sensor, it is attached to the human body and integrated with the surface of a radiofrequency ablation (RFA) needle with small radius of curvature. In the experiments, the proposed pressure sensor measured subtle pressure levels (pulse pressure) and high pressure levels (fingertip pressure) without losing conformal contact with the skin. In addition, when the pressure-sensor-integrated RFA needle was inserted into a bovine liver, successful detection of steam popping phenomenon was observed.

Transparent Body-Attachable Multifunctional Pressure, Thermal, and Proximity Sensor and Heater

A multifunctional sensor capable of simultaneous sensing of temperature, pressure, and proximity has been developed. This transparent and body-attachable device is also capable of providing heat under low voltage. The multi-sensor consists of metal fibers fabricated by electrospinning and electroplating. The device comprises randomly deposited metal fibers, which not only provide heating but also perform as thermal and proximity sensors, and orderly aligned metal fibers that act as a pressure sensor. The sensor is fabricated by weaving straight rectangular electrodes on a transparent substrate (a matrix). The sensitivity is readily enhanced by installing numerous matrices that facilitate higher sensing resolution. The convective heat transfer coefficient of the heater is h = 0.014 W·cm⁻²·°C⁻¹. The temperature coefficient of resistivity (TCR) and pressure sensitivity (η_P) are 0.038 °C⁻¹ and 5.3 × 10⁻³ kPa⁻¹, respectively. The superior sensitivity of the device is confirmed via quantitative comparison with similar devices. This multifunctional device also has a superior convective heat transfer coefficient than do other heaters reported in the literature.

Extending the pressure sensing range of porous polypyrrole with multiscale microstructures

Polymer-based piezoresistive sensors that combine the flexibility and stretchability of organic polymers have received considerable attention in flexible and wearable sensing systems. Generally, highly sensitive pressure sensors have a limited pressure sensing range, while pressure sensors with a wide pressure response range usually have limited pressure resolution. Herein, we used a polypyrrole (PPy) sponge with multiscale porous structures to extend the pressure sensing range of PPy-based piezoresistive sensors. The multiscale microstructures with different sizes will sink in sequence after increasing the external pressure and therefore exhibit a wide pressure response range. Our results show that the piezoresistive composite has a superior sensitivity of 28 kPa-1 and a broad stress range of 0-60 kPa. Moreover, the composite displays a stable, repeatable and durable performance over 16 000 cycles. It can be used to monitor diverse body part motions, including vocalization, pulse beating and joint bending. This work provides an effective strategy to extend the pressure sensing range of polymer-based piezoresistive sensors in the manner of structure design rather than modifying the intrinsic properties of active materials.

Ultra-Sensitive Flexible Pressure Sensor Based on Microstructured Electrode

Flexible pressure sensors with a high sensitivity in the lower zone of a subtle-pressure regime has shown great potential in the fields of electronic skin, human–computer interaction, wearable devices, intelligent prosthesis, and medical health. Adding microstructures on the dielectric layer on a capacitive pressure sensor has become a common and effective approach to enhance the performance of flexible pressure sensors. Here, we propose a method to further dramatically increase the sensitivity by adding elastic pyramidal microstructures on one side of the electrode and using a thin layer of a dielectric in a capacitive sensor. The sensitivity of the proposed device has been improved from 3.1 to 70.6 kPa⁻¹ compared to capacitive sensors having pyramidal microstructures in the same dimension on the dielectric layer. Moreover, a detection limit of 1 Pa was achieved. The finite element analysis performed based on electromechanical sequential coupling simulation for hyperelastic materials indicates that the microstructures on electrode are critical to achieve high sensitivity. The influence of the duty ratio of the micro-pyramids on the sensitivity of the sensor is analyzed by both simulation and experiment. The durability and robustness of the device was also demonstrated by pressure testing for 2000 cycles.

Laser direct writing of carbonaceous sensors on cardboard for human health and indoor environment monitoring

Carbonaceous sensors on cardboard can be used for human health and indoor environment monitoring.

Seeing pressure in color based on integration of highly sensitive pressure sensor and emission tunable light emitting diode

We demonstrate a highly sensitive, low-cost, environmental-friendly pressure sensor derived from a wool-based pressure sensor with wide pressure sensing range using wool bricks embedded with a Ag nano-wires. The easy fabrication and light weight allow portable and wearable device applications. Wth the integration of a light-emitting diode possessing multi-wavelength emission, we illustrate a hybrid multi-functional LED-integrated pressure sensor that is able to convert different applied pressures to light emission with different wavelengths. Due to the high sensitivity of the pressure sensor, the demonstration of acoustic signal detection has also been presented using sound of a metronome and a speaker playing a song. This multi-functional pressure sensor can be implemented to technologies such as smart lighting, health care, visible light communication (VLC), and other internet of things (IoT) applications.

Flexible Piezoresistive Sensor with the Microarray Structure Based on Self-Assembly of Multi-Walled Carbon Nanotubes

High-performance flexible pressure sensors have great application prospects in numerous fields, including the robot skin, intelligent prosthetic hands and wearable devices. In the present study, a novel type of flexible piezoresistive sensor is presented. The proposed sensor has remarkable superiorities, including high sensitivity, high repeatability, a simple manufacturing procedure and low initial cost. In this sensor, multi-walled carbon nanotubes were assembled onto a polydimethylsiloxane film with a pyramidal microarray structure through a layer-by-layer self-assembly system. It was found that when the applied external pressure deformed the pyramid microarray structure on the surface of the polydimethylsiloxane film, the resistance of the sensor varied linearly as the pressure changed. Tests that were performed on sensor samples with different self-assembled layers showed that the pressure sensitivity of the sensor could reach - 2.65 &nbsp; &nbsp; kPa - 1 , which ensured the high dynamic response ability and the high stability of the sensor. Moreover, it was proven that the sensor could be applied as a strain sensor under the tensile force to reflect the stretching extent or the bending object. Finally, a flexible pressure sensor was installed on five fingers and the back of the middle finger of a glove. The obtained results from grabbing different weights and different shapes of objects showed that the flexible pressure sensor not only reflected the change in the finger tactility during the grasping process, but also reflected the bending degree of fingers, which had a significant practical prospect.

Flexible and Highly Sensitive Strain Sensor Based on Laser-Induced Graphene Pattern Fabricated by 355 nm Pulsed Laser

A laser-induced-graphene (LIG) pattern fabricated using a 355 nm pulsed laser was applied to a strain sensor. Structural analysis and functional evaluation of the LIG strain sensor were performed by Raman spectroscopy, scanning electron microscopy (SEM) imaging, and electrical–mechanical coupled testing. The electrical characteristics of the sensor with respect to laser fluence and focal length were evaluated. The sensor responded sensitively to small deformations, had a high gauge factor of ~160, and underwent mechanical fracture at 30% tensile strain. In addition, we have applied the LIG sensor, which has high sensitivity, a simple manufacturing process, and good durability, to human finger motion monitoring.

Flexible capacitive pressure sensor with sensitivity and linear measuring range enhanced based on porous composite of carbon conductive paste and polydimethylsiloxane

In recent years, the development of electronic skin and smart wearable body sensors has put forward high requirements for flexible pressure sensors with high sensitivity and large linear measuring range. However, it turns out to be difficult to increase both of them simultaneously. In this paper, a flexible capacitive pressure sensor based on a porous carbon conductive paste-polydimethylsiloxane composite is reported, the sensitivity and the linear measuring range of which were developed using multiple methods including adjusting the stiffness of the dielectric layer material, fabricating a microstructure and increasing the dielectric permittivity of the dielectric layer. The capacitive pressure sensor reported here has a relatively high sensitivity of 1.1 kPa^-1 and a large linear measuring range of 10 kPa, making the product of the sensitivity and linear measuring range 11, which is higher than that of the most reported capacitive pressure sensors to our best knowledge. The sensor has a detection of limit of 4 Pa, response time of 60 ms and great stability. Some potential applications of the sensor were demonstrated, such as arterial pulse wave measuring and breath measuring, which shows it as a promising candidate for wearable biomedical devices. In addition, a pressure sensor array based on the material was also fabricated and it could identify objects in the shape of different letters clearly, which shows promising application in future electronic skins.

Graphene-based wearable sensors

The human body is a "delicate machine" full of sensors such as the fingers, nose, and mouth. In addition, numerous physiological signals are being created every moment, which can reflect the condition of the body. The quality and the quantity of the physiological signals are important for diagnoses and the execution of therapies. Due to the incompact interface between the sensors and the skin, the signals obtained by commercial rigid sensors do not bond well with the body; this decreases the quality of the signal. To increase the quantity of the data, it is important to detect physiological signals in real time during daily life. In recent years, there has been an obvious trend of applying graphene devices with excellent performance (flexibility, biocompatibility, and electronic characters) in wearable systems. In this review, we will first provide an introduction about the different methods of synthesis of graphene, and then techniques for graphene patterning will be outlined. Moreover, wearable graphene sensors to detect mechanical, electrophysiological, fluid, and gas signals will be introduced. Finally, the challenges and prospects of wearable graphene devices will be discussed. Wearable graphene sensors can improve the quality and quantity of the physiological signals and have great potential for health-care and telemedicine in the future.

Highly Sensitive and Wearable Liquid Metal-Based Pressure Sensor for Health Monitoring Applications: Integration of a 3D-Printed Microbump Array with the Microchannel

Wearable pressure sensors capable of sensitive, precise, and continuous measurement of physiological and physical signals have great potential for the monitoring of health status and the early diagnosis of diseases. This work introduces a 3D-printed rigid microbump-integrated liquid metal-based soft pressure sensor (3D-BLiPS) for wearable and health-monitoring applications. Using a 3D-printed master mold based on multimaterial fused deposition modeling, the fabrication of a liquid metal microchannel and the integration of a rigid microbump array above the microchannel are achieved in a one-step, direct process. The microbump array enhances the sensitivity of the pressure sensor (0.158 kPa^-1 ) by locally concentrating the deformation of the microchannel with negligible hysteresis and a stable signal response under cyclic loading. The 3D-BLiPS also demonstrates excellent robustness to 10 000 cycles of multidirectional stretching/bending, changes in temperature, and immersion in water. Finally, these characteristics are suitable for a wide range of applications in health monitoring systems, including a wristband for the continuous monitoring of the epidermal pulse rate for cuffless blood pressure estimation and a wireless wearable device for the monitoring of body pressure using a multiple pressure sensor array for the prevention of pressure ulcers.

Wearable, Stretchable, Transparent All-in-One Soft Sensor Formed from Supersonically Sprayed Silver Nanowires

The demand for wearable, stretchable soft electronics for human-machine interface applications continues to grow given the potential of these devices in humanoid robotics, prosthetics, and health-monitoring devices. We demonstrate fabrication of multifunctional sensors with simultaneous temperature-, pressure-, proximity-, and strain (or bending)-sensing capabilities, combined with heating and UV-protection features. These multifunctional sensors are flexible, light, and transparent and are thus body-attachable. Silver nanowires are supersonically sprayed on a large-scale transparent and flexible roll-to-roll substrate. The junctions between nanowires are physically fused by a strong impact resulting from supersonic spraying, which promotes adhesion and efficient deposition of the nanowire network. Accordingly, nanowires are strongly interconnected, facilitating efficient propagation of electric signals through the fused nanowire network, which allows simultaneous operation of such sensors while maintaining significant transparency. These multifunctional sensors are mechanically durable and retain long-term stability. A theoretical discussion is provided to explain the respective mechanisms of heating and proximity, pressure, and strain sensing.

Impact of Fabric Properties on Textile Pressure Sensors Performance

In recent years, wearable technologies have attracted great attention in physical and chemical sensing applications. Wearable pressure sensors with high sensitivity in low pressure range (<10 kPa) allow touch detection for human-computer interaction and the development of artificial hands for handling objects. Conversely, pressure sensors that perform in a high pressure range (up to 100 kPa), can be used to monitor the foot pressure distribution, the hand stress during movements of heavy weights or to evaluate the cyclist’s pressure pattern on a bicycle saddle. Recently, we developed a fully textile pressure sensor based on a conductive polymer, with simple fabrication and scalable features. In this paper, we intend to provide an extensive description on how the mechanical properties of several fabrics and different piezoresistive ink formulation may have an impact in the sensor’s response during a dynamic operation mode. These results highlight the complexity of the system due to the presence of various parameters such as the fabric used, the conductive polymer solution, the operation mode and the desired pressure range. Furthermore, this work can lead to a protocol for new improvements and optimizations useful for adapting textile pressure sensors to a large variety of applications.

A wearable and sensitive graphene-cotton based pressure sensor for human physiological signals monitoring

Cotton fiber is the most commonly used fabric in textiles and clothing. As compared to inorganic materials like foam, sponge and paper, cotton fibers boast higher levels of flexibility and toughness, which makes it more durable and be better integrated with clothes. In this study, a conductive cotton fiber material modified by reduced graphene oxide (rGO) was prepared, and applied in pressure sensor. The highest sensitivity of the pressure sensor constructed is 0.21 kPa⁻¹, and the pressure range covers up to 500 kPa, which demonstrates a combination of fine sensitivity and broader pressure range. The pressure sensor developed in this study demonstrates great performance in real-time monitoring of human physiological signals like pulse, breath rate and speech recognition, boasting great application value in wearable electronics and smart clothing.

Transparent and Flexible Mayan-Pyramid-based Pressure Sensor using Facile-Transferred Indium tin Oxide for Bimodal Sensor Applications

Transparent and conducting flexible electrodes have been successfully developed over the last few decades due to their potential applications in optoelectronics. However, recent developments in smart electronics, such as a direct human-machine interface, health-monitoring devices, motion-tracking sensors, and artificially electronic skin also require materials with multifunctional properties such as transparency, flexibility and good portability. In such devices, there remains room to develop transparent and flexible devices such as pressure sensors or temperature sensors. Herein, we demonstrate a fully transparent and flexible bimodal sensor using indium tin oxide (ITO), which is embedded in a plastic substrate. For the proposed pressure sensor, the embedded ITO is detached from its Mayan-pyramid-structured silicon mold by an environmentally friendly method which utilizes water-soluble sacrificial layers. The Mayan-pyramid-based pressure sensor is capable of six different pressure sensations with excellent sensitivity in the range of 100 Pa-10 kPa, high endurance of 105 cycles, and good pulse detection and tactile sensing data processing capabilities through machine learning (ML) algorithms for different surface textures. A 5 × 5-pixel pressure-temperature-based bimodal sensor array with a zigzag-shaped ITO temperature sensor on top of it is also demonstrated without a noticeable interface effect. This work demonstrates the potential to develop transparent bimodal sensors that can be employed for electronic skin (E-skin) applications.

Graphene Aerogel Broken to Fragments for a Piezoresistive Pressure Sensor with a Higher Sensitivity

The porous and elastic reduced graphene aerogel (rGA) is a promising active material for piezoresistive pressure sensors (PRSs) to realize an electronic skin. Due to the specific working mechanism and the limitation of the rGA's monolithic conductive network, the PRSs based on rGA suffer from a limited change of resistance with mechanical deformation, so they show poor sensitivity and cannot detect low pressures. Here we aim to improve the sensitivity of the PRS and make it suitable for a low-pressure system (0.5-8 kPa) through an effective method. The monolithic rGA is broken into small pieces by cutting (named as CGA). The sensitivity of the PRS based on CGA can be improved by 10 times that of the uncut rGA (named as UCGA). The resistance variation ratio of CGA (0.96) is 1.45 times of the resistance variation ratio of the UCGA (0.66). By using a package of elastic polypropylene thin films (PP), the cycle stability performance of CGA remains stable after 4200 cycles. The CGA can detect the pulse of a human being with sensitivity higher than the UCGA and the ordinary sensors. This method is simple, effective, and universal to improve the sensitivity of PRS based on porous and elastic materials.

Large area flexible pressure/strain sensors and arrays using nanomaterials and printing techniques

Sensors are becoming more demanding in all spheres of human activities for their advancement in terms of fabrication and cost. Several methods of fabrication and configurations exist which provide them myriad of applications. However, the advantage of fabrication for sensors lies with bulk fabrication and processing techniques. Exhaustive study for process advancement towards miniaturization from the advent of MEMS technology has been going on and progressing at high pace and has reached a highly advanced level wherein batch production and low cost alternatives provide a competitive performance. A look back to this advancement and thus understanding the route further is essential which is the core of this review in light of nanomaterials and printed technology based sensors. A subjective appraisal of these developments in sensor architecture from the advent of MEMS technology converging present date novel materials and process technologies through this article help us understand the path further.

Wearable Piezoresistive Sensors with Ultrawide Pressure Range and Circuit Compatibility Based on Conductive-Island-Bridging Nanonetworks

Wearable pressure sensors with wide operating pressure ranges and enhanced wearability via seamless integration with circuits can greatly improve the fields of digital healthcare, prosthetic limbs, and human-machine interfaces. Herein, we report an approach based on a conductive-island-bridging nanonetwork to realize wearable resistive pressure sensors that are operative over ultrawide pressure ranges >400 kPa and are circuit-compatible. The sensor has a simple two-layered structure, where nanonetworks of single-walled carbon nanotubes selectively patterned on a surface-modified elastomeric film interface and bridge conductive Au island patterns on printed circuit boards (PCBs). We show that varying the design of the Au islands and the conductivity of the nanonetworks systematically tunes the sensitivity, linearity, and the operation range of the pressure sensor. In addition, introducing microstructured lead contacts into the sensor based on a Au-island-bridging nanonetwork produces a record-high sensitivity of 0.06 kPa^-1 at 400 kPa. Furthermore, the PCB that serves as the bottom layer of the pressure sensor and contains embedded interconnects enables facile integration of the sensor with measurement circuits and wireless communication modules. The developed sensor enables the monitoring of wrist pulse waves. Moreover, an insole-shaped PCB-based pressure-sensing system wirelessly monitors pressure distributions and gait kinetics during walking. Our scheme can be extended to other nanomaterials and flexible PCBs and thus provides a simple yet powerful platform for emerging wearable applications.

Controllable Configuration of Sensing Band in a Pressure Sensor by Lenticular Pattern Deformation on Designated Electrodes

Resistive-type pressure sensor, which are mainly utilized in industry, are easy to manufacture and are not significantly affected by external electromagnetic fields, unlike capacitive type. However, the produce signal is not linear, and it is also difficult to measure a wide range of pressures using such a sensor. Therefore, before being utilized, the extracted nonlinear data from them need to be processed by. A resistive sensor that is capable of measuring a wide range of pressure of up to 4 MPa with constant linearity is presented. Moreover it can selectively control the sensing pressure band, or act as an on/off switch, without the need for any additional computer processing.

Bioinspired Interlocked Structure-Induced High Deformability for Two-Dimensional Titanium Carbide (MXene)/Natural Microcapsule-Based Flexible Pressure Sensors

Achieving high deformability in response to minimal external stimulation while maximizing human-machine interactions is a considerable challenge for wearable and flexible electronics applications. Various natural materials or living organisms consisting of hierarchical or interlocked structures exhibit combinations of properties (e.g., natural elasticity and flexibility) that do not occur in conventional materials. The interlocked epidermal-dermal microbridges in human skin have excellent elastic moduli, which enhance and amplify received tactile signal transport. Herein, we use the sensing mechanisms inspired by human skin to develop Ti₃C₂/natural microcapsule biocomposite films that are robust and deformable by mimicking the micro/nanoscale structure of human skin-such as the hierarchy, interlocking, and patterning. The interlocked hierarchical structures can be used to create biocomposite films with excellent elastic moduli (0.73 MPa), capable of high deformability in response to various external stimuli, as verified by employing theoretical studies. The flexible sensor with a hierarchical and interlocked structure (24.63 kPa^-1) achieves a 9.4-fold increase in pressure sensitivity compared to that of the planar structured Ti₃C₂-based flexible sensor (2.61 kPa^-1). This device also exhibits a rapid response rate (14 ms) and good cycling reproducibility and stability (5000 times). In addition, the flexible pressure device can be used to detect and discriminate signals ranging from finger motion and human pulses to voice recognition.

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.

Highly Sensitive Solvent-free Silver Nanoparticle Strain Sensors with Tunable Sensitivity Created Using an Aerodynamically Focused Nanoparticle Printer

We developed and presented highly sensitive solvent-free silver nanoparticle strain sensors fabricated using the aerodynamically focused nanoparticle (AFN) printer. The nanoparticles were printed in various conductive patterns. We explored how printer scan velocity affected pattern geometry and sensor sensitivity. The strain sensors were highly sensitive; the scan velocity afforded tunable sensitivity; and an analytical model predicted the behavior well under low-strain (<0.4%) conditions. We describe a prototype sensor that reliably measured composite beam tensile strain. We further enhanced the sensitivity by creating mechanical cracks, facilitating small dynamic signal measurements. The linear sensitivity of the sensor could be tuned from 18.60 to 290.62 by varying the scan velocity from 2 to 40 μm/s. The cracked sensor afforded the greatest sensitivity (1056) and captured small vibrations from a stringed instrument. We report highly sensitive and reliable measurements of dynamic behavior with simple tunability.

Laser Fabrication of Graphene-Based Electronic Skin

Graphene is promising for developing soft and flexible electronic skin. However, technologies for graphene processing is still at an early stage, which limits the applications of graphene in advanced electronics. Laser processing technologies permits mask-free and chemical-free patterning of graphene, revealing the potential for developing graphene-based electronics. In this minireview, we overviewed and summarized the recent progresses of laser enabled graphene-based electronic skins. Two typical strategies, laser reduction of graphene oxide (GO) and laser induced graphene (LIG) on polyimide (PI), have been introduced toward the fabrication of graphene electronic skins. The advancement of laser processing technology would push forward the rapid progress of graphene electronic skin.

Microstructured Porous Pyramid-Based Ultrahigh Sensitive Pressure Sensor Insensitive to Strain and Temperature

An ultrahigh sensitive capacitive pressure sensor based on a porous pyramid dielectric layer (PPDL) is reported. Compared to that of the conventional pyramid dielectric layer, the sensitivity was drastically increased to 44.5 kPa^-1 in the pressure range <100 Pa, an unprecedented sensitivity for capacitive pressure sensors. The enhanced sensitivity is attributed to a lower compressive modulus and larger change in an effective dielectric constant under pressure. By placing the pressure sensors on islands of hard elastomer embedded in a soft elastomer substrate, the sensors exhibited insensitivity to strain. The pressure sensors were also nonresponsive to temperature. Finally, a contact resistance-based pressure sensor is also demonstrated by chemically grafting PPDL with a conductive polymer, which also showed drastically enhanced sensitivity.

Temperature Characteristics of a Pressure Sensor Based on BN/Graphene/BN Heterostructure

Temperature is a significant factor in the application of graphene-based pressure sensors. The influence of temperature on graphene pressure sensors is twofold: an increase in temperature causes the substrates of graphene pressure sensors to thermally expand, and thus, the graphene membrane is stretched, leading to an increase in the device resistance; an increase in temperature also causes a change in the graphene electrophonon coupling, resulting in a decrease in device resistance. To investigate which effect dominates the influence of temperature on the pressure sensor based on the graphene–boron nitride (BN) heterostructure proposed in our previous work, the temperature characteristics of two BN/graphene/BN heterostructures with and without a microcavity beneath them were analyzed in the temperature range 30–150 °C. Experimental results showed that the resistance of the BN/graphene/BN heterostructure with a microcavity increased with the increase in temperature, and the temperature coefficient was up to 0.25%°C⁻¹, indicating the considerable influence of thermal expansion in such devices. In contrast, with an increase in temperature, the resistance of the BN/graphene/BN heterostructure without a microcavity decreased with a temperature coefficient of −0.16%°C⁻¹. The linearity of the resistance change rate (ΔR/R)–temperature curve of the BN/graphene/BN heterostructure without a microcavity was better than that of the BN/graphene/BN heterostructure with a microcavity. These results indicate that the influence of temperature on the pressure sensors based on BN/graphene/BN heterostructures should be considered, especially for devices with pressure microcavities. BN/graphene/BN heterostructures without microcavities can be used as high-performance temperature sensors.

Towards ultra-wide operation range and high sensitivity: Graphene film based pressure sensors for fingertips

Remarkable research efforts have been devoted to replicate the tactile sensitivity of human skin. Unfortunately, so far flexible pressure sensors reported barely fit the tactile requirements for fingertips, which could endure a pressure over 100 kPa and also can sense a gentle touch. It is vital to develop flexible pressure sensors which can ensure high sensitivity and wide operation range simultaneously, to satisfy the demands of mimicking the pressure sensing function of fingertips. In this work, a mini-size, light-weight but high-performance graphene film based pressure sensor is presented. Owing to the advanced structure with fluctuations on surface and fluffy-layered structure in cross-section of the graphene film, this pressure sensor shows an extraordinary performance of high sensitivity of 10.39 kPa^-1 (0-2 kPa), ultra-wide operation range up to 200 kPa, impressively stable repeatability, high working frequency, rapid response and recovery time. Moreover, the demonstrated results of the detection of traditional Chinese medicine wrist-pulse waveform and the bionic fingertip tactile sensors, suggest the great application potential of the obtain device in biomedical field and bionic skins field.

Flexible Sensors—From Materials to Applications

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Impact of pH on Regulating Ion Encapsulation of Graphene Oxide Nanoscroll for Pressure Sensing

Recently, graphene oxide nanoscroll (GONS) has attracted much attention due to its excellent properties. Encapsulation of nanomaterials in GONS can greatly enhance its performance while ion encapsulation is still unexplored. Herein, various ions including hydronium ion (H₃O⁺), Fe³⁺, Au³⁺, and Zn²⁺ were encapsulated in GONSs by molecular combing acidic graphene oxide (GO) solution. No GONS was obtained when the pH of the GO solution was greater than 9. A few GONSs without encapsulated ion were obtained at the pH of 5–8. When the pH decreased from 5 to 0.15, high-density GONSs with encapsulated ions were formed and the average height of GONS was increased from ~50 to ~190 nm. These results could be attributed to the varied repulsion between carboxylic acid groups located at the edges of GO nanosheets. Encapsulated metal ions were converted to nanoparticles in GONS after high-temperature annealing. The resistance-type device based on reduced GONS (rGONS) mesh with encapsulated H₃O⁺ showed good response for applied pressure from 600 to 8700 Pa, which manifested much better performance compared with that of a device based on rGONS mesh without H₃O⁺.

Flexible and Highly Sensitive Pressure Sensors Based on Microstructured Carbon Nanowalls Electrodes

Wearable pressure sensors have attracted widespread attention in recent years because of their great potential in human healthcare applications such as physiological signals monitoring. A desirable pressure sensor should possess the advantages of high sensitivity, a simple manufacturing process, and good stability. Here, we present a highly sensitive, simply fabricated wearable resistive pressure sensor based on three-dimensional microstructured carbon nanowalls (CNWs) embedded in a polydimethylsiloxane (PDMS) substrate. The method of using unpolished silicon wafers as templates provides an easy approach to fabricate the irregular microstructure of CNWs/PDMS electrodes, which plays a significant role in increasing the sensitivity and stability of resistive pressure sensors. The sensitivity of the CNWs/PDMS pressure sensor with irregular microstructures is as high as 6.64 kPa⁻¹ in the low-pressure regime, and remains fairly high (0.15 kPa⁻¹) in the high-pressure regime (~10 kPa). Both the relatively short response time of ~30 ms and good reproducibility over 1000 cycles of pressure loading and unloading tests illustrate the high performance of the proposed device. Our pressure sensor exhibits a superior minimal limit of detection of 0.6 Pa, which shows promising potential in detecting human physiological signals such as heart rate. Moreover, it can be turned into an 8 × 8 pixels array to map spatial pressure distribution and realize array sensing imaging.

Stretchable Graphene Pressure Sensors with Shar-Pei-like Hierarchical Wrinkles for Collision-Aware Surgical Robotics

Stretchable skin-like pressure sensing with minimized and distinguishable strain-induced interference is essential for the development of collision-aware surgical robotics to improve the safety and efficiency of minimally invasive surgery in a confined space. Inspired by the multidimensional wrinkles of Shar-Pei dog's skin for tactile sensing, we developed a stretchable pressure sensor consisting of reduced graphene oxide (rGO) electrodes with biomimetic topographies to improve the robot-tissue collision detections. A facile fabrication route for stretchable rGO electrodes was first demonstrated by harnessing the surface instability during the sequential deformation processes. The wrinkle-crumple rGO electrodes exhibited high stretchability (∼100%) and strain-insensitive resistance profiles [a gauge factor (GF) < 0.05], which were next utilized to fabricate piezoresistive pressure sensors. The rGO pressure sensors were highly stretchable and exhibited high sensitivity under uniaxial strains (1.37, 1.30, and 0.98 kPa^-1 at 0, 30, and 50% stretching states, respectively) along with distinguishable and reduced stretching responsiveness (a small GF ∼0.2 under 40% strains). The stretchable pressure sensors were next integrated with two surgical robots for the transoral robotic surgery procedure. During the cadaveric testing, the rGO sensors can detect the robot-tissue contacts under joint stretches in real time to enhance the surgeon's awareness for collision avoidance. The stretchable rGO pressure sensor that is highly sensitive under large strains provides great potential in the fields of wearable sensing and collision-aware humanoid robots to improve the human-machine interactions.

Structure-Property Relationships in Graphene-Based Strain and Pressure Sensors for Potential Artificial Intelligence Applications

Wearable electronic sensing devices are deemed to be a crucial technology of smart personal electronics. Strain and pressure sensors, one of the most popular research directions in recent years, are the key components of smart and flexible electronics. Graphene, as an advanced nanomaterial, exerts pre-eminent characteristics including high electrical conductivity, excellent mechanical properties, and flexibility. The above advantages of graphene provide great potential for applications in mechatronics, robotics, automation, human-machine interaction, etc.: graphene with diverse structures and leverages, strain and pressure sensors with new functionalities. Herein, the recent progress in graphene-based strain and pressure sensors is presented. The sensing materials are classified into four structures including 0D fullerene, 1D fiber, 2D film, and 3D porous structures. Different structures of graphene-based strain and pressure sensors provide various properties and multifunctions in crucial parameters such as sensitivity, linearity, and hysteresis. The recent and potential applications for graphene-based sensors are also discussed, especially in the field of human motion detection. Finally, the perspectives of graphene-based strain and pressure sensors used in human motion detection combined with artificial intelligence are surveyed. Challenges such as the biocompatibility, integration, and additivity of the sensors are discussed as well.

Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications

Recently, graphene has been extensively researched in fundamental science and engineering fields and has been developed for various electronic applications in emerging technologies owing to its outstanding material properties, including superior electronic, thermal, optical and mechanical properties. Thus, graphene has enabled substantial progress in the development of the current electronic systems. Here, we introduce the most important electronic and thermal properties of graphene, including its high conductivity, quantum Hall effect, Dirac fermions, high Seebeck coefficient and thermoelectric effects. We also present up-to-date graphene-based applications: optical devices, electronic and thermal sensors, and energy management systems. These applications pave the way for advanced biomedical engineering, reliable human therapy, and environmental protection. In this review, we show that the development of graphene suggests substantial improvements in current electronic technologies and applications in healthcare systems.

A Sprayed Graphene Pattern-Based Flexible Strain Sensor with High Sensitivity and Fast Response

Flexible strain sensors have a wide range of applications in biomedical science, aerospace industry, portable devices, precise manufacturing, etc. However, the manufacturing processes of most flexible strain sensors previously reported have usually required high manufacturing costs and harsh experimental conditions. Besides, research interests are often focused on improving a single attribute parameter while ignoring others. This work aims to propose a simple method of manufacturing flexible graphene-based strain sensors with high sensitivity and fast response. Firstly, oxygen plasma treats the substrate to improve the interfacial interaction between graphene and the substrate, thereby improving device performance. The graphene solution is then sprayed using a soft PET mask to define a pattern for making the sensitive layer. This flexible strain sensor exhibits high sensitivity (gauge factor ~100 at 1% strain), fast response (response time: 400–700 μs), good stability (1000 cycles), and low overshoot (<5%) as well. Those processes used are compatible with a variety of complexly curved substrates and is expected to broaden the application of flexible strain sensors.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.