Advanced Carbon for Flexible and Wearable Electronics

Flexible and wearable electronics are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Carbon materials have combined superiorities such as good electrical conductivity, intrinsic and structural flexibility, light weight, high chemical and thermal stability, ease of chemical functionalization, as well as potential mass production, enabling them to be promising candidate materials for flexible and wearable electronics. Consequently, great efforts are devoted to the controlled fabrication of carbon materials with rationally designed structures for applications in next‐generation electronics. Herein, the latest advances in the rational design and controlled fabrication of carbon materials toward applications in flexible and wearable electronics are reviewed. Various carbon materials (carbon nanotubes, graphene, natural‐biomaterial‐derived carbon, etc.) with controlled micro/nanostructures and designed macroscopic morphologies for high‐performance flexible electronics are introduced. The fabrication strategies, working mechanism, performance, and applications of carbon‐based flexible devices are reviewed and discussed, including strain/pressure sensors, temperature/humidity sensors, electrochemical sensors, flexible conductive electrodes/wires, and flexible power devices. Furthermore, the integration of multiple devices toward multifunctional wearable systems is briefly reviewed. Finally, the existing challenges and future opportunities in this field are summarized.     

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human–machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.     

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing,Integration, and Applications

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human–machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human–machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.     

Mechanically Flexible Conductors for Stretchable and Wearable E‐Skin and E‐Textile Devices

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of “Internet‐of‐Things” concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS₂), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4‐ethylenedioxythiophene):poly(4‐styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so‐called electronic skins and electronic textiles.     

Graphene-based wearable piezoresistive physical sensors

In the last two decades, wearable piezoresistive physical sensors have attracted tremendous attention due to their broad applications in individual health-monitoring, human–machine interfaces, robotics, sports and therapeutics. Many different nanostructured materials, including nanowires, nanoparticles, nanoribbons, carbon black, carbon nanotubes and graphene, have been explored to construct stretchable piezoresistive sensors on an elastomer substrate. Thanks to its unique two-dimensional geometry, lightweight, flexibility, semi-transparency and outstanding transport and mechanical properties, graphene and its derivatives in particular are considered among the most suitable candidates as wearable sensors. This paper reviews various design strategies established for fabricating flexible, wearable sensors using graphene. The current state-of-the-art developments are discussed of flexible sensors made of 1D fibrous, 2D planar and 3D cellular interconnected graphene architectures for detecting physiological strains, tactile pressures and temperatures. The working mechanisms along with existing applications of flexible sensors are presented. The challenges these sensors are currently facing and potential opportunities for novel applications are revealed to offer new insights into future prospects in this field.     

Bioinspired Dual-Mode Stretchable Strain Sensor Based on Magnetic Nanocomposites for Strain/Magnetic Discrimination

Recently, flexible stretchable sensors have been gaining attention for their excellent adaptability for electronic skin applications. However, the preparation of stretchable strain sensors that achieve dual-mode sensing while still retaining ultra-low detection limit of strain, high sensitivity, and low cost is a pressing task. Herein, a high-performance dual-mode stretchable strain sensor (DMSSS) based on biomimetic scorpion foot slit microstructures and multi-walled carbon nanotubes (MWCNTs)/graphene (GR)/silicone rubber (SR)/Fe₃O₄ nanocomposites is proposed, which can accurately sense strain and magnetic stimuli. The DMSSS exhibits a large strain detection range (≈160%), sensitivity up to 100.56 (130–160%), an ultra-low detection limit of strain (0.16% strain), and superior durability (9000 cycles of stretch/release). The sensor can accurately recognize sign language movement, as well as realize object proximity information perception and whole process information monitoring. Furthermore, human joint movements and micro-expressions can be monitored in real-time. Therefore, the DMSSS of this work opens up promising prospects for applications in sign language pose recognition, non-contact sensing, human-computer interaction, and electronic skin.     

Organic Photodetectors for Next-Generation Wearable Electronics

Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.     

Nanomaterial‐Enabled Stretchable Conductors: Strategies,Materials and Devices

Stretchable electronics are attracting intensive attention due to their promising applications in many areas where electronic devices undergo large deformation and/or form intimate contact with curvilinear surfaces. On the other hand, a plethora of nanomaterials with outstanding properties have emerged over the past decades. The understanding of nanoscale phenomena, materials, and devices has progressed to a point where substantial strides in nanomaterial‐enabled applications become realistic. This review summarizes recent advances in one such application, nanomaterial‐enabled stretchable conductors (one of the most important components for stretchable electronics) and related stretchable devices (e.g., capacitive sensors, supercapacitors and electroactive polymer actuators), over the past five years. Focusing on bottom‐up synthesized carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal nanomaterials (e.g., metal nanowires and nanoparticles), this review provides fundamental insights into the strategies for developing nanomaterial‐enabled highly conductive and stretchable conductors. Finally, some of the challenges and important directions in the area of nanomaterial‐enabled stretchable conductors and devices are discussed.     

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years.     

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line‐patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125–145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.     

Piezoresistive in-situ strain sensing of composite laminate structures

Various methods have been developed to monitor the health and strain state of carbon fiber reinforced polymers, each with a unique set of pros and cons. This research assesses the use of piezoresistive sensors for in situ strain measurement of carbon fiber and other composite structures in multidirectional laminates. The piezoresistive sensor material and the embedded circuitry are both evaluated. For the piezoresistive sensor, a conductive nickel nanocomposite sensor is compared with the piezoresistivity of the carbon fiber itself. For the circuit, the use of carbon fibers already present in the structure is compared with the use of nickel coated carbon fiber. Successful localized strain sensing is demonstrated for several sensor and circuitry configurations. Numerous engineering applications are possible in the ever-growing field of carbon-composites.     

Highly conductive, printable and stretchable composite films of carbon nanotubes and silver

Conductive films that are both stretchable and flexible could have applications in electronic devices, sensors, actuators and speakers. A substantial amount of research has been carried out on conductive polymer composites, metal electrode-integrated rubber substrates and materials based on carbon nanotubes and graphene. Here we present highly conductive, printable and stretchable hybrid composites composed of micrometre-sized silver flakes and multiwalled carbon nanotubes decorated with self-assembled silver nanoparticles. The nanotubes were used as one-dimensional, flexible and conductive scaffolds to construct effective electrical networks among the silver flakes. The nanocomposites, which included polyvinylidenefluoride copolymer, were created with a hot-rolling technique, and the maximum conductivities of the hybrid silver-nanotube composites were 5,710 S cm?1 at 0% strain and 20 S cm?1 at 140% strain, at which point the film ruptured. Three-dimensional percolation theory reveals that Poisson's ratio for the composite is a key parameter in determining how the conductivity changes upon stretching.     

Shape morphing smart 3D actuator materials for micro soft robot

Stimuli-responsive materials have been used in major applications such as sensors, actuators, wearable devices, and biomedical devices owing to their ability to respond to external stimuli including heat, light, electricity, humidity, and chemicals. Strategies to trigger the stimuli-responsive, shape-morphing of two-dimensional (2D) sheets into three-dimensional (3D) shapes are of significant interest for a variety of smart applications including soft robotics. Stimuli-responsive properties can be designed by the selection of materials, structures, and processing methods. This review outlines seven broad categories of stimuli-responsive 2D soft materials for 3D smart actuator applications, namely (1) carbon nanomaterials, (2) metal nanomaterials, (3) shape memory polymers, (4) liquid crystal polymers and elastomers, (5) azobenzenes, (6) hydrogels, and (7) bio-hybrids, along with their basic mechanisms, processing methods, and applications.     

Highly Stretchable,Adaptable, and Durable Strain Sensing Based on a Bioinspired Dynamically Cross‐Linked Graphene/Polymer Composite

Flexible strain sensors can detect physical signals (e.g., temperature, humidity, and flow) by sensing electrical deviation under dynamic deformation, and they have been used in diverse fields such as human motion detection, medical care, speech recognition, and robotics. Existing sensing materials have relatively low adaptability and durability and are not stretchable and flexible enough for complex tasks in motion detection. In this work, a highly flexible self‐healing conductive polymer composite consisting of graphene, poly(acrylic acid) and amorphous calcium carbonate is prepared via a biomineralization‐inspired process. The polymer composite shows good editability and processability and can be fabricated into stretchable strain sensors of various structures (sandwich structures, fibrous structures, self‐supporting structures, etc.). The developed sensors can be attached on different types of surfaces (e.g., flat, cambered) and work well both in air and under water in detecting various biosignals, including crawling, undulatory locomotion, and human body motion.     

Mechanics for stretchable sensors

In the past decade, high performance stretchable sensors have found many exciting applications including epidermal and in vivo monitors, minimally invasive surgical tools, as well as deployable structure health monitors (SHM). Although wafer based electronics are known to be rigid and planar, recent advances in manufacture and mechanics have made intrinsically stiff and brittle inorganic electronic materials stretchable and compliant. This review article summarizes the most recent mechanics studies on stretchable sensors composed of ceramic and metallic functional materials. The discussion will focus around the most popular “island plus serpentine” design where active electronic or sensing components are housed on an array of isolated, micro-scale islands which are interconnected by electrically conductive, stretchable, serpentine thin films. The mechanics of polymer supported islands, freestanding serpentines, and polymer supported serpentines will be introduced. The effects of feature geometry and polymer substrate on the stretchability, compliance, as well as functionality of the sensor system will be discussed in details. The tradeoff between mechanics and functionality gives rise to the challenge of simultaneously optimizing the structure and performance of stretchable sensors.     

基于微结构的柔性压力传感器设计、制备及性能

随着科技的快速发展,电子皮肤和柔性可穿戴设备由于在人体运动、健康监测、智能机器人等领域具有重要应用而引起了人们广泛的关注。传统的基于贵金属或金属氧化物半导体的压力传感器成本高、柔韧性差,而新型的基于微结构的柔性压力传感器具有灵敏度高、应变范围宽、低成本、低功耗、响应速度快等优势,在电子皮肤和柔性可穿戴设备等方面发挥重要作用,成为当前柔性电子材料与器件主要研究热点之一。本文系统总结了近年来颇受关注的基于金字塔形、微球形、微柱形、仿生结构、褶皱等不同柔性基底微结构和多孔导电聚合物材料的柔性压力传感器在材料选择、结构设计、制备方法、传感性能等方面取得的重要进展,并对柔性压力传感器的未来发展进行了展望。      

Carbon nanotube cement-based transducers for dynamic sensing of strain

A new type of sensor for structural health monitoring (SHM) has emerged since the birth and development of nanotechnology and is based on cementitious materials additioned with carbon nanoparticles that can provide measurable electrical responses to applied strain. The response of similar transducers was mainly investigated under slowly varying strains while applications in dynamics have not been yet documented.This paper is aimed at exploring the applicability of carbon nanotube–cement based sensors for measuring dynamically varying strain in concrete structures. Experiments are presented to investigate the electrical response of prismatic specimens made of carbon nanotube–cement composite when subjected to sinusoidal stress–strains in the typical frequency range of large civil structures. The results demonstrate that the sensors’ output retains all dynamic features of the input thus providing useful information for SHM and encouraging the transformation of structures into infinite sets of potential sensors with enhanced durability and limited access issues.     

Laser-induced graphene and carbon nanotubes as conductive carbon-based materials in environmental technology

Nanotechnology and nanomaterials have attracted interest due to their potential in mitigating contemporary environmental challenges, such as the stressors imposed by increased industrial and agricultural activities, and the deterioration of air, soil and water quality. In particular, advanced technologies that harness carbon-based nanomaterials are poised to emerge as tools that provide new solutions for the global water crises. These tools include, electrically conductive membrane processes, which uniquely combine a separation process with a functional surface. In this respect, laser-induced graphene (LIG) and carbon nanotubes (CNTs) are electrically conductive carbon nanomaterials that hold great utility in a multitude of environmental applications, including the development of fouling-resistant systems for desalination and water treatment, enhanced separation methods, and innovative pollutant sensing and electrocatalytic platforms. Consequently, this review article describes and compares some important recent advances in LIG- and CNT-based electroactive surfaces. The discussion of LIG as an emerging carbon material set in context with CNTs is intended to shed light on future directions and development possibilities to meet the growing global challenges in terms of water treatment applications of both materials as well as other electrically conductive carbon-based nanomaterials exhibiting exceptional performance and versatility.     

Carbon Nanotube Yarn for Fiber-Shaped Electrical Sensors,Actuators, and Energy Storage for Smart Systems

Smart systems are those that display autonomous or collaborative functionalities, and include the ability to sense multiple inputs, to respond with appropriate operations, and to control a given situation. In certain circumstances, it is also of great interest to retain flexible, stretchable, portable, wearable, and/or implantable attributes in smart electronic systems. Among the promising candidate smart materials, carbon nanotubes (CNTs) exhibit excellent electrical and mechanical properties, and structurally fabricated CNT-based fibers and yarns with coil and twist further introduce flexible and stretchable properties. A number of notable studies have demonstrated various functions of CNT yarns, including sensors, actuators, and energy storage. In particular, CNT yarns can operate as flexible electronic sensors and electrodes to monitor strain, temperature, ionic concentration, and the concentration of target biomolecules. Moreover, a twisted CNT yarn enables strong torsional actuation, and coiled CNT yarns generate large tensile strokes as an artificial muscle. Furthermore, the reversible actuation of CNT yarns can be used as an energy harvester and, when combined with a CNT supercapacitor, has promoted the next-generation of energy storage systems. Here, progressive advances of CNT yarns in electrical sensing, actuation, and energy storage are reported, and the future challenges in smart electronic systems considered.