Highly Sensitive,Flexible MEMS Based Pressure Sensor with Photoresist Insulation Layer

Pressure sensing is a crucial function for flexible and wearable electronics, such as artificial skin and health monitoring. Recent progress in material and device structure of pressure sensors has brought breakthroughs in flexibility, self‐healing, and sensitivity. However, the fabrication process of many pressure sensors is too complicated and difficult to integrate with traditional silicon‐based Micro‐Electro‐Mechanical System(MEMS). Here, this study demonstrates a scalable and integratable contact resistance‐based pressure sensor based on a carbon nanotube conductive network and a photoresist insulation layer. The pressure sensors have high sensitivity (95.5 kPa⁻¹), low sensing threshold (16 Pa), fast response speed (<16 ms), and zero power consumption when without loading pressure. The sensitivity, sensing threshold, and dynamic range are all tunable by conveniently modifying the hole diameter and thickness of insulation layer.     

MXenes在柔性力敏传感器中的应用研究进展

随着可穿戴柔性电子技术的发展, 高灵敏度和宽感应范围的柔性力敏传感器的需求量逐渐增大, 如何选择兼具高导电性和良好柔性的材料作为传感器的敏感材料是获得高性能传感器的关键。近年来, MXene材料因其导电性好、柔韧性高、亲水性好以及合成可控等优点成为一种极具潜力的导电敏感材料。本文就MXene基柔性力敏传感器的类型、敏感材料的微结构设计方式、传感性能及传感机理等方面的研究进展进行了阐述和总结。     

Wearable strain sensing textile based on one-dimensional stretchable and weavable yarn sensors

Flexible, wearable, and even stretchable sensors are the key components of smart electronic textiles. However, most reported flexible and wearable sensors for wearable electronics are usually fabricated in two-dimensional (2D) planar strip configurations, which cannot be properly integrated into textile structures and thus greatly degrade intrinsic properties such as the softness, flexibility, and air permeability of textiles and the aesthetic feeling of clothing. In this work, a new one-dimensional weavable strain sensing yarn consisting of an elastic polyurethane (PU) core, a conductive Ag-nanoparticles/graphene-microsheets composite sheath, and a silicone encapsulation layer was designed and fabricated through an easily manipulated protocol. Arising from the reasonable structural design and appropriate material selection, the as-fabricated strain sensor not only exhibited excellent flexibility, stretchability, and highly repeatable electromechanical stability (a repeatability error of 1.56%) but also possessed both high sensitivity (a gauge factor of nearly 500) and a relatively wide working range (0–50% applied strain) with good linearity (a correlation coefficient of 0.98). In addition, the facile, nearly all-solution-based fabrication protocol enabled the scalable production of long conductive yarns. Thus, the proper yarn length and superb mechanical properties endowed the stretchable conductive yarn with good weavability. The excellent wearability of the stretchable conductive yarn was derived from the outermost isolating, hydrophobic, and biocompatible silicone encapsulation layer. A wearable high-sensitivity strain sensing textile, fabricated by directly weaving the as-prepared yarn-based sensor, showed great potential for application to wearable textile sensors for real-time monitoring of human motions from vigorous walking to subtle and complex pronunciations.

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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.     

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。      

Mechano‐Based Transductive Sensing for Wearable Healthcare

Wearable healthcare presents exciting opportunities for continuous, real‐time, and noninvasive monitoring of health status. Even though electrochemical and optical sensing have already made great advances, there is still an urgent demand for alternative signal transformation in terms of miniaturization, wearability, conformability, and stretchability. Mechano‐based transductive sensing, referred to the efficient transformation of biosignals into measureable mechanical signals, is claimed to exhibit the aforementioned desirable properties, and ultrasensitivity. In this Concept, a focus on pressure, strain, deflection, and swelling transductive principles based on micro‐/nanostructures for wearable healthcare is presented. Special attention is paid to biophysical sensors based on pressure/strain, and biochemical sensors based on microfluidic pressure, microcantilever, and photonic crystals. There are still many challenges to be confronted in terms of sample collection, miniaturization, and wireless data readout. With continuing efforts toward solving those problems, it is anticipated that mechano‐based transduction will provide an accessible route for multimode wearable healthcare systems integrated with physical, electrophysiological, and biochemical sensors.     

导电水凝胶在柔性可穿戴传感器的应用研究进展

目的 概述导电水凝胶在柔性可穿戴传感器方面的研究情况,挖掘其作为传感器件的应用潜能。方法 查阅大量相关的文献,对导电水凝胶在柔性可穿戴传感领域的最新进展进行归纳与总结。按水凝胶网络分类的4种导电水凝胶,总结归纳其设计、合成、结构和潜在应用。讨论导电水凝胶的导电性、力学性能、黏附性、防冻性能、自愈性能和各式响应性等功能性能的影响因素,总结自黏性、防冻性、自修复和其他多种优秀性能的柔性可穿戴传感器。结论 导电水凝胶是一种具有多功能的独特刺激响应性的功能材料,在柔性可穿戴传感领域进行深入探究具有重大意义。     

Wearable Sensing Glove With Embedded Hetero-Core Fiber-Optic Nerves for Unconstrained Hand Motion Capture

In this paper, we present a wearable sensing glove with embedded hetero-core fiber-optic nerve sensors that detect finger flexion to achieve unconstrained hand motion monitoring. The hetero-core fiber sensor is suited to the wearable sensing glove because it is capable of optical intensity-based measurements with excellent stability and repeatability using single-mode transmission fibers and is unaffected by temperature fluctuations. The hetero-core sensor elements are located on the back of the hand so that they are not affected by random wrinkles in the glove at the joints. As a result, the hetero-core flexion sensor after calibration is capable of detecting the joint angles of the fingers regardless of differences in hand size, and the hetero-core sensing technique enables the sensing glove to be constructed with a minimum number of sensor points. The optical loss performance of the hetero-core sensors reveals monotonic characteristics with respect to the flexion angle of joints. The optical loss is 1.35 dB for a flexion angle of approximately 97.2$^{circ}$ with accuracy of 0.89$^{circ}$ in the detected flexion angle. Real-time hand motion capture was demonstrated by means of the proposed sensing glove without restricting natural human behavior.      

Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors

The development of a robust method for integrating high-performance semiconductors on flexible plastics could enable exciting avenues in fundamental research and novel applications. One area of vital relevance is chemical and biological sensing, which if implemented on biocompatible substrates, could yield breakthroughs in implantable or wearable monitoring systems. Semiconducting nanowires (and nanotubes) are particularly sensitive chemical sensors because of their high surface-to-volume ratios. Here, we present a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires are excellent field-effect transistors, and, as sensors, exhibit parts-per-billion sensitivity to NO2, a hazardous pollutant. We also use SiO2 surface chemistries to construct a 'nano-electronic nose' library, which can distinguish acetone and hexane vapours via distributed responses. The excellent sensing performance coupled with bendable plastic could open up opportunities in portable, wearable or even implantable sensors.     

Surface-Mediated Interconnections of Nanoparticles in Cellulosic Fibrous Materials toward 3D Sensors

Fibrous materials serve as an intriguing class of 3D materials to meet the growing demands for flexible, foldable, biocompatible, biodegradable, disposable, inexpensive, and wearable sensors and the rising desires for higher sensitivity, greater miniaturization, lower cost, and better wearability. The use of such materials for the creation of a fibrous sensor substrate that interfaces with a sensing film in 3D with the transducing electronics is however difficult by conventional photolithographic methods. Here, a highly effective pathway featuring surface-mediated interconnection (SMI) of metal nanoclusters (NCs) and nanoparticles (NPs) in fibrous materials at ambient conditions is demonstrated for fabricating fibrous sensor substrates or platforms. Bimodally distributed gold–copper alloy NCs and NPs are used as a model system to demonstrate the semiconductive-to-metallic conductivity transition, quantized capacitive charging, and anisotropic conductivity characteristics. Upon coupling SMI of NCs/NPs as electrically conductive microelectrodes and surface-mediated assembly (SMA) of the NCs/NPs as chemically sensitive interfaces, the resulting fibrous chemiresistors function as sensitive and selective sensors for gaseous and vaporous analytes. This new SMI–SMA strategy has significant implications for manufacturing high-performance fibrous platforms to meet the growing demands of the advanced multifunctional sensors and biosensors.     

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.     

Stretchable Electronic Sensors of Nanocomposite Network Films for Ultrasensitive Chemical Vapor Sensing

A stretchable, transparent, and body‐attachable chemical sensor is assembled from the stretchable nanocomposite network film for ultrasensitive chemical vapor sensing. The stretchable nanocomposite network film is fabricated by in situ preparation of polyaniline/MoS₂ (PANI/MoS₂) nanocomposite in MoS₂ suspension and simultaneously nanocomposite deposition onto prestrain elastomeric polydimethylsiloxane substrate. The assembled stretchable electronic sensor demonstrates ultrasensitive sensing performance as low as 50 ppb, robust sensing stability, and reliable stretchability for high‐performance chemical vapor sensing. The ultrasensitive sensing performance of the stretchable electronic sensors could be ascribed to the synergistic sensing advantages of MoS₂ and PANI, higher specific surface area, the reliable sensing channels of interconnected network, and the effectively exposed sensing materials. It is expected to hold great promise for assembling various flexible stretchable chemical vapor sensors with ultrasensitive sensing performance, superior sensing stability, reliable stretchability, and robust portability to be potentially integrated into wearable electronics for real‐time monitoring of environment safety and human healthcare.     

Electrohydrodynamic (EHD) inkjet printing flexible pressure sensors with a multilayer structure and periodically patterned Ag nanoparticles

Flexible pressure sensors are widely employed for accurate pressure sensing on geometrically complex surfaces. As sensing materials, silver nanoparticles (AgNPs) have high electrical conductivity but relatively poor sensitivity as a trade-off. In this work, electrohydrodynamic (EHD) inkjet printing was utilized to directly write patterns of AgNPs tracks with periodic geometries on the flex-substrate surface. The patterns in which the as-printed AgNPs tracks, with a width of several tens of micrometres, exhibited a piezoresistive effect. This work confirmed that introducing multilayered structures into the flexible pressure sensors with AgNPs patterns was a practical path to improve the sensing sensitivity, with the assistance of soft packaging material of Polydimethylsiloxane (PDMS). The sensitivity was improved more than tenfold after fourfold overlapping of the as-printed single-layer sensor. Experimental tests, formula calculations, and numerical simulations of the sensors were conducted. It was concluded that the as-printed single-layer sensor with the AgNPs pattern of concave regular hexagonal structure (CRHTS) had better sensing performance than that of grid-type structure (GTS) or wave-type structure (WTS). For the two-layered CRHTS sensor, the dynamic and quasi-static sensing response characteristics, response recovery duration, cyclic stability, and ability to discriminate different strain frequencies were further measured and analysed. The working principle of the flex sensors was discussed based on the Percolation Theory and the Tunneling Effect. Some application demonstrations of the sensors were also exhibited. The structural design and EHD inkjet printing fabrication path facilitate the development of more versatile flex sensors.

     

Advances in Rational Design and Materials of High‐Performance Stretchable Electromechanical Sensors

Stretchable and wearable sensor technology has attracted significant interests and created high technological impact on portable healthcare and smart human–machine interfaces. Wearable electromechanical systems are an important part of this technology that has recently witnessed tremendous progress toward high‐performance devices for commercialization. Over the past few years, great attention has been paid to simultaneously enhance the sensitivity and stretchability of the electromechanical sensors toward high sensitivity, ultra‐stretchability, low power consumption or self‐power functionalities, miniaturisation as well as simplicity in design and fabrication. This work presents state‐of‐the‐art advanced materials and rational designs of electromechanical sensors for wearable applications. Advances in various sensing concepts and structural designs for intrinsic stretchable conductive materials as well as advanced rational platforms are discussed. In addition, the practical applications and challenges in the development of stretchable electromechanical sensors are briefly mentioned and highlighted.     

Highly flexible and sensitive wearable strain and pressure sensor based on porous graphene paper for human motion

The demand for high-performance multifunctional wearable devices drives the rapid development of sensors with flexibility, sensitivity and easy preparation. Here, we report an efficient preparation method to fabricate a wearable strain and pressure sensor based on porous graphene paper (PGP), which is prepared by polymethylmethacrylate (PMMA) microsphere as a template. The prepared PGP-based strain and pressure sensor can detect multi-dimensional deformation and shows good flexibility even after more than 1000 s of repeated deformation cycles, while the rapid response time can be up to approximately 60 ms. Moreover, the obtained PGP-based sensor exhibits a good sensitivity that the gauge factor (GF) is up to 77 when the strain is in the range of 4–8%, much higher than other graphene materials. Importantly, the porous microstructure created by the PMMA microsphere in the PGP plays a vital role in the good comprehensive performance of the PGP-based sensor. The device shows potential applications in smart wearable devices to detect or monitor the posture and movement information of human beings.

     

Supercapacitive Iontronic Nanofabric Sensing

The study of wearable devices has become a popular research topic recently, where high‐sensitivity, noise proof sensing mechanisms with long‐term wearability play critical roles in a real‐world implementation, while the existing mechanical sensing technologies (i.e., resistive, capacitive, or piezoelectric) have yet offered a satisfactory solution to address them all. Here, we successfully introduced a flexible supercapacitive sensing modality to all‐fabric materials for wearable pressure and force sensing using an elastic ionic–electronic interface. Notably, an electrospun ionic fabric utilizing nanofibrous structures offers an extraordinarily high pressure‐to‐capacitance sensitivity (114 nF kPa^?1), which is at least 1000 times higher than any existing capacitive sensors and one order of magnitude higher than the previously reported ionic devices, with a pressure resolution of 2.4 Pa, achieving high levels of noise immunity and signal stability for wearable applications. In addition, its fabrication process is fully compatible with existing industrial manufacturing and can lead to cost‐effective production for its utility in emerging wearable uses in a foreseeable future.     

Flexible Transparent Electronic Gas Sensors

Flexible and transparent electronic gas sensors capable of real‐time, sensitive, and selective analysis at room‐temperature, have gained immense popularity in recent years for their potential to be integrated into various smart wearable electronics and display devices. Here, recent advances in flexible transparent sensors constructed from semiconducting oxides, carbon materials, conducting polymers, and their nanocomposites are presented. The sensing material selection, sensor device construction, and sensing mechanism of flexible transparent sensors are discussed in detail. The critical challenges and future development associated with flexible and transparent electronic gas sensors are presented. Smart wearable gas sensors are believed to have great potential in environmental monitoring and noninvasive health monitoring based on disease biomarkers in exhaled gas.     

Advanced Soft Materials,Sensor Integrations,and Applications of Wearable Flexible Hybrid Electronics in Healthcare,Energy, and Environment

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human–machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.     

Flexible strain sensors fabricated using carbon-based nanomaterials: A review

Flexible strain sensors have experienced growing demand due to their several potential applications, such as personalized health monitoring, human motion detection, structural health monitoring, smart garments, and robots. Recently, several academic results have been reported concerning flexible and stretchable strain sensors. These reports indicate that the materials and design methods have an important influence on the performance of strain sensors. Carbon-based nanomaterials including carbon-based nanofibers, carbon nanotubes, graphene, and carbon black nanoparticles play a key role in the fabrication of flexible strain sensors with excellent properties. In terms of design, carbon-based nanomaterials are generally combined with polymers to maintain the flexibility and stability of a strain sensor. Various combined methods were successfully developed using different assembly structures of carbon-based nanomaterials in polymers, such as uniform mixing and ordered structures, including films, fibers, nanofiber membranes, yarns, foams, and fabrics. The working mechanisms of the flexible strain sensors, including changing the conductive network between overlapped nanomaterials, tunneling effect, and crack propagation, are also different compared with that of traditional semiconductor and metal sensors. The effects of the carbon-based nanomaterial structures in polymers on the strain sensing performance have been comprehensively studied and analyzed. The potential applications of flexible strain sensors and current challenges have been summarized and evaluated. This review provides some suggestions for further development of flexible and stretchable strain sensors with outstanding performance.     

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.