Recent Progress of Fiber Shaped Lighting Devices for Smart Display Applications—A Fibertronic Perspective

Advances in material science and nanotechnology have fostered the miniaturization of devices. Over the past two decades, the form-factor of these devices has evolved from 3D rigid, volumetric devices through 2D film-based flexible electronics, finally to 1D fiber electronics (fibertronics). In this regard, fibertronic strategies toward wearable applications (e.g., electronic textiles (e-textiles)) have attracted considerable attention thanks to their capability to impart various functions into textiles with retaining textiles' intrinsic properties as well as imperceptible irritation by foreign matters. In recent years, extensive research has been carried out to develop various functional devices in the fiber form. Among various features, lighting and display features are the highly desirable functions in wearable electronics. This article discusses the recent progress of materials, architectural designs, and new fabrication technologies of fiber-shaped lighting devices and the current challenges corresponding to each device's operating mechanism. Moreover, opportunities and applications that the revolutionary convergence between the state-of-the-art fibertronic technology and age-long textile industry will bring in the future are also discussed.     

An Overview of Fiber-Shaped Batteries with a Focus on Multifunctionality,Scalability, and Technical Difficulties

Flexible and wearable energy storage devices are receiving increasing attention with the ever-growing market of wearable electronics. Fiber-shaped batteries display a unique 1D architecture with the merits of superior flexibility, miniaturization potential, adaptability to deformation, and compatibility with the traditional textile industry, which are especially advantageous for wearable applications. In the recent research frontier in the field of fiber-shaped batteries, in addition to higher performance, advances in multifunctional, scalable, and integrable systems are also the main themes. However, many difficulties exist, including difficult encapsulation and installation of separators, high internal resistance, and poor durability. Herein, the design principles (e.g., electrode preparation and battery assembly) and device performance (e.g., electrochemical and mechanical properties) of fiber-shaped batteries, including lithium-based batteries, zinc-based batteries, and some other representative systems, are summarized, with a focus on multifunctional devices with environmental adaptability, stimuli-responsive properties, and scalability up to energy textiles, with the hope of enlightening future research directions. Finally, technical challenges in the realistic wearable application of these batteries are also discussed with the aim of providing possible solutions and new insights for further improvement.     

Recent Advances in 1D Stretchable Electrodes and Devices for Textile and Wearable Electronics: Materials,Fabrications, and Applications

Research on wearable electronic devices that can be directly integrated into daily textiles or clothes has been explosively grown holding great potential for various practical wearable applications. These wearable electronic devices strongly demand 1D electronic devices that are light–weight, weavable, highly flexible, stretchable, and adaptable to comport to frequent deformations during usage in daily life. To this end, the development of 1D electrodes with high stretchability and electrical performance is fundamentally essential. Herein, the recent process of 1D stretchable electrodes for wearable and textile electronics is described, focusing on representative conductive materials, fabrication techniques for 1D stretchable electrodes with high performance, and designs and applications of various 1D stretchable electronic devices. To conclude, discussions are presented regarding limitations and perspectives of current materials and devices in terms of performance and scientific understanding that should be considered for further advances.     

Energy Harvesters for Wearable and Stretchable Electronics: From Flexibility to Stretchability

The rapid advancements of wearable electronics have caused a paradigm shift in consumer electronics, and the emerging development of stretchable electronics opens a new spectrum of applications for electronic systems. Playing a critical role as the power sources for independent electronic systems, energy harvesters with high flexibility or stretchability have been the focus of research efforts over the past decade. A large number of the flexible energy harvesters developed can only operate at very low strain level (≈0.1%), and their limited flexibility impedes their application in wearable or stretchable electronics. Here, the development of highly flexible and stretchable (stretchability >15% strain) energy harvesters is reviewed with emphasis on strategies of materials synthesis, device fabrication, and integration schemes for enhanced flexibility and stretchability. Due to their particular potential applications in wearable and stretchable electronics, energy‐harvesting devices based on piezoelectricity, triboelectricity, thermoelectricity, and dielectric elastomers have been largely developed and the progress is summarized. The challenges and opportunities of assembly and integration of energy harvesters into stretchable systems are also discussed.     

Materials and Designs for Wearable Photodetectors

Photodetectors (PDs), as an indispensable component in electronics, are highly desired to be flexible to meet the trend of next‐generation wearable electronics. Unfortunately, no in‐depth reviews on the design strategies, material exploration, and potential applications of wearable photodetectors are found in literature to date. Thus, this progress report first summarizes the fundamental design principles of turning “hard” photodetectors “soft,” including 2D (polymer and paper substrate‐based devices) and 1D PDs (fiber shaped devices). In short, the flexibility of PDs is realized through elaborate substrate modification, material selection, and device layout. More importantly, this report presents the current progress and specific requirements for wearable PDs according to the application: monitoring, imaging, and optical communication. Challenges and future research directions in these fields are proposed at the end. The purpose of this progress report is not only to shed light on the basic design principles of wearable PDs, but also serve as the roadmap for future exploration in wearable PDs in various applications, including health monitoring and Internet of Things.     

基于导电纤维的柔性电子器件研究进展

柔性电子器件具有优异的灵活性,实现了与服装的无缝集成,在各种实际的可穿戴应用中具有巨大的潜力。一维纤维状电子器件由于其优异的柔韧性、可编织性及舒适性成为智能可穿戴领域的研究热点。首先,综述了用于纤维状柔性电子器件的一维可拉伸电极的研究进展,然后详细介绍了高性能一维纤维状柔性电子器件制备过程中具有代表性的导电材料、制造技术及一维柔性纤维进一步应用于各类电子器件的主要制备方法,另外总结了近年来基于柔性纤维状电子器件在智能可穿戴领域的应用。最后对一维纤维基智能可穿戴电子器件的机遇和挑战进行了批判性思考。      

Graphene-Based Fibers: Recent Advances in Preparation and Application

Graphene-based fibers (GBFs) are macroscopic 1D assemblies formed by using microscopic 2D graphene sheets as building blocks. Their unique structure exhibits the same merits as graphene such as low weight, high specific surface area, excellent mechanical/electrical properties, and ease of functionalization. Furthermore, the fibrous nature of GBFs is intrinsically compatible with existing textile technologies, making them suitable for applications in flexible and wearable electronics. Recently, novel synthetic methods have endowed GBFs with new structures and functions, further improving their mechanical and electrical properties. These improvements have rapidly bridged the gaps between laboratory demonstrations and real-life applications in fiber-shaped batteries, supercapacitors, and electrochemical sensors. Recent advances in the fabrication, optimization, and application of GBFs are systematically reviewed and a perspective on their future development is given.     

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.     

Recent Advances in Fiber-Shaped Supercapacitors and Lithium-Ion Batteries

The rapid development in wearable electronics has spurred a great deal of interest in flexible energy storage devices, particularly fiber-shaped energy storage devices (FSESDs), such as fiber-shaped supercapacitors (FSSCs) and fiber-shaped batteries (FSBs). Depending on their electrode configurations, FSESDs can contain five differently structured electrodes, including parallel fiber electrodes (PFEs), twisted fiber electrodes (TFDs), wrapped fiber electrodes (WFEs), coaxial fiber devices (CFEs), and rolled electrodes (REs). Various rational methods have been devised to incorporate these fiber-shaped electrodes into multifunctional FSESDs, including fiber-shaped supercapacitors, lithium-ion batteries, lithium–sulfur batteries, lithium–air batteries, zinc–air batteries, and aluminum–air batteries. Although significant progress has been made in FSESDs, it remains a major challenge to make high-performance fiber-shaped devices at low cost. A focused and critical review of the recent advancements in fiber-shaped supercapacitors and lithium-ion batteries is provided here. The pros and cons for each of the aforementioned electrode configurations and FSESDs are discussed, along with current challenges and future opportunities for FSESDs.     

3D Printed Stretchable Tactile Sensors

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human–machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.     

Recent Progress of Textile‐Based Wearable Electronics: A Comprehensive Review of Materials,Devices, and Applications

Wearable electronics are emerging as a platform for next‐generation, human‐friendly, electronic devices. A new class of devices with various functionality and amenability for the human body is essential. These new conceptual devices are likely to be a set of various functional devices such as displays, sensors, batteries, etc., which have quite different working conditions, on or in the human body. In these aspects, electronic textiles seem to be a highly suitable possibility, due to the unique characteristics of textiles such as being light weight and flexible and their inherent warmth and the property to conform. Therefore, e‐textiles have evolved into fiber‐based electronic apparel or body attachable types in order to foster significant industrialization of the key components with adaptable formats. Although the advances are noteworthy, their electrical performance and device features are still unsatisfactory for consumer level e‐textile systems. To solve these issues, innovative structural and material designs, and novel processing technologies have been introduced into e‐textile systems. Recently reported and significantly developed functional materials and devices are summarized, including their enhanced optoelectrical and mechanical properties. Furthermore, the remaining challenges are discussed, and effective strategies to facilitate the full realization of e‐textile systems are suggested.     

Buckled Structures: Fabrication and Applications in Wearable Electronics

Wearable electronics have attracted a tremendous amount of attention due to their many potential applications, such as personalized health monitoring, motion detection, and smart clothing, where electronic devices must conformably form contacts with curvilinear surfaces and undergo large deformations. Structural design and material selection have been the key factors for the development of wearable electronics in the recent decades. As one of the most widely used geometries, buckling structures endow high stretchability, high mechanical durability, and comfortable contact for human–machine interaction via wearable devices. In addition, buckling structures that are derived from natural biosurfaces have high potential for use in cost‐effective and high‐grade wearable electronics. This review provides fundamental insights into buckling fabrication and discusses recent advancements for practical applications of buckled electronics, such as interconnects, sensors, transistors, energy storage, and conversion devices. In addition to the incorporation of desired functions, the simple and consecutive manipulation and advanced structural design of the buckled structures are discussed, which are important for advancing the field of wearable electronics. The remaining challenges and future perspectives for buckled electronics are briefly discussed in the final section.     

The Design of 3D‐Interface Architecture in an Ultralow‐Power,Electrospun Single‐Fiber Synaptic Transistor for Neuromorphic Computing

Synaptic electronics is a new technology for developing functional electronic devices that can mimic the structure and functions of biological counterparts. It has broad application prospects in wearable computing chips, human–machine interfaces, and neuron prostheses. These types of applications require synaptic devices with ultralow energy consumption as the effective energy supply for wearable electronics, which is still very difficult. Here, artificial synapse emulation is demonstrated by solid‐ion gated organic field‐effect transistors (OFETs) with a 3D‐interface conducting channel for ultralow‐power synaptic simulation. The basic features of the artificial synapse, excitatory postsynaptic current (EPSC), paired‐pulse facilitation (PPF), and high‐pass filtering, are successfully realized. Furthermore, the single‐fiber based artificial synapse can be operated by an ultralow presynaptic spike down to ?0.5 mV with an ultralow reading voltage at ?0.1 mV due to the large contact surface between the ionic electrolyte and fiber‐like semiconducting channel. Therefore, the ultralow energy consumption at one spike of the artificial synapse can be realized as low as ≈3.9 fJ, which provides great potential in a low‐power integrated synaptic circuit.     

Fabric Organic Electrochemical Transistors for Biosensors

Flexible fabric biosensors can find promising applications in wearable electronics. However, high‐performance fabric biosensors have been rarely reported due to many special requirements in device fabrication. Here, the preparation of organic electrochemical transistors (OECTs) on Nylon fibers is reported. By introducing metal/conductive polymer multilayer electrodes on the fibers, the OECTs show very stable performance during bending tests. The devices with functionalized gates are successfully used as various biosensors with high sensitivity and selectivity. The fiber‐based OECTs are woven together with cotton yarns successfully by using a conventional weaving machine, resulting in flexible and stretchable fabric biosensors with high performance. The fabric sensors show much more stable signals in the analysis of moving aqueous solutions than planar devices due to a capillary effect in fabrics. The fabric devices are integrated in a diaper and remotely operated by using a mobile phone, offering a unique platform for convenient wearable healthcare monitoring.     

Fiber/Fabric-Based Piezoelectric and Triboelectric Nanogenerators for Flexible/Stretchable and Wearable Electronics and Artificial Intelligence

Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile-based nanogenerators (NGs), which will inevitably promote the rapid development and widespread applications of next-generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self-powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric-based NGs with both excellent electrical output properties and outstanding textile-related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric-based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large-scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric-based NGs.     

Toward Environmentally Robust Organic Electronics: Approaches and Applications

Recent interest in flexible electronics has led to a paradigm shift in consumer electronics, and the emergent development of stretchable and wearable electronics is opening a new spectrum of ubiquitous applications for electronics. Organic electronic materials, such as π‐conjugated small molecules and polymers, are highly suitable for use in low‐cost wearable electronic devices, and their charge‐carrier mobilities have now exceeded that of amorphous silicon. However, their commercialization is minimal, mainly because of weaknesses in terms of operational stability, long‐term stability under ambient conditions, and chemical stability related to fabrication processes. Recently, however, many attempts have been made to overcome such instabilities of organic electronic materials. Here, an overview is provided of the strategies developed for environmentally robust organic electronics to overcome the detrimental effects of various critical factors such as oxygen, water, chemicals, heat, and light. Additionally, molecular design approaches to π‐conjugated small molecules and polymers that are highly stable under ambient and harsh conditions are explored; such materials will circumvent the need for encapsulation and provide a greater degree of freedom using simple solution‐based device‐fabrication techniques. Applications that are made possible through these strategies are highlighted.     

Mechanics of stretchable electronics

Recent advances in mechanics and materials provide routes to develop stretchable electronics that offer performance of conventional wafer-based devices but with the ability to be deformed to arbitrary shape. Many new applications become possible ranging from electronic eye cameras to wearable electronics, to bio-integrated therapeutic devices. This paper reviews mechanics of stretchable electronics in terms of two main forms of stretchable designs. One is wavy design, which can provide one-dimensional stretchability. The other is island-bridge design, which can be stretched in all directions. Mechanics models and their comparisons to experiments and finite element simulations are reviewed for these two designs. The results provide design guidelines for the development of stretchable electronics.     

3D Orthogonal Woven Triboelectric Nanogenerator for Effective Biomechanical Energy Harvesting and as Self‐Powered Active Motion Sensors

The development of wearable and large‐area energy‐harvesting textiles has received intensive attention due to their promising applications in next‐generation wearable functional electronics. However, the limited power outputs of conventional textiles have largely hindered their development. Here, in combination with the stainless steel/polyester fiber blended yarn, the polydimethylsiloxane‐coated energy‐harvesting yarn, and nonconductive binding yarn, a high‐power‐output textile triboelectric nanogenerator (TENG) with 3D orthogonal woven structure is developed for effective biomechanical energy harvesting and active motion signal tracking. Based on the advanced 3D structural design, the maximum peak power density of 3D textile can reach 263.36 mW m^?2 under the tapping frequency of 3 Hz, which is several times more than that of conventional 2D textile TENGs. Besides, its collected power is capable of lighting up a warning indicator, sustainably charging a commercial capacitor, and powering a smart watch. The 3D textile TENG can also be used as a self‐powered active motion sensor to constantly monitor the movement signals of human body. Furthermore, a smart dancing blanket is designed to simultaneously convert biomechanical energy and perceive body movement. This work provides a new direction for multifunctional self‐powered textiles with potential applications in wearable electronics, home security, and personalized healthcare.     

Textile Display for Electronic and Brain‐Interfaced Communications

Textile displays are poised to revolutionize current electronic devices, and reshape the future of electronics and related fields such as biomedicine and soft robotics. However, they remain unavailable due to the difficulty of directly constructing electroluminescent devices onto the textile‐like substrate to really display desired programmable patterns. Here, a novel textile display is developed from continuous electroluminescent fibers made by a one‐step extrusion process. The resulting displaying textile is flexible, stretchable, three‐dimensionally twistable, conformable to arbitrarily curved skins, and breathable, and can dynamically display a series of desired patterns, making it useful for bioinspired electronics, soft robotics, and electroluminescent skins, among other applications. It is demonstrated that these displaying textiles can also communicate with a computer and mouse brain for smart display and camouflage applications. This work may open up a new direction for the integration of wearable electroluminescent devices with the human body, providing new and promising communication platforms.     

Fiber-Based Energy Conversion Devices for Human-Body Energy Harvesting

Following the rapid development of lightweight and flexible smart electronic products, providing energy for these electronics has become a hot research topic. The human body produces considerable mechanical and thermal energy during daily activities, which could be used to power most wearable electronics. In this context, fiber-based energy conversion devices (FBECD) are proposed as candidates for effective conversion of human-body energy into electricity for powering wearable electronics. Herein, functional materials, fiber fabrication techniques, and device design strategies for different classes of FBECD based on piezoelectricity, triboelectricity, electrostaticity, and thermoelectricity are comprehensively reviewed. An overview of fiber-based self-powered systems and sensors according to their superior flexibility and cost-effectiveness is also presented. Finally, the challenges and opportunities in the field of fiber-based energy conversion are discussed.