Fiber-based structures are highly desirable for wearable electronics that are expected to be light-weight, long-lasting, flexible, and conformable Many fibrous structures have been manufactured by well-established lost-effective textile processing technologies, normally at ambient conditions The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens microns However, imparting electronic functions to porous, highly deformable and three-dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation This article attempts to critically review the current state-of-arts with respect to materials, fabrication techniques, and structural design of devices as well as applications of the fiber-based wearable electronic products In addition, this review elaborates the performance requirements of the fiber-based wearable electronic products, especially regarding the correlation among materials, fiber/textile structures and electronic as well as mechanical functionalities of fiber-based electronic devices Finally, discussions will be presented regarding to limitations of current materials, fabrication techniques, devices concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption

Fiber‐Based Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications

Covalent organic frameworks (COFs) are a class of crystalline porous polymer that allows the atomically precise integration of organic units into extended structures with periodic skeletons and ordered nanopores. One important feature of COFs is that they are designable; that is, the geometry and dimensions of the building blocks can be controlled to direct the topological evolution of structural periodicity. The diversity of building blocks and covalent linkage topology schemes make COFs an emerging materials platform for structural control and functional design. Indeed, COF architectures offer confined molecular spaces for the interplay of photons, excitons, electrons, holes, ions and guest molecules, thereby exhibiting unique properties and functions. In this Review, we summarize the major progress in the field of COFs and recent achievements in developing new design principles and synthetic strategies. We highlight cutting-edge functional designs and identify fundamental issues that need to be addressed in conjunction with future research directions from chemistry, physics and materials perspectives. Covalent organic frameworks are crystalline porous polymers with precisely ordered polygon architectures. In this Review we summarize recent advances in the design principles and synthetic reactions, highlight the current status in structural construction and functionality design, and predict challenging issues and future directions.

Covalent organic frameworks: a materials platform for structural and functional designs

Flexible energy-storage devices are attracting increasing attention as they show unique promising advantages, such as flexibility, shape diversity, light weight, and so on; these properties enable applications in portable, flexible, and even wearable electronic devices, including soft electronic products, roll-up displays, and wearable devices. Consequently, considerable effort has been made in recent years to fulfill the requirements of future flexible energy-storage devices, and much progress has been witnessed. This review describes the most recent advances in flexible energy-storage devices, including flexible lithium-ion batteries and flexible supercapacitors. The latest successful examples in flexible lithium-ion batteries and their technological innovations and challenges are reviewed first. This is followed by a detailed overview of the recent progress in flexible supercapacitors based on carbon materials and a number of composites and flexible micro-supercapacitors. Some of the latest achievements regarding interesting integrated energy-storage systems are also reviewed. Further research direction is also proposed to surpass existing technological bottle-necks and realize idealized flexible energy-storage devices.

Flexible Energy-Storage Devices: Design Consideration and Recent Progress

Recent advancements in supercapacitor technology

High-energy yarn supercapacitors are desirable for safe and wearable electronics. Here, Kou et al. use a coaxial wet-spinning assembly method to fabricate core-sheath fibres of polymer-wrapped carbon nanomaterials and demonstrate high-performance supercapacitor applications.

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Coaxial wet-spun yarn supercapacitors for high-energy density and safe wearable electronics

Fine count two-ply yarn supercapacitors are constructed from carbon nanotube yarns and polyaniline nanowires. The thread-like supercapacitor possess excellent electrochemical capacity and are very strong and flexible. When being woven or knitted into wearable electronic devices, alone or in combination with conventional textile yarns, the two-ply yarn supercapacitors can be flexed and stretched repeatedly without significant loss of capacitance.

High‐Performance Two‐Ply Yarn Supercapacitors Based on Carbon Nanotubes and Polyaniline Nanowire Arrays

A high-strength poly(vinyl alcohol) chemical hydrogel (PCH) film is prepared by coupling covalent crosslinking with a film-casting process. Conducting polyaniline (PANI) is then embedded in the PCH film by in situ growth to form a composite film with a PANI-hydrogel-PANI configuration, which leads to a new conceptual flexible supercapacitor with all-in-one configuration that exhibits superior electrochemical performance and mechanical flexibility.

Chemically Crosslinked Hydrogel Film Leads to Integrated Flexible Supercapacitors with Superior Performance

Single-walled carbon nanotubes and chitosan composite yarn is prepared using a wet-spinning method After thermal treatment, mesoporous all-carbon yarn is obtained Based on this material, flexible all-solid-state yarn micro-supercapacitors are fabricated Electrochemical results show high specific capacitance and energy density, good rate capability and stable cycling life Results of this research offer prospect for application in portable and wearable electronics © 2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

High-performance all-carbon yarn micro-supercapacitor for an integrated energy system.

Fabrication of a flexible organic electrode by growing polyimide nanoflakes on single-wall carbon nanotube films is presented. The flexible electrode exhibits high capacity and outstanding rate capability. This electrode is promising for the application in high-power flexible lithium-Ion batteries.

Flexible and binder-free organic cathode for high-performance lithium-ion batteries.

Flexible energy storage devices with excellent mechanical deformation performance are highly required to improve the integration degree of flexible electronics. Unlike those of traditional power sources, the mechanical reliability of flexible energy storage devices, including electrical performance retention and deformation endurance, has received much attention. To provide the guideline for the construction design of devices, the strain distribution and failure modes in the entire architecture should be comprehensively investigated during mechanical deformation. This review mainly focuses on the mechanical deformation characterization, analysis, and structural design strategies used in recent flexible lithium-ion batteries (LIBs) and supercapacitors (SCs). The primary theoretical calculation of bending strain in the devices is introduced first, and then several parameters to describe the bending status are summarized. Among those parameters, bending radius and its corresponding test methods and equipment are highlighted. Derivative strategies for structural design and an overview of their application progresses, such as selection of substrates, thickness of devices, employment of encapsulation, and novel architectural designs, are reviewed in detail. Finally, the challenges and prospects of flexible energy storage devices with reliable mechanical performance are discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.201700535

Qinghai Meng

Papers

High‐Performance Two‐Ply Yarn Supercapacitors Based on Carbon Nanotubes and Polyaniline Nanowire Arrays

Chemically Crosslinked Hydrogel Film Leads to Integrated Flexible Supercapacitors with Superior Performance

High-performance all-carbon yarn micro-supercapacitor for an integrated energy system.

Flexible and binder-free organic cathode for high-performance lithium-ion batteries.

Mechanical Analyses and Structural Design Requirements for Flexible Energy Storage Devices