Highly Stretchable Fiber-Shaped Supercapacitors Based on Ultrathin Gold Nanowires with Double-Helix Winding Design

A review of advanced helical fibers: formation mechanism, preparation, properties, and applications

As a unique structural form, helical structures have a wide range of application prospects. In the field of biology, helical structures are essential for the function of biological macromolecules such as proteins, so the study of helical structures can help to deeply understand life phenomena and develop new biotechnology. In materials science, helical structures can give rise to special physical and chemical properties, such as in the case of spiral nanotubes, helical fibers, etc., which are expected to be used in energy, environment, medical and other fields. The helical structure also has unique charm and application value in the fields of aesthetics and architecture. In addition, helical fibers have attracted a lot of attention because of their tendrils' vascular geometry and indispensable structural properties. In this paper, the development of helical fibers is briefly reviewed from the aspects of mechanism, synthesis process and application. Due to their good chemical and physical properties, helical fibers have a good application prospect in many fields. Potential problems and future opportunities for helical fibers are also presented for future studies.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

A Review on Recent Trends and Future Developments in Electrochemical Sensing

Electrochemical methods and devices have ignited prodigious interest for sensing and monitoring. The greatest challenge for science is far from meeting the expectations of consumers. Electrodes made of two-dimensional (2D) materials such as graphene, metal-organic frameworks, MXene, and transition metal dichalcogenides as well as alternative electrochemical sensing methods offer potential to improve selectivity, sensitivity, detection limit, and response time. Moreover, these advancements have accelerated the development of wearable and point-of-care electrochemical sensors, opening new possibilities and pathways for their applications. This Review presents a critical discussion of the recent developments and trends in electrochemical sensing.

Enhanced supercapacitor performance of a Cu-Fe2O3/g-C3N4 composite material: synthesis, characterization, and electrochemical analysis

A Cu-doped Fe2O3/g-C3N4 composite, synthesized via a straightforward hydrothermal process with controlled morphologies, represents a significant advancement in supercapacitor electrode materials. This study systematically analyzes the impact of Cu doping in Fe2O3 and its synergistic combination with g-C3N4 to understand their influence on the electrochemical performance of the resulting composite, focusing on Cu doping in Fe2O3 rather than varying Fe2O3/g-C3N4 content. The comprehensive characterization of these composites involved a suite of physicochemical techniques. X-ray diffraction (XRD) confirmed the successful synthesis of the composite, while field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were employed to investigate the morphological attributes of the synthesized materials. X-ray photoelectron spectroscopy (XPS) spectra confirmed the elemental composition of the composite with 6% Cu doped Fe2O3/g-C3N4. The composite electrode, which incorporated 6% Cu doped Fe2O3 with g-C3N4, exhibited exceptional cycling stability, retaining 94.22% of its capacity even after 2000 charge-discharge cycles at a current density of 5 mA cm-2. Furthermore, this Cu doped Fe2O3/g-C3N4 composite electrode demonstrated impressive electrochemical performance, boasting a specific capacitance of 244.0 F g-1 and an impressive maximum energy density of 5.31 W h kg-1 at a scan rate of 5 mV s-1. These findings highlight the substantial potential of the Cu doped Fe2O3/g-C3N4 electrode for supercapacitor applications.

A Review of Yarn-Based One-Dimensional Supercapacitors

Energy storage in a one-dimensional format is increasingly vital for the functionality of wearable technologies and is garnering attention from various sectors, such as smart apparel, the Internet of Things, e-vehicles, and robotics. Yarn-based supercapacitors are a particularly compelling solution for wearable energy reserves owing to their high power densities and adaptability to the human form. Furthermore, these supercapacitors can be seamlessly integrated into textile fabrics for practical utility across various types of clothing. The present review highlights the most recent innovations and research directions related to yarn-based supercapacitors. Initially, we explore different types of electrodes and active materials, ranging from carbon-based nanomaterials to metal oxides and conductive polymers, that are being used to optimize electrochemical capacitance. Subsequently, we survey different methodologies for loading these active materials onto yarn electrodes and summarize innovations in stretchable yarn designs, such as coiling and buckling. Finally, we outline a few pressing research challenges and future research directions in this field.

Molecular Modification of Lignin-Based Carbon Materials: Influence of Supramolecular Bonds on the Properties

For the application of lignin-based materials, it is necessary to develop simple and efficient chemical modification strategies for lignin. In this work, the iodization modification strategy is selected to improve the specific surface area and graphitization degree of lignin-based carbon fibers. The introduction of an iodine atom can effectively increase the π electron cloud density of the lignin aromatic hydrocarbon structure. High π electron cloud density can effectively enhance the π-π interaction force between lignin molecules (the supramolecular bonds). The biomass precursors with this intermolecular microstructure exhibit good thermal stability and can maintain the original fibrous morphology during high-temperature treatment, which is beneficial for increasing the specific surface area of biomass-based carbon materials. Furthermore, this intermolecular microstructure also contributes to the graphitization of biomass precursor materials and reduces the spacing of graphite micro-lamellae. The obtained lignin-based carbon fibers with iodization modification exhibit a specific capacitance of 333 F/g at a current density of 1 A/g in the three-electrode tests in 6 M KOH solution. As the assembled supercapacitor, the specific capacitance of lignin-based carbon fibers reaches 87 F/g in 1 M Na₂SO₄ solution. Compared to other modification processes for raw materials, this strategy is simple and efficient and has reference value for the synthesis of other high-performance biomass-based materials.

Advances in constructing silver nanowire-based conductive pathways for flexible and stretchable electronics

With their soaring technological demand, flexible and stretchable electronics have attracted many researchers' attention for a variety of applications. The challenge which was identified a decade ago and still remains, however, is that the conventional electrodes based on indium tin oxide (ITO) are not suitable for ultra-flexible electronic devices. The main reason is that ITO is brittle and expensive, limiting device performance and application. Thus, it is crucial to develop new materials and processes to construct flexible and stretchable electrodes with superior quality for next-generation soft devices. Herein, various types of conductive nanomaterials as candidates for flexible and stretchable electrodes are briefly reviewed. Among them, silver nanowire (AgNW) is selected as the focus of this review, on account of its excellent conductivity, superior flexibility, high technological maturity, and significant presence in the research community. To fabricate a reliable AgNW-based conductive network for electrodes, different processing technologies are introduced, and the corresponding characteristics are compared and discussed. Furthermore, this review summarizes strategies and the latest progress in enhancing the conductive pathway. Finally, we showcase some exemplary applications and provide some perspectives about the remaining technical challenges for future research.

Recent Progress in Metal Nanowires for Flexible Energy Storage Devices

With the rapid evolution of wearable electronics, the demand for flexible energy storage devices is gradually increasing. At present, the commonly used energy storage devices in life are based on rigid frames, which may lead to failure or explosion when mechanical deformation occurs. The main reason for this phenomenon is the insufficient elastic limit of the metal foil current collector with a simple plane structure inside the electrodes. Obviously, the design and introduction of innovative structural materials in current collectors is the key point to solving this problem. Several recent studies have shown that metal nanowires can be used as novel current collector materials to fabricate flexible energy storage devices. Herein, we review the applications of metal nanowires in the field of flexible energy storage devices by selecting the three most representative metals (Au, Ag, and Cu). By the analysis of the various typical literature, the advantages and disadvantages of these three metal nanowires (Au, Ag, and Cu) are discussed respectively. Finally, we look forward to the development direction of one-dimensional (1D) metal nanowires in flexible energy storage devices and show the personal opinions with a reference value, hoping to provide the experience and ideas for related research in the future.

Conductive Materials with Elaborate Micro/Nanostructures for Bioelectronics

Bioelectronics, an emerging field with the mutual penetration of biological systems and electronic sciences, allows the quantitative analysis of complicated biosignals together with the dynamic regulation of fateful biological functions. In this area, the development of conductive materials with elaborate micro/nanostructures has been of great significance to the improvement of high-performance bioelectronic devices. Thus, here, a comprehensive and up-to-date summary of relevant research studies on the fabrication and properties of conductive materials with micro/nanostructures and their promising applications and future opportunities in bioelectronic applications is presented. In addition, a critical analysis of the current opportunities and challenges regarding the future developments of conductive materials with elaborate micro/nanostructures for bioelectronic applications is also presented.

Fiber‐Shaped Electronic Devices

Textile electronics embedded in clothing represent an exciting new frontier for modern healthcare and communication systems. Fundamental to the development of these textile electronics is the development of the fibers forming the cloths into electronic devices. An electronic fiber must undergo diverse scrutiny for its selection for a multifunctional textile, viz., from the material selection to the device architecture, from the wearability to mechanical stresses, and from the environmental compatibility to the end‐use management. Herein, the performance requirements of fiber‐shaped electronics are reviewed considering the characteristics of single electronic fibers and their assemblies in smart clothing. Broadly, this article includes i) processing strategies of electronic fibers with required properties from precursor to material, ii) the state‐of‐art of current fiber‐shaped electronics emphasizing light‐emitting devices, solar cells, sensors, nanogenerators, supercapacitors storage, and chromatic devices, iii) mechanisms involved in the operation of the above devices, iv) limitations of the current materials and device manufacturing techniques to achieve the target performance, and v) the knowledge gap that must be minimized prior to their deployment. Lessons learned from this review with regard to the challenges and prospects for developing fiber‐shaped electronic components are presented as directions for future research on wearable electronics.

Soft gold nanowire sponge antenna for battery-free wireless pressure sensors

The past decade has witnessed growing interest in developing soft wearable pressure sensors with the ultimate goal of transforming today's hospital-centered diagnosis to tomorrow's patient-centered bio-diagnosis. In this context, battery-free wireless antenna-based pressure sensors will be highly advantageous for ubiquitous real-time health monitoring. However, current wireless antennas are largely based on thin films from traditional bulk metallic films or novel nanomaterials with an air-cavity design, which can only be operated in a limited pressure range due to the rigidity of active films and/or inherent cavity dimensions. Herein we report a soft battery-free wireless pressure sensor that is based on a three-dimensional (3D) porous gold nanowire foam-elastomer composite and is fabricated by solution-based conformal electroless plating technology, followed by elastomer encapsulation. We observe a transducer trade-off point for our foam antenna, below which the inductive effect and capacitive effect function together and above which the capacitive effect dominates. When an external pressure is applied, initially the inductance and capacitance increase simultaneously but the capacitance decreases afterwards. This can be transformed into a variable resonant frequency that first decreases linearly and then increases (in the capacitance domination pressure range). Importantly, the linear detection range of the sensor can be tuned simply by adjusting the thickness of the sponge or the rigidity of the elastomer (PDMS). We can achieve a wide pressure range of 0-248 kPa, which is the largest linear detection range reported in the literature (typically from 0 to 30 kPa) to the best of our knowledge. As a proof of concept, we further demonstrated that our gold nanowire foam sensor can be used to weigh people under both static and dynamic conditions.

Highly Stretchable Fiber-Based Potentiometric Ion Sensors for Multichannel Real-Time Analysis of Human Sweat

Wearable potentiometric ion sensors are attracting attention for real-time ion monitoring in biological fluids. One of the key challenges lies in keeping the analytical performances under a stretchable state. Herein, we report a highly stretchable fiber-based ion-selective electrode (ISE) prepared by coating an ion-selective membrane (ISM) on a stretchable gold fiber electrode. The fiber ISE ensures high stretchability up to 200% strain with only 2.1% increase in resistance of the fiber electrode. Owing to a strong attachment between the ISM and gold fiber electrode substrate, the ISE discloses favorable stability and potential repeatability. The Nernst slope of the ion response fluctuates from 59.2 to 57.4 mV/dec between 0 and 200% strain. Minor fluctuation of the intercept (E⁰) (±4.97 mV) also results. The ISE can endure 1000 cycles at the maximum stretch. Sodium, chloride, and pH fiber sensors were fabricated and integrated into a hairband for real-time analysis of human sweat. The result displays a high accuracy compared with ex situ analysis. The integrated sensors were calibrated before and just after on-body measurements, and they offer reliable results for sweat analysis.

Smart Glove Integrated with Tunable MWNTs/PDMS Fibers Made of a One-Step Extrusion Method for Finger Dexterity, Gesture, and Temperature Recognition

Flexible wearable devices have proven to be emerging tools for motion monitoring, personal healthcare, and rehabilitation training. The development of a multifunctional, flexible sensor and the integration of sensors and a smart chip for signal reading and transmission play a critical role in building a smart wearable device. In this work, a smart glove based on multiwalled carbon nanotubes/poly(dimethylsiloxane) (MWNTs/PDMS) fibers is developed for gesture and temperature recognition. First, the well-tunable, stretchable, and thermal-sensitive MWNTs/PDMS fibers are fabricated via a facile and cost-effective one-step extrusion method. The obtained fibers exhibit an outstanding linear relationship between resistance change and strain in the range of 0-120% and excellent cyclic stability and durability after 20 000 cycles of 50% tension. They also present a linear relationship of resistance change and temperature of 0.55% °C^-1 with a correlation coefficient of 0.998 in the range of 0-100 °C. The fibers, as parts of wearable sensors, are then integrated into a smart glove along with a custom-made data acquisition chip to recognize finger dexterity, gestures, and temperature signals and output them through a screen display, an audio system, and Bluetooth transmission. The highly integrated, low-cost, and multifunctional glove holds great potential for various applications, such as sign language recognition, rehabilitation training, and telemedicine in the Internet-of-Things era.

Disruptive, Soft, Wearable Sensors

The wearable industry is on the rise, with a myriad of technical applications ranging from real-time health monitoring, the Internet of Things, and robotics, to name but a few. However, there is a saying "wearable is not wearable" because the current market-available wearable sensors are largely bulky and rigid, leading to uncomfortable wearing experience, motion artefacts, and poor data accuracy. This has aroused a world-wide intensive research quest for novel materials, with the aim of fabricating next-generation ultra-lightweight and soft wearable devices. Such disruptive second-skin-like biosensing technologies may enable a paradigm shift from current wearable 1.0 to future wearable 2.0 products. Here, the state-of-the-art progress made in the key phases for future wearable technology, namely, wear → sense → communicate → analyze → interpret → decide, is summarized. Without a doubt, materials innovation is the key, which is the main focus of the discussion. In addition, emphasis is also given to wearable energy, multicomponent integration, and wireless communication.

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.

Application Challenges in Fiber and Textile Electronics

Modern electronic devices are moving toward miniaturization and integration with an emerging focus on wearable electronics. Due to their close contact with the human body, wearable electronics have new requirements including low weight, small size, and flexibility. Conventional 3D and 2D electronic devices fail to efficiently meet these requirements due to their rigidity and bulkiness. Hence, a new family of 1D fiber-shaped electronic devices including energy-harvesting devices, energy-storage devices, light-emitting devices, and sensing devices has risen to the challenge due to their small diameter, lightweight, flexibility, and weavability into soft textile electronics. The application challenges faced by fiber and textile electronics from single fiber-shaped devices to continuously scalable fabrication, to encapsulation and testing, and to application mode exploration, are discussed. The evolutionary trends of fiber and textile electronics are then summarized. Finally, future directions required to boost their commercialization are highlighted.

Highly Stretchable and Strain-Insensitive Fiber-Based Wearable Electrochemical Biosensor to Monitor Glucose in the Sweat

Development of high-performance fiber-shaped wearable sensors is of great significance for next-generation smart textiles for real-time and out-of-clinic health monitoring. The previous focus has been mainly on monitoring physical parameters such as pressure and strains associated with human activities. Development of an enzyme-based non-invasive wearable electrochemical sensor to monitor biochemical vital signs of health such as the glucose level in sweat has attracted increasing attention recently, due to the unmet clinical needs for the diabetic patients. To achieve this, the key challenge lies in the design of a highly stretchable fiber with high conductivity, facile enzyme immobilization, and strain-insensitive properties. Herein, we demonstrate an elastic gold fiber-based three-electrode electrochemical platform that can meet the aforementioned criteria toward wearable textile glucose biosensing. The gold fiber could be functionalized with Prussian blue and glucose oxidase to obtain the working electrode and modified by Ag/AgCl to serve as the reference electrode; and the nonmodified gold fiber could serve as the counter electrode. The as-fabricated textile glucose biosensors achieved a linear range of 0-500 μM and a sensitivity of 11.7 μA mM^-1 cm^-2. Importantly, such sensing performance could be maintained even under a large strain of 200%, indicating the potential applications in real-world wearable biochemical diagnostics from human sweat.