Fiber-Type Solar Cells, Nanogenerators, Batteries, and Supercapacitors for Wearable Applications

Large-Area Fabrication of Porous Graphene Networks on Carbon Fabric via Millisecond Photothermal Processing of Polyaniline for Supercapacitors

Supercapacitors demonstrate promising potential for flexible, multi-functional energy storage devices; however, their widespread adoption is confronted by fabrication challenges. To access a combination of desirable device qualities such as flexibility, lightweight, structural stability, and enhanced electrochemical performance, carbon fiber (CF) can be utilized as a current collector, alongside graphene as an electrochemically active material. Yet achieving a cost-effective, large-scale graphene production, particularly on CF, remains challenging. Here, a rapid (<1 min) photothermal approach is developed for the large-scale production of graphene directly onto CF, utilizing polyaniline (PANI) as a polymer precursor. The in situ electropolymerization of PANI on CF facilitates its rapid synthesis on large areas, followed by conversion into graphene networks, enabling the binder-free fabrication of supercapacitor devices. These devices exhibit an areal capacitance of 180 mF cm-2 (at 2 mA cm-2 in 1 m H2SO4), an order of magnitude higher than other fabric-based devices. Moreover, the devised photothermal strategy allows for one-step preparation of supercapacitor devices on areas exceeding 100 cm-2, yielding an absolute areal capacitance of 4.5 F. The proportional increase in capacitance with device area facilitates scaling and indicates the commercial viability of this approach for low-cost, energy-efficient, and high-throughput production of lightweight, high-performance graphene-based multi-functional supercapacitor devices.

A Review of Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Zinc-air batteries (ZABs) are gaining attention as an ideal option for various applications requiring high-capacity batteries, such as portable electronics, electric vehicles, and renewable energy storage. ZABs offer advantages such as low environmental impact, enhanced safety compared to Li-ion batteries, and cost-effectiveness due to the abundance of zinc. However, early research faced challenges due to parasitic reactions at the zinc anode and slow oxygen redox kinetics. Recent advancements in restructuring the anode, utilizing alternative electrolytes, and developing bifunctional oxygen catalysts have significantly improved ZABs. Scientists have achieved battery reversibility over thousands of cycles, introduced new electrolytes, and achieved energy efficiency records surpassing 70%. Despite these achievements, there are challenges related to lower power density, shorter lifespan, and air electrode corrosion leading to performance degradation. This review paper discusses different battery configurations, and reaction mechanisms for electrically and mechanically rechargeable ZABs, and proposes remedies to enhance overall battery performance. The paper also explores recent advancements, applications, and the future prospects of electrically/mechanically rechargeable ZABs.

Recent advances in integrated solar cell/supercapacitor devices: Fabrication, strategy and perspectives

BACKGROUND

Solar cell/supercapacitor integrated devices (SCSD) have made some progress in terms of device structure and electrode materials, but there are still many key challenges in controlling electrode performance and improving the efficiency of integrated devices.

AIM OF REVIEW

It is necessary to study how to balance the photoelectric conversion process and the storage process. From the microscopic mechanism of different functional unit materials to the mechanism of macroscopic devices, it is essential to conduct in-depth research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Here, the structures and preparation methods of various types of integrated SCSD were introduced. Then, the strategies for improving the overall performance of integrated devices were evaluated. Finally, the key objectives of reducing the cost of materials, increasing the stability and sustainability of devices were highlighted. Better matching of different functional units of devices was also prospected.

Advances in Smart Photovoltaic Textiles

Energy harvesting textiles have emerged as a promising solution to sustainably power wearable electronics. Textile-based solar cells (SCs) interconnected with on-body electronics have emerged to meet such needs. These technologies are lightweight, flexible, and easy to transport while leveraging the abundant natural sunlight in an eco-friendly way. In this Review, we comprehensively explore the working mechanisms, diverse types, and advanced fabrication strategies of photovoltaic textiles. Furthermore, we provide a detailed analysis of the recent progress made in various types of photovoltaic textiles, emphasizing their electrochemical performance. The focal point of this review centers on smart photovoltaic textiles for wearable electronic applications. Finally, we offer insights and perspectives on potential solutions to overcome the existing limitations of textile-based photovoltaics to promote their industrial commercialization.

Built-In Piezoelectric Nanogenerators Promote Sustainable and Flexible Supercapacitors: A Review

Energy storage devices such as supercapacitors (SCs), if equipped with built-in energy harvesters such as piezoelectric nanogenerators, will continuously power wearable electronics and become important enablers of the future Internet of Things. As wearable gadgets become flexible, energy items that can be fabricated with greater compliance will be crucial, and designing them with sustainable and flexible strategies for future use will be important. In this review, flexible supercapacitors designed with built-in nanogenerators, mainly piezoelectric nanogenerators, are discussed in terms of their operational principles, device configuration, and material selection, with a focus on their application in flexible wearable electronics. While the structural design and materials selection are highlighted, the current shortcomings and challenges in the emerging field of nanogenerators that can be integrated into flexible supercapacitors are also discussed to make wearable devices more comfortable and sustainable. We hope this work may provide references, future directions, and new perspectives for the development of electrochemical power sources that can charge themselves by harvesting mechanical energy from the ambient environment.

Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

The development of next-generation bioelectronics, as well as the powering of consumer and medical devices, require power sources that are soft, flexible, extensible, and even biocompatible. Traditional energy storage devices (typically, batteries and supercapacitors) are rigid, unrecyclable, offer short-lifetime, contain hazardous chemicals and possess poor biocompatibility, hindering their utilization in wearable electronics. Therefore, there is a genuine unmet need for a new generation of innovative energy-harvesting materials that are soft, flexible, bio-compatible, and bio-degradable. Piezoelectric gels or PiezoGels are a smart crystalline form of gels with polar ordered structures that belongs to the broader family of piezoelectric material, which generate electricity in response to mechanical stress or deformation. Given that PiezoGels are structurally similar to hydrogels, they offer several advantages including intrinsic chirality, crystallinity, degree of ordered structures, mechanical flexibility, biocompatibility, and biodegradability, emphasizing their potential applications ranging from power generation to bio-medical applications. Herein, we describe recent examples of new functional PiezoGel materials employed for energy harvesting, sensing, and wound dressing applications. First, this review focuses on the principles of piezoelectric generators (PEGs) and the advantages of using hydrogels as PiezoGels in energy and biomedical applications. Next, we provide a detailed discussion on the preparation, functionalization, and fabrication of PiezoGel-PEGs (P-PEGs) for the applications of energy harvesting, sensing and wound healing/dressing. Finally, this review concludes with a discussion of the current challenges and future directions of P-PEGs.

Micro/Nano-Fabrication of Flexible Poly(3,4-Ethylenedioxythiophene)-Based Conductive Films for High-Performance Microdevices

With the increasing demands for novel flexible organic electronic devices, conductive polymers are now becoming the rising star for reaching such targets, which has witnessed significant breakthroughs in the fields of thermoelectric devices, solar cells, sensors, and hydrogels during the past decade due to their outstanding conductivity, solution-processing ability, as well as tailorability. However, the commercialization of those devices still lags markedly behind the corresponding research advances, arising from the not high enough performance and limited manufacturing techniques. The conductivity and micro/nano-structure of conductive polymer films are two critical factors for achieving high-performance microdevices. In this review, the state-of-the-art technologies for developing organic devices by using conductive polymers are comprehensively summarized, which will begin with a description of the commonly used synthesis methods and mechanisms for conductive polymers. Next, the current techniques for the fabrication of conductive polymer films will be proffered and discussed. Subsequently, approaches for tailoring the nanostructures and microstructures of conductive polymer films are summarized and discussed. Then, the applications of micro/nano-fabricated conductive films-based devices in various fields are given and the role of the micro/nano-structures on the device performances is highlighted. Finally, the perspectives on future directions in this exciting field are presented.

Organic Solar Cells: Physical Principle and Recent Advances

Organic solar cells (OSC) based on organic semiconductor materials that convert solar energy into electric energy have been constantly developing at present, and also an effective way to solve the energy crisis and reduce carbon emissions. In the past several decades, efforts have been made to improve the power conversion efficiency (PCE) of OSCs. During this period, a variety of structural and material forms of OSCs have evolved. Commercializing OSCs, extending their service life and exploring their future development are promising but challenging. In this review, we first briefly introduce the development of OSCs and then summarize and analyze the working principle, performance parameters, and structural features of OSCs. Finally, we highlight some breakthrough related to OSCs in detail.

Highly Efficient and Stable Self-Powered Mixed Tin-Lead Perovskite Photodetector Used in Remote Wearable Health Monitoring Technology

Realization of remote wearable health monitoring (RWHM) technology for the flexible photodiodes is highly desirable in remote-sensing healthcare systems used in space stations, oceans, and forecasting warning, which demands high external quantum efficiency (EQE) and detectivity in NIR region. Traditional inorganic photodetectors (PDs) are mechanically rigid and expensive while the widely reported solution-processed mixed tin-lead (MSP) perovskite photodetectors (PPDs) exhibit a trade-off between EQE and detectivity in the NIR region. Herein, a novel functional passivating antioxidant (FPA) strategy has been introduced for the first time to simultaneously improve crystallization, restrain Sn2+ oxidization, and reduce defects in MSP perovskite films by multiple interactions between thiophene-2-carbohydrazide (TAH) molecules and cations/anions in MSP perovskite. The resultant solution-processed rigid mixed Sn-Pb PPD simultaneously achieves high EQE (75.4% at 840 nm), detectivity (1.8 × 1012 Jones at 840 nm), ultrafast response time (trise /tfall = 94 ns/97 ns), and improved stability. This work also highlights the demonstration of the first flexible photodiode using MSP perovskite and FPA strategy with remarkably high EQE (75% at 840 nm), and operational stability. Most importantly, the RWHM is implemented for the first time in the PIN MSP perovskite photodiodes to remotely monitor the heart rate of humans at rest and after-run conditions.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Energized Composites for Electric Vehicles: A Dual Function Energy-Storing Supercapacitor-Based Carbon Fiber Composite for the Body Panels

The current electric vehicles (EVs) face many challenges like limited charge capacity, low miles/charge, and long charging times. Herein, these issues are addressed by developing a dual-function supercapacitor-based energy-storing carbon fiber reinforced polymer (e-CFRP) that can store electrical energy and function as the structural component for the EV's body shell. This is achieved by developing a unique design, vertically aligned graphene sheets attached to carbon fiber electrodes on which different metal oxides are deposited to obtain high-energy density electrodes. A high-strength multilayer e-CFRP assembly is fabricated using an alternate layer patterning configuration of epoxy and polyacrylamide gel electrolyte. The e-CFRP so developed delivers a high areal energy density of 0.31 mWh cm^-2 at 0.3 mm thickness and a high tensile strength of 518 MPa, bending strength of 477 MPa, and impact strength of 2666 J m^-1 . To show its application in EVs, a toy car's body panel is fabricated with e-CFRP and the toy car is able to operate using the energy stored in its frame. Moreover, when integrated with a solar cell, this composite powers an Internet of Things device, showing its feasibility in communication satellites.

A leather-based electrolyte for all-in-one configured flexible supercapacitors

Leather based gel electrolytes were prepared from the top down method, and integrated flexible supercapacitors were developed by this method.

Ultra-Flexible Organic Solar Cell Based on Indium-Zinc-Tin Oxide Transparent Electrode for Power Source of Wearable Devices

We have developed a novel structure of ultra-flexible organic photovoltaics (UFOPVs) for application as a power source for wearable devices with excellent biocompatibility and flexibility. Parylene was applied as an ultra-flexible substrate through chemical vapor deposition. Indium-zinc-tin oxide (IZTO) thin film was used as a transparent electrode. The sputtering target composed of 70 at.% In₂O₃-15 at.% ZnO-15 at.% SnO₂ was used. It was fabricated at room temperature, using pulsed DC magnetron sputtering, with an amorphous structure. UFOPVs, in which a 1D grating pattern was introduced into the hole-transport and photoactive layers were fabricated, showed a 13.6% improvement (maximum power conversion efficiency (PCE): 8.35%) compared to the reference device, thereby minimizing reliance on the incident angle of the light. In addition, after 1000 compression/relaxation tests with a compression strain of 33%, the PCE of the UFOPVs maintained a maximum of 93.3% of their initial value.

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.

Fibre electronics: towards scaled-up manufacturing of integrated e-textile systems

The quest for a close human interaction with electronic devices for healthcare, safety, energy and security has driven giant leaps in portable and wearable technologies in recent years. Electronic textiles (e-textiles) are emerging as key enablers of wearable devices. Unlike conventional heavy, rigid, and hard-to-wear gadgets, e-textiles can lead to lightweight, flexible, soft, and breathable devices, which can be worn like everyday clothes. A new generation of fibre-based electronics is emerging which can be made into wearable e-textiles. A suite of start-of-the-art functional materials have been used to develop novel fibre-based devices (FBDs), which have shown excellent potential in creating wearable e-textiles. Recent research in this area has led to the development of fibre-based electronic, optoelectronic, energy harvesting, energy storage, and sensing devices, which have also been integrated into multifunctional e-textile systems. Here we review the key technological advancements in FBDs and provide an updated critical evaluation of the status of the research in this field. Focusing on various aspects of materials development, device fabrication, fibre processing, textile integration, and scaled-up manufacturing we discuss current limitations and present an outlook on how to address the future development of this field. The critical analysis of key challenges and existing opportunities in fibre electronics aims to define a roadmap for future applications in this area.

Tendril-Inspired 900% Ultrastretching Fiber-Based Zn-Ion Batteries for Wearable Energy Textiles

Flexible fiber-based Zn-ion batteries represent an ideal power platform for smart wearable energy textiles featuring safety, flexibility, and unique integration. However, the inevitably low elongation limits (<400%) of common fiber-based Zn-ion batteries may restrict applications in highly deformable wearable materials and lead to unstable energy storage performance during practical activities. Herein, an elastic graphene/polyaniline-Zn@silver fiber-based battery (eG/P-Zn@SFB) with a helical structure inspired by the biological structure of luffa tendril is reported. eG/P-Zn@SFB exhibits ultrastretching properties and can be stretched to 900% with a 71% capacity retention ratio. Moreover, the prefabricated battery delivers a high specific capacity of 32.56 mAh/cm³ at 10 mA/cm³ and an energy density of 36.04 mWh/cm³. As a proof of concept, the knitted integrated eG/P-Zn@SFB served as an effective power supply with different bending angles ranging from 0° to 180°, demonstrating potential applications and promising prospects in stretchable flexible electronics and wearable energy textiles.

Visualising the knowledge structure and evolution of wearable device research

In recent years, the literature associated with wearable devices has grown rapidly, but few studies have used bibliometrics and a visualisation approach to conduct deep mining and reveal a panorama of the wearable devices field. To explore the foundational knowledge and research hotspots of the wearable devices field, this study conducted a series of bibliometric analyses on the related literature, including papers' production trends in the field and the distribution of countries, a keyword co-occurrence analysis, theme evolution analysis and research hotspots and trends for the future. By conducting a literature content analysis and structure analysis, we found the following: (a) The subject evolution path includes sensor research, sensitivity research and multi-functional device research. (b) Wearable device research focuses on information collection, sensor materials, manufacturing technology and application, artificial intelligence technology application, energy supply and medical applications. The future development trend will be further studied in combination with big data analysis, telemedicine and personalised precision medical application.

Triboelectric Nanogenerators and Hybridized Systems for Enabling Next-Generation IoT Applications

In the past few years, triboelectric nanogenerator-based (TENG-based) hybrid generators and systems have experienced a widespread and flourishing development, ranging among almost every aspect of our lives, e.g., from industry to consumer, outdoor to indoor, and wearable to implantable applications. Although TENG technology has been extensively investigated for mechanical energy harvesting, most developed TENGs still have limitations of small output current, unstable power generation, and low energy utilization rate of multisource energies. To harvest the ubiquitous/coexisted energy forms including mechanical, thermal, and solar energy simultaneously, a promising direction is to integrate TENG with other transducing mechanisms, e.g., electromagnetic generator, piezoelectric nanogenerator, pyroelectric nanogenerator, thermoelectric generator, and solar cell, forming the hybrid generator for synergetic single-source and multisource energy harvesting. The resultant TENG-based hybrid generators utilizing integrated transducing mechanisms are able to compensate for the shortcomings of each mechanism and overcome the above limitations, toward achieving a maximum, reliable, and stable output generation. Hence, in this review, we systematically introduce the key technologies of the TENG-based hybrid generators and hybridized systems, in the aspects of operation principles, structure designs, optimization strategies, power management, and system integration. The recent progress of TENG-based hybrid generators and hybridized systems for the outdoor, indoor, wearable, and implantable applications is also provided. Lastly, we discuss our perspectives on the future development trend of hybrid generators and hybridized systems in environmental monitoring, human activity sensation, human-machine interaction, smart home, healthcare, wearables, implants, robotics, Internet of things (IoT), and many other fields.

Photoanodes for Aqueous Solar Cells: Exploring Additives and Formulations Starting from a Commercial TiO2 Paste

Whereas the commercialization of dye-sensitized solar cells (DSSCs) is finally proceeding taking advantage of their low cost and tunable optical features, such as colour and transparency for both indoor and building-integrated applications, the corresponding aqueous counterpart is still at its infancy. As the TiO₂ electrode is a fundamental component for hybrid solar cells, this work investigates the effect of different molecular (α-terpineol, propylene carbonate) and polymeric (polyethylene oxide, polyethylene glycol, carboxymethyl cellulose and xanthan gum) additives that can be introduced into a commercial TiO₂ paste for for screen-printing (or doctor blade). Among all, the addition of polyethylene glycol leads to the best cell performances, with markedly increased short-circuit current density (+18 %) and power conversion efficiency (+48 %) with respect to the pristine (commercial) counterpart. When further explored at different concentration levels, electrodes fabricated from polyethylene glycol-based pastes show different morphologies, thicknesses and performances, which are investigated through (photo)electrochemical, structural, physical-chemical and microscopic techniques.

MXene-Based Flexible Supercapacitors: Influence of an Organic Ionic Conductor Electrolyte on the Performance

Owing to the rise of miniaturized wearable electronic devices in the last decade, significant demands have arisen to obtain high-performance flexible supercapacitors (FSCs). Recently, a lot of research has been focused on developing smart components of FSCs and integrating them into new device configurations. In this work, FSCs based on a Ti₃C₂ nanosheet (NS) and an organic ionic conductor (OIC)-induced hydrogel as the electrode and the electrolyte, respectively, were used. The FSCs fabricated have three different configurations (sandwich, twisted fiber, and interdigitated) and a comparative study of their electrochemical performance was investigated in terms of cycle stability, bending stability, power density, and energy density. Finally, the experimental validation of practical application was conducted, which suggested excellent electrochemical stability of Ti₃C₂ NS FSCs for driven commercial electronic gadgets. This study presents mechanically robust, lightweight, high-performance FSCs, which can be assembled in different configurations for powering wearable electronic devices.

High-performance flexible asymmetric supercapacitor based on rGO anode and WO3/WS2 core/shell nanowire cathode

Flexible smart electronics require their energy storage device to be flexible in nature. Developing high-performance flexible energy storage devices require direct integration of electrode active materials on current collectors to satisfy the high electronic/ionic conductivity and long-term durability requirements. Herein, we develop a flexible all-solid-state asymmetric supercapacitor comprised of reduced graphene oxide (rGO) and core/shell tungsten trioxide/tungsten disulfide (WO₃/WS₂) nanowire based electrodes. The electrodes synthesized via electrochemical deposition and chemical vapor deposition avoided the necessity to use non-conductive binders and offered excellent cyclic stability. The structural integrity provided by the rGO and WO₃/WS₂ electrodes facilitated excellent electrochemical stability with capacitance retention of 90% and 100% after 10 000 charge-discharge cycles, respectively. An all-solid-state device provides a voltage window of 1.5 V and more than 70% capacitance retention after 10 000 charge-discharge cycles. Providing 97% capacitance retention upon mechanical bending reveals its potential to be used as an energy storage devices in flexible electronics.

Tungsten nitride-coated graphene fibers for high-performance wearable supercapacitors

Graphene-fiber (GF) supercapacitors have attracted significant research attention in the field of wearable devices. However, there is still a need for active materials with high energy density. Transition Metal Nitrides (TMNs) are promising candidates for this purpose compared with conventional Transition Metal Oxides (TMOs) or conducting polymers (CPs) owing to their higher electrical conductivity, stability and relevant electrochemical properties. We have successfully integrated Tungsten Nitride (WN) with reduced graphene oxide fibers (rGOF) and developed high-performance hybrid fiber (WN-rGOF) supercapacitors. These hybrid supercapacitors attained a high capacitance of 16.29 F cm-3 at 0.05 A cm-3 and an energy density of 1.448 mW h cm-3, which is 7.5 and 1.75 times higher than those of the pure rGOF supercapacitor and the Tungsten Oxide/rGO hybrid fiber (WO3-rGOF) supercapacitor, respectively. The energy density readily increased up to 2.896 mW h cm-3 when three WN-rGOF supercapacitors were connected in series. The WN-rGOF supercapacitor also showed high capacitance retention of 84.7% after 10 000 cycles along with appreciable performance under severe mechanical deformation.

Flexible supercapacitor electrodes using metal-organic frameworks

Advancements in the field of flexible and wearable devices require flexible energy storage devices to cater their power demands. Metal-ion batteries (such as lithium-ion batteries, sodium-ion batteries, etc.) and electrochemical capacitors (also called supercapacitors or ultracapacitors) have achieved great interest in the recent past due to their superior energy storage characteristics like high power density and long cycle life. A major bottleneck of using metal-ion batteries in wearable devices is their lack of flexibility. Low power density, toxicity and flammability due to organic electrolytes inhibit them from safe on-body device applications. On the other hand, supercapacitors can be made with aqueous electrolytes, making them a safer alternative for wearable applications. Metal-organic frameworks (MOFs) are novel candidates as electrode materials due to their salient features such as large surface area, three-dimensional porous architecture, permeability to foreign entities, structural tailorability, etc. Though pristine MOFs suffer from poor intrinsic conductivity, this can be rectified by preparing composites with other electronically conducting materials. MOF-based electrodes are highly promising for flexible and wearable supercapacitors since they exhibit good energy and power densities. This review focuses on the new developments in the field of MOF-based composite electrodes for developing flexible supercapacitors.

Electrophoretic Adhesion of Conductive Hydrogels

For the development of next-generation wearable and implantable devices that connect the human body and machines, the adhesion of a conductive hydrogel is required. In this study, a conductive hydrogel is adhered using an electrophoretic approach through polyion complex formation at the interface of the hydrogels. Cationic and anionic conductive hydrogels adhere to anionic and cationic hydrogels, respectively. Moreover, the cationic and anionic conductive hydrogels adhere strongly to each other and the adhered conductive hydrogels exhibit conductivity. De-adhesion is possible by adding a salt and re-adhesion is demonstrated under aqueous conditions. It is believed that this innovative adhesion strategy for conductive hydrogels will be a fundamental technology for the connecting "soft" people and "hard" machines.

A Multi-Source Harvesting System Applied to Sensor-Based Smart Garments for Monitoring Workers’ Bio-Physical Parameters in Harsh Environments

This paper describes the development and characterization of a smart garment for monitoring the environmental and biophysical parameters of the user wearing it; the wearable application is focused on the control to workers’ conditions in dangerous workplaces in order to prevent or reduce the consequences of accidents. The smart jacket includes flexible solar panels, thermoelectric generators and flexible piezoelectric harvesters to scavenge energy from the human body, thus ensuring the energy autonomy of the employed sensors and electronic boards. The hardware and firmware optimization allowed the correct interfacing of the heart rate and SpO2 sensor, accelerometers, temperature and electrochemical gas sensors with a modified Arduino Pro mini board. The latter stores and processes the sensor data and, in the event of abnormal parameters, sends an alarm to a cloud database, allowing company managers to check them via a web app. The characterization of the harvesting subsection has shown that ≈ 265 mW maximum power can be obtained in a real scenario, whereas the power consumption due to the acquisition, processing and BLE data transmission functions determined that a 10 mAh/day charge is required to ensure the device’s proper operation. By charging a 380 mAh Lipo battery in a few hours by means of the harvesting system, an energy autonomy of 23 days was obtained, in the absence of any further energy contribution.

A Route Toward Smart System Integration: From Fiber Design to Device Construction

Fiber is a symbol of human civilization, being ubiquitous but obscure in society over most of history. Fiber has been revived upon the advent of fiber-based electronic devices in the past two decades. This is due to its desirable lightweight, flexible, and conformable characteristics, which enable it to play a fundamental role in the electronic and information era. Numerous fiber-based electronic devices have sprung up in energy conversion, energy storage, sensing, actuation, etc. A possibility is thereby conceived that they can be integrated into smart systems compatible with the human body, consisting of biotic fiber-based organs and tissues, which possess similar but more advanced functions. However, the design of mono-/multifibers, the construction of fiber-based devices, and the integration of these smart systems represent great challenges in fundamental understanding and practical implementation. A systematic review of the current state of the art with respect to the design and fabrication of electronic fiber materials, construction of fiber-based devices, and integration of smart systems is presented. In addition, limitations of current fiber-based devices and perspectives are explored toward potential and promising smart integration.

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.

Control of thermal deformation with photonic sintering of ultrathin nanowire transparent electrodes

Development of electronic devices on ultrathin flexible plastic substrates is of great value in terms of portability, cost reduction, and mechanical flexibility. However, because thin plastic substrates with low heat capacity can be more easily damaged by thermal energy, their use is limited. Highly flexible nanowire (NW) transparent conductive electrodes on ultrathin (∼10 μm) low cost polyethylene terephthalate (PET) substrates are fabricated. The control of intense pulsed light (IPL) irradiation process parameters to induce NW welding for maximum conductivity and minimal thermal damage of the PET substrate is explored. For this purpose, trends in temperature variation of NW thin films irradiated by IPL under various operating conditions are numerically analyzed using commercial software. Simulations indicate that irradiating light operated at a higher voltage and for a shorter time, and use of multiple pulses of low frequency can reduce thermal deformation of the PET substrate. Furthermore, we experimentally confirm that NW transparent electrodes can be successfully fabricated with less thermal deformation of the ultrathin plastic substrate when light is irradiated under well-controlled conditions derived from the simulation. The highly flexible NW transparent conducting electrode exhibits excellent mechanical flexibility to withstand severe deformation and can be successfully implemented in flexible organic light-emitting diodes (OLEDs).

Flexible energy generation and storage devices: focus on key role of heterocyclic solid-state organic ionic conductors

This review addresses the vital role of solid-state electrolytes to develop highly efficient, customizable flexible energy generation and storage devices.

Advanced Artificial Muscle for Flexible Material-Based Reconfigurable Soft Robots

Flexible material-based soft robots are widely used in various areas. In many situations, the suitable soft robots should be rapidly fabricated to complete the urgent tasks (such as rescue), so the facile fabricating methods of the multifunctional soft robots are still in urgent needs. In this work, the origami structure is employed to design vacuum-powered silicone rubber artificial muscles, which can perform multiple motions, including contraction, bending, twisting, and radial motions. Artificial muscles can be used for rapid reconfiguration of different soft robots, just like the "building bricks". Based on these artificial muscles, four soft robots with different functions, including an omnidirectional quadruped robot, a flexible gripper, a flexible wrist, and a pipe-climbing robot, are reconfigured to complete different tasks. The proposed origami artificial muscles offer a facile and rapid fabricating method of flexible material-based soft robots, and also greatly improve the utilization rate of flexible materials.

Vertically Aligned Graphene-Carbon Fiber Hybrid Electrodes with Superlong Cycling Stability for Flexible Supercapacitors

Graphene electrode-based supercapacitors are in high demand due to their superior electrochemical characteristics. A major bottleneck of using the supercapacitors for commercial applications lies in their inferior electrode cycle life. Herein, a simple and facile method to fabricate highly efficient supercapacitor electrodes using pristine graphene sheets vertically stacked and electrically connected to the carbon fibers which can result in vertically aligned graphene-carbon fiber nanostructure is developed. The vertically aligned graphene-carbon fiber electrode prepared by electrophoretic deposition possesses a mesoporous 3D architecture which enabled faster and efficient electrolyte-ion diffusion with a gravimetric capacitance of 333.3 F g^-1 and an areal capacitance of 166 mF cm^-2 . The electrodes displayed superlong electrochemical cycling stability of more than 100 000 cycles with 100% capacitance retention hence promising for long-lasting supercapacitors. Apart from the electrochemical double layer charge storage, the oxygen-containing surface moieties and α-Ni(OH)₂ present on the graphene sheets enhance the charge storage by faradaic reactions. This enables the assembled device to provide an excellent gravimetric energy density of 76 W h kg^-1 with a 100% capacitance retention even after 1000 bending cycles. This study opens the door for developing high-performing flexible graphene electrodes for wearable energy storage applications.

Novel mesoporous electrode materials for symmetric, asymmetric and hybrid supercapacitors

Electrochemical capacitors or supercapacitors have achieved great interest in the recent past due to their potential applications ranging from microelectronic devices to hybrid electric vehicles. Supercapacitors can provide high power densities but their inherently low energy density remains a great challenge. The high-performance supercapacitors utilize large electrode surface area for electrochemical double-layer capacitance and/or pseudocapacitance. To enhance the performance of supercapacitors, various strategies have been adopted such as electrode nanostructuring, hybrid electrode designs using nanocomposite electrodes and hybrid supercapacitor (HSC) configurations. Nanoarchitecturing of electrode-active materials is an effective way of enhancing the performance of supercapacitors as it increases the effective electrode surface area for enhanced electrode/electrolyte interaction. In this review, we focus on the recent developments in the novel electrode materials and various hybrid designs used in supercapacitors for obtaining high specific capacitance and energy density. A family of electrode-active materials including carbon nanomaterials, transition metal-oxides, transition metal-nitrides, transition metal-hydroxides, electronically conducting polymers, and their nanocomposites are discussed in detail. The HSC configurations for attaining enhanced supercapacitor performance as well as strategies to integrate with other microelectronic devices/wearable fabrics are also included.

Coaxial Printing of Silicone Elastomer Composite Fibers for Stretchable and Wearable Piezoresistive Sensors

Despite the tremendous efforts dedicated to developing various wearable piezoresistive sensors with sufficient stretchability and high sensitivity, challenges remain pertaining to fabrication scalability, cost, and efficiency. In this study, a facile, scalable, and low-cost coaxial printing strategy is employed to fabricate stretchable and flexible fibers with a core–sheath structure for wearable strain sensors. The highly viscous silica-modified silicone elastomer solution is used to print the insulating sheath layer, and the silicone elastomer solutions containing multi-walled carbon nanotubes (CNTs) are used as the core inks to print the conductive inner layer. With the addition of silica powders as viscosifiers, silica-filled silicone ink (sheath ink) converts to printable ink. The dimensions of the printed coaxial fibers can be flexibly controlled via adjusting the extrusion pressure of the inks. In addition, the electro-mechanical responses of the fiber-shaped strain sensors are investigated. The printed stretchable and wearable fiber-like CNT-based strain sensor exhibits outstanding sensitivities with gauge factors (GFs) of 1.4 to 2.5 × 10⁶, a large stretchability of 150%, and excellent waterproof performance. Furthermore, the sensor can detect a strain of 0.1% and showed stable responses for over 15,000 cycles (high durability). The printed fiber-shaped sensor demonstrated capabilities of detecting and differentiating human joint movements and monitoring balloon inflation. These results obtained demonstrate that the one-step printed fiber-like strain sensors have potential applications in wearable devices, soft robotics, and electronic skins.

Two-Dimensional Mn3O4 Nanowalls Grown on Carbon Fibers as Electrodes for Flexible Supercapacitors

Emerging flexible and wearable electronic devices necessitates the development of fiber-type energy storage devices to power them. Supercapacitors received great attention for applications in flexible and wearable devices due to their scalability, safety, and miniature size. Herein, we report the fabrication of a flexible supercapacitor using manganese(II,III) oxide (Mn₃O₄) nanowalls (NWs) grown by electrochemical deposition on carbon fiber (CF) as electrode-active material. Here, CF serves as both a substrate for the growth of Mn₃O₄ NWs and a current collector for making a lightweight supercapacitor. Two-dimensional Mn₃O₄ NWs were uniformly grown on CF with high surface coverage. A three-dimensional nanostructured electrode is obtained using these individual two-dimensional Mn₃O₄ NWs. The Mn₃O₄ NWs grown on CF are characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. A symmetric sandwich-type supercapacitor is fabricated using two-dimensional Mn₃O₄ NW electrodes in an aqueous 1 M Na₂SO₄ electrolyte. The Mn₃O₄ NW supercapacitor electrode exhibits a specific capacitance of 300.7 F g^-1 at a scan rate of 5 mV s^-1. The assembled symmetric sandwich-type supercapacitor displayed high flexibility even at a bending angle of 180° without altering its performance. The Mn₃O₄ NW supercapacitor also displayed a long cycle life of 7500 cycles with 100% capacitance retention.

Electrochemical performance of antimony/chlorine-incorporated nickel foam

In this paper, an Sb/Cl-incorporated nickel foam (NF) electrode material was grown on acid-pretreated NF by one-step chemical vapor deposition using SbCl₃ as the Sb source.