Electrospun metal nanofiber webs as high-performance transparent electrode

Omnidirectional Bending Sensor with Bianisotropic Structure for Wearable Electronics

Bending sensors are critical to the advancement of wearable electronics and can be applied in the dynamic monitoring of flexible object morphology. However, current bending sensors are constrained by sensing range and precision, especially in full-range detection. The maximum sensing range of existing flexible bending sensors is 0-240°. This study introduces a bianisotropic responsive structure into the design of an all-textile bending sensor, thereby realizing 0-360° full-range omnidirectional bending sensing. First, the project elucidated the sensing mechanism of the piezoresistive bianisotropic structured bending sensor and identified critical factors through a numerical simulation method. Then, the bianisotropic structured bending sensors were produced through the stitch method and analyzed on their electromechanical performance. Further, the recognition model for both bending angle and direction parameters was developed via numerical calculation, achieving a high accuracy with an error rate of 2.82%. Last, according to the ergonomics of body joints, the sensors were customized and validated in body joint monitoring scenarios. This work significantly enhances the performance of flexible bending sensors in sensing range, accuracy, and comfort for the wearer. The versatility of this bending sensor positions it as a promising candidate to supplant traditional heavy equipment or rigid devices, particularly in wearable joint motion monitoring and soft robotics.

Patterning Techniques Based on Metallized Electrospun Nanofibers for Advanced Stretchable Electronics

Stretchable electronics have experienced remarkable progress, especially in sensors and wireless communication systems, attributed to their ability to conformably contact with rough or uneven surfaces. However, the development of complex, multifunctional, and high-precision stretchable electronics faces substantial challenges, including instability at rigid-soft interfaces and incompatibility with traditional high-precision patterning technologies. Metallized electrospun nanofibers emerge as a promising conductive filler, offering exceptional stretchability, electrical conductivity, transparency, and compatibility with existing patterning technologies. Here, this review focuses on the fundamental properties, preparation processes, patterning technologies, and application scenarios of conductive stretchable composites based on metallized nanofibers. Initially, it introduces the fabrication processes of metallized electrospun nanofibers and their advantages over alternative materials. It then highlights recent progress in patterning technologies, including collector collection, vapor deposition with masks, and lithography, emphasizing their role in enhancing precision and integration. Furthermore, the review shows the broad applicability and potential influence of metallized electrospun nanofibers in various fields through their use in sensors, wireless systems, semiconductor devices, and intelligent healthcare solutions. Ultimately, this review seeks to spark further innovation and address the prevailing challenges in stretchable electronics, paving the way for future breakthroughs in this dynamic field.

Highly Conductive PEDOT:PSS: Ag Nanowire-Based Nanofibers for Transparent Flexible Electronics

Highly conductive, transparent, and easily available materials are needed in a wide range of devices, such as sensors, solar cells, and touch screens, as alternatives to expensive and unsustainable materials such as indium tin oxide. Herein, electrospinning was employed to develop fibers of PEDOT:PSS/silver nanowire (AgNW) composites on various substrates, including poly(caprolactone) (PCL), cotton fabric, and Kapton. The influence of AgNWs, as well as the applied voltage of electrospinning on the conductivity of fibers, was thoroughly investigated. The developed fibers showed a sheet resistance of 7 Ω/sq, a conductivity of 354 S/cm, and a transmittance value of 77%, providing excellent optoelectrical properties. Further, the effect of bending on the fibers' electrical properties was analyzed. The sheet resistance of fibers on the PCL substrate increased slightly from 7 to 8 Ω/sq, after 1000 bending cycles. Subsequently, as a proof of concept, the nanofibers were evaluated as electrode material in a triboelectric nanogenerator (TENG)-based energy harvester, and they were observed to enhance the performance of the TENG device (78.83 V and 7 μA output voltage and current, respectively), as compared to the same device using copper electrodes. These experiments highlight the untapped potential of conductive electrospun fibers for flexible and transparent electronics.

Improvement strategies for oil/water separation based on electrospun SiO2 nanofibers

Oil spills and oily effluents from industry and daily life pose a great threat to all organisms in the ecosystem, while aggravating the problem of water scarcity, which has developed into a global challenge. Therefore, the development of advanced materials and technologies for oil/water separation has become a focus of attention. One-dimensional (1D) SiO2 nanofibers (SNFs) have become one of the most widely used inorganic nanomaterials in the past due to their stable chemical properties, excellent biocompatibility, and high temperature resistance etc. Meanwhile, electrospinning technique, as an emerging technology for treating oil/water emulsions, electrospun SNFs on this basis also has a number of advantages such as adjustable wettability, diverse structure and good connectivity. This review provides a systematic overview of the research progress of electrospun SNFs in different aspects. In this review, we first introduce the basic principles of electrospun SNFs, then focus on the design structures of various SNFs, propose corresponding strategies for the property improvement of SNFs, also analyze and consider the applications of SNFs. Finally, the challenges faced by electrospun SNFs in the field of oil/water separation are analyzed, and the future directions of electrospun SNFs are summarized and prospected.

Structural Control of Nanofibers According to Electrospinning Process Conditions and Their Applications

Nanofibers have gained much attention because of the large surface area they can provide. Thus, many fabrication methods that produce nanofiber materials have been proposed. Electrospinning is a spinning technique that can use an electric field to continuously and uniformly generate polymer and composite nanofibers. The structure of the electrospinning system can be modified, thus making changes to the structure, and also the alignment of nanofibers. Moreover, the nanofibers can also be treated, modifying the nanofiber structure. This paper thoroughly reviews the efforts to change the configuration of the electrospinning system and the effects of these configurations on the nanofibers. Excellent works in different fields of application that use electrospun nanofibers are also introduced. The studied materials functioned effectively in their application, thereby proving the potential for the future development of electrospinning nanofiber materials.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Shape tunability of copper nanocrystals deposited on nanorods

The significant role of metal particle geometry in dictating catalytic activity, selectivity, and stability is well established in heterocatalysis. However, this topic is rarely explored in semiconductor-metal hybrid photocatalytic systems, primarily due to the lack of synthetic control over this feature. Herein, we present a new synthetic route for the deposition of metallic Cu nanoparticles with spherical, elliptic, or cubic geometrical shapes, which are selectively grown on one side of the well-established CdSe@CdS nanorod photocatalytic system. An additional multipod morphology in which several nanorod branches are combined on a single Cu domain is presented as well. Cu is an earth-abundant low-cost catalyst known to promote a diverse gallery of organic transformations and is an excellent thermal and electrical conductor with interesting plasmonic properties. Its deposition on cadmium chalcogenide nanostructures is enabled here via mitigation of the reaction kinetics such that the cation exchange reaction is prevented. The structural diversity of these sophisticated nanoscale hybrid systems lays the foundations for shape-activity correlation studies and employment in various applications.

Halide-Salt-Free Synthesis of Silver Nanowires with High Yield and Purity for Transparent Conductive Films

To date, silver nanowires (AgNWs) are routinely synthesized. However, the controllable preparation of AgNWs without any halide salts has not reached a similar level. In particular, the halide-salt-free polyol synthesis of AgNWs commonly occurs above 413 K, and the property of AgNWs obtained is not so easy to control. In this study, a facile synthesis of AgNWs with a yield of up to ∼90% in an average length of 75 μm was successfully performed without any halide salts. The fabricated AgNW transparent conductive films (TCFs) show a transmittance of 81.7% (92.3% for the AgNW network only without substrate) at a sheet resistance of 12.25 Ω/square. In addition, the AgNW films show distinguished mechanical properties. More importantly, the reaction mechanism for AgNWs was briefly discussed, and the importance of reaction temperature, the mass ratio of poly(vinylpyrrolidone) (PVP)/AgNO3, and the atmosphere was emphasized. This knowledge will help enhance the reproducibility and scalability of polyol synthesis of high-quality AgNWs.

Engineered kirigami design of PVDF-Pt core-shell nanofiber network for flexible transparent electrode

Nanofiber networks comprising polymer-metal core-shell structures exhibit several advantages, such as high uniformities and considerable flexibilities. Additionally, the flexibility of the nanofiber network may be further enhanced by engineering the network topology. Therefore, in this study, the topologies of polyvinylidene fluoride (PVDF)-Pt core-shell nanofiber (CS NF) networks were engineered, and their performances as flexible transparent electrodes were comprehensively evaluated. Three distinct topologies of nanofiber networks were induced using circular, square, and rectangular electrode collectors. A highly uniform nanofiber network was obtained using the square electrode collector, which generated a high density of nanofiber junctions (nodes). Consequently, this nanofiber network exhibited the smallest sheet resistance [Formula: see text] and lowest optical transmittance [Formula: see text] among the three CS NF networks. In contrast, nanofiber bundles were frequently formed in the randomly aligned CS NF network prepared using the circular electrode collector, reducing the node density. As a result, it simultaneously exhibited a very small [Formula: see text] and high [Formula: see text], generating the largest percolation figure of merit [Formula: see text]. Under certain strain directions, the CS NF network with the engineered topology exhibited a significantly enhanced mechanical durability. Finally, a flexible piezoelectric pressure sensor with CS NF network electrodes was fabricated and its sensing performance was excellent.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

A Piezoresistive Sensor with High Sensitivity and Flexibility Based on Porous Sponge

Chemical plating has recently been employed for the preparation of flexible piezoresistive sensors; however, plating solutions and processes that affect the sensitivity still need further exploration. In the study, a sponge-based flexible sensor with copper as its conductive material is prepared using electroless plating. The variation in sponge resistance and sensitivity changes with different plating times are studied. It is found that, with the increasing plating time, the conductivity increases and the resistance of sample will decrease. Moreover, the range of resistance difference will decrease under compression, thus the sensitivity decreases. Furthermore, the sensor's applications were assessed, verifying the practicability of the developed preparation method. This study may bring ideas for the new development of flexible pressure sensors.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

From 1D Nanofibers to 3D Nanofibrous Aerogels: A Marvellous Evolution of Electrospun SiO2 Nanofibers for Emerging Applications

One-dimensional (1D) SiO2 nanofibers (SNFs), one of the most popular inorganic nanomaterials, have aroused widespread attention because of their excellent chemical stability, as well as unique optical and thermal characteristics. Electrospinning is a straightforward and versatile method to prepare 1D SNFs with programmable structures, manageable dimensions, and modifiable properties, which hold great potential in many cutting-edge applications including aerospace, nanodevice, and energy. In this review, substantial advances in the structural design, controllable synthesis, and multifunctional applications of electrospun SNFs are highlighted. We begin with a brief introduction to the fundamental principles, available raw materials, and typical apparatus of electrospun SNFs. We then discuss the strategies for preparing SNFs with diverse structures in detail, especially stressing the newly emerging three-dimensional SiO2 nanofibrous aerogels. We continue with focus on major breakthroughs about brittleness-to-flexibility transition of SNFs and the means to achieve their mechanical reinforcement. In addition, we showcase recent applications enabled by electrospun SNFs, with particular emphasis on physical protection, health care and water treatment. In the end, we summarize this review and provide some perspectives on the future development direction of electrospun SNFs.

Nanoscale plasmonic wires with maximal figure of merits as a superior flexible transparent conducting electrode for RGB colors

Based on incredibly increasing applications in modern optoelectronic devices, the demand for securing a superior conductive transparent electrode (TCE) candidate becomes significant and urgent. However, boosting both transmittance and conductance simultaneously is an intrinsic limitation. In this work, we present silver nanoscale plasmonic wires (Ag NPWs) to function as TCEs in the visible light region by lowering their corresponding plasma frequencies. By carefully designing geometric dimensions of the Ag NPWs, we also optimize the performance for red, green, and blue colors, respectively. The demonstrated figure of merits for RGB colors appeared respectively 443.29, 459.46, and 133.78 in simulation and 302.75, 344.11, and 348.02 in experiments. Evidently, our Ag NPWs offer much greater FoMs beyond conventional TCEs that are most frequently comprised of indium tin oxide and show further advantages of flexibility and less Moire effect for the applications of flexible and high-resolution optoelectronic devices.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Towards Transient Electronics through Heat Triggered Shattering of Off-the-Shelf Electronic Chips

With most of the critical data being stored in silicon (Si) based electronic devices, there is a need to develop such devices with a transient nature. Here, we have focused on developing a programmable and controllable heat triggered shattering transience mechanism for any off-the-shelf (OTS) Si microchip as a means to develop transient electronics which can then be safely and rapidly disabled on trigger when desired. This transience mechanism is based on irreversible and spontaneous propagation of cracks that are patterned on the back of the OTS chip in the form of grooves and then filled with thermally expandable (TE) material. Two types of TE materials were used in this study, commercially available microsphere particles and a developed elastomeric material. These materials expand >100 times their original volume on the application of heat which applies wedging stress of the groove boundaries and induces crack propagation resulting in the complete shattering of the OTS Si chip into tiny silicon pieces. Transience was controlled by temperature and can be triggered at ~160–190 °C. We also demonstrated the programmability of critical parameters such as transience time (0.35–12 s) and transience efficiency (5–60%) without the knowledge of material properties by modeling the swelling behavior using linear viscoelastic models.

Morphology, thermal and mechanical properties of electrospun polyvinylidene/polyethylene glycol composite nanofibers as form-stabilized phase change materials

Polyvinylidene fluoride (PVDF)/polyethylene glycol (PEG) form-stabilized composite phase change nanofibers with various mass percentage of PEG1000 (with average molecular weight of 1000) loadings were fabricated by electrospinning technique, in which PEG was served as phase change material (PCM) and PVDF as the supporting matrix to afford a mechanically strong structure. Effect of PEG1000 content on nanofiber morphology, phase transition properties, thermal stability, thermal energy storage and release performance, and mechanical properties were studied experimentally. The results showed that the incorporation of PEG1000 improved the spinning solution viscosity and high mass fraction of PEG1000 in the nanofibers led to decreased fiber diameter and melting temperature, and higher latent heat of fusion. In addition, mechanical test revealed that the fracture elongation of the electrospun PVDF/PEG composite nanofibrous membranes were initially increased with unobvious tensile strength changes, while the value of the elongation would decrease, and the tensile strength increase as the mass ratio of PVDF/PEG lower to 1:1. Furthermore, the melting temperature and maximum latent heat for PVDF/PEG were determined from DSC measurement as 51.8 °C, and 73.3 J/g, when the mass radio of PVDF/PEG was 1:1 (w/w), which was suitable utilized for thermo-regulating textiles or in heat storage devices.

Applying extrusion-based 3D printing technique accelerates fabricating complex biphasic calcium phosphate-based scaffolds for bone tissue regeneration

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Biphasic calcium phosphates offer a chemically similar biomaterial to the natural bone, which can significantly accelerate bone formation and reconstruction.

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Robocasting is a suitable technique to produce porous scaffolds supporting cell viability, proliferation, and differentiation.

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This review discusses materials and methods utilized for BCP robocasting, considering recent advancements and existing challenges in using additives for bioink preparation.

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Commercialization and marketing approach, in-vitro and in-vivo evaluations, biologic responses, and post-processing steps are also investigated.

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Possible strategies and opportunities for the use of BCP toward injured bone regeneration along with clinical applications are discussed.

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The study proposes that BCP possesses an acceptable level of bone substituting, considering its challenges and struggles.

Background

Aim of review

Key scientific concepts of review

Examining the Transmission of Visible Light through Electrospun Nanofibrous PCL Scaffolds for Corneal Tissue Engineering

The transparency of nanofibrous scaffolds is of highest interest for potential applications like corneal wound dressings in corneal tissue engineering. In this study, we provide a detailed analysis of light transmission through electrospun polycaprolactone (PCL) scaffolds. PCL scaffolds were produced via electrospinning, with fiber diameters in the range from (35 ± 13) nm to (167 ± 35) nm. Light transmission measurements were conducted using UV-vis spectroscopy in the range of visible light and analyzed with respect to the influence of scaffold thickness, fiber diameter, and surrounding medium. Contour plots were compiled for a straightforward access to light transmission values for arbitrary scaffold thicknesses. Depending on the fiber diameter, transmission values between 15% and 75% were observed for scaffold thicknesses of 10 µm. With a decreasing fiber diameter, light transmission could be improved, as well as with matching refractive indices of fiber material and medium. For corneal tissue engineering, scaffolds should be designed as thin as possible and fabricated from polymers with a matching refractive index to that of the human cornea. Concerning fiber diameter, smaller fiber diameters should be favored for maximizing graft transparency. Finally, a novel, semi-empirical formulation of light transmission through nanofibrous scaffolds is presented.

Fabrication of junction-free Cu nanowire networks via Ru-catalyzed electroless deposition and their application to transparent conducting electrodes

Over the past few years, metal nanowire networks have attracted attention as an alternative to transparent conducting oxide materials such as indium tin oxide for transparent conducting electrode applications. Recently, electrodeposition of metal on nanoscale template is widely used for formation of metal network. In the present work, junctionless Cu nanowire networks were simply fabricated on a substrate by forming a nanostructured Ru with 80 nm width as a seed layer, followed by direct electroless deposition of Cu. By controlling the density of Ru nanowires or the electroless deposition time, we readily achieve desired transmittance and sheet resistance values ranging from ∼1 kΩ sq^-1at 99% to 9 Ω sq^-1at 89%. After being transferred to flexible substrates, the nanowire networks exhibited no obvious increase in resistance during 8000 cycles of a bending test to a radius of 2.5 mm. The durability was verified by evaluation of its heating performance. The maximum temperature was greater than 180 °C at 3 V and remained constant after three repeated cycles and for 10 min. Transmission electron microscopy and x-ray diffraction studies revealed that the adhesion between the electrolessly deposited Cu and the seed Ru nanowires strongly influenced the durability of the core-shell structured nanowire-based heaters.

Improved Sheet Resistance of Nanofiber-Based Transparent Conducting Electrodes Using Silver Nanowires

There is an increased need for research on flexible transparent electrodes (FTEs) because they are critical to next-generation electronic devices, such as wearable computers. In this study, highly conductive transparent conducting electrodes, based on polyvinylidene fluoride (PVDF) nanofiber webs treated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and silver nanowires (AgNWs), were successfully fabricated. Transparent conducting electrodes (TCEs) were obtained by a brush-painting process using different weight ratios of a AgNWs to PEDOT:PSS solution, and the surface, electrical, optical, and chemical properties, as well as the tensile strength of the samples, were determined. It was found that the electrical conductivity of the samples improved as the AgNW content increased, but the light transmittance decreased. In this work, there was a slight decrease in the optical properties and a considerable increase in the electrical properties due to the hybridization of AgNWs and PEDOT:PSS, compared to using only PEDOT:PSS. When considering both transparency and electrical conductivity, which are essential parameters of TCEs, sample PA2, which was treated by mixing AgNWs and PEDOT:PSS/dimethyl sulfoxide (DMSO) in a ratio of 1:5 (16.67 wt% of AgNWs), was found to be the best sample, with a sheet resistance of 905 Ω/cm² and light transmittance of 79%.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Pushing the Limits of Flexibility and Stretchability of Solar Cells: A Review

Emerging forms of soft, flexible, and stretchable electronics promise to revolutionize the electronics industries of the future offering radically new products that combine multiple functionalities, including power generation, with arbitrary form factor. For example, skin-like electronics promise to transform the human-machine-interface, but the softness of the skin is incompatible with traditional electronic components. To address this issue, new strategies toward soft and wearable electronic systems are currently being pursued, which also include stretchable photovoltaics as self-powering systems for use in autonomous and stretchable electronics of the future. Here recent developments in the field of stretchable photovoltaics are reviewed and their potential for various emerging applications are examined. Emphasis is placed on the different strategies to induce stretchability including extrinsic and intrinsic approaches. In the former case, engineering and patterning of the materials and devices are key elements while intrinsically stretchable systems rely on mechanically compliant materials such as elastomers and organic conjugated polymers. The result is a review article that provides a comprehensive summary of the progress to date in the field of stretchable solar cells from the nanoscale to macroscopic functional devices. The article is concluded by discussing the emerging trends and future developments.

Mixed-Matrix Membrane Fabrication for Water Treatment

In recent years, technology for the fabrication of mixed-matrix membranes has received significant research interest due to the widespread use of mixed-matrix membranes (MMMs) for various separation processes, as well as biomedical applications. MMMs possess a wide range of properties, including selectivity, good permeability of desired liquid or gas, antifouling behavior, and desired mechanical strength, which makes them preferable for research nowadays. However, these properties of MMMs are due to their tailored and designed structure, which is possible due to a fabrication process with controlled fabrication parameters and a choice of appropriate materials, such as a polymer matrix with dispersed nanoparticulates based on a typical application. Therefore, several conventional fabrication methods such as a phase-inversion process, interfacial polymerization, co-casting, coating, electrospinning, etc., have been implemented for MMM preparation, and there is a drive for continuous modification of advanced, easy, and economic MMM fabrication technology for industrial-, small-, and bulk-scale production. This review focuses on different MMM fabrication processes and the importance of various parameter controls and membrane efficiency, as well as tackling membrane fouling with the use of nanomaterials in MMMs. Finally, future challenges and outlooks are highlighted.

A Review of Multiple Scale Fibrous and Composite Systems for Heating Applications

Different types of heating systems have been developed lately, representing a growing interest in both the academic and industrial sectors. Based on the Joule effect, fibrous structures can produce heat once an electrical current is passed, whereby different approaches have been followed. For that purpose, materials with electrical and thermal conductivity have been explored, such as carbon-based nanomaterials, metallic nanostructures, intrinsically conducting polymers, fibers or hybrids. We review the usage of these emerging nanomaterials at the nanoscale and processed up to the macroscale to create heaters. In addition to fibrous systems, the creation of composite systems for electrical and thermal conductivity enhancement has also been highly studied. Different techniques can be used to create thin film heaters or heating textiles, as opposed to the conventional textile technologies. The combination of nanoscale and microscale materials gives the best heating performances, and some applications have already been proven, even though some effort is still needed to reach the industry level.

PEDOT:PSS-glued MoO3 nanowire network for all-solid-state flexible transparent supercapacitors

Flexible transparent supercapacitors (FTSCs) are essential for the development of next-generation transparent electronics, however, a significant challenge is to achieve high-areal-capacitance FTSCs without sacrificing optical transparency. Herein, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-glued MoO₃ nanowires anchored on the Ag nanofiber (AgNF) network are employed as FTSC film electrodes, in which the AgNF network provides primary conducting pathways and guarantees rapid electron transport, while wide-bandgap semiconductor MoO₃ nanowires glued by the ultrathin PEDOT:PSS layer provide abundant redox-active sites to store energy. Benefiting from the PEDOT:PSS as the conducting glue to promote the connection at the junctions between AgNFs and MoO₃ nanowires, the as-prepared AgNFs/MoO₃/PEDOT:PSS (AMP) film electrode demonstrates a high transmittance (82.8%) and large areal capacitance (15.7 mF cm^-2), and has outperformed all the transparent conductive films known to date. Even after 11 000 charge/discharge cycles, the capacitance still remains at 92.4% of the initial value. The assembled all-solid-state FTSC device delivers an energy density of 0.623 μW h cm^-2, a power density of 40 μW cm^-2, and excellent mechanical robustness, implying a great potential in high performance FTSCs.

Stimulus-Responsive Graphene with Periodical Wrinkles on Grooved Microfiber Arrays: Simulation, Programmable Shape-Shifting, and Catalytic Applications

This work demonstrates a facile fabrication of stimulus-responsive, periodically wrinkled graphene sheets on grooved microfiber arrays with fast and reversible shape change, multiresponsiveness, and programmable deformation, with the aid of finite element analysis (FEA). The cellulose acetate (CA) microfibers, endowing responsiveness to humidity and solvents, are designed to grooved shape and assembled into a well-aligned fibrous mat by electrospinning. Under the guidance of FEA simulation, the stiff reduced graphene oxide (RGO) sheets, serving as a photoresponsive component, could ably bind on grooved CA microfibers with favorable interlocked interfacial-structure. Through simple direct-writing and hot-pressing, the grooved CA arrays interlocked the conformal RGO sheets by water-induced self-clamping, and enabled the generation of periodic wrinkles within RGO sheets to maximize interfacial areas. By simply adjusting the orientation of written RGO patterns relative to uniaxial CA microfibers, programmed and omnidirectional shape-shifting were obtained to minimize strain energy, consisting with the dynamic deformation process simulated by FEA. Upon remote light or contactless humidity stimuli, the RGO/CA mat shows a rapid response (≤1 s), large amplitude (angle change ≥150°, 1.62 cm^-1), sophisticated 3D motions, and lifts objects that weigh 12.7-times its own weight up to over 1/3 of own height within 1 s. After loading catalytical nanoparticles, the RGO/CA mat could rapidly move to the targeted position by continuous crawling even on a slippery surface, and served as a microchannel reactor to trigger a reaction in built-in microchannels with suppressing catalyst leaching while accelerating reaction kinetics by both nanoconfinement and photothermal effect.

Rapid Ag Nanofiber Formation Via Pt Nanoparticle-Assisted H2-Free Reduction of Ag+-Containing Polymers

One-dimensional Ag nanostructure-based networks have garnered significant attention as next-generation transparent conductive materials. Ag nanofibers (NFs) with high aspect ratios decrease the number density required for percolation; hence, they form qualitatively superior transparent conductive films. This study reports a novel method for rapidly fabricating Ag NFs via Pt nanoparticle-assisted H₂-free reduction of solid-state AgNO₃. Our results first indicated that polymers can be a source of hydrogen gas in the presence of Pt nanoparticles; Ag NFs with aspect ratios above 10⁵ were obtained herein by heating AgNO₃-containing polymer NFs in a short period of time and in an open-air environment. Our method not only successfully reduced the amount of polymer residue often encountered in spun NFs but also created an effective self-supporting reduction system that does not require an external reducing gas supply. The obtained Ag NF networks were highly conductive and transparent. Moreover, the mechanism of Ag NF formation was investigated. We demonstrate that the proposed method exhibits a high potential for producing high yields of Ag NFs in a simple and rapid manner.

Facile Post Treatment of Ag Nanowire/Polymer Composites for Flexible Transparent Electrodes and Thin Film Heaters

Typical polyol-based synthesis of silver nanowire employs insulating polymer as a surfactant for the silver nanowire growth, which limits direct contact between each nanowire and thus its optoelectronic properties. We herein demonstrate that a simple solvent treatment effectively removes the insulating polymer around Ag NWs, leading to significantly decreased sheet resistance (~12 Ω/sq) with an increased transmittance (81% @ T₅₅₀), as compared to other post-treatments. We successfully demonstrate the transparent film heaters using the solvent-treated Ag NWs network, which rapidly exhibited 150 °C under a bias of 5 V. Flexible film heaters on plastic substrate is also demonstrated, suggesting a great potential of the solvent treatment process of Ag NWs for flexible transparent electrode and film heater applications.

Preparation and Applications of Electrospun Optically Transparent Fibrous Membrane

The optically transparent electrospun fibrous membrane has been widely used in many fields due to its simple operation, flexible design, controllable structure, high specific surface area, high porosity, and unique excellent optical properties. This paper comprehensively summarizes the preparation methods and applications of an electrospun optically transparent fibrous membrane in view of the selection of raw materials and structure modulation during preparation. We start by the factors that affect transmittance among different materials and explain the light transmission mechanism of the fibrous membrane. This paper also provides an overview of the methods to fabricate a transparent nanofibrous membrane based on the electrospinning technology including direct electrospinning, solution treatment after electrospinning, heat treatment after electrospinning, and surface modification after electrospinning. It further summarizes the differences in the processes and mechanisms between different transparent fibrous membranes prepared by different methods. Additionally, we study the utilization of transparent as-spun membranes as flexible functional materials, namely alcohol dipstick, air purification, self-cleaning materials, biomedicine, sensors, energy and optoelectronics, oil-water separation, food packaging, anti-icing coating, and anti-corrosion materials. It demonstrates the high transparency of the nanofibers' effects on the applications as well as upgrades the product performance.

Titanium Nanopillar Arrays Functioning as Electron Transporting Layers for Efficient, Anti-Aging Perovskite Solar Cells

Electron transporting layers (ETLs), required to be optically transparent in perovskite solar cells (PSCs) having regular structures, possess a determinant effect on electron extraction and collection. Metal oxides (e.g., TiO₂ ) have overwhelmingly served as ETLs, but usually have low electron mobility (μ_e  < 10^-2  cm² V^-1 s^-1 ) not favorable for photovoltaic conversion. Here, metal oxides are replaced with metals (e.g., Ti with μ_e  ≈ 294 cm² V^-1 s^-1 ) that are sculptured via glancing angle deposition to be a close-packed nanopillar array (NaPA), which vertically protrudes on a transparent electrode to obtain sufficient optical transmission for light harvesting in perovskite. Ti NaPAs, whose rough surfaces are passivated with 5 nm thick TiO₂ (i.e., Ti NaPAs@TiO₂ ) to suppress exciton recombination, lead to the champion power conversion efficiency (PCE) of 18.89% that is superior to that of MAPbI₃ PSCs without Ti NaPAs@TiO₂ or containing TiO₂ NaPAs@TiO₂ , owing to high surface wettability, high μ_e , and relatively low work function of Ti. Furthermore, Ti NaPAs@TiO₂ effectively prevents the decomposition of MAPbI₃ to achieve long-term shelf stability whereby 50-day aging only causes 15% PCE degradation. This work paves the way toward widening the material spectrum, from semiconductors to metals, to generate a diverse range of ETLs for producing efficient optoelectronic devices with long-term shelf stability.

Free-Standing Carbon Nanofibers Protected by a Thin Metallic Iridium Layer for Extended Life-Cycle Li-Oxygen Batteries

It is evident that the exhaustive use of fossil fuels for decades has significantly contributed to global warming and environmental pollution. To mitigate the harm on the environment, lithium-oxygen batteries (LOBs) with a high theoretical energy density (3458 Wh kg^-1Li₂O₂) compared to that of Li-ion batteries (LIBs) have been considered as an attractive alternative to fossil fuels. For this purpose, porous carbon materials have been utilized as promising air cathodes owing to their low cost, lightness, easy fabrication process, and high performance. However, the challenge thus far lies in the uncontrollable formation of Li₂CO₃ at the interface between carbon and Li₂O₂, which is detrimental to the stable electrochemical performance of carbon-based cathodes in LOBs. In this work, we successfully protected the surface of the free-standing carbon nanofibers (CNFs) by coating it with a layer of iridium metal through direct sputtering (CNFs@Ir), which significantly improved the lifespan of LOBs. Moreover, the Ir would play a secondary role as an electrochemical catalyst. This all-in-one cathode was evaluated for the formation and decomposition of Li₂O₂ during (dis)charging processes. Compared with bare CNFs, the CNFs@Ir cathode showed two times longer lifespan with 0.2 V_Li lower overpotentials for the oxygen evolution reaction. We quantitatively calculated the contents of CO₃^2- in Li₂CO₃ formed on the different surfaces of the bare CNFs (63% reduced) and the protected CNFs@Ir (78% reduced) cathodes after charging. The protective effects and the reaction mechanism were elucidated by ex situ analyses, including scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.

Hydrogel-Assisted Electrospinning for Fabrication of a 3D Complex Tailored Nanofiber Macrostructure

Electrospinning has shown great potential in tissue engineering and regenerative medicine due to a high surface-area-to-volume ratio and an extracellular matrix-mimicking structure of electrospun nanofibers, but the fabrication of a complex three-dimensional (3D) macroscopic configuration with electrospun nanofibers remains challenging. In the present study, we developed a novel hydrogel-assisted electrospinning process (GelES) to fabricate a 3D nanofiber macrostructure with a 3D complex but tailored configuration by utilizing a 3D hydrogel structure as a grounded collector instead of a metal collector in conventional electrospinning. The 3D hydrogel collector was discovered to effectively concentrate the electric field toward itself similar to the metal collector, thereby depositing electrospun nanofibers directly on its exterior surface. Synergistic advantages of the hydrogel (e.g., biocompatibility and thermally reversible sol-gel transition) and the 3D nanofiber macrostructure (e.g., mechanical robustness and high permeability) provided by the GelES process were demonstrated in a highly permeable tubular tissue graft and a robust drug- or cell-encapsulation construct. GelES is expected to broaden potential applications of electrospinning to not only provide in vivo drug/cell delivery and tissue regeneration but also an in vitro drug testing platform by increasing the degree of freedom in the configuration of the 3D nanofiber macrostructure.

Performance-Enhancing Approaches for PEDOT:PSS-Si Hybrid Solar Cells

The emerging energy crisis has focused significant worldwide attention on solar cells. Although crystalline silicon solar cells are currently widely used, their high cost limits the development of solar power generation. Consequently, hybrid solar cells are becoming increasingly important, especially organic-Si hybrid solar cells (HSCs). Organic-Si HSCs combine a mature technology and high efficiency with the low-temperature manufacturing process and tunable optoelectronic properties of organic solar cells. The organic material can be P3HT, carbon nanotubes, graphene, and PEDOT:PSS. Here we review the performance of PEDOT:PSS/Si HSCs and methods for improving their efficiency, such as PEDOT:PSS modification, optimization of the trapping effect, passivation of the silicon surface, addition of an interface layer, improvement of a back contact, and optimization of the metal top electrode. This Review should help fill the gap in this area and provide perspectives for the future development of the PEDOT:PSS/Si HSCs.

Microwave-assisted calcination of electrospun indium-gallium-zinc oxide nanofibers for high-performance field-effect transistors

The effects of microwave-assisted calcination of electrospun In-Ga-Zn-O (IGZO) nanofibers intended for electronic devices are unclear. To this end, a systematic study was conducted on the effects of microwave-assisted calcination on the microstructure and optical and mechanical properties of electrospun IGZO nanofibers used in high-performance field-effect transistors (FETs). To clarify the role of microwave annealing (MWA) on the characteristics of the electrospun nanofibers, calcination was carried out using two techniques: MWA and conventional thermal annealing (CTA). The morphological differences between IGZO nanofibers calcined using the two techniques were analyzed by scanning electron microscopy (SEM); the diameter of nanofibers was significantly reduced through MWA as compared to CTA. After calcination, the optical transmittance in the visible region was slightly improved, with the MWA-calcined nanofibers exhibiting a higher transmittance than the CTA-calcined nanofibers. Scratch test results showed that the calcination improved the adhesion strength of the nanofibers to the SiO2 substrate; MWA was more effective in improving the mechanical properties than CTA. Furthermore, the effects of MWA calcination on the electrical properties of FETs fabricated using the electrospun IGZO nanofibers were investigated. The MWA-calcined devices showed better electrical characteristics and reliability than the CTA-calcined devices for IGZO nanofiber FETs.

Transparent and Freestanding Single-Walled Carbon Nanotube Films Synthesized Directly and Continuously via a Blown Aerosol Technique

Single-walled carbon nanotube (SWCNT) films are promising materials as flexible transparent conductive films (TCFs). Here, inspired by the extrusion blown plastic film technique and the SWCNT synthesis approach by floating catalyst chemical vapor deposition (FCCVD), a novel blown aerosol chemical vapor deposition (BACVD) method is reported to directly and continuously produce freestanding SWCNT TCFs at several hundred meters per hour. The synthesis mechanism, involving blowing a stable aerosol bubble and transforming the bubble into an aerogel, is investigated, and a general phase diagram is established for this method. For the SWCNT TCFs via BACVD, both carbon conversion efficiency and SWCNT TCF yield can reach three orders of magnitude higher than those with the conventional FCCVD. The film displays a sheet resistance of 40 ohm sq^-1 at 90% transmittance after being doped, representing the record performance based on large-scale SWCNT films. Transparent, flexible, and stretchable electrodes based on BACVD films are demonstrated. Moreover, this high-throughput method of producing SWCNT TCFs can be compatible with the roll-to-roll process for mass production of flexible displays, touch screens, solar cells, and solid-state lighting, and is expected to have a broad and long-term impact on many fields from consumer electronics to energy conversion and generation.

A Review of Recent Advancements in Electrospun Anode Materials to Improve Rechargeable Lithium Battery Performance

Although lithium-ion batteries have already had a considerable impact on making our lives smarter, healthier, and cleaner by powering smartphones, wearable devices, and electric vehicles, demands for significant improvement in battery performance have grown with the continuous development of electronic devices. Developing novel anode materials offers one of the most promising routes to meet these demands and to resolve issues present in existing graphite anodes, such as a low theoretical capacity and poor rate capabilities. Significant improvements over current commercial batteries have been identified using the electrospinning process, owing to a simple processing technique and a wide variety of electrospinnable materials. It is important to understand previous work on nanofiber anode materials to establish strategies that encourage the implementation of current technological developments into commercial lithium-ion battery production, and to advance the design of novel nanofiber anode materials that will be used in the next-generation of batteries. This review identifies previous research into electrospun nanofiber anode materials based on the type of electrochemical reactions present and provides insights that can be used to improve conventional lithium-ion battery performances and to pioneer novel manufacturing routes that can successfully produce the next generation of batteries.

Designing Polymeric Mixed Conductors and Their Application to Electrochemical-Transistor-Based Biosensors

Organic electrochemical transistors that employ polymeric mixed conductors as their active channels are one of the most prominent biosensor platforms because of their signal amplification capability, low fabrication cost, mechanical flexibility, and various properties tunable through molecular design. For application to biomedical devices, polymeric mixed conductors should fulfill several requirements, such as excellent conductivities of both holes/electrons and ions, long-term operation stability, and decent biocompatibility. However, trade-offs may exist, for instance, one between ionic conduction and overall device stability. In this report, the fundamental understanding of polymeric mixed conductors, the recent advance in enhancing their ionic and electrical conductivity, and their practical applications as biosensors based on organic electrochemical transistors are reviewed. Finally, key strategies are suggested for developing novel polymeric mixed conductors that may exceed the trade-off between device performance and stability.

Synthesis of Ultrathin Metal Nanowires with Chemically Exfoliated Tungsten Disulfide Nanosheets

Transition metal dichalcogenides (TMDs) have attracted great interest owing to their fascinating properties with atomically thin nature. Although TMDs have been exploited for diverse applications, the effective role of TMDs in the synthesis of metal nanowires has not been explored. Here, we propose a new approach to synthesize ultrathin metal nanowires using TMDs for the first time. High-quality ultrathin nanowires with an average diameter of 11.3 nm are successfully synthesized for realizing high-performance transparent conductors that exhibit excellent conductivity and transparency with low haze. The growth mechanism is carefully investigated using high-resolution transmission electron microscopy, and growth of nanowires with tunable diameters is achieved by controlling the nanosheet dimension. Finally, we unravel the important role of TMDs acting as both reducing and nucleating agents. Therefore, our work provides a new strategy of the TMD as an innovative material for the growth of metal nanowires as a promising building block in next-generation optoelectronics.

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics

The development of fiber-based smart electronics has provoked increasing demand for high-performance and multifunctional fiber materials. Carbon nanotube (CNT) fibers, the 1D macroassembly of CNTs, have extensively been utilized to construct wearable electronics due to their unique integration of high porosity/surface area, desirable mechanical/physical properties, and extraordinary structural flexibility, as well as their novel corrosion/oxidation resistivity. To take full advantage of CNT fibers, it is essential to understand their mechanical and conductive properties. Herein, the recent progress regarding the intrinsic structure-property relationship of CNT fibers, as well as the strategies of enhancing their mechanical and conductive properties are briefly summarized, providing helpful guidance for scouting ideally structured CNT fibers for specific flexible electronic applications.