Spinning Hierarchical Gold Nanowire Microfibers by Shear Alignment and Intermolecular Self-Assembly

Thermoelectric nanowires for dense 3D printed architectures

The large-scale employment of 3D printed inorganic thermoelectrics is primarily constrained because of their lower efficiencies as compared to those fabricated from conventional methods such as spark plasma sintering and hot-pressing. This originates from the significant challenge in the densification of printed parts, particularly through the direct-ink-writing fabrication process, which demands a high binder content for printability. To achieve high-density printed thermoelectrics, the ink formulation process often involves the addition of substantial filler content and sintering aids, coupled with prolonged sintering periods. Here, we propose a strategy to resolve the low densification issue of 3D printed thermoelectrics through a binder-less and sintering aid-free thermoelectric nanowire ink system that can achieve dense thermoelectric structures (up to 82.5% theoretical density). The increase in density and corresponding enhancement of thermoelectric material efficiency are attained in a more tunable and controlled manner without compromising the material composition. A high filler-derived density index (FDI) of 2.51 is also achieved, implying the potential to obtain high-density parts with minimal filler content, thus unlocking a cascade of profound impacts. Crucially, this advancement enables the possibilities of anisotropic engineering in thermoelectric materials, thereby shattering the limitations that have hindered the widespread adoption of 3D printed inorganic thermoelectrics.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

Fibrous wearable and implantable bioelectronics

Fibrous wearable and implantable devices have emerged as a promising technology, offering a range of new solutions for minimally invasive monitoring of human health. Compared to traditional biomedical devices, fibers offer a possibility for a modular design compatible with large-scale manufacturing and a plethora of advantages including mechanical compliance, breathability, and biocompatibility. The new generation of fibrous biomedical devices can revolutionize easy-to-use and accessible health monitoring systems by serving as building blocks for most common wearables such as fabrics and clothes. Despite significant progress in the fabrication, materials, and application of fibrous biomedical devices, there is still a notable absence of a comprehensive and systematic review on the subject. This review paper provides an overview of recent advancements in the development of fibrous wearable and implantable electronics. We categorized these advancements into three main areas: manufacturing processes, platforms, and applications, outlining their respective merits and limitations. The paper concludes by discussing the outlook and challenges that lie ahead for fiber bioelectronics, providing a holistic view of its current stage of development.

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.

Controlled filamentation instability as a scalable fabrication approach to flexible metamaterials

Long and flexible arrays of nanowires find impactful applications in sensing, photonics, and energy harvesting. Conventional manufacturing relies largely on lithographic methods limited in wafer size, rigidity, and machine write time. Here, we report a scalable process to generate encapsulated flexible nanowire arrays with high aspect ratios and excellent tunable size and periodicity. Our strategy is to control nanowire self-assembly into 2D and 3D architectures via the filamentation of a textured thin film under anisotropic stretching. This is achieved by coupling soft lithography, glancing angle deposition, and thermal drawing to obtain well-ordered meters-long nanowires with diameters down to 50 nanometers. We demonstrate that the nanowire diameter and period of the array can be decoupled and manipulated independently. We propose a filamentation criterion and perform numerical simulations implementing destabilizing long-range Van der Waals interactions. Applied to high-index chalcogenide glasses, we show that this decoupling allows for tuning diffraction. Finally, harnessing Mie resonance, we demonstrate the possibility of manufacturing macroscopic meta-grating superstructures for nanophotonic applications.

The simple and scalable fabrication of high aspect ratio encapsulated flexible nanowire arrays with controlled size and periodicity is demonstrated and modelled, enabling nanophotonics applications requiring long-range order over several meters.

Water-phase synthesis of Au and Au-Ag nanowires and their SERS activity

Metal nanowires (NWs) with a diameter of a few nanometers have attracted considerable attention as a promising one-dimensional nanomaterial due to their inherent flexibility and conductive properties and their weak plasmon absorption in the visible region. In a previous paper, we reported the synthesis of ultrathin 1.8 nm-diameter Au NWs using toluene-solubilized aqueous solutions of a long-chain amidoamine derivative (C18AA). This study investigates the effect of different organic solvents solubilized in C18AA aqueous solutions on the morphology of the Au products and demonstrates that solubilizing methylcyclohexane yields thick 2.7 nm-diameter Au NWs and 3.3 nm-diameter Au-Ag alloy NWs. Further, the surface-enhanced Raman scattering sensitivity of ultrathin Au NWs, thick Au NWs, and thick Au-Ag alloy NWs were assessed using 4-mercaptopyridine and found that their enhancement factors are 10⁴-10⁵ and the order is Au-Ag NWs > thick Au NWs > ultrathin Au NWs.

Programmable dynamic interfacial spinning of bioinspired microfibers with volumetric encoding

While artificially encoded microfibers inspired by biosynthetic fibrous microstructures are drawing considerable research attention, their practical applications are hindered by multiple limitations. Here, a programmable dynamic interfacial spinning (DIS) process is proposed for producing volume-encoded microfibers with superior encoding capacity and reliability. The produced microfibers comprise a sheath of deformed hydrogel encapsulating sequentially aligned droplets, with their morphologies controllable by adjusting the flow rates of the corresponding fluids and the vibration parameters of the spinning nozzle. In particular, microfibers with volumetric encoding of inner droplet sequence are constructed for information storage and encryption. With appropriate functionalization of volume-encoded microfibers, we have also demonstrated magnetic guidance and selective activation to simulate intravascular drug delivery. Our study implies the potential applications of the volume-encoded microfibers in information communication, drug delivery and biomedical engineering.

Sub-Nanometer Nanobelts Based on Titanium Dioxide/Zirconium Dioxide-Polyoxometalate Heterostructures

In addition to offering conformational flexibility, sub-nanometer nanobelts (SNBs) also outperform many larger nanobelts with large size owing to their ultrathin morphologies. However, to date, only a few monocomponent SNBs have been synthesized. This study presents a facile method for synthesizing ZrO₂ -PMoO (PMZ) SNBs and TiO₂ -PMoO (PMT) SNBs with heterostructures. The SNBs comprise ZrO₂ /TiO₂ and polyoxometalate (POM) nanoclusters, which are formed via the aggregation and subsequent transformation of nanoclusters. Significantly, these SNBs demonstrate high catalytic activity and stability in oxidative desulfurization reactions at room temperature. The impressive catalytic performance of the SNBs is aided by the POM nanoclusters, which not only coassemble with ZrO₂ /TiO₂ nuclei to form building blocks of PMZ SNBs/PMT SNBs but also serve as catalytic centers. The catalytic performance is further enhanced by the ZrO₂ /TiO₂ in the SNBs. Moreover, the proposed synthesis method can be utilized to produce other SNBs. Thus, this method provides valuable insights into the strong performance properties of SNBs created by combining metal oxides and POM nanoclusters into SNBs, which have great potential as redox catalysts.

Nonspherical Metal-Based Nanoarchitectures: Synthesis and Impact of Size, Shape, and Composition on Their Biological Activity

Metal-based nanoentities, apart from being indispensable research tools, have found extensive use in the industrial and biomedical arena. Because their biological impacts are governed by factors such as size, shape, and composition, such issues must be taken into account when these materials are incorporated into multi-component ensembles for clinical applications. The size and shape (rods, wires, sheets, tubes, and cages) of metallic nanostructures influence cell viability by virtue of their varied geometry and physicochemical interactions with mammalian cell membranes. The anisotropic properties of nonspherical metal-based nanoarchitectures render them exciting candidates for biomedical applications. Here, the size-, shape-, and composition-dependent properties of nonspherical metal-based nanoarchitectures are reviewed in the context of their potential applications in cancer diagnostics and therapeutics, as well as, in regenerative medicine. Strategies for the synthesis of nonspherical metal-based nanoarchitectures and their cytotoxicity and immunological profiles are also comprehensively appraised.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.

Entropy Can Bundle Nanowires in Good Solvents

Surfaces with surface-bound ligand molecules generally attract each other when immersed in poor solvents but repel each other in good solvents. While this common wisdom holds, for example, for oleylamine-ligated ultrathin nanowires in the poor solvent ethanol, the same nanowires were recently observed experimentally to bundle even when immersed in the good solvent n-hexane. To elucidate the respective binding mechanisms, we simulate both systems using molecular dynamics. In the case of ethanol, the solvent is completely depleted at the interface between two ligand shells so that their binding occurs, as expected, via direct interactions between ligands. In the case of n-hexane, ligands attached to different nanowires do not touch. The binding occurs because solvent molecules penetrating the shells preferentially orient their backbone normal to the wire, whereby they lose entropy. This entropy does not have to be summoned a second time when the molecules penetrate another nanowire. For the mechanism to be effective, the ligand density appears to best be intermediate, that is, small enough to allow solvent molecules to penetrate, but not so small that ligands do not possess a clear preferred orientation at the interface to the solvent. At the same time, solvent molecules may be neither too large nor too small for similar reasons. Experiments complementing the simulations confirm the predicted trends.

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

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

Bioinspired Macroscopic Ribbon Fibers with a Nacre-Mimetic Architecture Based on Highly Ordered Alignment of Ultralong Hydroxyapatite Nanowires

A variety of biological materials in natural organisms supply a rich source of structural design guidelines and inspirations for the construction of advanced structural materials with excellent mechanical properties. In this work, inspired by the natural nacre and human bone, a kind of flexible macroscopic ribbon fiber made from highly ordered alignment of ultralong hydroxyapatite (HAP) nanowires and sodium polyacrylate (PAAS) with a "brick-and-mortar" layered structure has been developed by a scalable and convenient wet-spinning method. The quasi-long-range orderly liquid crystal of one-dimensional ultralong hydroxyapatite nanowires is employed and spun into the continuous flexible macroscopic ribbon fiber. In this work, highly ordered ultralong HAP nanowires act as the hard "brick" and PAAS acts as the soft "mortar", and the nacre-mimetic layered architecture is obtained. The as-prepared flexible macroscopic HAP/PAAS ribbon fiber exhibits superior mechanical properties, and the maximum tensile strength and Young's modulus are as high as 203.58 ± 45.38 MPa and 24.56 ± 5.35 GPa, respectively. In addition, benefiting from the excellent flexibility and good knittability, the as-prepared macroscopic HAP/PAAS ribbon fiber can be woven into various flexible macroscopic architectures. Additionally, the as-prepared flexible macroscopic HAP/PAAS ribbon fiber can be further functionalized by incorporation of various functional components, such as magnetic and photoluminescent constituents. The as-prepared flexible macroscopic HAP/PAAS ribbon fiber has potential applications in various fields such as smart wearable devices, optical devices, magnetic devices, and biomedical engineering.

Nanowire Assemblies for Flexible Electronic Devices: Recent Advances and Perspectives

The fabrication of nanowire (NW)-based flexible electronics including wearable energy storage devices, flexible displays, electrical sensors, and health monitors has received great attention both in fundamental research and market requirements in our daily lives. Other than a disordered state after synthesis, NWs with designed and hierarchical structures would not only optimize the intrinsic performance, but also create new physical and chemical properties, and integration of individual NWs into well-defined structures over large areas is one of the most promising strategies to optimize the performance of NW-based flexible electronics. Here, the recent developments and achievements made in the field of flexible electronics composed of integrated NW structures are presented. The different assembly strategies for the construction of 1D, 2D, and 3D NW assemblies, especially the NW coassembly process for 2D NW assemblies, are comprehensively discussed. The improvements of different NW assemblies on flexible electronics structure and performance are described in detail to elucidate the advantages of well-defined NW assemblies. Finally, a short summary and outlook for future challenges and perspectives in this field are presented.

Facile Access to Wearable Device via Microfluidic Spinning of Robust and Aligned Fluorescent Microfibers

Microfluidic spinning technology (MST) has drawn much attention owing to its ideal platform for ordered fluorescent fibers, along with their large-scale manipulation, high efficiency, flexibility, and environment friendliness. Here, we employed the MST to fabricate a series of uniform fluorescent microfibers. By adjusting the microfluidic spinning parameters, the as-prepared microfibers of different diameters are successfully obtained. For more practice, these regular arranged fibers could be formed to versatile fluorescent codes by using various microfluidic chips. Also, these versatile fluorescent fibers could be further weaved into a white fluorescent film via continuous and cross-spinning process, which could be applied in a white light emitting diode (WLED) and a wearable device. Besides, we investigated the MST-directed microreactors to carry out green synthesis of CdSe quantum dots (QDs) fibers by the knot of Y-type microfluidic chip. The as-prepared CdSe QDs show nice optical property and are good candidate as phosphors in WLED. This strategy offers a facile and environment-friendly route to fluorescent hybrid microfibers and might open their potential application in optical devices, security, and fluorescent coding.

Size-Dependent Orientational Dynamics of Brownian Nanorods

Successful assembly of suspended nanoscale rod-like particles depends on fundamental phenomena controlling rotational and translational diffusion. Despite the significant developments in fluidic fabrication of nanostructured materials, the ability to quantify the dynamics in processing systems remains challenging. Here we demonstrate an experimental method for characterization of the orientation dynamics of nanorod suspensions in assembly flows using orientation relaxation. This relaxation, measured by birefringence and obtained after rapidly stopping the flow, is deconvoluted with an inverse Laplace transform to extract a length distribution of aligned nanorods. The methodology is illustrated using nanocelluloses as model systems, where the coupling of rotational diffusion coefficients to particle size distributions as well as flow-induced orientation mechanisms are elucidated.

Synthesis of Ultralong Copper Nanowires for High-Performance Flexible Transparent Conductive Electrodes: The Effects of Polyhydric Alcohols

Copper nanowires (Cu NWs) have become a promising material for flexible transparent conductive electrodes (FTCEs) owing to their outstanding transparency and conductivity properties. In this work, ultralong Cu NWs with an average length over 250 μm and a diameter of around 50 nm (aspect ratio ∼5000) were synthesized in a water/polyhydric alcohol cosolvent. The effects of polyhydric alcohols (including ethanol, ethylene glycol, and glycerol) on the aspect ratio of Cu NWs were investigated. The diameter of Cu NWs decreased with the increased number of hydroxyl groups of polyhydric alcohols. In addition, the capping ligands (oleylamine and oleic acid) and glucose also exhibit important effects on the dispersity and morphology of Cu NWs. The ultralong Cu NW-based poly(dimethylsiloxane) (PDMS) FTCEs exhibit high performance with a low sheet resistance of 92.1 Ω sq^-1 at a transmittance of 91.524%. Inspired by the stretchable ability of PDMS, wearable sensors were fabricated to detect the movement of the finger joint through the chronoamperometry method. The prepared sensors exhibit high sensitivity and a fast response time. The excellent performance of FTCEs and wearable sensors suggests that the ultralong Cu NWs have a bright future in the application of the next generation of flexible optoelectronic devices.

Bioinspired Nanocomposite Hydrogels with Highly Ordered Structures

In the human body, many soft tissues with hierarchically ordered composite structures, such as cartilage, skeletal muscle, the corneas, and blood vessels, exhibit highly anisotropic mechanical strength and functionality to adapt to complex environments. In artificial soft materials, hydrogels are analogous to these biological soft tissues due to their "soft and wet" properties, their biocompatibility, and their elastic performance. However, conventional hydrogel materials with unordered homogeneous structures inevitably lack high mechanical properties and anisotropic functional performances; thus, their further application is limited. Inspired by biological soft tissues with well-ordered structures, researchers have increasingly investigated highly ordered nanocomposite hydrogels as functional biological engineering soft materials with unique mechanical, optical, and biological properties. These hydrogels incorporate long-range ordered nanocomposite structures within hydrogel network matrixes. Here, the critical design criteria and the state-of-the-art fabrication strategies of nanocomposite hydrogels with highly ordered structures are systemically reviewed. Then, recent progress in applications in the fields of soft actuators, tissue engineering, and sensors is highlighted. The future development and prospective application of highly ordered nanocomposite hydrogels are also discussed.