Shape-Morphing in Oxide Ceramic Kirigami Nanomembranes

Interfacial strain engineering in ferroic nanomembranes can broaden the scope of ferroic nanomembrane assembly as well as facilitate the engineering of multiferroic-based devices with enhanced functionalities. Geometrical engineering in these material systems enables the realization of 3-D architectures with unconventional physical properties. Here, 3-D multiferroic architectures are introduced by incorporating barium titanate (BaTiO3, BTO) and cobalt ferrite (CoFe2O4, CFO) bilayer nanomembranes. Using photolithography and substrate etching techniques, complex 3-D microarchitectures including helices, arcs, and kirigami-inspired frames are developed. These 3-D architectures exhibit remarkable mechanical deformation capabilities, which can be attributed to the superelastic behavior of the membranes and geometric configurations. It is also demonstrated that dynamic shape reconfiguration of these nanomembrane architectures under electron beam exposure showcases their potential as electrically actuated microgrippers and for other micromechanical applications. This research highlights the versatility and promise of multi-dimensional ferroic nanomembrane architectures in the fields of micro actuation, soft robotics, and adaptive structures, paving the way for incorporating these architectures into stimulus-responsive materials and devices.

3D Printing of Metals with sub-10 µm Resolution

The ability to manufacture 3D metallic architectures with microscale resolution is greatly pursued because of their diverse applications in microelectromechanical systems (MEMS) including microelectronics, mechanical metamaterials, and biomedical devices. However, the well-developed photolithography and emerging metal additive manufacturing technologies have limited abilities in manufacturing micro-scaled metallic structures with freeform 3D geometries. Here, for the first time, the high-fidelity fabrication of arbitrary metallic motifs with sub-10 µm resolution is achieved by employing an embedded-writing embedded-sintering (EWES) process. A paraffin wax-based supporting matrix with high thermal stability is developed, which permits the printed silver nanoparticle ink to be pre-sintered at 175 °C to form metallic green bodies. Via carefully regulating the matrix components, the printing resolution is tuned down to ≈7 µm. The green bodies are then embedded in a supporting salt bath and further sintered to realize freeform 3D silver motifs with great structure fidelity. 3D printing of various micro-scaled silver architectures is demonstrated such as micro-spring arrays, BCC lattices, horn antenna, and rotatable windmills. This method can be extended to the high-fidelity 3D printing of other metals and metal oxides which require high-temperature sintering, providing the pathways toward the design and fabrication of 3D MEMS with complex geometries and functions.

An Approach to a Silver Conductive Ink for Inkjet Printer Technology

Silver-based metal-organic decomposition inks composed of silver salts, complexing agents and volatile solvents are now the subject of much research due to the simplicity and variability of their preparation, their high stability and their relatively low sintering temperature. The use of this type of ink in inkjet printing allows for improved cost-effective and environmentally friendly technology for the production of electrical devices, including flexible electronics. An approach to producing a silver salt-based reactive ink for jet printing has been developed. The test images were printed with an inkjet printer onto polyimide substrates, and two-stage thermal sintering was carried out at temperatures of 60 °C and 100-180 °C. The structure and electrical properties of the obtained conductive lines were investigated. As a result, under optimal conditions an electrically conductive film with low surface resistance of approximately 3 Ω/square can be formed.

Moving Meniscus-Assisted Template-Free Optothermofluidic Nanoparticle Patterning and Its Application in Optothermoconvective Particle Trapping

Moving meniscus-assisted vertical lifting is a commonly employed particle assembly technique to realize large-area particle patterning for the easy fabrication of colloidal photonic crystals and sensors. Though great success has been achieved for large-area patterning, inscribing desired patterns over the target substrate with precise control over the morphology remains a challenge. The target substrates need to be functionalized (physically or chemically) to realize desired patterns, which increases the complexity and limits their applicability to specific particle-liquid combinations. We demonstrate a new approach for the precise patterning of gold nanoparticles (Au NPs, diameter ∼60 nm) over solid substrates by the synergy of light-induced Marangoni flow and vertical lifting process (moving meniscus), without the requirement of photomasks or templates. The core idea relies on the particle accumulation due to light-induced Marangoni flow near the liquid meniscus in contact with a solid surface (due to plasmonic absorption of the particles) and the controlled lifting of the substrate. We present both the simulation and experimental results of the developed patterning technique. Various patterns such as continuous lines, intermittent lines with varying lengths, patterns with continuously varying widths, cross patterns, etc. are successfully inscribed. Dynamic control over the three-dimensional morphology of the deposited patterns is achieved by varying the lifting velocity, laser irradiation time, and lifting direction during the inscription process. Finally, we show the applicability of the developed plasmonically active surface for the large-area parallel manipulation of nonabsorbing microparticles based on optothermoconvective flow. The major advantage of the developed method compared to the existing light-controlled patterning techniques is its ability to inscribe patterns over large distances (up to several centimeters). We expect that the results presented in this paper will benefit different applications requiring precise particle patterning, such as optical elements, sensors, plasmonic substrates, microfluidic master templates, and electronic circuits.

Piezo inkjet formation of Ag nanoparticles from microdots arrays for surface plasmonic resonance

The study aims to explore a novel approach for fabricating plasmonic nanostructures to enhance the optical properties and performance of various optoelectronic devices. The research begins by employing a piezo-inkjet printing technique to deposit drops containing Ag nanoparticles (NPs) onto a glass substrate at a predefined equidistance, with the goal of obtaining arrays of Ag microdots (Ag-µdots) on the glass substrate. This process is followed by a thermal annealing treatment. The printing parameters are first optimized to achieve uniform deposition of different sizes of Ag-µdots arrays by controlling the number of Ag ink drops. Subsequently, the printed arrays undergo thermal annealing at various temperatures in air for 60 min, enabling precise and uniform control over nanoparticle formation. The printed Ag nanoparticles are characterized using field emission scanning electron microscopy and atomic force microscopy to analyze their morphological features, ensuring their suitability for plasmonic applications. UV-Vis spectrophotometry is employed to investigate the enhanced surface-plasmonic-resonance properties of the printed AgNPs. Measurements confirm that the equidistant arrays of AgNPs obtained from annealing Ag microdots exhibit enhanced light-matter interaction, leading to a surface plasmon resonance response dependent on the Ag NPs' specific surface area. These enhanced surface plasmonic resonances open avenues for developing cutting-edge optoelectronic devices that leverage the benefits of plasmonic nanostructures, thereby enabling new opportunities for future technological developments across various fields.

Evolution of the Microrobots: Stimuli-Responsive Materials and Additive Manufacturing Technologies Turn Small Structures into Microscale Robots

The development of functional microsystems and microrobots that have characterized the last decade is the result of a synergistic and effective interaction between the progress of fabrication techniques and the increased availability of smart and responsive materials to be employed in the latter. Functional structures on the microscale have been relevant for a vast plethora of technologies that find application in different sectors including automotive, sensing devices, and consumer electronics, but are now also entering medical clinics. Working on or inside the human body requires increasing complexity and functionality on an ever-smaller scale, which is becoming possible as a result of emerging technology and smart materials over the past decades. In recent years, additive manufacturing has risen to the forefront of this evolution as the most prominent method to fabricate complex 3D structures. In this review, we discuss the rapid 3D manufacturing techniques that have emerged and how they have enabled a great leap in microrobotic applications. The arrival of smart materials with inherent functionalities has propelled microrobots to great complexity and complex applications. We focus on which materials are important for actuation and what the possibilities are for supplying the required energy. Furthermore, we provide an updated view of a new generation of microrobots in terms of both materials and fabrication technology. While two-photon lithography may be the state-of-the-art technology at the moment, in terms of resolution and design freedom, new methods such as two-step are on the horizon. In the more distant future, innovations like molecular motors could make microscale robots redundant and bring about nanofabrication.

A Guide to Printed Stretchable Conductors

Printing of stretchable conductors enables the fabrication and rapid prototyping of stretchable electronic devices. For such applications, there are often specific process and material requirements such as print resolution, maximum strain, and electrical/ionic conductivity. This review highlights common printing methods and compatible inks that produce stretchable conductors. The review compares the capabilities, benefits, and limitations of each approach to help guide the selection of a suitable process and ink for an intended application. We also discuss methods to design and fabricate ink composites with the desired material properties (e.g., electrical conductance, viscosity, printability). This guide should help inform ongoing and future efforts to create soft, stretchable electronic devices for wearables, soft robots, e-skins, and sensors.

3D Printing Manufacturing of Lithium Batteries: Prospects and Challenges toward Practical Applications

The manufacturing and assembly of components within cells have a direct impact on the sample performance. Conventional processes restrict the shapes, dimensions, and structures of the commercially available batteries. 3D printing, a novel manufacturing process for precision and practicality, is expected to revolutionize the lithium battery industry owing to its advantages of customization, mechanization, and intelligence. This technique can be used to effectively construct intricate 3D structures that enhance the designability, integrity, and electrochemical performance of both liquid- and solid-state lithium batteries. In this study, an overview of the development of 3D printing technologies is provided and their suitability for comparison with conventional printing processes is assessed. Various 3D printing technologies applicable to lithium-ion batteries have been systematically introduced, especially more practical composite printing technologies. The practicality, limitations, and optimization of 3D printing are discussed dialectically for various battery modules, including electrodes, electrolytes, and functional architectures. In addition, all-printed batteries are emphatically introduced. Finally, the prospects and challenges of 3D printing in the battery industry are evaluated.

Rapid Fabrication of High-Resolution Flexible Electronics via Nanoparticle Self-Assembly and Transfer Printing

Printed electronic technology serves as a key component in flexible electronics and wearable devices, yet achieving compatibility with both high resolution and high efficiency remains a significant challenge. Here, we propose a rapid fabrication method of high-resolution nanoparticle microelectronics via self-assembly and transfer printing. The tension gradient-electrostatic attraction composite-induced nanoparticle self-assembly strategy is constructed, which can significantly enhance the self-assembly efficiency, stability, and coverage by leveraging the meniscus Marangoni effect and the electric double-layer effect. The close-packed nanoparticle self-assembly layer can be rapidly formed on microstructure surfaces over a large area. Inspired by ink printing, a transfer printing strategy is further proposed to transform the self-assembly layer into conformal micropatterns. Large-area, high-resolution (2 μm), and ultrathin (1 μm) nanoparticle microelectronics can be stably fabricated, yielding a significant improvement over fluid printing methods. The unique deformability, recoverability, and scalability of nanoparticle microelectronics are revealed, providing promising opportunities for various academic and real applications.

3D microprinting of inorganic porous materials by chemical linking-induced solidification of nanocrystals

Three-dimensional (3D) microprinting is considered a next-generation manufacturing process for the production of microscale components; however, the narrow range of suitable materials, which include mainly polymers, is a critical issue that limits the application of this process to functional inorganic materials. Herein, we develop a generalised microscale 3D printing method for the production of purely inorganic nanocrystal-based porous materials. Our process is designed to solidify all-inorganic nanocrystals via immediate dispersibility control and surface linking-induced interconnection in the nonsolvent linker bath and thereby creates multibranched gel networks. The process works with various inorganic materials, including metals, semiconductors, magnets, oxides, and multi-materials, not requiring organic binders or stereolithographic equipment. Filaments with a diameter of sub-10 μm are printed into designed complex 3D microarchitectures, which exhibit full nanocrystal functionality and high specific surface areas as well as hierarchical porous structures. This approach provides the platform technology for designing functional inorganics-based porous materials.

Omnidirectional Printing of PEDOT:PSS for High-Conductivity Spanning Structures

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a prominent conducting polymer, holds significance in both industry and academia. However, prevailing fabrication techniques struggle to build spanning features of PEDOT:PSS with both high electrical conductivity and fine resolution due to layerwise assembly in the xy plane. Here, we report an "omnidirectional printing and secondary doping" strategy to construct spanning, filamentary and out-of-plane 3D PEDOT:PSS with high conductivity. The pristine PEDOT:PSS suspension is homogeneously concentrated to form a printable ink with high solids (∼15 wt %) consisting of entangled PEDOT:PSS nanofibrils. Such ink shows a high storage modulus G' (43531 Pa) and a high yield stress τy (4325 Pa), thereby enabling omnidirectional printing. Secondary doping with sulfuric acid or other polar solvents is used to induce a synergetic process of PSS loss, conformational change, phase separation, and crystallinity enhancement in the printed structures, resulting in a remarkable enhancement of conductivity in dehydrated (65,378 S/m) and swollen (7190 S/m) states. As a proof-of-concept, 2D grids with a feature size of 15 μm and 3D overhanging arches are fabricated for high-performance transparent glass heaters and 3D interconnection, respectively. This work promises great potential for the development of advanced flexible electronics, wearable devices, and bioelectronics.

Assembled one-dimensional nanowires for flexible electronic devices via printing and coating: Techniques, applications, and perspectives

The rapid progress in flexible electronic devices has necessitated continual research into nanomaterials, structural design, and fabrication processes. One-dimensional nanowires, characterized by their distinct structures and exceptional properties, are considered essential components for various flexible electronic devices. Considerable attention has been directed toward the assembly of nanowires, which presents significant advantages. Printing and coating techniques can be used to assemble nanowires in a relatively simple, efficient, and cost-competitive manner and exhibit potential for scale-up production in the foreseeable future. This review aims to provide an overview of nanowire assembly using printing and coating techniques, such as bar coating, spray coating, dip coating, blade coating, 3D printing, and so forth. The application of assembled nanowires in flexible electronic devices is subsequently discussed. Finally, further discussion is presented on the potential and challenges of flexible electronic devices based on assembled nanowires via printing and coating.

Printed Eddy Current Testing Sensors: Toward Structural Health Monitoring Applications

Reliable measurements in structural health monitoring mean for the instrumentation to be set in perfect reproducible conditions. The solution described in this study consists of printing the sensors directly on the parts to be controlled. This method solves the reproducibility issue, limits human error, and can be used in confined or hazardous environments. This work was limited to eddy current testing, but the settings and conclusions are transposable to any non-destructive testing methods (ultrasounds, etc.). The first salve of tests was run to establish the best dielectric and conductive ink combination. The Dupont ink combination gave the best performances. Then, the dispenser- and the screen-printing methods were carried out to print flat spiral coils on flexible substrates. The resulting sensors were compared to flex-printed circuit boards (PCB-flex) using copper for the electrical circuit. The conductive ink methods were revealed to be just as efficient. The last stage of this work consisted of printing sensors on solid parts. For this, 20-turn spiral coils were printed on 3 mm thick stainless-steel plates. The permanent sensors showed good sensibility in the same range as the portative ones, demonstrating the method's feasibility.

3D printing of inorganic nanomaterials by photochemically bonding colloidal nanocrystals

3D printing of inorganic materials with nanoscale resolution offers a different materials processing pathway to explore devices with emergent functionalities. However, existing technologies typically involve photocurable resins that reduce material purity and degrade properties. We develop a general strategy for laser direct printing of inorganic nanomaterials, as exemplified by more than 10 semiconductors, metal oxides, metals, and their mixtures. Colloidal nanocrystals are used as building blocks and photochemically bonded through their native ligands. Without resins, this bonding process produces arbitrary three-dimensional (3D) structures with a large inorganic mass fraction (~90%) and high mechanical strength. The printed materials preserve the intrinsic properties of constituent nanocrystals and create structure-dictated functionalities, such as the broadband chiroptical responses with an anisotropic factor of ~0.24 for semiconducting cadmium chalcogenide nanohelical arrays.

Metal 3D nanoprinting with coupled fields

Metallized arrays of three-dimensional (3D) nanoarchitectures offer new and exciting prospects in nanophotonics and nanoelectronics. Engineering these repeating nanoarchitectures, which have dimensions smaller than the wavelength of the light source, enables in-depth investigation of unprecedented light-matter interactions. Conventional metal nanomanufacturing relies largely on lithographic methods that are limited regarding the choice of materials and machine write time and are restricted to flat patterns and rigid structures. Herein, we present a 3D nanoprinter devised to fabricate flexible arrays of 3D metallic nanoarchitectures over areas up to 4 × 4 mm2 within 20 min. By suitably adjusting the electric and flow fields, metal lines as narrow as 14 nm were printed. We also demonstrate the key ability to print a wide variety of materials ranging from single metals, alloys to multimaterials. In addition, the optical properties of the as-printed 3D nanoarchitectures can be tailored by varying the material, geometry, feature size, and periodic arrangement. The custom-designed and custom-built 3D nanoprinter not only combines metal 3D printing with nanoscale precision but also decouples the materials from the printing process, thereby yielding opportunities to advance future nanophotonics and semiconductor devices.

Microstructure-driven electrical conductivity optimization in additively manufactured microscale copper interconnects

As the microelectronics field pushes to increase device density through downscaling component dimensions, various novel micro- and nano-scale additive manufacturing technologies have emerged to expand the small scale design space. These techniques offer unprecedented freedom in designing 3D circuitry but have not yet delivered device-grade materials. To highlight the complex role of processing on the quality and microstructure of AM metals, we report the electrical properties of micrometer-scale copper interconnects fabricated by Fluid Force Microscopy (FluidFM) and Electrohydrodynamic-Redox Printing (EHD-RP). Using a thin film-based 4-terminal testing chip developed for the scope of this study, the electrical resistance of as-printed metals is directly related to print strategies and the specific morphological and microstructural features. Notably, the chip requires direct synthesis of conductive structures on an insulating substrate, which is shown for the first time in the case of FluidFM. Finally, we demonstrate the unique ability of EHD-RP to tune the materials resistivity by one order of magnitude solely through printing voltage. Through its novel electrical characterization approach, this study offers unique insight into the electrical properties of micro- and submicrometer-sized copper interconnects and steps towards a deeper understanding of micro AM metal properties for advanced electronics applications.

Silver-Nanowire-Based Elastic Conductors: Preparation Processes and Substrate Adhesion

The production of flexible electronic systems includes stretchable electrical interconnections and flexible electronic components, promoting the research and development of flexible conductors and stretchable conductive materials with large bending deformation or torsion resistance. Silver nanowires have the advantages of high conductivity, good transparency and flexibility in the development of flexible electronic products. In order to further prepare system-level flexible systems (such as autonomous full-software robots, etc.), it is necessary to focus on the conductivity of the system's composite conductor and the robustness of the system at the physical level. In terms of conductor preparation processes and substrate adhesion strategies, the more commonly used solutions are selected. Four kinds of elastic preparation processes (pretensioned/geometrically topological matrix, conductive fiber, aerogel composite, mixed percolation dopant) and five kinds of processes (coating, embedding, changing surface energy, chemical bond and force, adjusting tension and diffusion) to enhance the adhesion of composite conductors using silver nanowires as current-carrying channel substrates were reviewed. It is recommended to use the preparation process of mixed percolation doping and the adhesion mode of embedding/chemical bonding under non-special conditions. Developments in 3D printing and soft robots are also discussed.

Solution-Processed Flexible Transparent Electrodes for Printable Electronics

Flexible transparent electrodes (FTEs) have been widely witnessed in various printable electronic devices, especially those involving light. So far, solution processes have demonstrated increasing advantages in preparing FTEs not only in their mild operation conditions and high-throughput but also in the diversity in micropatterning conductive nanomaterials into networks. For the FTEs, both high transparency and high conductivity are desirable, which therefore create requirements for the conductive network by considering the trade-off relationship between the coverage and the micropatterns of the network. In addition, the conductive networks also affect the flexibility of FTEs due to the deformation during bending/stretching. Consequently, solution processes capable of micropatterning conductive nanomaterials including nanoparticles, nanowires/polymers, and graphene/MXene play a crucial role in determining the performance of FTEs. Here, we reviewed recent research progress on solution-processed FTEs, including the solution processes, the solution-processable conductive nanomaterials and the substrates for making FTEs, and applications of FTEs in flexible electronics. Finally, we proposed several perspective outlooks of the FTEs, which aim at not only the enhanced performance but also the performances in extreme conditions and in integration. We believe that the review would offer inspiration for developing functional FTEs.

Additively Manufactured 3D Micro-bioelectrodes for Enhanced Bioelectrocatalytic Operation

The drive toward miniaturization of enzyme-based bioelectronics established a need for three-dimensional (3D) microstructured electrodes, which are difficult to implement using conventional manufacturing processes. Additive manufacturing coupled with electroless metal plating enables the production of 3D conductive microarchitectures with high surface area for potential applications in such devices. However, interfacial delamination between the metal layer and the polymer structure is a major reliability concern, which leads to device performance degradation and eventually device failure. This work demonstrates a method to produce a highly conductive and robust metal layer on a 3D printed polymer microstructure with strong adhesion by introducing an interfacial adhesion layer. Prior to 3D printing, multifunctional acrylate monomers with alkoxysilane (-Si-(OCH₃)₃) were synthesized via the thiol-Michael addition reaction between pentaerythritol tetraacrylate (PETA) and 3-mercaptopropyltrimethoxysilane (MPTMS) with a 1:1 stoichiometric ratio. Alkoxysilane functionality remains intact during photopolymerization in a projection micro-stereolithography (PμSLA) system and is utilized for the sol-gel reaction with MPTMS during postfunctionalization of the 3D printed microstructure to build an interfacial adhesion layer. This leads to the implementation of abundant thiol functional groups on the surface of the 3D printed microstructure, which can act as a strong binding site for gold during electroless plating to improve interfacial adhesion. The 3D conductive microelectrode prepared by this technique exhibited excellent conductivity of 2.2 × 10⁷ S/m (53% of bulk gold) with strong adhesion between a gold layer and a polymer structure even after harsh sonication and an adhesion tape test. As a proof-of-concept, we examined the 3D gold diamond lattice microelectrode modified with glucose oxidase as a bioanode for a single enzymatic biofuel cell. The lattice-structured enzymatic electrode with high catalytic surface area was able to generate a current density of 2.5 μA/cm² at 0.35 V, which is an about 10 times increase in current output compared to a cube-shaped microelectrode.

Lignin-Based Materials for Additive Manufacturing: Chemistry, Processing, Structures, Properties, and Applications

The utilization of lignin, the most abundant aromatic biomass component, is at the forefront of sustainable engineering, energy, and environment research, where its abundance and low-cost features enable widespread application. Constructing lignin into material parts with controlled and desired macro- and microstructures and properties via additive manufacturing has been recognized as a promising technology and paves the way to the practical application of lignin. Considering the rapid development and significant progress recently achieved in this field, a comprehensive and critical review and outlook on three-dimensional (3D) printing of lignin is highly desirable. This article fulfils this demand with an overview on the structure of lignin and presents the state-of-the-art of 3D printing of pristine lignin and lignin-based composites, and highlights the key challenges. It is attempted to deliver better fundamental understanding of the impacts of morphology, microstructure, physical, chemical, and biological modifications, and composition/hybrids on the rheological behavior of lignin/polymer blends, as well as, on the mechanical, physical, and chemical performance of the 3D printed lignin-based materials. The main points toward future developments involve hybrid manufacturing, in situ polymerization, and surface tension or energy driven molecular segregation are also elaborated and discussed to promote the high-value utilization of lignin.

Optical response of plasmonic silver nanoparticles after treatment by a warm microwave plasma jet

This work investigates the effect of plasma treatment on the morphology and composition of 15 × 15 mm²silver nanoparticle (70-80 nm) thin films. The silver nanoparticles are deposited onto thermal silica (SiO₂/Si) substrates by spin-coating, then they are treated by an open-to-air microwave argon plasma jet characterized by a neutral gas temperature of 2200 ± 200 K. Scanning electron microscopy analysis reveals that the number of isolated nanoparticles in the film sample decreases after exposure to multiple jet passes, and that polygonal structures with sharp corners and edges are produced. Similar structures with much rounder edges are obtained after conventional thermal annealing at temperatures up to 1300 K. Based on localized surface plasmon resonance analysis in the range of 350-800 nm, the main extinction band of silver nanoparticles experiences a redshift after treatment with the plasma jet or with thermal annealing. Moreover, both treatments induce surface oxidation of the nanoparticles, as evidenced by x-ray photoelectron spectroscopy. However, only the plasma-exposed samples exhibit a significant rise in the surface-enhanced Raman scattering (SERS) signal of oxidized silver at 960 cm^-1. 29×29μm²mappings of hyperspectral Raman IMAging (RIMA) and multivariate curve resolution analysis by log-likelihood maximization demonstrate that the SERS signal is controlled by large-scale micrometer domains that exhibit sharp corners and edges.

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate's hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq^-1, 0.57 Ω sq^-1 and 19.7 Ω sq^-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Polylactide/Carbon Black Segregated Composites for 3D Printing of Conductive Products

One of the most important directions in the development of additive manufacturing or three-dimensional (3D) printing technologies is the creation of functional materials, which allow not only prototyping but also the manufacturing of products with functional properties. In this paper, poly-lactide acid (PLA) /carbon black (CB) composites with segregated (ordered) structure have been created. Computer simulation based on the Mamunya geometrical model showed that the CB content within φ = 2.5–5 vol.% in the polylactide matrix leads to the formation of a continuous electrically conductive phase with an increase of electrical conductivity σ_dc above the percolation threshold. The simulation results were experimentally confirmed by optical microscopy and studies of the electrical conductivity of the composites. It was found that increasing CB content from φ = 1 vol.% to φ = 7 vol.% in the composites causes insignificant (due to the segregated structure) phase changes in the polylactide matrix and improves the thermal properties of composites. Electrically conductive filaments for Fused Deposition 3D Printing (FDM) were developed from PLA/CB composites and then 3D printed. A correlation between the electrical conductivity σ_dc and the CB content φ for base composites, filaments produced from them, and final 3D samples, has been found. Conductivity varies within σ_dc = 3.1·10⁻¹¹ − 10·10⁻³ S/cm for the filaments and σ_dc = 3.6·10⁻¹¹ − 8.1·10⁻⁴ S/cm for the final 3D-products.

Bioadhesive ultrasound for long-term continuous imaging of diverse organs

Continuous imaging of internal organs over days could provide crucial information about health and diseases and enable insights into developmental biology. We report a bioadhesive ultrasound (BAUS) device that consists of a thin and rigid ultrasound probe robustly adhered to the skin via a couplant made of a soft, tough, antidehydrating, and bioadhesive hydrogel-elastomer hybrid. The BAUS device provides 48 hours of continuous imaging of diverse internal organs, including blood vessels, muscle, heart, gastrointestinal tract, diaphragm, and lung. The BAUS device could enable diagnostic and monitoring tools for various diseases.

Composites Additive Manufacturing for Space Applications: A Review

The assembly of 3D printed composites has a wide range of applications for ground preparation of space systems, in-orbit manufacturing, or even in-situ resource utilisation on planetary surfaces. The recent developments in composites additive manufacturing (AM) technologies include indoor experimentation on the International Space Station, and technological demonstrations will follow using satellite platforms on the Low Earth Orbits (LEOs) in the next few years. This review paper surveys AM technologies for varied off-Earth purposes where components or tools made of composite materials become necessary: mechanical, electrical, electrochemical and medical applications. Recommendations are also made on how to utilize AM technologies developed for ground applications, both commercial-off-the-shelf (COTS) and laboratory-based, to reduce development costs and promote sustainability.

Direct Ink Writing: A 3D Printing Technology for Diverse Materials

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in-depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects as a guideline toward possible futuristic innovations.

Recent Development of Morphology-Controlled Hybrid Nanomaterials for Triboelectric Nanogenerator: A Review

Being cognizant of modern electronic devices, the scientists are continuing to investigate renewable green-energy resources for a decade. Amid different energy harvesting systems, the triboelectric nanogenerators (TENGs) have been found to be the most promising mechanical harvesting technology and have drawn attention to generate electrical energy. Thanks to its instant output power, choice to opt for wide-ranging materials, low maintenance cost, easy fabrication process and environmentally friendly nature. Due to numerous working modes of TENGs, it is dedicated to desired application at ambient conditions. In this review, an advance correlation of TENGs have been explained based on the variety of nanostructures, including 0D, 1D, 2D, 3D, metal organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), and perovskite materials. Moreover, an overview of previous and current perspectives of various nanomaterials, synthesis, fabrication and their applications in potential fields have been discussed in detail.

High-resolution deposition of conductive and insulating materials at micrometer scale on complex substrates

Additive manufacturing transforms the landscape of modern microelectronics. Recent years have witnessed significant progress in the fabrication of 2D planar structures and free-standing 3D architectures. In this work, we present a much-needed intermediary approach: we introduce the Ultra-Precise Deposition (UPD) technology, a versatile platform for material deposition at micrometer scale on complex substrates. The versality of this approach is related to three aspects: material to be deposited (conductive or insulating), shape of the printed structures (lines, dots, arbitrary shapes), as well as type and shape of the substrate (rigid, flexible, hydrophilic, hydrophobic, substrates with pre-existing features). The process is based on the direct, maskless deposition of high-viscosity materials using narrow printing nozzles with the internal diameter in the range from 0.5 to 10 µm. For conductive structures we developed highly concentrated non-Newtonian pastes based on silver, copper, or gold nanoparticles. In this case, the feature size of the printed structures is in the range from 1 to 10 µm and their electrical conductivity is up to 40% of the bulk value, which is the record conductivity for metallic structures printed with spatial resolution below 10 µm. This result is the effect of the synergy between the printing process itself, formulation of the paste, and the proper sintering of the printed structures. We demonstrate a pathway to print such fine structures on complex substrates. We argue that this versatile and stable process paves the way for a widespread use of additive manufacturing for microfabrication.

Design Aspects of Additive Manufacturing at Microscale: A Review

Additive manufacturing (AM) technology has been researched and developed for almost three decades. Microscale AM is one of the fastest-growing fields of research within the AM area. Considerable progress has been made in the development and commercialization of new and innovative microscale AM processes, as well as several practical applications in a variety of fields. However, there are still significant challenges that exist in terms of design, available materials, processes, and the ability to fabricate true three-dimensional structures and systems at a microscale. For instance, microscale AM fabrication technologies are associated with certain limitations and constraints due to the scale aspect, which may require the establishment and use of specialized design methodologies in order to overcome them. The aim of this paper is to review the main processes, materials, and applications of the current microscale AM technology, to present future research needs for this technology, and to discuss the need for the introduction of a design methodology. Thus, one of the primary concerns of the current paper is to present the design aspects describing the comparative advantages and AM limitations at the microscale, as well as the selection of processes and materials.

Directly Printed Embedded Metal Mesh for Flexible Transparent Electrode via Liquid Substrate Electric-Field-Driven Jet

Flexible transparent electrodes (FTEs) with embedded metal meshes play an indispensable role in many optoelectronic devices due to their excellent mechanical stability and environmental adaptability. However, low-cost, simple, efficient, and environmental friendly integrated manufacturing of high-performance embedded metal meshes remains a huge challenge. Here, a facile and novel fabrication method is proposed for FTEs with an embedded metal mesh via liquid substrateelectric-field-driven microscale 3D printing process. This direct printing strategy avoids tedious processes and offers low-cost and high-volume production, enabling the fabrication of high-resolution, high-aspect ratio embedded metal meshes without sacrificing transparency. The final manufactured FTEs with 80 mm × 80 mm embedded metal mesh offers excellent optoelectronic performance with a sheet resistance (Rs ) of 6 Ω sq-1 and a transmittance (T) of 85.79%. The embedded metal structure still has excellent mechanical stability and good environmental suitability under different harsh working conditions. The practical feasibility of the FTEs is successfully demonstrated with a thermally driven 4D printing structure and a resistive transparent strain sensor. This method can be used to manufacture large areas with facile, high-efficiency, low-cost, and high-performance FTEs.

Combining printing and nanoparticle assembly: Methodology and application of nanoparticle patterning

Functional nanoparticles (NPs) with unique photoelectric, mechanical, magnetic, and chemical properties have attracted considerable attention. Aggregated NPs rather than individual NPs are generally required for sensing, electronics, and catalysis. However, the transformation of functional NP aggregates into scalable, controllable, and affordable functional devices remains challenging. Printing is a promising additive manufacturing technology for fabricating devices from NP building blocks because of its capabilities for rapid prototyping and versatile multifunctional manufacturing. This paper reviews recent advances in NP patterning based on the combination of self-assembly and printing technologies (including two-, three-, and four-dimensional printing), introduces the basic characteristics of these methods, and discusses various fields of NP patterning applications.

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Nanoparticles (NPs) printing assembly is a good solution for patterned devices

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NPs assembly can be combined with 2D, 3D, and 4D printing technologies

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A variety of ink-dispersed NPs are available for printing assembly

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NPs printing assembly technology is applied for nanosensing, energy storage, photodetector

Self-Folding PCB Kirigami: Rapid Prototyping of 3D Electronics via Laser Cutting and Forming

This paper demonstrates laser forming, localized heating with a laser to induce plastic deformation, can self-fold 2D printed circuit boards (PCBs) into 3D structures with electronic function. There are many methods for self-folding but few are compatible with electronic materials. We use a low-cost commercial laser writer to both cut and fold a commercial flexible PCB. Laser settings are tuned to select between cutting and folding with higher power resulting in cutting and lower power resulting in localized heating for folding into 3D shapes. Since the thin copper traces used in commercial PCBs are highly reflective and difficult to directly fold, two approaches are explored for enabling folding: plating with a nickel/gold coating or using a single, high-power laser exposure to oxidize the surface and improve laser absorption. We characterized the physical effect of the exposure on the sample as well as the fold angle as a function of laser passes and demonstrate the ability to lift weights comparable with circuit packages and passive components. This technique can form complex, multifold structures with integrated electronics; as a demonstrator, we fold a commercial board with a common timing circuit. Laser forming to add a third dimension to printed circuit boards is an important technology to enable the rapid prototyping of complex 3D electronics.

A Heating-Assisted Direct Ink Writing Method for Preparation of PDMS Cellular Structure with High Manufacturing Fidelity

In response to the fact that most of the current research on silicone 3D printing suffers from structure collapse and dimensional mismatch, this paper proposes a heating-assisted direct writing printing method for commercial silicone rubber materials for preparing silicone foam with enhanced fidelity. In the experimental processes, the effects of substrate temperature, printing pressure, and printing speed on the filament width were investigated using a controlled variable method. The results showed the following: (1) the diameter of silicone rubber filaments was positively correlated with the printing pressure and substrate temperature, but negatively correlated with the printing speed; (2) the filament collapse of the large filament spaced foams was significantly improved by the addition of the thermal field, which, in turn, improved the mechanical properties and manufacturing stability of the silicon foams. The heating-assisted direct writing process in this paper can facilitate the development of the field of microelectronics and the direct printing of biomaterials.

Ultrafast Shaped Laser Induced Synthesis of MXene Quantum Dots/Graphene for Transparent Supercapacitors

Ultratransparent electrodes have attracted considerable attention in optoelectronics and energy technology. However, balancing energy storage capability and transparency remains challenging. Herein, an in situ strategy employing a temporally and spatially shaped femtosecond laser is reported for photochemically synthesizing of MXene quantum dots (MQDs) uniformly attached to laser reduced graphene oxide (LRGO) with exceptional electrochemical capacitance and ultrahigh transparency. The mechanism and plasma dynamics of the synthesis process are analyzed and observed at the same time. The unique MQDs loaded on LRGO greatly improve the specific surface area of the electrode due to the nanoscale size and additional edge states. The MQD/LRGO supercapacitor has high flexibility and durability, ultrahigh energy density (2.04 × 10^-3  mWh cm^-2 ), long cycle life (97.6% after 12 000 cycles), and excellent capacitance (10.42 mF cm^-2 ) with both high transparency (transmittance over 90%) and high performance. Furthermore, this method provides a means of preparing nanostructured composite electrode materials and exploiting quantum capacitance effects for energy storage.

High-Resolution 3D Printing for Electronics

The ability to form arbitrary 3D structures provides the next level of complexity and a greater degree of freedom in the design of electronic devices. Since recent progress in electronics has expanded their applicability in various fields in which structural conformability and dynamic configuration are required, high-resolution 3D printing technologies can offer significant potential for freeform electronics. Here, the recent progress in novel 3D printing methods for freeform electronics is reviewed, with providing a comprehensive study on 3D-printable functional materials and processes for various device components. The latest advances in 3D-printed electronics are also reviewed to explain representative device components, including interconnects, batteries, antennas, and sensors. Furthermore, the key challenges and prospects for next-generation printed electronics are considered, and the future directions are explored based on research that has emerged recently.

Colloidal oxide nanoparticle inks for micrometer-resolution additive manufacturing of three-dimensional gas sensors

Micrometer-resolution 3D printing of functional oxides is of growing importance for the fabrication of micro-electromechanical systems (MEMSs) with customized 3D geometries. Compared to conventional microfabrication methods, additive manufacturing presents new opportunities for the low-cost, energy-saving, high-precision, and rapid manufacturing of electronics with complex 3D architectures. Despite these promises, methods for printable oxide inks are often hampered by challenges in achieving the printing resolution required by today's MEMS electronics and integration capabilities with various other electronic components. Here, a novel, facile ink design strategy is presented to overcome these challenges. Specifically, we first prepare a high-solid loading (∼78 wt%) colloidal suspension that contains polyethyleneimine (PEI)-coated stannic dioxide (SnO₂) nanoparticles, followed by PEI desorption that is induced by nitric acid (HNO₃) titration to optimize the rheological properties of the printable inks. Our achieved ∼3-5 μm printing resolution is at least an order of magnitude higher than those of other printed oxide studies employing nanoparticle ink-based printing methods demonstrated previously. Finally, various SnO₂ structures were directly printed on a MEMS-based microelectrode for acetylene detection application. The gas sensitivity measurements reveal that the device performance is strongly dependent on the printed SnO₂ structures. Specifically, the 3D structured SnO₂ gas sensor exhibits the highest response of ∼ 29.9 to 100 ppm acetylene with the fastest total response time of ∼ 65.8 s. This work presents a general ink formulation and printing strategy for functional oxides, which further provides a pathway for the additive manufacturing of oxide-based MEMSs.

A Printable and Conductive Yield-Stress Fluid as an Ultrastretchable Transparent Conductor

In contrast to ionically conductive liquids and gels, a new type of yield-stress fluid featuring reversible transitions between solid and liquid states is introduced in this study as a printable, ultrastretchable, and transparent conductor. The fluid is formulated by dispersing silica nanoparticles into the concentrated aqueous electrolyte. The as-printed features show solid-state appearances to allow facile encapsulation with elastomers. The transition into liquid-like behavior upon tensile deformations is the enabler for ultrahigh stretchability up to the fracture strain of the elastomer. Successful integrations of yield-stress fluid electrodes in highly stretchable strain sensors and light-emitting devices illustrate the practical suitability. The yield-stress fluid represents an attractive building block for stretchable electronic devices and systems in terms of giant deformability, high ionic conductivity, excellent optical transmittance, and compatibility with various elastomers.

Patterning of Metal Nanowire Networks: Methods and Applications

With the advance in flexible and stretchable electronics, one-dimensional nanomaterials such as metal nanowires have drawn much attention in the past 10 years or so. Metal nanowires, especially silver nanowires, have been recognized as promising candidate materials for flexible and stretchable electronics. Owing to their high electrical conductivity and high aspect ratio, metal nanowires can form electrical percolation networks, maintaining high electrical conductivity under deformation (e.g., bending and stretching). Apart from coating metal nanowires for making large-area transparent conductive films, many applications require patterned metal nanowires as electrodes and interconnects. Precise patterning of metal nanowire networks is crucial to achieve high device performances. Therefore, a high-resolution, designable, and scalable patterning of metal nanowire networks is important but remains a critical challenge for fabricating high-performance electronic devices. This review summarizes recent advances in patterning of metal nanowire networks, using subtractive methods, additive methods of nanowire dispersions, and printing methods. Representative device applications of the patterned metal nanowire networks are presented. Finally, challenges and important directions in the area of the patterning of metal nanowire networks for device applications are discussed.

Direct foam writing in microgravity

Herein we report 2D printing in microgravity of aqueous-based foams containing metal oxide nanoparticles. Such hierarchical foams have potential space applications, for example for in situ habitat repair work, or for UV shielding. Foam line patterns of a TiO₂-containing foam have been printed onto glass substrates via Direct Foam Writing (DFW) under microgravity conditions through a parabolic aircraft flight. Initial characterization of the foam properties (printed foam line width, bubble size, etc.) are presented. It has been found that gravity plays a significant role in the process of direct foam writing. The foam spread less over the substrate when deposited in microgravity as compared to Earth gravity. This had a direct impact on the cross-sectional area and surface roughness of the printed lines. Additionally, the contact angle of deionized water on a film exposed to microgravity was higher than that of a film not exposed to microgravity, due to the increased surface roughness of films exposed to microgravity.

Flexible Wearables for Plants

The excellent stretchability and biocompatibility of flexible sensors have inspired an emerging field of plant wearables, which enable intimate contact with the plants to continuously monitor the growth status and localized microclimate in real-time. Plant flexible wearables provide a promising platform for the development of plant phenotype and the construction of intelligent agriculture via monitoring and regulating the critical physiological parameters and microclimate of plants. Here, the emerging applications of plant flexible wearables together with their pros and cons from four aspects, including physiological indicators, surrounding environment, crop quality, and active control of growth, are highlighted. Self-powered energy supply systems and signal transmission mechanisms are also elucidated. Furthermore, the future opportunities and challenges of plant wearables are discussed in detail.

High-precision tabletop microplotter for flexible on-demand material deposition in printed electronics and device functionalization

Microstructuring, in particular, the additive functionalization of surfaces with, e.g., conductive or bioactive materials plays a crucial role in many applications in sensing or printed electronics. Mostly, the lithography steps are made prior to assembling functionalized surfaces into the desired places of use within a bigger device as a microfluidic channel or an electronic casing. However, when this is not possible, most lithography techniques struggle with access to recessed or inclined/vertical surfaces for geometrical reasons. In particular, for "on-the-fly" printing aiming to add microstructures to already existing devices on demand and maybe even for one-time trials, e.g., in prototyping, a flexible "micropencil" allowing for direct write under direct manual control and on arbitrarily positioned surfaces would be highly desirable. Here, we present a highly flexible, micromanipulator-based setup for capillary printing of conductive and biomaterial ink formulations that can address a wide range of geometries as exemplified on vertical, recessed surfaces and stacked 3D scaffolds as models for hard to access surfaces. A wide range of feature sizes from tens to hundreds of micrometer can be obtained by the choice of capillary sizes and the on-demand in situ writing capabilities are demonstrated with completion of a circuit structure by gold line interconnects deposited with the setup.

Printing thermoelectric inks toward next-generation energy and thermal devices

The ability of thermoelectric (TE) materials to convert thermal energy to electricity and vice versa highlights them as a promising candidate for sustainable energy applications. Despite considerable increases in the figure of merit zT of thermoelectric materials in the past two decades, there is still a prominent need to develop scalable synthesis and flexible manufacturing processes to convert high-efficiency materials into high-performance devices. Scalable printing techniques provide a versatile solution to not only fabricate both inorganic and organic TE materials with fine control over the compositions and microstructures, but also manufacture thermoelectric devices with optimized geometric and structural designs that lead to improved efficiency and system-level performances. In this review, we aim to provide a comprehensive framework of printing thermoelectric materials and devices by including recent breakthroughs and relevant discussions on TE materials chemistry, ink formulation, flexible or conformable device design, and processing strategies, with an emphasis on additive manufacturing techniques. In addition, we review recent innovations in the flexible, conformal, and stretchable device architectures and highlight state-of-the-art applications of these TE devices in energy harvesting and thermal management. Perspectives of emerging research opportunities and future directions are also discussed. While this review centers on thermoelectrics, the fundamental ink chemistry and printing processes possess the potential for applications to a broad range of energy, thermal and electronic devices.

Fabricating functional circuits on 3D freeform surfaces via intense pulsed light-induced zinc mass transfer

Deployment of functional circuits on a 3D freeform surface is of significant interest to wearable devices on curvilinear skin/tissue surfaces or smart Internet-of-Things with sensors on 3D objects. Here we present a new fabrication strategy that can directly print functional circuits either transient or long-lasting onto freeform surfaces by intense pulsed light-induced mass transfer of zinc nanoparticles (Zn NPs). The intense pulsed light can locally raise the temperature of Zn NPs to cause evaporation. Lamination of a kirigami-patterned soft semi-transparent polymer film with Zn NPs conforming to a 3D surface results in condensation of Zn NPs to form conductive yet degradable Zn patterns onto a 3D freeform surface for constructing transient electronics. Immersing the Zn patterns into a copper sulfate or silver nitrate solution can further convert the transient device to a long-lasting device with copper or silver. Functional circuits with integrated sensors and a wireless communication component on 3D glass beakers and seashells with complex surface geometries demonstrate the viability of this manufacturing strategy.

Femtosecond laser trapping nanoprinting of silver micro/nanostructures

In this paper, silver micro/nanostructures composed of sintered nanoparticles were printed by capturing silver nanoparticles in water with 800 nm femtosecond laser trapping. Relationships of laser power, scanning speed, nanoparticle concentration, and the width and morphology of fabricated silver wire were systematically investigated. It is found that low scanning speed and high nanoparticle concentration favor the printing of silver wire with good morphology. A silver wire with width of 305 nm was printed. Electrical resistivities of printed wires are about 24 times that of bulk silver. Silver grid structures and dot arrays were printed by using this technology. Several three-dimensional silver cuboid structures were also printed. This work provides a protocol for printing of three-dimensional metallic micro/nanostructures using laser trapping. These printed structures have great application prospects in metamaterials, flexible electronics, and SERS.