Multi-metal electrohydrodynamic redox 3D printing at the submicron scale

Structural insights into metal-metalloid glasses from mass spectrometry

Despite being studied for nearly 50 years, smallest chemically stable moieties in the metallic glass (MG) could not be found experimentally. Herein, we demonstrate a novel experimental approach based on electrochemical etching of amorphous alloys in inert solvent (acetonitrile) in the presence of a high voltage (1 kV) followed by detection of the ions using electrolytic spray ionization mass spectrometry (ESI MS). The experiment shows stable signals corresponding to Pd, PdSi and PdSi₂ ions, which emerges due to the electrochemical etching of the Pd₈₀Si₂₀ metallic glass electrode. These fragments are observed from the controlled dissolution of the Pd₈₀Si₂₀ melt-spun ribbon (MSR) electrode. Annealed electrode releases different fragments in the same experimental condition. These specific species are expected to be the smallest and most stable chemical units from the metallic glass which survived the chemical dissolution and complexation (with acetonitrile) process. Theoretically, these units can be produced from the cluster based models for the MG. Similar treatment on Pd₄₀Ni₄₀P₂₀ MSR resulted several complex peaks consisting of Pd, Ni and P in various combinations suggesting this can be adopted for any metal-metalloid glass.

3D electrohydrodynamic printing and characterisation of highly conductive gold nanowalls

3D printing research targets the creation of nanostructures beyond the limits of traditional micromachining. A proper characterisation of their functionalities is necessary to facilitate future implementation into applications. We fabricate, in an open atmosphere, high-aspect-ratio gold nanowalls by electrohydrodynamic rapid nanodripping, and comprehensively analyse their electronic performance by four-point probe measurements. We reveal the large-grained nanowall morphology by transmission electron microscopy and explain the measured low resistivities approaching those of bulk gold. This work is a significant advancement in contactless bottom-up 3D nanofabrication and characterisation and could also serve as a platform for fundamental studies of additively manufactured high-aspect-ratio out-of-plane metallic nanostructures.

Electrohydrodynamics of droplets and jets in multiphase microsystems

Electrohydrodynamics is among the most promising techniques for manipulating liquids in microsystems. The electric stress actuates, generates, and coalesces droplets of small sizes; it also accelerates, focuses, and controls the motion of fine jets. In this review, the current understanding of dynamic regimes of electrically driven drops and jets in multiphase microsystems is summarized. The experimental description and underlying mechanism of force interplay and instabilities are discussed. Conditions for controlled transitions among different regimes are also provided. Emerging new phenomena either due to special interfacial properties or geometric confinement are emphasized, and simple scaling arguments proposed in the literature are introduced. The review provides useful perspectives for investigations involving electrically driven droplets and jets.

Drop impact printing

Hydrodynamic collapse of a central air-cavity during the recoil phase of droplet impact on a superhydrophobic sieve leads to satellite-free generation of a single droplet through the sieve. Two modes of cavity formation and droplet ejection have been observed and explained. The volume of the generated droplet scales with the pore size. Based on this phenomenon, we propose a drop-on-demand printing technique. Despite significant advancements in inkjet technology, enhancement in mass-loading and particle-size have been limited due to clogging of the printhead nozzle. By replacing the nozzle with a sieve, we demonstrate printing of nanoparticle suspension with 71% mass-loading. Comparatively large particles of 20 μm diameter are dispensed in droplets of ~80 μm diameter. Printing is performed for surface tension as low as 32 mNm-1 and viscosity as high as 33 mPa∙s. In comparison to existing techniques, this way of printing is widely accessible as it is significantly simple and economical.

Accelerated microdroplet synthesis of benzimidazoles by nucleophilic addition to protonated carboxylic acids

We report a metal-free novel route for the accelerated synthesis of benzimidazole and its derivatives in the ambient atmosphere. The synthetic procedure involves 1,2-aromatic diamines and alkyl or aryl carboxylic acids reacting in electrostatically charged microdroplets generated using a nano-electrospray (nESI) ion source. The reactions are accelerated by orders of magnitude in comparison to the bulk. No other acid, base or catalyst is used. Online analysis of the microdroplet accelerated reaction products is performed by mass spectrometry. We provide evidence for an acid catalyzed reaction mechanism based on identification of the intermediate arylamides. Their dehydration to give benzimidazoles occurs in a subsequent thermally enhanced step. It is suggested that the extraordinary acidity at the droplet surface allows the carboxylic acid to function as a C-centered electrophile. Comparisons of this methodology with data from thin film and bulk synthesis lead to the proposal of three key steps in the reaction: (i) formation of an unusual reagent (protonated carboxylic acid) because of the extraordinary conditions at the droplet interface, (ii) accelerated bimolecular reaction because of limited solvation at the interface and (iii) thermally assisted elimination of water. Eleven examples are shown as evidence of the scope of this chemistry. The accelerated synthesis has been scaled-up to establish the substituent-dependence and to isolate products for NMR characterization.

Metals by Micro-Scale Additive Manufacturing: Comparison of Microstructure and Mechanical Properties

Many emerging applications in microscale engineering rely on the fabrication of 3D architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide flexible and facile access to these geometries. Yet, the synthesis of device-grade inorganic materials is still a key challenge toward the implementation of AM in microfabrication. Here, a comprehensive overview of the microstructural and mechanical properties of metals fabricated by most state-of-the-art AM methods that offer a spatial resolution ≤10 μm is presented. Standardized sets of samples are studied by cross-sectional electron microscopy, nanoindentation, and microcompression. It is shown that current microscale AM techniques synthesize metals with a wide range of microstructures and elastic and plastic properties, including materials of dense and crystalline microstructure with excellent mechanical properties that compare well to those of thin-film nanocrystalline materials. The large variation in materials' performance can be related to the individual microstructure, which in turn is coupled to the various physico-chemical principles exploited by the different printing methods. The study provides practical guidelines for users of small-scale additive methods and establishes a baseline for the future optimization of the properties of printed metallic objects-a significant step toward the potential establishment of AM techniques in microfabrication.

Evolution of 3D Printing Methods and Materials for Electrochemical Energy Storage

Additive manufacturing has revolutionized the building of materials, and 3D-printing has become a useful tool for complex electrode assembly for batteries and supercapacitors. The field initially grew from extrusion-based methods and quickly evolved to photopolymerization printing, while supercapacitor technologies less sensitive to solvents more often involved material jetting processes. The need to develop higher-resolution multimaterial printers is borne out in the performance data of recent 3D printed electrochemical energy storage devices. Underpinning every part of a 3D-printable battery are the printing method and the feed material. These influence material purity, printing fidelity, accuracy, complexity, and the ability to form conductive, ceramic, or solvent-stable materials. The future of 3D-printable batteries and electrochemical energy storage devices is reliant on materials and printing methods that are co-operatively informed by device design. Herein, the material and method requirements in 3D-printable batteries and supercapacitors are addressed and requirements for the future of the field are outlined by linking existing performance limitations to requirements for printable energy-storage materials, casings, and direct printing of electrodes and electrolytes. A guide to materials and printing method choice best suited for alternative-form-factor energy-storage devices to be designed and integrated into the devices they power is thus provided.

Hardening in Au-Ag nanoboxes from stacking fault-dislocation interactions

Porous, nano-architected metals with dimensions down to ~10 nm are predicted to have extraordinarily high strength and stiffness per weight, but have been challenging to fabricate and test experimentally. Here, we use colloidal synthesis to make ~140 nm length and ~15 nm wall thickness hollow Au-Ag nanoboxes with smooth and rough surfaces. In situ scanning electron microscope and transmission electron microscope testing of the smooth and rough nanoboxes show them to yield at 130 ± 45 MPa and 96 ± 31 MPa respectively, with significant strain hardening. A higher strain hardening rate is seen in rough nanoboxes than smooth nanoboxes. Finite element modeling is used to show that the structure of the nanoboxes is not responsible for the hardening behavior suggesting that material mechanisms are the source of observed hardening. Molecular dynamics simulations indicate that hardening is a result of interactions between dislocations and the associated increase in dislocation density.

Fabricating and mechanically testing nanoarchitected materials remains a challenge. Here, the authors use colloidal synthesis to fabricate Au-Ag hollow nanoboxes and investigate the effect of either a rough or a smooth nanobox surface on the mechanical properties.

Nanomaterial Patterning in 3D Printing

The synergistic integration of nanomaterials with 3D printing technologies can enable the creation of architecture and devices with an unprecedented level of functional integration. In particular, a multiscale 3D printing approach can seamlessly interweave nanomaterials with diverse classes of materials to impart, program, or modulate a wide range of functional properties in an otherwise passive 3D printed object. However, achieving such multiscale integration is challenging as it requires the ability to pattern, organize, or assemble nanomaterials in a 3D printing process. This review highlights the latest advances in the integration of nanomaterials with 3D printing, achieved by leveraging mechanical, electrical, magnetic, optical, or thermal phenomena. Ultimately, it is envisioned that such approaches can enable the creation of multifunctional constructs and devices that cannot be fabricated with conventional manufacturing approaches.

Parallel, Multi-Material Electrohydrodynamic 3D Nanoprinting

Direct mass-transfer via liquid nanodroplets is one of the most powerful approaches for additive micro/nanofabrication. Electrohydrodynamic (EHD) dispensing has made the delivery of nanosized droplets containing diverse materials a practical reality; however, in its serial form it has insufficient throughput for large-area processing. Here, a parallel, nanoscale EHD method is developed that offers both improved productivity and material diversity in 3D nanoprinting. The method exploits a double-barreled glass nanopipette filled with material inks to parallelize nanodripping ejections, enabling a dual 3D nanoprinting process. It is discovered that an unusual electric field distribution created by cross talk of neighboring pipette apertures can be used to steer the microscopic ejection paths of the ink at will, enabling on-demand control over shape, placement, and material mixing in 3D printed nanostructures. After thorough characterizations of the printing conditions, the parallel fabrication of nanomeshes and nanowalls of silver, CdSe/ZnS quantum dots, and their composites, with programmed designs is demonstrated. This method is expected to advance productivity in the heterogeneous integration of functional 3D nanodevices in a facile manner.

A Low-Cost Electrochemical Metal 3D Printer Based on a Microfluidic System for Printing Mesoscale Objects

For the additive manufacturing (AM) of metal objects, the powder-based fusion (PBF) method is routinely utilized to fabricate macroscale parts. On the other hand, electrochemical additive manufacturing (ECAM), in which metallic structures are deposited through the electrochemical reduction of metal ions, is a promising technique for producing micro- and nanoscale objects. However, a gap exists in terms of fabricating mesoscale objects within the current AM techniques. The PBF method is limited by fabrication precision due to pronounced residual stresses, and most current ECAM systems are difficult to scale up to print mesoscale objects. In the present paper, the novel design of a low-cost ECAM 3D printer based on a microfluidic system is proposed for fabricating mesoscale metal parts. The meniscus-guided electrodeposition approach is utilized, in which a meniscus is formed between the print head and substrate, and electrodeposition is confined within the meniscus. A 3D object is fabricated by the meniscus moving with the print head according to the programmed pattern and the material subsequently being deposited at the designated locations. The key to the proposed design is to maintain a mesoscale meniscus, which normally cannot be sustained by the electrolyte surface tension with a print nozzle having a mesoscale diameter. Therefore, a microfluidic system, called the fountain pen feed system, constituting a semi-open main channel and comb structure, was designed to maintain a mesoscale meniscus throughout the printing process. Two materials, copper and nickel, with various geometric shapes were attempted to print by the proposed ECAM system, and, during the printing process, both fluid leaking and meniscus breaking were completely prevented. Free standing tilted copper pillars with controlled angles were printed to show the ability of the proposed design in fabricating 3D structures. A copper circuit was also printed on a non-conductive substrate to demonstrate a possible application of the proposed ECAM system in the fabrication of functional electronics.

Ultrafast 3D printing with submicrometer features using electrostatic jet deflection

Additive manufacturing technologies based on layer-by-layer deposition of material ejected from a nozzle provide unmatched versatility but are limited in terms of printing speed and resolution. Electrohydrodynamic jetting uniquely allows generating submicrometer jets that can reach speeds above 1 m s^-1, but such jets cannot be precisely collected by too slow mechanical stages. Here, we demonstrate that controlling the voltage applied to electrodes located around the jet, its trajectory can be continuously adjusted with lateral accelerations up to 10⁶ m s^-2. Through electrostatically deflecting the jet, 3D objects with submicrometer features can be printed by stacking nanofibers on top of each other at layer-by-layer frequencies as high as 2000 Hz. The fast jet speed and large layer-by-layer frequencies achieved translate into printing speeds up to 0.5 m s^-1 in-plane and 0.4 mm s^-1 in the vertical direction, three to four orders of magnitude faster than techniques providing equivalent feature sizes.

3D Printing of a self-healing, high strength, and reprocessable thermoset

A furan-maleimide based 3D printing ink for the fabrication of a self-healing and high strength thermoset with recycling potential.

A Comprehensive Review on Bio-Nanomaterials for Medical Implants and Feasibility Studies on Fabrication of Such Implants by Additive Manufacturing Technique

Nanomaterials have allowed significant breakthroughs in bio-engineering and medical fields. In the present paper a holistic assessment on diverse biocompatible nanocomposites are studied. Their compatibility with advanced fabrication methods such as additive manufacturing for the design of functional medical implants is also critically reviewed. The significance of nanocomposites and processing techniques is also envisaged comprehensively in regard with the needs and futures of implantable medical device industries.

Additive Manufacturing of Sub-Micron to Sub-mm Metal Structures with Hollow AFM Cantilevers

We describe our force-controlled 3D printing method for layer-by-layer additive micromanufacturing (µAM) of metal microstructures. Hollow atomic force microscopy cantilevers are utilized to locally dispense metal ions in a standard 3-electrode electrochemical cell, enabling a confined electroplating reaction. The deflection feedback signal enables the live monitoring of the voxel growth and the consequent automation of the printing protocol in a layer-by-layer fashion for the fabrication of arbitrary-shaped geometries. In a second step, we investigated the effect of the free parameters (aperture diameter, applied pressure, and applied plating potential) on the voxel size, which enabled us to tune the voxel dimensions on-the-fly, as well as to produce objects spanning at least two orders of magnitude in each direction. As a concrete example, we printed two different replicas of Michelangelo's David. Copper was used as metal, but the process can in principle be extended to all metals that are macroscopically electroplated in a standard way.

Directed nanoscale metal deposition by the local perturbation of charge screening at the solid-liquid interface

Understanding and directing electrochemical reactions below the micrometer scale is a long-standing challenge in electrochemistry. Confining reactions to nanoscale areas paradoxically requires both isolation from and communication with the bulk electrolyte in terms of electrochemical potential and access of ions, respectively. Here, we demonstrate the directed electrochemical deposition of copper nanostructures by using an oscillating nanoelectrode operated with an atomic force microscope (AFM). Strikingly, the writing is only possible in highly dilute electrolytes and for a particular combination of AFM and electrochemical parameters. We propose a mechanism based on cyclic charging and discharging of the electrical double layer (EDL). The extended screening length and slower charge dynamics in dilute electrolytes allow the nanoelectrode to operate inside, and disturb, the EDL even for large oscillation amplitudes (∼100 nm). Our unique approach can not only be used for controlled additive nano-fabrication but also provides insights into ion behavior and EDL dynamics at the solid-liquid interface.