Microscale Additive Manufacturing Research Articles

Ultrafast Droplets Trajectories Modulation of Electrohydrodynamic Jet Printing for Pattern Refinement Using Electrostatic Deflection

AbstractElectrohydrodynamic jet (E‐Jet) printing is a highly promising non‐contact additive manufacturing technology. However, the fast jets generated by E‐Jet may cause droplet accumulation on small curvature features due to the slow motion control achieved by the mechanical stage, leading to a degradation of printed patterns in localized areas. Here, a method is proposed for ultrafast modulation of jet trajectories to achieve finely patterned E‐Jet printing by introducing jet‐deflecting electrodes around the jet, which can continuously transform the jet trajectory with lateral acceleration up to 104 ms−2. This extraordinary acceleration is two orders of magnitude higher than that can be achieved by using mechanical stage motion. Through a combination of numerical calculations and experiments, the effects of electric field parameters on the feature size and kinematic properties of jet droplets are investigated. In addition, deposition errors of <5 µm are fabricated by selecting appropriate working parameters and using UV‐curable adhesives as printing ink. Subsequently, complex micropatterns with feature curvatures <4 µm are successfully printed, demonstrating the consistency and reliability of this electrostatic deflection technique in achieving a uniform droplet deposition pitch. This innovative approach holds significant promise in the field of high‐quality microscale additive manufacturing.

Characterization of Photocurable IP-PDMS for Soft Micro Systems Fabricated by Two-Photon Polymerization 3D Printing.

Recent developments in micro-scale additive manufacturing (AM) have opened new possibilities in state-of-the-art areas, including microelectromechanical systems (MEMS) with intrinsically soft and compliant components. While fabrication with soft materials further complicates micro-scale AM, a soft photocurable polydimethylsiloxane (PDMS) resin, IP-PDMS, has recently entered the market of two-photon polymerization (2PP) AM. To facilitate the development of microdevices with soft components through the application of 2PP technique and IP-PDMS material, this research paper presents a comprehensive material characterization of IP-PDMS. The significance of this study lies in the scarcity of existing research on this material and the thorough investigation of its properties, many of which are reported here for the first time. Particularly, for uncured IP-PDMS resin, this work evaluates a surface tension of 26.7 ± 4.2 mN/m, a contact angle with glass of 11.5 ± 0.6°, spin-coating behavior, a transmittance of more than 90% above 440 nm wavelength, and FTIR with all the properties reported for the first time. For cured IP-PDMS, novel characterizations include a small mechanical creep, a velocity-dependent friction coefficient with glass, a typical dielectric permittivity value of 2.63 ± 0.02, a high dielectric/breakdown strength for 3D-printed elastomers of up to 73.3 ± 13.3 V/µm and typical values for a spin coated elastomer of 85.7 ± 12.4 V/µm, while the measured contact angle with water of 103.7 ± 0.5°, Young's modulus of 5.96 ± 0.2 MPa, and viscoelastic DMA mechanical characterization are compared with the previously reported values. Friction, permittivity, contact angle with water, and some of the breakdown strength measurements were performed with spin-coated cured IP-PDMS samples. Based on the performed characterization, IP-PDMS shows itself to be a promising material for micro-scale soft MEMS, including microfluidics, storage devices, and micro-scale smart material technologies.

Characterizing process window for microscale selective laser sintering

This paper outlines the experimental approach to create a sintering window for a microscale metal additive manufacturing (AM) process. The experimental framework discussed in the paper involves fabricating sintered features by varying pattern area and laser irradiance. The sintered features are then imaged under a microscope and the processed images are compared against an ideal image to quantify the uncertainty in near-net shape of the feature. Evaluating this data defines the process window within which good sintering and near-net shaped features can be expected in the microscale AM process.

Improving the dimensional accuracy of micro parts 3D printed with projection-based continuous vat photopolymerization using a model-based grayscale optimization method

Micro-scale additive manufacturing has seen significant growth over the past years, where improving the accuracy of complex micro-scale geometries is seen as an important challenge. Using grayscale images rather than black and white images during production is an effective method to improve the fabrication quality. This paper presents a model-based optimization method for improving the dimensional accuracy of parts using voxel-based grayscale dynamic optimization during continuous 3D printing. A detailed solidification model has been developed and used to estimate the curing dynamics of the resin used in 3D printing. The irradiance of the light beam projected for each pixel influences a larger volume on the resin than the targeted voxel. The proposed model-based method optimizes the images considering the light distribution from all closely related pixels to maintain the accuracy of the micro part. The results of this method have been applied to the printing of a complex 3D part to show that optimized grayscale images improve the areas with overcuring significantly. It is shown that the number of overcured voxels was reduced by 24.7% compared to the original images. Actual printing results from our experimental setup confirm the improvements in the accuracy and precision of the printing method.

Electrohydrodynamic Redox 3D Printing: Confined Electroplating of Alloys for Additive Manufacturing at the Submicron Scale

Combining the unprecedented design freedom in microscale additive manufacturing (AM) with the ability to control the chemical nature of each printed voxel could unlock unique possibilities for tailoring mechanical, chemical, electrical, optical and magnetic properties of metal microstructures. A variety of techniques for micro- and nanoscale AM has been proposed for the fabrication of device-grade metals and alloys [1]. Electrochemical approaches to small-scale AM generally lead to superior microstructures (in terms of porosity and contamination) compared with techniques that transfer pre-synthesized materials [2]. The deposition of alloys with controlled composition, however, remains a challenge with electrochemical small scale AM techniques.In this talk, we will present our work on additive manufacturing of alloyed structures using electrohydrodynamic redox (EHD-RP) 3D printing [3]. EHD-RP is based on the deposition of solvent droplets containing metal ions onto a conductive substrate, where the solvent evaporates and the ions are reduced. In general, this technique allows the direct deposition of polycrystalline 3D metal structures with a resolution of approx. 250 nm and a feature size down to 100 nm. We will present our work in expanding the materials range of EHD-RP from the limited range reported previously to a wide range of metals and subsequently discuss in detail the direct deposition of alloys. As it will be shown, the approach of spatially confining electrodeposition enables the fabrication of multi-metal and alloyed structures with a chemical voxel size <400 nm, hence making a step towards chemically architected materials. We will show how we can control the composition of the deposited material and the challenges involved in its characterization.In summary, we present a novel approach to the bottom-up manufacturing of locally alloyed microstructures, adding an additional parameter in the design of novel nano- and microstructured inorganic materials.[1] L. Hirt, A. Reiser, R. Spolenak & T. Zambelli. Additive Manufacturing of Metal Structures at the Micrometer Scale. Adv. Mater., 29(17), 2017.[2] A. Reiser, R. Spolenak et al. Metals by Micro-scale Additive Manufacturing: Comparison of Microstructure and Mechanical Properties. Adv. Funct. Mater., 30, 1910491, 2020[3] A. Reiser, M. Lindén, P. Rohner, A. Marchand, H. Galinski, A. S. Sologubenko, J. M. Wheeler, R. Zenobi, D. Poulikakos & R. Spolenak. Multi-metal electrohydrodynamic redox 3D printing at the submicron scale. Nat. Comm., 10(1):1-8, 2019. Figure 1

Dynamic cryo-mechanical properties of additively manufactured nanocrystalline nickel 3D microarchitectures

Template-assisted electrodeposition is a promising microscale additive manufacturing technique allowing to deposit pure metals with high resolution. To allow the application-relevant design of metamaterials, it is necessary to establish microstructure-mechanical property relationships under extreme conditions. In this work, a novel process based on two-photon lithography was used to synthesize arrays of nanocrystalline nickel micropillars and complex microlattices. This allowed high throughput mechanical testing using a newly developed in situ nanoindenter at unprecedented combination of cryogenic temperatures (160 to 300 K) and strain rates (0.001 to 500 s−1). Strain rate sensitivity was found to increase from ∼ 0.004 at 300 K to ∼ 0.008 at 160 K. Thermal activation analysis showed a decrease in activation volume from 122b3 at 300 K to 45b3 at 160 K and an activation energy of 0.59 eV in line with collective dislocation nucleation as the rate limiting mechanism. Transmission Kikuchi Diffraction allowed quantifying microstructural changes during deformation. As such, a deformation map along with the responsible deformation mechanisms has been ascertained for additively micromanufactured nanocrystalline nickel at unique combinations of extreme temperatures and strain rates. Further, rate-dependent compression of microlattices and complementary finite element simulations using the results from micropillars as constitutive models exemplified the promise of such metal microarchitectures in space and aviation applications.

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.

Submicron Metal 3D Printing by Ultrafast Laser Heating and Induced Ligand Transformation of Nanocrystals.

Currently, light-based three-dimensional (3D) printing with submicron features is mainly developed based on photosensitive polymers or inorganic-polymer composite materials. To eliminate polymer/organic additives, a strategy for direct 3D assembly and printing of metallic nanocrystals without additives is presented. Ultrafast laser with intensity in the range of 1 × 1010 to 1 × 1012 W/cm2 is used to nonequilibrium heat nanocrystals and induce ligand transformation, which triggers the spontaneous fusion and localized assembly of nanocrystals. The process is due to the operation of hot electrons as confirmed by a strong dependence of the printing rate on laser pulse duration varied in the range of electron-phonon relaxation time. Using the developed laser-induced ligand transformation (LILT) process, direct printing of 3D metallic structures at micro and submicron scales is demonstrated. Facile integration with other microscale additive manufacturing for printing 3D devices containing multiscale features is also demonstrated.

Bioprocess-inspired synthesis of multilayered chitosan/CaCO3 composites with nacre-like structures and high mechanical properties.

The formation of natural structures found in biological systems is wonderful and can be completed at ambient temperatures in contrast to artificial technologies wherein harsh conditions are common prerequisites. A new research direction, "bioprocess inspired manufacturing", is proposed for fabricating advanced materials with novel structures and functions. Nacre consists of an ordered multilayer structure of crystalline calcium carbonate lamellae separated by organic layers exhibiting mechanical toughness, which transcends that of its constituent components. Inspired by the nacre formation process, a microscale additive manufacturing mineralization method is proposed for achieving a multilayered organic-inorganic layered structure. In this work, layered calcite was synthesized on the surface of chitosan (CS) films at room temperature under the coordinated control of magnesium ions (Mg2+) and polyacrylic acid (PAA). The CS films and layered calcite are sequentially assembled in a layer-by-layer deposition approach to form an organic-inorganic hybrid structure. The nacre-like chitosan/CaCO3 (CS/CaCO3) composites exhibit high transparency and underwater superoleophobicity. Impressively, the hardness (2.35 ± 0.03 GPa) and Young's modulus (58.1 ± 0.5 GPa) of the as-prepared (CS/CaCO3) composites are comparable to those of their biological counterparts. This study provides a rational bioprocess-inspired room-temperature mineralization method to develop advanced composite materials with good performance.

Micro-scale Additive Manufacturing Using the Optical Potential Generated by a Bessel Beam

In this study, we proposed a novel micro-scale additive manufacturing method based on the optical potential formed by a Bessel beam. The proposed technique is expected to show no deterioration in manufacturing resolution due to heat generation, to be applicable to various materials, and to be able to be performed in an air environment. The basic principle of the proposed method involves accumulating and stacking particles dispersed in air by using optical radiation pressure. In this paper, the trajectory of the accumulated particles was numerically estimated and experimentally observed. The numerical and experimental results agreed well; specifically, the background flow carried the particles to the optical axis of the Bessel beam, and then the particles were localized at the bottom of the optical potential valley on the substrate. Finally, a pillar structure was fabricated with polystyrene particles having a diameter of 1 μm, which indicated that the proposed technique was promising for practical applications.

Investigating Porous Media for Relief Printing Using Micro‐Architected Materials

Advances in printed electronics are predicated on the integration of sophisticated printing technologies with functional materials. Although scalable manufacturing methods, such as letterpress and flexographic printing, have significant history in graphic arts printing, functional applications require sophisticated control and understanding of nanoscale transfer of fluid inks. Herein, a versatile platform is introduced to study and engineer printing forms, exploiting a microscale additive manufacturing process to design micro‐architected materials with controllable porosity and deformation. Building on this technology, controlled ink transfer for submicron functional films is demonstrated. The design freedom and high‐resolution 3D control afforded by this method provide a rich framework for studying mechanics of fluid transfer for advanced manufacturing processes.

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.

Aerosol-jet printing facilitates the rapid prototyping of microfluidic devices with versatile geometries and precise channel functionalization

Microfluidics has emerged as a powerful analytical tool for biology and biomedical research, with uses ranging from single-cell phenotyping to drug discovery and medical diagnostics, and only small sample volumes required for testing. The ability to rapidly prototype new designs is hugely beneficial in a research environment, but the high cost, slow turnaround, and wasteful nature of commonly used fabrication techniques, particularly for complex multi-layer geometries, severely impede the development process. In addition, microfluidic channels in most devices currently play a passive role and are typically used to direct flows. The ability to “functionalize” the channels with different materials in precise spatial locations would be a major advantage for a range of applications. This would involve incorporating functional materials directly within the channels that can partake in, guide or facilitate reactions in precisely controlled microenvironments. Here we demonstrate the use of Aerosol Jet Printing (AJP) to rapidly produce bespoke molds for microfluidic devices with a range of different geometries and precise “in-channel” functionalization. We show that such an advanced microscale additive manufacturing method can be used to rapidly design cost-efficient and customized microfluidic devices, with the ability to add functional coatings at specific locations within the microfluidic channels. We demonstrate the functionalization capabilities of our technique by specifically coating a section of a microfluidic channel with polyvinyl alcohol to render it hydrophilic. This versatile microfluidic device prototyping technique will be a powerful aid for biological and bio-medical research in both academic and industrial contexts.

A novel microscale selective laser sintering (\u03bc-SLS) process for the fabrication of microelectronic parts

One of the biggest challenges in microscale additive manufacturing is the production of three-dimensional, microscale metal parts with a high enough throughput to be relevant for commercial applications. This paper presents a new microscale additive manufacturing process called microscale selective laser sintering (μ-SLS) that can produce true 3D metal parts with sub-5 μm resolution and a throughput of greater than 60 mm3/hour. In μ-SLS, a layer of metal nanoparticle ink is first coated onto a substrate using a slot die coating system. The ink is then dried to produce a uniform nanoparticle layer. Next, the substrate is precisely positioned under an optical subsystem using a set of coarse and fine nanopositioning stages. In the optical subsystem, laser light that has been patterned using a digital micromirror array is used to heat and sinter the nanoparticles into the desired patterns. This set of steps is then repeated to build up each layer of the 3D part in the μ-SLS system. Overall, this new technology offers the potential to overcome many of the current limitations in microscale additive manufacturing of metals and become an important process in microelectronics packaging applications.

Bioprocess‐Inspired Microscale Additive Manufacturing of Multilayered TiO2/Polymer Composites with Enamel‐Like Structures and High Mechanical Properties

AbstractNatural structure‐forming processes found in biological systems are fantastic and perform at ambient temperatures, in contrast with anthropogenic technologies that commonly require harsh conditions. A new research direction “bioprocess‐inspired fabrication” is proposed to develop novel fabrication techniques for advanced materials. Enamel, an organic–inorganic composite biomaterial with outstanding mechanical performance and durability, is formed by repeating the basic blocks consisting of columnar hydroxyapatite or fluorapatite and an organic matrix. Inspired by the enamel formation process, a microscale additive manufacturing method is proposed for achieving a multilayered organic–inorganic columnar structure. In this approach, rutile titanium dioxide (TiO2) nanorods, polymers, and graphene oxide (GO) are sequentially assembled in a layer‐by‐layer fashion to form an organic–inorganic structure. In particular, GO serves as a substrate for TiO2 nanorods and interacts with polymers, jointly leading to the strength of the composites. Impressively, this enamel‐like structure material has hardness (1.56 ± 0.05 GPa) and ultrahigh Young's modulus (81.0 ± 2.7 GPa) comparable to natural enamel, and viscoelastic property (0.76 ± 0.12 GPa) superior to most solid materials. Consequently, this biomimetic synthetic approach provides an in‐depth understanding for the formation process of biomaterials and also enables the exploration of a new avenue for the preparation of organic–inorganic composite materials.

Volume-Averaged Electrochemical Performance Modeling of 3D Interpenetrating Battery Electrode Architectures

Recent advancements in micro-scale additive manufacturing techniques have created opportunities for design of novel electrode geometries that improve battery performance by deviating from the traditional layered battery design. These 3D batteries typically exhibit interpenetrating anode and cathode materials throughout the design space, but the existing well-established porous electrode theory models assume only one type of electrode is present in each battery layer. We therefore develop and demonstrate a multi-electrode volume-averaged electrochemical transport model to simulate transient discharge performance of these new interpenetrating electrode architectures. We implement the new reduced-order model in the PETSc framework and asses its accuracy by comparing predictions to corresponding mesoscale-resolved simulations that are orders of magnitude more computationally-intensive. For simple electrode designs such as alternating plates or cylinders, the volume-averaged model predicts performance within ∼2% for electrode feature sizes comparable to traditional particle sizes (5-10μm) at discharge rates up to 3C. When considering more complex geometries such as minimal surface designs (i.e. gyroid, Schwarz P), we show that using calibrated characteristic diffusion lengths for each design results in errors below 3% for discharge rates up to 3C. These comparisons verify that this novel model has made reliable cell-scale simulations of interpenetrating electrode designs possible.

A microscale additive manufacturing approach for in situ nanomechanics

In situ nanomechanics in scanning electron microscope (SEM) and transmission electron microscope (TEM) has been the gold-standard for direct observation of deformation mechanisms of metals and alloys. The extracted deformation mechanisms complement the process – microstructure – property relationship that is required for the full understanding of the mechanical behavior of these materials. Micro-pillar compression is perhaps the most frequently used method for such studies. Fabrication of micro-pillars from bulk materials relies on milling by the focused ion beam (FIB), which often requires several tens of hours of the equipment time, and the associated expenses. Additionally, the heavy ion bombardment by FIB may introduce damage into materials, which in turn may result in compromised interpretation of materials’ behavior. We introduce a microscale additive manufacturing (AM) approach that enables direct deposition of nano-pillars and micro-pillars of metals and alloys in room environment. In addition to the size, this process allows control over the microstructure of the deposited metals and alloys. Depending on the size and microstructure, a typical micro-pillar can be fabricated in a few minutes to tens of minutes at very low cost and without any beam-induced damages. When combined with in situ instrumentation, this approach may enable high-throughput investigation of the process – microstructure – property relationship, in particular for nano-crystalline and nano-twinned metals and alloys.

Aerosol Jet® Direct-Write for Microscale Additive Manufacturing

The unique capabilities of Aerosol Jet® technology for noncontact material deposition with in-flight adjustment of ink rheology in microdroplets are explained based on first principles of physics. The suitable range of ink droplet size is determined from the effectiveness for inertial impaction when depositing onto substrate and convenience for pneumatic manipulation, in-flight solvent evaporation, etc. The existence of a jet Reynolds number window is shown by a fluid dynamics analysis of impinging jets for Aerosol Jet® printing with long standoff between nozzle and substrate, which defines the operation range of gas flow rate according to the nozzle orifice diameter. The time scale for ink droplets to remove volatile solvent is shown to just coincide that for them to travel in the nozzle channel toward substrate after meeting the coflowing sheath gas, enabling the in-flight manipulation of ink properties for high aspect-ratio feature printing. With inks being able to solidify rapidly, 3D structures, such as tall micropillars and thin-wall boxes, can be fabricated with Aerosol Jet® printing. Having mist droplets in the range of 1–5 μm also makes it possible to print lines of width about 10 μm.

Mesoscale Electrochemical Performance Simulation of 3D Interpenetrating Lithium-Ion Battery Electrodes

Advancements in micro-scale additive manufacturing techniques have made it possible to fabricate intricate architectures including 3D interpenetrating electrode microstructures. A mesoscale electrochemical lithium-ion battery model is presented and implemented in the PETSc software framework using a finite volume scheme. The model is used to investigate interpenetrating 3D electrode architectures that offer potential energy density and power density improvements over traditional particle bed battery geometries. Using the computational model, a variety of battery electrode geometries are simulated and compared across various battery discharge rates and length scales to quantify performance trends and investigate geometrical factors that improve battery performance. The energy density vs. power density relationship of the electrode microstructures are compared in several ways, including a uniform surface area to volume ratio comparison as well as a comparison requiring a minimum manufacturable feature size. Significant performance improvements over traditional particle-bed electrode designs are predicted, and electrode microarchitectures derived from minimal surfaces are shown to be superior under a minimum feature size constraint, especially when subjected to high discharge currents. An average Thiele modulus formulation is presented as a back-of-the-envelope calculation to predict the performance trends of microbattery electrode geometries.

Toward Control of Microstructure in Microscale Additive Manufacturing of Copper Using Localized Electrodeposition

The progress in microscale additive manufacturing (μ‐AM) of metals requires engineering of the microstructure for various functional applications. In particular, achieving in situ control over the microstructure during 3D printing is critical to eliminate the need for post‐processing and annealing. Recent reports have demonstrated the possibility of electrochemical μ‐AM of nanotwinned metals, in which the presence of parallel arrays of twin boundaries (TBs) are known to enhance mechanical and electrical properties. For the first time, the authors report that the microstructure of metals printed using the microscale localized pulsed electrodeposition (L‐PED) process can be controlled in situ during 3D‐printing. In particular, the authors show that through electrochemical process parameters the density and the orientation of the TBs, as well as the grain size can be controlled. The results of the in situ SEM microcompression experiments on directly 3D‐printed micro‐pillars show that such control over microstructure directly correlates with the mechanical properties of the printed metal.