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Guest Editorial

J. Micro Nano-Manuf. 2017;5(4):040301-040301-1. doi:10.1115/1.4037876.
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Products with micro- and nano-scale features find widespread applications in industries including medical, automotive, optics, electronics, energy, and biotechnology sectors. The tendency toward miniaturization and development of products in many industries exposed the limitations of established micro- and nanomanufacturing methods in terms of processing capability, speed, flexibility, accuracy, scalability, etc. To respond to these challenges, many micro- and nano-additive manufacturing technologies have been developed. Compared to traditional micro/nanofabrication methods, additive manufacturing (AM) has the merits of simpler processing, shorter fabrication time, lower cost, and capability of fabricating high aspect ratio structures and almost any complicated freeform structures. The innovation of novel AM or hybrid processes for micro/nanofabrication is a field of active research throughout the world. Accordingly, characterization, control, modeling, and simulation of the manufacturing process are in great need for achieving accurate and reliable production of micro/nanoscale features. In addition, successful production of complicated features at micro- or nano-scale requires understanding of the materials used as feedstock and the relationships between feedstock materials, designs, processes, and properties of fabricated micro/nanoscale structures. Analysis of the micro/nano-AM process performance, in terms of manufacturing flexibility, reliability, cost, and quality, is also critical to enable more applications in fields including biomedical, mechanical, sensing and actuating industries, etc.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster

SPECIAL SECTION PAPERS

J. Micro Nano-Manuf. 2017;5(4):040901-040901-5. doi:10.1115/1.4037788.
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Three-dimensional (3D) printing of microscale structures with high-resolution (submicron) and low-cost is still a challenging work for the existing 3D printing techniques. Here, we report a direct writing process via near-field melt electrospinning (NFME) to achieve microscale printing of single filament wall structures. The process allows continuous direct writing due to the linear and stable jet trajectory in the electric near field. The layer-by-layer stacking of fibers, or self-assembly effect, is attributed to the attraction force from the molten deposited fibers and accumulated negative charges. We demonstrated successful printing of various 3D thin-wall structures with a minimal wall thickness less than 5 μm. By optimizing the process parameters of NFME, ultrafine poly (ε-caprolactone) (PCL) fibers have been stably generated and precisely stacked and fused into 3D thin-wall structures with an aspect ratio of more than 60. It is envisioned that the NFME can be transformed into a viable high-resolution and low-cost microscale 3D printing technology.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):040902-040902-8. doi:10.1115/1.4037769.

A new dry spraying additive manufacturing method for Li-ion batteries has been developed to replace the conventional slurry-casting technique for manufacturing Li-ion battery electrodes. A dry spray manufacturing process can allow for the elimination of the time- and energy-intensive slurry drying process needed due to the use solvents to make the electrodes. Previous studies into the new manufacturing method have shown successful fabrication of electrodes which have strong electrochemical and mechanical performance. Li-ion battery electrodes typically consist of three basic materials: active material (AM), binder particle additives (BPA), and conductive particle additives (CPA). In this paper, a discrete element method (DEM) simulation was developed and used to study the mixing characteristics of dry electrode powder materials. Due to the size of the particles being in the submicron to micron size range, the mixing characteristics are heavily dependent on van der Waals adhesive forces between the particles. Therefore, the effect the Li-ion battery electrode material surface energy has on the mixing characteristics was studied. Contour plots based on the DEM simulation results where the surface energy components of selected material types are changed were used to predict the mixing characteristics of different particle systems. For the cases studied, it is found that experimental mixing results are representative of the results of the DEM simulations.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):040903-040903-9. doi:10.1115/1.4037770.

Surface plasmon polaritons associated with light-nanoparticle interactions can result in dramatic enhancement of electromagnetic fields near and in the gaps between the particles, which can have a large effect on the sintering of these nanoparticles. For example, the plasmonic field enhancement within nanoparticle assemblies is affected by the particle size, spacing, interlayer distance, and light source properties. Computational analysis of plasmonic effects in three-dimensional (3D) nanoparticle packings are presented herein using 532 nm plane wave light. This analysis provides insight into the particle interactions both within and between adjacent layers for multilayer nanoparticle packings. Electric field enhancements up to 400-fold for transverse magnetic (TM) or X-polarized light and 26-fold for transverse electric (TE) or Y-polarized light are observed. It is observed that the thermo-optical properties of the nanoparticle packings change nonlinearly between 0 and 10 nm gap spacing due to the strong and nonlocal near-field interaction between the particles for TM polarized light, but this relationship is linear for TE polarized light. These studies help provide a foundation for understanding micro/nanoscale heating and heat transport for Cu nanoparticle packings under 532 nm light under different polarization for the photonic sintering of nanoparticle assemblies.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):040904-040904-10. doi:10.1115/1.4037831.

Multiscale and multimaterial three-dimensional (3D) printing is new frontier in additive manufacturing (AM). It has shown great potential to implement the simultaneous and full control for fabricated object including external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano-, micro-, to macro-scale, embedded components and electrocircuit, etc. Furthermore, it has the ability to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. That paves the way and may result in great breakthrough in various applications, e.g., functional tissue and organ, functionally graded (FG) material/structure, wearable devices, soft robot, functionally embedded electronics, metamaterial, multifunctionality product, etc. However, very few of the established AM processes have now the capability to implement the multimaterial and multiscale 3D printing. This paper presented a single nozzle-based multiscale and multimaterial 3D printing process by integrating the electrohydrodynamic jet printing and the active mixing multimaterial nozzle. The proposed AM technology has the capability to create multifunctional heterogeneously structured objects with control of the macroscale external geometry and microscale internal structures as well as functional surface features, particularly, the potential to dynamically mix, grade, and vary the ratios of different materials. An active mixing nozzle, as a core functional component of the 3D printer, is systematically investigated by combining with the theoretical analysis, numerical simulation, and experimental verification. The study aims at exploring a feasible solution to implement the multiscale and multimaterial 3D printing at low cost.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):040905-040905-10. doi:10.1115/1.4037832.

Current stereolithography (SL) can fabricate three-dimensional (3D) objects in a single-scale level, e.g., printing macroscale or microscale objects. However, it is difficult for the SL printers to fabricate a 3D macroscale object with microscale features. In the paper, a novel SL-based multiscale fabrication method is presented to address such a problem. The developed SL process can fabricate multiscale features by dynamically changing the shape and size of a laser beam. Different shaped beams are realized by switching apertures with different micropatterns. The laser beam without using micropatterns is used to fabricate macroscale features, while the shaped laser beams based on small apertures are used to fabricate micropatterned features. Accordingly, a tool path planning method for the multiscale fabrication process is presented to build macroscale and microscale features using different layer thicknesses, laser exposure time, and scanning paths. Compared with the conventional SL process using a fixed laser beam size, our process can manufacture multiscale features in a 3D object with fast fabrication speed and good surface quality.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):040906-040906-6. doi:10.1115/1.4037695.

In electrospray printing, a plume of highly charged droplets is created from a conductive ink. Printing occurs by positioning a target substrate (TS) in the path of the emitted material. Here, the ink used is a colloidal dispersion consisting of nanoparticles suspended in a volatile solvent. The selection of a volatile solvent allows for rapid evaporation of the droplets in-flight to produce dry nanoparticles. A net electric charge is imparted on the emitted particles during electrospray. The interaction of this charge with the global electric field and with other charged particles/droplets governs the particles' trajectory and determines the microstructure of the printed deposit. In this study, we characterized the structure of nanoparticle deposits printed using electrospray for deposits with low particle count. During printing, the TS was: (i) held stationary and (ii) translated with various short spray times and substrate velocities, respectively. Examination of both a static and translating TS provided fundamental insights into the printing process. Electrospray printing is capable of exerting much finer control over microstructure compared to other printing techniques. This has significant implications for the manufacturing of thin-films.

Commentary by Dr. Valentin Fuster

Research Papers

J. Micro Nano-Manuf. 2017;5(4):041001-041001-6. doi:10.1115/1.4037574.

This paper is focused on developing an in-process intervention technique that mitigates the effect of built-up edges (BUEs) during micromilling of aluminum. The technique relies on the intermittent removal of the BUEs formed during the machining process. This is achieved using a three-stage intervention that consists first of the mechanical removal of mesoscale BUEs, followed by an abrasive slurry treatment to remove the microscale BUEs. Finally, the tool is cleaned using a nonwoven fibrous mat to remove the slurry debris. An on-machine implementation of this intervention technique is demonstrated, followed by a study of its influence on key micromachining outcomes such as tool wear, cutting forces, part geometry, and burr formation. In general, all relevant machining measures are found to improve significantly with the intervention. The key attributes of this intervention that makes it viable for micromachining processes include the following: (i) an experimental setup that can be implemented within the working volume of the microscale machine tool; (ii) no removal of the tool from the spindle, which ensures that the intervention does not change critical process parameters such as tool runout and offset values; and (iii) implementation in the form of canned G-code subroutines dispersed within the regular micromachining operation.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041002-041002-10. doi:10.1115/1.4037124.

One of the greatest challenges in the manufacturing and development of nanotechnologies is the requirement for robust, reliable, and accurate characterization data. Presented here are the results of an interlaboratory comparison (ILC) brought about through multiple rounds of engagement with NanoSight Malvern and ten pan-European research facilities. Following refinement of the nanoparticle tracking analysis (NTA) technique, the size and concentration characterization of nanoparticles in liquid suspension was proven to be robust and reproducible for multiple sample types in monomodal, binary, or multimodal mixtures. The limits of measurement were shown to exceed the 30–600 nm range (with all system models), with percentage coefficients of variation (% CV) being calculated as sub 5% for monodisperse samples. Particle size distributions were also improved through the incorporation of the finite track length adjustment (FTLA) algorithm, which most noticeably acts to improve the resolution of multimodal sample mixtures. The addition of a software correction to account for variations between instruments also dramatically increased the accuracy and reproducibility of concentration measurements. When combined, the advances brought about during the interlaboratory comparisons allow for the simultaneous determination of accurate and precise nanoparticle sizing and concentration data in one measurement.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041003-041003-5. doi:10.1115/1.4037694.
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This paper reports a feasibility study that demonstrates the implementation of a computer-aided design and manufacturing (CAD/CAM) approach for producing two-dimensional (2D) patterns on the nanoscale using the atomic force microscope (AFM) tip-based nanomachining process. To achieve this, simple software tools and neutral file formats were used. A G-code postprocessor was also developed to ensure that the controller of the AFM equipment utilized could interpret the G-code representation of tip path trajectories generated using the computer-aided manufacturing (CAM) software. In addition, the error between a machined pattern and its theoretical geometry was also evaluated. The analyzed pattern covered an area of 20 μm × 20 μm. The average machined error in this case was estimated to be 66 nm. This value corresponds to 15% of the average width of machined grooves. Such machining errors are most likely due to the flexible nature of AFM probe cantilevers. Overall, it is anticipated that such a CAD/CAM approach could contribute to the development of a more flexible and portable solution for a range of tip-based nanofabrication tasks, which would not be restricted to particular customised software or AFM instruments. In the case of nanomachining operations, however, further work is required first to generate trajectories, which can compensate for the observed machining errors.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041004-041004-8. doi:10.1115/1.4037726.

This paper investigates the application of bioinspired serrated cutting edges in tissue cutting by biopsy punches (BPs) to reduce the insertion force. BPs are frequently used as a diagnostic tool in many minimally invasive procedures, for both tissue extraction and the delivery of medical fluids. The proposed work is inspired by the mosquito's maxilla that features microserrations on its cutting edges with the purpose of painlessly puncturing the human skin. The objective of this paper is to study the application of maxillalike microserrations on commercial BPs. The fundamental goal is the minimization of the puncture force at the BP tip during insertion procedures. Microserrations were created on the cutting edge by using a picosecond laser while cutting tests were implemented on a customized testbed on phantom tissue. A reduction of 20–30% in the insertion forces has been achieved with microserrated punches with different texture depths encouraging, thereby, further studies and applications in biomedical devices. Three-dimensional (3D) and two-dimensional (2D) finite element simulations were also developed to investigate the impact of microserrated cutting edges on the stresses in the contact area during soft tissue cutting.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041005-041005-10. doi:10.1115/1.4037768.

To adapt with today's rapidly changing world, fabrication of intricate microparts is becoming an urgent need. Manufacturing of these microparts with stringent requirements necessitates the early adoption of different microfabrication techniques. Wire electrochemical machining (WECM) is such a process which removes excess metal by dissolving it electrochemically. This process can easily generate features downscaled to micron ranges and offers several advantages like the requirement of very simple setup, fabrication of accurate complex microfeatures without undergoing any thermal stress, burr formation, and tool wear, which make it superior from other existing micromachining processes. However, this process is new, and little is known about its applicability and feasibility. Hence, the present work is directed towards developing suitable WECM setup to fabricate microfeatures by introducing proper means for enhancing the mass transport phenomenon. The tungsten tool wire for machining has been in situ etched to a diameter of 23.43 μm by a novel approach for retaining its regular cylindrical form and has been implemented during machining. Moreover, the influences of high duty ratio and applied frequency have been investigated on the corresponding width of the fabricated microslits and the experimental results have been represented graphically where the minimum width of the microslit is obtained as 44.85 μm. Furthermore, mathematical modeling has been developed to correlate duty ratio and applied frequency with generated slit width. Additionally, the mathematical modeling has been validated with practical results and complex stepped type microfeatures have been generated to establish process suitability.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041006-041006-14. doi:10.1115/1.4037645.

Laser microprocessing is a very attractive option for a growing number of industrial applications due to its intrinsic characteristics, such as high flexibility and process control and also capabilities for noncontact processing of a wide range of materials. However, there are some constrains that limit the applications of this technology, i.e., taper angles on sidewalls, edge quality, geometrical accuracy, and achievable aspect ratios of produced structures. To address these process limitations, a new method for two-side laser processing is proposed in this research. The method is described with a special focus on key enabling technologies for achieving high accuracy and repeatability in two-side laser drilling. The pilot implementation of the proposed processing configuration and technologies is discussed together with an in situ, on-machine inspection procedure to verify the achievable positional and geometrical accuracy. It is demonstrated that alignment accuracy better than 10 μm is achievable using this pilot two-side laser processing platform. In addition, the morphology of holes with circular and square cross sections produced with one-side laser drilling and the proposed method was compared in regard to achievable aspect ratios and holes' dimensional and geometrical accuracy and thus to make conclusions about its capabilities.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(4):041007-041007-9. doi:10.1115/1.4037646.

Multilayered encapsulation has been of great interest for various pharmaceutical, chemical, and food industries. Fabrication of well-defined capsules with more than one shell layer still poses a significant fabrication challenge. This study aims to investigate the feasibility of using a coaxial nozzle to fabricate double-layered (core–shell–shell) capsules during vibration-assisted dripping. A three-layered coaxial nozzle has been designed, manufactured, and tested for double-layered capsule fabrication when using sodium alginate solutions as the model liquid material for inner and outer shell layers and calcium chloride solution as the core fluid. To facilitate the droplet formation process, a vibrator has been integrated into the fabrication system to provide necessary perturbation for effective breakup of the fluid flow. It is demonstrated that double-layered alginate capsules can be successfully fabricated using the proposed three-layered coaxial nozzle fabrication system. During fabrication, increasing the core flow rate leads to an increase in capsule and core diameters while the inner and outer shell layer thicknesses decrease. Increasing annular flow rate results in an increase in capsule diameter and inner shell layer thickness while the outer shell layer thickness decreases. An increase in the sheath flow rate leads to an increase in capsule diameter and outer shell layer thickness but has no significant effect on the core diameter and inner shell layer thickness.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Micro Nano-Manuf. 2017;5(4):044501-044501-5. doi:10.1115/1.4037473.

The importance of coatings in modern science and industry is great, and the system presented in this manuscript attempts to provide a method of creating high quality nanoparticle coatings with in situ sintering of nanoparticles. Dual regime nozzle creates close to optimum conditions for particle delivery and deposition and the addition of in situ thermally assisted coating makes it more productive. Preliminary results show systems uniform coating and in situ sintering capability.

Commentary by Dr. Valentin Fuster

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