3D Printing Hierarchical Porous Materials

Hierarchical porous materials are commonly found in nature and have numerous applications as well, such as catalytic supports, biological scaffolds and lightweight structures. 3D printing has allowed for the fabrication of porous materials in the forms of lattices, cellular structures and foams across multiple length scales. However, according to a group of researchers in a paper entitled “3D printing of sacrificial templates into hierarchical porous materials,” current approaches “do not allow for the fast manufacturing of bulk porous materials featuring pore sizes that span broadly from macroscopic dimensions down to the nanoscale.”

In the research paper, the authors describe how they developed ink formulations to enable 3D printing of hierarchical materials displaying porosity at the nano-, micro- and macro-scales.

“Here, we 3D print inks that consist of nanoemulsions and other microtemplates to produce complex-shaped hierarchical materials with controlled pores ranging from hundreds of nanometers to millimetres in size,” the researchers state. “The pore size of the resulting porous materials can be easily tuned through the selection of the printing path and the size of the pore templating building blocks. Submicron pores are generated from particle-stabilized nanoemulsions, whereas larger droplets or sacrificial polymer particles are used to create pores in a size range varying from 10 to 100 µm. Finally, the macroscopic complex shape and the large-scale cellular architecture of the hierarchical porous material is determined by the 3D printing process.”

The researchers formed stable nanodroplets through a two-step emulsification process. These nanodroplets are stable enough to be concentrated by ultracentrifugation and form a dense jammed template that can be directly converted into a nanoporous structure upon drying or sintering depending on the oil volatility.

“Because the nanoparticles form a dense layer on the surface of the precursor droplets, closed nanopores are often obtained after drying and sintering,” the researchers continue. “However, open pores can also form if the emulsions are slightly destabilized during processing to generate droplet surfaces that are only partially covered by particles. For the emulsions investigated in this work, we found that such slight destabilization is possible by replacing corn oil by decane as the dispersed phase. The ability to tune the process to generate either open or close porosity after sintering enables tailoring of the porous structure according to the properties required by the aimed application.”

Since the nano- and micro-porosity are generated from the self-assembly of templating droplets and particles within the ink, as opposed to the slow sequential depositing of material, the 3D printing process is simple and fast. Because they are susceptible to coalescence during ink preparation, the templating droplets need to be stabilized by particles that will later form the walls of the pores created upon drying and consolidation.

“The zwitterionic nature of the surfactant used to promote this stabilization mechanism allows for the use of particles with a variety of distinct chemistries,” the researchers conclude. “Moreover, the dried printed structure can be consolidated either chemically or via heat treatment, depending on the ink formulation. Combined with the complex shaping capabilities of 3D printing, these features make the process highly tunable and open several new possibilities for the design and digital fabrication of hierarchical porous materials for a variety of applications.”

Authors of the paper include Lauriane Alison, Stefano Menasce, Florian Bouville, Elena Tervoort, Iacopo Mattich, Allesandro Ofner and André R. Studart.

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3D Printing Used to Create Holographic Color Prints for Enhanced Security

Researchers at the Singapore University of Technology and Design (SUTD) are constantly coming up with new discoveries and 3D printing applications, from 3D printed light shows to biocompatible hydrodgels. Now scientists at SUTD have used 3D printing to develop something that not only looks really cool, but can also be used to deter counterfeiting.

The device, which the scientists call “holographic color prints,” creates images that appear as a regular color print under white light. Under red, green or blue laser illumination, the device projects up to three different holograms. It is capable of modulating both the phase and the amplitude of light. The research is documented in a paper entitled “Holographic colour prints for enhanced optical security by combined phase and amplitude control.”

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. Microscopic color prints modulate the amplitude and holograms modulate the phase of light, but the researchers believe that this structure can be easily imitated. So they designed a pixel that overlays a structural color element onto a phase plate to control both the phase and amplitude of light. They then arrayed the pixels into monolithic prints, with each pixel strategically arranged on a plane. Nanostructured posts with different heights were used as structural colored filters to modulate the amplitude of light.

The researchers also created an algorithm that takes multiple images as its input and generates an output file to determine the positions of different phase and colored filter elements. They then used a nanoscale 3D printer to create a holoscopic print of painter Luigi Russolo’s 1910 painting “Perfume.” The color print is visible under ambient white light. Different thicknesses of polymerized cuboid were used to modulate the phase plates and form three multiplexed holograms, projected as a red thumbprint, a green key, and blue letters that read “SECURITY.” The images were embedded within the print.

The holographic color prints can be easily verified, according to the researchers, but are difficult to imitate.

“The relationship of holograms in combating counterfeiting is analogous to antibiotics against infections,” said professor Joel Yang. “Every so often, new technology is needed to deter counterfeiters as the old-fashioned holograms become easier to copy.”

The prints consist of nano 3D printed polymer structures and can be used in optical document security. Information in the prints is encoded only in the surface relief of a single polymeric material, so nanoscale 3D printing could then be used to mass manufacture customized masters by nanoimprint lithography.

“For the first time, multiple holograms that are color selective are ‘woven’ into a colorful image using advanced nanofabrication techniques,” Yang said. “We are hopeful that these new holographic color prints are user friendly but counterfeiter unfriendly: They are readily verified but challenging to copy, and can provide enhanced security in anticounterfeiting applications.”

Authors of the paper include Kevin T.P. Lim, Hailong Liu, Yejing Liu and Joel K.W. Yang.

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[Source: Photonics.com]

 

3D Printing Optical Illusions at the Nanoscale

Cytosurge is a spin-off company from ETH Zurich, and it has become known for its FluidFM 3D printing technology, which was commercialized and made consumer-friendly via the FluidFM 3D printer. FluidFM is a metal 3D printing technique capable of creating components at the nano level. It involves a moveable pipette, mounted on a cantilever leaf-spring, through which a liquid metal solution flows and exits through an aperture of 300 nanometers. An electrode produces a voltage under the aperture, which solidifies the metal as it exits the nozzle. Once the gap between the surface and the pipette is filled with solid metal, the pipette moves to the next position and repeats the process. In this way a 3D object is built.

FluidFM technology has been applied to many applications, from metal nanoprinting to medical research, and now it has been applied to something new: so-called impossible objects. Not to be confused with the company Impossible Objects, an impossible object is an optical illusion of a two-dimensional object that is interpreted by the viewer as a 3D image. It also appears to have an entirely different shape when viewed from different perspectives. The drawings of M.C. Escher are examples of such objects, and now Professor Kokichi Sugihara, a mathematical scientist at Meiji University, has found a way to 3D print these objects, using FluidFM technology.

Sugihara has a long history in mathematical engineering, having received a Bachelor, Master and Doctor of Engineering from the University of Tokyo. He worked at the Electrotechnical Laboratory in the Ministry of International Trade and Industry of Japan, Nagoya University and the University of Tokyo before moving into his current position at Meiji University. He has done a lot of work with computer vision, and that work led him to experimentation with impossible objects or impossible solids.

“A trompe l’oeil, referring to an art genre, is painted by using optical illusions,” Sugihara explains. “One day, I tried to confirm if my software determines that it is impossible to construct solids from stereoscopic objects depicted in a trompe l’oeil. Unexpectedly, the software reconstructed solids that we never could imagine are possible through human eyes. This interesting result inspired me to create ‘impossible solids’. We visualize a 3D object from a 2D drawing based on the preconceived assumption that is obtained through common sense and visual experience. The stereoscopic objects in the trompe l’oeil are painted by exploiting these preconceived assumptions and appear unlikely to our eyes to exist. However, the computer is not influenced by any assumption. The computer examines every possibility in order to reconstruct a 3D object and concludes that it is ‘Able to do it.’ I applied this theory also to create impossible motion.”

Sugihara won the Best Illusion of the Year contest three times, and finished in second place twice. Now he has teamed up with Cytosurge to 3D print one of those illusions, in three nanoscale sizes: 0.1 mm diameter, 0.03 mm and 0.01 mm, or the size of a red blood cell. The tiny objects were 3D printed from copper. The objects have been imaged at high resolution with an electron microscope, and you can see the illusion from different angles below:

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Microlight3D Offers a New Kind of Microscale 3D Printing

For 15 years, Patrice Baldeck and Michel Bouriau led intense research and development at the Université Grenoble Alpes. They were working on a two-photon polymerization 3D printing process that would become the basis of Microlight3D, founded in 2016. The process would be the first-ever non-additive two-photon polymerization direct laser writing technology. Microlight3D’s technology allows a laser to move freely in three dimensions, performing uninterrupted printing inside a polymer resist.

The benefits of the technology are many. It produces extremely high resolution and smooth surface finish comparable to injection molding. It also offers a great deal of design flexibility and eliminates the need for post-processing. It’s a fast technology that produces robust parts in any shape – 100 times smaller than a strand of hair. These microscopic parts have a wide variety of applications, including in micro-optics, microfluidics, micro-robotics, metamaterials and cell biology.

Microlight3D is the only company to commercialize 3D microprinters that use sub-nanosecond lasers. Compared to femtosecond lasers, sub-nanosecond lasers have long, energetic pulses, which enable a more efficient polymerization process.

The process begins with a 3D model, like any other 3D printing process. Microlight3D algorithms calculate the path that the laser will follow in order to 3D print the object, and then the focused laser solidifies the specified locations, with sub-micron resolution, within a liquid material bath. The free movement of the laser allows the fabrication of the part without the layer-by-layer limitations of typical additive manufacturing. Once the print is complete, a solvent bath washes off the excess monomer.

Microlight3D’s open 3D microfabrication platform, μFAB-3D, is geared toward research applications such as surface structuration, metamaterials, microfluidics and scaffolds for cell culture. It’s compatible with a wide range of materials, including biomaterials. It features a proprietary, intuitive software with customer-specific plug-ins, and can print objects up to 100 x 75 mm squared, on flat or non-flat substrates.

Once Baldeck and Bouriau decided to take their technology from the university to the commercial sector, things moved fast. Bouriau and Denis Barbier, formerly the CTO of Teem Photonics, led the transfer of the technology from the lab to the startup, which received support from SATT Linksium, a technology transfer acceleration network. The company then recruited Philippe Paliard, an applications engineer with end-user interface experience, and Gabriel Gonzalez, a software engineer. Within 18 months, Microlight3D’s technology had evolved from a prototype to a high-performance product.

Microlight3D can customize its 3D printers to customers’ needs, in terms of material, hardware and fabrication procedure plug-ins. The company’s printers are available worldwide, and have already been sold in Europe, the United States, China, Singapore and Taiwan. Microlight3D holds multiple patents and has already won several awards, and it has many goals for the future. The company plans to introduce a new product in November of this year, as well as to evolve its technology further. It intends to achieve a faster writing speed which would enable larger micro objects, as well as to improve accuracy, volume, and surface smoothness and to widen the already large array of compatible materials. Software improvements are also in the works, to enable more complex designs and manage bigger files.

Microlight3D also plans to open a commercial subsidiary and hire commercial personnel in the United States. The company is based in France.

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3D Printing Glass-Ceramics at the Nanoscale

Micro-graphs of different initial and treated structures (1000 ◦ C for 2h). Down-sizing of solid volumetric and free-form structures (with correspondingly high and low initial volume fractions of polymer). From top to bottom: (a) a free-form sculpture Vytis (Coat of arms of Lithuania), (b) homogeneous cube structure, (c) photonic crystal (periodic) structure with cage and (d) hexagonal scaffold.

Many methods are used to develop 3D printing materials, and the sources for new 3D printing materials are seemingly endless. In a study entitled “Additive Manufacturing of 3D Glass-Ceramics down to Nanoscale Resolution,” a group of researchers use a sol-gel resin to fabricate an inorganic ceramic.

Illustration of the main steps in synthesis of ceramics out of hybrid SZ2080 followed from laser induced polymerization that occurs during direct laser writing. During first stage of calcination, organic part is removed from the matrix and an inorganic glass matrix forms. As temperature is increased further, crystallization occurs and polycrystalline ceramic phase forms. Crystal structure of cristobalite and t-ZrO2 are shown in bottom row.

“Fabrication of a true-3D inorganic ceramic with resolution down to nanoscale using sol-gel resist precursor is demonstrated,” the researchers explain. “The method has an unrestricted free-form capability, control of the fill-factor, and high fabrication throughput. A systematic study of the proposed approach based on ultrafast laser 3D lithography of organic-inorganic hybrid sol-gel resin followed by a heat treatment enabled formation of inorganic amorphous and crystalline composites guided by the composition of the initial resin.”

A popular hybrid organic-inorganic sol-gel resist SZ2080 was converted into a material with entirely different properties through polymer-to-ceramic transition via high temperature sintering and oxidation. The silica and zirconia in the original material in the resist in the 20% inorganic part of the component led to the emergence of silica and zirconia crystalline phases in the final sintered ceramic material. In addition, “a proportional downscaling of the 3D polymerized object takes place with significant volume change of 40-50% dependent on annealing protocol without distortion of the proportions of the initial 3D design,” meaning that complex nanoscale patterns can be formed.

For the experiment, the researchers 3D printed different structures including bulk-cubes, periodic-3D woodpile micro lattices, free form structures, micro-sculptures, combining bulk and nanometer feature elements with complex bends, and macroscopic hexagonal 3D lattices which are usually used as artificial cell scaffolds.

“As the temperature increases the spectral shape changes and evolves via qualitatively two distinct form-factors,” the researchers state. “Close examination of the initial spectrum and comparison to that for T = 1000◦C reveals that they differ by the molecular vibrations which can be associated with the carbon-carbon, carbon-oxygen, carbon-hydrogen bonds. After heat-treatment those spectral lines vanishes. The new spectral form coincides with that typical for silica glass; we measured a control sample of fused silica.”

High temperature calcination of the 3D polymerized structures, created by 3D laser writing in the SZ2080 polymer resist, produced either silica-based glass or a polycrystalline ceramic pure inorganic material. A glass phase dominated in samples annealed at temperatures up to 1200°C, while formation of polycrystalline silica and zirconia was observed in samples annealed above 1200ºC.

“The presented modifications of silica-zirconia-rich resist SZ2080 from glass to polycrystalline ceramic by annealing shows a principle of the thermally guided 3D material printing which has nanoscale resolution,” the researchers conclude. “Isotropic down-sizing of the initial 3D polymerized objects with a volume fraction of 0.5-to-1 simplifies fabrication since there is no need to alter proportions of the initial material as it is widely used in DLW 3D nanolithography of photonic crystals, micro-optics and biomedical scaffolds in order to eliminate the effect of anisotropic shrinkage.”

Uniform 3D down-scaling by 3D nano-sintering. SEM micro-graph of a ceramic micro-sculpture after sintering at 1200◦C for one hour (right). Initial material SZ2080 resist (all dimensions were 1.7× larger; note the different scale bars)

The mechanical properties of the final structures, according to the researchers, acquire new features, such as resilience in harsh physical and chemical environments.

“Since nanoscale materials can initiate precipitation and guide growth of nano-crystallites, a wide field for experimentation horizons are widened by the presented modality of additive manufacturing,” they add.

Authors of the paper include Darius Gailevicius, Viktorija Padolskyte, Lina Mikoliūnaite, Simas Šakirzanovas, Saulius Juodkazis and Mangirdas Malinauskas.

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Lawrence Livermore National Laboratory Makes 3D Printed Nanoporous Gold That Could Change the Design of Electrochemical Reactors

[Image: Ryan Chen/LLNL]

Lawrence Livermore National Laboratory (LLNL) is known for doing impressive work with materials, particularly related to 3D printing. Whether it’s nanoscale 3D printing or 3D printed glass, the organization is constantly making new discoveries. In its latest study, entitled “Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing,” LLNL researchers along with those from Harvard University discuss the hierarchical printing of nanoporous gold. According to the researchers, this work could have a major impact on the design of chemical reactors.

Nanoporous metals are strong catalysts for chemical reactions, as they have a large surface area and high electrical conductivity. This makes them well-suited to applications such as electrochemical reactors, sensors and actuators.

“If you consider traditional machining processes, it’s time consuming and you waste a lot of materials — also, you don’t have the capability to create complex structures,” said LLNL postdoctoral researcher Zhen Qi, a co-author on the paper. “By using 3D printing we can realize macroporous structures with application-specific flow patterns. By creating hierarchical structures, we provide pathways for fast mass transport to take full advantage of the large surface area of nanoporous materials. It’s also a way to save materials, especially precious metals.”

[Image: Ryan Chen/LLNL]

The researchers combined an extrusion-based direct ink writing process with an alloying and dealloying process to engineer nanoporous gold into three distinct scales, from the microscale to the nanoscale. According to the team, the hierarchical structure “dramatically improves mass transport and reaction rates for both liquid and gases.” Being able to manipulate the catalyst’s surface area to generate electrochemical reactions through 3D printing could have a big impact on electrochemical plants, which currently rely mostly on thermal energy.

“By controlling the multiscale morphology and surface area of 3D porous materials, you can start to manipulate the mass transport properties of these materials,” said LLNL researcher Eric Duoss. “With hierarchal structures you have channels that can handle transfer of reactants and products for different reactions. It’s like transportation systems, where you go from seven-lane expressways down to multiple lane highways to thoroughfares and side streets, but instead of transporting vehicles we’re transporting molecules.”

LLNL researcher Cheng Zhu and former postdoctoral student Wen Chen made inks out of gold and silver nanoparticles, which were then 3D printed. The printed parts were placed into a furnace to allow the particles to coalesce into a gold-silver alloy. The team then put the parts into a chemical bath that removed the silver in a process called dealloying, leaving porous gold behind.

“The final part is a 3D hierarchical gold architecture comprising the macroscale printed pores and the nanoscale pores that result from dealloying,” said Chen, who is currently a professor at the University of Massachusetts-Amherst. “Such hierarchical 3D architectures allow us to digitally control the morphology of the macropores, which allowed us to realize the desired rapid mass transport behavior.”

According to Zhu and Chen, the method can also be applied to other metals such as magnesium, nickel and copper, opening up 3D printing applications in fields such as catalysis, batteries, supercapacitors and carbon dioxide reduction.

The challenge in catalysis, according to LLNL researcher Juergen Biener, is in combining high surface area with rapid transport.

“While additive manufacturing is an ideal tool to create complex macroscale structures, it remains extremely difficult to directly introduce the nanostructures that provide the required high surface area,” Biener said. “We overcame this challenge by developing a metallic ink-based approach that allowed us to introduce nanoporosity through a selective corrosion process called dealloying.”

Biener said that the team’s extrusion-based approach is universal and scalable, provides tooling-free control over the macroscopic sample shape, and enables integration of nanoporosity in an application-specific engineered macroporous network structure. These advantages open new design possibilities for chemical reactor and energy storage and conversion devices.

The project is part of a feasibility study into a proposed strategic initiative to create 3D electrochemical reactors in which scientists could have greater control over catalysts and reduce transport limitations. Instead of large electrochemical plants, which are located near oil refineries or in remote areas, modular reactor networks could be created in a series that could be easily replaceable and transportable to locations near sources of abundant renewable energy or carbon dioxide.

“There are a whole lot of scientific and engineering challenges left, but it could have significant impact,” said Chris Spadaccini, director of LLNL’s Center for Engineered Materials and Manufacturing. “Scaling up should be easier with small-scale reactors because you can parallelize. You could have an array of small 3D reactors together instead of one large vessel enabling you to control the chemical reaction process more effectively.”

The researchers are also starting to explore other materials that could be catalysts for other reactions.

Authors of the paper include Cheng Zhu, Zhen Qi, Victor A. Beck, Mathilde Luneau, Judith Lattimer, Wen Chen, Marcus A. Worsley, Jianchao Ye, Eric B. Duoss, Christopher M. Spadaccini,  Cynthia M. Friend and Juergen Biener. 

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[Source: Newswise]

French Artist Presents the Tiniest 3D Printed Self-Portrait You’ve Ever Seen (Or Not Seen)

Michel Paysant

There’s something magnificent about a mural or a massive statue, but when it comes to art, bigger does not always mean better. In fact, some of the most remarkable pieces of art are so small that they can’t even be seen by the naked eye. Consider this nanoscale nativity scene, or this itty-bitty Wall of China, or this miniscule pyramid. 3D printing has evolved to the point that it can create items of incredible detail at sizes smaller than a human hair, and that’s pretty cool, to put it simply.

Microlight3D is a French company that specializes in bioprinting, two-photon polymerization and 3D microfabrication. In a collaboration with artist Michel Paysant, the company has created what it is calling the smallest sculpture in the world. Paysant, who has exhibited at the Louvre, combines art with technology to create striking visual works including a series of self-portraits. One day, he decided to 3D print his own head. He certainly wouldn’t be the first one to do so, but he didn’t want to create just an ordinary 3D print, so he contacted Microlight3D after 3D scanning himself.

Microlight3D took the high-resolution scan and 3D printed it at a resolution of 0.2 microns, or 0.0002 millimeters. Michel Bouriau, CTO of the company, handled the 3D printing and came up with a work of art so small that it requires a microscope to see. Once you look through that microscope, however, you can easily see the amazing detail in the sculpture, which has a height of 80 microns, or 0.08 millimeters, about the size of an ant’s eye. Never thought much about the size of an ant’s eye? That’s because you can’t see it – not without a microscope.

Microlight3D is a young company that has only been selling its 3D printers since January 2017, but 15 years of research into two-photon polymerization at the University Grenoble-Alpes has led to a great deal of expertise in tiny 3D printing. Nanoscale 3D printing is a technology that is still in development, and it has a lot of potential for next-generation medical treatments, computer applications, aerospace engineering and more.

Just a few months ago, YouTube star James Bruton made the record books for creating the tallest 3D printed sculpture of a human. His statue came in at 3.62 meters, or nearly 12 feet, tall. If you enjoy math, I challenge you to calculate how many of Paysant’s microscopic sculptures could fit on Bruton’s giant one. When Bruton broke the record, it hadn’t been held for very long, and neither had the previous record before that. We’ll see how long Bruton holds it, because 3D printed creations just keep getting bigger and bigger. Size isn’t really a limitation when it comes to 3D printing, so it won’t be surprising if someone else comes up with a smaller sculpture than Paysant’s before long.

Paysant’s nanoscale sculpture will be on display to the public – with microscope handy – at the Artotheque FRAC Limousin New Aquitaine from June 27th to November 3rd.

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[Images: Microlight3D]