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[Image above] Complicated high-precision structures made of glass, like this pretzel, can be manufactured in a 3-D-printing process developed at Karslruhe Institute of Technology. Credit: Karslruhe Institute of Technology

Almost 2 years ago, Micron3DP demonstrated one of the earliest forays into 3-D printing with glass. And just a few months later, MIT backed up glass’s place in the additive manufacturing realm and showed just how beautiful the possibilities were.   

But although intriguing, those early demonstrations were only able to produce rather imprecise glass components with poor resolution—on the order of millimeters—because they printed in molten glass.

While that’s good enough for glass vases, bowls, and other artistic expressions, it just doesn’t cut it for the wide range of high-tech applications of glass that require intricate and precise microstructures.

To really open up the world of additive manufacturing for glass, we need techniques that can print with better resolution, precision, and detail, which is hard to achieve with molten glass.

Now, two new papers, one published in Nature and one in Advanced Materials, describe 3-D printing techniques that use silica nanoparticle inks—rather than molten glass itself—to fabricate optically clear glass components with micrometer-scale resolution, a huge leap forward for the integration of glass materials into additive manufacturing.

One of the techniques, developed by a team at Karslruhe Institute of Technology in Germany and published in Nature, prints precisely structured glass components via stereolithography with a UV-photocurable silica nanocomposite ink. About a year ago, I reported on the KIT team’s previous development of a photocurable liquid glass, which the team tweaked to develop the new printing method.

Stereolithography is a 3-D printing technique that uses light to polymerize molecules layer-by-layer, hardening an ink only in areas within a given design that are exposed to the light.

By developing an ink containing glass nanopowder suspended in a photocurable polymer, the KIT team was able to use stereolithography to print a desired design at room temperature using only UV light. Then, firing the printed nanocomposite part burns off the polymer and densifies the glass nanoparticles together at 1,300ºC, forming a fully glass structure, according to a KIT press release.

“The printed fused silica glass is non-porous, with the optical transparency of commercial fused silica glass, and has a smooth surface with a roughness of a few nanometers” according to the paper’s abstract.

The KIT logo, 3-D printed in glass. Credit: Karslruhe Institute of Technology

A minuscule 3-D printed glass castle. Credit: Karslruhe Institute of Technology

3-D printing can even be used to fabricate microfluidic devices in glass. Credit: Karslruhe Institute of Technology

The KIT team’s stereolithography technique currently can print glass components with a resolution of just a few tens of micrometers, but the authors think there’s still room for improvement. According to a C&EN article, “…when the ink is used with higher resolution printing methods, the ultimate resolution should be 150–500 nm, about ten times the size of the original silica particles.”

See more about this development in a short video available on KIT’s website.

But stereolithography isn’t the only way to 3-D print glass—a separate team of scientists at Lawrence Livermore National Lab, University of Minnesota, and Oklahoma State University also has developed a technique to 3-D print precise glass structures with sub-millimeter features, this time using direct ink writing. That development is published in Advanced Materials.

Similarly to KIT’s stereolithography method, the direct ink writing method also prints a silica-powder-infused liquid ink at room temperature, using a subsequent drying and sintering step “at temperatures well below the silica melting point,” according to the paper’s abstract, to form a final glass component.

“This is the first step to being able to print compositionally graded glass optics,” Rebecca Dylla-Spears, LLNL chemical engineer and project lead and the paper’s senior author, says in an LLNL news release.

Direct ink writing uses a precision nozzle to deposit the ink in the desired conformation, rather than using light to harden a photocurable ink. To produce a printable ink with optimal flow—too thin and it loses its structure, too thick and it doesn’t print well—the team varied the mixture of materials to get the ink composition just right.

“For printing high-quality optics, you shouldn’t be able to see any pores and lines, they have to be transparent,” LLNL materials engineer Du Nguyen says in an LLNL news release. “Once we got a general formulation to work, we were able to tweak it so the material could merge during the printing process. Most other groups that have printed glass melt the glass first and cool it down later, which has the potential for residual stress and cracking. Because we print at room temperature, that’s less of an issue.”

A new 3D printing technique could allow scientists to print glass that incorporates different refractive indices in a single flat optic, making finishing cheaper and easier. Credit: Jason Laurea; Lawrence Livermore National Lab

(From left) LLNL chemical engineer and project lead Rebecca Dylla-Spears and LLNL materials engineer Du Nguyen examine a 3-D printed piece of optical glass. Credit: Jason Laurea; Lawrence Livermore National Lab

Both 3-D printing techniques—stereolithography and direct ink writing—mean big possibilities for glass, because the material has properties that make it suitable for a wide range of potential applications, including biological and medical technologies, microfluidic devices like lab-on-a-chip systems, optics, and even components for next-generation electronics.

Beyond simply replacing existent methods to manufacture glass components, scientists also are optimistic that 3-D printing with glass could open new possibilities that haven’t yet been possible with other techniques for form and structure glass.

“The next plus one generation of computers will use light, which requires complicated processor structures; 3-D-technology could be used, for instance, to make small, complex structures out of a large number of very small optical components of different orientations,” Bastian Rapp, senior author of the Nature paper, says in the KIT press release.

Alternatively, the LLNL team is focusing on optical applications, indicating the possibility of fabricating compositional gradients that haven’t been possible with alternative techniques. The team even says that it may eventually be possible to print a single flat optic with different refractive indices.

“Optical fabrication research and development is trending toward freeform optics, which are optics that can be made virtually to any complex shape,” Tayyab Suratwala, LLNL’s program director for Optics and Material Science and Technology, says in the LLNL release. “Expanding this to 3-D-printed optics with compositional variation can greatly increase the capabilities of this new frontier.”

The Nature paper is “Three-dimensional printing of transparent fused silica glass” (DOI: 10.1038/nature22061).

The Advanced Materials paper is “3D-printed transparent glass” (DOI: 10.1002/adma.201701181).