A new silica-nanoparticle-based ink makes it possible to create intricate glass reaction vessels and optical components with a three-dimensional printer (Nature 2017, DOI: 10.1038/nature22061).
3-D printing has been growing in popularity, but the materials it’s been able to shape have been limited to polymers, ceramics, and metals, says Bastian Rapp, a polymer chemist at Karlsruhe Institute of Technology. The method creates elaborately structured objects that are difficult to form with a mold, and it enables mass customization. For example, Adidas and 3-D-printing company Carbon announced earlier this month that they plan to begin printing customized insoles for athletic shoes.
So far, it hasn’t been practical to put glass in a 3-D printer. Glass is useful—it’s transparent, resistant to chemical and thermal damage, and electrically insulating. But to work with the material, it’s necessary to heat it to more than 1,000 °C, then pour the melted substance into molds or work it by stretching and blowing.
“Making fine features in glass is challenging,” Rapp says. The glass must be chemically etched or mechanically polished. The complexity of the process means that manufacturers often choose to make optical parts such as the lenses inside cellphones from polymers instead. And the optical properties of those polymers are not as stellar.
Researchers have tried to 3-D print glass in the past, but the high melting temperature of the material has been a challenge. For instance, feeding and melting a glass filament in a 3-D printer or sintering a bed of silica particles has resulted in parts with low resolution—on the order of millimeters—and resulted in objects with rough surfaces not suitable for use in optics.
To overcome these difficulties, Rapp and coworkers adapted a “liquid glass” composite—a mixture of silica nanoparticles and ultraviolet-curable hydroxyethylmethacrylate monomers—that they’d previously developed for making molded glass objects. To make the material compatible with a 3-D printing method called stereolithography, they had to make a few modifications. Stereolithography uses projected patterns of UV light to turn monomers into polymers, building objects layer by layer. To make their ink more transparent, the researchers removed a solvent from the mixture. “If it scatters light, it’s a blurry mess,” Rapp says. They also had to make sure the material could mechanically withstand the printing process. So they made it stiffer by adding higher molecular weight triacrylates.
This material can be printed like a polymer but ultimately generates a pure glass object. After a 3-D printer generates an object with the liquid glass ink, the researchers heat it once to burn off the polymer, then a second time up to 1,300 °C to sinter the nanoparticles, leaving a pure, transparent fused-silica part.
Using these inks, the team made glass objects including a microlens array, a microfluidic chip, and a pretzel, with a resolution of tens of micrometers. Rapp says when the ink is used with higher resolution printing methods, the ultimate resolution should be 150 to 500 nm, about ten times the size of the original silica particles.
Nicholas Fang, a nanophotonics researcher at MIT, says the use of stereolithography to make optically transparent parts is “quite exciting.” Fang notes that the new ink could be used to print photonic crystals, which are usually made by more laborious methods. Photonic crystals use patterned nanostructures to control light and show promise for optical computing and other applications.
In addition, Rapp thinks the material could be used to create glass microreactors for making drugs and other compounds. Existing microreactor parts are made of metal or polymer and can’t withstand harsh reactions or extreme temperatures and pressures.
Rapp and coworkers are now adapting their composite ink to produce other kinds of glasses, including a bulletproof variety.