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Working with scientists from the Albert Ludwig University of Freiburg, Germany, the researchers expanded the capabilities of a 3D printing process they developed three years ago – computed axial lithography (CAL) – to print much finer features and to print in glass. They called the new system ‘micro-CAL’.
Glass is the preferred material for creating complex microscopic objects, including lenses in compact, high-quality cameras used in smartphones and endoscopes, as well as microfluidic devices used to analyse minute amounts of liquid. Current manufacturing methods can be slow, expensive, and limited in their ability to meet industry’s increasing demands, however.
Unlike conventional industrial 3D printing manufacturing processes, which are often time-intensive and result in rough surface textures, CAL prints the entire object simultaneously. Researchers use a laser to project patterns of light into a rotating volume of light-sensitive material, building up a 3D ‘light dose’ that then solidifies in the desired shape. The layer-less nature of the CAL process enables smooth surfaces and complex geometries.
The new study pushed the boundaries of CAL to demonstrate its ability to print microscale features. “When we first published this method in 2019, CAL could print objects into polymers with features down to about a third of a millimetre in size,” said principal investigator Hayden Taylor, professor of mechanical engineering at UC Berkeley. “Now with micro-CAL we can print objects in polymers with features down to about 20 millionths of a metre, or about a quarter of a human hair’s breadth. And for the first time, we have shown how this method can print not only into polymers but also into glass, with features down to about 50 millionths of a metre.”
To print the glass, Taylor and his research team collaborated with scientists from Albert Ludwig, who have developed a resin material containing nanoparticles of glass surrounded by a light-sensitive binder liquid. Digital light projections from the printer solidify the binder, then the researchers heat the printed object to remove the binder and fuse the particles together into a solid object of pure glass.
“The key enabler here is that the binder has a refractive index that is virtually identical to that of the glass, so that light passes through the material with virtually no scattering,” said Taylor. “The CAL printing process and this ‘glassomer’, GmbH-developed material, are a perfect match for each other.”
The research team, which included lead author Joseph Toombs, a PhD student in Taylor’s laboratory, also ran tests and discovered that the CAL-printed glass objects had more consistent strength than those made using a conventional layer-based printing process. “Glass objects tend to break more easily when they contain more flaws or cracks, or have a rough surface,” said Taylor. “CAL’s ability to make objects with smoother surfaces than other, layer-based 3D-printing processes is therefore a big potential advantage.”
The CAL 3D-printing method offers manufacturers of microscopic glass objects a more efficient way to meet customers’ demanding requirements for geometry, size and optical and mechanical properties, the researchers said. This includes manufacturers of microscopic optical components, which are a key part of compact cameras, virtual reality headsets, advanced microscopes and other scientific instruments. “Being able to make these components faster and with more geometric freedom could potentially lead to new device functions or lower-cost products,” said Taylor.
Microfluidic channels are also needed for ‘lab-on-a-chip’ systems for research and medical diagnostics. Until now, these have mostly been made of plastics, but they often cannot withstand high temperatures and aggressive chemicals. The new process has enabled complex channel systems manufactured from glass, said Freiburg materials scientist Frederik Kotz-Helmer: “Thanks to the thermal and chemical stability of glass, many new fields of application are opening up, especially in the area of chemistry on-a-chip synthesis.”
The study was funded by the US National Science Foundation, the European Research Council, the Carl Zeiss Foundation, the German Research Foundation and the US Department of Energy.
The research was published in Science.
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