Mar 28, 2018 | By Benedict

Researchers at Northwestern University have discovered a new method for speeding up the 3D printing of millimeter-sized imaging lenses. The technique could be used create customized contact lenses or to turn iPhones into disease-spotting microscopes.

Of all things you might attempt to 3D print, lenses—which need to be immaculately finished, transparent, and consistent—might not be high up your list. The inherent problems with the 3D printing of lenses are obvious: if your 3D printed lens has any visible stepping between layers, it becomes virtually useless (unless the aim of your lens is to obfuscate vision). Other fabrication methods would therefore seem more practical.

When engineers at Northwestern University in Illinois recently attempted to 3D print their own lenses, they ran into this very problem: the curved layers of their lenses, made from photo-curable resin, created visible stepping. They simply weren’t functional for any kind of imaging application.

Cheng Sun, an associate professor of mechanical engineering at Northwestern, described that moment of truth: “We realized that the layers on top of each other created surface roughness. The layer thickness is typically 5 microns, while the wavelength of visible light is around 0.5 microns. This creates an optically rough surface.”

Of course, this result was to be expected, so the researchers thought about other ways they could 3D print lenses so that they would not produce stepping. After lengthy research, they decided to attempt a two-step process of layering and polishing—if the DLP resin 3D printer wasn’t going to produce a perfectly smooth surface on its own, why not supplement that process with a helping hand?

“First, we used grayscale images to create more transitions between steps,” Sun said. “Then, we coated the surface with the same photo-curable resin. That then forms the meniscus that further smooths the surface.”

After over 100 attempts to get this technique right, the researchers finally achieved their goal: a 3D printed transparent lens with a smooth surface (less than 7-nanometer surface roughness) suitable for imaging applications.

But the exciting part of the 3D printing research isn’t just the fact that Northwestern has been able to 3D print functional lenses. After all, other groups, including German lithography specialist Nanoscribe, have already done this with some success. Rather, the unique achievement of the research is the sheer speed at which the researchers are now able to fabricate these lenses.

“If you want to make a lens, do you want to make it in two hours or two weeks?” asked Xiangfan Chen, a PhD candidate in mechanical engineering and lead author on the study. “We are very excited about this lens.”

Applications for the polished 3D printed lens are numerous. Sun says many devices in the field of optics and biomedical imaging could benefit from such lenses, and the researchers are now planning to make much larger 3D printed lenses.

But one of the most exciting future applications of these resin-made lenses could be within the field of medicine. The researchers think their quickly fabricated lenses could ultimately be used in endoscopes or optical microscopes, which could help detect diseases such as cancer.

The lenses could even be used by medical professionals in undeveloped areas for diagnostic imaging. On the other hand, they could also be used as customized contact lenses to counter the effects of keratoconus, a disease of the cornea.

“Up until now, we relied heavily on the time-consuming and costly process of polishing lenses,” Sun said. “With 3D printing, now you have the freedom to design and customize a lens quickly.”

The research, “High‐Speed 3D Printing of Millimeter‐Size Customized Aspheric Imaging Lenses with Sub 7 nm Surface Roughness,” has been published in Advanced Materials.



Posted in 3D Printing Technology



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