Fast, 3D-printed millimeter-sized lenses

Northwestern Engineering researchers have developed a 3D printing method to produce customized optical components, 5mm x 5mm diameter, in about 4 hours.

“Until now, we relied heavily on the time-consuming and costly process of polishing lenses,” says Cheng Sun, associate professor of mechanical engineering and whose lab developed the 3D printing process during two years of research. “With 3D printing, now you have the freedom to design and customize a lens quickly.”

Lens creation involves placing layer upon layer of material, which Sun likens to running a film projector.

“Instead of projecting one frame, one image after another, we layer one frame on top of another,” Sun says. “It is like playing a movie in a vertical fashion.”

But when researchers printed the lens, curved layers made of a photo-curable resin created visible stepping.

“The layers on top of each other created surface roughness. The layer thickness is typically 5µm, while the wavelength of visible light is around 0.5µm. This creates an optically rough surface,” Sun says. “That was the bottleneck. The roughness made the lens incapable of clear optics.”

This led Sun’s group to explore making the surface smooth without slowing printing speed. The result: a two-step process of layering and polishing.

The group used grayscale images to create more transitions between steps, then coated the surface with the same photo-curable resin to form a meniscus that smooths the surface.

“I must have tried more than 100 times to get this just right,” says Xiangfan Chen, a Ph.D. candidate in mechanical engineering and lead author on the study.

This is not the first 3D printed high-quality lens. Nanoscribe, a German-based company, has developed a high-precision femto-second 3D printer with 150µm precision, but that process builds a lens point-by-point (see TMD Sept. 2016: https://tinyurl.com/yc8b5q8t.)

“It is a time-consuming process. That is their limitation,” Chen says. “We wanted to make something comparable but faster and with better quality.”

Next, the group will experiment making larger lenses as well as investigating how to integrate the 3D-printed lens with medical devices, such as an endoscope or optical microscope.

Biqin Dong, a post-doctoral fellow focused on biomedical and mechanical engineering, who also worked on the research, envisions these lenses used by doctors in underdeveloped regions for diagnostic imaging or by field scientists as portable microscopes.

The lens also could be fashioned into a customized contact lens for people with distorted corneas caused by keratoconus.

“The contact lens would feature the customized surface, matching it to the shape of the patient’s cornea,” Sun says.

Northwestern University

This work was supported by the National Science Foundation under grant EEC-1520734 and DBI-1353952, along with a donation from the Farley Foundation.