Two years ago, researchers from Northwestern University in Illinois called on 3D printing technology to create a novel lens capable of working with frequencies on the terahertz scale. Now, a team from the university’s McCormick School of Engineering has developed a method that speeds up the 3D printing of a different kind of lens – one that could impact vision correction, optical imaging, and even disease diagnosis.
The researchers have used their new method to 3D print a low-cost, high-quality lens, measuring at 5 mm in height and 5 mm in diameter. The customized optical component, which only takes four hours to print, can make vision-correcting contact lenses and optical imaging lenses, and the team believes that one day, it could even be used to turn a smartphone into a microscope.
Cheng Sun, an Associate Professor of Mechanical Engineering at the university, said, “Up until now, we relied heavily on the time-consuming and costly process of polishing lenses. With 3D printing, now you have the freedom to design and customize a lens quickly.”
The 3D printing process, which was developed in Sun’s lab, differs from the high-quality 3D printed micro-optic lenses created by Nanoscribe last year. The German company developed its own 3D printer, a high-precision femto-second system with 150 nm precision. But while it builds the lens in a point-by-point fashion, Northwestern uses a layering technique.
“It is a time-consuming process. That is their limitation. We wanted to make something comparable but faster and with better quality,” explained Xiangfan Chen, a PhD candidate in mechanical engineering.
Sun described the process of fabricating the lens with 3D printing technology as being akin to running a film projector.
When the lens was first 3D printed, a visible stepping was created by its curved layers, which were made using a photocurable resin.
“Instead of projecting one frame, one image after another, we layer one frame on top of another. It is like playing a movie in a vertical fashion,” Sun said.
“We realized that the layers on top of each other created surface roughness,” Sun explained. “The layer thickness is typically 5 microns, while the wavelength of visible light is around 0.5 micron. This creates an optically rough surface. That was the bottleneck. The roughness made the lens incapable of clear optics.”
The team wondered if the surface of the lens could be made more smoothly without having to decrease the printing speed, and set about to solve the issue.
“If you want to make a lens, do you want to make it in two hours or two weeks? We are very excited about this lens,” Sun said.
They recently published a paper on their findings, titled “High-Speed 3D Printing of Millimeter-Size Customized Aspheric Imaging Lenses with Sub 7 nm Surface Roughness,” in the Advanced Materials journal; co-authors include Chen, Wenzhong Liu with Opticent Health, Biqin Dong, Jongwoo Lee, Henry Oliver T. Ware, Hao F. Zhang, and Sun.
Their work was supported by a donation from the Farley Foundation and two National Science Foundation grants.
The abstract reads, “Advancements in three‐dimensional (3D) printing technology have the potential to transform the manufacture of customized optical elements, which today relies heavily on time‐consuming and costly polishing and grinding processes. However the inherent speed‐accuracy trade‐off seriously constrains the practical applications of 3D‐printing technology in the optical realm. In addressing this issue, here, a new method featuring a significantly faster fabrication speed, at 24.54 mm3 h−1, without compromising the fabrication accuracy required to 3D‐print customized optical components is reported. A high‐speed 3D‐printing process with subvoxel‐scale precision (sub 5 µm) and deep subwavelength (sub 7 nm) surface roughness by employing the projection micro‐stereolithography process and the synergistic effects from grayscale photopolymerization and the meniscus equilibrium post‐curing methods is demonstrated. Fabricating a customized aspheric lens 5 mm in height and 3 mm in diameter is accomplished in four hours. The 3D‐printed singlet aspheric lens demonstrates a maximal imaging resolution of 373.2 lp mm−1 with low field distortion less than 0.13% across a 2 mm field of view. This lens is attached onto a cell phone camera and the colorful fine details of a sunset moth’s wing and the spot on a weevil’s elytra are captured.”
The team created a two-step process of layering and polishing to solve the challenge of fast, smooth 3D printing.
Sun explained, “First, we used grayscale images to create more transitions between steps. Then, we coated the surface with the same photo-curable resin. That then forms the meniscus that further smooths the surface.”
This resulted in a transparent lens with a smooth surface, still 3D printed at a high rate of speed. Included in the research are images, like a sunset moth’s wing or a spot on a weevil’s forewing, taken using the lens connected to an iPhone 6S.
Chen said, “I must have tried more than 100 times to get this just right.”
According to Sun, their new method could result in multiple devices with a range of applications in biomedical engineering and optics, such as making a custom contact lens for people with distorted corneas suffering from keratoconus.
“The contact lens would feature the customized surface, matching it to the shape of the patient’s cornea,” Sun said.
The team will now be attempting to 3D print larger lenses, and investigating how to integrate them with medical devices like microscopes. Then, they could be utilized in underdeveloped areas for diagnostic imaging.
Dong, a post-doctoral fellow in biomedical and mechanical engineering, said, “These lenses could help detect some genetic disease or cancer.”
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