Jan 4, 2018 | By Benedict

Researchers at Lawrence Livermore National Laboratory (LLNL) have found a way to improve two-photon lithography (TPL), a nanoscale 3D printing technique. The research could help doctors carry out X-ray CT scans to analyze 3D printed implants within the human body.

Unlike most laser 3D printing techniques, which are limited in resolution by the size of a 3D printer’s laser spot, two-photon lithography can take printing resolution to the extreme. That’s because TPL, which typically involves the use of a glass slide, a lens, and an immersion oil, uses a photoresist material that simultaneously absorbs two photons instead of one.

LLNL researchers recently found a way to drastically widen the capabilities of TPL—by making some important discoveries about the resist materials used in the 3D printing process. Their discovery was that turning the entire process upside down—applying the resist material directly to the lens and focusing the 3D printer’s laser through the resist—could result in the printing of 3D microstructures with features smaller than 150 nanometers but still multiple millimeters in height.

“In this paper, we have unlocked the secrets to making custom materials on two-photon lithography systems without losing resolution,” said LLNL researcher James Oakdale, a co-author on the paper.

But that’s just the start of the LLNL team’s exciting research project. In addition to dramatically improving the resolution of TPL by inverting part of the 3D printing system, the researchers also found they could improve the attenuation of the photopolymer resists used in the 3D printing process by more than 10 times, increasing (or decreasing) the number of X-rays the resists are able to absorb.

This was made possible by “index matching,” matching the refractive index of the resist material to the immersion medium of the lens, allowing the laser of the 3D printer to pass through with minimal disturbance. By deploying this index matching technique, the LLNL team say TPL could eventually be used to 3D print much larger parts, with features as small as 100 nanometers.

This widening of scope for TPL could have big practical implications, since scientists are constantly looking for ways to 3D print parts with incredibly fine resolutions, but which are still of a large enough size to be functional.

3D printed octet truss structures with submicron features

“Most researchers who want to use two-photon lithography for printing functional 3D structures want parts taller than 100 microns,” said Sourabh Saha, the paper’s lead author. “With these index-matched resists, you can print structures as tall as you want.”

The only limitation, Saha admits, is the speed of the TPL process. But now the researchers have a grasp of how they can modify and improve the process, they are confident they can “diagnose and improve” it.

There’s another important knock-on effect for the LLNL researchers’ new-found ability to tune a resist’s X-ray absorption. By making 3D printed objects that absorb more X-rays, researchers could theoretically make 3D printed bodily implants that can be more easily examined using an external X-ray CT scanner or other imaging equipment.

Since the tuned properties of the 3D printed implant would make it highly visible to CT scanners, doctors would not need to remove such an implant to see if it had, for example, internal defects.

3D printed woodpile lattices with submicron features

(Images: Jacob Long, Adam Connell, James Oakdale / LLNL)

There are uses for these tunable materials outside of medicine, too. The researchers say the optimized TPL process could be used to build (and then later examine) the internal structure of targets for the National Ignition Facility, LLNL’s large laser-based inertial confinement fusion (ICF) research device that is used to achieve fusion ignition.

Other uses for the process could include the 3D printing of optical and mechanical metamaterials and 3D printed electrochemical batteries.

The focus for now, however, is to speed up the printing process by parallelizing it, with the goal of eventually getting smaller features and higher functionality. The researchers believe the 3D printing process will someday be used to produce critical parts.

“It’s a very small piece of the puzzle that we solved, but we are much more confident in our abilities to start playing in this field now,” Saha said. “Our push for smaller and smaller features in larger and larger structures is bringing us closer to the forefront of scientific research that the rest of the world is doing. And on the application side, we’re developing new practical ways of printing things.”

The research, “Radiopaque Resists for Two-Photon Lithography To Enable Submicron 3D Imaging of Polymer Parts via X-ray Computed Tomography,” has been published in ACS Applied Materials & Interfaces.



Posted in 3D Printing Technology



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