Through the two-photon lithography (TPL) 3D printing process, researchers can print woodpile lattices with submicron features a fraction of the width of a human hair. [Image: James Oakdale/LLNL]

Lawrence Livermore National Laboratory (LLNL) is known for its in-depth materials research, which often involves advanced 3D printing techniques. Those techniques include 3D printing on the nanoscale, and the laboratory recently made a discovery that can increase the capability of a type of nanoscale 3D printing. Two-photon lithography is a high resolution 3D printing technique capable of producing nanoscale features smaller than one-hundredth the width of a human hair, and LLNL’s discovery extends its potential, as well as opening up the potential for X-ray computed tomography (CT) to noninvasively analyze stress or defects in embedded 3D printed medical devices or implants.

Two-photon lithography differs from other 3D printing methods because it’s able to produce features smaller than the point of the laser. It’s able to bypass the usual diffraction limit because the photoresist material that cures and hardens to create structures can simultaneously absorb two photons instead of one. Normally, the technique requires a thin glass slide, a lens and an immersion oil that helps the laser light focus to its fine point.

The LLNL team detailed its research in a paper entitled “Radiopaque Resists for Two-Photon Lithography To Enable Submicron 3D Imaging of Polymer Parts via X-ray Computed Tomography,” which you can access here. In the paper, the researchers describe how they cracked the code on resist materials optimized for two-photon lithography and forming 3D microstructures with features less than 150 nanometers. Past techniques would build structures from the ground up, limiting the size of the object because the distance between the glass slide and the lens was typically 200 microns or less. But by putting the resist material directly on the lens and focusing the laser through the resist, the researchers could 3D print objects multiple millimeters in height.

LLNL researchers printed octet truss structures with submicron features on top of a solid base with a diameter similar to human hair. [Image: James Oakdale/LLNL]

The researchers could also tune and increase the amount of X-rays the photopolymer resists could absorb, improving attenuation by more than 10 times over commonly used photoresists.

“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.

The laser light refracts as it passes through the photoresist material, so the key, according to the researchers, was “index matching,” or figuring out how to match the refractive index of the resist material to the immersion medium of the lens so the laser could pass through unimpeded. Index matching makes it possible to 3D print larger parts with features as small as 100 nm.

“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 is the speed. It’s a tradeoff, but now that we know how to do this, we can diagnose and improve the process.”

LLNL researchers can print woodpile lattices with submicron features a fraction of the width of a human hair. [Image: Jacob Long and Adam Connell/LLNL]

By tuning the material’s X-ray absorption, the researchers can use X-ray computed tomography as a diagnostic tool to examine the inside of parts without cutting them open, or to image 3D printed parts inside the body, like joint replacements, stents or bone scaffolds. They could also use the new techniques to create and probe the internal structure of targets for the National Ignition Facility as well as optical and mechanical metamaterials and 3D printed electrochemical batteries.

The next goal is to parallelize and speed up the process. The researchers also want to create even smaller features and add more functionality, eventually using the technique to 3D print mission-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. “We’re on a path where we know we have a potential solution for different types of applications. 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.”

Authors of the paper include Sourabh K. Saha, James S. Oakdale, Jefferson A. Cuadra, Chuck Divin, Jianchao Ye, Jean-Baptiste Forien, Leonardus B. Bayu Aji, Juergen Biener, and William L. Smith.

Discuss this and other 3D printing topics at or share your thoughts below. 

[Source: LLNL]


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