Manufacturing Bits: Jan. 9

Two-photon lithography
Lawrence Livermore National Laboratory (LLNL) has extended the capabilities of a high-resolution 3D printing technique called two-photon lithography (TPL).

TPL enables the development of 3D-printed objects. LLNL’s technology could enable 3D-printed embedded structures inside the body, such as stents, joint replacements or bone scaffolds. It could also one day be used to develop optical components, mechanical metamaterials and 3D-printed electrochemical batteries.

For years, the industry has been developing both single-photon lithography and TPL. In single-photon lithography or micro-stereolithography, a specialized printer is used. In the system, an ultraviolet light (UV) hits a material. This, in turn, builds a structure layer by layer.

In TPL, though, a specialized printer uses femtosecond pulsed laser beams. The printer also makes use of liquid resin or other materials. When exposed to the laser light, the resins harden into a polymer. For this to occur, “the molecules require the absorption of two photons of light at once,” according to researchers from the Vienna University of Technology.

The problem with TPL? It’s resolution limited.

LLNL, however, has developed new resist materials, enabling feature sizes less than 150nm. For this, researchers have synthesized iodinated acrylate monomers, which, in turn, create high-Z photoresist materials capable of developing 3D microstructures with sub-150nm feature sizes.

In addition, researchers made another discovery– index matching. In TPL, laser light refracts as it passes through the photoresist. Researchers discovered how to match the refractive index of the resist material to the immersion medium of the lens. As a result, the laser could pass through the resist unimpeded.

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. (Photo by James Oakdale/LLNL)

“Most researchers who want to use two-photon lithography for printing functional 3D structures want parts taller than 100 microns,” said LLNL researcher Sourabh Saha on the organization’s Web site. “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.

“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.”

LLNL researchers printed octet truss structures with submicron features on top of a solid base with a diameter similar to human hair. (Photo by James Oakdale/LLNL.)

Multi-beam 3D printing
In a separate announcement, LLNL as well as the University of California at Berkeley, the University of Rochester and the Massachusetts Institute of Technology (MIT) have developed a faster way to build parts with a 3D printer.

The technology, called “volumetric” 3D printing, uses a multiple laser beam approach to speed up the process. With this process, researchers have printed beams, planes, struts at arbitrary angles, lattices and other objects.

The technology makes use of a 3D printer. Then, within the system, there are various laser beams. “Volumetric 3D printing creates parts by overlapping three laser beams that define an object’s geometry from three different directions, creating a hologram-like 3D image suspended in the vat of resin,” according to LLNL.

Following those events, the laser light then hits the object for about 10 seconds. The part is cured and the excess resin is drained out of the vat, resulting in a 3D-printed part.

The LLNL logo in 3D printed technology. (Source: LLNL)

“It’s a demonstration of what the next generation of additive manufacturing may be,” said LLNL engineer Chris Spadaccini. “Most 3D printing and additive manufacturing technologies consist of either a one-dimensional or two-dimensional unit operation. This moves fabrication to a fully 3D operation, which has not been done before. The potential impact on throughput could be enormous and if you can do it well, you can still have a lot of complexity.”