A scanning electron microscope image of a nickel nano-architecture fabricated by the Greer lab (scale bar = 5 microns). Image credits: Greer lab.
In the world of structural engineering, smaller is stronger. As materials shrink, they often get stronger. But when we push metal components down to the nanoscale, the traditional rules of physics start to warp. In 3D printing, this is a nightmare, as even a tiny error can wreck an entire structure.
Now, a team at Caltech may have cracked the code. They’ve figured out how to engineer 3D metallic pieces with nanoscale precision. Their process works with almost any metal or alloy and produces components that are remarkably tough, even when they aren’t perfect.
The team, led by Wenxin Zhang, Zhi Li, Huajian Gao, and Julia R. Greer, used a process called hydrogel infusion additive manufacturing (HIAM). Think of it as a high-tech version of a kitchen sponge.
First, they use a specialized laser (two-photon lithography) to 3D print a scaffold made of a polymer gel. This gel is the ghost of the final structure.
Then, they soak this gel in a “metal juice” — an aqueous solution of nickel salts. “That’s where the magic happens,” says Greer, who is also the executive officer for applied physics and materials science at Caltech.
The metal ions seep into the gel, filling every nook and cranny. Finally, they bake the whole thing in a furnace. The gel burns away, the metal ions turn into solid nickel, and the structure shrinks down to its final, tiny form.
“Because of this thermal process, there’s a tremendous amount of shrinkage,” Greer says. Overall, the process can reduce the preheated volume by as much as 90%. This yields tiny lattices with overall dimensions smaller than 50 microns and building blocks measured in nanometers.
The result is complex 3D geometries: beams, shells, and even randomly organized structures that look like sea sponges, all on the nanoscale.
×
Thank you! One more thing…
Please check your inbox and confirm your subscription.
Overcoming Tiny Defects
In the world of big things like skyscrapers and bridges, engineers assume the material is smooth. They don’t need to worry about a single microscopic bubble in a steel beam because the beam is millions of times larger than the bubble. But at the nanoscale, every small imperfection matters.
Even the best nano-prints have flaws. The team used in situ nanocompression experiments to determine exactly where these faults are most likely to occur, essentially crushing the samples while observing them under a microscope. They found that even when flaws appeared, these nano-architected materials remained roughly 50 times stronger than their larger-scale counterparts.
“We put exactly the microstructure we uncovered into the models. It’s not an inference. It’s not representative. It’s the actual microstructure that we made,” Greer explains. As a result, for the first time, the models predict the correct, observed strengths of the fabricated parts.
“I think this work basically shows that in the future, even when we ‘nano-architect’ our world with custom parts, we’ll be able to reliably predict their properties, something society hasn’t been able to accomplish yet,” Greer says. “And we don’t have to disqualify a part simply because it contains defects.”
In addition to stronger micro-structures, this opens the door to a new era of “Nano-MEMS” (micro-electromechanical systems) and nanorobotics. Imagine medical nanobots that are strong enough to navigate through your bloodstream without crumbling, or flexible electronics that can be folded millions of times without the metal traces snapping.
But it’s not just the small things. This study is a roadmap for macro-scale engineering as well. By mastering the defects of the small, we are finally learning how to build the giants of the future.
The study “Nanoporosity-driven deformation of additively manufactured nano-architected metals” has been published in Nature Communications.