Metal 3D printing in midair

3D printing is becoming more commonplace, with new methods being introduced everyday to expand capabilities. Traditional 3D printing methods have some limitations when it comes to intricate structures. With the increasing demand for flexible devices and wearable electronics, a team at Harvard’s Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed a  new method to print complex metallic architectures.

This new method makes it appears as if structures are suspended in midair. Reported in the Proceedings of the National Academy of Sciences, this laser-assisted direct ink writing method allows microscopic metallic, free-standing 3D structures to be printed in one step without any support material required.

“I am truly excited by this latest advance from our lab, which allows one to 3D print and anneal flexible metal electrodes and complex architectures ‘on-the-fly,’ ” said Wyss Core Faculty member Jennifer Lewis, Sc.D., who is also the Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS.

The team developed the technique using an ink composed of silver nonparticles. The ink is sent through the printing nozzle and then annealed using a laser that applies the right amount of energy to drive the ink’s solidification. The printing nozzle moves  along x, y, and z axes and is combined with a rotary print stage to enable freeform curvature.

Laser-assisted direct ink writing allowed this delicate 3D butterfly to be printed without any auxiliary support structure. Credit: Lewis Lab / Wyss Institute at Harvard University

This method is able to produce uniquely shaped structures like tiny hemispherical shapes, spiral motifs, even a butterfly made of silver wires less than the width of a hair.

This technique is able to produce curvilinear, complex wire patterns in one step. The localized laser heating enables electrically conductive silver wires to be printed directly on low-cost plastic substrates.

One of the challenges the team faced was dealing with honing the technique to optimize the nozzle-to-laser separation distance, explains Wyss Institute Postdoctoral Fellow Mark Skylar-Scott, Ph.D.

“If the laser gets too close to the nozzle during printing, heat is conducted upstream which clogs the nozzle with solidified ink,” said Skylar-Scott. “To address this, we devised a heat transfer model to account for temperature distribution along a given silver wire pattern, allowing us to modulate the printing speed and distance between the nozzle and laser to elegantly control the laser annealing process ‘on the fly.'”

The result is that the method can produce not only sweeping curves and spirals but also sharp angular turns and directional changes written into thin air with silver inks. This will help develop new applications  in electronic and biomedical devices that rely on customized metallic architectures.

“This sophisticated use of laser technology to enhance 3D printing capabilities not only inspires new kinds of products, it moves the frontier of solid free-form fabrication into an exciting new realm, demonstrating once again that previously-accepted design limitations can be overcome by innovation,” said Wyss Institute Founding Director Donald Ingber. M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, as well as Professor of Bioengineering at SEAS.

www.wyss.harvard.edu