All futurists agree: the consumer electronics of the future will be flexible, wearable and packed with sensors and antennas to perform a wide range of (biomedical) functions. They sound great, but unfortunately today’s production techniques are suitable for little more than flat, bulky and rigid devices. But a new Harvard 3D printing innovation could change all that. One Harvard team led by Professor Jennifer A. Lewis has developed a potentially paradigm-shifting laser-DIW technique that 3D prints microscopic and conductive free-form metal structures in mid-air. Perfect for building complex electronics for the wearable and customizable electronics of tomorrow.
This fascinating breakthrough has been realized by a team of researchers from Harvard’s Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences (SEAS). Led by Professor of Biologically Inspired Engineering Jennifer Lewis, it also includes researchers Mark A. Skylar-Scott and Suman Gunasekaran. Their revolutionary 3D printing solution has been outlined in a paper entitled ‘Laser-assisted direct ink writing of planar and 3D metal architectures’, published in Proceedings of the National Academy of Sciences.
All images: Proceedings of the National Academy of Sciences.
As they explain in their paper, 3D printing is a perfect platform for creating the small scale metallic architectures that the consumer electronics industry needs. The only problem is that complex geometries are severely limited by support structures and printing speeds. They have therefore adapted existing direct ink writing (DIW) 3D printing techniques to create a laser-based solution that 3D prints at high speeds while simultaneously annealing the printed metal mid-air. This not only increases the accuracy of the prints, but also enables ‘on-the-fly’ free-form creation. To test this laser-DIW 3D printing technique, they have already built several free-form microscopic structures without auxiliary support material.
According to professor Lewis, this is a major breakthrough. “I am truly excited by this latest advance from our lab, which allows one to 3-D print and anneal flexible metal electrodes and complex architectures ‘on-the-fly,’” she said. Most importantly, it offers several key advantages over other metal 3D printing solutions. Firstly, patterning and annealing is done in a single step, giving the technique the ability to produce far more complex structures. What’s more, costs are decreased thanks to the high annealing speed, which enables users to rely on low-cost plastic substrates. Finally, the patterned structures are excellent conductors, rivaling the properties of bulk silver.
This last feature is enabled by a reliance on an ink composed of silver nanoparticles. The ink is annealed using a precisely programmed laser that drives solidification with exactly the right amount of energy. The nozzle can move along the x, y and z axes, and is combined with a rotary print stage that allows for free-form curvature. Printing is only limited by the fact that the curvilinear wire must always be patterned in a direction parallel to the laser–nozzle axis. But this hardly limits the technology. Hemispherical shapes, spiral motifs and a lot more complex geometries are easily realized – even decorative butterflies made from wires narrower than a hair’s width are possible. The large amounts of silver further embed excellent conductive properties into these microscopic creations, paving the way for a myriad of electronic devices.
This on-the-fly annealing process is enabled by an 808-nm IR laser, which is focused to a 100-µm spot adjacent to the glass nozzle. The concentrated silver nanoparticle ink is deposited mid-air, and rapidly heated and annealed by the laser to form mechanically robust, electrically conductive wires. These can have, as the researchers state in their paper, a diameter of anything from <1 µm to 20 µm, depending on the variable nozzle diameter, extrusion pressure and printing speed.
This makes the technique very tunable, something only further enhanced by the fact that the silver wires are also completely programmable in terms of electrical resistivity. And with 3D printing being possible on low-cost plastic substrates such as PET (perfect for electronic and photovoltaic applications), this technique could fundamentally change the way electronics are made, the researchers argue. “The ability to print high-resolution, functional metal electrodes and complex structures on demand may open up new avenues for creating customized electronics, MEMs, and biomedical devices,” they write.
According to postdoctoral researcher Mark Skylar-Scott, who is the study’s first author, optimizing the nozzle-to-laser separation distance was the most challenging part of their work. “If the laser gets too close to the nozzle during printing, heat is conducted upstream, which clogs the nozzle with solidified ink,” Skylar-Scott said. “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.’”
Wyss Institute Director Donald Ingber, who is also a professor of bioengineering at SEAS, believes this new technique could pave the way for a wide range of new applications. “This sophisticated use of laser technology to enhance 3-D 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,” he said. Could this be the breakthrough metal 3D printing technology we’ve all been waiting for? For more technical info on laser-DIW 3D printing, check out the full paper here.