"Flat" and "rigid" are terms typically used to describe electronic devices. But the increasing demand for flexible, wearable electronics, sensors, antennas and biomedical devices has led a team at Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS) and Wyss Institute for Biologically Inspired Engineering to innovate an eye-popping new way of printing complex metallic architectures – as though they are seemingly suspended in midair.
Reported online May 16 in the Proceedings of the National Academy of Sciences, this laser-assisted direct ink writing (laser-DIW) method allows microscopic metallic, free-standing 3D structures to be printed in one step without auxiliary support material. The research was led by Jennifer Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS and Wyss Core Faculty member.
As a demonstration of laser-DIW, the researches created arrays of conductive coils, resembling electrical inductors, on a silicon substrate. In each coil, the helix is oriented vertically, and the wire begins and ends on the substrate to facilitate component connectivity. When mechanically tested, the printed helical springs exhibit elastic and plastic behavior in both tension and compression. The force–displacement curve for an 800-µm-tall helix, with a wire diameter of 20 µm, exhibits a linear response up to a 50% macroscopic strain.
Such helical coils could not be printed solely by DIW, i.e., in the absence of laser annealing, because the as-printed ink is unable to retain the geometrical shape in midair. Next, 3D spiral arrays were created inspired by electrically small hemispherical spiral antenna designs. Using laser-DIW, high-fidelity spiral features were printed in midair, in the absence of an underlying hemispherical substrate. The construction of these shapes is highly repeatable leading to uniform spiral arrays.
As a final example, arbitrary 3D shapes were printed, such as a butterfly, formed by printing multiple woven curvilinear wires that depart from the underlying substrate. Interestingly, these curvilinear structures can be printed on flexible PET substrates, and by virtue of the strong adhesion at the silver wire–substrate interface resulting from HAZ formation, these structures can readily withstand cyclic bending of the substrate.
“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 Lewis. Lewis’ team used an ink composed of silver nanoparticles, sending it through a printing nozzle and then annealing it using a precisely programmed laser that applies just 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. In this way, tiny hemispherical shapes, spiral motifs, even a butterfly made of silver wires less than the width of a hair can be printed in free space within seconds. The printed wires exhibit excellent electrical conductivity, almost matching that of bulk silver.
When compared to conventional 3D printing techniques used to fabricate conductive metallic features, laser-assisted direct ink writing is not only superior in its ability to produce curvilinear, complex wire patterns in one step, but also in the sense that localized laser heating enables electrically conductive silver wires to be printed directly on low-cost plastic substrates.
According to the study’s first author, Wyss Institute Postdoctoral Fellow Mark Skylar-Scott, Ph.D., the most challenging aspect of honing the technique was optimizing the nozzle-to-laser separation distance.
“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.’ â€
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, opening up near limitless new potential applications in electronic and biomedical devices that rely on customized metallic architectures.
In addition to Lewis and Skylar-Scott, Suman Gunasekaran is a co-author on the study. Gunasekaran is undergraduate researcher studying chemistry and physics at SEAS. The work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy.
