3D printers can print almost anything these days, from production prototypes to artificial organs all the way to entire houses. Particularly on a microscopic scale, additive manufacturing, as 3D printing is also known, makes it possible to produce structures which cannot be realized using other techniques such as lithography. Up to now, a weak point of additive manufacturing has been the realization of metallic structures at the micrometre scale. A team of researchers at ETH Zurich led by Ralph Spolenak, professor at the Laboratory for Nanometallurgy of the Department of Materials, together with Dimos Poulikakos of the Department of Mechanical and Process Engineering as well as Renato Zenobi of the Department of Chemistry and Applied Biosciences, have now developed a technique that even allows one to print two metals at the same time with a spatial resolution of 250 nanometres.

Alternatives to ink

Conventional methods for 3D-metal printing are ink based, which means that the desired metal is dissolved as nanoparticles in a suspension and delivered to a surface through a printing nozzle. An advantage of such inks is that they can be made with a variety of materials, but they also have important disadvantages. “Those techniques require a post-printing treatment that involves heating, which results in a shrinking and pronounced porosity of the material,” explains Alain Reiser, a PhD student in Spolenak’s group and first author of the study recently published in Nature Communications. “Typically, this means that the metallic structures are less conductive, mechanically unstable and, moreover, often contaminated with the organic compounds of the liquid solvent.”

Printing with ions

To solve this problem, the researchers at ETH chose the direct route: the metals is no longer deposited as a nanoparticle, but rather transported in the shape of electrically charged metal ions. Those ions are created by applying an electric voltage to a “sacrificial anode” consisting of the desired metal inside the printing nozzle. The ions are then sprayed by electric forces inside a solvent onto the printing surface, where they lose their electric charge and reassemble as a metal.