Microfabrication is typically limited to 2D, planar designs. Such limitations can be overcome by microscale additive manufacturing (AM) techniques which have been developed in the last decade. Amongst them are several methods for the mask-less 3D deposition of metals . Many of those techniques have proven their value for e.g. the deposition of microscale electronic conductors, optical metamaterials or mechanical components. Yet, there are considerable differences between the individual techniques – minimal feature size, speed or material quality vary notably.
On the one hand, electrochemical microscale AM methods enable the fabrication of materials with dense, nanocrystalline microstructures and excellent material properties. Unfortunately, they share a common disadvantage: average deposition speeds are two orders of magnitude lower than for many other techniques. On the other hand, ink-based techniques are much faster, but often struggle with synthesizing dense metals. Additionally, they often require post-deposition heat treatments that can cause warping and shrinkage of the printed geometries.
Here, we combine the best features of these techniques: the high quality as-deposited material of electrochemical techniques with the high speed of ink-based methods. We present a new approach that combines electrohydrodynamic (EHD) printing [2,3] with electrochemistry. The EHD ejection of solvent provides increased mass-transport, which is shown to result in deposition speeds an order of magnitude higher than previous electrochemical microscale AM methods. The electrochemical reduction provides metallic, conductive, and mechanically stable deposits without any additional processing.
We demonstrate that electrochemical EHD-printing can be used to deposit gold, copper and silver structures of various geometry (lines, pillars, and overhangs) with minimum feature sizes < 500 nm. The obtainable microstructure is polycrystalline with a density varying from ~50% to > 90%, depending on the ejection voltage.
The presented technique is purely electrochemical and requires no inks. Key benefits are the simple working principle of the technique and its potential applicability to a large range of metallic materials. Furthermore, we envision facile multimetal and alloy printing. This could provide valuable tools for engineering the local composition and microstructure and thus properties of additively manufactured parts at the microscale.
 A. Reiser, L. Hirt, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.
 P. Galliker, J. Schneider, H. Eghlidi, S. Kress et al., Nat. Commun. 2012, 3, 890.
 J.-U. Park, M. Hardy, S. J. Kang, K. Barton, K. Adair, et al., Nat. Mater. 2007, 6, 782.