A rapidly expanding area of research in materials science and engineering involves the fabrication of complex three-dimensional structures with feature sizes in the micro- and submicrometer range. Such 3D geometries enable a wide range of emerging applications in, e.g., microelectromechanical systems (MEMS), electronics and energy storage, and boosted the field of optical and mechanical metamaterials.

Advanced fabrication methods are a major driving force for progress in small-scale 3D materials. This drive is exemplarily illustrated by the major acceleration the field experienced after the commercialization of two-photon lithography approximately a decade ago. All of a sudden, almost arbitrarily complex 3D structures with submicron feature size became widely available, basically empowering for example the field of 3D mechanical metamaterials.

Yet, alternatives to 3D photopolymerization are needed. Although it is an ideal solution for many applications, the lithographical approach lacks direct access to inorganic materials. Hence a number of alternative methods for the direct printing of inorganic materials have been suggested based on a variety of physico-chemical principles (for a comprehensive overview, have a look at our latest review article on small-scale AM of metals) 

Fig. 1: Principle of multi-metal printing with EHD-RP. Solvated metal ions M^z+ are generated within the printing nozzle via electrocorrosion of a metal electrode M⁰ immersed in a liquid solvent. (2) Ion-loaded solvent droplets are ejected by electrohydrodynamic forces. (3) Upon landing, M^z+ ions are reduced to zero valence metal M⁰ through electron transfer from the substrate. Switching the oxidative voltage between different electrodes in a multichannel nozzle enables on-the-fly modulation of the printed chemistry.

About six years ago, we started to work on electrochemical AM methods for the 3D deposition of metals at small scales. We were motivated by the fact that electrochemical approaches undoubtedly offer the best materials performance compared to other small-scale printing techniques – in our view a critical criterion for establishing inorganic AM in state-of-the-art microfabrication.

Unfortunately, electrochemical techniques were known to lack deposition speed, and the deposition in a wet electrochemical environment is excruciatingly complicated compared to much simpler alternative techniques based on colloidal inks (direct ink writing and others) – drawbacks that we experienced in our daily work (for example during a nevertheless very fruitful collaboration with Prof. Tomaso Zambelli and Dr. Luca Hirt to develop microscale electrochemical printing with the FluidFM).

Consequently, almost three years ago, we concluded that only a radical rethinking of electrochemical deposition could open up electrochemical small-scale printing for real world applications. We had to consider a target audience that presumably is not attracted to deal with the cleanliness of an electrochemical cell or issues with stable reference potentials, and most importantly, painfully long printing times.

Thus, we came up with the idea to merge the best of two worlds: the good materials quality provided by the electrochemical principle, and the user friendliness and speed of ink-based methods that work in atmospheric environment. To do so, we teamed up with the group of Prof. Poulikakos, an expert on ink-based electrohydrodynamic nanoprinting (a technique that allows facile, contactless printing at high speeds with a resolution of 100 nm). The idea of our collaboration: replace the charged particles used in standard electrohydrodynamic printing with charged ions to enable electrochemical electrohydrodynamic printing.

However, our first approach was unsatisfying: inspired by previous work by Prof. Rogers, whose group printed salt solutions for the fabrication of positively or negatively charged pixels, we tried to deposit aqueous solutions of copper sulfate. The result was not very motivating: slow printing of sparsely distributed copper nanoparticles covered in huge blobs of salt due to the recombination of an- and cations. It was obvious that we had to get rid of the anions if we wanted to synthesize pure metals.

We found the decisive input in mass-spectrometry literature on electrospraying of metal cations generated by the corrosion of sacrificial metal anodes. Especially the work by the Cooks group on nanoparticle deposition from ions convinced us that material synthesis via sprayed ions is possible. Involving the group of Prof. Zenobi as a third collaborator and expert on mass spectrometry, we quickly found that the combination of electrohydrodynamic printing with the anodic corrosion of sacrificial metal anodes is a practical route for small-scale printing of metals.

The resulting technique – electrohydrodynamic redox printing (EHD-RP) – fulfilled the original goal of the project: it was an order of magnitude faster than other electrochemical techniques (see video), provided a minimal feature size of 100 nm, and synthesized dense metals with properties that can compete with thin-film materials synthesized by standard processes used in microfabrication.

However, during development, we realized that the most powerful and distinguishing feature of EHD-RP was neither its speed, its resolution nor its materials quality, but its multi-metal capabilities. Simply adding a second anode wire all of a sudden enabled switching of the ions to be printed at the mere flick of a switch (see Fig. 1 and video). For the first time, this elevates an electrochemical method into the ranks of multi-material 3D printing techniques. And in contrast to other multi-material methods, EHD-RP allows extremely rapid switching at up to 10Hz as well as mixing of metals to control alloy composition. This facilitates the additive control of printed chemical composition on a voxel-by-voxel basis with a chemical voxel size of approx. 250 nm – a level of detail previously unseen for any multi-material technique compatible with inorganic materials (Fig. 2).

Fig. 2: Side-view EDX-map of a 3D-printed Cu wall with the letters “Ag” imprinted in silver. The smallest chemical feature is <400 nm.

Control of the local chemistry directly leads to control of local materials properties via 3D printing. We used this ability to print sacrificial support materials (tuning the local reactivity of printed metals) as well as structures with locally modulated mechanical strength (Fig. 3).

Fig. 3: Cu-Ag / Ag pillars turn into nanoporous / dense pillars after chemical dissolution of Cu. This modulates the mechanical strength of the printed material and is a simple demonstrator of local properties tuning by multi-metal 3D printing.

After three years of work, we are excited to have this unique technique at hand that enables the design of materials with complex 3D gradients in chemistry and microstructure. And we are amazed by how unexpectedly the technique’s prospects changed from the simple acceleration of electrochemical printing to the additive control of chemistry and properties in 3D dimensions at the nanoscale – simply by adding a second wire.

The research was supported by grant No. ETH 47 14-2 and partially supported by grants No. SNF 200021-146180 and SNF 200020-178765.