In Situ Actuators with Gallium Liquid Metal Alloys and Polypyrrole-Coated Electrodes

Gallium liquid metal alloys (GLMAs) such as Galinstan and gallium-indium eutectic (EGaIn) are interesting materials due to their high surface tensions, low viscosities, and electrical conductivities comparable to classical solid metals. They have been used for applications in microelectromechanical systems (MEMS) and, more recently, liquid metal microfluidics (LMMF) for setting up devices like actuators. However, their high tendency to alloy with the most common metals used for electrodes such as gold (Au), platinum (Pt), titanium (Ti), nickel (Ni), and tungsten-titanium (WTi) is a major problem limiting the scaleup and applicability, e.g., liquid metal actuators. Stable electrodes are key elements for many applications and thus, the lack of an electrode material compatible with GLMAs is detrimental for many potential application scenarios. In this work, we study the effect of actuating Galinstan on various solid metal electrodes and present an electrode protection methodology that, first, prevents alloying and, second, prevents electrode corrosion. We demonstrate reproducible actuation of GLMA segments in LMMF, showcasing the stability of the proposed protective coating. We investigated a range of electrode materials including Au, Pt, Ti, Ni, and WTi, all in aqueous environments, and present the resulting corrosion/alloying effects by studying the interface morphology. Our proposed protective coating is based on a simple method to electrodeposit electrically conductive polypyrrole (PPy) on the electrodes to provide a conductive alloying-barrier layer for applications involving direct contact between GLMAs and electrodes. We demonstrate the versatility of this approach by direct three-dimensional (3D) printing of a 500 μm microfluidic chip on a set of electrodes onto which PPy is electrodeposited in situ for actuation of Galinstan plugs. The developed protection protocol will provide a generic, widely applicable strategy to protect a wide range of electrodes from alloying and corrosion and thus form a key element in future applications of GLMAs.