Oxygen-Controlled Printing and Interfacing of Liquid Metals for Reliable Robotic Electronic Skins

Gallium-based liquid metals (LMs) such as EGaIn and Galinstan combine high electrical conductivity with fluidic deformability, making them promising conductors for stretchable electronics, electronic skins, and soft robotic systems. Yet their processing and integration are governed by a nanoscale oxide skin that forms spontaneously in the presence of oxygen. This oxide plays a paradoxical role: it enables stable printing and adhesion to substrates, but can also inhibit electrical contact formation, increase contact resistance, and influence mechanical fatigue. In this PhD project, you will investigate oxygen and oxide dynamics as controllable manufacturing parameters across the complete process chain: from LM printing and substrate adhesion to electrical contacting and long-term reliability. The resulting insights will be validated in printed electronic skins for humanoid and soft robotic applications. 

Gallium-based LMs rapidly form a thin gallium-rich oxide skin upon exposure to oxygen. Although only nanometres thick, this layer fundamentally changes their interfacial and mechanical behaviour. During direct ink writing and stencil spraying, the oxide stabilizes the LM, promotes adhesion, and enables continuous pattern formation. Preliminary observations within our research group indicate that structures printed under low-oxygen glovebox conditions show reduced adhesion and earlier failure during stretching compared with structures printed in air. However, excessive oxidation may impair flow, increase resistance, create mechanically fragile regions, and reduce fatigue lifetime. A systematic relationship between oxygen level during printing, oxide properties, adhesion, electrical resistance, and cyclic reliability is still missing.

Oxide is equally critical when printed LM conductors are connected to solid metallic electrodes, sensors, or electronic components. It can inhibit metallic contact, wetting, and interfacial diffusion, while controlled oxide disruption may expose fresh LM and enable low-resistance contacts, reactive wetting, or intermetallic formation. Previous approaches have explored LM soldering at elevated temperatures, but these conditions are often incompatible with polymer substrates and printed devices. The scientific challenge is therefore to determine whether oxidation should be promoted for printing, suppressed or locally disrupted for contacting, and controlled during operation to maximize reliability. 

Fundamental questions remain unresolved: How much oxygen is required for reliable printing and adhesion, and when does oxidation become detrimental? How does oxide history influence contact resistance and LM–metal bonding? Which failure mechanisms dominate during cyclic deformation: delamination, oxide fracture, cohesive LM failure, contact degradation, or encapsulation failure? Can spatially and temporally controlled oxidation provide different optimal interface states for printing, electrical contacting, and device operation?