Multi-scale processing: large-area film coating and high-resolution patterning.
Multi-scale processing: large-area film coating and high-resolution patterning.

Soft electronic devices are essential for a new generation of soft robotics and wearable or implantable healthcare devices. Now researchers have fabricated stretchable displays, multilayer pressure sensors, and soft artificial fingers that use electrodes and interconnects made from a liquid metal [Lee et al., Materials Today (2023), ].

Stretchable electrodes and interconnects are currently fabricated either from serpentine patterns of conventional metals or flexible conductive materials such as carbon or metal nanowire composites and conductive polymers. Since both approaches have drawbacks, attention has recently turned to liquid metals like eutectic gallium indium (EGaIn), which is highly conductive, deformable, and stretchable. Although EGaIn is challenging to use in its liquid state, in particle form it has the necessary mechanical and chemical stability. Until now, however, EGaIn particles have required annealing at high temperatures or embedding in an elastomer matrix, as well as an activation step to induce conductivity.

“We have developed a solution process that enables precise patterning of highly stretchable, stable, and initially conductive liquid metal using the conventional ‘gold standard’ electronic fabrication processes,” says first author of the study, Gun-Hee Lee of Korea Advanced Institute of Science and Technology (KAIST), which was performed with colleagues at Seoul National University, Korea University, and Stanford University.

The approach involves covering EGaIn microparticles with a polymer, polystyrene sulfonate (PSS), to create a stable, conductive film. By employing the solution shearing method, conductive films can be coated onto large, wafer-sized substrates and patterned with conventional semiconductor processes such as photolithography and multi-layer structure integration using reactive ion etching. A water-based suspension ensures that the photoresist is not damaged during the coating process, enabling the precise patterning of features as small as 10 microns. Using the organic solvent DMSO makes lift off so easy that EGaIn particle-based interconnects and electrodes can be patterned directly onto the substrate.

“We discovered that solution-shearing of the polymer-attached liquid metal particles effectively preserves their morphology after film deposition,” explains Lee. “The film can be easily patterned using a lift-off process employing a polar organic, [which also] induces cohesion between the liquid metal particles.”

It is this cohesion between EGaIn microparticles that ensures high initial conductivity without any activation process.

“Our work represents the first successful demonstration of an ‘unstable’ liquid metal in conventional fabrication processes by imposing stability onto it, which allows the realization of ‘elastic’ circuit boards,” points out Lee.