3D printing metal alloys for flexible electronics

Oregon State University

RESEARCHERS at Oregon State University have developed a method to 3D-print metal alloys which could be used to make flexible electronics.

The development of flexible electronics such as bendable displays, wearable sensor suits, and biomedical sensors, is dependent on having circuits that still function even when they’re stretched. Gallium alloys are typically injected into microchannels to act as conducting wires, but there are limitations to using liquid metal.

Gallium alloys will oxidise quickly on the surface upon exposure to air, and the gallium oxide layer has a high surface tension that provides the only structural support. Such structures can break apart easily or else coalesce with only a small perturbation of the oxide skin, which limits the liquid metal structures to 2D.

By mixing nickel nanoparticles into the liquid metal using the energy of sound – known as sonification – researchers at Oregon State University changed the gallium alloy into a paste which can be printed layer by layer to make a 3D structure. The sonification causes the gallium oxide to become mixed throughout the material, which helps it retain its structure, and the dispersed oxides keep the nickel in suspension.  

“The runny alloy was impossible to layer into tall structures,” said Yiğit Mengüç, assistant professor of mechanical engineering. “With the paste-like texture, it can be layered while maintaining its capacity to flow, and to stretch inside of rubber tubes. We demonstrated the potential of our discovery by 3D printing a very stretchy two-layered circuit whose layers weave in and out of each other without touching.”

The paste retains the high electrical conductivity and stretchability of pure liquid metal while still holding its shape after printing. It also has the same “self-healing” properties so that broken circuits can easily be reattached. The researchers printed several structures up to 1 cm high, and also created a stacked circuit enclosed in Ecoflex, which is a soft silicon rubber gel. The circuit continued to light an LED bulb even when the Ecoflex was stretched.

“Liquid metal printing is integral to the flexible electronics field,” said study co-author Doğan Yirmibeşoğlu, a robotics PhD student. “Additive manufacturing enables fast fabrication of intricate designs and circuitry.”

Future work will explore the exact structure of the paste in order to understand how the nickel particles are stabilised in the liquid metal and how the structure changes as the paste ages.

Advanced Materials Technologies http://doi.org/cmbk