Liquid metals went to work

Unusual properties of gallium alloys opened a door to stretchable electronics and soldering without heat

Mitch Jacoby

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Dickey and coworkers create random patterns by dispensing a gallium-based liquid metal from a nozzle. The metal shapes are stable and free-standing thanks to a thin oxide shell that forms spontaneously in air.
Credit: Michael Dickey/NCSU

Liquid metals, which have long been treated as scientific curiosities, drew renewed attention this year with novel applications, for example in flexible, stretchable electronics for wearable and implantable devices.

This small group of materials mainly includes gallium and a few of its alloys. On exposure to air, the liquid spontaneously forms a thin oxide skin that mechanically stabilizes droplets and arbitrary patterns that researchers create. If the material is jostled, the skin breaks and the metal flows momentarily until the skin re-forms around the liquid.

A team led by Michael D. Dickey of North Carolina State University took advantage of this unusual property to make 10-μm-wide polymer-encased wires of eGaIn, a eutectic mixture of gallium and indium that’s a liquid at room temperature. Unlike ordinary wires, the ones made with eGaIn can easily be stretched, bent, and shaped while maintaining electrical conductivity (Extreme Mech. Lett. 2016, DOI: 10.1016/­j.eml.2016.03.010).

In another example from this year, Stéphanie P. Lacour and coworkers at the Swiss Federal Institute of Technology, Lausanne (EPFL), devised a method for making a two-phase material consisting of solid AuGa2 clusters interspersed with microscopic liquid gallium droplets. They used the material to fabricate stretchable devices containing stacked layers of LEDs and sensors embedded in polymeric skin patches that can track the subtle motions of fingers (Adv. Mater. 2016, DOI: 10.1002/adma.201506234).

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Developed by Purdue University’s Rebecca K. Kramer and coworkers, the stretchable eGaIn-based circuitry inkjet printed on this nitrile glove measures strain induced by finger motions.
Credit: Adv. Mater.

Martin Thuo’s group at Iowa State University exploited the spontaneously forming oxide skin of bismuth-indium-tin and related alloys to keep microscopic liquid metal droplets from solidifying, even at temperatures below their melting points. The researchers showed that applying a gentle force to the droplets breaks the shells, causing the metal to briefly flow before the skin re-forms. They used that property to bond metal parts together at room temperature, in effect soldering without electricity or heat (Sci. Rep. 2016, DOI: 10.1038/srep21864).

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Thuo’s group scrapes away part of a thin shell of a Bi-In-Sn alloy (orange, false color) with an ion beam to allow encapsulated liquid metal droplets (left) to flow and immediately solidify (right), like solder.
Credit: Sci. Rep.

CORRECTION: This story was updated on Dec. 15, 2016, to correct the description of the work by the EPFL group and the attribution of the work depicted in the photograph of the nitrile glove.

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