Nanoparticle liquid-metal ink allows standard inkjet printers to create flexible circuits

Using a standard inkjet-printer, researchers claim to be able to produce flexible electronic circuits that could advance bendable electronics for clothing, soft robotics, and wearable devices (Image: Alex Bottiglio/Purdue University)

Researchers at Purdue University have shown how standard inkjet-printers can be employed to produce flexible electronic circuits from liquid-metal nanoparticle inks. This simple printing solution promises faster, cheaper, and easier production of stretchable, bendable electronics for clothing, soft robotics, and wearable devices.

Making circuits elastic means that many rigid platforms could be made pliable, opening up a range of potential new applications for electronic devices. To make this possible, however, new manufacturing techniques are required. Nanoparticle ink-jet printed ductile circuits may well solve this problem.

"We want to create stretchable electronics that might be compatible with soft machines, such as robots that need to squeeze through small spaces, or wearable technologies that aren't restrictive of motion," says Rebecca Kramer, assistant professor of mechanical engineering at Purdue University. "Conductors made from liquid metal can stretch and deform without breaking. This process now allows us to print flexible and stretchable conductors onto anything, including elastic materials and fabrics."

To create the flexible metal ink, liquid metal is distributed evenly through a non-metallic solvent by using ultrasonic energy to fracture the liquid metal into nanoparticles. This nanoparticle suspension is then usable in an ink-jet printer, as if it was ordinary ink.

"Liquid metal in its native form is not inkjet-able," adds Kramer. "So what we do is create liquid metal nanoparticles that are small enough to pass through an inkjet nozzle. Sonicating liquid metal in a carrier solvent, such as ethanol, both creates the nanoparticles and disperses them in the solvent. Then we can print the ink onto any substrate. The ethanol evaporates away so we are just left with liquid metal nanoparticles on a surface."

The oxidized gallium (Ga2O3) "skin" originally applied to the liquid-metal nanoparticles to keep them electrically inert during processing needs to be removed once printing has finished, allowing the nanoparticles to recombine and the material to become conductive.

"But it's a fragile skin, so when you apply pressure it breaks the skin and everything coalesces into one uniform film," said professor Kramer. "We can do this either by stamping or by dragging something across the surface, such as the sharp edge of a silicon tip."

Being able to activate various sections of a device individually makes the technique extremely versatile. It may one day be conceivable to produce a generic blank nanoparticle film that could be easily customized to suit a large range of possible applications.

"We selectively activate what electronics we want to turn on by applying pressure to just those areas," says professor Kramer.

One major advantage of this circuit activation process, unlike similar graphene printing techniques, is that the resultant printed material does not need to be heated to exceptionally high temperatures to activate it. This makes the Purdue technique more readily accessible to ordinary circuit designers.

This isn't the first application of nanoparticle conductive liquid in an ink-jet printer – back in 2013 Gizmag reported on a system developed at Georgia Tech that prints conductive circuits on PET film and photographic paper. However the Purdue University technique is the first claimed to be able to print on stretchable materials and retain its integrity when stretched or flexed.

Kramer says that the team plans to investigate various properties of the new ink and how it interacts with the surface being printed on.

"For example, how do the nanoparticles orient themselves on hydrophobic versus hydrophilic surfaces? How can we formulate the ink and exploit its interaction with a surface to enable self-assembly of the particles?”

The results of the research will be published in the journal Advanced Materials on April 18.

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