Inkjet-printed liquid metal could lead to new wearable tech, soft robotics

April 8, 2015

Inkjet-functionalized nitrile glove with arrays of electronic strain gauges, intricate wiring, and contact pads (credit: John William Boley et al./Advanced Materials)

Purdue University researchers have developed a potential manufacturing method called “mechanically sintered gallium-indium nanoparticles” that can inkjet-print flexible, stretchable conductors onto anything — including elastic materials and fabrics — and can mass-produce electronic circuits made of liquid-metal alloys for “soft robots” and flexible electronics.

The method uses ultrasound to break up liquid metal into nanoparticles in ethanol solvent to make ink that is compatible with inkjet printing.

Elastic technologies could make possible a new class of pliable robots and stretchable garments that people might wear to interact with computers or for therapeutic purposes.

“Liquid metal in its native form is not inkjet-able,” said Rebecca Kramer, an assistant professor of mechanical engineering at Purdue. “So what we do is create gallium-indium liquid metal nanoparticles that are small enough to pass through an inkjet nozzle.

“Sonicating [using ultrasound] 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.”

After printing, the nanoparticles must be rejoined by applying light pressure, which renders the material conductive. This step is necessary because the liquid-metal nanoparticles are initially coated with oxidized gallium, which acts as a skin that prevents electrical conductivity.

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

Custom-designed electronics

The approach makes it possible to select which portions to activate depending on particular designs, suggesting that a blank film might be manufactured for a multitude of potential applications.

“We selectively activate what electronics we want to turn on by applying pressure to just those areas,” said Kramer. The process could make it possible to rapidly mass-produce large quantities of the film.

Future research will explore how the interaction between the ink and the surface being printed could lead production of specific types of devices.

“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?” she said.

The researchers also will study and model how individual particles rupture when pressure is applied, providing information that could allow the manufacture of ultrathin traces and new types of sensors.

A research paper about the method will appear on April 18 in the journal Advanced Materials.

Abstract of Mechanically Sintered Gallium–Indium Nanoparticles

Liquid metal nanoparticles that are mechanically sintered at and below room temperature are introduced. This material can be sintered globally on large areas of entire deposits or locally to create liquid traces within deposits. The metallic nanoparticles are fabricated by dispersing a liquid metal in a carrier solvent via sonication. The resulting dispersion is compatible with inkjet printing, a process not applicable to the bulk liquid metal in air.