New approach could create very flexible electronic circuitry.

New research being done at Purdue University demonstrates that inkjet-printing technology can be used to mass-produce electronic circuits made of liquid-metal alloys that may be used for “soft robots” and flexible electronics. These elastic technologies could be used to create a new class of pliable robots and stretchable garments that people might wear to interact with computers or for therapeutic purposes.

Fig. 1 – This artistic rendering depicts elastic technologies that could make possible a new class of pliable robots and stretchable garments that people might wear to interact with computers or for therapeutic purposes. (Credit: Alex Bottiglio. Purdue University)
However, warns Rebecca Kramer, an assistant professor of mechanical engineering, new manufacturing techniques must be developed before soft machines become commercially feasible.

“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,” Kramer says. “Conductors made from liquid metal can stretch and deform without breaking.”

A new potential manufacturing approach focuses on harnessing inkjet printing to create devices made of liquid alloys. “This process now allows us to print flexible and stretchable conductors onto anything, including elastic materials and fabrics,” Kramer explains. (See Figure 1)

Kramer’s team of researchers used a method called mechanically sintered gallium-indium nanoparticles. A printable ink is made by dispersing the liquid metal in a non-metallic solvent using ultrasound, which breaks up the bulk liquid metal into nanoparticles. This nanoparticle-filled ink is then compatible with inkjet printing.

“Liquid metal in its native form is not inkjet-able,” Kramer says. “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.”

Then, after printing, the nanoparticles must be rejoined by applying light pressure, rendering the material conductive, because the liquid-metal nanoparticles are initially coated with oxidized gallium, which acts as a skin that prevents electrical conductivity. Applying pressure breaks up the skin and allows everything to coalesce into one uniform film. This can be accomplished, she says either by stamping or by dragging something across the surface. The approach makes it possible to select which portions to activate depending on particular designs.

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