Inkjet prints liquid metal for wearables

The lines between men and machines are getting increasingly blurry. Wearable technology and soft robotics are just two areas exemplifying this trend. However, in many cases manufacturing techniques are not yet advanced enough to fully realize the potential of a new interconnected world. Researchers at Purdue University are attempting to change that. They are harnessing inkjet printing technology to create devices made of liquid alloys printed onto pliable, stretchable surfaces. This breakthrough makes liquid metal compatible with large-scale manufacturing.

Conductors made from liquid metal can stretch and deform without breaking, which make them ideal for technology that is seamlessly integrated into the human environment. The new process will enable manufacturers to print semiconductors onto anything, including stretchable film and fabrics. “Inkjet printing enables ultra-low-cost, large area electronics. Liquid metal is known to be good conductor and highly deformable, and therefore well suited to flexible and stretchable electronics,” says Rebecca Krone Kramer, Assistant Professor at the School of Mechanical Engineering at Purdue University. “Enabling compatibility between liquid metal and inkjet printing makes this material more accessible from a manufacturing standpoint.”

Liquid metal in its usual form can’t be used as ink since the metal particles would clog the printer’s nozzle. Thus, the metal first needs to be broken down into nanoparticles. The research team at Purdue achieved this by dispersing the liquid metal in non-metallic solvent using ultrasound to fragment the metals. The nanoparticle-filled ink can be used in inkjet printers and printed onto any substrate. The solvent, ethanol for example, evaporates after printing, leaving metal nanoparticles on the surface.

A research paper about the method will appear on April 18 in the journal Advanced Materials (“Mechanically Sintered Gallium–Indium Nanoparticles”). The oxidized gallium “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. This can be done by applying pressure to the surface so that the skin breaks and the nanoparticles coalesce and become conductive.  One of the advantages of this method over graphene printing is that the printed material does not have to be heated at high temperatures to activate it and maintains its integrity when twisted and stretched.

Being able to activate various sections on a printed surface individually makes the technique extremely versatile. In the future, nano-coated films could be mass-produced and then turned into a variety of devices by activating different sections.  Possible real-life applications include search-and-rescue robots that are impact resistant and can deform to squeeze through cracks and crevices to aid in natural disasters or wearable technologies such as fabrics and skins that can give proprioceptive feedback to the wearer or assist with motions and prolong endurance without restricting the natural mechanics of motion. The technology could also be used in surgical tools that match the pliability of human tissue and pose lower-risk during invasive procedures.