Inkjet Technology Could Herald Wearable Biometrics

Inkjet-printing technology can be used to mass-produce electronic circuits made of liquid-metal alloys for flexible electronics, according to a new study.

Researchers at Purdue University (Lafayette, IN, USA) have created a new class of metallic nanoparticles (MNPs) that can be mechanically sintered at and below room temperature. To do so, liquid gallium-indium (EGaIn) is dispersed in a nonmetallic solvent (such as ethanol) using ultrasound, which breaks up the bulk liquid metal into the nanoparticles. The EGaIn MNPs are then coated with oxidized gallium (Ga2O3), which acts as a protective skin that prevents electrical conductivity.

The liquid MNPs created are thus small enough to pass through an inkjet printer nozzle and can be sprayed onto any substrate. The ethanol then evaporates away, leaving the MNPs on the surface. After printing, the nanoparticles must be rejoined by applying light pressure, which ruptures the particle coatings and releases the low-viscosity liquid-metal, making it electrically conductive. The process is achieved thanks to the unique properties of EGaIn and the semiconductive nature of Ga2O3.

According to the researchers, the EGaIn MNPs could be used for applications across a broad array of fields such as soft robotics, conformable electronics, wireless communications, micro- and nano-fluidics, wearable and implantable devices, and energy storage and transport systems. The researchers also demonstrated that besides inkjet printing, the MNPs could be used to fabricate flexible or stretchable integrated devices across multiple length scales. The study was published on April 18, 2015, in Advanced Materials.

“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,” said lead author, Assistant Professor of Mechanical Engineering Rebecca Kramer, PhD. “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.”

According to the researchers, future studies will explore how the interaction between the ink and the surface it is being printed on might be conducive to the production of specific types of devices. They will also 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.

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