To create parts made of metal in electronic devices, engineers have traditionally used heat, as metals must be molten to be formed. Researchers now have devised a way that eliminates the need for heat when creating electronic interconnects, facilitating the design of flexible and wearable electronics, they said.

Researchers at Iowa State University demonstrated a way to print flexible electronics that doesn’t require heat. Shown here are electronic traces printed on gelatin as part of this research. (Source: Iowa State University)

A team at the designed the method, which pre-heats metal—in this case, an alloy of bismuth, indium, and tin–and keeps it as a liquid, creating conductive interfaces without damaging the substrate, explained University of Iowa Professor Martin Thuo, who led the research team.

“We make metallic conductive traces at room temperature and without melting the metal,” Thuo told Design News. “Unlike the silver pastes and types of conductive traces, our systems are 100 percent metal–i.e., they are exactly the same as you would make on a motherboard today, except you do not need to melt the metal when you are making this.”

Thuo also is an associate of the U.S. Department of Energy’s Ames Laboratory and a co-founder of the Ames startup, SAFI-Tech Inc. He originally launched the project three years ago as an exercise for his graduate students, and the work took off from there, resulting in the new method.

No Heat, More Options

In the process, liquid metal is trapped below its melting point in polished, oxide shells, creating particles about 10 millionths of a meter across. When the shells are broken—which is done using mechanical pressure or chemical dissolving–the metal inside flows and solidifies.

This creates a heat-free weld or—in the case of the work of Thuo’s team—the ability to print conductive, metallic lines and traces on various materials, he said. The method gives researchers a greater range of substrates for metal interconnects than was possible before, as they don’t have to worry about heat damage, Thuo told Design News.

“We can now use any substrate without damage,” he said. “When heat is involved, the yield is also low–the output of a reflow oven can significantly reduce the overall yield (output) of a fab facility. It also limits the kinds of materials that can be used in electronic devices.”

The method can be used in a range of new applications and uses, some of which researchers demonstrated. The team printed metal traces on a rose; a flexible, programmable LED display on a curled sheet of paper; and metal traces on a gelatin cylinder.

In addition to interconnects for flexible electronics, engineers could use the heat-free process to create sensors to measure the structural integrity of a building or the growth of crops, or to develop electrical contacts for solar cells, researchers said.

The team published a paper on its work in the journal Advanced Functional Materials.

Thuo and his researchers plan to continue to develop the process by exploring other uses of the materials as well as extending the degree of undercooling to see what effect that has, he told Design News. They also are expanding uses of the method to develop sensors for agriculture- and infrastructure-health monitoring, Thuo added.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.