Iowa State University (ISU) engineers have developed a flexible, stretchable and tunable “meta-skin” that uses rows of small, liquid-metal devices to make an object invisible to radar.

The meta-skin takes its name from metamaterials, which are composites that have properties not found in nature and can manipulate electromagnetic waves. By stretching and flexing the polymer meta-skin, it can be tuned to reduce the reflection of a wide range of radar frequencies.

This meta-skin uses rows of small, liquid-metal devices to make an object invisible to radar. Image credit: Liang Dong.Researchers led by professor Jiming Song and associate professor Liang Dong, both of ISU's Department of Electrical and Computer Engineering, were hoping to prove an idea: that electromagnetic waves—perhaps even the shorter wavelengths of visible light—can be suppressed with flexible, tunable liquid-metal technologies.

What they came up with are rows of split-ring resonators embedded inside layers of silicone sheets. The electric resonators are filled with galinstan, a metal alloy that is liquid at room temperature and less toxic than other liquid metals such as mercury.

Those resonators are small rings with an outer radius of 2.5 millimeters and a thickness of half a millimeter. They have a 1-millimeter gap, essentially creating a small, curved segment of liquid wire.

The rings create electric inductors, and the gaps create electric capacitors. Together, they create a resonator that can trap and suppress radar waves at a certain frequency. Stretching the meta-skin changes the size of the liquid metal rings inside and changes the frequency the devices suppress.

Tests showed radar suppression was about 75% in the frequency range of 8 to 10 gigahertz. When objects are wrapped in the meta-skin, the radar waves are suppressed in all incident directions and observation angles.

According to the researchers, the meta-skin technology is different from traditional stealth technologies that often reduce only the backscattering, i.e., the power reflected back to a probing radar.

While the meta-skin could coat the surface of the next generation of stealth aircraft, the researchers are hoping for even more—a cloak of invisibility.

“The long-term goal is to shrink the size of these devices,” says Dong. “Then hopefully we can do this with higher-frequency electromagnetic waves such as visible or infrared light."

While that would require advanced nanomanufacturing technologies and appropriate structural modifications, Dong says this study proves the concept of frequency tuning and broadening, as well as multidirectional wave suppression with skin-type metamaterials.