This Antenna Bends but Won't Break
Injecting liquid metal into a polymer results in a twistable, stretchable antenna.
Engineers at North Carolina State University have created a highly efficient, flexible, and self-healing antenna using a metal alloy that’s a liquid at room temperature.
Most of the materials that go into electronic devices are brittle, inflexible, and prone to damage, including the copper used most frequently to make antennas. The new liquid-metal antenna could make it easier to send and receive data from flexible electronics. Possible uses include sensors incorporated into clothing or other textiles, pliant electronic paper, or implantable biomedical devices.
Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State, was working with a gallium-indium alloy, which is liquid at room temperature, researching how it behaves in microchannels with a view to electronics fabrication applications. Hunting for other possible uses, he hit on the idea of making a flexible antenna. In collaboration with electrical engineer Gianluca Lazzi–then at NC State, now chair of the department of electrical and computer engineering at the University of Utah–Dickey and his students used the alloy and a common flexible polymer called polydimethylsiloxane (PDMS) to make a simple dipole antenna–essentially a straight rod, like the old-fashioned “bunny ear” antennas used for analog TV.
The researchers poured liquid PDMS into a mold that left it with a single internal channel once cured. They then injected the liquid gallium-indium mixture into the channel and sealed it. “It’s all pretty straightforward,” Dickey says.
Researchers at Lazzi’s lab tested the antenna’s performance and found that they could create an electrical contact with the device simply by jabbing a wire into the liquid, eliminating the need for solder. In the lab, the antenna radiated over a broad frequency range at about 90 percent efficiency–equivalent to the efficiency of a similar antenna made of copper. “That’s the first thing we were surprised by,” says Lazzi. The antenna also remained functional while the engineers bent, twisted, and folded it in half; they even stretched it an additional 40 percent beyond its normal length. When the stress was released, the PDMS snapped back to its original shape.
When the length of the antenna is changed by stretching it, however, the device responds to different frequencies of radio waves. Stretching the device eight millimeters shifted its peak response by over 200 megahertz. Lazzi says that this could be a novel way to tune the antenna or to create a combined antenna-sensor. Embedded in machinery or in a concrete structure such as a bridge, the antenna could monitor it for strain over time.
The liquid-metal antenna could also “heal” itself when damaged. When exposed to air, the alloy forms a thin oxidized coat that keeps it from flowing freely. Knowing this, Dickey cut through the antenna with a razor blade to test its ability to heal. The oxide layer kept the liquid metal within the PDMS, and once the razor was removed, in many cases the two ends spontaneously reformed a single, conducting wire. In the other instances, the researchers had to press the severed ends together to reestablish a connection.
Dickey says that it would be easy to mass-produce this kind of antenna: a whole sheet of PDMS forms could be made at once and then cut up into individual devices. The researchers are also evaluating other polymers, since PDMS might interfere with the efficiency of some forms of antennas, such as the loops, helices, or patches used more commonly in communications devices such as cell phones and radio or TV transceivers. In addition, Dickey says, other polymers could allow the antennas to stretch even further than PDMS before tearing apart.
Researchers who are developing flexible electronics are interested in the possibilities opened up by the new antenna. “It’s a really clever way to address an important problem,” say John Rogers, an engineering professor at the University of Illinois at Urabana-Champaign who is developing soft materials for flexible and stretchable electronics.
Juan Hinestroza, an assistant professor of fiber science at Cornell University, raises the idea of incorporating Bluetooth or cell-phone antennas into clothing. “Because of the flexibility, there will be negligible effects on the draping properties of the material, and the antenna can go unnoticed to the observers and the wearer,” he says.