Fluidic Antenna Has the Properties of a Rubber Band

Author: 
Bob Michaels
When people think of antennas, they often imagine a slender metal rod protruding from the body of a car, a TV, or a radio. But that was then. Now, Michael Dickey, Ju-Hee So, Jacob Thelen, Amit Qusba, Gerard Hayes, and Gianlucca Lazzi from North Carolina State University (NC State; Raleigh, NC; www.ncsu.edu) have invented an antenna with a liquid conductive element, making it potentially suitable for flexible electronic medical devices.

A flexible antenna with a liquid conductive element is potentially suitable for flexible electronic applications, including medical devices.
“The antenna has the properties of a rubber band,” notes Dickey, a professor in the department of chemical and biomolecular engineering at NC State and project team leader. “It can be stretched, twisted, and bent, yet it returns to its original form without any degradation in performance.” The antenna’s durability results from its fluidic properties. Unlike solid metals, which are damaged irreversibly when they become deformed, liquids can flow in response to stress. As a result, the mechanical properties of the antenna are defined by an encasing material and not the antenna itself. “One of the nice features of these antennas is that they ‘self-heal’ after they are cut because the metal can flow and reconnect after being severed,” Dickey explains.

Conventional antennas are fabricated by milling or etching rigid sheets of copper into a static shape that dictates a singular function. While it forms efficient antennas, copper is poorly suited for flexible electronics because it suffers fatigue when bent, experiencing irreversible deformation beyond strains of ≈2%. In contrast, Dickey’s antenna has a fluid conductive element and mechanical properties and shape that are defined by elastomeric channels composed of polydimethylsiloxane (PDMS). Thus, it can withstand mechanical deformations such as stretching, bending, rolling, and twisting and then return to its original state. The ability of the fluid metal to flow while the PDMS material undergoes deformation ensures electrical continuity.

Because it is based on flexible materials, the antenna also exhibits flexible frequency tunability. “The geometry of an antenna helps determine its resonant frequency,” Dickey comments. “Since our antennas are stretchable, we can alter the shape in real time and dynamically adjust the resonant frequency. Simply put, by making the antenna longer, we can change the frequency to lower values.”

Based on microfluidics, the antenna is constructed by forming very small channels—the encasing material—and injecting fluid metal into them, Dickey says. “The trick to our work is that the liquid metal forms an oxide ‘skin’ composed of gallium that holds the metal inside the channels.” While most liquids bead up much like water on a waxed car, this liquid is different because of the presence of the skin. The lack of liquid beading enables the formation of nonspherical structures.

“The antenna resembles a waterbed in that there is liquid in the middle and a skin holding it together, but the skin ruptures under low force, easing liquid flow,” Dickey comments. “Once the fluid metal is inside the channel, the skin reforms and provides mechanical stability to the otherwise low-viscosity, high-surface-tension liquid.”

Since PDMS features a low modulus and low surface energy, the antenna can conform to a variety of surfaces and substrates. And it can be incorporated into devices through ex situ fabrication and subsequent integration or through winding or weaving. “The real benefit of this system for biomedical applications is that it can form ‘soft’ antennas that are highly durable,” Dickey remarks.