Ancient 3D paper art, kirigami, could shape modern wireless technology

Recently published in the journal Nature Communications, research from the Drexel-UBC team showed how kirigami -- a variation of origami -- can transform a single sheet of acetate coated with conductive MXene ink into a flexible 3D microwave antenna whose transmission frequency can be adjusted simply by pulling or squeezing to slightly shift its shape.

The proof of concept is significant, according to the researchers, because it represents a new way to quickly and cost-effectively manufacture an antenna by simply coating aqueous MXene ink onto a clear elastic polymer substrate material.

"For wireless technology to support advancements in fields like soft robotics and aerospace, antennas need to be designed for tunable performance and with ease of fabrication," said Yury Gogotsi, PhD, Distinguished University and Bach Professor in Drexel's College of Engineering, and a co-author of the research. "Kirigami is a natural model for a manufacturing process, due to the simplicity with which complex 3D forms can be created from a single 2D piece of material."

Standard microwave antennas can be reconfigured either electronically or by altering their physical shape. However, adding the necessary circuitry to control an antenna electronically can increase its complexity, making the antenna bulkier, more vulnerable to malfunction and more expensive to manufacture. By contrast, the process demonstrated in this joint work leverages physical shape change and can create antennas in a variety of intricate shapes and forms. These antennas are flexible, lightweight and durable, which are crucial factors for their survivability on movable robotics and aerospace components.

To create the test antennas, the researchers first coated a sheet of acetate with a special conductive ink, composed of a titanium carbide MXene, to create frequency-selective patterns. MXene ink is particularly useful in this application because its chemical composition allows it to adhere strongly to the substrate for a durable antenna and can be adjusted to reconfigure the transmission specifications of the antenna.

MXenes are a family of two-dimensional nanomaterials discovered by Drexel researchers in 2011 whose physical and electrochemical properties can be adjusted by slightly altering their chemical composition. MXenes have been widely used in the last decade for applications that require materials with precise physiochemical behavior, such as electromagnetic shielding, biofiltration and energy storage. They have also been explored for telecommunications applications for many years due to their efficiency in transmitting radio waves and their ability to be adjusted to selectively block and allow transmission of electromagnetic waves.

Using kirigami techniques, originally developed in Japan the 4th and 5th centuries A.D., the researchers made a series of parallel cuts in the MXene-coated surface. Pulling at the edges of the sheet triggered an array of square-shaped resonator antennas to spring from its two-dimensional surface. Varying the tension caused the angle of the array to shift -- a capability that could be deployed to quickly adjust the communications configuration of the antennas.

The researchers assembled two kirigami antenna arrays for testing. They also created a prototype of a co-planar resonator -- a component used in sensors that naturally produces waves of a certain frequency -- to showcase the versatility of the approach. In addition to communication applications, resonators and reconfigurable antennas could also be used for strain-sensing, according to the team.

"Frequency selective surfaces, like these antennas, are periodic structures that selectively transmit, reflect, or absorb electromagnetic waves at specific frequencies," said Mohammad Zarifi, principal research chair, an associate professor at UBC, who helped lead the research. "They have active and/or passive structures and are commonly used in applications such as antennas, radomes, and reflectors to control wave propagation direction in wireless communication at 5G and beyond platforms."

The kirigami antennas proved effective at transmitting signals in three commonly used microwave frequency bands: 2-4 GHz, 4-8 GHz and 8-12 GHz. Additionally, the team found that shifting the geometry and direction of the substrate could redirect the waves from each resonator.

The frequency produced by the resonator shifted by 400 MHz as its shape was deformed under strain conditions -- demonstrating that it could perform effectively as a strain sensor for monitoring the condition of infrastructure and buildings.

According to the team, these findings are the first step toward integrating the components on relevant structures and wireless devices. With kirigami's myriad forms as their inspiration, the team will now seek to optimize the performance of the antennas by exploring new shapes, substrates and movements.

"Our goal here was to simultaneously improve the adjustability of antenna performance as well as create a simple manufacturing process for new microwave components by incorporating a versatile MXene nanomaterial with kirigami-inspired designs," said Omid Niksan, PhD, from University of British Columbia, who was an author of the paper. "The next phase of this research will explore new materials and geometries for the antennas."