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New research led by Penn State engineers is paving the way for wireless, wearable health monitoring devices that can collect and transfer information. Image: Image provided by Huanyu Cheng

By Gabrielle Stewart

Current research on flexible electronics is paving the way for wireless sensors that can be worn on the body and collect a variety of medical data. But where do the data go? Without a similar flexible transmitting device, these sensors would require wired connections to transmit health data.

Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Assistant Professor of Engineering Science and Mechanics in the Penn State College of Engineering, and two international teams of researchers are developing devices to explore the possibilities of wearable, flexible antennas. They published two papers in April in Nano-Micro Letters and Materials & Design

Wearable antenna bends, stretches, compresses without compromising function

Like wearable sensors, a wearable transmitter needs to be safe for use on human skin, functional at room temperature and able to withstand twisting, compression and stretching. The flexibility of the transmitter, though, poses a unique challenge: When antennas are compressed or stretched, their resonance frequency (RF) changes and they transmit radio signals at wavelengths that may not match those of the antenna’s intended receivers.

“Changing the geometry of an antenna will change its performance,” Cheng said. “We wanted to target a geometric structure that would allow for movement while leaving the transmitting frequency unchanged.”

The research team created the flexible transmitter in layers. Building upon previous research, they fabricated a copper mesh with a pattern of overlapping, wavy lines. This mesh makes up the bottom layer, which touches the skin, and the top layer, which serves as the radiating element in the antenna. The top layer creates a double arch when compressed and stretches when pulled — and moves between these stages in an ordered set of steps. The structured process through which the antenna mesh arches, flattens and stretches improves the overall flexibility of the layer and reduces RF fluctuations between the antenna’s states, according to Cheng. 

Energy efficiency was another priority. The bottom mesh layer keeps radio signals from interacting with the skin. This implementation, beyond preventing tissue damage, avoids a loss of energy caused by tissue degrading the signal. The antenna’s ability to maintain a steady RF also allows the transmitter to collect energy from radio waves, Cheng said, potentially lowering energy consumption from outside sources.

The transmitter, which can send wireless data at a range of nearly 300 feet, can easily integrate a number of computer chips or sensors, Cheng said. With further research, it could have applications in health monitoring and clinical treatments, as well as energy generation and storage.

“We’ve demonstrated robust wireless communication in a stretchable transmitter,” Cheng said. “To our knowledge, this is the first wearable antenna that exhibits almost completely unchanged resonance frequency over a relatively large range of stretching.”

The wearable transmitter is designed to compress its top layer in a double arch pattern, shown here, to respond to movement without compromising signal transmission. IMAGE: Image provided by Huanyu Cheng

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