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 without a similar flexible transmitting device, these sensors would require wired connections to transmit health data.

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.

Researchers have created a flexible, wearable transmitter in layers. Building upon previous work, 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.

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, 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. With further research, it could have applications in health monitoring and clinical treatments as well as energy generation and storage.

After developing the stretchable antenna prototype, the researchers analyzed it to identify new fundamental pathways for fine-tuning such a device that could be applied to similar, future research. The team fabricated an antenna with layers and a mesh pattern similar to their previous prototype but lacking the double-arch compression structure. They measured the deformation of the antenna as the mesh was stretched at different intervals, then used computer simulations to examine the relationship between the deformation and the antenna performance.

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: Huanyu Cheng)

To simplify the analysis of the antenna’s radio signal transmission, the researchers used a mathematical technique to convert certain measurements — such as the width and angle of the repeating mesh pattern — into constant values. With this process, called normalization, researchers can focus on the relationship between specific variables by negating the influence of the normalized variables.

The team found that the normalization of different variables provided several avenues for customizing the antenna’s performance. They also found that the simulated geometry of the mesh could produce different outcomes, even with the same set of normalized variables.

Though the researchers analyzed wearable antenna properties, the methods could be applied to other radio frequency devices. The research could lead to networks of sensors and transmitters worn on the body, all communicating with each other and external devices.

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This article first appeared in the September, 2021 issue of Tech Briefs Magazine.

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