Current research in 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 does the data go? Without a similar flexible transmission device, these sensors would require wired connections to transmit health data.
Huanyu “Larry” Cheng, Dorothy Quiggle Assistant Professor of Career Development in Engineering and Mechanics at Penn State College of Engineering, and two international teams of researchers are developing devices to explore the possibilities of portable and flexible antennas. They published two articles in April in Nano-Micro Letters and Materials and design.
Portable antenna bends, stretches, compresses without compromising function
Like wearable sensors, a wearable transmitter must be safe for use on human skin, functional at room temperature, and able to resist twisting, compression, and stretching. The flexibility of the transmitter, however, poses a unique challenge: When the antennas are compressed or stretched, their resonant frequency (RF) changes and they transmit radio signals at wavelengths that may not match those of the Antenna receivers.
“Changing the geometry of an antenna will change its performance,” Cheng said. “We wanted to target a geometric structure that would allow movement while leaving the transmit frequency unchanged.”
The research team created the flexible emitter in layers. Building on previous research, they made a copper mesh with a pattern of overlapping wavy lines. This mesh constitutes the lower layer, which touches the skin, and the upper layer, which serves as a radiating element in the antenna. The top layer creates a double arch when compressed and stretches when pulled – and moves between these steps in a set of ordered steps. The structured process by which the antenna mesh arches, flattens and stretches improves the overall flexibility of the layer and reduces RF fluctuations between antenna states, according to Cheng.
Energy efficiency was another priority. The lower mesh layer prevents radio signals from interacting with the skin. This implementation, beyond the prevention of tissue damage, avoids a loss of energy caused by tissue degrading the signal. The antenna’s ability to maintain constant RF also allows the transmitter to collect energy from radio waves, Cheng said, potentially reducing power consumption from outside sources.
The transmitter, which can send data wirelessly at a range of nearly 90 meters, 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 in energy production and storage.
“We have demonstrated robust wireless communication in an expandable transmitter,” said Cheng. “To our knowledge, this is the first portable antenna to exhibit an almost completely unchanged resonant frequency over a relatively wide stretch range.”
Allow additional customization of the antenna with constant variables
After developing the extendable antenna prototype, Cheng analyzed it with another research team. The researchers aimed to identify new fundamental ways to refine such a device that could be applied to similar future research.
“We wanted to study the problem by examining the link between mechanical properties and electromagnetic behavior,” Cheng said. “Highlighting this relationship can reveal information about the influence of different parameters on antenna performance.”
The team made an antenna with layers and a mesh pattern similar to their previous prototype but lacking the dual arch compression structure. They measured the antenna strain when the mesh was stretched at different intervals, and then used computer simulations to examine the relationship between strain and antenna performance.
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 – to constant values. With this process, called normalization, researchers can focus on the relationship between specific variables by canceling the influence of the normalized variables.
The team found that normalizing different variables offered several ways to customize antenna performance. They also found that the simulated geometry of the mesh could produce different results, even with the same set of normalized variables.
Although the researchers analyzed the properties of portable antennas, Cheng pointed out that their methods could be applied to other radio frequency devices.
“We’ve shown that you don’t have to limit yourself to exploring the effects of a normalized variable,” Cheng said. “Using this method, we can customize the properties of other antennas or devices that communicate using microwaves.”
Look to the future
Cheng and his collaborators will continue to seek ways to facilitate the development of these devices through application-based studies as well as other fundamental explorations to optimize the design process.
“We’re really excited that this research may one day lead to arrays of body-worn sensors and transmitters, all of which communicate with each other and with external devices,” Cheng said. “What we imagine is science fiction at the moment, but we are working to make it happen.”