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CUE 2006

Electrical & Computer Engineering: 'Wallpaper' Harvests Ambient Energy for Low-Power Electronics

Several new arrays of micro-sized rectifying antennas, which resemble swatches of differently patterned wallpaper, are attracting international attention—both for their novelty and for potential applications ranging from wireless sensor networks to battery-less biomedical devices.

The customizable "rectenna" arrays were developed by CU Professor Zoya Popovic, who heads the Microwave Antenna and Circuits Group in the electrical and computer engineering department. With her colleague Regan Zane, who specializes in the management of power, Popovic has completed a number of projects for industry and government agencies demonstrating the new technology.

For example, a rectenna array mounted on a flexible substrate just under the surface of an aircraft wing is helping the U.S. Navy to monitor the possible formation of micro-cracks due to corrosion. Popovic's low-profile rectenna array powers a network of piezoelectric sensors without the need for battery replacement. When the aircraft is on a carrier or in a hangar, the rectenna is illuminated with 10 GHz radio waves, activating the sensors that provide the structural data.

Popovic also has worked with a European company that manufactures "smart" water pumps to develop a rectenna array that could power the sensors monitoring water flow, temperature, and pressure in its products. The rectenna arrays are attractive for a range of uses because they are low in cost and provide a source of power that is wireless and battery-less.

Professor Diego Restrepo at the University of Colorado Health Sciences Center is working with Popovic on a rectenna that would meet the power needs of implanted medical devices, such as probes that monitor neural signals. Popovic says the technology also could extend the lifetime of a pacemaker battery, requiring less frequent surgical replacements.

Biomedical applications of the technology are more difficult than industrial ones due to more stringent regulations related to human exposure, as well as absorption of radiowaves by different body tissues, which results in poorer antenna efficiency. But Popovic is investigating ways to work around these problems.

 "I am toying with embedding rectennas in clothing," she says, explaining that rectennas could provide some of the power needs for a medical implant through recycling of ambient energy already in the environment.

Scavenging of ambient energy is the most challenging application of the new devices because available power is low, the frequency is unknown, and polarization changes over time. One broadband rectenna that Popovic designed for a multi-path environment uses circularly polarized antennas to extract power at frequencies ranging from 1 to 18 GHz, and recycle it for a specific purpose, such as monitoring a medical or elderly patient's vital signs in a health care setting.

"We are basically extracting radio-frequency noise out of the environment," Popovic says. Each rectenna array is densely packed with antennas and diodes to try to achieve the greatest efficiency in the anticipated frequency and power range.

Now that the concept has been demonstrated and devices developed for several different applications, Popovic and her colleagues are working to develop a reconfigurable rectenna array, which could adapt automatically to changes in voltage and current and extract the most power under each condition.

"If our eyes were adapted to seeing the radio-wave instead of the optical part of the electromagnetic spectrum, you would see that we are swimming in radio waves over a broad frequency range. I want to capture the part that's not being used," she says.

For more information visit: http://nemes.colorado.edu/microwave
Department web site: http://ece.colorado.edu