Virtually all urban areas are absolutely inundated with radio waves from cell phone towers, WiFi routers, radio station transmitters, and more. Despite what conspiracy theorists will tell you, there is no reason to believe that those radio waves cause any physical harm to people. But radio waves do carry energy and that energy can be utilized. RFID (Radio-Frequency Identification) tags are, for example, powered entirely by the energy from the radio waves coming from the RFID reader, which is why RFID tags and cards do not require batteries. Now, a team of researchers from Pennsylvania State University are developing similar technology that can power wearable devices with ambient radio waves.
To understand how this works, consider a microphone. Sound waves move through the air and collide with a very sensitive diaphragm in the microphone. The sound waves make the diaphragm vibrate, which causes an attached magnet to move relative to a coil. That induces an electrical current, which is the audio signal. RFID and this technology work in the same way, just with radio waves. Radio waves with the proper frequency induce a current in the antenna. For FM radio, the frequency of those radio waves is modulated to carry an audio signal. In this case, however, the current induced by the radio waves is used for power. It’s a very small amount of power, but still enough to be useful.
The Penn State researchers demonstrated this technology with a special wearable patch that has an embedded antenna. That is a wideband antenna that is tuned to pick up a range of radio wave frequencies and to continue doing so even when the patch is stretched or bent. As long as the wearer is in an area with plenty of ambient radio waves, as most urban locations are, the patch will continuously harvest a small of energy. That energy can be used to power low-power sensors. Realistically, that is only enough power for something like a temperature sensor or a heart rate sensor, which makes this technology well-suited to wearable patches that perform medical monitoring. But the technology will just become more useful for other wearable devices as we develop more power-efficient chips and sensors. This technology won’t replace solar power or other sustainable means of producing electricity, but it is very promising for low-power wearable devices that are inconvenient to charge or that don’t have the room for a battery.