From microwave ovens to Wi-Fi connections, the radio waves that permeate the environment are not just signals of energy consumed but are also sources of energy themselves. An international team of researchers, led by Huanyu "Larry" Cheng, Dorothy Quiggle Career Development Professor in the Penn State Department of Engineering Science and Mechanics, has developed a way to harvest energy from radio waves to power wearable devices.
source https://techxplore.com/news/2021-03-harvest-energy-radio-power-wearable.html