An anonymous reader quotes HotHardware:
Researchers from the University of Washington are looking to make batteries a thing of the past when it comes to mobile phones. The team has developed a phone that uses "almost zero power" according to associate professor Shyam Gollakota, who co-authored a paper which detailed the breakthrough... The researchers designed the phone to harvest microwatts of power from RF signals transmitted from a base station that is 31 feet away. Additional power is harnessed via ambient light through the use of miniature photodiodes that are about the size of a grain of rice. While in use, the phone consumes about 3.5 microwatts of power and is capable of communicating with a custom base station that is up to 50 feet away to send and receive calls... The phone ditches the traditional analog-to-digital converter, which turns your voice into data, in favor of a system that uses the vibrations from a microphone or speaker to perform the same task. An antenna then converts that motion into radio signals in such a way that very little power is consumed.
There's two drawbacks. First, modern smartphones "need a lot more than a 3.5-microwatt power budget for blazing fast processor, copious amounts of RAM and internal storage, and power-hungry displays." And more importantly, "you have to press a button to switch between transmissions and listening modes with the phone."