You’re talking on your phone, and you hear that dreaded “BEEP” that warns you that the battery is about to die. Sure, you had a dozen chances to plug it in throughout the day, but now it’s too late. Looks like you’re going to need a bigger battery. Or maybe you don’t need any battery at all!

Researchers at the University of Washington set out to design a phone with minimal energy requirements. They used a lot of clever tricks, such as waiting to sense speech, and switching between transmit and receive modes. It uses energy-efficient earphones instead of a speaker. All of this was done using off-the-shelf components, and results in a device that requires just 3.5 microWatts when operating. (A standard AAA alkaline battery could provide this much power continuously for 46 years.) The power requirement is that the developers are able to pull it out of thin air. They can harvest enough energy from ambient radio waves to power the phone. Adding a solar cell to harvest light energy, or a system to convert motion into energy would also be able to provide the required electricity. Note that this device does not have any display, but there are low-power display options available.

The reason that this design is so significant is that it could also provide a model for wearable devices that could communicate sensor data to a smartphone or other receiver, all without requiring any batteries. This would achieve the essential goal of having these devices “disappear” into our lives, because they do not need to be recharged. And it is important to note that it was created using existing components. It could be relatively easy to create devices that are more integrated to target specific use cases, which could result in lower costs and tinier packages that would make remote monitoring of bio-information practical and convenient.