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This is Scientific American — 60-Second Science. I'm Christopher Intagliata.

One of the biggest drawbacks to wearing a smartwatch is how often you have to ta?ke it off to charge it. But here's an idea. How about charging it with a power source that's pretty ubiquitous nowadays: wifi?

"Wifi signals are all around us and most of them are just wasted." Xu Zhang, an electrical engineer at MIT. Writing in the journal Nature, he and his colleagues describe a device, called a "rectenna", designed to capture energy from wifi signals — and turn it into direct current electricity.

The rectenna c?onsists of a small gold antenna¹ — about the size of an SD card — which converts a variety of wireless² signals — wi-fi, bluetooth, cellular³ LTE — into an AC signal. Next, a three-atom-thick layer of molybdenum disulfide converts that AC signal into usable DC electricity. That layer is called a rectifier. Slap it onto the antenna, and the result is the "rectenna."

The device is flexible and, using typical home wi-fi signals, it spits out about 40 microwatts. Enough to light up a simple LED display or power a biosensor. It's not enough juice for power-hungry smartwatches and smartphones just yet. But Zhang says their next goal is to build an array of rectennas to power larger devices.

The scientists also envision a smart city where buildings, bridges and highways are studded with tiny sensors⁵ to monitor their structural⁶ health, each sensor⁴ with its own rectenna, so it never goes dark. "We can in some sense bring intelligence to almost every object around us, and that can enable ubiquitous sensing."

Because a "smart city" becomes a lot less intelligent when it runs out of juice.

Thanks for listening for Scientific American — 60-Second Science. I'm Christopher Intagliata.