Inspired by a manta ray, scientists have built a small, squishy robotic fish that can swim and turn by remote control. Using the surrounding water as an electrode to capture and distribute electrical energy, the self-powered machine treads water twice as fast as similar robots and can function in a range of water temperatures. The biggest advance, though, is its energy efficiency — the robot can operate for three hours without recharging.

"[It] is an elegant demonstration of the emerging capabilities of soft robots," says Harvard's Michael Wehner, who last year built an octopus robot powered by hydrogen peroxide.

Why it matters: Robots will soon move out of warehouses and hospitals and into harsher, less predictable environments where they'll have to navigate small spaces and interact with humans (they may eventually even move through our bodies to diagnose disease in a next generation of endoscopies). To make it, they'll need to be compact, soft and flexible.