A shoe insert that harnesses the power of each step and can be used to charge cell phones is in development.

The idea is simple enough: When a person’s heel hits the ground, it produces 20 watts of energy. Capturing that energy, however, poses serious challenges. But scientists Tom Krupenkin and Ashely J. Taylor believe they have developed system, in which droplets of metal get pushed against an electrode, that can capture about 2 watts of that energy and then transfer it to electronic devices.

According to Krupenkin, a two-mile walk with the insert would be enough to charge a cell phone battery.

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