A group of British scientists collaborating with Cranfield University and others has created what is essentially a knee brace that generates electricity from movement. Originally designed for military use, the "pizzicato knee-joint energy harvester" places a disk at the joint of the knee, where four piezoelectric arms are "plucked" by 72 plectrum as the leg moves. These piezo arms (called bimorphs) generate electricity under stress, and in this setup the scientists are getting around 2 milliwatts out of their prototype, though they hope to get up to 30mW once the design is complete.
This technology could be a boon to soldiers, whose equipment could be lightened significantly without some of the weighty batteries they usually carry. Using one of the harvesters on each leg would only output up to 60mW, which is still far below the 20-1300mW power needs of modern smartphones, though a low-power GPS might be possible. The scientists estimate that once the device reaches large-scale production that it could sell for as little as £10, or roughly $15.50, including 16-20 bimorphs to boost the power. What the researchers don't mention is a time frame, so there could be a little bit of work left to do.