Source: http://www.engadget.com/2014/01/23/tiny-...

The prototype machine generates electricity through the movement of nanoribbons that are tiny enough to be sewn into an organ's surface, but efficient enough to power a pacemaker solely through heartbeats. The device could also keep implants going through the motion of a diaphragm or lung. Scientists still need to test the long-term viability of this miniscule energy factory; there's no guarantee that it's safe enough to last for years inside a human body.