Our favorite German researchers over at the Fraunhofer Institute have developed "entire electronic systems" capable of operating battery-free from body heat alone. The picture above shows a wireless transmitter powered by the human hand. The 200 millivolts required to drive the device is supplied by a thermoelectric generator (TEG) which extracts electrical energy from hot and a cold temperature differentials of just a few degrees Celsius. Of course, the application processor alone in modern handhelds requires about 1W to operate so 2mW is a long way off from powering our portable electronics. Still, progress is progress.