The German Fraunhofer Institute have developed "entire electronic systems" capable of operating on their thermoelectric generator (TEG) technology. TEG extracts elecrical energy from temperature differential (e.g. the temperature differential between the inside and outside of a down jacket.)


Shown above is a wireless transmitter powered by the heat differential between a man's hand and the ambient room temperature. Only 200 millivolts is required to drive the device which is just about what such a limited differential of just a few degrees (Celsius) can provide. Single processor units in modern handhelds alone require about 1W to operate so 2mW is a long way off.

For more information visit:
http://www.fraunhofer.de/fhg/EN/press/pi/2007/08/Researchnews82007Topic1.jsp