I know that recent advances in technology are doing amazing things with very low amounts of electricity, with RFID being one of the most familiar examples, but I'm not very familiar with the principles involved. It makes me wonder: is it possible (with current technology, not "is it theoretically possible?") to use ambient sources (including body heat) to generate enough power to light up an LED, all in a device small enough for a person to hold in their hand? If so, how small could the whole assembly be made?
I've actually got a very specific application in mind, but if it turns out we don't have the capability for it, that'll save a lot of time right there.