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This one is actually powered by RF transfer, so the power is all external.

This link explains it better: http://www.youtube.com/watch?v=d6g581tJ7bM#t=10m15s

Few years ago I saw a feature in IEEE spectrum about contact lens displays/computers. Apparently, because they're so small and have such low power requirements, they can power themselves off of the ambient radio / wifi / cell / etc signals

I'm wondering this too. Could it pick up on an electrical potential from the eye itself? Or could the reading be gathered with only as much of a signal as an RFID tag gets?

Good question. Maybe from body heat. A polymer thermoelectric converter seems like it could be a good candidate.

This is the most interesting question. And failsafe. You couldn't rely on this, unless there was a way to ensure it is working or had a fixed lifespan.

I believe the thermoelectric converter works by exploiting a temperature gradient, which I suspect would not be sufficient at this scale and location.