[ewolfe]

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

[neumann]

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.