Check this out:
Nanogenerator Could Draw Energy From Human Blood
It seems researchers at the Georgia Institue of Technology are working on a nanogenerator that harvests energy from the human body.
"Our bodies are good at converting chemical energy from glucose into the mechanical energy of our muscles ... These nanogenerators can take that mechanical energy and convert it to electrical energy for powering devices inside the body ...
It sets a solid foundation for self-powering implantable and wireless nanodevices and nanosystems in biofluid and any other type of liquid."
Uh huh. Talk to me when you have a foolproof method for removing those nanodevices, OK?