The most clever scheme for powering pacemakers I have heard of is to power them with oxygen and carbon from the bloodstream. Like small, artificial organs. I do not know where that project is but it seems like a neat idea. The power lasts as long as the person does.
There are some other schemes to scavenge a little power from the body: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3146093/ - Jed
