Various elements have been used to power pacemakers, which, externally, overdrive suppress ectopic or insufficiently fast automaticity foci, or kick in when there are no impulses whatsoever.
Plutonium has a pretty long half-life and radiates well, so some scientists figured "why not?!"
oops....
some are still in people!
http://www.orau.org/ptp/collection/miscellaneous/pacemaker.htm