Yes they did sometime in the 60's. I was actually involved with research in the early eighties where one option for an artificial heart (not a pace-maker) was t

Yes they did sometime in the 60's. I was actually involved with research in the early eighties where one option for an artificial heart (not a pace-maker) was to use a tiny Stirling Engine powered by an isotope heat source. The general public has a revulsion for anything 'radioactive', however radio-active material if handled properly is quite safe. For many years people did not realized it, but all Smoke detectors had radioactive americium 241. If you ever renovate an old home just don't put them in your pockets :)