In today's episode of "weird shit I stumbled onto on the internet", I bring you: nuclear-powered pacemakers.
Some of the earlier pacemakers made in the US, around the 70's, were powered by a very small amount of plutonium. If you've ever heard of the term radioisotope thermoelectric generator or RTG in relation to eg. satellites, that's what the pacemakers used. The upside of using an RTG was that the device could run for decades without needing to get its power source replaced. The downside is that you now have plutonium sown in to your chest cavity – which actually isn't as bad as it sounds considering the amounts used, but it's still a highly radioactive element and presents some fun challenges, some of which are discussed in the article.
Here's an article on the technical details on how they, and thermoelectric kajiggers in general, work https://blog.plover.com/tech/seebeck-effect.html
Huh, that's interesting. I found a wiki article that describes a diamond battery that generates current from beta particles emitted by carbon-14, but that one's only good for microwatts. I also found some pages that referred to diamond batteries that use nuclear waste, like this one, which might be what you were thinking of? No idea what the wattage of those would be, but I'd guess more than ^14^C