Actually … technically, the technology is there. Granted, it’s not fusion, but it is nuclear.
The only problem is that you’d have a bit of plutonium strapped to your wrist.
(It might sound like science fiction, but it’s real. I don’t think one’s been implanted in decades, but there are people out there with plutonium-powered heart pacemakers.)
Of course, it wouldn’t necessarily be powered by plutonium; there are lots of other isotopes, and something with a shorter half-life that produced more power would probably be preferred. I’m not a nuclear engineer; go ask one if you’re really interested.
Note to readers: you most emphatically do not want, and never will have, a nuclear-powered wristwatch. Unless, of course, you count my fusion-powered Ultra. And, by fusion-powered, I, of course, mean solar-powered. With the solar panels very conveniently mounted on the roof of my house, where they can collect power all day long without me having to haul them around, which is nice considering how big and bulky and heavy they are. All I have to do is pop the watch on its charger while I take a shower, and — miracle of miracles! — some of the energy from millions-of-years-ago hydrogen fusion deep in the heart of the Sun trickles into the watch, more than enough to power the watch for another day or three. And, oh-by-the-way, also collect enough energy to power the rest of the house. And two cars. Pretty cool, eh? Driving around in a fusion-powered car?
b&