Anyone who's watched the new Captain America: Civil War (you should, it's ace) will almost certainly have had this thought process at one point during its climactic battles: Tony Stark isn't really a superhero.
Sure, he's smart. Super smart. And super rich. But he owes all his 'power' to an atomic reactor strapped into his chest, powering a small electromagnet that... well, you've seen the film.
And so, it would seem, has the US government. Back in 1967, two government-funded teams from the National Heart Institute (NHI) and the Atomic Energy Agency (AEC) set about trying to build an 'atomic heart'.
Summarised in Shelley McKellar's recent Physics Today article, the teams spent a decade trying to come up with a reliable artificial heart that wasn't reliant on short-lived batteries that had a tendency to overheat.
The resulting heart converted heat from plutonium-238 - an element that releases 87 years' worth of stable heat - surrounded by insulation to protect the user's body. This power unit was then attached to a small blood pump that would take over from a failed heart.
"Surgeons would squeeze the pump into a chest whose diseased biological heart had been removed and then connect it through the flexible drive shaft to the heat converter implanted in the abdomen," explains McKellar. However, there were issues with the design: "Both heat converter and blood pump were too big, heavy, inefficient, and unable to meet the body’s energy needs."
Another atomic heart prototype was successfully transferred into several calves in 1972. However, "exaggerated claims" concerning the atomic hearts, "only made the public more suspicious at a time when media reports, lawsuits, and vocal public outrage abounded concerning reported deaths and injuries from defective heart valves, pacemakers, and other medical devices."
The projects were closed in 1977 - mercifully, we never got to see what sort of damage an atomic heart might do to the poor sod who'd have one implanted inside their chest.
Best keep this tech to the comic books.