In the very first MCU Iron Man movie, Tony Stark (Robert Downey Jr.) builds a miniature arc reactor to keep his heart beating after a bomb explosion leaves his chest full of shrapnel. The reactor also powers his Mark I Iron Man suit and helps him escape from his captors – but once Tony gets back to the U.S., he builds a more advanced arc reactor and tells his assistant Pepper Potts to throw the original away.
Instead, Pepper places the life-saving device in a glass box and engraves the words, “Proof That Tony Stark Has A Heart” on the arc reactor, revealing that she knows her boss is a decent man beneath his arrogant swagger. Her act turns out to be a life-saving one when Tony is forced to use the reactor to save his life after his treacherous partner Obadiah Stane steals his new one.
The original arc reactor makes a final appearance at the end of Avengers: Endgame during Tony’s funeral when Pepper places it onto a bed of flowers and lets it float away in a stream. It’s an emotional scene that brings closure to Iron Man’s long cinematic journey, but is it related to anything from Iron Man’s comic book roots?
Actually yes – but only slightly. In the comics, Tony did build a device to keep his heart beating after it was injured by shrapnel – but instead of being the sleek, compact arc reactor, it was a bulky chest plate that he needed to wear at all times to keep himself alive. Like the arc reactor, the chest plate functioned as a battery for the rest of Tony’s Iron Man armor. Unlike the arc reactor, however, the chest plate ran low on power constantly, forcing Tony to keep plugging himself into outlets to get a recharge. (Considering the original comic book Iron Man armor was powered by transistors, it’s not surprising that the first battery wasn’t that reliable).
Tony would eventually get a heart transplant using a special “synthetic tissue,” negating his need for the arc reactor (much like the way MCU Tony had heart surgery performed on him in Iron Man 3). This happened in Iron Man #19, fairly early in his career, showing that Tony didn’t need a special pacemaker for his heart as long as his MCU counterpart did. Tony would continue to have health problems, however, ranging from alcoholism to paralysis to a brain tumor. Yet while Tony was free of his heart condition, heart issues would affect someone very close to him.
In The Invincible Iron Man, Pepper Potts also encountered an accident that left life-threatening shrapnel inside her chest. To save her life, Pepper was outfitted with an arc reactor (or magnetic stabilizer disk) of her own, which later powered her own suit of Iron Man armor and enabled her to become the armored hero Rescue.
Thus, while the “Proof That Tony Stark Has A Heart” arc reactor was a lovely memento that served as a symbol for Tony Stark’s rebirth into a more socially-conscious man, in the comics, his life-saving device basically discarded. Granted, Tony tends to save many of his early Iron Man suits as trophies, indicating the original chest plate could be gathering dust in some corner of Stark Industries, but it didn’t serve anywhere near an important role as the MCU arc reactor did.
