Steve Rogers is one of a kind, but there have still been many, many attempts in the Marvel Cinematic Universe to replicate the super soldier serum that transformed him into Captain America. Whether because of flaws in the science, poorly chosen test subjects, or ill treatment of the super soldiers who received it, super soldier serums have arguably done a lot more harm than good.

That's certainly the opinion of Baron Helmut Zemo, who was broken out of prison in The Falcon and the Winter Soldier and temporarily teamed up with Sam and Bucky against the Flag-Smashers. Considered freedom fighters by some and terrorists by others, the Flag-Smashers have used their newfound superpowers to fight on behalf of the refugees who have been displaced in the wake of billions of people suddenly returning after the Blip. The new Captain America, John Walker, has now also taken the super soldier serum in the hopes of being as strong as Steve Rogers - and has proven that it takes more than just physical strength to be Captain America.

Related: Who Is Madame Hydra? Julia Louis-Dreyfus' MCU Future Explained

Zemo has argued for the destruction of all super soldiers and the serum that creates them, saying that the desire to create super soldiers is inherently supremacist. Whether he's right or not, there's no denying that super soldier serums often end in disaster. Here's every variation of Abraham Erskine's super soldier serum that has appeared so far in the MCU, and why so many of them failed.

Abraham Erskine's Super Soldier Serum

Dr Erskine and Steve Rogers

The original super soldier serum was created by the scientist Abraham Erskine, who hand-picked Steve Rogers to be the soldier who received it based on his scrawny body and great strength of character. Erskine emphasized that his serum enhanced not just the body, but the mind as well. "The serum amplifies everything that is inside, so good becomes great; bad becomes worse," he told Steve Rogers. "The strong man who has known power all his life may lose respect for that power, but a weak man knows the value of strength, and knows compassion." Erskine's super soldier serum had to be used in combination with vita radiation in order to have safe and stable effects, so Steve Rogers was simultaneously injected with the serum and bombarded with the vita rays.

Though the original plan was for Steve to be the first of many super soldiers serving in the U.S. military, he ended up being the only person ever to receive the Erskine serum, since HYDRA assassinated Erskine and destroyed the serum. Therefore it's unknown whether this super soldier serum had a uniquely positive outcome because of the quality of Erskine's work, or his choice of recipient. Most likel it was a combination of both.

The Red Skull's Super Soldier Serum

Johann Schmidt Super Soldier Serum

Erskine first began working on the super soldier serum in 1930, and when the Nazis came into power both they and HYDRA forced him to continue developing his formula for their purposes. Nazi leader and head of HYDRA Johann Schmidt injected himself with an earlier version of Erskine's super soldier serum, but without the stabilizing effect of vita radiation the serum caused the skin on Schmidt's face to burn and turn red, earning him his supervillain moniker: the Red Skull. Despite this unfortunate side effect, Erskine said that the serum did not fail, but did exactly what it was designed to do; just as the serum would later amplify Steve Rogers' goodness, bravery and compassion, it amplified Johann Schmidt's evil and lust for power. The Red Skull also didn't consider himself a failure, declaring that he, not Captain America, was Erskine's "greatest success."

Related: Why Bucky Is So Much Weaker Than John Walker

Arnim Zola's Super Soldier Serum

Captain America The Winter Soldier Bucky Unmasked

After Erskine escaped and began developing his super soldier serum for the United States instead, scientist Arnim Zola took over the job of trying to perfect HYDRA's version. He used captured American soldiers as test subjects for his experimentation with the super soldier serum, and one of these test subjects was Bucky Barnes. With Captain America's help, Bucky was able to escape HYDRA's clutches - but only for a short while. When he fell off a train and down a mountainside, it was his super soldier physiology that allowed him to survive the fall, albeit with the loss of his arm. He was returned to HYDRA, and a combination of cybernetics and brainwashing was used to transform him into the Winter Soldier.

HYDRA would no doubt argue that Zola's super soldier serum wasn't a failure, since Bucky served the organization in exactly the way that he was supposed to and was the scourge of their enemies over the following 70 years. If the Red Skull is an example of the harm that super soldier serum can done when given to the wrong person, then Bucky is an example of the harm it can do when the super soldier is molded by the wrong people. Had Bucky not fallen from the train, he could have become as worthy a superhero as Steve Rogers was. Since being freed from HYDRA's influence by the Wakandans, he has made every effort to use his abilities to help people and make amends.

Isaiah Bradley's Super Soldier Serum

Isaiah Bradley angrily speaks to Bucky and Sam

After the death of Abraham Erskine and the later disappearance of Captain America, the United States began trying to recreate the super soldier serum and used Black soldiers as test subjects. In the limited comic book series Truth: Red, White and Black (upon which The Falcon and the Winter Soldier is partly based), these new versions of the serum frequently resulted in horrifying deaths for the men who received them. Isaiah Bradley was one of the few test subjects for whom the serum worked, and the only super soldier who ultimately survived. He was sent to Goyang during the Korean War in the hopes that his enhanced strength would allow him to destroy the Winter Soldier, but was only successful in tearing off Bucky's arm.

Like the Winter Soldier, Isaiah Bradley is an example of a super soldier serum that worked and had the potential for positive outcomes, but failed because of abuse from those in power. As a "reward" for his service, Isaiah Bradley was imprisoned and experimented upon, with both the CIA and HYDRA hoping to extract the recipe for a successful super soldier serum from his blood. Isaiah's super strength persisted into his old age, but he was left jaded and embittered towards the system that had used and abused him, and killed his friends.

Related: MCU Theory: John Walker Becomes Hydra’s Captain America

Howard Stark's Super Soldier Serum

HYDRA Winter Soldiers

In 1991 a new version of the super soldier serum was developed by Iron Man's father, Howard Stark. Before it could be used on any American test subjects, however, Howard was murdered by the Winter Soldier and the five doses of his super soldier serum were delivered to HYDRA. The serum was given to HYDRA's elite death squad, who had "more kills than anyone in HYDRA's history," according to Bucky. As Erskine had warned back in the 1940s, giving the serum to ruthless killers had the effect of amplifying their lust for violence and HYDRA found that they were impossible to control. This attempt to create more Winter Soldiers was deemed a failure, and the test subjects were put on ice in HYDRA's Siberia facility until they were eventually killed by Zemo.

Bruce Banner's Super Soldier Serum

Edward Norton as Bruce Banner in The Incredible Hulk

The American government resurrected Project Rebirth, the code name used for developing the super soldier serum, after the turn of the 21st century. Thaddeus Ross recruited scientist Bruce Banner to recreate Erskine's formula, and Banner developed a new serum that was used in combination with gamma radiation instead of vita radiation. Unfortunately this proved to be anything but stable, transforming Banner into a giant green rage monster who became known as the Incredible Hulk. Rather than enhancing his personality, as the previous versions of the super soldier serum had, Banner's formula had the effect of splitting his personality between the mild-mannered Bruce and the ill-mannered Hulk.

In Bruce Banner's case the problem wasn't necessarily with the science, but with the deceptive circumstances under which the serum was developed. Ross told Bruce that he was working on a treatment for radiation resistance, not a super soldier serum, and Bruce didn't know what he was really doing when he injected himself with it. Years later, and in the wake of Thanos snapping away half of all life in the universe, Bruce and the Hulk were able to reconcile and "Smart Hulk" was born - all of the giant green monster, minus the rage, and complete with Bruce Banner's personality and brains.

Emil Blonsky's Super Soldier Serum

Abomination in The Incredible Hulk

Thaddeus Ross didn't give up on the super soldier serum even after the Hulk nearly killed him and his daughter. His new version of Project Rebirth continued working on serums until he found one that was "very promising." This was offered to Captain Emil Blonsky at a very low dose, and seemed to have the desired effect of enhancing speed, strength and agility without any negative side effects. Unfortunately for Blonsky, he wasn't yet strong enough to beat the Incredible Hulk. After he healed from having most of the bones in his body shattered, Blonsky was given a larger dose of the new serum that began to mutate his body and unravel his mind. Now lusting for power to match the Incredible Hulk, Blonsky injected himself with Bruce Banner's gamma-irradiated blood and transformed into a monstrosity called the Abomination. So many poor decisions were made with this iteration of the super soldier serum that it's hard to pin down which contributed the most to its failure.

Related: Why Every Super Soldier After Steve Rogers Has Gone Wrong

The Centipede Serum

Agents of SHIELD Deathlok

The next super soldier serum, which appeared as part of a recurring storyline in Agents of SHIELD, was somehow even messier than the one that created the Hulk and the Abomination. HYDRA was back at it again, this time with the Centipede Project, which combined the elements of the super soldier serum, gamma radiation, Extremis, Inhuman DNA, blood from a pyrokinetic - basically, as Jemma Simmons put it, "Every known source of superpower thrown in a blender." Unlike other versions of the serum, which were designed to be injected only once, the Centipede serum was delivered continuously via a device on the recipient's arm.

The most notable recipient of the Centipede Serum was Deathlok, whose super soldier abilities were enhanced with cybernetics. Deathlok endured great suffering at the hands of HYDRA, but because he was a good man at heart he avoided becoming an outright villain and ultimately sought redemption.

Dr. Wilfred Nagel's Super Soldier Serum

John Walker holding the shield in The Falcon and the Winter Soldier

The Falcon and the Winter Soldier introduces the latest iteration of the super soldier serum, this time developed by scientist Dr. Wilfred Nagel and reverse-engineered from Isaiah Bradley's blood. Nagel first worked for HYDRA, then for the CIA, and then finally for the Power Broker in Madripoor while developing his super soldier serum, which he believed to be superior to Erskine's. The serum doesn't require vita radiation to be stabilized, and doesn't physically change the appearance of the recipient's body. However, it does still appear to have the affect of enhancing a person's fundamental personality - though whether that's a physical effect or a psychological response to suddenly having great power is uncertain.

Karli Morgenthau, the leader of the Flag-Smashers, tried to use her newfound strength, speed and agility for good. She took stockpiled resources from the Global Repatriation Council and distributed them among refugees who were desperately in need of them. However, she also came to believe that violence was justified as a means to an end, and she's shown a willingness to kill people whom she deems guilty by their association with the GRC. Though Karli is one of the more sympathetic recipients of the super soldier serum, she's also let her anger and righteousness lead her to justify acts of terrorism.

The other notable recipient of Nagel's super soldier serum has also fallen prey to his anger and arrogance. One of the first things John Walker did with his newfound strength was brutally execute one of the Flag-Smashers in front of a horrified crowd. Before taking the serum he had some anger problems, but now he is truly unstable and fixated on his belief that he is rightfully Captain America, even after being officially stripped of that title. Perhaps the lesson John Walker teaches is that Sam Wilson doesn't need to be a super soldier to become Captain America.

More: Every Falcon & Winter Soldier Easter Egg In Episode 5

Key Release Dates