After the Hulk had suffered brain damage upon being shot in the head by the Order of the Shield- the assassin having been carefully trained to target Bruce at just the right part of the brain to incapacitate him without triggering a transformation- Iron Man used the Extremis to cure the Hulk. This procedure also increased Banner's mental capacity, which gave him the intelligence to tweak the Extremis virus within him and unleash a new persona for the Hulk: the super-intelligent Doc Green.