Nitramene is a chemical compound developed by Howard Stark after WWII.
While Felix Blake was an agent of S.H.I.E.L.D., he experimented with the nitramene formula and found a way to use it as a weapon. After he became leader of the Watchdogs, he created nitramene bullets for them.
In "Watchdogs", the Watchdogs use nitramene to implode an ATCU storage facility and later used the nitramene bullets again in a shootout with S.H.I.E.L.D. During the fight, Fitz is shot in the neck by one of the bullets, but is able to slow down the time to implosion with sodium-hydrogen-acetate. Daisy is then able to stop the nitramene from imploding by freezing it with liquid nitrogen.
Causes any object that comes in contact with it to implode in on itself. However, the implosion can be prevented with sodium-hydrogen-acetate.