In the Batman: Assault on Arkham animated movie, they had explosives planted in their necks. In the movie, they drink nano-bots, but Flag says that his head will explode. Are the nano-bots a lie, and they surgically implanted something in their neck instead?
Evidence suggests The Joker is using the CPR electric pads to remove one, when he says "it really, really hurts" (paraphrasing).
Or do the nano-bots grow into an explosive and move to the head?