To save his comrades who are infected with Dark Energon, Optimus Prime had his body covered with Energon Defense Coating. This black coating is an alloy Ratchet created by analyzing and converting MECH's equipment, and is capable of shielding Transformers from Dark Energon or any biological weapons such as a virus. Optimus' mobility is reduced due to the added weight, but this is the only form he can guard himself from Dark Energon.
