The Neutral-buoyancy gel tank is a piece of human medical technology created in the 26th century. It is a mobile life-support system used to sustain a person's life in case of crippling injury. After their failed augmentation procedures that left their bones so twisted they "no longer looked human," the SPARTAN-IIs René-081 and Kirk-018 had to be placed into neutral-buoyancy gel tanks to keep them alive.