Neutronium-alloy fiber
The neutronium-alloy fiber was a type of material, made from neutronium alloy, that was used in the construction of 31st century Federation starships. This material was theoretical in the 2250s.
In 3189, the USS Discovery detected ships in the fleet stationed at Federation Headquarters with neutronium-alloy fibers in their hulls. (DIS: "Die Trying")