User blog comment:Mineko Charat Lucky/Touhou LoL - Toyohime, the Technological Moon-Princess/@comment-5980721-20130204174926/@comment-5791947-20130205112153

Femtofiber is made of (probably) malleable lunar ilmenite. 1 rope of femtofiber is made of 2 ilmenite fibers twisted together.

Now, each ilmenite fiber used to make femtofiber is created by 2 smaller ilmenite fibers twisted together. These 2 fibers are made out of 2 even smaller ilmenite fibers. Keep on continuing until each strand of fiber is an atom across.

That's femtofiber.