fiber is a man-made elastane fiber.
From Wikipedia (en.wikipedia.org/.../Spandex):
Spandex—or elastane—is a synthetic fiber known for its exceptional elasticity. It is stronger and more durable than rubber, its major non-synthetic competitor. It is a polyurethane-polyurea copolymer that was invented in 1959 by DuPont chemist Joseph Shivers. When first introduced, it revolutionized many areas of the clothing industry.
"Spandex" is a generic name and not derived from the chemical name of the fiber, as are most manufactured fibers, but an anagram of the word expands. "Spandex" is the preferred name in North America; elsewhere it is referred to as "elastane".citation needed]
The most famous brand name associated with spandex is Lycra, a trademark of Invista (formerly part of DuPont). Such is the prominence of the Lycra brand that it has become a genericized trademark in many parts of the world, used to describe any kind of spandex. Invista discourages such use, protecting its trademark vigorously.
