That's where his team's engineered ACL comes in. Its main body is made from braided polyester fibers, with a tensile strength similar to that of the natural ligament. At either end of it, however, those fibers are blended into a mixture of a porous antioxidant biomaterial developed previously in his lab, and hydroxyapatite (a form of calcium) nanocrystals ?" these occur naturally in bones and teeth.