Vein vs. Symthetic
Typically natural bypass grafts are your own vein or artery.
It is also possible to use cryopreserved vein or artery from a cadaver (less common but an option). Synthetic grafts are made either or gore-tex /ptfe or woven polyester (dacron). The natural grafts are less likely to clot when there is poor circulation and they are less prone to infection.