An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
St Jude Medical â Trifectaâ¢ Valve. The Trifecta aortic valve replacement is a three-leaflet tissue valve made from a polyester and tissue covered titanium stent. The leaflets are made of pericardial tissue that are connected to the outside of the stent so that they can open more fully than in previous designs.