The material should be nonirritating to the tissue, be resistant to platelet and thrombus deposition, be nondegradable in the physiological environment, and neither absorb blood constituents nor release foreign substances into the bloodstream. Fixed bovine pericardial tissue is also used to construct heart valves in which design characteristics such as orice area, valve height, and degree of coaptation can be specied and controlled. Bioprosthetic valves are prone to calcification and leaet tear with an average lifetime of about 10 years before replacement is necessary, and is generally attributed to the tissue xation process. Self-expanding stents exert a continuous, radially outward directed force on periluminal tissues, whereas balloon-expandable stents assume a xed diameter that resists recoil of the surrounding periluminal tissues. Balloon-expandable stents are much more versatile when it comes to conforming to irregular vascular morphologies because their diameter can be radially adjusted via balloon dilatation.