Heart valves are flaps of tissue within the heart that open to allow blood to flow from one of the heart's four chambers to the next and then close to prevent any blood from leaking back. When any one of the heart's four valves becomes too diseased or damaged to function adequately, the only effective treatment is valve replacement. This was not possible until the advent of open-heart surgery in the 1950s. Researchers then set out to design a valve that could be easily implanted and tolerated by surrounding tissue, would not promote clot formation, and would be durable. A precursor of the artificial heart valve was developed by American surgeon Charles A. Hufnagel (1916-), who inserted a tube-and-float device in a patient's descending aorta in 1952, to prevent aortic backflow. An artificial cardiac aortic valve (a caged-ball device) was implanted into a human being...