Expert: Carolyn Primus - 10/3/2009

Question
What are some common materials used in making artificial heart valves? Which are the most effective in simulating the mechanics of a biological heart valve?

Answer
Several materials are used in heart valves, and several designs have been tested.
Artificial heart valves consist of an orifice, through which blood flows, and a mechanism that closes and opens the orifice. Mechanical valves are of three types based on the opening and closing mechanism: a reciprocating ball, a tilting disk, or two semicircular hinged leaflets. The ball-in-cage design, which uses a rubber ball that oscillates in a metal cage made from a cobalt-chromium alloy. The tilting disk valve uses a circular disk retained by wire-like arms that project into the orifice. When the disk opens, the primary orifice is separated into two unequal orifices. The most current is the hinged leaflet design consists of two semicircular leaflets connected to the orifice housing by a hinge mechanism.

Most artificial valves are made of titanium, graphite, pyrolytic carbon, and polyester. Titanium is used for its strength and biocompatibility. The titanium is used for the housing or outer ring, graphite coated with pyrolytic carbon is used for the bileaflets, and 100% pyrolytic carbon is used for the inner ring. The pyrolytic carbon is sometimes impregnated with tungsten so that the valve can easily be seen by x-rays following implantation). The sewing cuff, used to attach the valve to the heart, is made out of double velour polyester- and sometimes Teflon or Dacron.

Many of the complications associated with mechanical heart valves can be explained through fluid mechanics. For example, thrombus formation is a debilitating side effect of high shear stresses created by the design of the valves. One of the major drawbacks of mechanical heart valves is that patients with these implants require consistent anti-coagulation therapy. Clots formed by red blood cell (RBC) and platelet damage can block up blood vessels and lead to very serious consequences. Clotting occurs in one of three basic pathways: tissue factor exposure, platelet activation, or contact activation by foreign materials.

An ideal heart valve from an engineering perspective would produce minimal pressure drops, have small regurgitation volumes, minimize turbulence, reduce prevalence of high stresses, and not create flow separations in the vicinity of the valve.