Elastomers for Use in Heart Valves

Approximately 0.1 per cent of silicone rubber balls from the estimated 18,000 implanted Starr-Edwards heart valve prostheses have been found to be variant. This material variation has initiated investigations of present material and accelerated the search for new and improved elastomers. Descriptions of various types of artificial heart valve prostheses, environmental conditions, design requirements and test apparatus are presented. Variant balls exhibit external physical damage by surface grooves and splits. Internal changes and discolorations from absorbed lipid materials have also been observed. Variant silicone rubber balls have poor physical properties compared with nonvariant or non-implanted balls. This reduction lowers the resistance to damage. The engineering laboratory evaluation program of elastomeric materials includes the compilation of test data on balls molded from silicone, butyl, ethylene propylene, polybutadiene and chlorohydrin rubbers. Butyl and polybutadiene rubbers were selected to demonstrate the extremes of the resilience spectrum with respect to the hydraulic performance of the ball type prosthesis. Compounds with low resilience for less ball rebound appear to be advantageous in this application To date, butyl rubber has exhibited excellent wear resistance in laboratory durability testers.