Unalloyed pyrolytic carbon for implanted mechanical heart valves

Area of Science:

• Biomaterials Science
• Mechanical Engineering
• Materials Science

Background:

• Implanted heart valves require exceptional mechanical reliability for lifelong function in harsh physiological environments.
• Isotropic pyrolytic carbon (PyC) is a widely used material for heart valve prostheses, necessitating a thorough understanding of its long-term mechanical behavior.

Purpose of the Study:

• To investigate the fatigue strength, crack growth threshold, and fracture mechanics of pyrolytic carbon (PyC) with and without silicon alloying.
• To assess the statistical probability of ultra-high survival under cyclic stress for PyC heart valve materials.

Main Methods:

• Cyclic stressing tests were performed to evaluate fatigue strength at 10^9 cycles.
• Fracture mechanics principles were applied to determine the threshold stress intensity factor for crack growth.
• Static strength tests were conducted after cyclic loading to assess material degradation.

Main Results:

• Cyclic stressing within the static strength scatter band did not initiate cracks in PyC.
• Artificially induced cracks remained stable below a determined threshold stress intensity factor, valid across various crack lengths.
• No failures were observed in specimens tested above service stress for 6 x 10^8 cycles, and static strength remained unaffected by cyclic loading.

Conclusions:

• PyC exhibits excellent fatigue resistance and a critical threshold for crack growth, ensuring reliability for implanted heart valves.
• The service stress is significantly lower than the fracture strength, allowing for high survival probabilities through proof testing.