Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
18.104.22.168 Heart Valve Biomaterials
An example of the successful development of a critical implant technology is the artificial heart valve [260, 6050]. Although poor heart valve designs resulted in clinical failures in the past, by 2002 the limiting factor for long-term success had become the materials themselves. Two types of materials (hard man-made and soft bioprosthetic) were commonly used for artificial heart valves [232, 1147-1152], though a third type – polymer valves  – were also being investigated.
First and most popular (~60% of implants) are the hard man-made materials used in mechanical heart valves. The principal problems with mechanical heart valves are thrombosis , which may be revealed as a thromboembolism, with the formation of a stationary (thrombus) or free (embolus) clot, or hemorrhaging as a result of inappropriately elevated levels of anticoagulation. Graphite coated with pyrolytic carbon (Section 22.214.171.124) has become the material of choice for mechanical heart valves because of its excellent resistance to thrombosis (thromboresistance) [261, 955, 4839]. During 1969-1994, an estimated 2 million components were successfully implanted, resulting in at least ~10 million patient-years of additional life . It has been suggested that the service lives of pyrolytic carbon heart valves may be limited both by cyclical fatigue [263, 940] and by cavitation stress* due to turbulent flow , because cyclic crack growth  is possible in this material . However, double-leaflet pyrolytic carbon valves subjected to accelerated ex vivo wear testing have demonstrated up to 2.1 billion cycles (~52.5 human years) without mechanical failure or loss of functionality , and recent experiments  suggest that isotropic pyrolytic carbons may be fatigue-free in the physiologically relevant stress regimes encountered in contemporary bi-leaflet artificial heart-valve designs, for ~109 cycles.** Other drawbacks include excessive noise, catastrophic failure modes, and the need for lifelong anticoagulant therapy to prevent incidence of embolism (stroke) due to clot formation . A blood pump with diamond-like carbon on all blood-contacting surfaces has been developed as an implantable left ventricular assist system .
* Lin et al  used a high-speed video camera and an ultrasonic monitoring system to observe cavitation and gas bubble release on the inflow valve surfaces of a Medtronic-Hall pyrolytic carbon disk valve in a mock circulatory loop. In the absence of cavitation, no stable gas bubbles were formed, but when gas bubbles were formed, they were first seen a few milliseconds after and in the vicinity of a cavitation collapse. Bubble volume increased with both increased cavitation intensity and increased concentration of CO2 (the most soluble blood gas), which is believed to be the major component of stable gas bubbles because no correlation was observed between O2 concentration and bubble volume.
** Medtronics’ and CarboMedics’ pyrolytic carbon valves have a projected wear-related half-life of 570 years .
The second most common heart valve materials are the soft bioprosthetic materials (~40% of implants) or tissue valves [267, 1147], such as denatured porcine aortic valves , bovine  or autologous [270, 271] pericardium, human aortic valve homografts [272, 273], or tissue engineered biovalves . It is believed that autologous pericardium, being still alive, should not degrade as fast as fixed porcine valves. Bioprosthetic valves , the only option for children, often fail due to calcification [275, 276] (bloodstream calcium forms deposits on the implant), which can result in mechanical dysfunction, vascular obstruction, or embolization of calcific deposits [277, 278]. Bioprosthetic valves may have low thrombogenicity and immunoreactivity but are also susceptible to mechanical fatigue – cyclical valve loading can facilitate fatigue crack growth, often resulting in catastrophic failure . The major unresolved problem with tissue-based heart valves is their limited durability, generally 5-15 years .
A review of several large comparative studies on clinical valve performance finds that valve infection (prosthetic valve endocarditis), nonstructural dysfunction, and overall results after 10 years were about equal for tissue and mechanical valves [1147, 1152].
Last updated on 30 April 2004