Soft Tissue Replacements - Biomaterials - page 193

FIGURE 9.1 A caged-ball heart valve prosthesis. (Courtesy of Baxter Health Care, Irvine, CA.)

years of implantation, it was also found to swell when exposed to humid environments such as autoclav-
ing and blood contact. To avoid design and manufacturing difficulties due to the swelling phenomenon,
the Delrin disk was soon replaced by the pyrolytic carbon disk and has become the preferred material for
mechanical valve prosthesis occluders to date. Pyrolytic carbons are formed in a fluidized bed by pyrolysis
of a gaseous hydrocarbon in the range 1000-2400°C. For biomedical applications, carbon is deposited
onto a preformed polycrystalline graphite substrate at temperatures below 1500°C (low-temperature iso-
tropic pyrolytic carbon, LTI Pyrolite ® ). Increase in strength and wear resistance is obtained by codeposit-
ing silicone (up to 10 wt%) with carbon in applications for heart valve prostheses. The pyrolytic carbon
disks exhibit excellent blood compatibility, as well as wear and fatigue resistance. The guiding struts of
tilting disk valves are made of titanium or Co-Cr alloys ( Haynes 25 ® and Stellite 21). The Co-Cr-based
alloys, along with pure titanium and its alloy (Ti6A14V), exhibit excellent mechanical properties as well as
resistance to corrosion and thrombus deposition. A typical commercially available tilting disk valve with
a pyrolytic carbon disk is shown in Figure 9.2a. A tilting disk valve with the leaflet made of ultra-high-
molecular-weight polyethylene (Chitra valve—Figure 9.2b) is currently marketed in India. The advantages
of lealets with relatively more flexibility compared to pyrolytic carbon leaflets are discussed in Chandran
et al. (1994a). Another new concept in a tilting disk valve design introduced by Reul et al. (1995) has an
S-shaped leaflet with leading and trailing edges being parallel to the direction of blood flow. The housing
for the valve is nozzle-shaped to minimize flow separation at the inlet and energy loss in flow across the
valve. Results from in vitro evaluation and animal implantation have been encouraging.
In the late 1970s, a bileaflet design was introduced for mechanical valve prostheses and several dif-
ferent bileaflet models are being introduced into the market today. The leaflets as well as the housing of
the bileaflet valves are made of pyrolytic carbon, and the bileaflet valves show improved hemodynamic
characteristics, especially in smaller sizes compared to tilting disk valves. A typical bileaflet valve is
shown in Figure 9.3. Design features to improve the hydrodynamic characteristics of the mechanical
valves include the opening angle of the leaflets (Baldwin et al., 1997) as well as having an open-pivot
design in which the pivot area protrudes into the orifice and is exposed to the washing action of flowing
blood (Drogue and Villafana, 1997). Other design modifications to improve the mechanical valve func-
tion include the use of double polyester (Dacron ® ) velour material for the suture ring to encourage rapid
and controlled tissue ingrowth, and mounting the cuff on a rotation ring that surrounds the orifice ring
to protect the cuff mounting mechanism from deeply placed annulus sutures. A PTFE (Teflon) insert in
the cuff provides pliability without excessive drag on the sutures. Tungsten (20 wt%) is incorporated into
the leaflet substrate in order to visualize the leaflet motion in vivo .
Another attempt to design a mechanical valve that mimics the geometry and function of the trileaflet
aortic valve is that of Lapeyre et al. (1994) (Figure 9.4a,b). The geometry of the valve affords true central