Biomedical Engineering Reference
In-Depth Information
A
A natural tissue heart valve
B
A synthetic heart valve
FIGURE 5.17
Two types of artificial valves that are used to replace diseased or malformed human valves.
(a) The tissue valve generally contains valve leaflets from pigs, while (b) the synthetic valve is composed entirely
of manufactured materials.
mechanism, such as a cage, that constrains the occluder when the valve is open; and a
sewing ring that permits attachment of the valve to the heart. The occluder bounces back
and forth from the seating ring to the capture mechanism with each heartbeat. The more
promptly it moves, the more efficient the opening and closing. Thus, weight, or mass, of
the occluder and wear resistance of the occluder material are critical features. Early materi-
als that were used included lightweight plastics or hollow metal balls. Silicone rubber
proved very effective as a ball. The advent of low-profile disc valves for the mitral position
set a new material constraint—namely, stiffness. Silicone rubber was too soft for the disc
designs. Polyoxymethylene (POM) or polyacetal is stronger and stiffer than silicone rubber
and therefore was used in early disc designs. However, it had a problem with wear. The
discs were supposed to be free to spin in the cage so they could distribute wear evenly
around the edge of the disc. However, as the disc moved up and down in the cage, wear
tracks developed on the edge, preventing spinning and leading to the development of deep
wear tracks and valve failure. Due to its high fatigue strength and wear resistance, pyrolytic
carbon was selected as one of the prime materials for the occluder.
The capture mechanism and seating ring required strength and stiffness to maintain their
shape. Furthermore, they had to be made of a material that could be sprung open for inser-
tion of the occluders. Metal has typically been used, either as machined parts or as separate
parts that are welded together. Early designs used the cobalt alloys due to their strength
and corrosion resistance. More recently, titanium alloys have been used, due in part to their
being lighter than cobalt alloys. Concerns of allergic reactions to cobalt alloy cages have also
been expressed over the years.
