Biomedical Engineering Reference
In-Depth Information
premature failure of the valves due to processing issues. These problems were
corrected and a small number of these valves are still implanted today. It is not
uncommon that explants are recovered 30 to 40 years after implantation showing
no degradation, minor wear, and only discoloration of the silicone ball. Many of
the silicones currently used are platinum-cured systems.
Metallic alloys commonly used in mechanical heart valves include austenitic
stainless steels, cobalt±chrome±molybdenum (Co-Cr-Mo), tantalum, and
titanium. Stainless steels are not as widely used for permanent implants because
their passive layer is not as durable as the other implant metals and alloys.
Furthermore, the release of nickel ions can result in nickel hypersensitivity
(Gibbons, 1980). However, 316 LVM (low carbon vacuum melt) is currently
used for endovascular stents and the stented portion of percutaneous valves. This
material provides the needed mechanical and biocompatibility requirements for
these applications.
Titanium alloys are used for heart valves and artificial heart structural
components because of their low density, high strength, low corrosion rate, and
biocompatibility. The Sorin heart valve uses a Ti-6V-4Cr alloy coated with a low
temperature isotropic carbon for the orifice. Percutaneous heart valves also use a
nickel±titanium alloy referred to as Nitinol, which allows self-expansion of a
stent as it remembers its original shape. Finishing techniques for Nitinol are very
important in imparting corrosion resistance and biocompatibility to the material.
Co-Cr alloys have also been used as cast or machined orifices for heart valves.
5.2.3 Advances in mechanical heart valves
The focus is on two approaches:
· Reduction of the mechanical damage to blood elements in the pivot points of
the valve, by reduction of cavitation effects and closing of the valve leafets.
This approach relies heavily on computational fluid dynamics and designs
that demonstrate improved hemodynamics.
· New designs based on polymeric leaflets of biostable polymers of poly-
urethane or new generation materials such as styrene±isobutylene±styrene
(SIBS) block copolymers (Duraiswamy et al., 2009). The key to making this
approach successful is the lack of thrombus and emboli, prevention of leaflet
tearing and prevention of material calcification. These valves would then
have the theoretical advantage of a bioprosthetic valve without the failure
modes of bioprostheses.
5.3
Tissue valves
There are several primary components to consider in the success of tissue
valves, including the tissue, the stent, and how the tissue is processed. Each of
