Biomedical Engineering Reference
In-Depth Information
thrombosis. This requires permanent anticoagulation, typically with warfarin
(Coumadin) for the rest of the patient's life. Tissue integration is required at the
sewing cuff and fabrics of polyethylene terephthalate (PET) or polytetra-
fluoroethylene (PTFE) are commonly employed. The sewing cuff may be a site
for thrombosis and bacterial attachment and growth, so it must promote rapid
tissue integration and endothelial cell coverage without extensive thrombus and
platelet attachment.
5.2.1 Design considerations
A detailed list of heart valve design criteria is contained in Table 5.2. The
hemodynamics of the mechanical heart valve is extraordinarily critical. If
regions of low flow exist around some areas of the valve, thrombotic com-
plications are likely. The pivot point (the region where the leaflet's pivot
protrusion rests within the ring support's pivot cavity) may be such a region of
low flow, which is poorly washed by the incoming blood. Thrombosis may
result and this region may become a nidus for calcification, which may restrict
the leaflet's movement. Areas of turbulent flow, as well as the cavitation that
results when the valve opens and closes, can cause platelet and cellular
destruction, with potential thrombosis and embolization. Thus, the materials
requirements and the design of the device are intimately connected.
5.2.2 Mechanical heart valve materials
Pyrolytic carbon is a glassy carbon, deposited at high temperatures by pyrolysis
of a carbon gas such as acetylene, used as heart valve components, particularly
as pyrolytic carbon in the leaflets and housings of mechanical valves. These
materials demonstrate good biocompatibility and thromboresistance, as well as
high lubricity and resistance to wear, in this application. Graphite is used as the
substrate for many of the pyrolytic carbon coatings. Typically the pyrolytic
carbon is deposited on the graphite substrate and then machined and polished to
the final shape. Conversely, the orifice can be deposited on a graphite core that
is machined out, so that the orifice has no graphite substrate. Strength and
durability are imparted by the pyrolytic coatings. The use of a graphite substrate
can reduce residual stresses that become significant in thick pyrolytic coatings.
The substrate has the potential to act as a barrier to crack propagation within the
pyrolytic coating. Low-temperature isotropic coatings (LTI) can be used to coat
more heat-sensitive polymeric substrates such as the heart valve sewing cuff.
Silicone is a rubber-like polymer. It is normally crosslinked in a mold or
during extrusion. The most common silicone used is room temperature vulcan-
izing (or RTV) silicone. The first commercially-viable heart valve, the Starr±
Edwards valve, used a peroxide heat-cured silicone ball in a cage. However,
when first used,
these silicone balls absorbed lipids and swelled, causing
