Biomedical Engineering Reference
In-Depth Information
urethane. Polyurethane is one of the most used polymer elastomer for cardiac
devices. Its excellent properties, such as inherent blood compatibility, flexure
endurance and abrasion resistance, made it the most favoured synthetic polymer
for successful cardiac devices (Szycher and Poirier, 1983; Zdrahala and
Zdrahala, 1999). Polyurethanes have been used for over 20 years in medicine
and no records have been found related to its degradation (Pinchuk, 1994).
There are over 30 commercially available biomedical polyurethanes (Lamba
et al., 1998). However, only a few of them have been used for artificial medical
devices, i.e. Biomer (Ethicon, Inc., Somervilee, NJ), Cardiothane (Kontron, Inc.,
Everett, MA), Pellethane (Upjohn Chemical, North Haven, CT) and Tecoflex
(Thermedics Inc., Walthan, MA). Biomer is one of the first biomedical grade
polyurethane which has been used for artificial hearts (Guelcher, 2006).
To test for advantages of Biomer, Poirier (1997a) investigated flexure
endurance evaluation of materials for artificial heart diaphragms. Among seven
different materials, SRI (linear segmented, ether-type urethane), Silastic
(silicone rubber), TecothaneB (2-component polyester, cross-link urethane),
Tecoflex HR (aliphatic, polyether-based, linear-segmented elastomer containing
100% linkage), Hexsyn (polyolefin rubber), Pellethane (thermoplastic polymer)
and Biomer (segmented polyether urethane), Biomer performed longer,
implying suitability for long-term cardiac assist devices.
However, a few drawbacks have been reported related to its functionality
within the artificial hearts and VADs. One of the limitations of polyurethane is
its water permeablility, which may affect the functionality of electrically
actuated blood pumps due to water transfer into the system (Lamba et al., 1998).
A composite of butyl rubber and Biomer has been used as a diaphragm in an
intra-abdominally positioned blood pump (E type ALVAD) by McGee et al.
(1980) in order to prevent water interaction with actuator. Biomer (0.020 inches;
0.5mm)/butyl rubber (0.010 inches; 0.25mm)/Biomer (0.010 inches; 0.25 mm)
composites provide a sufficient barrier to fluid and provide a flexible blood
interface.
The deposition of calcium minerals onto polyurethane is considered to be
another limitation. Calcification is a process of building up calcium minerals on
a material or soft tissue. Pierce et al. (1980) have shown that polyurethane is
often affected by calcification in which calcium phosphate deposits mostly on
the flexion surface, resulting in flexure failure. Perforations and stiffening of
polyurethanes are other effects of these deposits.
Dacron and expanded polytetrafluoroethylene (ePTFE)
Dacron graft, a synthetic material, and expanded polytetrafluoroethylene
(ePTFE) are used for manufacturing inlet and outlet cannula of blood circulatory
devices. Dacron graft and ePTFE cannula have been used in a few devices such
as EvaHeart, ThoratecHeartMate II LVAS, ThoratecHeartMate XVE LVAS,
