Biomedical Engineering Reference
In-Depth Information
BerlinHeart and Novacor as listed in Tables 8.1 and 8.2. High crystallinity and
hydrophobicity are the advantages of both materials. These two characteristics
of ePTFE and Dacron ensure the prevention of hydrolysis (Xue and Greisler,
2003). Diameters of more than 6 mm of both materials provide optimum
functional performance. Clinical grade Decron is usually available in woven or
knitted form. Outflow graft bend relief is a polyester graft to avoid twisting and
abrasion and its life cycle is about 10 years without corrosion.
HeartMate VE LVAS, a commercially available assist device includes
Decron for its inflow and outflow conduits. Improvements on reliability and
durability have been achieved by changing the materials of percutaneous-lead
connection of external controller to internal pump, of the HeartMate VE LVAS
(Dowling et al., 2004). Materials with improved percutaneous-lead include high
flex cadmium±copper alloy, high strength silicone, hardenable stainless steel,
fibres, and polycarbonate-based polyurethane and two independent coil springs.
Vent adapter is made of acetal.
Ultrahigh molecular weight polyethylene (UHMWPE)
Ultrahigh molecular weight polyethylene (UHMWPE) is used as a female pivot
for pivot-bearing system of centrifugal pump (Takami et al., 1997). The male
pivot of the system is high grade alumina (Al
2
O
3
) ceramic with a purity of
99.7%. Even though a combination of these two materials is usually used for
joint implants for long-term biomedical devices, they have proven useful as a
blood-contacting surface for centrifugal pumps as well. Al
2
O
3
ceramic counter
bearing male-pivot decreases wear of UHMWPE.
Synthetic materials have a propensity of increasing the possibility of stroke in
humans (Lamba et al., 1998). For example, polyurethane has a limitation due to
occlusion of a blood vessel as mentioned below.
8.4.2 Metals
Titanium
Titanium has been widely used as a material for implant devices since the late
1930s. It has been a preferred metallic biomaterial for the investigation of
human tissue±biomaterial interactions (Kasemo, 1983). Mechanochemical
properties of titanium such as lightness, corrosion resistance by the oxide layer,
strength and inertness and that it is not magnetic made it the mostly used metal
for cardiac devices. There are four grades of titanium, which are graded based
on the percentage of impurities such as nitrogen, carbon, hydrogen, iron, oxygen
(Park and Lakes, 1992). For medical implants especially for VADs, extra low
interstitial alloy is generally used (i.e. Ti6-Al4-V ELI) and the main elements of
the alloy are aluminium (5.5 to 6.75% (m/m)) and vanadium (3.5 to 4.5% (m/m)).
