Biomedical Engineering Reference
In-Depth Information
because of its shape memory effects (Biehl et al., 2002; Liu et al., 2004). With
respect to its unique properties (Duerig et al., 1999), nickel±titanium is recom-
mended as a potential biomaterial for VADs by Levi et al. (2008). The report
demonstrated that fluid pumps manufactured from thin film NiTi showed higher
power production per unit volume when compared with other small-scale
pumps. NiTi performed better than AISI 316 LVM stainless steel in terms of
resistance to chemical breakdown of passivity (Wever et al., 1998).
Even though titanium and its alloys have excellent biocompatibility, there is
an absolute need for the evaluation of haemocompatibility based on stringent
clinical requirements. Improvement of titanium and its alloys with respect to
haemocompatibility can be successfully achieved with surface coatings (see
Section 8.5) in order to reduce the complications outlined in Section 8.3 (Ebel et
al., 2005).
Reaction of titanium and oxygen is immediate when titanium contacts with
air (Ellingsen and Lyngstadaas, 2003). This reaction forms a 5±6 nm oxide
layer, which is mostly TiO
2
and Ti
2
O
3
on the surface (Kasemo, 1983). Con-
sequently, tissues interface mainly with TiO
2
for titanium implantation and
TiO
2
properties are different from pure Ti as TiO
2
behaves like ceramics
(Ellingsen, 1991). In addition, studies showed that the mole ratio of fibrinogen/
albumin adsorption to oxide layer is very low, which leads to higher
haemocompatibility (Sunny and Sharma, 1991) since thromboses are mainly
related to fibrinogen adsorption (Dion et al., 1993c). Ellingsen (1991) reported
that TiO
2
has a possible biocompatibility because same proteins, e.g. albumin,
prealbumin and IgG, are adsorbed from human serum to TiO
2
as they absorbed
to hydroxyapatite. The albumin proteins binds Ca
2+
ions which behave as a
joint between the negatively charged albumin and TiO
2
surface (Klinger et al.,
1997). Sunny and Sharma (1991) also reported that blood compatibility of
titanium depends slightly on the stable oxide layer on it. Different thickness of
oxide layers (26, 40.8, 126.8 nm) were investigated and the results showed that
oxide layer could increase hydrophobicity of titanium, which causes greater
adsorption of protein (Ellingsen, 1991). Several other authors also indicate an
increase of blood compatibility of titanium with oxide layer (Huang et al.,
1997, 2003).
8.4.3 Ceramics
Ceramics which have been utilized to repair or replace body parts, especially
bone, are usually called bioceramics (Hench, 1991). They are inorganic,
chemically inert, thermodynamically stable hydrophilic, highly wettable, hard
and tribologically excellent (Christel, 1992). Owing to these properties, bio-
ceramics are widely used for many orthopaedic procedures, especially articulat-
ing components of total joint prosthesis and dental applications. In addition, they
have been considered as a biomaterial for blood-contacting applications.
