Biomedical Engineering Reference
In-Depth Information
improved due to high density and ine grain structure (four-point
bending lexural strength of the new generation alumina has
reached 630 MPa). Further improvement of the fracture toughness
and lexure strength was made in alumina with additions of 17-20
vol.% of zirconia (ZrO 2 ) and some other oxides (e.g., chromium
oxide). Zirconia toughened alumina (ZTA) has lexural strength of
1100 MPa.
The biocompatibility of alumina ceramic has been tested. Takami
et al . investigated Al 2 O 3 ceramic as a blood-contacting material
using a standard assessment in vitro and in vivo analysis [125]. The
examined items were systemic toxicity, sensitization (guinea pig
maximization test), cytotoxicity (elution test), mutagenicity (Ames
test), direct contact hemolysis, and thrombogenicity. These indings
indicate that Al 2 O 3 is biocompatible material for double-pivot
bearings in the centrifugal blood pump.
3 . 3 . 1 . 2 Zirconia
Zirconia ceramics has been used in medical applications since
1985 [35, 36, 107, 108, 134]. Polycrystalline tetragonal zirconia
possesses high fracture toughness and lexural strength due to its
dense and ine grain structure (Table 3.24). Pure tetragonal zirconia
is metastable. Under an inluence of temperature and mechanical
stress, tetragonal zirconia may transform to the monoclinic form.
The phase transformation is associated with volume expansion of
approximately 3-4%. This means that components made of pure
zirconium oxide would burst due to volume increase of grains and
tension. In order to depress possible allotropic transformation of
tetragonal zirconia, it is doped with yttria (about 5.15%). Yttrium
tetragonal zirconia polycrystals (Y-TZP) based systems are the more
recent addition in to the high-strength all-ceramic systems that are
used for crowns and ixed partial dentures [108].
Ta b l e 3 . 24 Mechanical properties of TZP [108]
Property
TZP material
Color
White
Chemical compositions
Zirconium oxide and Yttrium oxide 3
mol%
Hafnium oxide < 2% Aluminum oxide
+ Silicone oxide <1% Total 100%
Density g cm -3
>6
Porosity %
<0.1
( Contd )
 
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