Biomedical Engineering Reference
In-Depth Information
surface and migration of bone cells over the implant surfaces [Steinmann,
1980 ].
During implantation, titanium releases corrosion products—mainly titanium
oxide or titanium hydro-oxide—into the surrounding tissue and fl uids even
though it is covered by a thermodynamically stable oxide fi lm [Ferguson et al.,
1962; Meachim et al., 1973; Ducheyne et al., 1984]. An increase in oxide thickness,
as well as incorporation of elements from the extra-cellular fl uid (P, Ca, and S)
into the oxide, has been observed as a function of implantation time [Sundgren
et al., 1986]. Moreover, changes in the oxide stoichiometry, composition, and
thickness have been associated with the release of titanium corrosion products
in vitro
[Ducheyne et al., 1988]. Properties of the oxide, such as stoichiometry,
defect density, crystal structure and orientation, surface defects, and impurities
were suggested as factors determining biological performance [Fraker et al.,
1973; Albrektsson et al., 1983; Albrektsson et al., 1986].
The performance of titanium and its alloys in surgical implant applications
can be evaluated with respect to their biocompatibility and capability to with-
stand the corrosive species involved in fl uids within the human body [Solar, 1979].
This may be considered as an electrolyte in an electrochemical reaction. It is well
documented that the excellent corrosion resistance of titanium materials is due
to the formation of a dense, protective, and strongly-adhered fi lm, called a passive
fi lm. Such a surface situation is referred to as passivity or a passivation state. The
exact composition and structure of the passive fi lm covering titanium and its
alloys is controversial. This is the case not only for the “natural” air oxide, but
also for fi lms formed during exposure to various solutions, as well as those formed
anodically. The “ natural ” oxide fi lm on titanium ranges in thickness from 2-to-
7 nm, depending on such parameters as the composition of the metal and sur-
rounding medium, the maximum temperature reached during the working of the
metal, the surface fi nish, and so on.
The excellent corrosion resistance associated with titanium materials is due
to the stability of surface titanium oxide fi lms. If such oxide fi lms possess protec-
tiveness against the hostile environments, strong adherence to the Ti substrate,
and dense structure, they are said to be passive fi lm. To form such passive fi lms,
chemical or electrochemical treatment is normally conducted. Oxides formed on
Ti materials are varied with a general form; TiO
X
(1
<
x
<
2). Depending on
x
values, there are fi ve different crystalline oxides, including:
(1) cubic TiO (a
o
= 4.24 Å ),
(2) hexagonal Ti
2
O
3
(a
o
= 5.37 Å , ( = 56 ° 48
),
(3) tetragonal TiO
2
(anatase) (a
o
= 3.78 Å , c
o
= 9.50 Å ),
(4) tetragonal TiO
2
(rutile) (a
o
= 4.58 Å , c
o
= 2.98 Å ), and
(5) orthorhombic TiO
2
(brookite) (a
o
= 9.17 Å , b
o
= 5.43 Å , c
o
= 5.13 Å ).
′
Besides these, there are non-stoichiometric oxide (when
x
is not integral)
and amorphous oxides. It is widely believed that, among these oxides, only rutile
Search WWH ::
Custom Search