Biomedical Engineering Reference
In-Depth Information
surface forms on the TiO
2
. When the hydrogen PIII TiO
2
surface is soaked in SBF, Ti
OH reacts
with the hydroxyl ion in the SBF to produce a negatively charged surface with the functional group
Ti
-
O
-
as follows:
-
OH
-
O
-
+
Ti
-
OH
+
→
Ti
-
H
2
O
(19.3)
Adsorption of ions and molecules on the solid surface is a critical step in crystal growth. The
deposition of calcium ions is the fi rst and most crucial step of apatite nucleation in an ionic solution.
This process is believed to initiate the growth of bone-like apatite on the surface of biocompat-
ible implants [68]. The formation of a negatively charged surface gives rise to apatite precipitation
because positive calcium ions are attracted from the solution [65]. On the hydrogen PIII nano-TiO
2
coating, in addition to hydrogen implantation that leads to a negatively charged surface, the forma-
tion of a nanostructured surface composed of enough small particles is required for the formation
of apatite. It can be demonstrated by thermodynamic analysis that the surface or interfacial tension
diminishes with decreasing particle size as a result of the increase in the potential energy of the bulk
atoms of the particles [69]. Smaller particles with increased molar free energy are more likely to
adsorb molecules or ions per unit area onto their surfaces in order to decrease the total free energy
and to become more stable. The overall effect is a suffi ciently high adsorption coeffi cient on the
nano-TiO
2
surface so that the apatite can be formed.
19.3.2 Ca/Na PIIID
OF
T
ITANIUM
Titanium and its alloys are generally the preferred materials for orthopedic and dental applica-
tions due to their relatively low modulus, excellent fatigue strength, excellent formability, good
machinability, superior biocompatibility, and reasonable corrosion resistance. However, titanium
also has relatively poor wear resistance and bioactivity [70]. Ion implantation has been used to
harden the surface and reduce the friction coeffi cient of titanium in tribological applications, but
the metal surface still requires further modifi cation in order to achieve enhanced bioactivity or
bone conductivity [71].
In protein-free solutions, it is possible to precipitate HA on an activated Ti surface with Ti
-
OH groups. The primary step is the formation of Ti
-
OH groups on the titanium surface. These
Ti-OH groups are considered as nucleation points for calcium phosphate from supersaturated solu-
tions [72,73], but this nucleation does not work in the presence of proteins [74]. The wet chemical
method of Kokubo uses NaOH treatment of titanium for the induction of the Ti
Ti
-
OH groups and
activation of the titanium surface, so that HA precipitates spontaneously from an SBF possessing
an ion composition similar to blood plasma [75,76]. It has also been shown that HA precipitation
after Na beam-line ion implantation follows the same chemistry as the Kokubo method. The Na-
implanted Ti surface forms a sodium titanate layer when oxidized at 600°C in air and sodium
titanate generates a Ti
-
OH hydrogel on the surface in an aqueous solution. The OH groups act as
nucleation points for calcium phosphate precipitation from a supersaturated solution, among them is
the HA [73,77]. Hanawa et al. investigated early bone formation on calcium ion-implanted titanium
inserted into rat tibia. Their results reveal that Ca
2
+
-implanted titanium is superior to unimplanted
titanium from the perspective of bone conduction [78].
As an alternative to conventional ion implantation, there are advantages of using PIIID to pro-
duce the functional titanium surface for better bone conduction. It is a nonline-of-sight process as
opposed to conventional beam-line ion implantation. Therefore, under proper conditions, samples
with a complicated shape can be treated with good conformality and uniformity without the need
to resort to ion beam scanning and special target manipulation. In addition, since PIIID is usually
conducted at low temperature and the target can be cooled, thermal deformation of the specimens
can usually be minimized [62].
-
