Biomedical Engineering Reference
In-Depth Information
roughness. Suficiently thick oxide layer restrains corrosion and
helps release harmful compounds. Porous implant layer has lower
density than respective bulk substrate, and good mechanical strength
is provided by bulk substrate. Hence, that material is attractive with
respect to bulk titanium alloys or austenitic steels. The porous
layer on the Ti substrate is necessary for fast osseointegration with
bone [43], which is normally not provided by native oxide. Anodic
oxidation usually proceeds at high DC voltages, for example in
the range of 90-180 V and spark-discharge occurred at voltages
higher than 105 V [109]. On the titanium surface, TiO
2
with anatase
structure was observed at 90 V [109], but for higher voltages 155 V
and 180 V, mixture of anatase and rutile and single rutile phase was
formed, respectively [109].
Generally an unequivocal explanation of pore formation does
not exist. The ield-assisted dissolution has been considered a
predominant mechanism of porous anodic ilm formation [12, 19,
25, 85, 105]. Transportation of speciic ions in oxide is considered
as well for explanation of the pore formation [7]. Transformation
of amorphous phase into crystalline generates stress, resulting
in dissolution of oxides leading to pore formation, too [37]. Most of
the models are true, but only for the speciic processing conditions
is useful for pore formation explanation.
Titania coating can be prepared by a few other methods in
addition to anodization, such as heat treatment, sol-gel, chemical
treatment, and plasma spraying [22, 27, 28]. Rohanizadeh [76] found
that the titanium pretreated in H
2
O
2
solution shows the highest
adhesion to the titanium substrate.
Shih
et al
. [79] investigated the effect of titanium hydride on
the formation of nanoporous TiO
2
on Ti during anodization. They
prepared titanium hydride TiH
2
during cathodization (potentials
negative to the ocp), followed to oxide layer and nanocrystalline
TiO
2
structure after anodization. A multi-nanoporous TiO
2
layer
was formed on the titanium. The titanium hydride nanostructure
is directly changed to nanoporous TiO
2
by a dissolution reaction
during anodization. The nanostructural layer of TiH
2
formed during
cathodization plays a crucial role in forming the nanoporous TiO
2
layer. Anodization treatment with cathodic pretreatment transforms
the titanium surface into a nanostructured titanium oxide surface.
