Biomedical Engineering Reference
In-Depth Information
A possible growth mechanism of the nanotubes compared with
the nanoporous structure was proposed by Yu
et al
. [118] and is
presented in Fig. 9.38 (based on the assumptions of Fig. 9.26). For
samples anodized in aqueous HF solutions, a compact oxide layer is
formed at the initial stage of anodization, followed by the random
generation of small pits on it (Fig. 9.38a). Then at steady-state
stage of dissolution (Fig. 9.26), pores grow on the basis of such pits
(Fig. 9.38b) and a porous structure forms. For samples anodized in
F
−
-containing neutral electrolytes (with NH
4
F), the interpore regions
are also attacked by the F
−
ions (Fig. 9.38c). Slits are generated at
these parts and cause the formation of the nanotubes (Fig. 9.38d)
[118].
(a)
(c)
(d)
(b)
Figure 9.38
Schematic diagram of (a) pits formed on the compact oxide,
(b) growth of nanoporous structures, (c) dissolution at the
inter-pore region and (d) formation of discrete nanotubes
[118].
Crawford and Chawla [13] also proposed a comparable, more
evaluated mechanism of surface changes during anodization of Ti
(Fig. 9.39). They fabricated the TiO
2
porous and tubular coatings
by anodic oxidation in 1M H
2
SO
4
+ 0.1M NaF solution. They found
that oxidized ilm can consist of large (1 to 20 μm) pores and small
nanotubes (50 nm diameter). For the bioactive applications, the
nanoporous structure enhances bioactivity and osteoblast function
[41, 67, 68, 90, 114] and microporous structure improve a bone
ingrowth and mechanical ixation [8].
Crawford and Chawla described the evolution of the TiO
2
microstructure in four stages (Fig. 9.39). In the initial stage 1 of
anodization, a large decrease in the current density with time
