Biomedical Engineering Reference
In-Depth Information
2. Electrochemical anodization
In general, the mechanism of TiO
2
nanotube formation in fluorine-ion based electrolytes is
said to occur as a result of three simultaneous processes: the field assisted oxidation of Ti
metal to form titanium dioxide, the field assisted dissolution of Ti metal ions in the
electrolyte, and the chemical dissolution of Ti and TiO
2
due to etching by fluoride ions,
which is enhanced by the presence of H
+
ions (Shankar et al. 2007). TiO
2
nanotubes are not
formed on the pure Ti surface but on the thin TiO
2
oxide layer naturally present on the Ti
surface. Therefore, the mechanism of TiO
2
nanotubes formation is related to oxidation and
dissolution kinetics. Schematic diagram of the formation of TiO
2
nanotubes by anodization
process is shown in Figure 1. For a description of the process displayed in Figure 1, the
anodization mechanism for creating the nanotube structure is as follows:
a.
Before anodization, a nano scale TiO
2
passivation layer is on the Ti surface.
b.
When constant voltage is applied, a pit is formed on the TiO2 layer.
c.
As anodization time increases, the pit grows longer and larger, and then it becomes a
nanopore.
d.
Nanopores and small pits undergo continuous barrier layer formation. (e) After specific
anodization time, completely developed nanotubes are formed on the Ti surface.
Fig. 1. Schematic illustration of TiO
2
nanotube formation.
Furthermore, based on the mechanism of nanotube formation, it is inherent that the nano-
tubular structure formation depends on both the intensity of applied voltage and the
concentration of fluorine ions in the electrolyte solution. It is well-known that by increasing
the applied voltage, larger diameter nanotubes can be formed. This aspect of diameter
manipulation using applied voltage will be further emphasized and the effects on cell
function and fate is also discussed.
3. Nanotube size effects
The Jin lab was the first to demonstrate that TiO
2
nanotubes can significantly accelerate
osteoblast (bone cell) adhesion and proliferation at the biomaterial/tissue interface and
enhance bone mineral formation. The TiO
2
nanotubes are formed as vertically aligned
configuration, with an average diameter of ~100 nm, a height of ~300 nm, and a wall
thickness of ~10 nm. According to published research (Oh S 2006), nanotube arrays on
titanium surfaces induced proliferation of osteoblasts by as much as 300 - 400% compared
to non-modified titanium surfaces. In other research groups studying nanoporous materials,
