Biomedical Engineering Reference
In-Depth Information
a coating with 25-nm particle size and 40 μm thick is deposited. It is found that the smaller
the particle size in the powder, the denser is the coatings (Fan et al. 2006). As the coatings
are deposited at a low temperature, poor bonding strength is suspected.
The plasma-sprayed coatings deposited on stainless and titanium alloys are intended
to enhance the bioactivity and tribological performance. In the study conducted by Liu et
al. (2005), an HA layer about 20 μm thick are grown on the nanosized coatings fabricated
by plasma spraying after exposure in simulated body fluids for 4 weeks. Liu et al. (2008b)
have tried to further modify the as-prepared coating by ultraviolet light irradiation and
hydrogen plasma immersion ion implantation. These two treatments increase the amount
of Ti-OH groups that are considered to play an important role in the growth of HA. It is
widely accepted that the atomic coordination on the TiO 2 surface differs from that in the
bulk due to truncation of atomic arrangements on the surface. The perfect surface is con-
structed from 5-coordinated Ti atoms and 3-coordinated O atoms, which are more ener-
getically reactive than the 6-coordinated Ti and 3-coordinated O atoms in the bulk. By UV
illumination, oxygen vacancies are most likely created at the 2-coordinated bridging sites,
resulting in the conversion of the corresponding Ti 4+ sites to Ti 3+ sites. Ti 3+ sites bode well
for dissociation of water that is absorbed from the atmosphere (Hugenschmidt et al. 1994).
This leads to an abundance of Ti-OH groups at the bridging oxygen sites on the surface of
titania coating. The reactions can be explained as follows:
2Ti 4+ + O O + h v →2Ti 3+ + V O + 1/2O 2 ,
Ti 3+ + H 2 O →Ti 4+ − OH + 1/2H 2 .
Sol-Gel Processing
The sol-gel process can be employed to fabricate metal oxide and ceramic materials with
high purity and high homogeneity. This technique allows good control of the composition
and structure at the molecular level. In general, sol-gel processing involves the generation
of a colloidal suspension (sol), which is subsequently converted to various gels and solid
materials. Thin sol-gel coatings can also be obtained by a surface process in which the
sol-gel reaction proceeds only on the surface of substrate to form a monolayer of thin TiO 2
on each circle (Ichinose et al. 1996; Chen 2007; Biju and Jain 2008). The process proceeds
in a sequential manner involving chemisorption to an OH functionalized surface in a
titanium alkoxide solution followed by rinsing, hydrolysis, and drying of the film. A calci-
nation treatment may be needed if a denser, more crystalline coating is desired. This heat
treatment can also remove organic groups from the coating. This process is readily appli-
cable to any hydroxylated surface using metal alkoxide to react with hydroxyl groups. The
sol-gel process can be conducted separately. That is, each different cycle allows the indi-
vidual layer to be nanostructured (Acharya and Kunitake 2003; He et al. 2002; Advincula
et al. 2006). As coatings produced by the sol-gel process are usually prone to cracking
during densification and crystallization, chemicals such as PEG, polyvinyl alcohol (PVA)
(Larbot et al. 1988), and hydroxypropyl-cellulose (HPC) (Lucic and Turkovi 2002) have
been introduced into the colloidal solution to improve the drying properties of the gel,
adjust the viscosity of sol, and increase the strength of the materials to prevent crack for-
mation (Larbot et al. 1988; Maria et al. 2006). The coatings produced by the sol-gel method
have abundant hydroxyl groups and possess the desirable capability of inducing growth
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