Biomedical Engineering Reference
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behavior). Uniformity will occur when viscous forces balance centrifugal forces. The final
thickness of a spin coated film is given by Scriven (1988):
1 3
/
3
2
e
ρ
ρ
η
ρω
h
=
(2.3)
0
0
where
ρ
0
is the initial solvent volume fraction,
ρ
is the solvent volume fraction in the film,
η
is the viscosity of the solution,
ω
is the angular velocity, and
e
is the evaporation rate. This
equation shows that the film thickness can be controlled by adjusting the viscosity and
solids content of the solution and the deposition conditions (spin speed).
Sol-GelSynthesisofNanohydroxyapatite
Nanocrystalline hydroxyapatite can be produced by a number of production methods to
be used as nanocoatings, monolithic solid ceramic products, or as nanosized powders and
platelets for a number of applications (Figure 2.5).
To prepare nanocrystalline apatites, methods of wet chemical precipitation, sol-gel syn-
thesis, coprecipitation, hydrothermal synthesis, mechanochemical synthesis, mechanical
alloying, ball milling, radio frequency induction plasma, vibromilling of bones, flame
spray pyrolysis, liquid-solid-solution synthesis, electrocrystallization, microwave process-
ing, hydrolysis of other calcium orthophosphates, double step stirring, emulsion-based, or
solvothermal syntheses and several other techniques are known. Continuous preparation
procedures are also available. Furthermore, nanodimensional HAp might be manufac-
tured by a laser-induced fragmentation of HAp microparticles in water and in solvent-
containing aqueous solutions, while dense nanocrystalline HAp films might be produced
by radio frequency magnetron sputtering (Dorozhkin 2009). A comparison between the
sol-gel synthesis and wet chemical precipitation technique has been performed and both
methods appear to be suitable for synthesis of nanocrystalline apatite.
Over the past 40 years, synthetic production methods of crystalline monolithic hydroxy-
apatite have been extensive, especially once it was discovered that hydroxyapatite has
nearly the same mineral component as bone and that it can be implemented as a bone
substitute material (Hulbert et al. 1970; Heimke and Griss 1980; LeGeros 1991; Hench 1991;
Liu et al. 2003).
Most published information on hydroxyapatite is classified under calcium phosphate, to
which hydroxyapatite belongs. As a result, the chemical properties will be viewed from the
standpoint that hydroxyapatite is a calcium phosphate, even though it has different reac-
tivity and solubility to other calcium phosphates within the physiological environment.
Calcium phosphates are characterized by particular solubilities, for example when bond-
ing to the surrounding tissue, and their ability to degrade and be replaced by advancing
bone growth. The solubilities of various calcium phosphates can be shown as (LeGeros
1991; LeGeros 1993):
amorphous calcium phosphate (ACP) > dibasic calcium phosphate (DCP) > tetracalcium
phosphate (TTCP) > α-tricalcium phosphate (α-TCP) > β-tricalcium phosphate (β-TCP) >
hydroxyapatite
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