Biomedical Engineering Reference
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nanohydroxyapatite (HAp)-polyvinyl alcohol (PVA). The hybrid was prepared from phos-
phoric acid solution and saturated calcium hydroxide solution, while the PVA was dis-
solved in distilled water. They discovered that mixing a suitable quantity of PVA in HAp
sol can prevent the HAp coating from cracking and enhance its stability. The properties of
the nano-HAp-PVA hybrid were affected by the concentration of PVA.
HAp and tricalcium phosphate (TCP) powders and coatings were synthesized by
Tkalcec et al. (2001) via the sol-gel technique to examine the formation mechanism of
crystalline phases in the firing processes of coatings with different Ca/P molar ratios.
Calcium hydroxide was suspended in ethanol and ethylhexanoic acid (EHA) was added
dropwise to this suspension. The solution was then filtered by pressure filtering to obtain
a clear solution of calcium 2-ethylhexanoate (Ca(O
2
C
8
H
15
)
2
). Calcium 2-ethylhexanoate and
2-ethyl-hexyl-phosphate were used as calcium and phosphorus precursors, respectively.
Coatings were deposited on Si-wafer and Ti-alloy substrates by dipping the substrates
into sols at room temperature. The results from the work showed that dip-coating and
sintering in two cycles yielded a homogeneous and dense coated film with a thickness of
250 nm.
Calcium Nitrate
Calcium nitrate (Ca(NO
3
)
2
) has been a popular calcium precursor for the synthesis of
hydroxyapatite.
The citric acid sol-gel combustion method has been employed by Han et al. (2004) for the
synthesis of nanocrystalline HAp powders from calcium nitrate, diammonium hydrogen
phosphate, and citric acid. HAp powder was used to sinter a monolithic ceramic product in
order to illustrate its sinterability, open porosity, flexural strength, and structural rigidity.
Microscopy revealed that there were many pores in the micropore sizes ranging between
1 and 5 μm of irregular shape. Although the open porosity of the resulting ceramic was
about 19%, the pore size is not good for bone ingrowth. After sintering at 1200°C, the grain
size is about 3 μm.
Weng et al. (2002) also examined the effect on the addition of citric acid on the forma-
tion of sol-gel derived HAp. In order to improve the gelation in the sol-gel preparation of
HAp by using Ca(NO
3
)
2
and PO(OH)
3−
x
(OEt)
x
as precursors, citric acid was selected as an
enhancing gelation additive. HAp derived from the mixed precursor solutions with citric
acid showed a different reaction path from that without citric acid. They suggest that citric
acid plays a role in enhancing gelation through the strong coordination ability of Ca ions
with citrate groups. They have also concluded that the addition of citric acid can provide
a way to synthesize a pure oxy-HAp or an apatite with carbonated HAp, HAp, and minor
β-TCP, which might have good bioactivity.
Balamurugan et al. (2006) developed a different sol-gel technique for the synthesis phase
of pure HAp powder. Triethyl phosphite and calcium nitrate were used as phosphorus and
calcium precursors. The powders obtained were dried and calcined at different tempera-
tures up to 900°C. X-ray diffraction analysis and Raman spectra were reported to show the
presence of pure HAp.
Kim et al. (2005) coated hydroxyapatite composites with titania (TiO
2
) on titanium (Ti)
substrate by a sol-gel route, and the mechanical and biological properties of the coating
systems were examined. Calcium nitrate tetrahydrate (Ca(NO
3
)
2
·4H
2
O) and triethyl phos-
phite (P(OC
2
H
5
)
3
) were hydrolyzed together with ethanol and distilled water. Ammonium
hydroxide (NH
4
OH) was added stepwise to the mixture. To produce a TiO
2
sol, tita-
nium propoxide (Ti(OCH
2
CH
2
CH
3
)
4
) was hydrolyzed within an ethanol-based solution
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