Biomedical Engineering Reference
In-Depth Information
In 2001, Cheng et al. (2001) developed a sol-gel method to synthesize a (HAp)/(FA) solid
solution. Calcium nitrate-4 hydrate, phosphoric pentoxide, and trifluoroacetic acid (TFA)
were used as the precursors. Triethanolamine (TEA) was used as a promoter for incorpo-
rating fluorine into Ca phosphates. Mixed ethanol solutions of the Ca and P precursors
in Ca/P ratio of 1.67 with different amounts of TFA and TEA were prepared; the mixed
solutions were dried on a hot plate to convert them to the as-prepared powders. HAp/
FA solid solutions were obtained after the powders were calcined at temperatures up to
900°C.
Cheng et al. (2004) utilized a sol-gel method to synthesize a fluoridated hydroxyapa-
tite (FHAp) phase. Calcium nitrate tetrahydrate, phosphoric pentoxide, and ammonium
hexafluorophosphate were used as precursors. The Ca, P, and F precursors were mixed
under designated proportions to form solutions with a Ca/P ratio of 1.67. In order to obtain
an FHA phase with various fluorine contents, different amounts of ammonium hexafluo-
rophosphate were added in the Ca-P mixed solutions.
The synthesis of HAp nanopowders using a sol-gel route with calcium nitrate and
ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively,
were described by Bose and Saha (2003). Sucrose was used as the template material, and
alumina was added as a dopant to study its effects on particle size and surface area. The
average particle size of porous HAp samples was between 30 and 50 nm.
Hsieh et al. (2002) successfully developed a simple rapid-heating method for calcium
phosphate coatings on Ti-6Al-4V substrates deposited by using a sol-gel derived pre-
cursor. The preparation of the precursor was carried out by mixing Ca(NO
3
)
2
·4H
2
O and
(C
2
H
5
O)
3
PO in 2-methoxy ethanol. Upon aging, the as-prepared solution was closely
capped and placed in an oil bath for 16 h. Upon gelation (drying), the solvent was evapo-
rated in the same oil bath so that a viscous precursor was obtained. Adhesive strength tests
were conducted and the results indicated that, at the first coating layer using either spin
or dip coating, the breakages occur at the glue-coating interface, representing an adhesive
strength higher than 90 MPa. Thus, the first layer is firmly adhered to the substrate. Hsieh
et al. also found that a porous structure, with a pore size of 10 to 20 μm, was formed on the
outermost coating surface. It was reported that this structure is due to the fast decomposi-
tion during rapid heating of the precursor deposited on the substrate, and is very suitable
for ingrowth of living cells. Although this comment is true for cell penetration it does not
allow vascularization, which requires pore sizes of 140 to 500 μm.
Lim et al. (2001) investigated the bioactivities of the coating by analyzing the variation of
ion concentrations of Ca and P in simulated body fluid after soaking, using an inductively
coupled plasma-atomic emission spectrometer. Ti/HAp coating solutions with variable
HAp concentrations were derived from calcium nitrate (Ca(NO
3
)
2
·4H
2
O) and phosphoric
acid (H
3
PO
4
) that were dissolved in ethylene glycol monomethyl ether (CH
3
OCH
2
CH
2
OH).
Coating surfaces, after soaking in simulated body fluid, indicated significant morphologi-
cal changes when investigated by field emission-scanning electron microscopy.
Dicalcium Phosphate Dihydrate
The crystal growth and morphology of the nanosized HAp powders synthesized from
dicalcium phosphate dihydrate (CaHPO
4
·2H
2
O) and CaCO
3
have been investigated by Shin
et al. (2004). The nanosized HAp powders were obtained from the hydrolysis of dicalcium
phosphate dihydrate and CaCO
3
with NaOH. They discovered that the only product syn-
thesized from dicalcium phosphate dihydrate was HAp, and the crystallinity of the HAp
was improved with increasing annealing temperature. The crystallite size of the HAp
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