Biomedical Engineering Reference
In-Depth Information
with its preparative medium. They found that electron diffraction
patterns of ACP taken later in the precipitation reaction no longer
were diffuse but resembled patterns of a poorly crystalline CDHA.
Further investigations revealed that this amorphous-to-crystalline
transition was not gradual but occurred rather precipitously.
Initially, there is a period of a relative stability, where surfaces of the
high-contrast spherules generally remain smooth and regular [181].
However, as seen from the data of Table 2.1 [132], some yet unknown
changes occur with the precipitated ACP during this time. Afterward,
the transition follows a sigmoid evolution by the solid phase rapidly
progressing from being barely crystalline to where the amorphous
features disappear. Once the first crystals appear on the surface
of the spherules, the transition proceeds rapidly to completion.
Simultaneously, dramatic declines in ionic concentrations of calcium
and orthophosphate ions occur in the mother solution. The time
it takes to reach this amorphous-to-crystalline boundary various
considerably with the preparation conditions, being particularly
sensitive to temperature and solution pH [68, 130]. For example,
at pH ~ 7.4, ACP converts five times faster at 37°C than at 20°C
[239]. The pH dependency is somewhat more complex than that
for temperature. Namely, at 25°C the aqueous lifetime of freshly
precipitated ACP is less than 0.3 h at pH ~ 7.4. It increases to a
maximum lifetime of over 9 h between pH ~ 10.0 and pH ~ 10.5,
then rapidly decreases until at pH ~ 12.8 the lifetime is nearly as
short as that at pH ~ 7.4 [207, 240]. The solution lifetime of ACP
can be greatly extended by inclusion of simple inorganic ions such as
Mg
2+
2+
, silicates, carbonates, and pyrophosphates [40, 41, 43, 133,
179, 204, 216-219, 241, 242]. As an extreme example, ACP prepared
from Mg
, Zr
2+
-containing solutions at pH = 10.0 and 32.5°C remained
in a gel-like amorphous state for up to 20 weeks when the reactant
Mg/Ca molar ratio was set at 0.2 [179]. Other substances those can
increase the stability of ACP in aqueous solutions include F
−
[239,
240], various polyelectrolytes [67, 68], polyalcohols and polyglycols
[131, 144, 146], phospholipids [243], dentin phosphoprotein
[244], phosvitin [220, 244], glycochenodeoxycholic acid [245],
biomacromolecules such as casein phosphopeptide [69], as well
as adenosine di- and triphosphates (but not the monophosphate)
[246, 247]. On the other hand, excess of Ca
ions in the solution
accelerates the transformation of ACP into a crystalline CDHA [136].
2+
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