Biomedical Engineering Reference
In-Depth Information
Figure 2.12
A proposed mechanism of a CDHA/amino acid biocomposite
from an ACP/amino acid biocomposite. Reprinted from Ref.
[72] with permission.
Having summarized findings of their own and previously
published results by other researchers, Wang et al., [236] have
depicted a following picture of the main events that take place during
the induction period and finally trigger the rapid precipitation of
calcium orthophosphates from supersaturated aqueous solutions:
“Calcium and orthophosphate ions form pairs and clusters
successively in the first few seconds. These ions, pairs, and clusters
then compose the initial solid phase that is heavily hydrated and
contains hydrogenorthophosphates. Growing and aggregating, the
solid increases in size and quantity without affecting the solution
pH. During the induction period, the solid particles exhibit a steady
size distribution, which is around 300 nm at the early stage and
shifts toward 1000 nm with time. These particles are agglomerates
of primary particles of 60-100 nm in diameter and are originally
amorphous in structure. At multiple sites inside a particle, crystalline
domains develop from ion pairs and/or clusters by taking up
calcium and releasing hydrated proton, possibly through a stage
at which the more compact cluster Ca
presents. Since the
expansion of crystalline domains consumes surrounding calcium
and orthophosphate ions (or their pairs and clusters) and releases
hydrated protons, the mechanic strength decreases in the inter-
domain regions. Finally, under the action of the shearing strength of
the fluid, these primary particles collapse and the liberated crystallites
induce the rapid precipitation of calcium orthophosphates, together
with the previously trapped hydrated protons in primary particles,
resulting in the abrupt pH drop. Indeed, it is the crystallization at
multiple sites inside amorphous particles that finally triggered the
rapid precipitation of calcium phosphate from the supersaturated
solution.” (Ref. [236], pp. 2625-2626).
(PO
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