Biomedical Engineering Reference
In-Depth Information
The observation in Fig. 7.13 a can be understood as follows: the occurrence of
daughter crystals on the parent crystals is via self-epitaxial nucleation. This belongs
to a special type of 3D nucleation, and there is a 3D nucleation barrier
.G mis /
asso-
ciated with it. For such a process, suppose there are two events of the self-epitaxial
nucleation occurring at the same site, one with a poor structural match
.G mis /
and
.G mis D f match .m/G mis ;f match .m/ 1/
asshowninFig. 7.13 b. Due to the high nucleation barrier at low supersaturations,
it is very difficult for the mismatch epitaxial nucleation to occur as
the other with a good structural match
G mis is
very high (i.e., ( 7.16 ) and Fig. 7.13 b). Under such conditions, only the epitaxial
nucleation with a good structural match may occur, as the good structural match
substantially lowers the nucleation barrier (i.e., f match ( m )
1, Fig. 7.13 b). One has
the formation of the ordered calcite crystallite assembly observed in Fig. 7.13 a.
As the supersaturation increases, the nucleation barrier for mismatch epitaxial
nucleation
G mis drops rapidly (Fig. 7.13 b). At relatively high supersaturations, the
requirement of adopting the good structural match between the daughter crystals
and the substrates becomes less demanded, in the view of a low nucleation barrier
G mis . Due to the shadow effect, the mismatch self-epitaxial nucleation becomes
kinetically more favorable. The shadow region in Fig. 7.13 b illustrates the region
the mismatch self-epitaxial nucleation may occur. This facilitates self-epitaxial
nucleation leading to formation of the assembly of HAP crystallites with small
mismatch (see Fig. 7.13 a,b). As the supersaturation increases further, the nucleation
barrier for mismatch epitaxial nucleation totally collapses. Self-epitaxial nucleation
occurs much more easily, resulting in a severe interfacial structural mismatch. In
this case, the crystallite assembly will often be randomly and highly branched
(see Fig. 7.13 a,b). This causes the supersaturation-driven interfacial structural
mismatch .
As indicated above, as
progressively increases from low supersaturations to
high supersaturations, the self-epitaxial nucleation will be governed by a sequence
of progressive heterogeneous processes associated with increasing f ( m ). In analogy
with the above analysis, the biomineralization of HAP in simulated body fluids
was carried out [ 84 ]. Correspondingly, we should obtain a set of pairwise inter-
secting straight lines if ln t s is plotted against 1/[ln(1
)] 2 ( t s : induction time; cf.
Fig. 7.13 c,d). Since for the crystalline phase, m ,and f ( m ) take on only those values
that correspond to some crystallographically preferential orientations, f ( m )orthe
slope of the straight lines will take on discrete values, and f ( m ) will increase as
C ยข
increases.
As shown in Fig. 7.13 c and d, the interfacial correlation factor f ( m , R 0 ) subse-
quently increases from 3.9/
, as supersaturation increases from 1.5 to 5.
This result unambiguously confirms that the increase in supersaturation will drive
the substrates/biominerals from an interfacial structural match state (a lower f ( m ))
to an interfacial structure mismatch state (a higher f ( m )). The abrupt changes from
one state to the other at certain supersaturations (such as A, B,
to 12.5/
:::
in Fig. 7.13 d) are
due to the anisotropy of the crystalline phase.
To explore the effects of the self-epitaxial nucleation-induced assembly and
the supersaturation-driven interfacial structural mismatch at the single particle
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