Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 6.5 (a) Cross-section of a 13-93B3 bioactive borate glass fiber converted to
HCA after six weeks in vivo . (b) Cross-section of a CSZF fiber converted in vivo
for six weeks that formed HA at the outer edge and calcite (white) at the centre.
(Images modified from Ref. [8].)
A new phenomenon in bioactive glass conversion has been discovered
with the bioactive borate glass family. As mentioned earlier, calcium-
containing bioactive glasses react to form HCA when implanted in a
phosphate-containing solution such as body fluids. It has been discovered
that the addition of certain elements can shift the reaction from HCA
to calcium carbonate [9]. This shift may seem undesirable, but in fact
calcium carbonate is a precursor to HCA, and can be dissolved by
osteoclasts at a higher rate than HCA [16]. Theoretically, this means
that the body can remodel the biomaterial into natural tissue at a higher
rate, leaving behind no evidence that an implant was ever there.
Figure 6.5 shows scanning electron microscope (SEM) cross-sections
of two bioactive borate glass fibers that have been implanted in vivo
for six weeks. The undoped 13-93B3 glass (a) formed HCA from the
outer edge to the centre. The CSZF glass (b), which has been doped with
copper, strontium, zinc, and iron, formed a thin (2-3
m) layer of HCA
at the outer edge, but then the remaining material (bright white) was
identified as calcium carbonate in the form of calcite by X-ray diffraction
(XRD) [9].
This phenomenon may be partially explained by the schematic shown
in Figure 6.6. The size of the dopant and the dopant concentration have
been identified in vitro [17] and in vivo [9] to have caused inhibition
of HCA formation. Atoms with ionic radii smaller than
μ
0.8 ˚ A appear
to have moderate to strong inhibition on HCA formation depending
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