Biomedical Engineering Reference
In-Depth Information
will subsequently dissolve even faster, as its degradation rate increases
with decreasing pH. This process is commonly known as autocatalysis.
It is important to note that, while a buffered system can control pH
changes to some extent, local pH changes near the glass surface can
be quite dramatic (even in buffered systems such as
in vivo
), resulting
in rapid degradation and failure of the implant. Fluid flow around an
implant is therefore an important factor, as fluid flow would remove the
cations from the implant surface, reducing the autocatalytic effect.
Apart from the phosphate content and the pH of the surrounding
solution, the charge-to-size ratio of the modifier cation was also found
to affect phosphate glass solubility. As described above, an increase
in the charge-to-size ratio results in more effective cross-linking of
the phosphate units, and the stronger this cross-linking, the lower the
glass solubility. The solubility of phosphate glasses is therefore directly
related to their composition. If, in a phosphate glass in the system
P
2
O
5
-CaO-Na
2
O, calcium is gradually replaced by sodium, that is, if
the alkali content increases, the solubility increases and the durability
decreases [9].
Glass solubility also increases with increasing ionic radius by constant
charge, as in the series Li
+
K
+
[8]. The effect of the
charge-to-size ratio also explains why TiO
2
was found to significantly
decrease the solubility of phosphate glasses [10]: the Ti
4
+
cation very
effectively cross-links the phosphate units, and therefore the solubility
decreases if TiO
2
is incorporated into a phosphate glass (Figure 4.15).
Na
+
→
→
4.6 CELL COMPATIBILITY OF GLASSES
Cell tests on phosphate glasses show a strong relationship to glass
dissolution rate, pH and ion release, and controlling glass solubility and
degradation seems to be critical in order to design phosphate glasses for
use as biomaterials.
If, in a phosphate glass in the system P
2
O
5
-CaO-Na
2
O, sodium is
gradually replaced by calcium, cell attachment and cell proliferation
increase, which is due to a reduction in glass solubility by increased ionic
cross-linking through Ca
2
+
compared to Na
+
ions. If glass solubility is
very high, due to very high Na
2
O contents, cell growth can be inhibited
[9]. Cell compatibility can therefore be improved by improving the
durability of the glasses.
TiO
2
, which very effectively controls the solubility of phosphate
glasses, can modulate cell adhesion and proliferation. Cells cultured
on TiO
2
-containing metaphosphate glasses show enhanced proliferation
