Biomedical Engineering Reference
In-Depth Information
4-ring
3-ring
4-ring
2-ring
NBO
Figure 5.6
Structure of simulated surface of vitreous silica. Various structural
features are identified, which are generally not seen in the bulk glass structure and
are therefore considered to be defects.
already contain NBOs, so their appearance on the surface is no surprise.
What is known is that there is an excess of sodium at the surface, with
respect to the bulk, and this is the case, too, with bioactive glasses [5].
Also, the surface contains more smaller sized rings, particularly two-
and three-membered rings, as well as under-coordinated silicon, as was
found for pure silica, described in the previous paragraph. However,
the fraction of two- and three-membered rings is lower in the more bio-
reactive glasses than in the less bio-reactive glasses, because the larger
amounts of modifiers present in the former allow the surface structure to
relax more to bulk-like configurations than the higher silica-containing
less bioactive glasses [6].
In addition, for bioactive glasses, the surface concentration of Na
+
seems to be greater than for bio-inert glasses, though this is primarily
because of the higher bulk concentration of Na in the more bioactive
glasses. There is an association between the Na
+
and the surface NBOs
to form Na
+
ยทยทยท
NBO pairs, sites that promote the dissociation of water
through protonation of the NBOs. These sites, along with the under-
coordinated silicon, which also promote protonation of the NBOs,
provide for a more hydrophilic surface for the more bioactive glasses.
