Biomedical Engineering Reference
In-Depth Information
processes: the first two steps are the exchange of Na
+
with proton
species in the ambient fluid, and the disruption of the glass structure
through hydrolysis (reaction of H
2
O with the tetrahedral network
to form silanol groups, Chapter 2). Thus, in order to appreciate the
bioactivity of glasses, we need to develop an understanding of the nature
of their surfaces.
5.4 GLASS SURFACES
Whereas it is not difficult to visualize the termination of a crystalline
structure, which can be characterized by the Miller indices of the surface
plane, glasses offer no such opportunity. Experimental studies of glass
surfaces have not yet been able to achieve atomic-scale resolution, so
much of what has been learnt is inferred indirectly. On the other hand,
computer simulations can offer an atomic-scale picture, as we will see.
In some respects, terminating the structure of a glass at its surface
may be easier to conceptualize than for a crystal, because one is not con-
strained by considerations of translational periodicity. 'Slicing' through
a bulk glass structure to form a surface will result in some 'dangling'
bonds, which are Si-O bonds that were cut during the slicing process.
Thus, the electrons in an Si-O bond suddenly find themselves without
an oxygen with which to be shared. This is not an energetically favored
state, so Nature will try to rearrange the structure to remove such dis-
rupted bonds, and to restore, where possible, the optimum coordination
environment of the silicon cations. The consequence is that the tetrahe-
dral network structure at the surface will contain tetrahedra connected
in smaller sized rings than in the bulk. In some cases, the tetrahedral
coordination geometry cannot be restored - that is, sometimes it is not
possible for a silicon ion to find four conveniently located oxygen atoms,
and it has to be satisfied with just three. In such a case, the Si will not be
in a tetrahedral geometry, but will form more of a planar coordination
environment. In addition, in pure silica, NBOs may be formed at the
surface, as depicted in Figure 5.6. All of these structural features - two-
and three-membered rings, NBOs, and under-coordinated silicon - are
defect structures, compared to the bulk structure, and are high-energy,
reactive sites (recall that in bulk silica all of the oxygens are bridg-
ing). These are the surface features that will be the first to react with
environmental species, such as water [3, 4].
In polycomponent glasses, the picture that we have of the surface
structure is a bit more complicated, because, for example, they usually
