The Structure of Bioactive Glasses and Their Surfaces - Bio-Glasses: An Introduction - page 70

Figure 5.5 The tetrahedral fragments remaining when the largest fragment is
removed from the structure shown in Figure 5.4. For a better understanding of the
figure, please refer to the colour section (Figure 6).
connected to the SiO 4 tetrahedral network, which (because of the Q 0 and
Q 1 species) is more connected than an NC value of two, that is, a simple
collection of strings of tetrahedra, would imply. The fact that some PO 4
units are not Q 0 means that there must be some P-O-Si bonds, and
this point is somewhat controversial. While solid-state nuclear magnetic
resonance (NMR) experiments can provide relative proportions of Q
units, the proportions are averaged over the sample, giving a binary
distribution of Q n species; the computer simulations indicate a more
diverse distribution, and this is supported by other spectroscopic data.
It should, perhaps, be noted that P-O-Si are relatively rare in other
compounds, but are not unknown.
While we have a reasonable picture of the atomic arrangement in bulk
bioactive glasses, and can correlate, loosely, the degree of fragmentation
of the tetrahedral network with the degree of bioactivity, the actual
structural features that initially control the bio-reactivity are those on
the surface of the biomaterial. Recall Hench's sequence of reaction