The Structure of Bioactive Glasses and Their Surfaces - Bio-Glasses: An Introduction - page 68

(Chapter 4). How the PO 4 tetrahedra are incorporated into the silica
tetrahedral network is a matter of discussion, to which we will return
later. Because of the amount of Na and Ca in these glasses, there is a
large concentration of NBOs. Their number is too large to be distributed
simply one to a tetrahedron, and so the structures contain Q 3 ,Q 2 ,
Q 1 and Q 0 units. The PO 4 tetrahedra are particularly prone to having
four NBOs, and, when they do, are called orthophosphate units. These
structural units, along with their SiO 4 counterparts, are believed to play
an important part in the bioactivity of the glasses.
The more NBOs that are present in a glass, the more depolymerized
becomes its tetrahedral network. Bioactive glasses thus tend to be quite
depolymerized, and it has been argued, for example, that a degree of
depolymerization corresponding to an NC of around two is optimum
for bioactivity (larger NC leading to reduced bioactivity). This is related
to the formation of the hydroxyapatite layer on the glass surface,
which requires the (partial) dissolution of the structure, and so is
facilitated when the structure is not completely polymerized, because
fewer Si-O-Si bonds need to be broken (Chapter 2). Recall that an
NC of two corresponds, on average, to a structure containing just
strings of tetrahedra in which each tetrahedron is connected only to two
other tetrahedra; in other words, NC 2 represents a structure that is

adequately depolymerized, or fragmented, to promote hydroxyapatite
formation.

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5.3 COMPUTER MODELING (THEORETICAL
SIMULATION) OF BIOACTIVE GLASSES

Although experimental probes of glass structure are limited in the
information that they can provide, atomistic computer simulations have
played a significant role in developing our current understanding of the
structure of glasses, particularly silicates, including bioactive glasses. For
a recent review, see [2].
Figure 5.4 shows the tetrahedral structure of a simulated bioactive
glass, 45S5 Bioglass ® , when all of the modifier cations (Na and Ca) have
been removed from the picture. It can readily be seen that, in addition to
the isolated orthophosphate groups, there are orthosilicate groups and P
in Q 1 sites, as well as Q 3 and Q 4 Si sites. Naturally, the presence of the
modifier cations, Na and Ca, as well as the NBOs, causes an increase
in the variety of interatomic bond lengths that are needed to describe
the structure. This makes it even more problematic to extract such