Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 5.3
(a) Two tetrahedra (Q
4
species) with a common, bridging, oxygen ion
are shown, along with an Na
2
O molecule. (b) The two tetrahedra are now both Q
3
species, and each has a non-bridging oxygen ion, as the Si-O-Si bond is disrupted
by the addition of Na
2
O.
NBOs, but nevertheless NC has been found to be useful in predicting
some properties, particularly the tendency of a glass toward bioactivity
(see below and Chapter 2).
In addition to the properties of the network formed by the tetrahedra,
other structural considerations include interatomic bond lengths and
bond angles, and the coordination number of the ions, particularly the
cations, but also the oxygen anions. In vitreous silica (without any
network modifiers added), the Si-O bond length has a very narrow
distribution because of the strength of the bond, and is around 1.6
˚
A.
In addition, the O-O bond length does not vary much because of the
tetrahedral shape of the SiO
4
unit, which causes the O-Si-O bond angle
to be fixed at 109.4
◦
, the internal tetrahedral angle. What does vary,
however, is the angle through which adjacent tetrahedra are related, the
Si-O-Si bond angle.
5.2 STRUCTURE OF BIOACTIVE GLASSES
Melt-derived bioactive glasses contain considerable amounts of modi-
fiers: the composition of the eponymous Bioglass
®
is (in mol%) 24.35
Na
2
O, 26.9 CaO, 2.57 P
2
O
5
, and 46.1 SiO
2
. In addition, note the small,
but critical, phosphorus content. The coordination number of phospho-
rus in the glass is four, and it is found in a tetrahedral environment, like
silicon. This makes sense, as phosphate can also form a glass network
