Biomedical Engineering Reference
In-Depth Information
2.2 NETWORK CONNECTIVITY AND BIOACTIVITY
The NC of an inorganic glass is the number of bridging oxygens per
[SiO
4
]
4
−
tetrahedron. As this is a measure of the degree of cross-linking
and connectivity of the silica, it is a useful tool for predicting glass
properties from the composition, including bioactivity [9]. NC can be
calculated by the following equation:
BO
−
NBO
G
NC
=
2
+
(2.1)
where BO is the total number of bridging oxygens per network-forming
ion, NBO is the total number of non-bridging oxygens per network
modifier ion and
G
is the total number of glass-forming units. For
an SiO
2
-Na
2
O-CaO-P
2
O
5
system, where P is assumed to enter the
glass network (i.e. formation of P-O-Si bonds), NC can be calculated
as follows:
[(2
×
SiO
2
)
+
(2
×
P
2
O
5
)]
−
[(2
×
Na
2
O)
+
(2
×
CaO)]
=
+
NC
2
[SiO
2
+
(2
×
P
2
O
5
)]
(2.2)
For 45S5 Bioglass (46.13SiO
2
-24.35Na
2
O-26.91CaO-2.6P
2
O
5
mol%),
NC is equal to:
[(2
×
46
.
13)
+
(2
×
2
.
6)]
−
[(2
×
24
.
35)
+
(2
×
26
.
91)]
NC
=
2
+
.
+
×
.
[46
13
(2
2
6)]
=
.
1
90
(2.3)
In melt-derived bioactive glass systems, phosphate typically forms
Q
0
[PO
4
]
3
−
units rather than forming part of the silicate network. This
phosphate complex requires three positive charges from other cations
to charge-balance itself [10]. The lower the NC, the more susceptible a
glass is to degradation, and therefore the more bioactive it is likely to be.
Therefore, the calculation must be modified, and NC can be calculated
as follows:
[2
×
SiO
2
]
−
[(2
×
Na
2
O)
+
(2
×
CaO)]
+
(2
×
P
2
O
5
×
3)
NC
=
2
+
[SiO
2
]
(2.4)