Biomedical Engineering Reference
In-Depth Information
Figure 2.1
Bioactivity map of compositions in the SiO
2
-Na
2
O-CaO system (6wt%
of P
2
O
5
) showing regions of bioactive response.
and all those in region A bond to bone. Glasses such as window and
container glass would fall in region B and are effectively bio-inert and
implantation would result in scar tissue encapsulation. This is a result
of the silica network being too dense and resistant to reaction with
body fluid. Glasses in region C are the opposite and degrade too rapidly
(
30 days), as the silica content is too low and the soda content too high,
leading to a highly disrupted network. Region D contain compositions
with such low silica that either a glass network cannot form or crystals
form. Glasses within the dashed line (region E within region A) strongly
adhere to collagenous (soft) tissue as well as the inorganic part of bone.
45S5 Bioglass falls within regions A and E.
The reason behind the composition affecting dissolution rate in body
fluid is the connectivity of the silicate network. A highly connected
network containing a large proportion of bridging oxygen bonds (silica
tetrahedra covalently bonded to other silica tetrahedra via -O-Si-O-
bonds) is durable in body fluid. High connectivity is created by high
silica content, such that melt-derived glasses with more than 60mol%
silica are not bioactive. Connectivity is lowered by adding the network-
modifying (network-disrupting) cations such as sodium and calcium.
The network connectivity (NC) (number of bridging oxygen bonds per
silicon atom) of 45S5 Bioglass is approximately 2, making the glass
susceptible to degradation. Chapter 5 discusses the atomic structure of
bioactive glasses in more detail.
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