Melt-Derived Bioactive Glass - Bio-Glasses: An Introduction - page 22

intermediate oxide in highly disrupted bioactive glasses owing to its
charge-to-size ratio (field strength). When magnesium is substituted for
calcium in bioactive glasses, approximately 14% of the magnesium
forms [MgO 4 ] 2 − units, which, like the Q 0 phosphate units, remove
network modifiers (e.g. Ca 2 + ) from the silicate network, increasing the
connectivity of the silicate network and reducing its solubility and reac-
tivity [14]. Therefore, above a critical concentration, magnesium should
be considered a network former in NC calculations, but that threshold
will depend on the rest of the glass composition.
2.4 IN VITRO STUDIES
Bioglass has been studied extensively in vitro . The glass has been shown
to be bioactive using the SBF test and forms biomimetic nanocrystalline
HCA within a few hours. The SBF test [15] is used extensively in
the literature. However, there are a number of problems with the
test [16], such as how closely the solution actually mimics body fluid
(actually, it is an aqueous solution with ionic concentrations similar
to blood plasma). The cytotoxicity and osteogenic behaviour have also
been assessed in vitro using various cell lines. Osteogenic cells, in
the presence of Bioglass, or its dissolution products, show increased
metabolic activity and produce more bone matrix and mineralised bone
nodules than other bioceramics without the addition of growth factors
or hormones. Cells seem to like the nanotopography and chemistry
of the HCA layer and, perhaps more importantly, they respond to
critical concentrations of soluble silica and calcium ions released by the
glasses. Bioglass has also been shown to up-regulate genes associated
with bone formation, including cMyc -responsive growth-related gene,
cell cycle regulators, apoptosis regulators, cell surface receptors and
extracellular matrix regulators [17]. The effect is dose-dependent. This
is important, as too many ions would raise the pH too high and start
damaging cells. It is a challenge to engineers, surgeons and biologists to
work together to understand exactly how different cells respond to the
different topographies and chemistries of materials.
2.5 IN VIVO STUDIES AND COMMERCIAL PRODUCTS
2.5.1 Animal Studies
Animal studies are needed before new medical devices can be implanted
in people. To minimise the number of animals used and to obtain