Hench developed the first bioactive material, Bioglass®, based on a soda-lime phospho-silicate
glass. Most materials, elicit a neutral response when implanted into the human body. Bioglass®,
however, was seen to create a positive response by depositing the body’s natural bone
substance, Hydroxyapatite on its surface. Although it is recognised that compositional
modifications effect bioactivity, there is very little comprehension of the composition-structureproperty
relationships that result in such bioactivity. The objective of this investigation,
therefore, was to study such fundamental relationships with respect to two components often
found in bioactive glass compositions – P2O5 and MgO.
The first component studied was P2O5. The design of two series was undertaken – the first, a
straight substitution of silicon for phosphorus, varying the network connectivity, NC, and the
second, a charge compensating series, keeping the NC constant. 31P and 29Si MAS NMR of the
two series provided evidence that the glasses were phase separated, with a predominantly Q2
silicate structure co-existing with phosphorus in a predominantly Q0 orthophosphate
environment. Raman, FTIR, density measurements, differential thermal analysis and
dilatometric analysis all further supported the existence of this structure. Dissolution studies in
SBF highlighted the importance of phosphorus on bioactivity, with the glass dissolution rates of
both series increasing with the addition of phosphorus. Instead of the dissolution of a glass
depending solely on ion exchange reactions, as previously thought, it is proposed that
dissolution depends upon the balance existing between the NC of the silicate phase and the
existence of isolated orthophosphate rich domains. It is hypothesised that phosphorus in a phase
separated structure is far more important than previously suspected, with its ability to
preferentially dissolve into solution, dominating over the effect of NC on the resultant
bioactivity of the glass and apatite formation.
The second component investigated was magnesium oxide and its influence on the glass
structure when substituted for calcium oxide. Two series of glasses were designed, the first
series with a high sodium content and the second series with a low sodium content. In order to
eliminate any influence due to silicate network disruption, all glasses were designed to have a
constant NC of 2.04. All physical parameters were seen to be related strongly to the
substitution of magnesium oxide, in both series. 31P, 29Si and 25Mg MAS-NMR of the high
sodium magnesium glasses highlighted that magnesium, rather than acting to depolymerise the
silicate network by acting as a network modifier, was acting partially as an intermediate oxide
with a proportion entering the network as MgO4 tetrahedra. The decreasing Tg and Ts values
and increasing thermal expansion coefficients, with increasing MgO substitution, supported this
theory; with the significantly weaker bond strength of Mg-O, compared to Si-O, explaining the
experimentally observed weakening of the network. The corresponding results for the low
sodium magnesium glasses also pointed to magnesium acting as an intermediate oxide,
however, with a smaller proportion entering the silicate network as MgO4 tetrahedra. It is
suggested that magnesium acts as an intermediate oxide in highly disrupted glasses, with a more
disrupted glass giving a higher proportion of MgO4. Dissolution studies in SBF settled the
previously controversial subject of magnesium and bioactivity, with the addition of magnesium
resulting in decreased glass dissolution rates and apatite formation in both series.
This work has highlighted the importance of having a detailed understanding of the
composition-structure-property relationships which exist in a bioactive glass. It is suggested
that, from the contribution this work makes to this understanding, coupled with the knowledge
gained from parallel studies, we are now at the point where a specific bioactive glass
composition could be engineered, and tailored for a particular biomedical application.