Biomedical Engineering Reference
In-Depth Information
responsible for producing new bone, resulting in loss of bone density.
Strontium incorporation in sol-gel glasses is an effective way to deliver
a steady supply of strontium ions to a bone defect site [14].
A further advantage is that melt-quenched glasses cannot be bioactive
if they have a silica content in excess of 60mol%. For sol-gel glasses
this is extended to 90mol% [15]. The reason for this is the nanoporous
network and the connectivity of the silica network. Owing to the wet
nature of the process and the way the nanoparticles assemble, nanopores
are left behind and the silica network is disrupted by protons (H
+
ions).
The protons act as additional network modifiers. This means that the
70S30C composition is not only 70mol% SiO
2
and 30mol% CaO,
but it also contains OH groups. Thus the network connectivity of the
glass is lower than it would be for a melt-derived glass. The amount of
OH groups decreases as the thermal processing temperature increases
and therefore the real composition of the glass changes with thermal
processing. The nanopores also cause the sol-gel glasses to have a surface
area two orders of magnitude higher than that for similar melt-quenched
compositions, which increases the degradation rate and therefore the
rate of formation of the HCA layer that forms a bond to the apatite
in bone. Tailoring the nanoporosity also provides an easy route to
controlling the degradation rate of the glasses.
The addition of calcium to a silica sol-gel glass reduces the network
connectivity of the glass and also increases the nanopore size. Calcium
nitrate is the most common precursor for introducing calcium into a
silica glass, as it is very soluble in the sol. However, calcium does not
automatically incorporate into the silica network at room temperature.
The calcium only enters the network when the temperature reaches
400
◦
C. This has an impact on low-temperature synthesis, such as that
used for hybrid synthesis (Chapter 10).
A great benefit of the sol-gel process is that the process can be modified
to produce porous scaffolds for bone repair or tissue engineering appli-
cations, either by foaming the sol (Figure 3.7), which produces foam-like
scaffolds with pore structures similar to porous bone [16], or by feeding
the sol into an electrospinning system to create fine fibrous membranes
(Figure 3.8). Chapter 12 gives further details on these methods.
It is not only silica glasses that can be produced by the sol-gel process.
Amorphous titania [17] and phosphate glasses [18] can also be produced
by the sol-gel process. However, the titania is usually limited to thin
films, and sol-gel phosphate glasses have, as yet, been very (too) soluble
and fragile.
