Biomedical Engineering Reference
In-Depth Information
particles are linked to each other (Figure 3.5). The gelation continues
during ageing. Figure 3.5 demonstrates that gel forms by particles
coalescing and then bonding together. Spaces therefore form between
the particles as they come together, as packing of spheres always leaves
interstitial gaps between the spheres. When the gel is formed and is still
wet, the by-products of hydrolysis and condensation remain in the pores
and are called 'pore liquor'. During drying, the pore liquor evaporates,
leaving behind the nanoporosity (Figure 3.1). Pore size is related to
particle size, which in turn depends on reaction conditions and glass
composition. The 'surfaces' of the pores are covered in Si-OH groups
after drying following the evaporation of water. Thermal processing
can be used to drive off the -OH groups and densify the silica network,
which also reduces the pore size. However, for bioactive compositions,
the thermal processing temperature is usually minimised to maintain
nanoporosity.
Silica gels are mostly linear structures when formed under acidic condi-
tions owing to the low degree of cross-linking due to steric crowding. In
contrast, under basic conditions, the distribution of polysilicate species
is broad and characteristic of branched polymers with a high degree
of cross-linking [2]. Fast hydrolysis and slow condensation favour the
formation of linear polymers. Slow hydrolysis and fast condensation
result in larger, bulkier and more ramified networks [2]. Larger particles
are anticipated when alcohol is used as a solvent (lower rate of depoly-
merisation). Removing the pore liquor from the nanoporosity without
cracking the glass can be a challenge though.
3.6 MAKING SOL-GEL MONOLITHS
The greatest challenge in the sol-gel process is the production of crack-
free monoliths. Discs, rods or blocks with diameters in excess of 1 cm are
generally difficult to produce by the sol-gel process. This is particularly
difficult if the glass has network modifiers, that is, if the composition is
anything other than silica.
The primary problem that had to be overcome from the early days
of the sol-gel process was cracking during drying. The cracking is due
to two reasons: the large shrinkage that occurs during drying; and the
evaporation of the liquid by-products of the condensation reaction.
When pore liquids are removed from the gels, the vapour must travel
from within the gel to the gel surface via the interconnected pore
network. This can cause capillary stresses within the pore network and
therefore cracking. For small cross-sections, such as in powders, coatings
