Bioactive Glass as Synthetic Bone Grafts and Scaffolds for Tissue Engineering - Bio-Glasses: An Introduction - page 182

12.5.3 Direct Foaming

A way to avoid having hollow struts is to directly foam the slurry
so a polymer foam is not needed. This technique involves the use of
surfactants to stabilise bubbles that are created in a liquid by vigorous
agitation. The bubbles must then be gelled (solidified) to maintain the
porous structure. This is a key step in the process, as the bubbles
must be maintained, but solidified. The process is similar to what is
used to produce Actifuse, and is the latest technique for producing
porous bioactive glasses with similar interconnected pore structures and
mechanical strengths to porous bone.
For direct foaming, either melt-derived or sol-gel glasses can be used.
Melt-derived glasses are foamed by the gel-cast foaming process and
sol-gel by the sol-gel foaming processes. The processes have many
similarities. The main ones are that in both cases a solution or slurry
is foamed under vigorous agitation with a surfactant to form bubbles.
The bubbles are gelled and are poured into moulds immediately prior
to gelling. The main differences are that the melt-derived glass gel-cast
foaming process uses in situ polymerisation to gel the bubbles. The
sol-gel process is different in that, rather than needing a polymer to do
the gelling, it gels itself, which simplifies the process. Surfactants are
'surface active agents'. They are molecules that have a hydrophilic end
and a hydrophobic end and are the active ingredients of detergents.
When surfactants are added to water, they lower the surface tension
because the hydrophilic end of the molecule affiliates itself with the
water, and the hydrophobic end is in the air. This stabilises the bubbles
(Figure 12.7) that grow by agitation.

12.5.4 Gel-Cast Foaming

For melt-derived glass, fine particles ( m) of a sinterable composi-

tion, such as 13-93 or ICIE16, are added to water to produce a slurry.
A surfactant is then added and the slurry is foamed under vigorous
agitation. This could be with a whisk, rather like making a meringue.
Figure 12.8 shows a schematic of the process. For the process to succeed,
the viscosity must increase and then the slurry must be gelled to bind the
particles around the bubbles and permanently fix them in place. In the
gel-cast foaming process, the gelation is achieved by in situ polymerisa-
tion, that is, a polymer is formed while the foaming is being carried out.
Monomers (usually acrylates) are usually used, which are polymerised by

< 38 μ