Biomedical Engineering Reference
In-Depth Information
Ceramic powder
Add
Dispersant, surfactant,
monomer, cross-linker
Prepare suspension from the powder
Foam the suspension using one of the
foaming techniques in Table 2.16
Add
Initiator, catalyst
While the foamed suspension is poly
merized to form a gel, cast the gel
Dry and sinter the green body
Ceramic foam
FIGURE 1.5
Flowchart of the gelcasting method to produce a ceramic foam.
respectively, in the case of porous hydroxyapatite [173,181,183,185]. It is worthwhile noticing that
there is a narrow time-temperature window for densifi cation of foams made from bioactive glasses,
which are prone to crystallize while sintering by viscous fl ow. Hence the production of bioactive
glass foams by powder-based methods presents diffi culties [80].
1.4.1.2 Sol-Gel Techniques
1.4.1.2.1
Sol-Gel Process and Synthesis of Aerogel Ceramics
The sol-gel process is a well-developed, robust, and versatile “wet” technique for the synthesis of
ceramics and glasses. By applying the sol-gel process, it is possible to fabricate inorganic mate-
rials in various forms: ultrafi ne or spherical shaped powders, thin fi lm coatings, ceramic fi bers,
microporous inorganic membranes, monolithic ceramics and glasses, and extremely porous aerogel
materials [186].
The processing path of aerogel ceramics starts with an alkoxide precursor. Alkoxide precursors,
such as tetraethyl orthosilicate (TEOS) and triethoxyl orthophosphate (TEP), undergo hydrolysis
and condensation reactions to form a sol. In case of silicate precursors, polymerization of -Si-OH
groups continues after hydrolysis is complete, beginning the formation of the silicate (-Si-O-Si-)
network. The network connectivity increases until it spans throughout the solvent medium. Eventually
a wet gel forms. The wet gel is then subjected to controlled thermal processes of aging to strengthen
the gel, drying to remove the liquid by-product of the polycondensation reaction, and thermal sta-
bilization (or sintering) to remove organic species from the surface of the material; and as a result,
a porous aerogel forms [2,187].
1.4.1.2.2
Production of Highly Porous Glasses
Highly porous glasses (or glass foams) have been developed by a slightly modifi ed sol-gel pro-
cess [188]. The sol-gel process is based on the polymerization reactions of metal alkoxide precur-
sors (usually TEOS and TEP). These precursors are dissolved in a solvent, and a gel is formed by
hydrolysis and condensation reactions. The gel is then subjected to controlled thermal processes of
aging to strengthen the gel, drying to remove the liquid by-product of the polycondensation reac-
tion, and thermal stabilization/sintering to remove organic species from the surface of the material
(500-800°C). Sol-gel derived glass scaffolds are obtained by directly foaming the sol with the use
