Biomedical Engineering Reference
In-Depth Information
greater homogeneity, along with net shape casting, represent major
advances resulting from sol-gel processing of monoliths. The process
also allows the synthesis of glasses with a variety of pore structures from
the nanometre to the millimetre range, which gives sol-gel glasses great
potential in biomedical applications.
Fully densified sol-gel silica has physical properties and structural
characteristics similar to high-grade fused silica but offers the advantages
of near net shape casting, including internal cavities, and a lower thermal
expansion coefficient of 0.2
10
6
for conventional processing methods. Optically transparent porous gel
silica optics have a density as low as 60% of fused silica and can be
impregnated with up to 30-40 vol% of a second-phase optically active
organic or inorganic compound.
Centrifugal deposition of 12-40 nm colloidal silica powders can be
used to produce synthetic silica tubes used in the manufacture of optical
telecommunication fibres. One of the main advantages of the sol-gel
technique is very high accuracies for the diameter, cross-sectional area
and wall thickness of the tubes.
10
6
cm/cm compared with 0.55
×
×
3.3 SOL-GEL PROCESS PRINCIPLES
A common sol-gel process is the use of organic precursors that can
undergo polymer-type reactions to form an inorganic network rather like
a highly cross-linked amorphous inorganic polymer. Although modern
compared to the discovery of traditional glass, which pre-dates history,
the sol-gel process is over 150 years old. Work began in 1846 [4],
with the discovery that the alkoxide tetraethyl orthosilicate (TEOS)
Si(OC
2
H
5
) reacted with water (hydrolysis) under acidic conditions.
Polymer condensation reactions then occurred, forming an inorganic
silica network (gel) with aqueous by-products. Drying the gel produced
a glassy SiO
2
. Unfortunately, there was little wider interest, as the drying
times were in excess of a year to avoid the gels breaking into tiny pieces.
However, 100 years later, the interest was rekindled. The processing of
powders, fibres and coatings that did not crack during drying and the
hydrolysis of TEOS became the basis for many sol-gel glass systems.
Gels that are dried under ambient conditions, or under carefully
controlled heating, are referred to either as xerogels or simply as gels.
However, it is still difficult to obtain crack-free monoliths under these
conditions. When the pore liquid is removed as a gas phase from the
interconnected solid gel network under hypercritical conditions (critical-
point drying), the network does not collapse and a low-density aerogel
