Biomedical Engineering Reference
In-Depth Information
100 nm in size, they are denoted as colloid particles, and the solution is
called a sol. Under acidic pH, the particles are attracted to each other
and agglomerate. Further progress in hydrolysis and polycondensation
causes the nanoparticles to bond to each other, and the sol sets to
become a gel (a wet covalently bonded network). The time since the
beginning of the hydrolysis and condensation reactions up to yielding
the gel is called the gelation time or gelation period. As it is largely
a room-temperature process, a polymer can be incorporated prior to
gelation to create a hybrid with molecular-scale interactions between the
silica and the polymer chains.
In the synthesis of a silica glass, condensation between Si-OH groups
to form a silicate network represents condensation between the same
inorganic species, hence it is a homo-condensation reaction.
Condensation may also take place between different kinds of M-OH
groups when a different alkoxide M(OR) n
is present in the reaction
system, like
>
Si-OH
+
HO-M
< >
Si-O-M
<
(10.1)
where M represents metals other than Si, for example, Ti or Zr, creating
an inorganic hybrid. The reaction is denoted as hetero-condensation.
Such a hetero-condensation reaction is difficult owing to the difference in
the rate of hydrolysis reaction and the reactivity of the resulting M-OH
group. For example, Ti(OR) 4 is more highly susceptible to hydrolysis
than the corresponding Si(OR) 4 , where R stands for an alkyl group.
The hydrolysis of Si-OR is much more sluggish to yield Si-OH and,
if yielded, the Si-OH group is less reactive than Ti-OH. Thus, many
Ti-OH groups are produced and are condensed favorably to similar
groups to form homo-clusters, Ti-O-Ti bonds, before they meet Si-OH
to form Ti-O-Si bonds.
In an intermediate stage of the procedure to fabricate sol-gel hybrids,
many kinds of species with different degrees in hydrolysis or conden-
sation appear in the sol or gel systems. Owing to flexibility of the sol
or gel matrix of those systems, the hybrid gels can hold molecules,
large or small, like drugs, DNA or its fragments, and proteins (growth
factors or enzymes), or even microorganisms. Consequently, the sol-gel
processes are greatly adaptable in components or ingredients, and hence
synergetic effects are expected from the hybridization among inorganic
and organic species.
Organic polymers can also be introduced into the sol to improve
the mechanical properties of the inorganic component, for example,
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