Biomedical Engineering Reference
In-Depth Information
TEOS
Gelator
(a)
(b)
(c)
(d)
(e)
Figure 5.1
Schematic description of the
creation of hollow fiber silica by sol-gel
polycondensation of tetraethyl orthosilicate
(TEOS) with gel fibril as a template: (a) a
mixture of TEOS and gelator molecules;
(b) gelation; (c) sol-gel polycondensation
of TEOS and adsorption onto the cationic
gelator fibril; (d) before calcination; and
(e) hollow silica fiber formation after cal-
cination. Reprinted with permission from
Ref. [11b]. Copyright 2000 The American
Chemical Society.
(a)
(b)
(c)
200 nm
250 nm
200 nm
Figure 5.2
Tubular silica (a), left-handed (b), and right-handed (c) silica fibers obtained
by sol-gel transcription in organogels. (a) is reprinted with permission from Ref. [11b];
(b) and (c) are reprinted with permission from Ref. [14b]. Copyright 2000 American
Chemical Society.
silica. It is worth mentioning that in general the transcription (condensation) does
not start simultaneously all over the gel fibers. Instead, it begins on some points
on the fiber and grows in a spherical manner until two adjacent spheres meet,
which was evidenced by the pearl-necklace silica obtained at the early stage of
transcription [13]. Continuous growth then leads to the formation of silica fibers.
More detailed structural features of gel fibers have been imprinted into silica.
Based on the observation that some cholesterol derivatives can gelate silanol,
various cholesterol-based gelators were synthesized by the researchers [11b, 12,
15]. Since fibers formed by this class of gelators are frequently helical, helical silica
fibers were obtained (Figure 5.2b,c). These researchers also designed several chiral
diaminocyclohexane derivatives [14] which gelate a wide range of organic solvents.
More interestingly, both left- and right-handed silica fibers were obtained by using
cholesterol-based chiral gelators. The molecular structures of the chiral gelators
are shown in Scheme 5.1a. Gels based on other gelators such as sugar-appended
azonaphthol (Scheme 5.1b) and sugar-appended porphyrin were also successfully
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