Molecular Gels for Tissue Engineering - Soft Fibrillar Materials: Fabrication and Applications - page 129

gelator molecules to condense, leading to one of the three possible situations:
crystallization as a result of highly ordered aggregates, amorphous precipitates
derived from random aggregation of gelator molecules, or gelation as a result of
an aggregation process intermediate between the above-mentioned two processes
[6]. Similar to protein structural nomenclatures, primary, secondary, and tertiary
structures of a gel, ranging from angstrom to micron, have been proposed to
explain the mechanism of gel formation [10].
The primary structure of a gel refers to its structure on angstrom to nanometer
scale, and is determined by molecular recognitions between complementary donor
and acceptor groups in the gelator molecules. During gel formation, LMWGs
interact with each other through intermolecular non-covalent interactions, leading
to the formation of anisotropic aggregation in one or two dimensions, which serve
as platforms for higher-order organization. The driving force for the formation
of primary structure of a gel is usually hydrogen bonding in organogels and
hydrophobic interactions in hydrogels, as hydrogen bonding loses its strength in
aqueous solution [13].
The secondary structure of a gel is defined as the morphology, on nano- to micro-
meter scale, of aggregates such as tapes, ribbons, micelles, and fibers. It is directly
influenced by the structure of the gelator molecules. Gelators such as amphiphiles
organize in water to generate molecular aggregates in the formof bilayers, spherical
or tubular vesicles, and micelles [14]. Some LMWGs form micelles at the critical
micellar concentration. As the concentration of gelators increases, these micelles
convert into ellipsoidal micelles (disks), and further into cylindrical micellar fibers
(rods). These fibers, however, do not necessarily form a gel due to the presence of
electrostatic repulsion between charged surfaces.
The tertiary structure of a gel involves the interactions among individual aggre-
gates onmicro- tomilli-meter scale and determines whether gel or fiber precipitates
are formed in a given condition. The formation of a gel, instead of fiber aggregates,
is determined by the type of interactions that can occur among the fibers. Com-
pared to shorter fibers, long and flexible fibers are more likely to trap solvent and
form a gel. Gels with different properties are made by manipulating the gelation
conditions by adding additives or changing the solution temperature to adjust the
fibers' morphologies.
In self assembly, LMWGs with complementary donor and acceptor groups can
interact with adjacent gelator molecules to forma dimer, which further interact with
other dimers to form oligomers. Oligomers extend into fibrils and bundle further
into fibers, which in turn entangle into a three-dimensional network, or SAFIN
[15]. The self-assembly of an LMWG such as a peptide in a

β -sheet conformation

is demonstrated in Figure 4.3. In this -sheet structure, the complementary donor

and acceptor groups line up on opposing sides, and interactions between these
donor and acceptor groups enable the peptide molecules to assemble themselves in
solution into rod-like monomers. These rod-like monomers serve as the foundation
of higher hierarchical structures and assemble themselves via recognitions among
the complementary donor and acceptor groups into β-sheet tapes and, with
increasing concentration, into ribbons (double tapes), fibrils (twisted stacks of

β