Molecular Gels for Tissue Engineering - Soft Fibrillar Materials: Fabrication and Applications - page 128

manner, non-covalent interactions can also be achieved by ionic cross-linking or
molecular recognition events such as antibody-antigen and protein-polysaccharide
interactions. The reversible bindings between antigen and antibody have been
used as a cross-linking mechanism by introducing a type of antigen and its spe-
cific antibody into a hydrogel. A gel formed through antigen-antibody binding
is stimuli-responsive and swells when it is exposed to a new type of antigen
that binds to the same antibody. The non-covalent interactions that form the gel
change with the dissociation of the binding between the antibody with its specific
antigen and the establishment of binding between the antibodies and the newly
introduced antigens [11]. Glycosylated amino acetate types of hydrogelator can
form hydrogels with nano-fibers possessing well-developed hydrophobic domains
and micro-sized cavities that are filled with immobilized water in a semi-wet
hydrogel state. The host-guest binding between the aqueous microcavities and
the hydrophobic nanofibers determines the dynamic redistribution of receptor
molecules, and these hydrogels can be designed to recognize and discriminate
between phosphate derivatives [12].

4.3
Self-Assembly and Gel Structures

Self-assembly is a hierarchical process to form an organized structure from gelator
molecules via intermolecular non-covalent interactions, including hydrogen bonds,
π stacking, and van der Waals interactions. The final structure occurs in a

stepwise manner from intermediate structures formed on the nano-scale through
inter-gelator interactions. Gelation is a balance between the tendency of molecules
to dissolve or to aggregate in a given solvent. Molecular gels are usually made by
heating the gelators in a solvent and cooling down the supersaturated solution
to room temperature. As shown in Figure 4.2, the action of cooling causes the

- π

Precipitate

Gel

Cool Cool

Super-saturated
solution

Crystal

Figure 4.2 Schematic representation of aggregation mode. (Figure adapted from Ref. [6]
(Copyright 2004) The Royal Society of Chemistry.)