Molecular Gels for Tissue Engineering - Soft Fibrillar Materials: Fabrication and Applications - page 127

aggregates. These aggregates, or self-assembled fibrillar networks on nano-scale,
are formed as a result of a combination of non-covalent interactions among these
low-molecular-weight compounds. As gelation is generally difficult to fully predict,
these compounds, which can form gels in organic or aqueous solvents without
photo- or ionicinitiation, provide a model system to study the relationship between
the structure of a gelator and the gel properties [3]. The studies on these small
molecular weight compounds subsequently gained growing popularity, emphasis
being on their gelation mechanisms and applications in the medical and phar-
maceutical industries, such as controlled drug delivery and scaffolding for tissue
engineering [4, 5].
Gels derived from those low-molecular-weight gelators (LMWGs) are collectively
referred to as molecular gels . However, the definition of molecular gels is not
as straightforward as that of the other types of gels. Molecular gels have been
referred to as gelations derived from LMWGs [6]. This definition is consistent
in general with other definitions referring to molecular gels as gelations formed
from certain LMWGs through non-covalent interactions [7, 8]. In the classification
of gels in the above-mentioned review [6], physical gels, which are gels that
are formed through physical non-covalent interactions, are proposed to consist
of supramolecular and macromolecular gels. Macromolecular gels refer to gels
derived from high-molecular-weight compounds such as polymers and collagens,
and supramolecular gels refer to gels derived from LMWGs [6]. However, unlike
the above classification, that equates molecular gels to supramolecular gels [6],
another review considered supramolecular gels to be synonymous with physical
gels [9]. In this chapter, we equate supramolecular gels to physical gels (Figure 4.1).
By this definition, molecular gels constitute a subgroup of physical gels that are
specifically formed from LMWGs with a molecular mass of less than 3000 Da [9].
A number of gelators with molecular weight more than 3000 Da are also included
in this review for the potential applications in tissue engineering [10].
It is worth noting that at the heart of molecular gels are non-covalent in-
teractions. Although most gelators form fibrillar networks in a self-assembled

Gels

Chemical gels Physical gels/ Supramolecular gels

Low molecular
weight gels

High molecular
weight gels

Macromolecular
gels

Low molecular
weight
gels/Molecular
gels

Figure 4.1 Classification of gels.