Molecular Gels for Tissue Engineering - Soft Fibrillar Materials: Fabrication and Applications - page 132

Non-ionic surfactants such as polyoxyethylene sorbitan monooleate and sorbitan
monolaurate increase the degree of fiber branching by selectively adsorb at the
growing GP-1 fiber tips and hinder normal fiber growth in one-dimensional axial
orientation. Formations of stronger interconnected fiber networks are promoted
without affecting the fiber's crystalline nature [26, 27]. Similarly to surfactants,
rigid polymer additives such as ethylene/vinyl acetate copolymer and poly(methyl
methacrylate comethacrylic acid) also adsorb strongly to GP-1 fibers and promote
the formation of highly branched multidomain spherulitic networks. Adsorp-
tion of polymer molecules also suppresses primary nucleation and inhibits the
formation of less highly branched fibers at an early stage of cooling where
the temperature is higher and the degree of supersaturation is relatively lower,
forming uniform and homogenous highly branched spherulite only networks
[28]. Lower nucleation rate also leads to the development of a smaller number
of larger spherulites, reducing boundary area and improving network integra-
tion, leading to higher viscoelasticity [29]. Ultrasound is another technique to
induce stronger gel formation. Ultrasound promotes gelation below critical gela-
tion concentration and favors the formation of homogenous interconnected fiber
networks [27]. As the macroscopic properties of molecular gels are highly depen-
dent on their microscopic network structures, the ability to flexibly adjust the
gels' structural network makes LMWG an excellent candidate for soft materials
development where different physical properties can be attained to suit specific
applications.
While most molecular gels have been made in organic solvents, molecular
organogel by default is not compatible with the aqueous in vivo environment. This
review therefore focuses mainly on applications of molecular hydrogel in tissue
engineering, and readers are referred to a recent review on the applications of
molecular organogel [30].
Interestingly, biomaterials such as amino acids, lipids, and nucleic acids can
arrange themselves spontaneously to form highly organized structures when they
are used as building blocks. Based on the studies on self-assembly of these bio-
materials, a number of variations have been designed and introduced into these
biomaterials to make them self-assemble in a controllable manner. Starting with
3-

-cholesteryl-4-(2-anthryl)butanoate (CAB), the discoveries of most LMWGs were
serendipitous, and new gelators were developed by modifying the structure of
existing parent gelators [31]. There are a number of ways to classify LMWGs,
depending on the type and placement of the polar groups [10] or on the essential
groups contributing to the non-covalent interactions that maintain the molecular
gel [32]. Based on the nature of the interactions responsible for self-assembly,
gelators for molecular gels have been classified as conventional amphiphiles, bo-
laamphiphiles, Gemini surfactants, sugar-based systems, and others [10]. Based
on the functional groups that supply the major non-covalent interactions among
gelator molecules, the building blocks for molecular gels have been classified into
seven groups [32]. Based on the class of bio-functional molecules incorporated
for self-assembly, molecular gels have been classified into four groups, namely,
peptide/amino acid-based, saccharide-based, lipid-based, and nucleobase-based

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