Supramolecular Soft Biomaterials for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 118

create polyoxometalate-based (POM-based) supramolecular hydrogels that could
potentially play important roles in optics, medicine, catalysis, as well as magnet-
ism [ 48 ]. They made use of the dispersion of POM-based building blocks in cer-
tain solvents to create the even hybrid gels. To achieve this, POMs were modified
through the exchange of their counter ions with cationic surfactants, resulting in
the formation of surfactant-encapsulated (SEP) POM complexes. This allowed for
hybrid self-assemblings as the surface properties of the inorganic clusters were
largely improved. With these properties, grafting into both organic and inorganic
matrices was also made possible, and the SEPs could also be used to fabricate
organogels. Thus, through appropriate selections of particular solvents and SEPs,
POM-based hybrid supramolecular hydrogels could be fabricated. In addition, the
relationships between the properties of the gelators and the alkyl chain density,
alkyl chain length SEP, as well as the shape of the POM were investigated, and
the authors have concluded that the formation of the hybrid gels was due to the
effect of the electrostatic interaction and solvents. This research could hence serve
as basis for the development of organic-inorganic hybrid gels through complexes
with similar structures to SEPs in hydrophobic environments.
Schubert et al. reported the synthesis of poly(methyl methacrylate) based
copolymers with different amounts of terpyridine units in the side chain [ 49 ].
The addition of transition metal ions, Fe (II) or Zn (II), was used to trigger supra-
molecular crosslinking in diluted solutions. It was found that the stability of the
metal-terpyridine complexes affects the physical properties of the gel concentra-
tions. The dissolution of the gel could be accomplished by the addition of a strong
competing ligand to obtain the soluble phase. Another report focuses on poly(vinyl
chloride) crosslinking in organic solvent using a variety of transition metal ions,
leading to an increase in the polymer molecular weight [ 50 ]. In another report,
the gelation of the amphiphilic quaternary ammonium oligoether-based ionic liq-
uid with water is presented [ 51 ]. The thermoreversible ionogels have high ionic
conductivity (up to 60 mS cm − 1 ), and storage moduli above 10 5 Pa. Upon heating,
these gels melt with melting points ranging from 20 to 53 °C. These properties can
be easily tuned in a broad range by varying the aqueous (and/or inorganic salts)
concentration in the ionogels. The observed gelation phenomenon is purported
to occur via the formation of a hydrogen bonded network between water and the
ionic liquid.
Weck et al. reported the preparation of complementary hydrogen bonded
cross-linked polymer networks based on two distinct hydrogen bonding recogni-
tion motifs [ 52 ]. The hydrogen bonding recognition units were based on either
three-point cyanuric acid-2,4-diaminotriazine or six-point cyanuric acid-Hamilton
wedge interactions. The polymer scaffold, which was functionalized with cyanu-
ric acid functional groups, was cross-linked in 1-chloronaphthalene through com-
plementary interchain hydrogen bonding interactions. These gels are thermally
reversible and possess tunable mechanical properties that are controlled by the
molecular structure of the cross-linking agent. In related work by the same group,
side-chain-functionalized polymers containing hydrogen bonding and metal coor-
dination sites were synthesized [ 53 ]. The crosslinking of these polymers can be