Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 148

'click' chemistry [ 6 ], or by radical crosslinking copolymerization of suitable
monomers and crosslinkers [ 2 , 7 , 8 ], both resulting in permanent polymer net-
works, as illustrated in Fig. 1 a. Due to the irreversibility of chain connection, the
mechanical properties of chemical gels are determined by their crosslinking den-
sity and chain flexibility [ 9 ]. Many materials, such as polymeric adhesives [ 10 ],
artificial lenses [ 11 , 12 ], and cements [ 13 ], require gels to be stable and tough;
this is achieved by dense crosslinking. However, dense crosslinking also entails
gel brittleness [ 14 ] and turbidity [ 15 ]. A decisive downside of covalent hydro-
gels is that extensive mechanical stress can cause irreversible bond breakage in
chemical gels, impairing their utility.
The drawbacks of chemical gels are overcome by physical gels. In these mate-
rials, chain crosslinking occurs by transient and reversible supramolecular asso-
ciation, as illustrated in Fig. 1 b. As a result, the interchain junctions continuously
break and rearrange on experimental timescales; this causes the transient bonds
to be susceptible to shear, decreasing the stability of the hydrogels [ 16 ]. This
dynamic characteristic leads to two unique abilities of gels: shear-thinning [ 16 ]
and self-healing [ 17 , 18 ]. At shear-thinning, physical hydrogels display decrease
of their viscosity upon application of stress. This effect makes them suitable for
implantation via syringe, whereafter they can regain their original properties; it
also allows them to be processed to obtain desired shapes in materials engineering.
In self-healing, disrupted supramolecular bonds exhibit a tendency to reassociate
upon contact. This effect allows the hydrogel to restore its original mechanical
strength after damage; it also allows fusion of two independent gel fragments to a
new composite [ 19 ]. As a complement to self-healing, another feature of supramo-
lecular hydrogels is that they can be disintegrated by external stimulation, includ-
ing change of pH [ 20 , 21 ], temperature [ 22 , 23 ], or solvent composition [ 24 ],
thereby degrading them to their precursor polymers or building blocks. Because of

(a) (b)

supramolecular
crosslink

covalent
crosslink

Fig. 1 Schematic structures of a permanent, chemically crosslinked and b transient, physically
crosslinked polymer networks