the cyclodextrin hosts on the surface of the vesicles. Hence, cyclodextrin vesicles
interconnect the polymer chains into a three-dimensional network and act as multivalent non-covalent responsive junctions. The obtained gel shows significant shearthinning and self-healing properties, which make it highly suitable for applications
that require injectability.
Another investigation on shape-memory and self-healing DNA-based
carboxymethyl cellulose hydrogels has been reported by Wang et al. [168]. They
introduced carboxymethyl cellulose (CMC) as the backbone matrix to provide
effective anchoring sites for the conjugation of a variety of functional tethers. The
first system included the cross-linking of the hydrogel by trans-azobenzene/β-CD
supramolecular complexes and by duplex nucleic acid bridges. The stiffness of the
hydrogel was shifted between high and low values by the cyclic switchable
photoisomerization of the trans-azobenzene units to the cis-azobenzene units that
lack binding affinity toward β-CD. These features were used to apply the photoresponsive hydrogel as a shape-memory and as a self-healing functional matrix. The
second hydrogel system included a carboxymethyl cellulose backbone cross-linked
by two stimuli-responsive units composed of the K
+
-stabilized G-quadruplex bridging units and trans-azobenzene/β-CD complexes. The two types of cross-linkers
were triggered individually to be separated by light or by chemical means, which
enabled to control the stiffness properties and thus stimulate the shape-memory or
self-healing process of hydrogels using two different codes.
The cucurbit[8]uril (CB[8]) are a family of macrocyclic host molecules consisting
of methylene-linked oligomers of glycoluril that have a symmetric “barrel” shape
with two identical portal regions laced by ureidocarbonyl oxygens [169, 170]. CB
[8] could form reversible host-guest interactions involving 1:1:1 heteroternary
complexation with two guest motifs and design dynamic cellulose-based hydrogels
[171–173]. Scherman’s group has achieved significant progress in the preparation of
supramolecularly cross-linked viscoelastic materials from relatively low molecular
weight synthetic polymers with CB[8]. For instance, they produced an ultrahigh
water content hydrogel (up to 99.7% water by weight) driven by strong host-guest
complexation with CB[8] via a facile, rapid, and scalable conjugation technique.
Cellulosic derivatives and commodity polymers were modified with strongly binding guests for CB[8] ternary complex formation of host-guest interactions. A
transparent cellulosed-hydrogel was formed instantaneously when these polymers
were mixed in the presence of CB[8]. The supramolecular nature of these hydrogels
affords them with highly tunable mechanical properties, and the dynamics of the CB
[8] ternary complex cross-links allows for rapid self-healing of the materials after
damage caused by stress deformation. Moreover, these hydrogels display response
to a multitude of external stimuli, including temperature, chemical potential, and
competing guests.
In addition, to address the great challenge of achieving high stiffness with
properties requiring high molecular dynamics, McKee et al. [174] prepared healable,
stable, and stiff nanocomposite supramolecular hydrogels, which bonded polymer
brush-modified “hard” cellulose nanocrystals (CNCs) and “soft” polymeric domains
by the dynamic host-guest interactions of CB[8] supramolecular cross-links
(Fig. 10). CNCs with nanometer-scale lateral dimensions are mechanically strong
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
341
interconnect the polymer chains into a three-dimensional network and act as multivalent non-covalent responsive junctions. The obtained gel shows significant shearthinning and self-healing properties, which make it highly suitable for applications
that require injectability.
Another investigation on shape-memory and self-healing DNA-based
carboxymethyl cellulose hydrogels has been reported by Wang et al. [168]. They
introduced carboxymethyl cellulose (CMC) as the backbone matrix to provide
effective anchoring sites for the conjugation of a variety of functional tethers. The
first system included the cross-linking of the hydrogel by trans-azobenzene/β-CD
supramolecular complexes and by duplex nucleic acid bridges. The stiffness of the
hydrogel was shifted between high and low values by the cyclic switchable
photoisomerization of the trans-azobenzene units to the cis-azobenzene units that
lack binding affinity toward β-CD. These features were used to apply the photoresponsive hydrogel as a shape-memory and as a self-healing functional matrix. The
second hydrogel system included a carboxymethyl cellulose backbone cross-linked
by two stimuli-responsive units composed of the K
+
-stabilized G-quadruplex bridging units and trans-azobenzene/β-CD complexes. The two types of cross-linkers
were triggered individually to be separated by light or by chemical means, which
enabled to control the stiffness properties and thus stimulate the shape-memory or
self-healing process of hydrogels using two different codes.
The cucurbit[8]uril (CB[8]) are a family of macrocyclic host molecules consisting
of methylene-linked oligomers of glycoluril that have a symmetric “barrel” shape
with two identical portal regions laced by ureidocarbonyl oxygens [169, 170]. CB
[8] could form reversible host-guest interactions involving 1:1:1 heteroternary
complexation with two guest motifs and design dynamic cellulose-based hydrogels
[171–173]. Scherman’s group has achieved significant progress in the preparation of
supramolecularly cross-linked viscoelastic materials from relatively low molecular
weight synthetic polymers with CB[8]. For instance, they produced an ultrahigh
water content hydrogel (up to 99.7% water by weight) driven by strong host-guest
complexation with CB[8] via a facile, rapid, and scalable conjugation technique.
Cellulosic derivatives and commodity polymers were modified with strongly binding guests for CB[8] ternary complex formation of host-guest interactions. A
transparent cellulosed-hydrogel was formed instantaneously when these polymers
were mixed in the presence of CB[8]. The supramolecular nature of these hydrogels
affords them with highly tunable mechanical properties, and the dynamics of the CB
[8] ternary complex cross-links allows for rapid self-healing of the materials after
damage caused by stress deformation. Moreover, these hydrogels display response
to a multitude of external stimuli, including temperature, chemical potential, and
competing guests.
In addition, to address the great challenge of achieving high stiffness with
properties requiring high molecular dynamics, McKee et al. [174] prepared healable,
stable, and stiff nanocomposite supramolecular hydrogels, which bonded polymer
brush-modified “hard” cellulose nanocrystals (CNCs) and “soft” polymeric domains
by the dynamic host-guest interactions of CB[8] supramolecular cross-links
(Fig. 10). CNCs with nanometer-scale lateral dimensions are mechanically strong
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
341