The NC gel was cross-linked by dialdehyde cellulose nanocrystals (DACNCs), and
acylhydrazine-terminated polyethylene glycol (HZ-PEG-HZ) acted as dual crosslinkers via dynamic reversible acylhydrazone bonds, which displayed remarkable
mechanical properties with tensile strength and compressive strength up to 141 kPa
and 580 kPa, respectively. Interestingly, the obtained hydrogels presented excellent
self-healing ability without additional stimuli, whose healing efficiency was higher
than 90% derived from the dynamic reversible exchange of acylhydrazone bond
under acid catalysis.
3.1.3 Diels-Alder Reactions
The Diels-Alder (DA) reaction, as one of the “click chemistry” reactions, is a highly
chemo-selective [4 + 2] cycloaddition between a diene and a dienophile, and the
presence of water has a rate-accelerating effect on the reaction process [137–
139]. DA reaction provides high selectivity and yields without any side reactions
and by-products, which plays an essential role in the preparation of functional gels
with reversible networks because of the unique thermo-reversibility [140–142]. For
instance, Wang et al. [143] fabricated a hydroxypropyl methylcellulose-based
hydrogel via Diels-Alder reaction. Furfurylamine and N-maleoyl alanine were
initially used to modify hydroxypropyl methylcellulose (HPMC), and then HPMCbased hydrogels were fabricated by DA click chemistry in water without an initiator
or catalyst. The results indicated that the gelation time decreased with increase in
temperature and concentration of the solution, where water demonstrated a rateaccelerating effect on DA reaction. The obtained HPMC-based hydrogels displayed
a high temperature-dependent swelling ratio in water, implying the potential application in the fields of tissue engineering and drug-controlled release carriers.
Besides, in order to explore the applicability of the Diels-Alder reaction for the
preparation of completely renewable biobased hydrogels, Gabilondo’s group [144]
used covalently bound maleimide-functionalized CNCs as nanofillers, and the DielsAlder cycloaddition was then employed as a mild covalent strategy for their binding
with furan-modified gelatin. Subsequently the second cross-linking based on the
amide coupling between CS and gelatin was performed to further stabilize the
collected hydrogel, leading to the formation of stiff networks with a lower swelling
ratio.
The retro-DA reactions typically are induced by elevated temperature to result in
the disconnection of diene and dienophile, which can be utilized to build the crosslinked polymer network. The first use of Diels-Alder chemistry for healable materials was reported in 2002 by Chen et al. [142], in which the furan and maleimide
moieties were incorporated into the polymer backbone as pendant diene and
dienophile groups. Previous works of the polysaccharide hydrogels cross-linked
by DA reaction provided fruitful platform for designing cellulose-based hydrogels
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
335
acylhydrazine-terminated polyethylene glycol (HZ-PEG-HZ) acted as dual crosslinkers via dynamic reversible acylhydrazone bonds, which displayed remarkable
mechanical properties with tensile strength and compressive strength up to 141 kPa
and 580 kPa, respectively. Interestingly, the obtained hydrogels presented excellent
self-healing ability without additional stimuli, whose healing efficiency was higher
than 90% derived from the dynamic reversible exchange of acylhydrazone bond
under acid catalysis.
3.1.3 Diels-Alder Reactions
The Diels-Alder (DA) reaction, as one of the “click chemistry” reactions, is a highly
chemo-selective [4 + 2] cycloaddition between a diene and a dienophile, and the
presence of water has a rate-accelerating effect on the reaction process [137–
139]. DA reaction provides high selectivity and yields without any side reactions
and by-products, which plays an essential role in the preparation of functional gels
with reversible networks because of the unique thermo-reversibility [140–142]. For
instance, Wang et al. [143] fabricated a hydroxypropyl methylcellulose-based
hydrogel via Diels-Alder reaction. Furfurylamine and N-maleoyl alanine were
initially used to modify hydroxypropyl methylcellulose (HPMC), and then HPMCbased hydrogels were fabricated by DA click chemistry in water without an initiator
or catalyst. The results indicated that the gelation time decreased with increase in
temperature and concentration of the solution, where water demonstrated a rateaccelerating effect on DA reaction. The obtained HPMC-based hydrogels displayed
a high temperature-dependent swelling ratio in water, implying the potential application in the fields of tissue engineering and drug-controlled release carriers.
Besides, in order to explore the applicability of the Diels-Alder reaction for the
preparation of completely renewable biobased hydrogels, Gabilondo’s group [144]
used covalently bound maleimide-functionalized CNCs as nanofillers, and the DielsAlder cycloaddition was then employed as a mild covalent strategy for their binding
with furan-modified gelatin. Subsequently the second cross-linking based on the
amide coupling between CS and gelatin was performed to further stabilize the
collected hydrogel, leading to the formation of stiff networks with a lower swelling
ratio.
The retro-DA reactions typically are induced by elevated temperature to result in
the disconnection of diene and dienophile, which can be utilized to build the crosslinked polymer network. The first use of Diels-Alder chemistry for healable materials was reported in 2002 by Chen et al. [142], in which the furan and maleimide
moieties were incorporated into the polymer backbone as pendant diene and
dienophile groups. Previous works of the polysaccharide hydrogels cross-linked
by DA reaction provided fruitful platform for designing cellulose-based hydrogels
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
335