network structure. In general, chemical cross-links in hydrogels are often irreversible, e.g., those formed in photopolymerized gels. Once the interchain cross-links are
broken, the network structure does not revert back to its original state, and the loss of
mechanical and other related properties is irrecoverable. In recent years, it has been
established that this problem can be addressed through utilization of cross-linking
strategies that involve dynamic chemical linkages. These chemical bonds upon
breakage can undergo reformation, and thus the original network structure can be
restored. The reformation of the ruptured bond can occur in an autonomous manner,
i.e., by itself without the need of any external stimuli, or in an externally triggered
fashion which would require external stimuli such as heat or light, either to enable
fast recovery or to recreate conditions necessary for bond formation. To date, most
dynamic covalently cross-linked hydrogels undergo self-healing through application
of external stimuli, which generally recreate the bond formation conditions that are
often the same as the ones employed during their fabrication.
A variety of reversible chemical linkages have been employed to date to install
covalent cross-linkages required for network formation. The commonly used reversible bond formations involve heteroatom-based imine, acylhydrazone, boronate
ester, disulfide bonds, along with carbon-carbon bonds formed through [4+2]
Diels-Alder cycloaddition and [2+2] and [4+4] cycloaddition reactions (Fig. 1).
The subsequent sections of this chapter are divided into parts that highlight the
synthesis and self-healing properties of hydrogels based on the specific type of
dynamic chemistry utilized to fabricate such gels through examples. While the
main focus is on the chemistry of cross-linking and the conditions under which
self-healing was achieved, a brief discussion on the method utilized to ascertain the
realization of self-healing has also been provided by mentioning the mechanical or
rheological data of these materials. Furthermore, information related to possible
application of the hydrogels under investigations as provided by the researchers
has also been included to provide the readers with intended or possible utility of such
materials.
2 Imine Bond Formation-Based Hydrogels
Dynamic imine bonds formed through reaction between an aldehyde and a primary
amine group, also known as Schiff base reaction, have been widely exploited to
install stimuli-responsive cross-links in hydrogels. Imine bond forms upon nucleophilic addition of the amine to the carbonyl of aldehyde through three types of
reactions which are imine condensation, exchange, and metathesis. It is a favorable
reaction for polymer cross-linking to obtain hydrogels since it does not involve any
intermediate step. The cleavage and formation of the bond occur instantly within the
hydrogel network allowing autonomous self-healing. The dynamic equilibrium of
these hydrogels can be tuned by varying the electronic and steric nature of the
carbonyl and amine groups involved, as well as through the choice of solvent,
concentration, pH, and temperature of the environment [11].
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
245
broken, the network structure does not revert back to its original state, and the loss of
mechanical and other related properties is irrecoverable. In recent years, it has been
established that this problem can be addressed through utilization of cross-linking
strategies that involve dynamic chemical linkages. These chemical bonds upon
breakage can undergo reformation, and thus the original network structure can be
restored. The reformation of the ruptured bond can occur in an autonomous manner,
i.e., by itself without the need of any external stimuli, or in an externally triggered
fashion which would require external stimuli such as heat or light, either to enable
fast recovery or to recreate conditions necessary for bond formation. To date, most
dynamic covalently cross-linked hydrogels undergo self-healing through application
of external stimuli, which generally recreate the bond formation conditions that are
often the same as the ones employed during their fabrication.
A variety of reversible chemical linkages have been employed to date to install
covalent cross-linkages required for network formation. The commonly used reversible bond formations involve heteroatom-based imine, acylhydrazone, boronate
ester, disulfide bonds, along with carbon-carbon bonds formed through [4+2]
Diels-Alder cycloaddition and [2+2] and [4+4] cycloaddition reactions (Fig. 1).
The subsequent sections of this chapter are divided into parts that highlight the
synthesis and self-healing properties of hydrogels based on the specific type of
dynamic chemistry utilized to fabricate such gels through examples. While the
main focus is on the chemistry of cross-linking and the conditions under which
self-healing was achieved, a brief discussion on the method utilized to ascertain the
realization of self-healing has also been provided by mentioning the mechanical or
rheological data of these materials. Furthermore, information related to possible
application of the hydrogels under investigations as provided by the researchers
has also been included to provide the readers with intended or possible utility of such
materials.
2 Imine Bond Formation-Based Hydrogels
Dynamic imine bonds formed through reaction between an aldehyde and a primary
amine group, also known as Schiff base reaction, have been widely exploited to
install stimuli-responsive cross-links in hydrogels. Imine bond forms upon nucleophilic addition of the amine to the carbonyl of aldehyde through three types of
reactions which are imine condensation, exchange, and metathesis. It is a favorable
reaction for polymer cross-linking to obtain hydrogels since it does not involve any
intermediate step. The cleavage and formation of the bond occur instantly within the
hydrogel network allowing autonomous self-healing. The dynamic equilibrium of
these hydrogels can be tuned by varying the electronic and steric nature of the
carbonyl and amine groups involved, as well as through the choice of solvent,
concentration, pH, and temperature of the environment [11].
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
245