deformed under high strain and lost its integrity, yet, restored back completely to
initial modulus values upon reducing strain (Fig. 34b). Additionally, visual selfhealing experiments were carried by putting two halves of hydrogels in contact at
room temperature for 12 h (Fig. 34c).
In another recent report, a double-network, acid-resistive self-healing polysaccharide-based hydrogel by combining dynamic Schiff base and Diels-Alder reaction
was reported by Wang and coworkers [81]. Hydrogel was obtained by cross-linking
N-(furfural) carboxymethyl chitosan (FCC) and oxidized dextran (Odex) through
Schiff base reaction, followed with a Diels-Alder reaction upon the addition of MalPEG-Mal (Fig. 35a). Self-healing ability was endowed by the reversible imine
bonds, while the Diels-Alder reaction-based cross-linking provided mechanical
strength and acid resistance to the hydrogel construct. To demonstrate the selfhealing property of hydrogel, gel was cut into two halves and then pressed for 3 h
at 25
C. Self-healed hydrogel was found to withstand its own weight. Frequency test
showed that damaged gel regained storage modulus of its original value after the
healing process (Fig. 35b).
9 Conclusions
As evident from the abovementioned examples, the interest in fabrication of selfhealing hydrogels has intensified in recent years. The design of materials with the
ability to mend themselves is a long sought goal since it would ensure their proper
functioning over time without loss of performance, as well as prevent any possible
catastrophic failures which often start as microscopic cracks. As mentioned earlier,
the healing process needs to initiate at the molecular level, since upon damage, the
cracks in chemically cross-linked materials propagate through the rupture of the
weakest chemical linkages. It is only in recent years that the advances in the
chemistry of polymeric materials have enabled fabrication of functional materials
from the viewpoint of integrating elements of self-healing as a basic design aspect.
While most examples investigate the loss and recovery of mechanical properties of
self-healing materials, some of the examples discussed demonstrate that mechanical
damage induced in the materials can lead to loss of other properties such as electrical
conductivity, arising due to the discontinuity of the matrix. With ever-growing
applications of hydrogels in various areas of biomedical sciences such as
bioelectronics, e.g., devices like strain sensors or neural implant interfaces, the
importance of conductive soft matrices is increasing. Thus, the incentives of introducing self-healing modalities into hydrogels go much beyond the loss of their
mechanical behavior. As illustrated in the sections above, to date, several types of
reversible chemical linkages have been employed effectively to obtain self-healing
materials. But, one can anticipate that the variation in the chemical composition of
the hydrogel matrix will play a crucial role in the appropriate choice of the underlying dynamic chemistry to achieve fast and full recovery of properties. Since
hydrogels with vastly diverse compositions are needed to fulfill the demands of
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
289
initial modulus values upon reducing strain (Fig. 34b). Additionally, visual selfhealing experiments were carried by putting two halves of hydrogels in contact at
room temperature for 12 h (Fig. 34c).
In another recent report, a double-network, acid-resistive self-healing polysaccharide-based hydrogel by combining dynamic Schiff base and Diels-Alder reaction
was reported by Wang and coworkers [81]. Hydrogel was obtained by cross-linking
N-(furfural) carboxymethyl chitosan (FCC) and oxidized dextran (Odex) through
Schiff base reaction, followed with a Diels-Alder reaction upon the addition of MalPEG-Mal (Fig. 35a). Self-healing ability was endowed by the reversible imine
bonds, while the Diels-Alder reaction-based cross-linking provided mechanical
strength and acid resistance to the hydrogel construct. To demonstrate the selfhealing property of hydrogel, gel was cut into two halves and then pressed for 3 h
at 25
C. Self-healed hydrogel was found to withstand its own weight. Frequency test
showed that damaged gel regained storage modulus of its original value after the
healing process (Fig. 35b).
9 Conclusions
As evident from the abovementioned examples, the interest in fabrication of selfhealing hydrogels has intensified in recent years. The design of materials with the
ability to mend themselves is a long sought goal since it would ensure their proper
functioning over time without loss of performance, as well as prevent any possible
catastrophic failures which often start as microscopic cracks. As mentioned earlier,
the healing process needs to initiate at the molecular level, since upon damage, the
cracks in chemically cross-linked materials propagate through the rupture of the
weakest chemical linkages. It is only in recent years that the advances in the
chemistry of polymeric materials have enabled fabrication of functional materials
from the viewpoint of integrating elements of self-healing as a basic design aspect.
While most examples investigate the loss and recovery of mechanical properties of
self-healing materials, some of the examples discussed demonstrate that mechanical
damage induced in the materials can lead to loss of other properties such as electrical
conductivity, arising due to the discontinuity of the matrix. With ever-growing
applications of hydrogels in various areas of biomedical sciences such as
bioelectronics, e.g., devices like strain sensors or neural implant interfaces, the
importance of conductive soft matrices is increasing. Thus, the incentives of introducing self-healing modalities into hydrogels go much beyond the loss of their
mechanical behavior. As illustrated in the sections above, to date, several types of
reversible chemical linkages have been employed effectively to obtain self-healing
materials. But, one can anticipate that the variation in the chemical composition of
the hydrogel matrix will play a crucial role in the appropriate choice of the underlying dynamic chemistry to achieve fast and full recovery of properties. Since
hydrogels with vastly diverse compositions are needed to fulfill the demands of
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
289