the healing efficiencies was also quantitatively evaluated and deduced to be as high
as 96.1 Æ 3.0%, using wedge-shaped strain compression test.
In 2017, Zhang and coworkers synthesized SN hydrogels sensitive to pH, redox,
and glucose based on dynamic boronate bonds [78]. Hydrogels were obtained
through cross-linking of a diol-containing polymer (PVA) or catechol-bearing
polymer P(AM-DOPMA) (poly-(acrylamide-co-dopamine methacrylamide) with a
disulfide-containing boronic acid-based cross-linker (BPBAC) in DMSO/water
mixture (Fig. 33a). Both PVA and P(AM-DOPMA) hydrogels cross-linked with
BPBAC showed self-healing behavior. Visual self-healing experiments of P
(AM-DOPMA)/BPBAC hydrogel demonstrated combination of two pieces within
1 min with complete disappearance of the cut line in 5 min. To demonstrate selfhealing, visual experiments were carried out by putting two pieces of hydrogels in
contact (Fig. 33b). Also, time sweep tests were done on self-healed gels right after
deforming gels under 800% strain, with self-healed gels possessing G
0 and G
00 values
similar to pristine hydrogel (Fig. 33c).
A combination of dynamic covalent chemistries was recently used by Lu and
coworkers [79]. They designed a chondroitin sulfate-based double-network, injectable self-healing hydrogel for cranial bone repair. Chondroitin sulfate is a natural
polymer derived from animal cartilage and is known for improving wound healing
and facilitating chondrogenesis. Three different hydrogel systems were prepared as
AA (F127@ChS-PEG), ABC (F127@ChS-PEG-OChS), and BC (F127@ChSOChS) hydrogels which were cross-linked by Diels-Alder chemistry (using a
bismaleimide-containing polymer PEG-AMI, M n ¼ 2 kDa), dynamic acylhydrazone
bonds and Diels-Alder chemistry, and dynamic acylhydrazone bonds (through crosslinking with oxidized chondroitin sulfide), respectively. The ABC type of hydrogel
Fig. 32 (a) Preparation of disulfide and acylhydrazone cross-linked hydrogels, (b) photographs
and microscopic images of self-healing of DN hydrogel, (c) compressive stress-strain test of
original and self-healed hydrogels. Adapted with permission [77]. Copyright 2017 Wiley-VCH
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
285
as 96.1 Æ 3.0%, using wedge-shaped strain compression test.
In 2017, Zhang and coworkers synthesized SN hydrogels sensitive to pH, redox,
and glucose based on dynamic boronate bonds [78]. Hydrogels were obtained
through cross-linking of a diol-containing polymer (PVA) or catechol-bearing
polymer P(AM-DOPMA) (poly-(acrylamide-co-dopamine methacrylamide) with a
disulfide-containing boronic acid-based cross-linker (BPBAC) in DMSO/water
mixture (Fig. 33a). Both PVA and P(AM-DOPMA) hydrogels cross-linked with
BPBAC showed self-healing behavior. Visual self-healing experiments of P
(AM-DOPMA)/BPBAC hydrogel demonstrated combination of two pieces within
1 min with complete disappearance of the cut line in 5 min. To demonstrate selfhealing, visual experiments were carried out by putting two pieces of hydrogels in
contact (Fig. 33b). Also, time sweep tests were done on self-healed gels right after
deforming gels under 800% strain, with self-healed gels possessing G
0 and G
00 values
similar to pristine hydrogel (Fig. 33c).
A combination of dynamic covalent chemistries was recently used by Lu and
coworkers [79]. They designed a chondroitin sulfate-based double-network, injectable self-healing hydrogel for cranial bone repair. Chondroitin sulfate is a natural
polymer derived from animal cartilage and is known for improving wound healing
and facilitating chondrogenesis. Three different hydrogel systems were prepared as
AA (F127@ChS-PEG), ABC (F127@ChS-PEG-OChS), and BC (F127@ChSOChS) hydrogels which were cross-linked by Diels-Alder chemistry (using a
bismaleimide-containing polymer PEG-AMI, M n ¼ 2 kDa), dynamic acylhydrazone
bonds and Diels-Alder chemistry, and dynamic acylhydrazone bonds (through crosslinking with oxidized chondroitin sulfide), respectively. The ABC type of hydrogel
Fig. 32 (a) Preparation of disulfide and acylhydrazone cross-linked hydrogels, (b) photographs
and microscopic images of self-healing of DN hydrogel, (c) compressive stress-strain test of
original and self-healed hydrogels. Adapted with permission [77]. Copyright 2017 Wiley-VCH
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
285