where photo-triggered dynamic disulfide metathesis reaction took place under UV
exposure for 2 h to result in photo-healing of hydrogel [71]. A zwitterionic block
copolymer consisting of a poly(furfuryl methacrylate) (PFMA)-based core-forming
block and a poly(dimethyl[3(2-methyl-acryloylamino)-propyl]-(3-sulfopropyl)
ammonium) (poly(sulfobetaine))-based shell was cross-linked using a disulfidecontaining bismaleimide unit. Hydrogel films formed by assembly of the micelles
were demonstrated to be self-healable due to photo-triggered dynamic disulfide
metathesis reaction and zwitterionic interactions using atomic force microscopy
analysis of closure of an induced scratch on the surface.
Coumarin, a class of benzopyrone which is present in many natural products such
as cinnamon, undergoes [2+2] cycloaddition to form dimers under long wavelength
UV irradiation. The photo-dimer undergoes cleavage upon exposure to shortwavelength UV irradiation. In 2016, Xing and Zhong utilized the photo-reversible
dimerization of coumarin to obtain a photo-induced self-healing cytocompatible
hydrogel system [72]. Hydrogels were fabricated through the copolymerization of
acrylamide, 7-(2-methacryloyloxyethoxy)-4-methylcoumarin, and agmatine
containing poly(amidoamine) in the presence of ammonium-persulfate/N,N,N
0 ,N0 -tetramethylethylenediamine (APS/TEMED) in dimethyl sulfoxide (DMSO) at
65
C (Fig. 27a). Obtained hydrogels were cut into two halves and put in contact
for 10, 20, 30, and 60 min under UV irradiation (365 nm) to demonstrate self-healing
(Fig. 27b). Tensile stress, breaking elongations, and tensile modulus tests also
revealed that coumarin-containing hydrogels restored their mechanical properties
upon UV curing. Moreover, photo-reversibility of coumarin dimer in network was
investigated in healed gels which were cut and cured under 365 nm UV irradiation
for 30 min. Gels were further exposed to 254 nm UV irradiation (where photocleavage of the formed cyclobutane rings occurs) for 10 or 20 min. It was reported
that gel strength was reduced by increasing exposure time according to stress-strain
tests (Fig. 27c). While the photodissociation of hydrogels may appear opposite to the
intention of introducing self-healable junctions, presence of such dissociable linkages allows the modulation of hydrogel stiffness through simple exposure to UV
irradiation.
A very different approach was recently reported by Scott and coworkers, where
they utilized photo-reversible cleavage of hexaarylbiimidazole (HABI), an imidazole derivative which produces triarylimidazolyl radicals that recombine to yield the
dimer [68]. A hydrogel was obtained through the Cu(I)-catalyzed azide-alkyne
cycloaddition reaction of alkyne-functionalized HABI (HHy-HABI) and four-arm
PEG tetra-azide in DMF (Fig. 28a). Photo-reversibility of HABI dimers through the
cleavage under UV and their reformation in the dark was examined by rheological
tests, where strain response of hydrogels was recorded under constant shear stress.
An increased deformation upon UV exposure was observed while the strain was
constant in the dark, while a bisphenol A-based gel system prepared as control,
which does not contain any photosensitive moieties, did not exhibit such switchable
deformation (Fig. 28b).
Recently, Kalow and coworkers reported a novel hydrogel system involving
photo-triggered dynamic covalent bond [73]. In their work, hydrogels were crossSelf-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
279
exposure for 2 h to result in photo-healing of hydrogel [71]. A zwitterionic block
copolymer consisting of a poly(furfuryl methacrylate) (PFMA)-based core-forming
block and a poly(dimethyl[3(2-methyl-acryloylamino)-propyl]-(3-sulfopropyl)
ammonium) (poly(sulfobetaine))-based shell was cross-linked using a disulfidecontaining bismaleimide unit. Hydrogel films formed by assembly of the micelles
were demonstrated to be self-healable due to photo-triggered dynamic disulfide
metathesis reaction and zwitterionic interactions using atomic force microscopy
analysis of closure of an induced scratch on the surface.
Coumarin, a class of benzopyrone which is present in many natural products such
as cinnamon, undergoes [2+2] cycloaddition to form dimers under long wavelength
UV irradiation. The photo-dimer undergoes cleavage upon exposure to shortwavelength UV irradiation. In 2016, Xing and Zhong utilized the photo-reversible
dimerization of coumarin to obtain a photo-induced self-healing cytocompatible
hydrogel system [72]. Hydrogels were fabricated through the copolymerization of
acrylamide, 7-(2-methacryloyloxyethoxy)-4-methylcoumarin, and agmatine
containing poly(amidoamine) in the presence of ammonium-persulfate/N,N,N
0 ,N0 -tetramethylethylenediamine (APS/TEMED) in dimethyl sulfoxide (DMSO) at
65
C (Fig. 27a). Obtained hydrogels were cut into two halves and put in contact
for 10, 20, 30, and 60 min under UV irradiation (365 nm) to demonstrate self-healing
(Fig. 27b). Tensile stress, breaking elongations, and tensile modulus tests also
revealed that coumarin-containing hydrogels restored their mechanical properties
upon UV curing. Moreover, photo-reversibility of coumarin dimer in network was
investigated in healed gels which were cut and cured under 365 nm UV irradiation
for 30 min. Gels were further exposed to 254 nm UV irradiation (where photocleavage of the formed cyclobutane rings occurs) for 10 or 20 min. It was reported
that gel strength was reduced by increasing exposure time according to stress-strain
tests (Fig. 27c). While the photodissociation of hydrogels may appear opposite to the
intention of introducing self-healable junctions, presence of such dissociable linkages allows the modulation of hydrogel stiffness through simple exposure to UV
irradiation.
A very different approach was recently reported by Scott and coworkers, where
they utilized photo-reversible cleavage of hexaarylbiimidazole (HABI), an imidazole derivative which produces triarylimidazolyl radicals that recombine to yield the
dimer [68]. A hydrogel was obtained through the Cu(I)-catalyzed azide-alkyne
cycloaddition reaction of alkyne-functionalized HABI (HHy-HABI) and four-arm
PEG tetra-azide in DMF (Fig. 28a). Photo-reversibility of HABI dimers through the
cleavage under UV and their reformation in the dark was examined by rheological
tests, where strain response of hydrogels was recorded under constant shear stress.
An increased deformation upon UV exposure was observed while the strain was
constant in the dark, while a bisphenol A-based gel system prepared as control,
which does not contain any photosensitive moieties, did not exhibit such switchable
deformation (Fig. 28b).
Recently, Kalow and coworkers reported a novel hydrogel system involving
photo-triggered dynamic covalent bond [73]. In their work, hydrogels were crossSelf-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
279