linked gels. Repetitive step strain measurements were performed at high (γ ¼ 200%
for TMCLA or 800% for TMCDT/TMCLA containing copolymers) and low
(γ ¼ 1% for all) strains to demonstrate self-healing properties of mono-thiol or
dithiol cross-linked hydrogels (Fig. 26b). Physical properties of hydrogels such as
stress relaxation rate, degradation profile, and strength were found to be tunable by
changing the ratio of dithiolane monomers.
7 Photo-Responsive Linkage-Based Hydrogels
Photochemical fabrication of hydrogels is operationally simple reactions that usually
proceed with rapid kinetics under mild and environmentally benign conditions.
Furthermore, any photochemical addition reactions are reversible in nature, i.e.,
yield products that can revert back to starting components and thus can be
reconnected using external stimuli to self-heal the material. Thus, utilization of
photo-responsive systems to yield self-healing polymeric materials such as elastomers, rubbers, plastics, and even hydrogels has drawn considerable attention in
recent years.
Photo-induced self-healing of polymeric materials can be achieved through
several chemical processes such as photo-reversible cycloaddition, photoisomerization, etc. Incorporation of cinnamoyl groups such as cross-linking junctions is widely used to obtain photochemical healing where [2+2] cycloadduct of
cinnamoyl groups reform under UV irradiation to re-cross-link the polymeric network [62, 63]. Thymine is another motif that can undergo photo-dimerization upon
irradiation above 270 nm and has been incorporated to yield multi-responsive
hydrogels [64]. Anthracene derivatives [65, 66], styrylpyrene [67], and
hexaarylbiimidazoles [68, 69] are other functional groups that have been employed
to obtain photo-cleavable and healable materials such as photodynamic hydrogels
[67], photovoltaics [63], photoplastics [69], and elastomers [66].
While photo-dimerization reactions are most commonly employed approach to
introduce reversible cross-linking, other reactions that involve photo-induced cleavage and bond formations have emerged in recent years. For example, a radicalmediated disulfide fragmentation reaction was utilized by Bowman, Anseth, and
coworkers to yield self-healing hydrogels [70]. Hydrogel was obtained by mixing
tetra-arm PEG tetra-thiol in DI water containing hydrogen peroxide and sodium
iodide (1 mM). It was observed that photodegradation of these gels occurs when they
are exposed to 365 nm in the presence of lithium acylphosphinate, a photoinitiator.
The radicals formed from the cleavage of the photoinitiator cleave the disulfide
bonds in the hydrogel. Importantly, it was postulated that if the amount of radicals
formed are much less compared to the sulfur atoms in the gel, the hydrogel will adapt
to applied strain. Such radical generation-induced disulfide exchange reactions were
used to induce self-healing of an interface. Parallel-plate rheometry tests were
conducted to assess mechanical properties of gels before and after photo-healing
process. As another example, Singha and coworkers designed a hydrogel system
278
R. Kilic and A. Sanyal
for TMCLA or 800% for TMCDT/TMCLA containing copolymers) and low
(γ ¼ 1% for all) strains to demonstrate self-healing properties of mono-thiol or
dithiol cross-linked hydrogels (Fig. 26b). Physical properties of hydrogels such as
stress relaxation rate, degradation profile, and strength were found to be tunable by
changing the ratio of dithiolane monomers.
7 Photo-Responsive Linkage-Based Hydrogels
Photochemical fabrication of hydrogels is operationally simple reactions that usually
proceed with rapid kinetics under mild and environmentally benign conditions.
Furthermore, any photochemical addition reactions are reversible in nature, i.e.,
yield products that can revert back to starting components and thus can be
reconnected using external stimuli to self-heal the material. Thus, utilization of
photo-responsive systems to yield self-healing polymeric materials such as elastomers, rubbers, plastics, and even hydrogels has drawn considerable attention in
recent years.
Photo-induced self-healing of polymeric materials can be achieved through
several chemical processes such as photo-reversible cycloaddition, photoisomerization, etc. Incorporation of cinnamoyl groups such as cross-linking junctions is widely used to obtain photochemical healing where [2+2] cycloadduct of
cinnamoyl groups reform under UV irradiation to re-cross-link the polymeric network [62, 63]. Thymine is another motif that can undergo photo-dimerization upon
irradiation above 270 nm and has been incorporated to yield multi-responsive
hydrogels [64]. Anthracene derivatives [65, 66], styrylpyrene [67], and
hexaarylbiimidazoles [68, 69] are other functional groups that have been employed
to obtain photo-cleavable and healable materials such as photodynamic hydrogels
[67], photovoltaics [63], photoplastics [69], and elastomers [66].
While photo-dimerization reactions are most commonly employed approach to
introduce reversible cross-linking, other reactions that involve photo-induced cleavage and bond formations have emerged in recent years. For example, a radicalmediated disulfide fragmentation reaction was utilized by Bowman, Anseth, and
coworkers to yield self-healing hydrogels [70]. Hydrogel was obtained by mixing
tetra-arm PEG tetra-thiol in DI water containing hydrogen peroxide and sodium
iodide (1 mM). It was observed that photodegradation of these gels occurs when they
are exposed to 365 nm in the presence of lithium acylphosphinate, a photoinitiator.
The radicals formed from the cleavage of the photoinitiator cleave the disulfide
bonds in the hydrogel. Importantly, it was postulated that if the amount of radicals
formed are much less compared to the sulfur atoms in the gel, the hydrogel will adapt
to applied strain. Such radical generation-induced disulfide exchange reactions were
used to induce self-healing of an interface. Parallel-plate rheometry tests were
conducted to assess mechanical properties of gels before and after photo-healing
process. As another example, Singha and coworkers designed a hydrogel system
278
R. Kilic and A. Sanyal