physiological pH by using hyaluronic acid (HA), a high molecular weight negatively
charged polysaccharide (Fig. 22a) [52]. Dynamic hydrogels were formed by crosslinking maltose and PBA-modified HA in aqueous media at pH 7.4. Self-healing
property of hydrogel was demonstrated by rheological test as well as visual
experiments. Upon applying an oscillatory stress above the LVE region, hydrogel
structure was deformed (50% decrease in G
0 ), and recovery of G
0 occurs (within less
than 10 min) without a significant loss in G
0 value when stress was reduced for five
repeating cycles (Fig. 22b).
Another example utilizing guar gum was reported by Chen and coworkers to
fabricate a multi-responsive self-healing hydrogel by cross-linking it with borax in
aqueous media at room temperature [53]. This reformable and injectable gel possessed rapid and repeatable self-healing ability, as demonstrated by continuous strain
test to assess the changes in G
0 and G
00 under high strain (100%) and low (10%)
strain. Strain-induced decrease in G
0 by 66% was recorded under high strain (100%)
which regained its original value immediately upon reducing strain to 10% G
0 over a
cyclic test. Additionally, several visual experiments where two different hydrogels
were cut, rejoined, stretched, bended, and remolded were demonstrated.
5 Diels-Alder Cycloaddition-Based Hydrogels
Diels-Alder cycloaddition reaction between a diene and dienophile is highly specific
and has been extensively exploited in fabrication of self-healing polymeric materials
[54]. It is thermally reversible due to the instability of Diels-Alder product under
high temperature. Diels-Alder reaction has been utilized to fabricate thermally
healable materials that heal on demand by applying the appropriate thermal
treatment.
The seminal contribution in the area of self-healing materials was made in 2002
by Wudl and coworkers, who utilized Diels-Alder cycloaddition chemistry to obtain
mechanically strong and thermally re-mendable materials by mixing multi-furan and
multi-maleimide monomeric precursors [55]. Re-mending upon damage was examined through compact tension test where two pieces of fractured polymeric material
were held together for 2 h at 120–150
C under nitrogen and cooled to room
temperature. A quick survey of reports on self-healing materials shows that most
of the work reported to date have investigated self-healing properties of
non-hydrophilic cross-linked networks. For example, several self-healing gel systems based on furan-containing methacrylate polymers cross-linked using
bismaleimide linkers have been reported [56]. Although interesting, utilization of
high temperatures to achieve self-healing can be a limitation in many applications. It
can be anticipated that the maleimide functional group will undergo decomposition
through hydrolysis and ring opening to a certain extent upon heating in aqueous
environment.
One of the advantages that the Diels-Alder reaction offers for fabrication of
hydrogels is that it can be carried out in water, is catalyst-free, has tolerance toward
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
273
charged polysaccharide (Fig. 22a) [52]. Dynamic hydrogels were formed by crosslinking maltose and PBA-modified HA in aqueous media at pH 7.4. Self-healing
property of hydrogel was demonstrated by rheological test as well as visual
experiments. Upon applying an oscillatory stress above the LVE region, hydrogel
structure was deformed (50% decrease in G
0 ), and recovery of G
0 occurs (within less
than 10 min) without a significant loss in G
0 value when stress was reduced for five
repeating cycles (Fig. 22b).
Another example utilizing guar gum was reported by Chen and coworkers to
fabricate a multi-responsive self-healing hydrogel by cross-linking it with borax in
aqueous media at room temperature [53]. This reformable and injectable gel possessed rapid and repeatable self-healing ability, as demonstrated by continuous strain
test to assess the changes in G
0 and G
00 under high strain (100%) and low (10%)
strain. Strain-induced decrease in G
0 by 66% was recorded under high strain (100%)
which regained its original value immediately upon reducing strain to 10% G
0 over a
cyclic test. Additionally, several visual experiments where two different hydrogels
were cut, rejoined, stretched, bended, and remolded were demonstrated.
5 Diels-Alder Cycloaddition-Based Hydrogels
Diels-Alder cycloaddition reaction between a diene and dienophile is highly specific
and has been extensively exploited in fabrication of self-healing polymeric materials
[54]. It is thermally reversible due to the instability of Diels-Alder product under
high temperature. Diels-Alder reaction has been utilized to fabricate thermally
healable materials that heal on demand by applying the appropriate thermal
treatment.
The seminal contribution in the area of self-healing materials was made in 2002
by Wudl and coworkers, who utilized Diels-Alder cycloaddition chemistry to obtain
mechanically strong and thermally re-mendable materials by mixing multi-furan and
multi-maleimide monomeric precursors [55]. Re-mending upon damage was examined through compact tension test where two pieces of fractured polymeric material
were held together for 2 h at 120–150
C under nitrogen and cooled to room
temperature. A quick survey of reports on self-healing materials shows that most
of the work reported to date have investigated self-healing properties of
non-hydrophilic cross-linked networks. For example, several self-healing gel systems based on furan-containing methacrylate polymers cross-linked using
bismaleimide linkers have been reported [56]. Although interesting, utilization of
high temperatures to achieve self-healing can be a limitation in many applications. It
can be anticipated that the maleimide functional group will undergo decomposition
through hydrolysis and ring opening to a certain extent upon heating in aqueous
environment.
One of the advantages that the Diels-Alder reaction offers for fabrication of
hydrogels is that it can be carried out in water, is catalyst-free, has tolerance toward
Self-Healing Hydrogels Based on Reversible Covalent Linkages: A Survey of. . .
273