of DN hydrogel are comparable to some soft load-bearing tissues. Using the
state-of-the-art double-network approach to enhance the mechanical toughness
has successfully resulted in a series of tough hydrogels and elastomers, such as
polyethylene-oxide/polyacrylic acid double-network gels, tough double-network
hydrogel from biopolymers, multiple network elastomers, etc. [28–31].
However, a disadvantage of DN gel is that the sample shows permanent softening
after experiencing large deformation since the rupture of the brittle networks is the
breaking of the covalent bonds. Thus, DN hydrogels show excellent mechanical
performance but lack self-healing behavior, compared to the loading-bearing
bio-tissues [27]. To address this problem, alternatively it is possible to replace the
irreversible covalent bonds in the brittle network by the dynamic bonds, including
reversible physical bonds and reversible covalent bonds [32]. These reversible bonds
serve as the reversible sacrificial bonds. When the sample is loaded, the reversible
bond is broken to dissipate energy, imparting the toughness of the sample. When the
sample is unloaded, the ruptured reversible bonds are able to re-form, imparting the
internal and surface self-healing of the damaged materials. Along this line, several
tough hydrogels with partial or full self-healing behavior have been fabricated using
either the reversible physical bonds or covalent bonds. For example, the reversible
physical bonds have been used to design self-healing materials, including agar/
polyacrylamide double-network hydrogel and supramolecular hydrogel from the
association of ureidopyrimidinone units based on hydrogen bonds [33–37];
polyion complex hydrogel, polyampholyte hydrogel, and metal-ion-cross-linked
alginate/polyacrylamide double-network hydrogel based on ionic bonds [38–
40]; poly(dodecyl glyceryl itaconate)/polyacrylamide double-network hydrogel,
poly(stearyl methacrylate-co-acrylamide) hydrogel, and poly(dococylacrylate-coacrylamide) hydrogel based on hydrophobic interaction [41–43]; supramolecular
hydrogel from the cyclodextrins based on host-gust interaction [44–46]; the healable
supramolecular polymers based on π – π packing [47, 48]; and so on. The reversible
covalent bonds are used to design self-healing materials, including poly(ethylene
oxide)/tris[(4-formylphenoxy)methyl]ethane supramolecular hydrogel based on
acylhydrazone bonds [49], self-healing gels from copolymerization of n-butyl
acrylate based on trithiocarbonate units [50], self-healing gels based on
diarylbibenzofuranone units [51], etc.
Recently, we develop a tough and self-healing hydrogel from polyampholytes
(PAs) that bears the cationic groups and anionic groups distributed along the main
chain [39]. The random copolymerization process makes the ionic monomers
randomly distributed along the backbones, resulting in the formation of ionic
bonds with a wide strength distribution via inter- and intra-chain complexation in
the polymer network. These ionic bonds were divided into two groups, weak ionic
bonds (dynamic bonds) and strong ionic bonds (Fig. 1a), according to the competition between bonds strength and experimental observation time. The strong ionic
bonds can maintain the integrity of the hydrogel over much longer time scale,
imparting the elastic behavior, while the weak ionic bonds can break to dissipate
energy to give toughness and re-form to enable self-healing behavior. Therefore,
polyampholyte hydrogels are very strong and tough with failure tensile stress of
Tough and Self-Healing Hydrogels from Polyampholytes
297
state-of-the-art double-network approach to enhance the mechanical toughness
has successfully resulted in a series of tough hydrogels and elastomers, such as
polyethylene-oxide/polyacrylic acid double-network gels, tough double-network
hydrogel from biopolymers, multiple network elastomers, etc. [28–31].
However, a disadvantage of DN gel is that the sample shows permanent softening
after experiencing large deformation since the rupture of the brittle networks is the
breaking of the covalent bonds. Thus, DN hydrogels show excellent mechanical
performance but lack self-healing behavior, compared to the loading-bearing
bio-tissues [27]. To address this problem, alternatively it is possible to replace the
irreversible covalent bonds in the brittle network by the dynamic bonds, including
reversible physical bonds and reversible covalent bonds [32]. These reversible bonds
serve as the reversible sacrificial bonds. When the sample is loaded, the reversible
bond is broken to dissipate energy, imparting the toughness of the sample. When the
sample is unloaded, the ruptured reversible bonds are able to re-form, imparting the
internal and surface self-healing of the damaged materials. Along this line, several
tough hydrogels with partial or full self-healing behavior have been fabricated using
either the reversible physical bonds or covalent bonds. For example, the reversible
physical bonds have been used to design self-healing materials, including agar/
polyacrylamide double-network hydrogel and supramolecular hydrogel from the
association of ureidopyrimidinone units based on hydrogen bonds [33–37];
polyion complex hydrogel, polyampholyte hydrogel, and metal-ion-cross-linked
alginate/polyacrylamide double-network hydrogel based on ionic bonds [38–
40]; poly(dodecyl glyceryl itaconate)/polyacrylamide double-network hydrogel,
poly(stearyl methacrylate-co-acrylamide) hydrogel, and poly(dococylacrylate-coacrylamide) hydrogel based on hydrophobic interaction [41–43]; supramolecular
hydrogel from the cyclodextrins based on host-gust interaction [44–46]; the healable
supramolecular polymers based on π – π packing [47, 48]; and so on. The reversible
covalent bonds are used to design self-healing materials, including poly(ethylene
oxide)/tris[(4-formylphenoxy)methyl]ethane supramolecular hydrogel based on
acylhydrazone bonds [49], self-healing gels from copolymerization of n-butyl
acrylate based on trithiocarbonate units [50], self-healing gels based on
diarylbibenzofuranone units [51], etc.
Recently, we develop a tough and self-healing hydrogel from polyampholytes
(PAs) that bears the cationic groups and anionic groups distributed along the main
chain [39]. The random copolymerization process makes the ionic monomers
randomly distributed along the backbones, resulting in the formation of ionic
bonds with a wide strength distribution via inter- and intra-chain complexation in
the polymer network. These ionic bonds were divided into two groups, weak ionic
bonds (dynamic bonds) and strong ionic bonds (Fig. 1a), according to the competition between bonds strength and experimental observation time. The strong ionic
bonds can maintain the integrity of the hydrogel over much longer time scale,
imparting the elastic behavior, while the weak ionic bonds can break to dissipate
energy to give toughness and re-form to enable self-healing behavior. Therefore,
polyampholyte hydrogels are very strong and tough with failure tensile stress of
Tough and Self-Healing Hydrogels from Polyampholytes
297