surfaces so that, after cooling below T m , ds-DNA bridges formed between the
surfaces provide healing of the hydrogels (Fig. 8c) [15].
Self-recovery hydrogels via both dipole-dipole and H-bonding interactions were
fabricated by copolymerization of the dipole acrylonitrile, H-bonding acrylamide
(AAm), anionic AMPS, and a hydrophilic cross-linker in DMSO as the solvent,
which was replaced with water after the reaction [70]. It was shown that the amount
of ionic AMPS segments in the gel network regulates the water content between
32 and 98%, whereas the collaborative effect of H-bonding and dipole-dipole
interactions leads to mechanically robust hydrogels with a tensile strength and
elongation at break up to ~8 MPa and 700%, respectively. Self-recovery doublenetwork hydrogels based on poly(acrylic acid) and poly(N-isopropyl acrylamide)
were prepared by both chemical cross-links and H-bonds [71]. Young’s modulus of
the hydrogels is around 226 MPa at room temperature, but it significantly decreases
at elevated temperature indicating that the cooperative H-bonds mainly determine
the cross-link density of the hydrogels.
Hydrogels via both electrostatic and H-bonding interactions were prepared by
mixing of a solution of the cationic polyelectrolytes poly(diallyldimethylammonium
chloride) and branched poly(ethylenimine) (PEI) with another solution of anionic
polyelectrolytes poly(sodium 4-styrenesulfonate) and poly(acrylic acid), followed
by molding, drying, and rehydration [72]. The hydrogels with 42% water content
having oppositely charged ionic groups and H-bond forming sites exhibit a modulus,
tensile strength, and elongation at break of 0.4 MPa, 1 MPa, and 2,400%, respectively, and a complete healing efficiency after immersing in water at room temperature for 14 h. Mixing of aqueous solutions or hydrogels of ionic polymers with
oppositely charged ions was also utilized to create hydrogels via reversible ionic
bonds enabling an efficient energy dissipation [73, 74]. Immersion of a loosely
cross-linked poly(acrylamide-co-acrylic acid) hydrogel in aqueous FeCl 3 solution to
form the physical cross-links followed by washing with water to remove the excess
ions leads to self-recoverable hydrogels with a high modulus (3 MPa), tensile
strength (6 MPa), and 500% elongation at break [75]. Ionic nanocomposite selfhealing hydrogels were prepared from acrylic acid (AAc), vinyl hybrid silica
nanoparticles (VSNPs), and Fe
3+ ions by free-radical polymerization [76]. Physical
cross-links between PAAc chains and Fe
3+ ions lead to nanocomposite physical
hydrogels with a tensile strength and elongation at break of 0.9 MPa and 2,300%,
respectively, exhibiting self-healing ability at elevated temperature recovering
1,800% elongation at break and 0.56 MPa tensile strength [76]. Self-healing
hydrogels were also prepared using Fe
3+ ions and carboxylated cellulose nanofibrils
(CNFs) as physical cross-linkers [77]. Carboxylated CNFs form H-bonds with poly
(acrylic acid) (PAAc) chains, whereas Fe
3+ forms ionic bonds with the carboxylic
groups of both PAAc and carboxylated CNFs. The hydrogels exhibit a relatively
high tensile strength, elongation at break, and healing efficiency of 4 MPa, 180%,
and 87%, respectively [77]. Hydrogels based on cationic polyacrylamides reinforced
with graphene oxide (GO) exhibit an efficient energy dissipation under stress due to
the H-bonding and ionic interactions between AAm-GO and GO-cationic segments,
respectively [78]. Both the amount of GO and the copolymer composition are the
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
35
surfaces provide healing of the hydrogels (Fig. 8c) [15].
Self-recovery hydrogels via both dipole-dipole and H-bonding interactions were
fabricated by copolymerization of the dipole acrylonitrile, H-bonding acrylamide
(AAm), anionic AMPS, and a hydrophilic cross-linker in DMSO as the solvent,
which was replaced with water after the reaction [70]. It was shown that the amount
of ionic AMPS segments in the gel network regulates the water content between
32 and 98%, whereas the collaborative effect of H-bonding and dipole-dipole
interactions leads to mechanically robust hydrogels with a tensile strength and
elongation at break up to ~8 MPa and 700%, respectively. Self-recovery doublenetwork hydrogels based on poly(acrylic acid) and poly(N-isopropyl acrylamide)
were prepared by both chemical cross-links and H-bonds [71]. Young’s modulus of
the hydrogels is around 226 MPa at room temperature, but it significantly decreases
at elevated temperature indicating that the cooperative H-bonds mainly determine
the cross-link density of the hydrogels.
Hydrogels via both electrostatic and H-bonding interactions were prepared by
mixing of a solution of the cationic polyelectrolytes poly(diallyldimethylammonium
chloride) and branched poly(ethylenimine) (PEI) with another solution of anionic
polyelectrolytes poly(sodium 4-styrenesulfonate) and poly(acrylic acid), followed
by molding, drying, and rehydration [72]. The hydrogels with 42% water content
having oppositely charged ionic groups and H-bond forming sites exhibit a modulus,
tensile strength, and elongation at break of 0.4 MPa, 1 MPa, and 2,400%, respectively, and a complete healing efficiency after immersing in water at room temperature for 14 h. Mixing of aqueous solutions or hydrogels of ionic polymers with
oppositely charged ions was also utilized to create hydrogels via reversible ionic
bonds enabling an efficient energy dissipation [73, 74]. Immersion of a loosely
cross-linked poly(acrylamide-co-acrylic acid) hydrogel in aqueous FeCl 3 solution to
form the physical cross-links followed by washing with water to remove the excess
ions leads to self-recoverable hydrogels with a high modulus (3 MPa), tensile
strength (6 MPa), and 500% elongation at break [75]. Ionic nanocomposite selfhealing hydrogels were prepared from acrylic acid (AAc), vinyl hybrid silica
nanoparticles (VSNPs), and Fe
3+ ions by free-radical polymerization [76]. Physical
cross-links between PAAc chains and Fe
3+ ions lead to nanocomposite physical
hydrogels with a tensile strength and elongation at break of 0.9 MPa and 2,300%,
respectively, exhibiting self-healing ability at elevated temperature recovering
1,800% elongation at break and 0.56 MPa tensile strength [76]. Self-healing
hydrogels were also prepared using Fe
3+ ions and carboxylated cellulose nanofibrils
(CNFs) as physical cross-linkers [77]. Carboxylated CNFs form H-bonds with poly
(acrylic acid) (PAAc) chains, whereas Fe
3+ forms ionic bonds with the carboxylic
groups of both PAAc and carboxylated CNFs. The hydrogels exhibit a relatively
high tensile strength, elongation at break, and healing efficiency of 4 MPa, 180%,
and 87%, respectively [77]. Hydrogels based on cationic polyacrylamides reinforced
with graphene oxide (GO) exhibit an efficient energy dissipation under stress due to
the H-bonding and ionic interactions between AAm-GO and GO-cationic segments,
respectively [78]. Both the amount of GO and the copolymer composition are the
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
35