NIPAM
N-Isopropylacrylamide
PAAc
Poly(AAc)
PAAm
Poly(AAm)
PAMPS
Poly(AMPS)
PDMAA Poly(DMAA)
PEG
Poly(ethylene glycol)
PVP
Poly(N-vinylpyrrolidone)
SDS
Sodium dodecyl sulfate
SFS
Scanning force microscopy
tan δ
Loss factor (¼G
00 /G
0 )
T m
Melting temperature
U hys
Hysteresis energy
UPy
Ureidopyrimidinone
WLMs
Worm-like micelles
1 Introduction
Hydrogels are 3D networks of chemically and/or physically cross-linked polymer
chains swollen in water. They are soft and smart materials with a variety of
applications including superabsorbents, tissue engineering, drug delivery, soft contact lenses, and so on [1–4]. Because the first-generation classical hydrogels prepared using a chemical cross-linker were too weak or brittle in nature, extensive
studies conducted in the past decade explored a new design principle for the
fabrication of mechanically strong hydrogels of high toughness [5–12]. This principle bases on creating an effective energy dissipation in the hydrogel network using
sacrificial or reversible bonds that prevent propagation of crack and hence a catastrophic damage even under large strain. Otherwise, that is, if the crack energy
localizes around the crack tip and cannot be dissipated as in the classical hydrogels,
rapid crack propagation leads to the fracture of the whole hydrogel. By manipulating
the gel structure to induce dissipative mechanisms at the molecular level, the secondgeneration hydrogels developed so far exhibit Young’s moduli and tensile strengths
in the range of MPa and hence are a good candidate for the replacement of loadbearing tissues such as cartilage, tendons, and ligaments [2]. For example, doublenetworking strategy developed by Gong and co-workers bases on creating two
interpenetrated and interconnected networks in a single hydrogel material, namely,
highly and loosely chemically cross-linked polymer networks acting as brittle and
ductile components, respectively [6, 13]. The sacrificial bonds of the brittle network
break under a low strain to produce many microcracks by dissipating energy, while
the ductile network keeps the macroscopic sample together.
Another important challenge emerging in the field of mechanically robust
hydrogels is to generate self-healing or self-recovery ability in these materials.
Self-healing, which is an inherent property of many biological systems, is defined
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
23
N-Isopropylacrylamide
PAAc
Poly(AAc)
PAAm
Poly(AAm)
PAMPS
Poly(AMPS)
PDMAA Poly(DMAA)
PEG
Poly(ethylene glycol)
PVP
Poly(N-vinylpyrrolidone)
SDS
Sodium dodecyl sulfate
SFS
Scanning force microscopy
tan δ
Loss factor (¼G
00 /G
0 )
T m
Melting temperature
U hys
Hysteresis energy
UPy
Ureidopyrimidinone
WLMs
Worm-like micelles
1 Introduction
Hydrogels are 3D networks of chemically and/or physically cross-linked polymer
chains swollen in water. They are soft and smart materials with a variety of
applications including superabsorbents, tissue engineering, drug delivery, soft contact lenses, and so on [1–4]. Because the first-generation classical hydrogels prepared using a chemical cross-linker were too weak or brittle in nature, extensive
studies conducted in the past decade explored a new design principle for the
fabrication of mechanically strong hydrogels of high toughness [5–12]. This principle bases on creating an effective energy dissipation in the hydrogel network using
sacrificial or reversible bonds that prevent propagation of crack and hence a catastrophic damage even under large strain. Otherwise, that is, if the crack energy
localizes around the crack tip and cannot be dissipated as in the classical hydrogels,
rapid crack propagation leads to the fracture of the whole hydrogel. By manipulating
the gel structure to induce dissipative mechanisms at the molecular level, the secondgeneration hydrogels developed so far exhibit Young’s moduli and tensile strengths
in the range of MPa and hence are a good candidate for the replacement of loadbearing tissues such as cartilage, tendons, and ligaments [2]. For example, doublenetworking strategy developed by Gong and co-workers bases on creating two
interpenetrated and interconnected networks in a single hydrogel material, namely,
highly and loosely chemically cross-linked polymer networks acting as brittle and
ductile components, respectively [6, 13]. The sacrificial bonds of the brittle network
break under a low strain to produce many microcracks by dissipating energy, while
the ductile network keeps the macroscopic sample together.
Another important challenge emerging in the field of mechanically robust
hydrogels is to generate self-healing or self-recovery ability in these materials.
Self-healing, which is an inherent property of many biological systems, is defined
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
23