such as microplastics causing serious pollution in the seas [16]. It is also important
for hydrogels as they are used as biomaterials, scaffolds in tissue engineering and
drug delivery systems, and superabsorbents where their extended service time is of
prime importance.
The number of the works published on self-healing/self-recovery hydrogels has
grown almost exponentially during the past 10 years (Fig. 1c). Several reviews have
been published covering exhaustive collection of all results reported so far [17–
24]. A critical survey over the published works reveals that many studies deal with
the preparation of self-healing physical hydrogels exhibiting a frequency-dependent
low storage modulus and a Young’s modulus and fracture stress in the Pa to kPa
range. Because self-healing and mechanical strength are inversely related, it is not
surprising to detect self-healing in such weak hydrogels having short-lived crosslinks. However, hydrogels with a good mechanical performance such as cartilage
require existence of cross-links with long lifetimes together with an efficient energy
dissipation mechanism to prevent crack propagation. Because self-healing efficiency
decreases with increasing lifetime of cross-links, it seems a challenge to generate
self-healing in high-strength hydrogels with modulus and tensile strength in the
range of MPa. This review tries to answer the question “How to design both
mechanically strong and self-healable hydrogels?”.
The capability to self-heal in hydrogels is generated by forming a reversible 3D
network of polymer chains via dynamic covalent bonds or non-covalent interactions.
Thus, instead of chemical cross-links, intermolecular bonds with finite lifetimes are
used to build a hydrogel network. The type of the physical bonds and their lifetimes
are key elements determining many of the properties of self-healing physical
hydrogels. Dynamic covalent bonds such as phenylboronic ester, acylhydrazone,
disulfide, dynamic imine bonds, as well as reversible radical and Diels-Alder
reactions have been used to create self-healing in hydrogels. One may expect that
such bonds will combine the strength and reversibility of covalent and non-covalent
bonds, respectively. However, hydrogels formed via dynamic covalent bonds generally exhibit insufficient mechanical properties for load-bearing applications, and
their preparation often requires sophisticated synthetic procedures [17]. Therefore,
this review covers publications on creating mechanically strong self-healing/selfrecovery hydrogels via non-covalent interactions. We focus here on hydrophobic
and hydrogen-bonding interactions as well as on their combinations with ionic
interactions. Self-healable polyampholyte hydrogels formed via ionic bonds [25],
and physical double-network hydrogels have been reviewed by Sun and Cui in this
volume [26].
Because of the inverse proportionality between self-healing efficiency and lifetime of intermolecular cross-links, one may argue that an effective self-healing could
not be achieved in a mechanically strong hydrogel with long-lived cross-links.
However, it could be achieved in a short period of time if an external trigger induces
a significant, reversible hard-to-soft, or first-order transition from order to disorder in
the cross-link domains of the hydrogels leading to a dramatic change in the crosslink lifetime. For example, if a semicrystalline physical hydrogel with a modulus and
tensile strength in the MPa range is damaged, heating the damaged region above the
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
25
for hydrogels as they are used as biomaterials, scaffolds in tissue engineering and
drug delivery systems, and superabsorbents where their extended service time is of
prime importance.
The number of the works published on self-healing/self-recovery hydrogels has
grown almost exponentially during the past 10 years (Fig. 1c). Several reviews have
been published covering exhaustive collection of all results reported so far [17–
24]. A critical survey over the published works reveals that many studies deal with
the preparation of self-healing physical hydrogels exhibiting a frequency-dependent
low storage modulus and a Young’s modulus and fracture stress in the Pa to kPa
range. Because self-healing and mechanical strength are inversely related, it is not
surprising to detect self-healing in such weak hydrogels having short-lived crosslinks. However, hydrogels with a good mechanical performance such as cartilage
require existence of cross-links with long lifetimes together with an efficient energy
dissipation mechanism to prevent crack propagation. Because self-healing efficiency
decreases with increasing lifetime of cross-links, it seems a challenge to generate
self-healing in high-strength hydrogels with modulus and tensile strength in the
range of MPa. This review tries to answer the question “How to design both
mechanically strong and self-healable hydrogels?”.
The capability to self-heal in hydrogels is generated by forming a reversible 3D
network of polymer chains via dynamic covalent bonds or non-covalent interactions.
Thus, instead of chemical cross-links, intermolecular bonds with finite lifetimes are
used to build a hydrogel network. The type of the physical bonds and their lifetimes
are key elements determining many of the properties of self-healing physical
hydrogels. Dynamic covalent bonds such as phenylboronic ester, acylhydrazone,
disulfide, dynamic imine bonds, as well as reversible radical and Diels-Alder
reactions have been used to create self-healing in hydrogels. One may expect that
such bonds will combine the strength and reversibility of covalent and non-covalent
bonds, respectively. However, hydrogels formed via dynamic covalent bonds generally exhibit insufficient mechanical properties for load-bearing applications, and
their preparation often requires sophisticated synthetic procedures [17]. Therefore,
this review covers publications on creating mechanically strong self-healing/selfrecovery hydrogels via non-covalent interactions. We focus here on hydrophobic
and hydrogen-bonding interactions as well as on their combinations with ionic
interactions. Self-healable polyampholyte hydrogels formed via ionic bonds [25],
and physical double-network hydrogels have been reviewed by Sun and Cui in this
volume [26].
Because of the inverse proportionality between self-healing efficiency and lifetime of intermolecular cross-links, one may argue that an effective self-healing could
not be achieved in a mechanically strong hydrogel with long-lived cross-links.
However, it could be achieved in a short period of time if an external trigger induces
a significant, reversible hard-to-soft, or first-order transition from order to disorder in
the cross-link domains of the hydrogels leading to a dramatic change in the crosslink lifetime. For example, if a semicrystalline physical hydrogel with a modulus and
tensile strength in the MPa range is damaged, heating the damaged region above the
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
25