4.1 Biological Archetypes
The self-healing possibilities based on metal–ligand interactions can also be
observed in nature. In 2001, Vaccaro and Waite described the ability of mussel
byssus threads to heal after an inflicted damage [82]. In the following decade, more
detailed insights into the mechanism and the parameters that influence this natural
system were collected. Meanwhile, it could be shown that the self-healing behavior
is based on an interaction between iron(III) ions and 3,4-dihydroxyphenylalanine
(dopa) [83]. The iron center can bind one, two, or three catcheol-based ligands,
which are connected with a polymer backbone. The number of bonded ligands
depends on the pH value [84–86]. At low pH (below 5), the mono-dopa iron(III)
complex is formed, which does not lead to a crosslinking of the polymer chains. The
result is an extensible material. By contrast, increasing pH values lead to a more
crosslinked polymer network, which provides an increased hardness [83, 87–90].
This principle, which can be found in nature, could also be mimicked by synthetic
polymers. For this purpose, poly(ethylene glycol) (PEG) was functionalized with
catechol units and Holten-Andersen et al. were able to show the reversibility of the
metal–ligand interaction [84, 85].
Beside the iron–dopa interaction, it could be proven that the zinc-histidine
system also leads to self-healing behavior of mussel byssus threads, which feature
a hierarchical structure [91–93]; a fiber containing a middle block of collagen,
which is flanked by histidine-rich parts, in an environment with a high content of
zinc ions leads to a reversible crosslinking [93, 94]. The recovery of the mechanical
properties of the mussel byssus thread could be demonstrated (Fig. 4) and, furthermore, it was possible to design a model system based on a PEG star functionalized
with histidine moieties [95, 96]. These systems were utilized for the generation of
metal-containing hydrogels.
4.2 Synthetic Self-Healing Metallopolymers
Several synthetic metallopolymers have been synthesized to obtain self-healing
behavior based on metal–ligand interactions. In 2005, Varghese et al. were able to
show that a gel based on acryloyl-6-amino caproic acid (A6ACA) can heal
scratches if the polymer is dipped into an aqueous copper(II) chloride solution
[97]. However, the study did not answer the question of the influence of hydrogen
bonds on the self-healing effect, particularly because a medium was selected in
which hydrogen bonds exist. Moreover, the mobility of the gel itself could also
affect the self-healing behavior; therefore, the particular influence of the
metal–ligand interaction on the self-healing process is still unclear.
In addition to the incorporation of the ligand function in the side chain, it is also
possible to install the ligand in the main chain of the polymer. Yuan et al. showed
that a self-healing behavior can be generated by polymers that feature ligands
246
B. Sandmann et al.
The self-healing possibilities based on metal–ligand interactions can also be
observed in nature. In 2001, Vaccaro and Waite described the ability of mussel
byssus threads to heal after an inflicted damage [82]. In the following decade, more
detailed insights into the mechanism and the parameters that influence this natural
system were collected. Meanwhile, it could be shown that the self-healing behavior
is based on an interaction between iron(III) ions and 3,4-dihydroxyphenylalanine
(dopa) [83]. The iron center can bind one, two, or three catcheol-based ligands,
which are connected with a polymer backbone. The number of bonded ligands
depends on the pH value [84–86]. At low pH (below 5), the mono-dopa iron(III)
complex is formed, which does not lead to a crosslinking of the polymer chains. The
result is an extensible material. By contrast, increasing pH values lead to a more
crosslinked polymer network, which provides an increased hardness [83, 87–90].
This principle, which can be found in nature, could also be mimicked by synthetic
polymers. For this purpose, poly(ethylene glycol) (PEG) was functionalized with
catechol units and Holten-Andersen et al. were able to show the reversibility of the
metal–ligand interaction [84, 85].
Beside the iron–dopa interaction, it could be proven that the zinc-histidine
system also leads to self-healing behavior of mussel byssus threads, which feature
a hierarchical structure [91–93]; a fiber containing a middle block of collagen,
which is flanked by histidine-rich parts, in an environment with a high content of
zinc ions leads to a reversible crosslinking [93, 94]. The recovery of the mechanical
properties of the mussel byssus thread could be demonstrated (Fig. 4) and, furthermore, it was possible to design a model system based on a PEG star functionalized
with histidine moieties [95, 96]. These systems were utilized for the generation of
metal-containing hydrogels.
4.2 Synthetic Self-Healing Metallopolymers
Several synthetic metallopolymers have been synthesized to obtain self-healing
behavior based on metal–ligand interactions. In 2005, Varghese et al. were able to
show that a gel based on acryloyl-6-amino caproic acid (A6ACA) can heal
scratches if the polymer is dipped into an aqueous copper(II) chloride solution
[97]. However, the study did not answer the question of the influence of hydrogen
bonds on the self-healing effect, particularly because a medium was selected in
which hydrogen bonds exist. Moreover, the mobility of the gel itself could also
affect the self-healing behavior; therefore, the particular influence of the
metal–ligand interaction on the self-healing process is still unclear.
In addition to the incorporation of the ligand function in the side chain, it is also
possible to install the ligand in the main chain of the polymer. Yuan et al. showed
that a self-healing behavior can be generated by polymers that feature ligands
246
B. Sandmann et al.