repulsive charges on the nanofibrils and to dominate gel properties through ionic
cross-linking. Binding energies of cations with carboxylate groups were calculated
from molecular models of CNF intrafibril and interfibril bonding to validate the
correlation and provide further insight information for the cross-linked network
structure. Taking advantage of their native biocompatibility and dynamic metalligand cross-linked network structure, the cellulose nanofibril-based hydrogels present promising biomedical applications as a tissue engineering substrate with physically adsorbed and covalently attached fibronectin protein onto the hydrogel surface
to improve cell adhesion [183].
Utilization of reversible metal-ligand coordination interactions as sacrificial
bonds in biopolymers is critical for the integral synthesis of mechanically superior
biological materials. On the basis of the sacrificial bond theory, Yang et al. [177]
designed cellulose nanofibrils (CNFs)-reinforced covalent polyacrylamide (PAAm)
composite hydrogels by immersing into various multivalent cation aqueous solutions to form metal-ligand coordination association among CNFs, leading to the
ionic-covalent cross-linked hydrogels (Fig. 11b). The cations promote the formation
of porous networks of nanofibrils by screening the repulsive negative charges on
CNF surface, dominating the superior mechanical properties with excellent tensile
strength and toughness. Another advantageous feature of metal-ligand coordination
is its ability to undergo repeated association to endow excellent self-recovery ability
of the obtained ionic gels. The in situ Raman spectroscopy during stretching is
utilized to corroborate the stress transfer medium of CNF in the microscopic
deformation behavior of the gels. The microscopic morphologies of stable crack
propagation validate that the multiple toughening mechanisms occur in a balanced
energy dissipation manner, enabling synergistic combination of stiffness and toughness. Moreover, the creep behavior of the ionic gel in indentation test also suggests
that the CNF ionic coordination contributes simultaneous improvement in hardness
and elasticity compared to those pristine gels. This work provides a facile and
straightforward ionic cross-linking strategy for CNF-based hydrogels with tunable
dynamic properties, which may enrich exploration in biomedical field of highbearing cellulose-based soft materials.
Dynamic nature of reversible coordination interactions not only toughens the
mechanical performance via a sacrificial manner but also effectively heals the
damage by reversible break and reform. Physical gels can realize self-healing
process autonomously but often suffer from poor mechanical properties; thus
constructing the fully physically cross-linked network without covalent bonds is
an ideal strategy for integrating gels with efficient self-healing property and good
mechanical performance. A few pioneering efforts on self-healing nanocellulosebased metallogel design have been made [178, 184]. For example, Yang et al.
developed a simple one-pot strategy to prepare a fully physically cross-linked
nanocomposite hydrogel through the formation of the hydrogen bonds and dual
metal-carboxylate coordination bonds within supramolecular networks [184]. The
iron ions (Fe
3+ ) and TEMPO-oxidized cellulose nanofibrils (CNFs) acted as dual
cross-linkers and led to the improved mechanical properties with excellent fracture
strength (1.37 MPa), fracture elongation (1803%), and toughness (11.05 MJ/m
3 ).
344
C. Shao and J. Yang
cross-linking. Binding energies of cations with carboxylate groups were calculated
from molecular models of CNF intrafibril and interfibril bonding to validate the
correlation and provide further insight information for the cross-linked network
structure. Taking advantage of their native biocompatibility and dynamic metalligand cross-linked network structure, the cellulose nanofibril-based hydrogels present promising biomedical applications as a tissue engineering substrate with physically adsorbed and covalently attached fibronectin protein onto the hydrogel surface
to improve cell adhesion [183].
Utilization of reversible metal-ligand coordination interactions as sacrificial
bonds in biopolymers is critical for the integral synthesis of mechanically superior
biological materials. On the basis of the sacrificial bond theory, Yang et al. [177]
designed cellulose nanofibrils (CNFs)-reinforced covalent polyacrylamide (PAAm)
composite hydrogels by immersing into various multivalent cation aqueous solutions to form metal-ligand coordination association among CNFs, leading to the
ionic-covalent cross-linked hydrogels (Fig. 11b). The cations promote the formation
of porous networks of nanofibrils by screening the repulsive negative charges on
CNF surface, dominating the superior mechanical properties with excellent tensile
strength and toughness. Another advantageous feature of metal-ligand coordination
is its ability to undergo repeated association to endow excellent self-recovery ability
of the obtained ionic gels. The in situ Raman spectroscopy during stretching is
utilized to corroborate the stress transfer medium of CNF in the microscopic
deformation behavior of the gels. The microscopic morphologies of stable crack
propagation validate that the multiple toughening mechanisms occur in a balanced
energy dissipation manner, enabling synergistic combination of stiffness and toughness. Moreover, the creep behavior of the ionic gel in indentation test also suggests
that the CNF ionic coordination contributes simultaneous improvement in hardness
and elasticity compared to those pristine gels. This work provides a facile and
straightforward ionic cross-linking strategy for CNF-based hydrogels with tunable
dynamic properties, which may enrich exploration in biomedical field of highbearing cellulose-based soft materials.
Dynamic nature of reversible coordination interactions not only toughens the
mechanical performance via a sacrificial manner but also effectively heals the
damage by reversible break and reform. Physical gels can realize self-healing
process autonomously but often suffer from poor mechanical properties; thus
constructing the fully physically cross-linked network without covalent bonds is
an ideal strategy for integrating gels with efficient self-healing property and good
mechanical performance. A few pioneering efforts on self-healing nanocellulosebased metallogel design have been made [178, 184]. For example, Yang et al.
developed a simple one-pot strategy to prepare a fully physically cross-linked
nanocomposite hydrogel through the formation of the hydrogen bonds and dual
metal-carboxylate coordination bonds within supramolecular networks [184]. The
iron ions (Fe
3+ ) and TEMPO-oxidized cellulose nanofibrils (CNFs) acted as dual
cross-linkers and led to the improved mechanical properties with excellent fracture
strength (1.37 MPa), fracture elongation (1803%), and toughness (11.05 MJ/m
3 ).
344
C. Shao and J. Yang