aggregation, while CNCs functionalized with carboxylic acids (CNC-CO 2 H) exhibit
an increase in modulus at low pH and form gels. In both cases, the neutral or little
charged CNCs show better mechanical reinforcement than their highly charged
counterparts. Thus, this work suggests for the first time that it is possible to alter
the surface chemistry of the CNCs and therefore change the stimulus that can be used
to alter the CNC interactions and thus the mechanical properties of their
corresponding suspensions, gels, and nanocomposites.
Based on the electrostatic interactions, Lin et al. [199] developed a biocompatible
hydrogel with a double-membrane structure from cationic cellulose nanocrystals
(CCNC) and anionic alginate as novel drug carrier. The presence of CCNC in the
inner membrane can enhance the structural stability of the hydrogel through electrostatic interactions between cationic nanoparticles and anionic alginate. The thickness of the outer layer can be tuned by the adsorption duration of neat alginate, and
the shape of the inner layer can directly determine the morphology and dimensions
of the double-membrane hydrogel. The biocompatibility and nontoxicity derived
from natural polysaccharide components as the building blocks can be preserved for
the double-membrane hydrogel, which contribute to the complex drug release with
the first quick release of one drug and the successively slow release of another drug.
4 Summary and Outlook
Hydrogels based on biopolymer cellulose are a unique functional soft material with a
cross-linked hydrophilic network entrapping large amounts of water and thus possess a great variety of properties. The combination of versatile physicochemical
properties allows cellulose-based hydrogels to have a wide range of industrial and
biomedical applications, attracting great scientific and industrial interest across the
globe. Given the renewable, biocompatible, and biodegradable characteristics, cellulose and its derivatives with fascinating structures and properties offer a versatile
platform for rational design of cellulose-based functional hydrogels. Much fundamental research has been conducted on the preparation of cellulose-based hydrogels,
such as the development of solvent systems for hydrogels prepared directly from a
native cellulose solution, hydrogel formation from cellulose derivatives by physical
or chemical strategies, and cellulose-based composite hydrogels made by mixing
natural biopolymers, synthetic polymers, or inorganics with cellulose or its derivatives to achieve a new structural design and functional properties integrated with the
advantages of both components. In fact, more attention is paid on the development of
the dynamically cross-linked cellulose-based hydrogels. The dynamic chemical
cellulose-based gels contain reversible dynamic covalent bonds that can break and
reform, such as imine bonds, disulfide bonds, Diels-Alder (DA) bonds, and disulfide
bonds. Dynamically physical cellulose-based gels re-establish networks through
dynamic formation of attractive non-covalent interactions, including hydrogen
bonds, host-guest interactions, metal-ligand coordination, hydrophobic interactions,
and electrostatic interactions. Dynamic chemistry provides a fascinating strategy to
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C. Shao and J. Yang
an increase in modulus at low pH and form gels. In both cases, the neutral or little
charged CNCs show better mechanical reinforcement than their highly charged
counterparts. Thus, this work suggests for the first time that it is possible to alter
the surface chemistry of the CNCs and therefore change the stimulus that can be used
to alter the CNC interactions and thus the mechanical properties of their
corresponding suspensions, gels, and nanocomposites.
Based on the electrostatic interactions, Lin et al. [199] developed a biocompatible
hydrogel with a double-membrane structure from cationic cellulose nanocrystals
(CCNC) and anionic alginate as novel drug carrier. The presence of CCNC in the
inner membrane can enhance the structural stability of the hydrogel through electrostatic interactions between cationic nanoparticles and anionic alginate. The thickness of the outer layer can be tuned by the adsorption duration of neat alginate, and
the shape of the inner layer can directly determine the morphology and dimensions
of the double-membrane hydrogel. The biocompatibility and nontoxicity derived
from natural polysaccharide components as the building blocks can be preserved for
the double-membrane hydrogel, which contribute to the complex drug release with
the first quick release of one drug and the successively slow release of another drug.
4 Summary and Outlook
Hydrogels based on biopolymer cellulose are a unique functional soft material with a
cross-linked hydrophilic network entrapping large amounts of water and thus possess a great variety of properties. The combination of versatile physicochemical
properties allows cellulose-based hydrogels to have a wide range of industrial and
biomedical applications, attracting great scientific and industrial interest across the
globe. Given the renewable, biocompatible, and biodegradable characteristics, cellulose and its derivatives with fascinating structures and properties offer a versatile
platform for rational design of cellulose-based functional hydrogels. Much fundamental research has been conducted on the preparation of cellulose-based hydrogels,
such as the development of solvent systems for hydrogels prepared directly from a
native cellulose solution, hydrogel formation from cellulose derivatives by physical
or chemical strategies, and cellulose-based composite hydrogels made by mixing
natural biopolymers, synthetic polymers, or inorganics with cellulose or its derivatives to achieve a new structural design and functional properties integrated with the
advantages of both components. In fact, more attention is paid on the development of
the dynamically cross-linked cellulose-based hydrogels. The dynamic chemical
cellulose-based gels contain reversible dynamic covalent bonds that can break and
reform, such as imine bonds, disulfide bonds, Diels-Alder (DA) bonds, and disulfide
bonds. Dynamically physical cellulose-based gels re-establish networks through
dynamic formation of attractive non-covalent interactions, including hydrogen
bonds, host-guest interactions, metal-ligand coordination, hydrophobic interactions,
and electrostatic interactions. Dynamic chemistry provides a fascinating strategy to
348
C. Shao and J. Yang