changed, and its microfibrils are fused into a relatively homogeneous
macrostructure. By changing the regeneration process, regenerated cellulose materials can be adopted in various forms, such as film, beads, and gels [61]. Cellulose
hydrogels have been prepared by regenerating the cellulose solution from its ionic
liquid (AMIMCl) [60] using deionized water as coagulant. Kadokawa et al. also
reported flexible gels by dissolving cellulose in BMIMCl and keeping it at room
temperature for 7 days [59].
2.1.3 Alkali/(Thio)Urea Aqueous Systems
Another new solvent system for cellulose dissolution and gelation is the alkali/alkali/
(thio)urea aqueous systems developed by Zhang’s group [66, 67]. Typically, the
dissolution power of alkali/urea solvent systems is in the order NaOH/thiourea >
LiOH/urea > NaOH/urea >> KOH/urea aqueous solution [67, 68]. Different from
that in other solvents mentioned above, the dissolution process in alkali/urea systems
needs a low temperature treatment procedure (precooling the solvent [65] or freezing/thawing the mixture [66]), which arises as a fast dynamic self-assembly process
among solvent small molecules (NaOH, urea, and water) and the cellulose macromolecules. The widely accepted mechanism of the dissolution of cellulose is that
hydrogen bond acceptors (N–O, Cl
À , OAc
À , etc.) and/or donors (–NH 2 in urea or
thiourea) of the solvent break up the intra- and intermolecular hydrogen bonds in the
biopolymer chains upon stirring, heating, or low temperature treatment
[64, 74]. Notably, cellulose hydrogels can be formed via destructing the stability
of cellulose solution in 7 wt% NaOH/12 wt% urea system by increasing the
temperature to 50
C or reducing it to À20
C [64]. The irreversible gelation behavior
of cellulose solution is very sensitive to temperature change and cellulose molecular
weight and concentrations. In the NaOH/thiourea system, the sol-gel transition of
cellulose is partially reversible in the range from 10 to 30
C. The formed gels at
30
C can be transformed to the liquid state at À5
C after stirring for a long period of
time, which is caused by the reversible hydrogen-bonding networks between cellulose and solvent [75]. Thus, either at a higher temperature or a longer time, the
gelation of cellulose solution can occur but only obtain weak hydrogels by using the
preparation method from pure cellulose solution.
2.1.4 Hydrogels Prepared Directly from Bacterial Cellulose
Bacterial cellulose (BC) is formed by aerobic bacteria, such as acetic acid bacteria of
the genus Gluconacetobacter, producing a thick gel composed of cellulose microfibrils and ~97% water, called pellicle [76]. The BC is proved to be a very pure
cellulose with a high weight-average molecular weight (M w ), high crystallinity,
excellent water-holding capacity, and good mechanical stability, which expand the
wide range of applications of BC-based hydrogels in the biomaterials fields, such as
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
325
macrostructure. By changing the regeneration process, regenerated cellulose materials can be adopted in various forms, such as film, beads, and gels [61]. Cellulose
hydrogels have been prepared by regenerating the cellulose solution from its ionic
liquid (AMIMCl) [60] using deionized water as coagulant. Kadokawa et al. also
reported flexible gels by dissolving cellulose in BMIMCl and keeping it at room
temperature for 7 days [59].
2.1.3 Alkali/(Thio)Urea Aqueous Systems
Another new solvent system for cellulose dissolution and gelation is the alkali/alkali/
(thio)urea aqueous systems developed by Zhang’s group [66, 67]. Typically, the
dissolution power of alkali/urea solvent systems is in the order NaOH/thiourea >
LiOH/urea > NaOH/urea >> KOH/urea aqueous solution [67, 68]. Different from
that in other solvents mentioned above, the dissolution process in alkali/urea systems
needs a low temperature treatment procedure (precooling the solvent [65] or freezing/thawing the mixture [66]), which arises as a fast dynamic self-assembly process
among solvent small molecules (NaOH, urea, and water) and the cellulose macromolecules. The widely accepted mechanism of the dissolution of cellulose is that
hydrogen bond acceptors (N–O, Cl
À , OAc
À , etc.) and/or donors (–NH 2 in urea or
thiourea) of the solvent break up the intra- and intermolecular hydrogen bonds in the
biopolymer chains upon stirring, heating, or low temperature treatment
[64, 74]. Notably, cellulose hydrogels can be formed via destructing the stability
of cellulose solution in 7 wt% NaOH/12 wt% urea system by increasing the
temperature to 50
C or reducing it to À20
C [64]. The irreversible gelation behavior
of cellulose solution is very sensitive to temperature change and cellulose molecular
weight and concentrations. In the NaOH/thiourea system, the sol-gel transition of
cellulose is partially reversible in the range from 10 to 30
C. The formed gels at
30
C can be transformed to the liquid state at À5
C after stirring for a long period of
time, which is caused by the reversible hydrogen-bonding networks between cellulose and solvent [75]. Thus, either at a higher temperature or a longer time, the
gelation of cellulose solution can occur but only obtain weak hydrogels by using the
preparation method from pure cellulose solution.
2.1.4 Hydrogels Prepared Directly from Bacterial Cellulose
Bacterial cellulose (BC) is formed by aerobic bacteria, such as acetic acid bacteria of
the genus Gluconacetobacter, producing a thick gel composed of cellulose microfibrils and ~97% water, called pellicle [76]. The BC is proved to be a very pure
cellulose with a high weight-average molecular weight (M w ), high crystallinity,
excellent water-holding capacity, and good mechanical stability, which expand the
wide range of applications of BC-based hydrogels in the biomaterials fields, such as
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
325