structural design and functional properties integrated with the advantages of both
components [96].
2.3.1 Cellulose-Based Composite Hydrogels with Biopolymer
Cellulose (or its derivatives) mixed with natural biopolymers, including chitin,
chitosan, starch, alginates, and hyaluronic acid, has created novel materials to
meet the special demand in various fields, such as mixing with chitosan for heavy
metal removal, with starch for foods, and with alginates for tissue engineering. The
preparation processes of composite hydrogels focus on dissolution (or dispersion)
and cross-linking of components. In contrast to the pure hydrogels, formation of
natural polymer-based composite hydrogels needs to mix two biopolymer solutions
in the same solvent before gelation. Cellulose powder and chitosan solution are
mixed together to form cellulose/chitosan hydrogel beads, which are cross-linked
with ethylene glycol diglycidyl ether to improve cross-linking density and chemically stability for Cu adsorption applications [97]. Sodium alginate has been used as
a pore size expander for the blending with cellulose to create cellulose-sodium
alginate hydrogels [98]. These hydrogels exhibited macroporous structure, excellent
mechanical strength, and high equilibrium swelling ratio in water, as shown in
Fig. 3. The introduction of sodium alginate into cellulose hydrogels significantly
increases the pore size and swelling ratio of hydrogel samples, while cellulose
contributes to enhance the mechanical properties of the hydrogels.
2.3.2 Cellulose-Based Composite Hydrogels with Synthetic Polymer
When mixing with synthetic polymers such as polyethylene glycol (PEG) and
polyvinyl alcohol (PVA) [99, 100], the preparation methods are similar to those of
cellulose-based composite hydrogels with biopolymer. Cellulose is used as the first
network, and the second network is formed by in situ polymerization of the monomers with the assistance of an initiator upon heating or irradiation, which is called
sequential interpenetrating polymer networks (IPNs) [101]. In some cases, the
monomer was initially grafted to the biopolymer via esterification that acts as the
active grafting sites on the chains for polymerization. Thus if cellulose or its
derivative is linear or branched in a cross-linked network, it is called as semi-IPN
hydrogel [102].
By employing cellulose hydrogel as the first network and in situ polymerizing/
cross-linking N-isopropylacrylamide (NIPAAm), Chang et al. [103] have successfully prepared IPN hydrogels based on cellulose and poly(N-isopropylacrylamide)
(PNIPAAm). The first network was created by chemically cross-linking cellulose in
NaOH/urea aqueous solution, and PNIPAAm was polymerized/cross-linked as the
second network with N,N-methylenebisacrylamide (BIS) as cross-linker and
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components [96].
2.3.1 Cellulose-Based Composite Hydrogels with Biopolymer
Cellulose (or its derivatives) mixed with natural biopolymers, including chitin,
chitosan, starch, alginates, and hyaluronic acid, has created novel materials to
meet the special demand in various fields, such as mixing with chitosan for heavy
metal removal, with starch for foods, and with alginates for tissue engineering. The
preparation processes of composite hydrogels focus on dissolution (or dispersion)
and cross-linking of components. In contrast to the pure hydrogels, formation of
natural polymer-based composite hydrogels needs to mix two biopolymer solutions
in the same solvent before gelation. Cellulose powder and chitosan solution are
mixed together to form cellulose/chitosan hydrogel beads, which are cross-linked
with ethylene glycol diglycidyl ether to improve cross-linking density and chemically stability for Cu adsorption applications [97]. Sodium alginate has been used as
a pore size expander for the blending with cellulose to create cellulose-sodium
alginate hydrogels [98]. These hydrogels exhibited macroporous structure, excellent
mechanical strength, and high equilibrium swelling ratio in water, as shown in
Fig. 3. The introduction of sodium alginate into cellulose hydrogels significantly
increases the pore size and swelling ratio of hydrogel samples, while cellulose
contributes to enhance the mechanical properties of the hydrogels.
2.3.2 Cellulose-Based Composite Hydrogels with Synthetic Polymer
When mixing with synthetic polymers such as polyethylene glycol (PEG) and
polyvinyl alcohol (PVA) [99, 100], the preparation methods are similar to those of
cellulose-based composite hydrogels with biopolymer. Cellulose is used as the first
network, and the second network is formed by in situ polymerization of the monomers with the assistance of an initiator upon heating or irradiation, which is called
sequential interpenetrating polymer networks (IPNs) [101]. In some cases, the
monomer was initially grafted to the biopolymer via esterification that acts as the
active grafting sites on the chains for polymerization. Thus if cellulose or its
derivative is linear or branched in a cross-linked network, it is called as semi-IPN
hydrogel [102].
By employing cellulose hydrogel as the first network and in situ polymerizing/
cross-linking N-isopropylacrylamide (NIPAAm), Chang et al. [103] have successfully prepared IPN hydrogels based on cellulose and poly(N-isopropylacrylamide)
(PNIPAAm). The first network was created by chemically cross-linking cellulose in
NaOH/urea aqueous solution, and PNIPAAm was polymerized/cross-linked as the
second network with N,N-methylenebisacrylamide (BIS) as cross-linker and
328
C. Shao and J. Yang