Cellulose-Based Composite Hydrogels: Preparation, Structures, and Applications
In this chapter, cellulose-based composite hydrogels were summarized in three categories according to the components. Synthetic polymer/cellulose composite hydrogels combine the advantages of synthetic polymers and cellulose. Soluble cellulose derivatives are feasible to construct the composite hydrogels with polyacrylamide, polyvinyl alcohol, polyacrylic acid, and so on. The composite hydrogels are normally applied as superabsorbents for heavy metal ions and dyes because the abundant functional groups in the hydrogels can act as binding sites. Due to most of the crosslinked polymeric hydrogels suffering from poor mechanical performance, low breaking strain, and sensitivity to fracture, cellulose nanocrystal can be combined into the hydrogels to enhance the mechanical properties significantly in order to obtain the mechanically strong, tough, or highly stretchable nanocomposite hydrogels. Natural macromolecules/cellulose composite hydrogels have a great potential for applications in tissue engineering, drug delivery, sensors, and purification for their excellent biocompatible, biodegradable, and nontoxic properties. Cellulose hydrogels have high mechanical strength and good permeability for liquids, gases, and electrolytes, the composite hydrogels which combine cellulose and extracellular matrixes are very promising scaf folds for the tissue repair and regeneration. Chitosan, alginate, and other polysaccharides are popular natural macromolecules for the composite hydrogels. Inorganics/cellulose composite hydrogels have recently received considerable attentions in both academic research and industrial application due to their excellent hybrid properties. Montmorillonite, clay, and bentonite are traditional inorganic minerals to fabricate the composite hydrogels as superabsorbents for water treatment, personal care, and agriculture. Nanoparticles of ZnO and Ag are also incorporated into the cellulose hydrogels to render the antimicrobial activity to biomedical materials. Recently, the novel cellulose-based composite hydrogels with graphene oxide, carbon nanotube, and carbon dots show potential applications in supercapacitors and biosensors.
KeywordsComposite hydrogel Synthetic polymer Natural macromolecules Inorganic; Preparation Structures and applications
Cellulose composite hydrogels can be prepared from cellulose and macromolecules or inorganic materials with physical and chemical crosslinking. Physical hydrogels are formed by molecular self-assembly through ionic or hydrogen bonds, while chemical hydrogels are formed by covalent bonds . Cellulose can take part in the formation of 3D network or act as reinforcement. Interpenetrating polymer networks (IPNs) are common structures for cellulose-based composite hydrogels, especially for synthetic polymer/cellulose composite hydrogels. IPNs can be divided into two types: sequential IPN and semi-IPN. For sequential IPN, cellulose is used as the first network, and the second network is formed by polymerizing in the presence of the cellulose network. When cellulose or its derivative is linear or branched in a crosslinked network, it is called as semi-IPN hydrogel . Nanocellulose is an excellent reinforcement for both synthetic polymer and natural macromolecule hydrogels. Inorganic/cellulose composite hydrogels combine the advantages of both components which are very promising materials in biomedicals, tissue scaffold engineering, superabsorbent, and so on. The main purposes of incorporation of inorganics are reducing production cost, improving the properties, and rendering special features. This chapter discusses about cellulose-based composite hydrogels including synthetic polymer/cellulose, natural macromolecules/cellulose, and inorganics/cellulose composite hydrogels with the emphasis on the latter two categories.
2 Synthetic Polymer/Cellulose Composite Hydrogels
2.1 Water-Soluble Cellulose Derivatives
Naturally available cellulose is not water-soluble due to the hydrogen bonding between molecular chains; the strong intermolecular and intramolecular hydrogen bonds between the hydroxyl groups not only limit the water solubility but also lead to the poor reactivity of cellulose. However, water-soluble cellulose can be prepared by esterification and etherification reaction of the hydroxyl groups. Methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), and carboxymethyl cellulose (CMC) are common water-soluble cellulose derivatives. The concentrated aqueous solution of cellulose derivatives can be crosslinked under irradiation to prepare cellulose-based hydrogels . It is feasible to incorporate some synthetic polymers with cellulose to fabricate the composite hydrogels by chemical crosslinking via crosslinker and irradiation, physical crosslinking, and ionic crosslinking.
MC is normally obtained by heating cellulose with a solution of sodium hydroxide and methyl chloride to replace the hydroxyl groups of cellulose by methoxyl groups. MC can form a thermoreversible physical hydrogel itself. At low temperatures, the hydrogen bonds formed and the polar water molecules form cages around the hydrophobic methoxyl groups of MC; upon heating, the hydrogen bonds and the water cages were broken to expose the hydrophobic side groups of MC chains, inducing hydrophobic associations and thus gelation . MC-based injectable hydrogels for postoperation anti-adhesion were fabricated by mixing CMC, polyethylene glycol (PEG), and chitosan sulfate (CS-SO3) in the MC matrix with a low critical solution temperature (LCST) at 36 °C . PAM/MC/montmorillonite (MMT) composite hydrogels were obtained through chemical polymerization of the AM monomer in aqueous solution containing MC and MMT which were appropriate for the controlled release of fertilizers with very high fertilizer loading in their structures . PAM/MC semi-IPNs composite hydrogels were prepared by aqueous radical polymerization at room temperature, and MBA was used as a crossliking agent [34, 35]. Frontal polymerization (FP) was exploited as an alternative and convenient technique to prepare PAM/MC composite hydrogels swollen either in water or glycerol . MC was modified with functional methacrylate groups and photocrosslinked to produce hydrogels for potential applications in plastic and reconstructive surgery . Polythiophene-g-poly(dimethylaminoethyl methacrylate)-doped MC hydrogel behaved like a polymeric AND logic gate .
HPMC is produced by the addition of methyl and hydroxypropyl groups to the cellulose backbone. PAM/HPMC composite hydrogels were synthesized by radical polymerization technique which demonstrated controlled drug release behavior due to the hydrophobicity as well as lower rate of erosion [39, 40]. Thermosensitive PNIPAM/HPMC hydrogels were prepared, and swelling ratio of the hydrogel decreased with increasing temperature . HPMC hydrogels containing PEG as crosslinks were prepared by reacting HPMC with polyethylene glycol dichloride which was a promising drug delivery system for the drugs to be delivered to the colon .
HEC is obtained by reacting cellulose with sodium hydroxide, and hydroxyl groups of cellulose are replaced by hydroxyethyl groups. Double-layered gels, consisting of HEC cryogel core and poly(ethylene oxide) (PEO) hydrogel shell, were synthesized with UV irradiation which were used as carriers for immobilization by entrapment of cells . Composite hydrogels from poly(N-vinyl pyrrolidone) (PVP) and methyl hydroxyethyl cellulose (MHEC) were obtained via gamma irradiation . PAA/HEC composite hydrogels were prepared by in situ method, and moduli of the composite hydrogels were improved dramatically compared to HEC . HEC-g-PAA/vermiculite superabsorbent nanocomposites were prepared by radical solution polymerization among HEC, AA, and vermiculite in the presence of MBA as crosslinker . PNIPAM/HEC composite hydrogels were prepared by copolymerization with NIPAM onto HEC, and LCST of hydrogels was enhanced by the introduction of HEC .
Nanocellulose (NC) with high aspect ratio is partially to fully crystalline and exhibits impressive mechanical properties and tunable surface chemistries. Recently, the use of NC as a reinforcing agent for hydrogels has attracted significant research interest, especially in applications of tissue engineering scaffolds, wound healing materials, and drug delivery systems. There are three main reasons for the improved performance of the polymer hydrogels by incorporating NC, i.e., increased relative surface area (aspect ratios) and its associated quantum effects exhibited by NC, which formed polymer-filler interactions such as hydrogen bond, enhanced the elastic modulus with the rigid nanofibrils . NC can be physically entrapped in the polymer network as a filler or chemically crosslinked with the polymer matrix in the composite hydrogels. Due to their rigidity and mechanical strength, NC acts as effective fillers in the composite hydrogels, and the majority of the NC composite hydrogel research has focused on the physical incorporation of NC as fillers. Common preparation methods include simple homogenization and physical entanglement of polymer networks, free-radical polymerization of polymeric species within NC suspensions, and UV/ion-mediated crosslinking of the polymeric network around the NC and cyclic freeze-thaw processing .
Incorporation of NC into synthetic polymer hydrogels can significantly improve the degradation and the mechanical strength of the hydrogels. The orientation of the reinforcing NC from the anisotropic to highly isotropic state under applied stress is the main mechanism of strain-dependent behavior of the composite hydrogels . Polyvinyl alcohol (PVA) is among the most promising polymer candidates for the composite hydrogels because it is biocompatible, nontoxic, and readily crosslinked polymer. With the increase of NC concentration in PVA hydrogels, the composite hydrogels greatly improved the mechanical strength while maintaining remarkable ductility [58, 59]. PVA/NC composite hydrogels obtained by freezing-thawing (F-T) technique revealed increased thermal stability, mechanical properties , and proper water vapor transmission rate in the potential application of wound dressing . The porous structure of PVA/NC hydrogels can be controlled by adding PEG with various molecular weights . Hyperelastic, rubberlike PVA/NC composite hydrogels mimicking collagenous soft tissues are promising materials for ophthalmic applications [57, 63, 64]. The stiffness of PVA/NC composite hydrogels was significantly enhanced by utilizing three-component recognition-driven supramolecular crosslinks . NC can also be chemically crosslinked with PVA to obtain the composite hydrogels [66, 67]. NC can be incorporated into PVA/chitosan  or PVA/alginate  to form the ternary composite hydrogels.
3 Natural Macromolecules/Cellulose Composite Hydrogels
There are an abundance of amino groups in chitosan molecules, and they are feasible to react with aldehyde groups in oxidized cellulose derivatives to form a Schiff base which is an important chemical crosslinking to produce chitosan/cellulose composite hydrogels. Sodium periodate is a normal agent to cleave the C2-C3 bond of the anhydroglucose unit in cellulose and results in the formation of two aldehyde groups. Hydrogen peroxide was also used as an oxidant which oxidized CMC in the presence of sulfate copper, and the amount of aldehydes increased with the CuSO4 . Due to the good water solubility, CMC was extensively employed to be oxidized to dialdehyde carboxymethylcellulose (DCMC) and self-crosslinked with chitosan or its derivatives based on Schiff base formation [100, 101, 102, 103]. The chitosan/DCMC composite hydrogels displayed significantly better thermostability, swelling capacity, and cytocompatibility compared with glutaraldehyde (GA) crosslinked carboxymethyl chitosan . Chitosan/cellulose composite hydrogels with gelation driven by Schiff base reactions are highly porous materials which are capable of loading various potential therapeutic agents including cells, drugs, and growth factors, as drug delivery carriers of an anticancer drug, drug loading and releasing can be easily modulated by tuning the composition of the composite hydrogels. Protonation of carboxylic groups in the microsphere matrix at acidic conditions resulted in an accelerated release of doxorubicin, which is desired for treating tumors . Chemical stability of the composite hydrogels was improved remarkably by grafting chitosan onto cellulose hydrogels prepared by dissolution-regeneration in LiOH/urea .
Irradiation is a useful method for the formation of covalent bonding between chitosan and cellulose to obtain the composite hydrogels. The advantages of this method are high purity of the hydrogel products without crosslinkers and tedious process to prepare polymer derivatives. High concentrated paste-like condition was favorable for irradiation crosslinking of chitosan or cellulose; meanwhile, degradation of the polymer chains competed with crosslinking during irradiation . Carboxymethyl chitosan/CMC composite hydrogels were prepared by γ-irradiation of a high concentrated mixture solution revealing the high-adsorption capacities for metal ions. It was found that the maximum strength depended on the composition of the composite hydrogels. The behavior of the tensile strength of the composite hydrogels with increasing dose was similar to that of pure hydrogels [106, 107, 108]. Additionally, MBA as a radical crosslinker could be added to the mixed solution before irradiation to enhance the gelation efficiency . It was reported that macroradicals emerged preferentially in weakened C1 and C4 positions of cellulose as a result of the fracture of C-H bonds upon irradiation, while macroradicals of the derivatives were created in the side chains during radical crosslinking [110, 111].
Chitosan/cellulose composite hydrogels can be fabricated by physical interactions as well such as hydrogen bonds, electronic or hydrophobic associations, chain entanglements, and van der Waals forces. Cellulose ethers such as MC, HEC, and HPMC are the most important water-soluble derivatives of cellulose which can form physical crosslinking composite hydrogels with chitosan via hydrophobic associations and hydrogen bonding. HPMC and MC are temperature-sensitive and will undergo abrupt change in solubility in response to increases in environmental temperature at LCST which is governed by the balance of hydrophilic and hydrophobic moieties on the polymer chains. At the LCST, phase separation occurred because the balance of hydrophilic and hydrophobic moieties on the polymer chain was broken which resulted in a more hydrophobic structure of the polymer. Thermoreversible chitosan/HPMC composite hydrogels were fabricated, and LCST was increased with the increment of chitosan proportion in the composite hydrogels. For HPMC gelation, hydrophobic associations among the macromolecules possessing the methoxy group are the main mechanism. As temperature increases, hydrated and simply entangled polymer chains gradually lose their water of hydration with hydrophobic associations and form the hydrogel network. By adding chitosan, the concentration of HPMC was decreased, and hydrogen bond between the OH groups of HPMC and NH2 groups of chitosan enhanced the interactions of two macromolecules. The ionization of the amino groups extended the distance between the chitosan molecules through cationic repulsion and favored the osmotic flow of the water into the network. Therefore, the hydrophilicity of the composite was enhanced resulting in the increased LCST . The composite hydrogels displayed excellent thermal stability and good mechanical properties . The composite hydrogels physically crosslinked by hydrogen bonding of carboxymethyl chitosan with HEC and MC were prepared for controlled drug release system. Better interaction is demonstrated in HEC composite hydrogels than that in MC composite hydrogels, owing to stronger intermolecular hydrogen bonding . Polyol was also introduced into chitosan/HEC composite hydrogels to strengthen the hydrophobic interactions resulting in the incorporation of hydrophobic network into the hydrophilic network via IPN. The hydrophobic microenvironment can not only limit the swelling degree of the composite hydrogels but also act as reservoir of the drug . The pH-responsive composite hydrogels slowed down the drug release rate by the moieties formed by hydrophobic network which was a promising approach to cover up the drawbacks of drug burst-release kinetics from the hydrogels . The gelation temperature of CS-SO3/MC/CMC was around body temperature which made it a good candidate for injectable thermosensitive hydrogels for postoperation anti-adhesion . Chitosan/cellulose composite hydrogels can also be fabricated through the different solubilities of the macromolecules in various solvents. Both cellulose and chitosan can be dissolved in ionic liquid and coagulated to composite hydrogels by adding certain antisolvents such as water and ethanol. By this method, the lipase could be entrapped into chitosan/cellulose composite hydrogels with higher immobilization yields than cellulose hydrogels . Chitosan/cellulose composite hydrogels also exhibited good cell compatibility and non-cytotoxicity. Magnetic hybrid hydrogels were obtained by coating cellulose and chitosan in ionic liquid on the surface of Fe3O4 which revealed high adsorption capacities for different heavy metal ions .
Alginate, a water-soluble polysaccharide composed of D-mannuronic and L-guluronic acid residues, is an abundantly available biopolymer extensively used for hydrogels, and the ionotropic gelation of alginate can be achieved with several cations. Alkaline earth metals, transition metals, and certain post-transition metals are normally applied to form alginate hydrogels. Most of alginate/cellulose composite hydrogels were fabricated by utilization of the gelation of alginate with calcium ions, and cellulose was incorporated by dispersing in the composite hydrogels without taking part in forming the network. Water-soluble CMC is feasible to be mixed with alginate and crosslinked by Ca2+ to form alginate/cellulose composite hydrogels in order to remove some heavy metal ions such as Zn(II) , Chromium(VI) , and Hg(II) . Both alginate and CMC are anionic in nature due to the presence of negatively charged carboxyl groups at pH > 5. These negative charges allow the polymers to shrink in the acidic pH and to swell when they are exposed to neutral or basic pH. Therefore, alginate/CMC composite hydrogels prepared by the ionic gelation are good candidates of the drug delivery . The crosslinking rate for trivalent ions was faster than that of divalent ions due to their higher valency; the Al3+ crosslinked alginate/MC composite hydrogels improved not only the drug encapsulation efficiency but also drug release in alkaline media . Addition of CMC to alginate gel significantly improved the yield of lactase immobilization and the activity of the fixed enzyme . CMC and its grafted copolymer were also incorporated into the alginate hydrogels by ionic crosslinking at the presence of inorganics to form the ternary composite hydrogels [128, 129].
Recently, alginate/bacteria cellulose (BC) composite hydrogels are gaining more and more attentions as scaffolds for tissue engineering by combining beneficial properties of both BC and alginate. BC has advantages in terms of biocompatibility, nontoxicity, high mechanical strength, high swelling ability, and high stability to pH variations. Alginate can provide mechanical integrity in tissue engineering applications for its hydrogel properties. Several therapeutic agents, including antibiotics, enzymes, growth factors, and DNA, have already been successfully incorporated in alginate hydrogels in order to maintain high biological activity . Normally, the BC pellicles were crushed to form BC slurry and mixed with the alginate solution, then crosslinked by Ca2+ to obtain the composite hydrogels for scaffold. The compression strength and chemical stability of the alginate/BC composites were increased compared with the alginate hydrogels ; cell proliferation and migration on the surface of the scaffolds were promoted as well [130, 132]. Silver sulfadiazine particles, topical antibacterial agents, were embedded into the alginate/BC composite hydrogels to render the enhanced antibacterial property for utility as potential wound dressings . Multi-walled carbon nanotube was incorporated into alginate/BC composite hydrogels to get a pH and electric field dual-stimulus responsive drug delivery system . Alginate/BC composite hydrogels were also good immobilization carriers of lipase  and yeast . In application in lipase immobilization, a novel in situ method was employed. Alginate solution was mixed with G. xylinus suspension firstly, and the resulting mixture was then added into BaCl2 solution to gain the hydrogels. G. xylinus entrapped in the hydrogels successfully produced BC with a narrow distribution of fiber diameters. The prepared alginate/BC nanocomposite hydrogel beads showed consistent sizes and regular spherical shapes .
Alginate/cellulose composite hydrogels can be fabricated by ionic crosslinking by some trivalent cations with both macromolecules taking part in the formation of network. Trivalent cations such as Fe3+ and Al3+ can induce the gelation of alginate; meanwhile, CMC can produce hydrogels by coordinating with these trivalent cations  due to numerous carboxyls and hydroxyls in the molecular chains. The trivalent iron can be a more suitable crossliking agent for alginate/CMC composite hydrogels than divalent metal ions in drug delivery system due to the faster crossliking rate which can result in the higher load capacity and sustained release [128, 138]. Protein, drugs, and some bioactive compounds can be encapsulated into the composite hydrogels and controlled released from them. Albumin was embedded into the Fe3+-crosslinked alginate/CMC composite hydrogels, encapsulation efficiency was higher, and protein release was reduced in the gastric environment because of the pH sensitivity, which demonstrated that the alginate/CMC composite hydrogels presented a promising protein therapeutic carrier for the oral delivery . The composite hydrogels also exhibited pH sensitive for delivering metformin hydrochloride (MH); the swelling degree and in vitro release profiles were higher by two orders at gastric conditions compared to an acidic environment . Hesperidin-loaded efficiency in the alginate/CMC composite hydrogels was high, and in vitro release of hesperidin was prolonged. The composite hydrogels exhibited the ability to protect the encapsulated flavonoid glycoside during its passage through the simulated gastric environment, while under the simulated intestinal conditions, the transportation systems exhibited a pH-triggered release behavior . The alginate/CMC composite hydrogels are also good absorbents to remove lead ions by physical, chemical, and electrostatic adsorptions .
High-energy irradiation technique is becoming an important approach in the crosslinking of polymers as it can efficiently replace the chemical crosslinking agents and produce hydrogels with high purity that do not need the removal of hazardous chemical crosslinkers. It allows the chemical crosslinking without the use of crosslinking agents which provides a convenient approach to crosslink polysaccharide at high concentrated aqueous solution mainly due to the mobility of side chains [153, 154, 155]. Side-chain radicals were formed mostly via indirect effects, by the abstraction of H atoms in the intermediate products of water radiolysis. Electron beam irradiation (EB) was used to prepare composite hydrogels from aqueous solutions of plasticized starch (PLST), cellulose acetate (CA), and CMC. A semi-IPN structure could be formed in which the network was mainly formed by crosslinked CMC and CA with PLST physically included . Carboxymethyl sago pulp (CMSP)/carboxymethyl sago starch (CMSS) hydrogel was also synthesized by electron beam irradiation to produce the composite hydrogels with sustained release of drugs. The radicals on CMSP and CMSS chains can combine and form the intermolecular crosslinking between the molecular chains . Gamma irradiation was also applied to prepare starch/CMC composite hydrogels as superabsorbent gels, in which the starch granules also participated in the crosslinking process resulting in the improved gelation. CMC chains reacted with the granule surface through the recombination of the radicals, and the reaction was not hindered by electrostatic repulsion during the crosslink formation between two CMC chains. The partial replacement of CMC with starch improved the gel fraction; however, very high starch content had a negative impact on the gelation .
Hydroxypropyl-β-cyclodextrin (HPβCD) was crosslinked with HPMC via EGDE under mild conditions to prepare β-CD/cellulose composite hydrogels . 1,4-butanediol diglycidyl ether (BDGE) was used as crosslinker to form β-CD/HPMC composite hydrogels which were conjugated with antimicrobial gallic acid to prevent wound infection . β-CD and CMC were graft polymerized with AA, in the presence of MBA as a crosslinker to prepare β-CD/CMC-g-PAA composite hydrogels. The pH-sensitive composite hydrogels were conferred good mechanical properties with improved drug loading and release capability for the antiviral drug acyclovir .
β-CD can be incorporated into β-CD/cellulose composite hydrogels by noncovalent interactions as well. Hydrophobic cavities of β-CD can act as moieties capable of generating physical crosslinks, by making inclusion complexes with hydrophobically modified polymers or with amphiphilic polymers with hydrophobic chains, which combined to form host-guest complexes networks. β-CD can also be modified by some anionic or cationic groups to make it charged and form the composite hydrogels with opposite charged polymers via electrostatic interaction.
Chitosan/HEC hydrogels were prepared by crosslinking firstly, the negative sulfonatedbutyl (SEB) β-CD (SEB-β-CD) was loaded into the hydrogels simply by immersing the hydrogels into the SEB-β-CD solution. The electrostatic interaction of cationic chitosan and anionic SEB-β-CD was the main driving force to immobilize the β-CD into the hydrogels. Moreover, the encaged SEB-β-CD can be controlled and released from the composite hydrogels with ion-exchange method by immersing it in electrolyte solution readily . In addition, hydroxypropyl-β-CD (HP-β-CD) was immobilized into HEC , HPC , and HPMC  hydrogels at the same time of the crosslinking of cellulose derivatives.
3.2 Extracellular Matrix
The extracellular matrix (ECM) is the noncellular component presenting within all tissues and organs that not only provides essential physical scaffolding for the cellular constituents but also initiates crucial biochemical and biomechanical cues [175, 176]. The composite hydrogels-combined cellulose and ECM are very promising scaffolds for the tissue repair and regeneration, especially for the injectable self-healing hydrogels with the dynamic interactions. Collagen as a key ECM component for promoting cell cultivation is most widely used to fabricate the composite hydrogels, especially for bone reparation because of its weak antigenicity, excellent biocompatibility, and unique fibril-forming properties. Collagen is a kind of protein forming a characteristic triple helix of three polypeptide chains, and all members of the collagen family form these supramolecular structures in the extracellular matrix although their size, function, and tissue distribution vary considerably .
The cellulose hydrogels were prepared from cellulose solution dissolved in LiOH/urea aqueous system by crosslinking with ECH and then thermal crosslinked collagen by neutralizing the collagen solution to obtain collagen/cellulose composite hydrogels. The composite hydrogels integrated microfluidics with well-controlled pore size, good mechanical durability, cytocompatibility in cell culture, and excellent dimensional stability . The amino groups in collagen provide Cu(II) binding sites through their chelating performance . Collagen/cellulose hydrogel beads reconstituted from ionic liquid showed high Cu(II) adsorption capacity for removal of it from wastewater .
BC displays many unique properties including high water uptake capacity and an ultrafine nanofibril network structure which are essential for scaffolds in tissue engineering. To improve the biocompatibility and cell affinity of BC as wound dressing material, BC can be modified through the incorporation of collagen, and the collagen/BC composite hydrogels are very promising materials in skin tissue engineering. Collagen/BC composite hydrogels can be prepared by simply immersing wet BC pellicle into collagen solution followed by a freeze-drying process. The collagen molecules were not only coated on the BC fibrils surface but also penetrated inside BC, and hydrogen bond interactions were thus formed between BC and collagen . According to the in vivo test and macroscopic evaluation, collagen/BC composite hydrogels showed better reparation ability of wounds and promoted statistically significant differences of tissue repair between treatments after surgery . BC formed semi-penetrate hydrogels with alginate and immobilized collagen to ternary composite hydrogels which supported the growth of human skin fibroblast as deliver vehicles for therapeutic compounds during wound healing . In order to improve thermal and mechanical properties of collagen/BC composite hydrogels, both GA and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride were used as crosslinkers to form the chemical bonds between collagen and BC .
3.3 Nanocellulose/Natural Macromolecules
NC can also be incorporated into natural macromolecules hydrogels such as chitosan, alginate, and gelatin which targeted biological or biomedical applications. GA was used as a crosslinker to chitosan because of its high reactivity toward the amine groups, and NC was incorporated into the network to improve mechanical properties and pH sensitivity of the chitosan/NC composite hydrogels [190, 191]. Carboxymethylated chitosan/NC aerogel for dye removal was prepared through Schiff base reaction. NC, bearing aldehyde and carboxylic acid groups, facilitated the crosslinking with chitosan through imine bond formation while providing negatively charged functional groups, and chitosan was modified to accommodate carboxylic acid . Chitosan/NC sponge was prepared by lyophilization to be used as a new absorbable hemostat . The carboxyl groups of NC and amino groups of chitosn were linked with peptide bonds forming stable hydrogel network . Chitosan-g-PAA/NC composite hydrogels obtained by in situ polymerization showed responsive behavior in relation to pH and the salt solution . Chitosan/NC/xanthan  and chitosan/BC/MMT  ternary composite hydrogels were also fabricated by lyophilization. Chitosan/BC composite hydrogels were prepared by co-dissolution of BC and chitosan in 1-Ethyl-3-methylimidazolium acetate ([Emim][Ac]) and reconstitution with water  or in situ method during biosynthesis which showed better effect on cell growth, cell proliferation, and biocompatibility .
NC was employed to reinforce the cellulose and its derivative hydrogels such as CMC/HEC  and MC . Injectable hydrogels based on CMC and dextran, reinforced with rigid rodlike NC as simple fillers and aldehyde-functionalized NC as chemical crosslinkers, were prepared with potential applications such as drug delivery vehicles or tissue engineering matrices . Composite hydrogels based on CMC-g-P(AA-co-AM) and carboxylate NC were prepared by in situ graft polymerization in the presence of NC, and results showed that the composite hydrogels comprised a crosslinking structure of NC and CMC with side chains carrying AA and AM . Gelatin [210, 211], collagen [212, 213, 214, 215], and starch [148, 149, 216] were also applied to fabricate the composite hydrogels with NC.
4 Inorganics/Cellulose Composite Hydrogels
4.1 Inorganic Minerals
Vinyl monomers such as AA and AM can be in situ polymerized and grafted onto the chains of cellulose derivatives in the presence of the layered silicate to prepare the composite hydrogels. The obtained superabsorbents revealed that the inorganic particles could be incorporated into the hydrogels matrix by reacting through the active groups on the surface. Silanol groups on the attapulgite (APT)  and hydroxyl groups on the MMT  make them facilitate to participate in the graft polymerization of CMC side chain of vinyl monomers. The remarkable pH sensitivity swelling behavior of the composite hydrogels was observed. Compared to the effects of cellulose derivates on swelling properties of the composite hydrogels, equilibrium water absorbency and the swelling rate in distilled water and saline solution are always in the order CMC > HPMC > MC > HEC. It was revealed that the influence of functional groups on swelling properties was great and the swelling properties of composite hydrogels were related to the hydrophilicity and the rigidity of functional groups . Carboxyl group is an ion hydrophilic group which can be ionized; therefore, the polymeric network tends to swell, and the space for absorbing and holding water increases due to electrostatic repulsion. Moreover, the higher osmotic swelling pressure and chain relaxation process of CMC make it possess the higher swelling rate than other cellulose derivatives. Therefore, CMC is suitable to fabricate the inorganics/cellulose composite hydrogels for superabsorbent.
MMT is one of the most widely used layered silicates because its lamellar elements display high in-plane strength, stiffness, and high aspect ratio, which are beneficail to the properties of the composite hydrogels such as swelling ability, mechanical strength, and thermal stability. It has a 2:1 layer structure consisting of an octahedral alumina sheet sandwiched between two opposing tetrahedral silica sheets. The bonding between two silica sheets is very weak, which permits the water, exchangeable ions, and polymer chains to enter, and this leads to the development of nanocomposite hydrogels and their swelling capacity. The reactive hydroxyl groups and exchangeable cations on the surface of MMT can also make it act as a crosslinking agent between cellulose chains  and in situ intercalative polymerization [222, 223].
Cellulose-g-P(AA-co-AM-co-AMPS)/MMT composite hydrogels for superabsorbent were synthesized by using potassium persulfate as a free-radical initiator, in the presence of MBA as a crosslinking agent. The composite hydrogels comprised a crosslink structure of MMT and CMC with side chains that carried carboxylate, carboxamide, and sulfate . CMC, MMT, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) could be incorporated into AM hydrogels during free-radical solution polymerization; meanwhile the multifunctional crosslinker such as poly(ethylene glycol)diacrylate was used at the polymerization process to fabricate the highly swollen composite hydrogels . Cellulose-g-PAA/MMT nanocomposite hydrogels were proved to be effective in controlled release of macro- and micronutrients; furthermore, a hydrolysis treatment improved their physicochemical properties. The presence of MMT not only remarkably enhanced their capacities of loading nutrient solutions but also acted as an effective barrier to control and retard the release of nutrients to the medium as well as played an important role in reducing production costs . CMC-g-poly(NIPAM-co-AA)/MMT hydrogels were prepared by graft copolymerization of NIPAM and AA onto CMC in MMT/water suspension media, which were thermoresponsive and pH-sensitive hydrogels for removal of heavy metal ions from aqueous solutions . MC was also used to prepare the nanocomposite hydrogels with PAM and MMT to slowly release of fertilizers . Cellulose/MMT composite hydrogels were fabricated by chemical crosslinking cellulose and CMC with ECH in the presence of MMT nanosheets. MMT was first modified in order to make it take part in the formation of hydrogel network by reacting with cellulose. In this case, cellulose chains entered the galleries of MMT, and intercalated nanocomposite hydrogels were fabricated for highly efficient removal of dye in water .
Bentonite is a type of clay mainly composed of the crystalline structure of MMT and other additional crystalline structures. Composite hydrogels were synthesized by the graft copolymerization reaction among cellulose, polymethacrylic acid (PMAA), and bentonite; the inorganic bentonite particles in the network acted as additional network points. The PMAA-g-cellulose/bentonite could be effectively used as an adsorbent for the removal of dyes from water and wastewater . CA-crosslinked CMC/bentonite composite hydrogels were prepared to develop base triggered release formulations of thiamethoxam due to their high swelling sensitivity under the basic pH condition, low cost, and environment-benign nature. Bentonite was added in the composite hydrogels to improve the barrier properties of formulations with enhanced rheological properties . A new composite matrix for the controlled release of herbicides was prepared by adding bentonite or its acid-activated form into CMC gel .
Halloysite nanotubes (HNTs) with similar chemical formula as kaolinite possess unique empty lumen tubular morphology with outer diameters of 50 nm and inner lumens of 20 nm and 200–1000 nm long. They are biocompatible and widely available in nature, which make them good candidates for application in biomedicine by protecting the drug from enzyme degradation and delivery of the drugs to enter the cell. HNTs can be easily incorporated via solution mixing for cellulose composite hydrogels. HNTs were mixed into cellulose NaOH/urea solution to prepare composite hydrogels using ECH crosslinking at elevated temperature, and curcumin-loaded composite hydrogels showed a strong inhibition effect on the cancer cells and good cytocompatibility . HNTs were firstly modified by the reaction of the external silanol groups with maleic anhydride and further reacted with adipic acid dihydrazide to introduce a hydrazine moiety on the clay surface. Functionalized HNTs were mixed with oxidized CMC to form biodegradable hydrazone bond to prepare the injectable composites hydrogels with potential applications in bone medication .
Similar to MMT, carclazyte is a layered aluminum silicate with exchangeable cations and reactive -OH groups on the surface. CMC-g-poly(AA-co-AM)/carclazyte composite hydrogels for superabsorbent were synthesized by graft polymerization of AA and AM onto CMC and the introduction of carclazyte. Additionally, ammonium persulfate (APS) and MBA were used as a free-radical initiator and a hydrophilic crosslinking agent, respectively . Medicinal stone is a special igneous rock composed of silicic acid, alumina oxide, and more than 50 kinds of constant and trace elements. Vermiculite is a trioctahedral 2:1-layered silicate similar in texture to mica, and it is abundant and much cheaper in comparison with MMT . Both medicinal stone and vermiculite can be incorporated with HEC-g-PAA to fabricate the composite hydrogels by the free-radical polymerization among HEC and AA which exhibited the enhanced swelling capability, swelling rate, excellent pH-responsive and good deswelling capability as potential water-manageable materials [46, 235].
4.2 Antimicrobial Nanoparticles
The synthesis of cellulose/nanoparticles hybrid composite hydrogels has attracted increasing attention especially for the nanoparticles (NPs) with special functions. Metallic NPs with inherent antimicrobial properties such as Ag, Zn, Au, and Cu are known to have broad-spectrum antimicrobial nature and pose minimum toxicity to humans. Particularly, nanosilver-based wound dressings have received approval for clinical applications, but dermal toxicity was reported ; therefore, the combination of hydrogels with silver NPs (AgNPs) is a better choice for the treatment of wounds . There are free spaces within the cellulose hydrogels that could be utilized to incorporate NPs; meanwhile the incorporation of NPs into the hydrogel matrix reduces their agglomeration. Development of cellulose/NPs composite hydrogels is important to control growth of pathogenic microorganisms for the prevention of infectious diseases with good biocompatibility in the medical field. However, NPs have a tendency to aggregate to minimize their surface energy, which decreases their antimicrobial activity  and hinders the large-scale production with cost-effective dispersion methods. The methods of composite hydrogels preparation have a great influence on the structure and properties of the resulting composite hydrogels. For the preparation of cellulose/NP composite hydrogels, metals NP can be incorporated via two approaches: (1) ex situ, in which pre-synthesized NPs are incorporated into the cellulose hydrogel matrix and (2) in situ, in which cellulose matrix acts as a nano-reactor for synthesizing NPs. The antimicrobial NPs must be integrated in isolated and well-dispersed primary NPs inside the cellulose matrix to obtain the homogeneous composite hydrogels with good mechanical strength and enhanced antimicrobial activity.
CMC [240, 241] and hydroxypropyl CMC  can be used as reducing and stabilizing agents for green preparation of AgNPs, the hydroxyl groups form coordination bonds with silver ions, and carboxylic groups have electrostatic interaction with Ag ions which facilitate the reduction of Ag ions by CMC. A two-step process was the most common mechanism for synthesis of AgNPs, i.e., single-atom formation and then polymerization of the atoms to a particle. Firstly, a portion of metal ions in a solution is reduced by the available reducing groups of the cellulose and its derivatives; the produced atoms act as nucleation centers and catalyze the reduction of the other metal ions remaining in the bulk of solution. Subsequently, these formed atoms coalesce together to form metal clusters, and the ions attracted to the surfaces of these clusters are reduced further to aggregate to NPs. The formed particles are stabilized by a layer from the polymer matrix, thus preventing further coalescence of these NPs to larger particles [242, 243]. Synthesis procedures using microwave irradiation were also employed to accelerate the process. Microwave radiation of CMC and silver nitrates solution produced uniform AgNPs that were stable for 2 months at room temperature .
In situ: Cellulose and its derivatives can be chemically crosslinked into hydrogels, and AgNPs can further be incorporated into hydrogels matrix by in situ synthesis. CMC was converted to hydrogels using ECH as a cross linking agent. Two strategies were employed to obtain the CMC/AgNPs composite hydrogels: the first involved reaction of ECH with CMC in alkaline medium containing silver nitrate to synthesize AgNPs and form CMC networks simultaneously; the second entailed previous preparation of CMC hydrogel and reduction Ag+ ions into AgNPs within the swollen hydrogels. Both of CMC/AgNPs composite hydrogels showed uniform distribution of NPs in the hydrogels matrix, and the composite hydrogels obtained by the first strategy exhibited high antibacterial activity against Gram-positive and Gram-negative bacteria . Fumaric acid was also used as a crosslinker to prepare the CMC/AgNPs composite hydrogels with the first strategy to render cotton fabrics for antibacterial property with the potential applications in wound dressings . With the second strategy, hydrogels with CMC, PVA, and the crosslinker EGDE were fabricated, and AgNPs were incorporated by using trisodium citrate as a reducing agent under microwave radiation. The hydrogel matrix mainly provides steric protection to the Ag+ due to the polymeric network by direct bonding with these electron donor sites. The obtained composites hydrogels exhibited high antibacterial activity against urinary tract infection (UTI) pathogens . CMC-poly(diallyldimethylammonium chloride) (DADMAC) hydrogels were fabricated by graft copolymerization of DADMAC onto CMC using APS as a free-radical initiator and MBA as a crosslinker. AgNPs or CuO NPs were incorporated into the hydrogels by immersing the CMC-poly(DADMAC) hydrogels into AgNO3 solution or copper sulfate solution and reducing or oxidizing to NPs in situ. The prepared composite hydrogels can be used in different medical fields, i.e., drug delivery and wound dressing as well as wound healing . Semi-IPN hydrogels based on crosslinked PAM through an optimized rapid redox solution polymerization with MBA in presence of CMC were synthesized, and silver ions were in situ reduced using Azadirachta indica (Neem) plant extract under atmospheric conditions. The obtained pH-responsive composite hydrogels were suitable to safely transfer the drug through the stomach with acidic pH and release it successfully in the basic environment of the colon . AgNPs can be transformed into Ag3PO4 NPs by the reaction of Ag and HPO42− to prepare Ag3PO4/cellulose composite hydrogels .
NC can also form the composite hydrogels with AgNPs by in situ reduction of Ag ions [252, 253]. The negatively charged surface carboxylate groups of TEMPO-oxidized NC provide high binding capability to Ag ions which triggers rapid gelation of the NC because transition metals are capable of resulting in carboxylate metal ion complex formation. Simultaneously, Ag+ was reduced slowly into AgNPs with prevalent hydroxyl groups on NC without additional reducing agents . BC fibers are assembled by bundles of thinner cellulosic fibers with diameter sizes down to micro- and nanoscale. An extended network is observed via both intramolecular and intermolecular hydrogen bonds  enabling the production of sheets with high surface area and porosity. BC network has a very high affinity for water which results in hydrogel-like properties; therefore, BC hydrogel can embed the NPs to fabricate the composite hydrogels. GO-AgNPs/BC hydrogel microfibers were prepared with the microfluidic technology which exhibited a well-defined coaxial cable-like structure, with GO-AgNPs in the inner-core layer and BC hydrogel in the outer-shell layer. GO-AgNPs/BC hydrogel microfibers revealed efficient activity to sterilize both Gram-positive and Gram-negative bacterial strains with low cytotoxicity . AgNPs/BC composite hydrogels could be obtained by UV light irradiation in situ with AgNPs photochemically deposited onto the BC gel network as well as chemically bonded to the cellulose fiber surfaces .
Ex situ: The super-macroporous polymer hydrogels based on HPC and PAM were obtained by UV irradiation of moderately frozen systems, using hydrogen peroxide as a source of radicals. H2O2 generated hydroxyl radicals during its photo-homolysis which reacted with the polymer molecules giving rise to macroradicals. The crosslinking occurred by intermolecular recombination of two macroradicals. AgNPs were immobilized in the channels of the gel by immersing a freeze-dried HPC cryogel in aqueous dispersion of AgNPs or incorporated into the polymer matrix by mixing of the AgNPs dispersion and HPC followed by freezing and subsequent crosslinking. The two types of composite hydrogels exhibited completely different releasing process of AgNPs; the gels containing AgNPs only in the channels could release them immediately by compression, while the gels with AgNPs embedded in the walls exhibited a slow release of Ag for months because AgNPs were physically entrapped within the dense polymer network. Thus, composite hydrogels with different release profiles could be obtained by different ex situ incorporation methods which were beneficial to controllable release of AgNPs . Cellulose-based sponges were developed by freeze drying of regenerated cellulose gels and incorporated the prepared AgNPs by immersing the sponges into the AgNPs suspension. The composite sponges showed excellent antibacterial activity with prolonged release of AgNPs, high sorption of simulated wound fluids, and high water vapor transmission ability due to hydrophylicity of cellulose and porous structure, resulting in the cellulose-based sponges as promising wound dressing materials for fester and infected wounds . AgNPs can also be coated with PEG, sodium dodecyl sulfate, and β-CD before loading into the cellulose hydrogels in order to improve the stability of the NPs and reduce the growth of the particles via trapping of seeds and steric repulsion .
4.2.2 ZnO NPs
ZnO NPs possess antibacterial activity to both Gram-positive and Gram-negative bacteria which may be related to the induction of oxidative stress due to generation of reactive oxygen species and the degradation of the membrane structure of the cell. They even have antibacterial activity against spores that are high-temperature-resistant and high-pressure-resistant . Due to the lower cost, lack of color, and UV-blocking properties of ZnO NPs, they are used as alternatives to AgNPs to incorporate with cellulose hydrogels [261, 262]. Like AgNPs, ZnO NPs can be embedded into the cellulose hydrogels by in situ formation or ex situ process.
CMC can form complexes with Zn2+ due to its large number of coordinating functional groups in its molecular chains, and the precipitations of nuclei start as the concentrations of Zn2+ and OH− ions exceed the critical values. The precipitated Zn(OH)2 can be transformed into the ZnO crystals via the simple chemical reaction by heat . ZnO NPs/cellulose composite hydrogel particles were fabricated by using ZnCl2 aqueous solution as both the solvent of cellulose and the zinc source of ZnO nanoflakes. ZnCl2 aqueous solution can effectively dissolve cellulose, and reactive hydroxyl groups in the cellulose chains can accelerate the formation of ZnO nano-structures by reacting with Zn2+ to form Zn(OH)42−/cellulose complex through co-gelation process and transforming to ZnO nanocrystals at low temperature. ZnO nanoflakes are hexagonal structured nanoflakes with lamellar thickness of about 100 nm. Cellulose dissolved in ZnCl2 contributed not only to the growth of ZnO nanoflakes but also to restrain the formation of impurity phase . Cellulose solution dissolved in NaOH/urea revealed that the sol-gel transition and intra- and intermolecular hydrogen bonds of cellulose tended to be increased as a result of its strong self-association tendency to cause aggregation and chain entanglements for the gelation . ZnO NPs were grown in the network of cellulose hydrogels in situ to form the composite hydrogels . Like AgNPs, ZnO was also incorporated into BC hydrogel-like pellicle by in situ synthesis of ZnO via solution plasma process without the addition of a reducing agent .
4.3 Carbon Nanomaterials
Graphene and its derivates such as graphene oxide (GO) have been targeted on mechanically or electrically enhanced cellulose hydrogels as nanofillers. Specifically, GO shows the strong functionalities and processibilities due to the oxygen-containing functional groups on its basal planes and edges. The layered structure with oxygen-containing groups, i.e., hydroxyl, epoxide, and carboxyl groups, makes GO to be a favorable candidate for improving the mechanical strength of the cellulose hydrogels.
Normally, GO dispersion can be mixed with the cellulose solutions firstly and fabricated the GO/cellulose composite hydrogels further by crosslinking of cellulose. GO was mixed into cellulose dissolved in NaOH/PEG, and then the composite hydrogels were regenerated with the introduction of hydrochloric acid which neutralized NaOH and caused the reconnect and entanglement of cellulose chains by hydrogen bonds . Both GO and cellulose dissolved in ionic liquids were mixed and regenerated using water as a coagulant to composite hydrogels which significantly enhanced mechanical strength and thermal stability . CMC could be chemically crosslinked with CA in the presence of GO to obtain the GO/CMC composite hydrogels, and the introduction of GO also significantly improved the adsorption performances of the composite hydrogels toward dyes and heavy metals . GO/CMC-g-PAA and GO/CMC-g-PAM composite hydrogels were fabricated by in situ copolymerization of monomers on CMC backbone with MBA as a crosslinker in the presence of GO as fillers [276, 277]. GO/CMC composite hydrogels via electron beam radiation-assisted polymerization with MBA as a crosslinking agent showed larger gel fraction, higher mechanical strength and swelling capabilities compared to those prepared by solution polymerization with ECH as a crosslinker . Moreover, GO and the dissolved cellulose in the NaOH/urea were crosslinked with ECH; the incorporation of GO chemically increased the compressive strength of the composite hydrogels and significantly improved their adsorption capacities for the metal ions .
GO/CMC composite hydrogels physically crosslinked with Fe3+ and Ca2+ ions were prepared via the coordination between metal ions and the carboxyl groups in the CMC side chains. Due to the biocompatibility and the anionic-exchange properties of GO, the composite hydrogels revealed higher loading capacity and controlled release of the drugs [280, 281]. GO/CMC/PAM composite hydrogels with double networks consisting of ionically (Al3+) crosslinked CMC and covalently crosslinked PAM exhibited superior mechanical properties by combination of double networks and nanofiller reinforcement . In order to decrease aggregation of GO, HPC was grafted onto GO before fabricating the composite hydrogels .
The recent emergence of three-dimensional (3D) graphene hydrogels has provided a promising avenue to explore the performance of 3D porous material on electrochemistry [284, 285]. GO/cellulose composite hydrogels could be fabricated by ball milling of the mixture of GO hydrogels and cellulose solution in the presence of hydrazine, template shaping, coagulating, and freeze drying. The hydrogen bond interactions and the shearing force collaboratively resulted in sufficient mixing of GO and cellulose and the subsequent formation of homogenous GO/cellulose hydrogels. It is a scalable preparation method of GO/cellulose composite hydrogels with application in supercapacitors.
GO could be incorporated into BC hydrogels with ex situ composites synthesis [286, 287, 288]; however intrinsically 3D structure of BC was broken . In situ BC composites synthesis is becoming a main approach in which reinforcement materials are added to BC culture media at the beginning of the BC synthetic process. Microfibrils of BC become denser with time and produce a network structure that can trap various materials added to the BC synthetic media. The encaged materials become part of the BC fibril network, resulting in BC composites. GO/BC composite hydrogels were prepared by in situ biosynthesis of BC in graphene-dispersed culture medium. The addition of graphene reduced the crystallinity of BC without changing the entangled network structure [289, 290]. GO/BC composite hydrogels followed a non-Fickian diffusion mechanism in the drug release with good cell viability for drug delivery system .
Carbon nanotubes (CNTs) demonstrate very interesting properties because of sp2 hybridization of carbon-carbon bonds and the resultant cylindrical arrangement of the graphene sheets. In the realm of drug delivery, CNTs have gained tremendous attentions as promising nanocarriers, owing to their distinct characteristics, such as high surface area, enhanced cellular uptake, and the possibility to be easily conjugated with therapeutics, including both small molecules and biologics, displaying superior efficacy, enhanced specificity, and diminished side effects . Carbon nanodots (C-dots) as new carbon nanomaterials have obtained increasing attentions for the applications of bioimaging, sensing, and catalysis fields, due to their exceptional properties such as highly fluorescent feature, low toxicity, excellent dispersibility in water, flexible functionalization, and good biocompatibility. C-dots possess numerous oxygen-containing functional groups on their surfaces such as hydroxyl groups, carboxyl groups, and epoxide groups, which indicate that C-dots have the promising potential application as physical crosslinkers in the preparation of NC hydrogels .
CNT/CMC composite hydrogels were fabricated simply by mixing CMC and CNTs at room temperature by ultrasonication; the strong hydrogen bonding as well as electrostatic interactions between acid groups of CNT and hydroxyl/carboxyl groups of CMC were believed to be the main reasons to form the composite hydrogels . High-strength composite hydrogels were designed and synthesized by introducing CNTs into cellulose/NaOH/urea aqueous solution and chemically crosslinked by ECH . Cellulose nanofibrils and BC were also used to fabricate the CNT/cellulose composite hydrogels [296, 297, 298, 299, 300].
Three categories of cellulose-based composite hydrogels were summarized in this chapter including synthetic polymer/cellulose, natural macromolecules/cellulose, and inorganics/cellulose composite hydrogels. The composite hydrogels combine the advantages of two components and obtain the new features for broadening applications especially in biomedical and tissue engineering, drug delivery, biosensors, and superabsorbent. It is feasible to incorporate some synthetic polymers with water-soluble cellulose derivatives to fabricate the synthetic polymer/cellulose composite hydrogels by chemical, physical, and ionic crosslinking. In situ free-radical polymerization of acrylate monomers onto cellulose derivatives is one of main methods to introduce synthetic polymers into cellulose-based hydrogels. Nanocellulose with high-aspect ratio can be physically entrapped in the polymer network as a filler or chemically crosslinked with the polymer matrix in order to improve the degradation and the mechanical strength of the composite hydrogels. Chitosan, alginate, starch, and extracellular matrix are popular natural macromolecules for the fabrication of natural macromolecules/cellulose composite hydrogels. Chitosan is the most important polysaccharide to form the composite hydrogels with cellulose by chemical crosslinking. Chitosan can crosslink with cellulose via crosslinkers such as epichlorohydrin or reaction between reactive groups in chitosan and cellulose derivatives. Inorganics/cellulose composite hydrogels are a kind of special hydrogels combining both inorganic and organic properties. Montmorillonite and other layered silicates can form the nanocomposite hydrogels with cellulose for applications of superabsorbent and controlled release of nutrients. Metallic nanoparticles with inherent antimicrobial properties such as Ag were also incorporated into the cellulose composite hydrogels via ex situ and in situ methods to render antimicrobial. Carbon nanomaterials such as graphene oxide can be incorporated into the cellulose hydrogels as fillers, and the composite hydrogels show potential applications in supercapacitors and biosensors. Cellulose-based composite hydrogels are attracting considerable attentions in both academic research and industrial application due to their excellent hybrid properties.
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