Cellulose-Based Hydrogel for Industrial Applications
Cellulose-based superabsorbent hydrogels can absorb and retain huge amounts of water or aqueous solutions. They have a wide range of industrial applications including (a) hygienic and bio-related uses (more specifically in disposable diapers); (b) agricultural uses (such as water reserving in soil, soil conditioning, and controlled release of agrochemicals); (c) pharmaceutical dosage forms; (d) separation technology; (e) textile, leather, and paper industries (such as in wastewater treatment); (f) water-swelling rubbers; (g) soft actuators/valves; (h) electrical applications; (i) construction, packaging, and artificial snow; (j) sludge/coal dewatering; and (k) fire extinguishing gels. Many new advanced technologies are evolving by the day to cope with rigorous industrial-scale applications to ensure improved technical feasibilities. This chapter will briefly cover some of the selected aspects of cellulose-based hydrogels and their industrial applications.
KeywordsCellulose Hydrogels Superabsorbent hydrogels Composite Stimuli-responsive hydrogels Carboxymethylcellulose
Hydrogels have three-dimensional structure with elastic network that can span the volume in aqueous media. Hydrogels can be both natural and man-made [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Natural hydrogels are important components of different complex organisms that include (a) the bodies of jellyfish, (b) connective tissues in joints, (c) cornea in the eye, and (d) nuclear pore complexes inside cells. Synthetic or man-made hydrogels have a wide range of applications in different areas of life sciences including biomedical applications, where some of the important uses are (i) contact lenses, (ii) drug delivery, and (iii) tissue engineering [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. Hydrogels are mostly polymer-based, and their networks usually contain covalently cross-linked natural or synthetic polymers, whereas the supramolecular hydrogels use nanofibers to form the self-assembly of small molecules (such as hydrogelators), which have many promising applications [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. In this context, natural polymers such as cellulose are very popular and frequently used for a variety of applications in hydrogel formulations apart from their numerous other uses in different areas of science and technology. One of the many practical aspects of the use of cellulose in hydrogel formulations is its abundant availability (as a natural biomaterial) in the form of a living terrestrial biomass suitable for a wide range of industrial applications [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27].
2 Cellulose-Based Hydrogels
Nature is the main source of cellulose. It is a carbohydrate-type natural polymer, and it also has other attractive properties which include (a) biocompatibility, (b) biodegradability, (c) environmentally benign, and (d) cheap and renewable. In addition, cellulose contains hydroxyl groups in its structure that provide a unique suitability to produce superabsorbent hydrogels with attractive structures and desired characters. Since the advent of superabsorbent polymers in 1950, notable progress has been achieved within the last few decades in the field of superabsorbent hydrogels. One of the main driving forces behind this is the main concern of continuously increasing customer demands for high-quality superabsorbent materials for industrial applications primarily in the sanitary industry. Superabsorbent hydrogels have hydrophilic networks that provide higher capacity in water uptake and facilitate the capacity for water absorption and swelling. A good quality superabsorbent hydrogel can retain aqueous solutions up to hundreds of times compared to the weight of dry hydrogel [1, 2, 3]. Usually superabsorbent hydrogels are produced from synthetic polymers; however, there is a current trend to replace these polymers using environmentally benign and sustainable green alternatives such as natural polymers like cellulose and their products . Cellulose-based superabsorbent hydrogels have been successful to attract current active research interest due to their many practical and potential industrial applications. For example, when celluloses or their derivatives are used in the synthesis of superabsorbent hydrogels, they provide the opportunity to overcome different demerits associated with synthetic-based superabsorbent hydrogels and also contribute to meet many requirements relating to environmental issues and other desired set criteria for specific applications . In addition, it is possible to add multifunctional character on hydrogels by incorporating functional molecules into the structure of the hydrogels. Additionally, cellulose-based superabsorbent hydrogels typically show higher properties in comparison to properties observed from synthetic superabsorbent hydrogels; these properties include (a) absorbency, (b) strength, (c) biodegradation capability, (d) biocompatibility, and (e) good salt resistance [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39].
2.1 The Emergence of Hydrogels as Biomaterials
Hydrogels are one of the few ever-evolving biomaterials which have been rigorously studied due to their wide range of application potentials in different areas of science and technologies including biomedical, therapeutic, and diagnostic application purposes. Hydrogels provide many unique feasibilities to incorporate vivid range of materials (such as biopolymers, synthetic polymers, nanoparticles, hybrid materials, composites, and nanocomposites) to form their tailor-oriented suitable structural morphologies using different techniques in order to serve specific desired uses. Besides these wider advantages, there are also some factors (such as proper and stable dispersion of hydrogels and nanoparticulate systems) that limit their uses in different areas. From a comprehensive study on the recent development trends on different hydrogel systems, it is quite transparent to observe that the incorporation of nanostructured fillers into hydrogels has been systematically enhanced using novel techniques to synthesize novel hydrogels that can successfully retain different functionalities to address different challenges. This chapter concentrates on the applications of carboxymethylcellulose (CMC)-based hydrogels along with other related hydrogel systems such as CMC-based composite, nanocomposite, hybrid, and functional hydrogel systems and their general method of fabrications as well as their applications. In addition, it also shed some light on the fundamentals of hydrogels and nanoparticles (NPs) along with recent advances in the field of design, synthesis, functionalization, and application of nanocomposite hydrogels with improved characters (such as mechanical, biological, and physicochemical behaviors). Besides this, it also briefly states the current challenges and future opportunities for the successful applications of hydrogels (including composite- and nanocomposite-based hydrogel systems) in particular areas (such as industrial applications of cellulose-based superabsorbent hydrogels) [35, 37, 38, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65].
3 Cellulose Composite-Based Hydrogels
Cellulose, the most abundant natural biomass, possesses very promising properties, some of which include mechanical robustness, hydrophilicity, biocompatibility, and biodegradability. Cellulose-based composites have many applications, for example, in drug delivery systems, hydrogels, electronic active papers, sensors, shape memory materials, and smart membranes. It also offers interesting potential improved properties and new functionalities when used in hydrogel formulations using appropriate methods. For example, cellulose-based stimuli-responsive smart materials can be used for hydrogel synthesis, and the hydrogels produced from these materials have many advantages, some of the most important ones include regulation of stimuli-responsive behaviors during the reactions with the environmental stimuli and wide application potentials of these smart materials in different fields (for instance, as biomaterials). Different methods can be used to incorporate different stimuli- responsive materials into the structures of cellulose composites in order to regulate different properties of the bulk composite structures which can be tailored to make them suitable properties for their use in composite-based hydrogel systems [66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132]. Environmentally stimuli-responsive cellulose composite-based hydrogels have many conventional and high-tech applications, but there are quite a lot of challenges in order to produce high-quality stimuli-responsive cellulose composite-based hydrogels that meet desired criteria and mitigate challenges. This chapter discusses different selective aspects of environmentally stimuli-responsive cellulose composites and their uses in hydrogel productions and their potential industrial applications.
3.1 Cellulose-Based Stimuli-Responsive Hydrogels
The abundance of hydrophilic groups on the chains and slightly cross-linked structure hydrogels allows them to absorb large amounts of water which they can release at dry conditions. Hydrogels are also widely used in food, biomaterials, agriculture, and other industrial applications [6, 10, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247]. The syntheses and uses of hydrogels based on cellulose have been reviewed by other researchers [6, 10, 247], so some selective studies on stimuli-responsive hydrogels based on cellulose are briefly discussed. The superabsorbent nanocomposites based on CMC and rectorite also exhibited saline, pH, and organic solvent-responsive behaviors. Stimuli-responsive smart hydrogels based on cellulose nanocrystals contributed to improve the swell capacity and the mechanical strength of the hydrogels [133, 224]. Based on titanium dioxide-based coated, native nanocellulose aerogel networks, a novel photo-controlled switching between the water superabsorbent and water-repellent states has been reported . In the stable state, titanium dioxide-based coated aerogels did not show any water absorption; however, the original absorption and wetting properties slowly recovered when stored in the dark. Besides this photo-induced absorption and wetting behavior, titanium oxide coated nanocellulose aerogels also exhibited photocatalytic activity and capability to decompose an organic material (such as methylene blue). Electrospinning or coating techniques have been used to prepare pH-responsive hydrogel fibers from cellulose which have potential applications in the fields of cotton knitwear and biomaterials [134, 156].
3.2 Applications of Environmental Stimuli-Responsive Cellulose Composites in Combination with Cellulose-Based Hydrogels
Cellulose-based biocomposites and nanocomposites have a very wide range of potential applications. For example, nanocellulose-based materials are extensively used in various applications including in the manufacture of paper and packaging products as well as in construction, automotive, furniture, electronics, pharmaceuticals, cosmetics, and biomedical applications. Additionally, they have very useful properties (e.g., the higher strength and stiffness, the higher surface reactivity due to the presence of numerous hydroxyl groups, the specific organization of the small dimensions of nanocellulose) suitable for reinforcement with nanofibers to enhance desired mechanical and physical properties for high-tech applications. Cellulose-based hydrogels can be used for coating and pretreatment on cellulose composite product for their various applications in different areas including (a) pharmaceutical products and packing, (b) specific high-quality papers for electronic and sensoric applications, (c) cellulose-based diagnostic chips for biomedical applications, and (d) cellulose-based specialty product for high-quality printing and packaging. Additionally, cellulose-based hydrogels produced from stimuli-responsive cellulose-based nanocomposites (responsive to pH, temperature, redox potential, light, magnetic field, and electrical field) have many potential applications in drug delivery systems, some of which include (a) promotion in controlled drug release and (b) controlled delivery in specific intracellular locations or to targeted tissues. Stimuli-responsive smart drug delivery systems have been intensively investigated and reviewed in recent years [237, 238, 239, 240, 241, 242, 243, 244, 245]. Additionally, when hydrogels produced from cellulose-based nanocomposites with capability to show responsive behaviors have unique properties, for example, (a) biocompatibility, (b) biodegradability, and (c) biological functions. This type of products has been successful to draw the attentions of many researchers all around the world to explore these materials for practical industrial-scale exploitations because of their attractive uses including drug delivery systems, tissue engineering, and related biomedical applications. For example, stimuli-induced self-assembly and post-assembly triggering strategies can be used as an alternative approach to manipulate self-assembled architectures of synthetic polymeric aggregates in drug delivery systems. The assembly of polymeric aggregates contributes to transmit and change hydrodynamic radius changes, so drugs loaded in the assembled polymeric aggregates can be used for a controlled release when they disassemble after they are exposed to particular environmental stimuli (such as temperature and pH). For example, Sui et al. studied the temperature- and pH-sensitive characteristic of cellulose-g-PDMAEMA by using UV detection and dynamic light scattering technique . The lower critical solution temperature (LCST) of aqueous cellulose-g-PDMAEMA solution was measured to be 42 °C, and when the thermal environment was below this temperature, the solution was transparent, and the value of hydrodynamic radius increased slightly with temperature increase (25–40 °C). On the contrary, when the temperature was raised to the range of 42–55 °C, the solution became opaque, and the value of hydrodynamic radius increased abruptly. At a low temperature in the surrounding environment, the cellulose-g-PDMAEMA copolymer chains exhibited random coil conformation because of the hydrogen-bonding interactions between the copolymer and water molecules. However, when the temperature was raised to LCST, polymer chains showed a shrinking into a globular structure due to the hydrophobic interactions between N,N-dimethylaminoethyl groups. In addition, the cellulose-g-PDMAEMA was dissolved in aqueous hydrochloric acid (with pH 2.0 at room temperature) and was precipitated in an aqueous alkaline condition (pH 12.0). Moreover, at lower acidic pH, due to the geometrical constraint and the electrostatic repulsion between polymer chains, the PDMAEMA chains were entirely protonated and highly stretched along the radial direction. Yuan et al. reported the synthesis of amphiphilic ethyl cellulose brush polymers with mono and dual side chains, which showed promising properties with dual temperature and pH response .
Stimuli-responsive cellulose-based hydrogels or similar types of other hydrogel-based products with the capability to exhibit swelling or shrinking when exposed to external stimuli (such as pH, temperature, light, magnetic field, etc.) have many potential biomedical applications because of their unique characteristic properties (e.g., biocompatibility, biodegradability, and biological functionality) [10, 247]. The swelling and shrinking mechanism of hydrogels is similar to that of aggregate assembly usually observed during stimulus-induced intermolecular and intramolecular hydrogen-bonding changes. Targeted drugs can be released controllably from drug-loaded hydrogel carriers when hydrogels swell to more loose structures when exposed to environmental stimuli (with a proper control on this stimulus). Similarly, redox-responsive hydrogels have many application potentials for controlled drug release . In addition, stimuli-responsive cellulose-based hydrogels incorporated with functional microcapsules and nanoparticles materials have many practical application potentials in pharmaceutical industry. For example, magnetic-responsive systems are widely used to trigger drug release at target sites and can also be applied to concentrate the drug-specific-responsive carriers. Magnetic cores encapsulated with ethyl cellulose have the potential to improve the biocompatibility and 5-fluorouracil loaded in the nanoparticles for a controlled release on specific sites during treatment of cancer . Magnetic particles coated with hydroxy propyl cellulose exhibited response to thermal and magnetic environments which have many potentials for nanomedical applications (e.g., remote-controlled drug carriers) . This type of products has been investigated to functionalize hydrogels for advanced life science applications. Lastly, controllable pore size of stimuli-responsive membranes is highly used in drug delivery. For example, cellulose-based membranes produced by incorporating stimuli-responsive materials can change their pore size according to the environmental conditions [6, 155]. So, drug delivery systems based on smart stimuli-responsive membranes can be used for a controllable drug release by an effective control on the stimulus that control the nature of diffusion through the membranes. For example, when CMC ethers mixed with aspirin were pressed to the tablet membrane, they showed zero-order release of the drug based on pH-responsive properties. This reversible glucose responsiveness was attributed to the reversibility of swelling and shrinking of the nanoparticles in response to changes of pH . They can also be used in cellulose-based stimuli-responsive hydrogel fabrications. Cellulose derivative-based membranes embedded with liquid crystal molecules also showed temperature-responsive drug permeation characteristics [183, 184]. Suedee et al. prepared an MIP incorporated cellulose membrane for enantioselective-controlled delivery of racemic drugs with pH-responsive characters where (S)-omeprazole was used as an imprinting molecule conferring stereoselectivity upon the polymers . Cellulose-based superabsorbent hydrogels have the potentials to be incorporated with this type of advanced systems to offer higher functionality suitable for high-tech applications in different areas of biomedical and life sciences [6, 10, 133, 134, 135, 136, 137, 138, 139, 140, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 193, 194, 195, 196, 197, 198, 199, 200, 201, 205, 206, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264]. Generally, different environmental stimuli (such as light, temperature, pH, and ionic strength) can be used as triggers for the formation of supramolecular hydrogels. Additionally, inherent biological processes can be used to synthesize supramolecular hydrogels that are useful for biomedical applications. Biomimicking biomacromolecular self-assembly (e.g., formation of collagen fibrils, the integration of enzymatic reactions with self-assembly of small molecules) is an important technique for formation of nanofiber networks that can be used to produce hydrogels under various conditions. Enzymatic reaction can be used to initiate the self-assembly of a derivative of Taxol in aqueous solution to form nanofibers for producing supramolecular hydrogel. Cellulose derivatives can also be used for producing composite-type biomimetic hydrogels [253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270].
4 Cellulose-Based Functional Hydrogels
Composite hydrogels are produced using a variety of ways where some principle techniques include the incorporation of a range of suitable compatible materials into hydrogel structure or the assembly of hydrogels with other suitable structures (such as electrospun nanofibers) for serving specific purposes. This type of composite hydrogels can be used in various uses in everyday life (e.g., superabsorbers, contact lenses, drug delivery) [3, 4, 5, 6, 7, 8, 9, 10, 11, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181]. This type of composite hydrogels can be further functionalized by incorporating additional functionalities to produce functionalized hydrogels for many conventional and high-tech applications. For example, some particular type of hydrogels can be easily functionalized by introducing responsive characters which can respond to external stimuli. Some cross-linked polymers are responsive to various stimuli including light, temperature, pH, pressure, chemical (certain), electrical field, magnetic field, enzyme, and biological agent. By the careful and controlled use of responsive characters of the hydrogels produced by using these responsive polymers, they (hydrogels) can be applied in many fields like drug delivery, tissue engineering, and purification and implementation as actuators and biosensors or for medical coatings. In addition, cellulose can be used in combination with other suitable polymers to produce composites which can be used for hydrogel preparations [6, 10, 133, 134, 135, 136, 137, 138, 139, 140, 193, 194, 195, 196, 197, 198, 199, 200, 201, 205, 206, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264].
4.1 CMC-Based Analyte-Sensitive Hydrogels
Carboxymethylcellulose (CMC) and their derivatives have the potentials for producing analyte-sensitive hydrogels that incorporate optical structures, and this type of hydrogels is particularly attractive for many emerging applications including their uses as sensing platforms for point-of-care diagnostics. In this case, the optical properties of the hydrogel sensors need to be rationally designed and fabricated using different techniques including (a) self-assembly, (b) microfabrication, and (c) laser writing. Main merits of CMC-based or similar other polymer-based photonic hydrogel sensors over the typically used conventional assay formats include (a) reusability, (b) free from labeling, and (c) quantitative as well as continuous measurement capability with potential for integration to equipment-free text or image display applications [253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264]. Here in this section of this chapter, we briefly explain different aspects of photonic hydrogels and their applications as sensors; we also present different important ways for the syntheses of CMC and other celluloses or polymers as well as quantum dot-doped composite hydrogel systems and their industrial applications. When photonic structures (such as quantum dots or functional molecules that can be modulated for photonic uses) are incorporated into CMC, cellulose, or other polymer-based hydrogel structures, this embedding or introduction causes certain level of change that contributes to alter the water contents and volumes of hydrogels and also changes the level of interactions with specific analyses. This phenomenon offers a potential new platform to fabricate different biomedical diagnostic devices (such as IVD devices). In general consideration, hydrogels are 3D polymer networks that can be used for variable volume changes by controlling the variations of Donnan osmotic pressures applied on the hydrogel systems [254, 255]. By careful design to incorporate different suitable materials (such as quantum dot and responsive polymers) into hydrogel structures, sensitive hydrogels can be prepared that will be responsive to a range of stimuli as well as clinically relevant analytes [255, 256]. When functional molecules (such as bioactive recognition molecules and stimuli-responsive polymers) are incorporated into hydrogels to provide bioactive functional properties, the produced hydrogels show responsive characters on exposure to external stimuli by changing their physical and chemical characters to certain extent [255, 257]. For example, functional materials like quantum dot-doped hydrogels are useful for the fabrication of optical signal transduction and reporting within one device. Different reports that deal with bottom-up or top-down nano-/microfabrication methods are available that studied the feasibility of fabrication techniques for the miniaturization and multiplexing hydrogel-based photonic structures [258, 259, 260, 261, 262, 263, 264, 265]. By the interaction of a target analyte and modulating reflection, diffraction, refraction, surface plasmon resonance, or emission  characteristics, the volumetric change in the hydrogel structure can be monitored, and the change in optical properties can be used to work as transducers that provide the opportunity for spectroscopic analysis (such as the concentration of the analyte, when this type of hydrogels are used for quantitative analysis). Besides this, CMC- or similar other polymer-based photonic hydrogels have the potentials to be used as sensors for tuning and reporting visually distinguishable color changes with a scope for semiquantitative determination without the use of an equipment. CMC- or their derivative-based photonic hydrogel sensors can be used for different biomedical applications. These photonic hydrogels may also have optically active elements with capabilities in displaying 3D images or writing [267, 268, 269]. Hence, the development of photonic hydrogel sensors has immense potential for both equipment-free semiquantitative diagnostics and quantitative analyzers that are compatible with mobile spectrophotometers and smartphone readers [270, 271, 272, 273, 274, 275, 276, 277, 278]. The potential applications of photonic hydrogel sensors are not limited to medical diagnostics but also include veterinary testing, pharmaceutical bioassays, and biohazard and environmental monitoring. However, the main focus area of hydrogel sensors has been in the detection and/or quantification of chemicals and cells in medical diagnostics. For example, their potential applications in biochemistry and biology are monitoring enzyme activity and metabolites  and serum albumin ligand binding [280, 281, 282]. Another potential area of application of photonic hydrogel sensors includes the detection of biocontaminants, heavy metals, and nanoparticles in water or air. The development of environmental sensors is aligned with the strict regulations imposed by the European Union and the United States. Reusable hydrogel-based sensing of environmental contaminants is an emerging area that can significantly reduce the costs and turnaround time at resource-limited settings.
4.2 Quantum Dot (QD)-Doped Cellulose-Based Hydrogels
Quantum dots are often used for the functionalization of hydrogels including cellulose-based hydrogels. For example, Baruah et al. reported the synthesis of functionalized graphene oxide quantum dots (GOQDs)-poly(vinyl alcohol) (PVA) hybrid hydrogels by using a simple, facile, and cost-effective strategy . They introduced GOQDs bearing different surface functional groups as the cross-linking agent into the PVA matrix for gelation. They synthesized four different types of hybrid hydrogels by using graphene oxide, reduced graphene oxide, ester functionalized graphene oxide, and amine functionalized GOQDs as cross-linking agents. They found that the hybrid hydrogel prepared with amine functionalized GOQDs exhibited the highest stability compared to other hydrogels prepared by using other cross-linking agents. They used this stable hydrogel to explore the feasibility for an easy, simple, effective, and sensitive method for optical detection of M2+ (Fe2+, Co2+, and Cu2+) in aqueous media involving colorimetric detection. They observed that when the amine functionalized GOQD-PVA hybrid hydrogel was exposed to the corresponding solution of Fe2+, Co2+, and Cu2+, it showed, respectively, brown, orange, and blue color changes of the solutions and contributed to detect the presence of Fe2+, Co2+, and Cu2+ ions in the solutions. They also noted that this system can also be applied for sensing a mixture of coexisting ions in solution. It is possible that this piece of work can be extended to produce cellulose- and PVA-based composite systems for hydrogel formulations for detecting metal ions from solutions. El-Salmawi synthesized polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC)-based hydrogel using three different techniques (freezing and thawing, electron beam irradiation or combined freezing and thawing, and electron beam irradiation) and then applied the hydrogel for particular applications . Then she carried out a comparative study between the three techniques in terms of gel fraction (%) and swelling (%). She observed that the physical properties of the hydrogel showed improved results when the combination of freezing and thawing and irradiation was applied rather than just freezing and thawing or irradiation only. She examined the effects of temperature and soil fertilizers on swelling (%) in order to evaluate the usefulness of the hydrogel as a superabsorbent material for its applications in the soil. She also observed that the swelling ratio increased as the composition of CMC increased in the blend. During the study, she used the blend having the composition 80/20 (CMC/PVA) and examined its behaviors as a superabsorbent in the soil for agricultural applications. She also noticed that the water retention increased in the soil containing this hydrogel. Thus, this type of hydrogel can be used to increase water retention in desert regions .
4.3 Cellulose Nanocrystal (CNC)-Based Hydrogels
Cellulose-based nanocrystals are also used for the formulation of different types of hydrogels due to many advantages that include (a) low cost, (b) biocompatibility, (c) biodegradability, and (d) good mechanical properties and chemically reactive surfaces. In addition, cellulose nanocrystals are a promising class of nanomaterials that have a wide range of application potentials for a variety of industrial uses. Some of the main industrial uses include (a) food packaging, (b) personal care, (c) biomedical devices, (d) textiles, (e) separation technologies, (f) construction materials, and (g) paper and similar paper-based product industries [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 195, 196, 197, 198, 199, 200, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264]. Cellulose nanocrystals have some significantly important characters (such as large aspect ratio, high strength, and lightweight) and also have the required capability to be chemical cross-linked in order to modify that chemical and physical properties. All these attractive features make cellulose nanocrystals attractive for their use in hydrogel formulations using different strategies including the injection of these materials into the structure of hydrogels and aerogels to produce desired technical characters (such as robust mechanical properties). Thus, it is a popular trend to use chemically cross-linkable cellulose nanocrystals as nanofillers to reinforce injectable hydrogels and also to use them in the production of nanofibers for all-cellulose nanocrystal-based hydrogels and aerogels for a wide range of conventional and high-tech applications. In addition, it is possible to produce a wide range of cellulose nanocrystal reinforced nanocomposites based on injectable polysaccharide hydrogels filled with aldehyde and functionalized cellulose nanocrystals that can simultaneously work both as nanofillers and cross-linkers to provide required level of technical performances to hydrogels produced by using these systems that have many industrial application potentials . For example, in one investigation, hydrogels were produced using this theme that showed highly enhanced elastic moduli (>140% increase at peak strength) along with improved dimensional stability during swelling experimentations and also exhibited strong coherence, while unfilled hydrogels showed degradation over the same time period during experimentations. Besides this, hydrogels produced in this way were also suitable for cell culture and tissue engineering applications [161, 285].
4.4 Cellulose- and Other Polymer-Based Supramolecular Hybrid Hydrogels
Cellulose- and other polymer-based supramolecular hybrid hydrogels have been successful to draw active current research interests for a plethora of potential industrial applications. For example, functional materials like quantum dot (such as CdSe, CdSe/Zns, and CdTe)-doped hydrogels produced by using cellulose or other types of polymers have been actively investigated for different applications including biosensing or bio-labeling. Xie et al. fabricated a fluorescent supramolecular hydrogel by doping with CdTe . In this study, they used mercaptan-ended poly(ethylene glycol)-poly(ε-caprolactone)-based synthetic amphiphilic block copolymer to stabilize colloidal QDs and studied the stability and fluorescent properties of the resultant colloidal QDs. Using a host-guest self-assembly technique between the amphiphilic block copolymers on the QD surface with the addition of cyclic oligosaccharide host molecule (α-cyclodextrin or α-CD), they fabricated a fluorescent supramolecular hydrogel and investigated its physical and chemical characters (such as spectral properties, rheological characters, gelation kinetics, and mechanical strength). This study showed new prospects to develop biocompatible optical materials with tunable fluorescent properties and mechanical properties suitable for various potential industrial applications . For the first time, Chang et al. successfully fabricated strongly fluorescent hydrogels with quantum dots (CdSe/ZnS nanoparticles) using a mild chemical cross-linking method where CdSe/ZnS nanoparticles were embedded firmly in the cellulose matrices due to strong interactions between the CdSe/ZnS nanoparticles and cellulose after the hydrolysis of QD ligands. The cellulose networks in the hydrogels had a significant contribution to protect the structural integrity and photoluminescent properties of CdSe/ZnS in the hydrogel network. These composite hydrogels (QD-doped cellulose matrix-based hydrogels) showed strong photoluminescent properties that demonstrated different color changes depending on the size of the QDs. In addition, the hybrid hydrogels were transparent and had good compressive strength with a scope to fabricate safe and biocompatible biopolymer-quantum dot-based hydrogels with high level of photoluminescence for different potential applications .
4.5 Cellulose Hydrogels for Enzyme Sensing
Palomero et al. fabricated fluorescent nanocellulosic hydrogels based on graphene quantum dots for sensing enzymes (such as laccase) . They developed a novel low-cost fluorimetric platform into the nanocellulosic hydrogel (using sulfur and nitrogen-codoped graphene quantum dots immersed into hydrogels) structure and applied it for the detection of the laccase enzyme. It is novel in the sense that usually used methods generally use catalytic activities for the detection of laccase that depend on surrounding environmental parameters. However, they used the fluorescence response of hydrogels that have graphene quantum dots (or GQDs) which worked as a luminophore for laccase. This type of hydrogels can be easily prepared, and they have enhanced fluorescence signal of GQDs that can avoid their self-quenching with stabilized fluorescence signals with relatively higher level of sensitivity toward laccase. They attributed this behavior to the noncovalent interactions between the sensor and the analyte that caused this significant quenching without peak shifts of GQD fluorescence through energy transfer. It is a simple cost-effective method that can be used to detect and stabilize laccase with a value addition to store and recycle enzymes .
5 Hybrid Cellulose Composites and Hydrogels
Both cellulose-based composites and nanocomposites are suitable candidates for different types of advanced composite hydrogel formulations for many potential industrial applications. This type of composite hydrogels provides a very good opportunity to functionalize them using many advanced materials (such as quantum dot, carbon nanotube, cyclodextrin, 2D layered materials, and biomaterials) for both conventional and high-tech applications. A composite or nanocomposite-based hydrogel has the beauty of the material often derived from the combination of dissimilar entities [131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 285, 286, 287, 288, 289]. Different other techniques also can be used for the preparation of functionalized composite- and nanocomposite-based hydrogels, for example, (a) composite hydrogel systems doped with functional materials and (b) the functionalized composite or nanocomposite systems for the preparation of hydrogels. In this context, different aspects of quantum dot-doped cellulose nanocomposite systems and their potentials for hydrogel formulations along with their applications are briefly discussed here. Different ways are usually used to produce QD-doped cellulose-based nanocomposites for different industrial applications. For example, Hassan et al. investigated the preparation of new dendronized cellulose derivatives, which were used in the preparation of cadmium sulfide quantum dots/cellulose nanocomposites . In addition, Ruan et al. investigated the preparation and properties of CdS nanocrystals/regenerated cellulose nanocomposites by using in situ synthesizing method during cellulose dissolution in NaOH/thiourea system . Small et al. investigated the preparation of ZnS-doped nanocrystals and mechanical and optical properties of cellulose-ZnS: Cu and cellulose-ZnS: Mn nanocomposites . In addition, Hasan et al. also reported CdS, ZnS, CdS/ZnS, ZnS/CdS (core/shell nanostructures), CdS/ZnS/CdS, and ZnS/CdS/ZnS multilayered nanostructures which were prepared at low temperature using cadmium chloride, zinc chloride, and sodium sulfide in the presence of hyperbranched polyethyleneimine (PEI) polymer. They also reported the method of the preparation of PEI-stabilized nanoparticle-doped functional cellulosic materials which could be used in different optical and electrical applications . From these studies, it was reported that cellulose fibers/semiconductor nanocomposites made from impregnating bagasse pulp fibers and the prepared semiconductor nanostructures exhibited lower-strength properties than blank cellulose fiber sheets despite of the very low fiber loading with the nanoparticles. In addition, the optical properties of different cellulose/semiconductor nanocomposites were close to each other. Loading of cellulosic fiber with the prepared nanostructured semiconductors has no significant impact on the degradation onset temperature but increases the rate of thermal degradation. This kind of nanocomposite and the properties of the interphases have strong influence on the dielectric properties and interfacial polarization of cellulose fiber/semiconductor nanocomposites. All these nanocomposite systems have the potentials to be processed in suitable way to use them in composite- and nanocomposite-based hydrogel formulations for various applications [35, 58, 59, 60, 61, 62, 63, 64, 65, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185].
5.1 Modification of Hydrogel Properties by the Incorporations of Nanomaterials or Cellulose Nanocrystals into the Structures of Cellulose Composite or Nanocomposites
An important opportunity for the modification of hydrogel properties is simply by the incorporations of different functional materials (such as nanomaterials and cellulose nanocrystals) into the structures of cellulose composite or nanocomposites that are finally used in the hydrogel formulations using particular techniques. For example, different research groups used cellulose and cellulose polymers as a matrix for nanoparticles for a variety of reasons, including the natural abundance and biodegradability of cellulose. The application of cellulose as a polymeric matrix usually requires either the dispersion or the dissolution of cellulose and the concurrent or subsequent addition of nanoparticles/nanoparticle precursors. As, for example, clay particles have been added to NMMO-based solutions of cellulose, microcrystalline cellulose (MCC)-hydroxyapatite nanocomposites were synthesized by using a microwave assisted, one-step reaction where CaCl2, NaH2PO4, and MCC were added to N,N-dimethylacetamide solvent. CdS particles were prepared in NaOH/urea cellulose solutions and the regenerated cellulose films cast from the dispersion which showed the optical properties of CdS. In addition, all-cellulose nanocomposites were prepared by the selective surface dissolution of bacterial cellulose sheets and by the electrospinning of core-shell fibrous mats in which CNCs were sheathed by a shell composed of regenerated cellulose. There are a number of reports on the in situ synthesis of nanoparticles within solid cellulose scaffold, for example, (a) the preparation of superparamagnetic nanocomposite films by synthesizing iron oxide (Fe2O3) nanoparticles within the pores of regenerated cellulose films; (b) the production of wet spun cellulose fibers from solution in NaOH/urea/H2O, followed by treatment in FeCl3 and NaOH in order to generate iron oxide particles using in situ technique; and (c) the preparation of CaCO3 cellulose nanocomposites by synthesizing CaCO3 nanoparticles in the presence of hardwood bleached Kraft pulp and carboxymethylated cellulose fibers [106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126]. The limited solubility of the cellulose curbs the applicability of cellulose nanocomposites, despite the successes achieved from using cellulose to prepare interesting and functional nanocomposites.
In this context, cellulose derivatives, which are soluble in solvents ranging from water to nonpolar organics, are more versatile and have the potential for improved compatibility with nanoparticles. Cellulose acetate has interesting properties, such as (a) transparency, (b) flexibility, and (c) easy processability. It has been used for the production of cellulose-based nanocomposites, for example, (a) melt intercalation of polymer, (b) extrusion followed by either injection or compression molding to create cellulose acetate-clay nanocomposites, and (c) incorporation of TiO2 particles into CA films in order to promote the enzymatic biodegradation of CA by cellulose. Carboxymethyl cellulose (CMC), a water-soluble cellulose derivative, has been reported to stabilize ZnO nanoparticle dispersions in glycerol plasticized-pea starch. CMC has also been used as the matrix in pH-sensitive superabsorbent nanocomposites containing attapulgites, in nanocomposites containing metals (Cu, Ag, In, and Fe), and in hydrogels cross-linked with poly(N-isopropylacrylamide) which contained clay [110, 111, 112, 113, 114, 125, 126]. So, this type of modified composite and nanocomposites is interesting candidate for cellulose-based hydrogel formulations for different specific industrial applications.
5.1.1 Modifications of Polymer Characteristics by the Incorporation of Cellulose Nanocrystals in Different Polymer-Based Composites and Nanocomposites and Their Uses in Hydrogel Formulations for Various Industrial Applications
Different ways can be used to modify the characteristics of composite or nanocomposites, for example, by incorporating cellulose nanocrystals in structural compositions of composites and nanocomposites, and these modified systems can then be used for the fabrication of hydrogels for various industrial applications. The interesting properties of cellulose nanocrystals (CNCs), such as the high strength, high aspect ratio, and huge surface area, have the potential to significantly improve the mechanical properties of nanocomposites at low CNC filler loadings. Percolation theory has been used to state the surprising reinforcement effects observed at low loadings of fibrous elements. Mechanical properties (e.g., strength and modulus) are optimal at or above the percolation threshold, where each CNC is, on average, in close contact with two others and a rigid, 3D, hydrogen-bonded network is formed within the polymeric matrix . As with all nanocomposite materials, the main challenges are (a) the uniform dispersion of CNCs and (b) achieving good interfacial adhesion between CNCs and matrix, particularly if hydrophobic. Generally, CNC nanocomposites are either processed into films, by solvent casting, or fibers/fibrous mats, by electrospinning. Electrospinning is very promising due to the alignment of CNCs in the fibers which can enhance axisymmetric properties. One of the most straightforward methods for the preparation of polymeric nanocomposites which contain CNCs is the direct addition of CNCs into either a pre-polymer or polymeric solution. In order for a successful approach in this method, the CNCs must show dispensability in the polymeric phase.
CNCs synthesized from sulfuric acid hydrolysis which show colloidal stability in water are suitable for direct addition to an aqueous system, for instance, some CNC-based nanocomposites (which depend on the aqueous stability inherent to sulfuric acid hydrolyzed samples) are (a) films cast from mixtures of CNCs and aqueous poly(oxyethylene) solutions, (b) films of oriented CNCs in a polyvinyl alcohol (PVA) matrix prepared by application of a 7T magnetic field during solvent evaporation, (c) fibrous mats electrospun from CNCs dispersed in aqueous PVA, and (d) films cast from CNCs which were dispersed in aqueous polyurethane solutions. Different methods are often used to disperse CNCs in nonaqueous media (such as hydrophobic solvents and polymeric matrices), some of which are (a) vigorous sonication to disperse freeze and/or vacuum dried CNC powders in solvents (e.g., ethanol), furfuryl alcohol, formic acid, N,N-dimethylformamide (DMF) [76, 77, 78, 135, 136, 137, 138, 139, 140, 141], or dimethyl sulfoxide (DMSO), which are compatible with a pre-polymeric/polymeric matrix, (b) the use of a surfactant, (c) solvent exchange of water for organic media such as toluene or DMF, and (d) surface grafting reactions to improve CNC compatibility with hydrophobic polymers. Different research groups have achieved some degree of success with each of these methods. However, the drying of CNCs results in aggregation and is very difficult to obtain; a complete reversion on re-dispersal remains uncertain whether graded solvent exchanges are truly able to produce stable CNC dispersions in organic media. For example, the stability of freeze-dried CNCs re-dispersed in polar organic solvents (e.g., DMF, DMSO) was attributed to the presence of residual water (~0.1%). Besides this, the surface modification of the CNCs is challenging due to limited dispersibility of CNCs in organic reaction media which is required to be surface specific in order to retain the colloidal nature of the particles. Additionally, the characterization of the modified product is also challenging due to the limitation of the use of usual techniques to colloidal particles; otherwise they may not possess the degree of sensitivity required to detect changes in surface groups which represent a small fraction of total atoms. As a result, direct observation of surface-modified CNCs is rarely reported in the literature but is instead inferred from observed changes in fundamental characteristic properties (e.g., stability, dispersibility, charge, the presence or absence of LC ordering). Research in this area is still in the initial stage, and it is expected that in the near future, suitable techniques for reliable production of stable dispersions at the percolation threshold will be discovered, as they are essential to produce homogeneous CNC-reinforced hydrophobic nanomaterials [123, 124, 125, 126, 135, 136]. It is highly envisaged that cellulose nanocrystal can be used along with quantum dots, and this combination can be used to dope a range of natural and synthetic polymers and processed accordingly for producing electrospun fibers from these doped polymers for a variety of applications. Electrospun nanofibers are popular as fillers for both composite and nanocomposite systems which can be processed to be used in hydrogel formulations. Additionally, electrospinning technique can also be used for fiber formations from certain type of specially designed hydrogels. Co-electrospinning technique has the feasibility to produce different composite scaffolds from polymer mixtures containing both hydrogels and solution or dispersion of another polymer for producing special type of scaffolds with selective targeted applications. Besides this, ultrashort protein fibers or similar type of biological or particular type of polymer-based fibers produced by electrospinning can also be directly used as special type of component in hydrogel fabrications [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 248, 249, 250, 251, 252, 253, 255].
5.1.2 Future Trends in the Developments of Functional Material (Such as Quantum Dot-Doped Polymers) and Cellulose Composite- and Nanocomposite-Based Hydrogels for Specific High-Tech Applications
Cellulose composites and nanocomposites have an increasing tendency of functionalization using a wide variety of materials, and they can also be extended for hydrogel formulation for large-scale industrial explorations for many interesting properties. Some of these properties include (a) the natural abundance of raw materials, (b) renewability, (c) low cost, (d) biodegradability, (e) biocompatibility, (f) low density, (g) outstanding elastic modulus (of such polymers), and (h) environmentally benign in character. These are some of the most important characters from different industrial points of view. In terms of functionalization, new materials evolve by the day due to tremendous advancement in different areas of science and technology. For example, new generation of 2D layered materials and nanotubes, advanced fluorescent materials, stimuli-responsive materials and polymers, polymer dot, carbon dot, and quantum dot is being targeted for the future focused advanced use of these materials. For instance, different selective features of quantum dot-doped cellulose-based hydrogels and their future application trends are discussed here.
Many future materials and devices based on QDs require their incorporation and organization in polymeric matrices (such as synthetic polymers, natural polymers like cellulose, and cellulose composites as additives). As a result, there is a growing need for photonic materials and devices which provide the required encouragement to the development of many different strategies to produce polymer/QD hybrid structures. The appropriate selection of a strategy depends primarily on the final application of the hybrid materials. For example, if the material is targeted for biological applications, in addition to its luminescence efficiency, the compatibility of the material with the biological systems is very important (such as some cellulose products show biocompatibility and are suitable to be used in biological applications when incorporated with quantum dots). The fabrication methods which have been reported to date have many merits and demerits. As a result, the development of new routes for the production of QD/polymer nanohybrid materials (e.g., quantum dots doped in the composition of cellulose and another suitable polymer-based nanocomposite) is a very important research topic as the simultaneous control over the size and shape of the matrix and over the amount, spatial distribution/localization, separation, and orientation of the QDs within the matrix still remains a challenge to be solved. In this context, it is important to note that an important study on flexible luminescent CdSe/bacterial cellulose nanocomposite membranes has been reported . They successfully fabricated flexible luminescent membranes based on bacterial cellulose (BC) by using the in situ synthesis of the CdSe nanoparticles on the BC nanofibers . By using X-ray diffraction (XRD) patterns and field emission scanning electron microscopy (FE-SEM), they observed that CdSe nanoparticles were homogeneously dispersed on the BC nanofibers. They also observed that the thermal stability of BC was greatly increased with the inclusion of CdSe nanoparticles. In addition, they noted that the CdSe/BC nanocomposite exhibited good photoluminescent properties and excellent mechanical properties. This study provided an effective method for the construction of flexible BC membranes with photoluminescent properties, which have promising application potentials in the fields of security papers, sensors, and flexible luminescent membranes. Additionally, the introduction of colloidal quantum dots into cellulose and other suitable polymer-based nanocomposite structures using different techniques including electrospinning technique provides many opportunities due to their potential industrial application potentials. For example, colloidal quantum dots (QDs) have garnered much attention in the recent times due to their attractive spectral properties leading to a wide range of potential applications in bio-imaging/sensing, display, telecommunication, and quantum cryptography. The colloidal QDs allow spin-coating-based processing, possibility of self-assembly, compatibility with silicon platform, and tunability in absorption and emission spectra and have become one of the most attractive nanoscale fluorescent emitters. While colloidal QDs have become one of the most attractive nanoscale fluorescent emitters, they have still not found widespread application in practical photonic devices. This is in part due to the difficulty in incorporating these QDs into photonic structures. In addition, practical ultrafast all-optical switches, modulators, flexible emitters, and room temperature single-photon sources using these QDs can be realized if they can be patterned into waveguide or microcavity structures. Recently there have been several attempts to achieve this goal by embedding QDs in a variety of photonic structures and hosts [86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 127, 128, 129, 130]. A significant challenge here is the incorporation of QDs into transparent host matrices without affecting their optical properties [101, 102, 103, 127, 128, 129, 130]. The achievement of monodispersity, high fill factor, and efficient charge injection is highly desired. In addition, embedding photon emitters in microcavities alters their emission properties due to the ability of these structures to confine and enhance electromagnetic fields. Recently, there have been several attempts to achieve this goal in QDs by embedding them in poly(methylmethacrylate) (PMMA) spheres, silica microspheres, one-dimensional microcavities, two- and three-dimensional photonic crystals, and microdisk structures [86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]. Most of the one-dimensional microcavity structures reported to date have used sputtered or thermally evaporated distributed Bragg reflector (DBR) mirrors. While they do give good reflectivity, often they have undesirable effects on the optical properties of the QDs through surface defects. In addition, these techniques also require multiple deposition systems. Hence, it is highly attractive to develop a low-cost technique for the fabrication of the microcavities which is compatible with the solution processing of the colloidal QDs. In addition, solution processing allows the fabrication of the microcavity on a wide variety of substrates including flexible ones (such as cellulose, paper, and cellulose composites). Besides this, there are studies which report the realization of a one-dimensional microcavity laser using colloidal InGaP quantum dots as the gain medium fabricated using spin coating and the development of photonic integrated circuits using colloidal CdSe quantum dot composites fabricated through soft lithography. These studies also discuss the effect of the host matrix on the luminescent properties of the QDs using steady-state and time-resolved luminescence measurements. Currently, polymer composites based on natural fillers (such as cellulose, natural polymers, etc.) have been successful to attract active research interest as alternative materials to glass- or synthetic fiber-reinforced plastics in several applications, mostly for automotive, appliance, and packaging products. In this case, one of the major advantages of using natural fibers is that these are biodegradable and renewable and exhibit low cost, low density, and high toughness. However, the low compatibility between fibers and polymer matrix contributes to the weak mechanical performances which limit the use of these materials. Surface modification of the fibers and/or polymer functionalization, as well as addition of compatibilizers, is usually required to improve the interfacial interactions between the components. In addition, when a quantum dot is used in order to make composite materials for advanced applications, a special care is needed to ensure that the functional behaviors are properly retained after the incorporation of quantum dots into the nanocomposite structure and their use in hydrogel fabrications using appropriate steps and techniques. This type of materials can also be used for aerogel fabrications. For example, photoluminescent cellulose aerogels of variable shape containing homogeneously dispersed and surface-immobilized alloyed (ZnS)x(Cu-InS2)1-x/ZnS (core/shell) quantum dots (QDs) have been reported by using different techniques, some of which are (a) dissolution of hardwood pre-hydrolysis Kraft pulp in the ionic liquid 1-hexyl-3-methyl-1H-imidazolium chloride, (b) addition of a homogenous dispersion of quantum dots in the same solvent, (c) molding, (d) coagulation of cellulose using ethanol as antisolvent, and (e) supercritical carbon dioxide (scCO2) drying of the resulting composite aerogels . Wang et al. achieved both compatibilization with the cellulose solvent and covalent attachment of the quantum dots onto the cellulose surface through replacement of 1-mercaptododecyl ligands typically used in synthesis of (ZnS)x(CuInS2)1-x/ZnS (core-shell) QDs by 1-mercapto-3-(trimethoxysilyl)-propyl ligands. They also obtained cellulose – quantum dot hybrid aerogels where apparent densities were from 37.9 to –57.2 mg cm−3. In addition, their BET surface areas range from 296 to 686 m2 g−1 comparable with non-luminescent cellulose aerogels obtained via the NMMO, TBAF/DMSO, or Ca(SCN)2 route. They also observed that depending mainly on the ratio of QD core constituents and to a minor extent on the cellulose/QD ratio, the emission wavelength of the novel aerogels could be controlled within a wide range of the visible-light spectra. They also observed that higher QD contents led to bathochromic PL shifts and a hypsochromic shift with an increase in the amount of cellulose at constant QD content. In addition, the reinforcement of the cellulose aerogels and hence significantly reduced shrinkage during scCO2 drying is a beneficial side effect when using α-mercapto-ω-(trialkoxysilyl) alkyl ligands for QD capping and covalent QD immobilization onto the cellulose surface . This type of functionalized cellulose composites has the potentials for advanced hydrogel or aerogel fabrications for many high-tech applications. For example, when the respective QDs are furnished with suitable functional groups grafting QDs onto the large surface of aerogels, it is possible that they can form covalent linkages with the solid aerogel network structure. In addition, synthesis of QDs through thermolysis in high boiling solvents is commonly accomplished by simultaneous introduction of nonpolar, hydrophobic ligands to support surface deactivation for preventing QDs from agglomeration which would negatively impact their photoluminescent properties. Hence covalent immobilization of QDs on the surface of solids requires the introduction of moieties that carry respective anchor groups which can be achieved either by inclusion of hydrophobic QDs into amphiphilic micelles leading to an interdigitated bilayer or by ligand replacement . This type of cellulose-based gels is envisaged that can be made suitable to produce quantum dot-doped electrospun nanofibers using proper adjustment of chemistry, rheology, and electrospinning methods [127, 128, 129, 130]. Moreover, cellulose-based quantum dot-doped electrospun nanofibers can be used in particular type of composite hydrogel fabrications which are useful for light-emitting purposes with potential use in lab-on-a-chip devices and also in optical sensing applications [6, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272].
Electrospun polymeric nanofibers and their incorporation in different composite systems including hydrogels are attracting burgeoning interest as innovative nanoscale structures, exhibiting peculiar, smart properties useful for many applications, including sensing, tissue engineering, optoelectronics, and photonics [255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272]. In addition, the interest toward this class of nanomaterials relies on their physicochemical properties, on their high-surface-to-volume ratio, and on the availability of low-cost production techniques. Among different techniques, electrospinning offers a valuable compromise between throughput and control of the nanostructure composition, shape, and size. Hence, the application of the use of this technique in different aspects of hydrogel fabrications has some real significance in different areas. For example, the high stretching of the liquid jet using the electrospinning process can lead to anisotropic physical properties of the collected nanofibers and to enhanced optical and electronic features because of the peculiar macromolecular packing within the electrospun fibers. As emerging field, the development of light-emitting electrospun nanofibers for optoelectronics and photonics can be employed by using either optically inert polymers doped with low-molar-mass fluorescent molecules (typically organic dyes), inorganic quantum dots, etc. or organic semiconducting polymers or particular type of composite- or nanocomposite-based hydrogel systems. Hydrogel systems doped with polymeric nanomaterials have potential applications as submicron light sources, as waveguides, as active components of lasers and transistor devices with electrooptical interplay, and as nanoscale sensing elements with potentials for different industrial applications [6, 118, 161, 182, 183, 184, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285].
6 Selected Industrial Applications of Cellulose-Based Hydrogels
6.1 Applications in Agricultural Sector
Cellulose-based superabsorbent hydrogels have the capability to retain enormous quantity of water in their structure which can be a blessing to the agricultural sectors in different part of the world where there is a chronic shortage of water especially in the area of irrigation and other similar uses. Many studies focused on the uses of cellulose-based superabsorbent hydrogels for enhancing the conditions of soils and related other properties which include (a) the changes of water content, (b) microbial activity of soil, and (c) comparative study on the nature of the crop yield from the soil before and after treatment with biomass. For example, one of the similar investigations revealed that the use of cellulose-based superabsorbent hydrogels was useful to the physical properties of the soil that contributed in crop yield . A separate study observed the impact of the controlled uses of water resources in agriculture where cellulose-based superabsorbent hydrogels were rigorously studied for their efficient storage and a sustained release of water to the soil and plant roots and illustrated the potential use of superabsorbent hydrogels as water reservoir for controlled applications in agriculture . Cellulose-based superabsorbent hydrogels can be used as carriers for pesticides which have many economic and sustainable implications in agricultural sector due to the possibility to encapsulate herbicides into the structure of the hydrogels in order to have effective influence on the release of herbicides. It is one of the cost-effective ways to control pest and weed in agriculture to avoid potential adverse environmental impacts . For example, similar other related studies also focused on the uses of cellulose-based superabsorbent hydrogels on agriculture [32, 33, 34].
6.2 Personal Healthcare
One of the most common uses of superabsorbent hydrogels is in the field of personal healthcare since they can absorb and contain large amounts of fluid (e.g., urine, blood, secreted fluid from wound). Some of the usually used hygiene products include (a) disposable diapers, (b) female napkins, and (c) special type of absorbing materials used in wound dressing . Currently, most widely used superabsorbent hydrogels for producing sanitary napkins are mainly based on acrylic acid- or acrylamide-based products, but they have a number of limitations including (a) higher cost, (b) poor biodegradability, and (c) less environmentally friendly. However, there are quite a few reports to enhance the quality of hydrogels and also overcome the limitations. For example, Liu et al. incorporated flax yarn waste into the structure of superabsorbent hydrogels suitable to use and develop sanitary products (such as sanitary napkins) with relatively higher level of biodegradability, superabsorbency, and fluid retention capability . Over the times, relatively more convenient and comfortable disposable healthcare products have been developed, and now some of them are commercially available [36, 37, 38]. However, no commercially developed products are available in the market which show complete biodegradability; thus, cellulose-based superabsorbent hydrogels may provide some future product which can overcome some of these limitations.
6.3 Water Treatments
Cellulose-based superabsorbent hydrogels provide the opportunity to be used in water purification by removing pollutants (such as heavy metals) from the water sources and also by separating certain elements of the contaminated water. In order to carry out this type of activities, hydrogels are usually functionalized, for example, Zhou et al. synthesized novel magnetic hydrogel beads by blending chitosan with amine functionalized magnetite nanoparticles, carboxylated cellulose nanofibrils, and polyvinyl alcohol using instantaneous gelation technique. The resultant magnetic hydrogel beads showed the capability to absorb lead ions from polluted water due to the presence of numerous carboxylate groups and abundant hydroxyl and amino groups in the composite functionalized hydrogel structure . Many other reports also indicated the scope of water purification to some extent using cellulose-based superabsorbent hydrogels [40, 41, 42]. However, new strategies are still required to develop water treatment using cellulose-based superabsorbent hydrogels.
6.4 Biomedical Applications
Cellulose-based superabsorbent hydrogels are good candidate, when biodegradability of a hydrogel is required or recommended because of their interesting properties including (a) low cost, (b) availability in large quantity, (c) biocompatibility, and (d) stimuli-responsive behaviors of some cellulose and their derivatives (present in the hydrogel) when exposed to external stimuli. In fact, cellulose-based superabsorbent hydrogels are frequently used in different areas of biomedical field, some of which include in drug delivery, tissue engineering, cell bioreactors, and micropatterning neural cell cultures. For example, He et al. fabricated the onion-like, multilayered tubular cellulose-based superabsorbent hydrogels. This study showed that the L929 cell could survive and proliferate in the larger interior space of the multilayer cellulose-based superabsorbent hydrogels which proved a great potential biomedical application . Many similar studies also showed the practical and potential biomedical applications [34, 49, 50, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84].
6.5 Miscellaneous Industrial Exploitations
Besides the usual applications of cellulose-based superabsorbent hydrogels in hygienic and bio-related and agricultural uses, they also have applications in other areas including (a) pharmaceutical dosage forms; (b) separation technology; (c) textile, leather, and paper industries (such as in wastewater treatment); (d) water-swelling rubbers; (e) soft actuators/valves; (f) electrical applications; (g) construction, packaging, and artificial snow; (h) sludge/coal dewatering; (i) fire extinguishing gels; and (j) sensoric materials. In addition, many new advanced technologies are surfacing by the day which have industrial application potentials [6, 35, 61, 62, 63, 64, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 285, 286, 287, 288, 289, 297].
7 Future Trends and Perspectives in the Applications of Cellulose-Based Hydrogels
Different selected aspects of superabsorbent hydrogels based on cellulose have been briefly discussed within this chapter. Cellulose-based superabsorbent hydrogels show some attractive characters that include (a) hydrophilicity, (b) biodegradability, (c) biocompatibility, (d) transparency, (e) low cost, and (f) environmentally friendly. As a result, this type of hydrogels is popular for various industrial applications which include (a) biomedical applications, (b) applications in the human body, (c) water purification, (c) applications in agriculture and horticulture, (d) personal healthcare, (e) water treatment, and (f) biomedical applications. The trend in future research on cellulose-based superabsorbent hydrogels is mainly focused on the design of novel materials and product in order to cater demands for different desired properties and also to demonstrate new functionalities with higher performances. These functional characters are likely to enhance their capacity to make them suitable for various applications including electronics and catalysis and also as chemical and biomedical sensors. In industrial applications’ point of view, cellulose-based superabsorbent hydrogels provide a new extended field of research to design and realize enhanced cost-effective product performance with respect to a number of areas including (a) biocompatibility and biodegradation, (b) higher mechanical, and (c) environmentally benign characteristics. In addition, another current research trend is to develop cellulose-based environmentally friendly cost-effective products with a desired design capability to replace available petroleum-based products in the foreseeable future. Continuous active research activities relating to cellulose-based superabsorbent hydrogels will contribute to realize this type of products in the near future [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 131, 161, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 284, 285, 286, 287, 288, 289].
Hydrogels based on cellulose composite and nanocomposites have also seen a notable upsurge in continuous active research activities for a wide range of industrial application potentials. During the past several decades, hydrogels and nanoparticles have already exerted a dramatic impact in biological, biomedical, pharmaceutical, and diagnostic fields. However, their some intrinsic shortcomings severely restrict their practical applications. Significant efforts have been paid to improve the performance of hydrogels and NPs. The fabrication of composite materials by combining two or more components in a single entity can surmount individual shortcomings and give rise to synergistic functions that are absent in the individual components. The incorporation of NPs in three-dimensional polymeric hydrogel matrix as an innovative means to obtain nanocomposite hydrogels with improved properties and multiple functionalities has gained enormous attention in many areas. On the basis of this review, we can found that various types of NPs, such as carbon-based NPs, silicon-based NPs, metal NPs, and polymeric NPs, are combined with the polymeric hydrogel network to create multicomponent systems. The porous structure and free space within the hydrogel networks can not only provide an ideal hydrated environment for the stabilization of NPs without aggregations or disintegration and the protection of NPs from degradation or denaturation but also work as a reservoir to localize NPs at the target site. More importantly, the coatings of hydrogels around the NPs endow a hydration layer, which often is an essential prerequisite for the biomedical and biological applications of inorganic and metal NPs, as the hydration layer can significantly improve the biocompatibility and reduce the cytotoxicity [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 71, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269].
On the other hand, the incorporation of NPs into hydrogels can not only markedly improve their mechanical, elastic, and adhesive properties as well as physicochemical and thermal stability but also promote the cell attachment and proliferation and improve drug loading capacity and drug release profiles. More significantly, the encapsulation of carbon, metal colloidal particles or quantum dots into polymer hydrogel networks will impart them with exclusive thermal, sonic, optical, electrical, or magnetic properties, which are not achieved by individual polymeric systems and are highly appropriate for various applications, especially for therapeutic and diagnostic applications. Apparently, the benefits of the combination of NPs and hydrogels have resulted in generation of a new class of advanced materials with unique properties that have a wide spectral range of biomedical applications, ranging from controlled drug delivery depots, cell and tissue adhesive matrices, wound dressing and tissue engineering scaffolds, stem cell engineering and regenerative medicines, biosensors, actuators, and other biomedical devices. However, despite the vast potential applications of nanocomposite hydrogels in biomedical fields, there are still lots of challenges to be overcome before they can be applied in clinical use. For example, the improved performance of the nanocomposite hydrogels is mainly ascribed to the enhanced interactions between the NPs and polymer chains. Therefore, apart from particle parameters, the quantity and homogeneity of NP integration are still a matter of concern. To obtain excellent performance, NPs should be abundantly and homogeneously dispersed within the hydrogel matrix. However, due to a large surface area, hydrophobic NPs physically embedded in the hydrophilic polymer matrix often tend to aggregate, leading to the failure of anticipative enhancement of properties. Therefore, there is still a need to elucidate the mechanisms and interactions between NPs and polymer chains inside the nanocomposite hydrogels, and the simple, cost-effective, scalable, and reproducible preparations of nanocomposite hydrogels with desirable properties need to be investigated thoroughly. Apparently, the nanocomposite hydrogels combining the individual functions of NPs and hydrogels are ideal candidates for multimodal drug delivery platform with simultaneous capabilities in drug encapsulation, targeting delivery, photothermal therapy, and in vivo imaging. This synergistic performance is usually not be achieved by an individual. However, the more detailed pharmacological studies need to demonstrate the therapeutic efficacy and biological responses. In addition, most of the nanocomposite hydrogels have been performed only in vitro. Therefore, in future studies, the detailed performances (gelation time, swelling, elastic modulus, responsivity and functionality), long-term toxicity and biodegradability, as well as the biological properties such as protein adsorption, cell adhesion, tissue compatibility, and whole-body effect of such hydrogels under in vivo conditions should be clearly addressed. In summary, there should be a coordinated and comprehensive research to establish fundamental interactions among nanoparticulate materials, hydrogel matrices, and biological systems, before hydrogels will become practical and useful in this exciting field. All these challenges will drive the effective collaboration of scientists from the fields of chemistry, materials, engineering, biology, medicine, and nanoscience. We look forward to seeing many exciting research accomplishments in this burgeoning field of bio-related nanocomposite hydrogels [161, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 284, 285, 286, 287, 288, 289]. In addition, electrospun cellulose nanofibers have many exciting potential applications as reinforcements in nanocomposites and their applications in cellulose-based hydrogel fabrications in association with other natural and synthetic polymers in hybrid systems. These types of hybrid systems have a great variety of application potentials as very important materials where some of which include (a) materials for robust structures, (b) hierarchical materials, (c) electronic display materials, and (d) materials for energy harvesting. The potential mechanical properties of cellulose nanofibers compete well with other engineering materials and have many useful high-end technological applications. Proper dispersion of cellulose or other components of the cellulose-based nanocomposites is one of the critical steps in the production of cellulose nanocomposites and their use in the hydrogel fabrication for desired applications. For example, the layer-by-layer deposition technique offers a facile route to overcoming this, with remarkable percolation of cellulose nanowhiskers interacting with each other, and with the surrounding matrix, in a way that greatly improves the mechanical properties of the resultant material (such as when quantum dot-doped cellulose is also used as one of the constituents in the cellulose nanocomposite structure). Additionally, there are also some issues with the toxicity of quantum dots. However, many pieces of active research are going on in order to overcome these drawbacks. Besides this, cellulose nanowhiskers have a high-surface-area-to-volume ratio where the surface plays a vital role in not only the mechanical efficiency of stress transfer in a nanocomposite but also the ability to modify the surface chemistry. By grafting quantum dots to the surface of cellulose nanowhiskers, it may be possible to utilize self-assembly methods to generate new forms of composite biomaterials for a variety of applications (including biosensing, security, electronic display, and sensitive membranes for wastewater treatments). When the surface of nanowhiskers is chemically coupled with chromophores, it provides a route for successive dispersion of cellulose nanowhiskers in nanocomposite materials, and TEMPO oxidation also proves a means for isolating nanofibers. Similar type of technique can be used for chemical coupling of the surface of nanowhiskers using suitable quantum dots for a wide range of applications. Bacterial cellulose (BC) nanofibers are useful for the generation of hierarchical composites which offers a way for long micrometer-sized fibers for an effective use in composites by improving coupling between the fiber surface and the surrounding resin. BC nanofibers have also been shown to be useful for the generation of optically transparent and flexible composite films with low thermal expansion coefficients. Additionally, by the combination of these high stiffness fibrils with a cellulose matrix, excellent mechanical properties can be achieved for all-cellulose nanocomposites which can be further functionalized with the use of functional materials such as quantum dots for high-tech applications. This chapter has attempted to provide some specific information on the behavior of quantum dots and their applications along with the scope of doping them into polymers using different techniques using a special emphasis electrospinning technique and the use of quantum dot and cellulose for many advanced uses. This chapter has provided some very useful information on the scope and uses for quantum dot-doped cellulose nanofibers and their potential advanced applications in both composite- and nanocomposite-based hydrogels or as active components of the hydrogel where active current research is going on to realize the practical industrial application potentials. So, in the near future, we will see quite a lot of new progresses in the use of quantum dot-doped cellulose nanofibers or similar other functional material-based advanced systems for their applications in composites, nanocomposites, and hydrogel fabrications which can effectively overcome many of the current practical difficulties [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 254, 255, 256, 257, 258, 259].
Lastly, cellulose-based hydrogels are biocompatible and biodegradable materials which show promise for a number of industrial uses, especially in cases where environmental issues are concerned, as well as biomedical applications. Several water-soluble cellulose derivatives can be used, singularly or in combination, to form hydrogel networks possessing specific properties in terms of swelling capability and sensitivity to external stimuli. The current trend in the design of cellulose hydrogels is related to the use of nontoxic cross-linking agents or cross-linking treatments, to further improve the safety of both the final product and the manufacturing process. The smart behavior of some cellulose derivatives in response to physiologically relevant variables makes the resulting hydrogels particularly appealing for in vivo applications. In spite of the non-bioresorbability of cellulose, the possibility to functionalize cellulose-based hydrogels with bioactive and biodegradable extracellular matrix domains suggests that in the near future, such hydrogels might be ideal platforms for the design of scaffolding biomaterials in the field of tissue engineering and regenerative medicine [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247].
Different selected aspects of superabsorbent hydrogels based on cellulose have been briefly discussed within this chapter. Cellulose-based superabsorbent hydrogels show some very attractive characters suitable for a wide range of industrial exploitations. This chapter has selectively covered some of these most commonly used industrial applications of cellulose-based hydrogels. Due to the limitation of the scope of this current chapter, detailed discussions on various aspects of different industrial applications are carefully avoided. However, some updated information along with references have been provided on different aspects of these industrial uses of cellulose-based hydrogels in order to provide more detailed information for interested readers.
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