Cellulose-Based Superabsorbent Hydrogels
Hydrogels are polymeric three-dimensional networks able to absorb and release water solutions. Sometimes, this behavior is reversed in response to definite environmental stimuli, i.e., temperature, pH, ionic strength, etc. Such stimuli-responsive behavior makes hydrogels attractive candidates for the design of “smart” devices, applicable in a variety of technological fields. In particular, when concerning either ecological or biocompatibility issues, the biodegradability of the hydrogel network, combined with the control of the degradation rate, may add more value to the developed device. Development of new products and materials, particularly those which are based on renewable organic resources using innovative sustainable processes, represents an increasing interest in both academic and industrial research. Cellulose and its derivatives – with numerous hydroxyl groups – have established to be flexible materials with unique chemical structure which provides a good platform for the creation of hydrogel networks with distinctive properties with respect to swelling ability and sensibility to external stimuli. Consequently, cellulose-based hydrogels are attractive materials, biodegradable, biocompatible, and low cost, which exhibit properties that make them promising in many applications, particularly in biomedical and environmental applications. This article reviews the design and the applications of cellulose-based hydrogels, which are extensively investigated due to cellulose availability in nature, the intrinsic degradability of cellulose, and the smart behavior displayed by some cellulose derivatives.
KeywordsCellulose Carboxymethyl cellulose Hydrogels Smart polymers
The modification of natural polymers is a promising method for the preparation of new materials. Graft copolymerization of vinyl monomers onto natural polymers is an efficient approach to achieve these materials. Among these materials, hydrogels have attracted great attention due to their wide applications [1, 2, 3, 4, 5, 6, 7]. Hydrogels are three-dimensional (3D) materials with the ability to absorb large amounts of water while maintaining their dimensional stability and network structure. The amount of water absorbed in hydrogels is related to the presence of specific groups such as –COOH, –OH, –CONH2, –CONH–, and –SO3H.
Lignin, lipids, shellac
Among natural polymers, cellulose is the most abundant organic raw material and finds numerous applications in different areas as composite materials, textiles, drug delivery systems, and personal care products. Since it was first characterized in 1838 [9, 10] it has received a great deal of attention for its physical properties and chemical reactivity. Moreover, cellulose is an inexpensive, biodegradable, and renewable material. Many properties of cellulose, both physical and chemical, are significantly different from those of synthetic polymers. Despite all its well-established and interesting properties, cellulose lacks some of the versatile properties of synthetic polymers.
Modification of biofibers has motivated to increase their functionality and the scope of their use. Different chemical modifications are available, but the most predominant type is modification by graft polymerization which provides a means of altering the physical and chemical properties of cellulose and improving its functionality. With the recent progresses in polymer synthesis, new routes are now available for the production of functional and sustainable cellulose-based materials. In this chapter, the structure of cellulose and its reactivity, together with highlights of the recent advances in techniques for cellulose grafting, are considered.
1.1 Chemical Structure of Cellulose
The linearity of structure arises from the b-glucose link at C1–O–C4 to yield cellobiose units. This linear structure can contain up to 1000–1500 b-glucose units . The degree of linearity enables the molecules to draw near together. Thus, cellulose has a high cohesive energy that is greatly enhanced by the fact that the hydroxyl groups are capable of forming extensive hydrogen bond networks between the chains and within the chains.
1.2 Cellulose Reactivity
As the DS indicates the average number of reactive groups in the molecule that have been substituted, one can say that the DS of the anhydroglucose unit of the cellulose molecule is three . However, the reaction of cellulose should not simply be considered as being that of a trihydric alcohol that is similar in its chemistry to trihydric sugar. This is due to that cellulose is a fiber-forming and a high molar mass substance. The reactivity of these three hydroxyl groups under is mainly affected by their intrinsic chemical reactivity, by steric effects that are produced by the reacting agent, and by steric effects that are derived from the supramolecular structure of cellulose. Generally, the relative reactivity of the hydroxyl groups can be expressed as OH–C6 ≫ OH–C2 > OH–C3 [13, 14].
2 Chemical Modification of Cellulose
Cellulose is a distinctive natural polymer that possesses several attributes such as a fine cross section, the ability to absorb moisture, high strength and durability, high thermal stability, good biocompatibility, relatively low cost, low density, and good mechanical properties . Yet, there are some drawbacks for cellulose. These include poor solubility in common solvents, poor dimensional stability, lack of thermoplasticity, and lack of antimicrobial properties. Thus controlled physical and/or chemical modification of the cellulose structure is necessary to prevail over such drawbacks .
The modification of cellulose with bi- or polyfunctional compounds to form crosslinked or network structure provides another possible attempt of modifying the structure of cellulose . These methods can bring stability to the structure of cellulose and can induce crease-resistance (or “durable press” properties) to cellulose . Among the methods of modification of polymers, graft copolymerization offers a smart and adaptable means of imparting a range of functional groups to a polymer molecule. A graft copolymer generally consists of a long chain of one monomer, referred to as the backbone polymer (main chain) with one or more branches (grafts) of long sequences of a different monomer .
This chemical modification can provide polymeric materials with valuable properties and different chemical structures. It can also permits one to combine the best properties of two or more polymers in one physical unit This can be achieved by controlling some parameters such as the polymer types, the degree of polymerization and the polydispersities of the main chain and the side chains, the graft density (average spacing in between the side chains), and the distribution of the grafts (graft uniformity) .
3 Cellulose-Based Smart Hydrogels
Because cellulose has many hydroxyl groups which can form hydrogen bonding linked network easily, various designations of cellulose-based hydrogels can be tailored. Water-soluble cellulose derivatives are mostly biocompatible which can be used as thickener, binding agents, emulsifiers, film formers, suspension aids, surfactants, lubricants, and stabilizers, especially as additives in food, pharmaceutical, and cosmetic industries.
The most common cellulose derivatives
3.1 Thermo-Responsive Cellulose-Based Hydrogels
Temperature-responsive hydrogels have gained considerable attention in the endless applications. Some molecular interactions, such as hydrophobic associations and hydrogen bonds, play a vital role in the immediate volume change of these hydrogels at the critical solution temperature (CST). In the swollen state, water molecules form hydrogen bonds with polar groups of polymer backbone within the hydrogels and arrange themselves around hydrophobic groups.
Physically crosslinked, thermo-reversible gels were prepared from water solutions of methylcellulose and/or hydroxypropyl methylcellulose (in a concentration of 1–10% by weight) . The gelation mechanism involved hydrophobic associations among the macromolecules possessing the methoxy group. At low temperatures, polymer chains in solution are hydrated and simply entwined with one another. By increasing temperature, water of hydration is lost gradually, until polymer-polymer hydrophobic associations take place, thus forming the hydrogel network.
The glass transition temperature (Tg) was dependent on the degree of substitution of the cellulose ethers as well as on the addition of salts. A higher degree of substitution of the cellulose derivatives provided them a more hydrophobic character, thus reducing the glass transition temperature at which hydrophobic associations take place. A similar effect was noticed when adding salts to the polymer solution. This may be due to that salts reduce the hydration level of macromolecules by recalling the presence of water molecules around themselves. Both the degree of substitution and the salt concentration could be properly adjusted to obtain specific formulations gelling at 37 °C and thus potentially useful for biomedical applications [35, 36, 37].
Selective cellulose derivatives, including methyl cellulose (MC), hydroxypropyl cellulose (HPC), hydroxypropylmethyl cellulose (HPMC), and carboxymethyl cellulose (CMC), have been used to construct cellulose-based hydrogels through physical crosslinking and chemical crosslinking. The mostly studied temperature-responsive hydrogels among cellulose derivatives were methylcellulose  and hydroxypropyl methylcellulose .
In the case of physical crosslinked gels, there is no covalent bonding formation or breakage, and the crosslinked network is formed through ionic bonding, hydrogen bonding, or an associative polymer-polymer interaction . In general, chemical crosslinked hydrogels are prepared through crosslinking two or more kinds of polymer chains with a functionalized crosslinker  or under UV light . However, physically crosslinked hydrogels are reversible  thus might flow under given conditions (e.g., mechanical loading) and might degrade in an uncontrollable manner. Due to such drawbacks, physical hydrogels based on MC and HPMC are not recommended for use in vivo. In vitro, MC hydrogels have been recently proposed as novel cell sheet harvest systems .
This also suggests that large-granule hydrogels are likely to yield better results than fine-granule ones, if suitably mixed with the soil (indeed different spatial configurations for the soil and hydrogel particles are possible, depending on their densities and the soil-hydrogel and hydrogel-hydrogel interactions).
Cellulose-based hydrogels fit perfectly in the current trend to develop environmentally friendly alternatives to acrylate-based superabsorbent hydrogels [48, 49, 50]. Sannino and coworkers recently developed a novel class of totally biodegradable and biocompatible microporous cellulose-based superabsorbent hydrogels . In biomedical applications, cellulose derivatives are used in preparation of thermo-responsive hydrogels used in drug delivery systems, as found in dressing and in bioengineering.
In this respect, Trong et al.  prepared hydrogel membranes mainly composed of three kinds of latex particles within carboxymethyl cellulose (CMC) matrix for the purpose of transdermal drug release. To give a thermo-responsive behavior in swelling, poly(N-isopropyl acrylamide) latex and its copolymers were synthesized by polymerization of N-isopropyl acrylamide with different amounts of acrylic acid, in which lower critical solution temperature (LCST) could be modulated. Morphology, structures, and swelling capability of prepared hydrogel membranes were then examined. Caffeine, used as the model drug, was incorporated into membranes, and the drug release behavior at different temperatures was evaluated. These prepared hydrogel membranes have potential in the application of transdermal drug delivery system.
Recently, semi-interpenetrating polymer network (SIPN) strategy was employed to fabricate a kind of novel hydrogels composed of cellulose and poly(N-isopropylacrylamide) (PNIPAAm) in the presence of N,N-methylenebisacrylamide (MBAAm) as the crosslinker and benzoyl peroxide (BPO) as the initiator. The results from FTIR and TGA indicated that the network indeed existed in the SIPN hydrogels. The data from experiments, those associated to the swelling behavior of the hydrogels at different temperatures in particular, proved the thermal sensitivity of these hydrogels. The impact of crosslinker concentration on the hydrogel properties was discussed as well. The swelling ratio of hydrogels decreased with increasing the content of MBAAm. Besides, the loading and releasing behavior of the hydrogels was examined using dimethyl methylene blue as a model drug. These novel hydrogels combining the advantages of natural polymer with thermal-responsive behavior are of great potential to be applied to drug delivery and control release systems .
Graft copolymerization of suitable vinyl monomer(s) on polysaccharide in the presence of a crosslinker
Direct crosslinking of polysaccharide
In graft copolymerization, generally a polysaccharide enters reaction with initiator by either of two separate ways. First, the neighboring OHs on the saccharide units and the initiator (commonly Ce4+) interact to form redox pair-based complexes. These complexes are subsequently dissociated to produce carbon radicals on the polysaccharide substrate via homogeneous cleavage of the saccharide C–C bonds. These free radicals initiate the graft polymerization of the vinyl monomers and crosslinker on the substrate. In the second way of initiation, an initiator such as persulfate may abstract hydrogen radicals from the OHs of the polysaccharide to produce the initiating radicals on the polysaccharide backbone.
Due to employing a thermal initiator, this reaction is more affected by temperature compared to previous method. In agriculture, polymer complexes of crosslinked carboxymethyl cellulose (CMC) and starch were synthesized to form superabsorbent polymers (SAP) and their performances as a water retaining aid for irrigation were assessed . Starch from vegetables and chemically modified cellulose fibers were used as the basis for the polymer structure because of their biodegradability and the sustainability of their sources. These polymers were found to release the absorbed water at 34 °C, i.e., the LCST of N-isopropylacrylamide.
3.2 pH-Responsive Cellulose-Based Hydrogels
Variations in pH are known to occur at several body sites, such as the gastrointestinal tract , vagina , and blood vessels, and these can provide a suitable base for pH-responsive drug release. pH- sensitive polymers are polyelectrolytes that bear in their structure weak acidic or basic groups that either accept or release protons in response to changes in environmental pH. The pendant acidic or basic groups on polyelectrolytes undergo ionization just like acidic or basic groups of monoacids or monobases. However, complete ionization on polyelectrolytes is more difficult due to electrostatic effects exerted by other adjacent ionized groups. Most commonly studied ionic polymers for pH-responsive behavior include poly (acrylamide) (PAAm), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), poly(diethylaminoethyl methacrylate) (PDEAEMA), and poly(dimethylaminoethyl methacrylate) (PDMAEMA).
Furthermore, Lim et al.  prepared a novel pH-sensitive hydrogel with superior thermal stability, composed of poly(acrylic acid) (PAA) and cellulose nanocrystal (CNC). CNC was extracted from kenaf fiber through a series of alkali and bleaching treatments followed by acid hydrolysis. PAA was then subjected to chemical crosslinking using the crosslinking agent (N,N-methylenebisacrylamide) in CNC suspension. A disk shape hydrogel was obtained by casting the mixture onto a petri dish. PAA/cellulose hydrogel with the same composition ratio was also prepared as control. The effect of reaction conditions such as the ratio of PAA and CNC on the swelling behavior of the hydrogel obtained toward pH was studied. The obtained hydrogel was further subjected to different tests such as thermogravimetric analysis (TGA) to investigate the thermal behavior, Fourier transform infrared for functional group detection, and swelling test for swelling behavior at different pH. The crosslinking of PAA was established with FTIR with the the absence of C=C double bond. In TGA test, PAA/CNC hydrogel showed significantly higher thermal stability compared with pure PAA hydrogel. The hydrogel obtained showed excellent pH sensitivity and experienced maximum swelling at pH 7. The PAA/CNC hydrogel can be developed further as drug carrier.
3.3 Biodegradable Cellulose-Based Hydrogels
An important focus of the research in this field is the material’s biodegradability. Modern superabsorbents are non-biodegradable acrylamide-based products. The renewed attention of institutions and public opinion toward environmental protection issues has awoken some producers to the development of biodegradable superabsorbents. Potential biodegradable cellulose-based superabsorbent, with sorption properties similar to those displayed for acrylate-based products, can be prepared by crosslinking reaction of cellulose polyelectrolyte derivatives, carboxymethyl cellulose (CMC), hydroxyethyl cellulose, etc.
Biodegradable superabsorbent hydrogels involve crosslinked poly (amino acid)s such as poly(γ-glutamic acid) and poly(aspartic acid) [60, 61] and crosslinked sodium salt of carboxymethyl cellulose (CMC). The crosslinking of CMC has been investigated with various methods, such as crosslinking agents  and ionizing irradiation [63, 64]. As concerns about environmental problems are rising today, various naturally occurring polymers should be used instead of synthetic ones. Among them, cellulose exists most abundantly on the earth and is used for various applications. Cotton is one of the most accessible types of cellulose in our daily life and is produced in large amounts every year  One of the typical features of cotton cellulose is its extremely high molecular weight in the cellulosic family, over 1,000,000 . Thus, cotton cellulose is a suitable starting material for superabsorbent hydrogels because the extremely high molecular weight of the polymer is one of the indispensable factors for attaining high water absorbency. The most representative cellulosic derivative containing sodium carboxylate is CMC, which contains it via an ether linkage .
The green chemistry approach pushed researches and researchers toward replacing petroleum-based products with natural polymers for environmental concerns. In this respect, polymer networks of crosslinked cellulose derivatives were synthesized to form hydrogels for different purposes starting from sanitary pads and hygienic products to advanced applications such as biomedical field, drug delivery, pharmaceutical ground, and agriculture. In these specific applications, “smart” polymers are required to respond to different environmental changes to induce the required effect. These smart polymers are cheap, adaptable, and biodegradable so that extensive work is continued in order to introduce new generations of these materials to serve in different fields.
5 Future Perspectives
Shortage of fresh water resources is of worldwide concern. Efforts are directed to solve this problem by focusing on hydrogel industry. Hydrogels can be made by chemical modification of natural polymers such as cellulose, carboxymethyl cellulose, starch, guar gum, and so on. They can be used in water holding applications such as retained irrigation. They can be used as vehicles for delivering nutrients and pesticides to the root. Furthermore, they can be applied in wastewater treatment and removal of heavy metals. The new application of hydrogel utilization is in greenhouse industry where they regulate the amount of sunlight that can pass into the shelter and control the water loss.
The authors express their gratitude for the Egyptian Petroleum Research Institute for supporting this work.
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