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Cellulose-Based Superabsorbent Hydrogels

  • Abdulraheim M. A. Hasan
  • Manar El-Sayed Abdel-Raouf
Living reference work entry
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Part of the Polymers and Polymeric Composites: A Reference Series book series (POPOC)

Abstract

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.

Keywords

Cellulose Carboxymethyl cellulose Hydrogels Smart polymers 
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1 Introduction

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.

The 3D structure of hydrogels in their swollen state is maintained either by physical or chemical crosslinking [8]. These crosslinking points within the network keep the three-dimensional integrity of hydrogels in their swollen state. In chemically crosslinked hydrogels, the linear polymer chains are covalently bonded with each other via crosslinking agents; different natural polymers, either of plant or animal origin, were used in preparation of hydrogels for different applications. Some of these hydrogels are conventional hydrogels such as those used in diapers. Whereas others are “smart,” i.e., they show specific response toward certain stimuli. Thus, hydrogels can be classified according to different bases. Some of these bases are given in Fig. 1.
Fig. 1

Some bases of classification of hydrogels

Indeed, the invention of smart hydrogels represents a great revolution in science. Due to many environmental concerns, polymers of animal or plant sources are now incorporated in the network structure. The natural polymers derived from animal or plants are given in Table 1. The substitution of petroleum-based products with bio-based materials is not an economical solution. Some of the possible solutions are blending biopolymers with synthetic polymers and reinforcing natural fibers with synthetic polymers (termed bio-composites), which are viable alternatives to glass fiber composites.
Table 1

Natural polymers

Polysaccharides

 Starch

 Cellulose

 Chitin

 Chitosan

 Guar gum

Proteins

 Collagen/gelatin

 Casein, albumin

 Fibrogen, silks

Polyesters

 Poly(hydroxyalkanoates)

Other polymers

 Lignin, lipids, shellac

 Natural rubber

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 chemical and physical properties of the cellulose biopolymer are mainly dependent on its particular structure. The polymeric structure of cellulose was first verified by Staudinger in 1920. Understanding the structure of cellulose is very important in controlling its modification. Figure 2 shows the molecular structure of cellulose generated from repeating β-d-anhydroglucopyranose units that are joined together covalently through acetal functions between the equatorial group of the C4 carbon atom and the C1 carbon atom (β-1,4-glycosidic bonds).
Fig. 2

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 [11]. 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

The presence of three hydroxyl groups in each glucose residue gives cellulose high reactivity. In the most cases, the hydroxyl groups at the second and third positions act as secondary alcohols, while the hydroxyl group in the sixth position behaves as a primary alcohol (Fig. 3). These hydroxyl groups are principally responsible for the chemical reactions and chemical modification of cellulose.
Fig. 3

The numbering system for carbon atoms in anhydroglucose unit of cellulose molecule

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 [12]. 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 [14]. 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 [15].

Introducing functional groups into cellulose molecules through chemical modification is one of the key ways of adding new properties to the cellulose without destroying its many attractive intrinsic properties. For instance, the formation of cellulose nitrate involves the esterification of cellulose with nitric acid in the presence of sulfuric acid, phosphoric acid, or acetic acid. Currently, other commercially important cellulose derivatives include hydroxyl ethyl cellulose, carboxymethyl cellulose, etc. Some cellulose derivatives are given in Fig. 4.
Fig. 4

(a) Repeating unit of cellulose “cellobiose.” (b) Repeating unit of cellulose derivatives. The substituent group “R” is indicated for (MC), (HPMC), (EC), (HEC), and (NaCMC)

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 [16]. These methods can bring stability to the structure of cellulose and can induce crease-resistance (or “durable press” properties) to cellulose [17]. 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 [18].

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) [19].

The creation of cellulose graft copolymers is one of the key ways of modifying the physical properties and chemical properties of cellulose [19]. This involves modification of the cellulose molecules through the formation of branches (grafts) of synthetic polymers that impart specific properties onto the cellulose substrate, without destroying its intrinsic properties (Fig. 5). These grafts can be linked together via their functional groups to form a three-dimensional network structure known as “gel.” Cellulose hydrogels will be discussed in more details in the next sections.
Fig. 5

An illustration of cellulose graft copolymer

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.

Cellulose-based hydrogels can be obtained via either physical or chemical stabilization of aqueous solutions of cellulosics. Additional natural and/or synthetic polymers might be combined with cellulose to obtain composite hydrogels with specific properties [20, 21]. Some cellulose derivatives and their most common applications are given in Table 2.
Table 2

The most common cellulose derivatives

Cellulose derivatives

Applications

Ref.

Carboxy methyl cellulose

Biomedical and agriculture

[22, 23]

Methyl cellulose

Releasing fertilizers

[24, 25]

Hydroxy ethyl cellulose

Smart materials

[26, 27]

Hydroxypropyl methyl cellulose

Controlled release

[28, 29]

Cellulose acetate

Drug carrier system

[30, 31]

Stimuli-responsive or smart hydrogels are those hydrogels that undergo reasonably large and abrupt changes in their network structure, swelling behavior, permeability, and/or mechanical strength in response to small environmental changes (Fig. 6). Stimuli-responsive hydrogels are also known as environmentally sensitive hydrogels [32, 33]. Stimuli-responsive hydrogels could be further classified as either physical or chemical stimuli-responsive hydrogels as shown in Fig. 7 [34]. Chemical stimuli, such as pH, ionic factors, and chemical agents, change the interactions between polymer chains or between polymer chains and solvent at the molecular level, whereas physical stimuli, such as temperature and electric or magnetic fields, and mechanical stress will influence the molecular interactions at critical onset points.
Fig. 6

Stimuli-responsive hydrogel

Fig. 7

Classification of smart hydrogels. (Modified from [34])

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) [21]. 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 [38] and hydroxypropyl methylcellulose [39].

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 [40]. In general, chemical crosslinked hydrogels are prepared through crosslinking two or more kinds of polymer chains with a functionalized crosslinker [41] or under UV light [42]. However, physically crosslinked hydrogels are reversible [39] 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 [43].

In agriculture, there is an increasing interest in using superabsorbent hydrogels. This is mainly due to the call for a reduction of water consumption and to optimize water resources in agriculture and horticulture and has a role in the endorsement of a novel advance of human habit and culture toward water, to be treated as a benefit to save and not as an excess to waste. During the swelling process of a superabsorbent hydrogel, the material changes from a glassy to a rubber-like state, which is able to pile up large amount of water and release the stored water under significant conditions. The controlled release system is formed from carboxymethyl cellulose (Fig. 8), a low-cost and completely biocompatible polymer that can be produced in a sustainable way from natural sources.
Fig. 8

Chemical structure of CMC

In this respect, Bao et al. [44] investigated the reaction process of cellulose-based inorganic/organic nanocomposite superabsorbent hydrogels by solution polymerization. First, potassium persulfate was used as an initiator to produce free radicals under heating, and then these radicals abstracted hydrogen from the hydroxyl groups on the cellulose substrate to create the alkoxy radicals. The alkoxy radicals attacked the acrylic monomers leading to chain initiation. Consequently, these small molecule radicals acted as free-radical donors to the neighboring molecules. Furthermore, in the presence of the crosslinker, N,N- methylenebisacrylamide (MBA), and a filler, inorganic sodium montmorillonite (Na-MMT), the chain propagation developed promptly. Finally, the reaction ceases by the coupling of macromolecules. The formation mechanisms of cellulose-g-poly(AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel are shown in Fig. 9.
Fig. 9

Mechanism of formation of CMC-based superabsorbent hydrogels

During or after preparation, the xerogel might also be loaded with nutrients and/or plant pharmaceuticals. When irrigating plants, the water is absorbed by the hydrogel, which then affected externally to release water and nutrients to the soil as needed, thus keeping the soil moist over long periods of time. This process is a highly water-saving process that the water is not lost soon after the watering due to evaporation and drainage and a redistribution of the water resources available for cultivation in other applications [45, 46, 47]. An extra advantage in using hydrogels in this application is related to the effect of the swelling itself on the soil. Indeed the hydrogel granules, which in the dry form have almost the same dimensions of the substrate granules, increase their dimension after swelling, thus increasing soil porosity and providing a better oxygenation to the plant roots (Fig. 10) [45].
Fig. 10

Compactness of soil particles due to addition of hydrogels [45]

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 [47]. 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. [51] 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 [52].

Mohammed and Kourosh [53] suggested the following mechanism for preparation of carbohydrate-based superabsorbent hydrogel (Fig. 11):
  1. (a)

    Graft copolymerization of suitable vinyl monomer(s) on polysaccharide in the presence of a crosslinker

     
  2. (b)

    Direct crosslinking of polysaccharide

     
Fig. 11

The mechanism of preparation of polysaccharide-g-PAN copolymer superabsorbent

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 [54]. 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 [55], vagina [56], 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).

In this respect, a new set of pH-, temperature-, and redox-responsive hydrogels were prepared from carboxymethylcellulose (CMC) and poly(N-isopropylacrylamide). Copolymeric (CP) hydrogels were synthesized by copolymerizing N-isopropylacrylamide (NIPA) and methacrylated carboxymethylcellulose; semi-interpenetrating network (SIPN) hydrogels were prepared by polymerizing NIPA in the presence of CMC. Two types of crosslinkers were used, viz., N,N′-Methylenebisacrylamide (MBA) and N,N′-Bis(acryloyl)cystamine (BAC), a redox sensitive crosslinker. The structures of the hydrogels were characterized by FTIR and SEM studies. The CP hydrogels were proved to be more porous than analogous SIPNs which resulted in higher swelling for the CP hydrogels. Swelling for both the hydrogels was established to increase with CMC content. While the swelling of SIPN hydrogels showed discontinuous temperature dependency, CP hydrogels showed gradual decrease in water retention values with increase in temperature. CBA crosslinked hydrogels showed higher swelling in comparison to BIS crosslinked hydrogels. Additionally, lysozyme was loaded in the hydrogels, and its in vitro release was studied in various pH, temperature, and in the presence of a reducing agent, glutathione (GSH). The release rate was found to be maximum at lower temperature, lower pH, and in the presence of GSH [57] (Figs. 12 and 13).
Fig. 12

Synthesis of methacrylated carboxymethyl cellulose sodium salt (MACMC)

Fig. 13

Synthesis of N,N′-Bis(acryloyl)cystamine (BAC)

Furthermore, Lim et al. [58] 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.

Recently, Gholamreza et al. [59] developed magnetic/pH-sensitive nanocomposite hydrogel-based carboxymethyl cellulose-g-polyacrylamide/montmorillonite for colon-targeted drug delivery (Fig. 14).
Fig. 14

Schematic representation for the synthesis of CMC-g-Am/mMT hydrogel [59]

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 [62] 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 [65] One of the typical features of cotton cellulose is its extremely high molecular weight in the cellulosic family, over 1,000,000 [66]. 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 [67].

The introduction of sodium carboxylate groups into cotton cellulose was investigated by esterification because esterification proceeds under milder conditions than etherification and ester linkages are more susceptible to hydrolysis and biodegradation than ether linkages [68] (Fig. 15). These features are suitable for the design of biodegradable superabsorbent hydrogels. Thus, the synthesis of superabsorbent hydrogels by the esterification of cotton cellulose with succinic anhydride (SA) was studied. It was found that a hydrogel could be obtained without any crosslinker when 4-dimethylaminopyridine (DMAP) was used as an esterification catalyst (Fig. 16).
Fig. 15

Synthetic route of sodium carboxylate from cotton cellulose and SA [68]

Fig. 16

Synthesis scheme of biodegradable superabsorbent hydrogel

Furthermore, Francesco et al. [69] evaluated a novel class of cellulose-based superabsorbent hydrogels, totally biodegradable and biocompatible, for agricultural use. Briefly, two cellulose derivatives, sodium carboxymethyl cellulose (CMCNa) and hydroxyethyl cellulose (HEC), were used for superabsorbent hydrogel preparation; citric acid (CA), a crosslinking agent able to overcome toxicity and costs associated with other crosslinking reagents, was selected in a heat-activated reaction. The objectives of their study were (1) to validate the ability of the hydrogel to modify the water retention properties of the growing media (soils and soilless substrates), (2) to investigate the effects on the growth of plants grown on media amended with the hydrogel, and (3) to find out the biodegradability of the prepared hydrogels. The applied tests revealed the absence of phytotoxicity of the hydrogel, and cultivation trials on cucumber (on soil) and sweet basil (in soilless conditions) showed a general overall enhancement of plant growth and quality when hydrogel was added to growing media. The tested hydrogel showed a high suitability for potential use in agriculture. The scheme for crosslinking reaction is illustrated in Fig. 17 [70].
Fig. 17

Crosslinking reaction of cellulose derivatives with citric acid [70]

Chunyu et al. demonstrated [71] two novel methods to prepare cellulose/PVA hydrogels with different functional properties via a green process. They introduced a series of hydrogels that were prepared from cellulose and PVA in NaOH/urea aqueous solution using both physical and chemical crosslinking methods. The hydrogels were secure and biodegradable materials. The results showed that all of the cellulose/PVA hydrogels exhibited homogeneous porous structures and a certain miscibility. The swelling degree and water uptake of the chemical hydrogels were markedly higher than those of the physical hydrogel (Fig. 18).
Fig. 18

Proposed mechanism for crosslinking reaction of ECH with cellulose and PVA

4 Conclusion

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.

Notes

Acknowledgments

The authors express their gratitude for the Egyptian Petroleum Research Institute for supporting this work.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Abdulraheim M. A. Hasan
    • 1
  • Manar El-Sayed Abdel-Raouf
    • 1
  1. 1.Egyptian Petroleum Research InstituteCairoEgypt

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