Advances in cellulose-based superabsorbent hydrogels

Abstract

This contribution provides a brief overview of recent progress in cellulose-based superabsorbent hydrogels, fabrication approaches, materials and promising applications. First, different synthesis methods are introduced, including physical, as well as chemical cross-linking. Second, some of the cellulose series original materials were introduced in this work. In addition, some applications and future research in cellulose-based superabsorbent hydrogels are also discussed in this review.

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Jianzhong Ma

Professor Jianzhong Ma received his PhD degree in Polymer Science and Engineering from Zhejiang University in 1998. He then worked as a postdoctoral fellow at the Eastern Regional Research Center, ARS, USDA, from 1999 to 2000. Currently, he works at the College of Resource and Environment, Shaanxi University of Science and Technology, China. His current research interests include synthesis, characterization and properties of organic–inorganic nanocomposite materials, and development of functional coatings and functional polymer materials.

Xiaolu Li

Xiaolu Li will receive her Master's degree under the supervision of Professor Jianzhong Ma in 2016. Her current research interests focus on the preparation, characterization and properties of novel organic–inorganic hybrid biodegradable superabsorbent materials.

Yan Bao

Professor Yan Bao received her PhD degree from Shaanxi University of Science and Technology in July, 2008. Then, she joined the college of Resource and Environment, Shaanxi University of Science and Technology, China. Her current research interests include preparation and design organic/inorganic nanocomposite functional biodegradable chemicals.

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1. Introduction

Since the appearance of the new concept of a “superabsorbent polymer” in the 1950s, considerable interest has been devoted to it within the scientific community as well as the industrial world, and rapid progress has been made in the past few decades because of the tremendous demand for superabsorbent materials in the sanitary industry. Superabsorbent hydrogels are hydrophilic networks with a high capacity for water uptake, which can absorb, swell and retain aqueous solutions up to hundreds of times their own weight (dry sample).^1–3 Even though most of the superabsorbent hydrogels are produced from synthetic polymers (essentially acrylics) for their superior price-to-efficiency balance,⁴ the tendency for replacing these synthetics with “greener” alternatives is more overwhelming due to the poor degradability and biocompatibility of synthetic superabsorbents.

Cellulose, one of the carbohydrate polymers, is the most abundant resource in nature, and is biocompatible, biodegradable, non-toxic, low cost and renewable. Cellulose, which has abundant hydroxyl groups, can be used to prepare superabsorbent hydrogels easily with fascinating structures and properties.

Most recently, cellulose-based superabsorbent hydrogels have become ubiquitous and indispensable materials in many applications. Introducing cellulose series materials into the superabsorbent hydrogels can overcome the disadvantages of synthetic-based superabsorbent hydrogels in satisfying the utilization requirements and can endow the final products with excellent properties.⁵ Thus, multifunctional superabsorbent materials could be achieved. Compared with the synthetic superabsorbent hydrogels, cellulose-based superabsorbent hydrogels have high absorbency, high strength, good salt resistance, excellent biodegradable ability and biocompatibility, and other special functions that promise a wide range of applications in many fields.

Only few reviews about cellulose-based superabsorbent hydrogels on its different category have appeared in the literature.⁶ This review aims to highlight the recent developments in cellulose-based superabsorbent hydrogels with emphasis on the preparation methods, the original material of cellulose and the possible applications.

2. Preparation methods

Various preparation methods are used to obtain the target superabsorbent hydrogels. In general, they can be classified into two types: chemical methods and physical methods. Chemical methods include aqueous solution polymerization, inverse-phase suspension polymerization, and even microwave irradiation methods. Moreover, physical cross-link techniques include freeze/thaw cycle technology and hydrogen bond cross-linking, which are also adapted in some cases.⁷ It is worth noting that new techniques such as interface contact technology⁸ and in situ photo-polymerization⁹ have emerged in recent years.

2.1 Chemical synthesis methods

Chemical synthesis methods are widely used to fabricate cellulose-based superabsorbent hydrogels for the formation of covalent linkages. The typical chemical synthesis methods are stated below.

2.1.1 Aqueous solution polymerization. Among the homogeneous polymerizations, solution polymerization is preferred due to better control of the heat of polymerization, lower cost and added convenience. Most of the cellulose-based superabsorbent hydrogels are produced in this way. Generally, the cellulose series macromolecular, monomer(s), initiator, and cross-linker(s) are freely soluble in water or have good solubility in water. Once the initiator is induced by temperature or radiation, the polymerization process starts. After a certain time, the product of this reaction can be dried and pulverized for various applications.

The mechanism for the solution polymerization synthesis of cellulose-based superabsorbent hydrogels is mainly attributed to free-radical induced polymerization. The free radical polymerization is a process in which monomers are polymerized through the action of initiators. This type of polymerization has been used so extensively because it has high polymerization rate and happens in an aqueous medium, which is safe and harmless. The cellulose macromolecule produces the free radical initiated by the initiator and then interacts with the monomers forming the graft copolymer. The prominent induction approach is chemical induction, containing mono-induced systems (for example, persulfate¹⁰), bi-induced systems (such as redox induced systems^11–13), and even ternary-induced systems.¹⁴ In addition, physical induction can also be adapted such as Co-60 γ radiation-induced plasma,¹⁵ Ce(IV) induced plasma,¹⁶ microwave irradiation induced plasma,¹⁷ and radiation-induced plasma.¹⁸

Bao et al. elaborated the reaction process¹⁹ of cellulose-based inorganic/organic nanocomposite superabsorbent hydrogels by solution polymerization. First, potassium persulfate was used to produce the initial free radicals under heating, and then these radicals captured hydrogen from the hydroxyl groups on the cellulose substrate to generate the alkoxy radicals. The alkoxy radicals attacked the acrylic monomers in close vicinity of the reaction sites, leading to chain initiation. Subsequently, these small molecule radicals became free-radical donors to the neighboring molecules. Furthermore, in the presence of the cross-linker, N,N-methylenebisacrylamide (NMBA), and a filler, powdery Na-MMT, the chain propagation developed quickly. Finally, the reaction ends by the coupling of macromolecules. The formation mechanisms of cellulose-g-poly(AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel are shown in Fig. 1. Other works on the solution polymerization process contain almost the same procedure with the nitrogen line purged before the reaction.

Fig. 1 Proposed reaction mechanism for the synthesis of cellulose-g-poly(AA-co-AM-co-AMPS)/MMT superabsorbent hydrogels. Reprinted from Carbohydrate Polymers, 84(1), Bao Y., Ma J. and Li N., Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly(AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel, 76–82. Copyright (2011), with permission from Elsevier.

Except for polymerization in pure water, interaction of the mixture in aqueous media is assumed to be another route to get cellulose-based superabsorbent hydrogels. Similarly, a cellulose-based superabsorbent hydrogel was fabricated by the interaction between cellulose and carboxymethyl cellulose sodium (CMC) in the alkaline/urea aqueous medium.²⁰ The process proceeded with nucleophilic attack of the cross-linker epichlorohydrin between cellulose and CMC.

2.1.2 Inverse-phase suspension polymerization. Inverse-phase suspension polymerization is conducted in the dispersed and continuous phases. The dispersed phase is aqueous and the continuous phase is organic. The monomer is usually dissolved in the dispersed phase and a surfactant is used to help the monomer and other aqueous reagents to be effectively dispersed throughout the continuous phase. Although particles with desirable sizes can be obtained by this technique, the removal of the organic solvents such as n-hexane and toluene is a very challenging problem. This technique is appropriate for the polymerization of highly hydrophilic monomers such as salts of acrylic and methacrylic acids, as well as acrylamide.²¹

When the superabsorbent hydrogels are used in controlled release or chromatography, they are needed in the form of particulates. To avoid the “gel blocking” caused by the irregular shaped pieces generated from the grinding process, Liu et al.²² produced hydroxypropyl methylcellulose (HPMC)-based porous gels in bead form by inverse-phase suspension polymerization, in which cyclohexane was used as the continuous phase and HPMC solution (10 wt%) was used as the dispersed phase.

For the industry, inverse-phase suspension polymerization is the second choice compared with solution polymerization in aqueous solution because of its complexity and higher costs.²³ Searching among the recent literature over the past five years, inverse-phase suspension polymerization tends to be used minimally.

2.1.3 Microwave irradiation polymerization. Microwave irradiation technology as an emerging polymerization approach, compared with traditional approaches, displays stronger penetrating ability, faster heating, is cleaner and of higher efficiency. The differences between the abovementioned polymerization approaches are listed in Table 1.

Table 1 Comparison among the three polymerization approaches

Polymerization type	Characterization
Aqueous solution polymerization	Easy control, lower cost and stable; mass shape products
Inverse-phase suspension polymerization	Complex, higher costs and unstable; particle products
Microwave irradiation polymerization	Fast heat, high efficiency and clean; mass shape products

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Because it is simple and is performed without waste drainage, it is promising for the cleaner production of cellulose-based superabsorbent hydrogels. Giachi et al.²⁴ reported that the microwave-synthesized product possessed faster swelling and shrinking kinetics in comparison to the superabsorbent hydrogels prepared by conventional methods. Jelena et al.²⁵ investigated the influence of microwave synthesis on the kinetics of polymerization and found that the polymerization rate increased significantly in comparison to the normal heating method. They deduced that this may be due to decreased activation energy and increased inherent energy of the cross-linker. Afterwards, M. Pandey et al.²⁶ synthesized bacterial cellulose (BC)/acrylamide (Am) hydrogels using the microwave irradiation method and the product showed non-cytotoxic and hemocompatible properties. In addition, a comparative study was made among freezing, microwave irradiation and a combination of both methods.²⁷ Feng et al.²⁸ also prepared the cellulose-based superabsorbent hydrogels using flax shives under microwave irradiation. They chose potassium persulfate and N,N′-methylenebisarylamide (NMBA) as initiator and cross-linker, respectively. Moreover, Wan et al.²⁹ grafted a copolymer of methyl methacrylate onto bamboo cellulose under microwave irradiation using ceric ammonium nitrate as a cross-linker. The effect of microwave power, microwave exposure time and initiator concentration on the graft copolymerization reactions were estimated and the optimum conditions of 160 W microwave power and 9 min exposure time were obtained for graft copolymerization. They found that the moisture absorption capacity of the graft copolymers decreased significantly with increase in grafting percentage.

2.2 Physical synthesis methods

Unlike chemical synthesis methods, physical synthesis methods always refer to the molecular assembly cross-linked by hydrogen bonds or ionic bonds between the polymers or by the interaction between the polymers.

Cryogenic treatment was applied to obtain the cellulose-based superabsorbent hydrogels, which is in contrast with the methodology used at ambient temperature. The superabsorbent hydrogels from this process are the so-called “cryogels,” which form by the association of strong hydrogen bonds. This strong hydrogen bond may be formed during one of the stages of the freeze/thaw cycles: either during freezing of the initial system, during storage of the samples in the frozen state, or during thawing of the frozen specimens. Guan et al.³⁰ prepared a novel cellulose-based superabsorbent hydrogel by repeating the freeze/thaw cycles, which induced physically cross-linked chain packing among these polymers. Then, phase separation caused the formation of a compact structure after multiple freeze/thaw cycles, resulting in high mechanical strength and thermal stability. The highest compressive strength of 10.5 MPa was achieved by 9 freeze/thaw cycles.

By combining UV irradiation and cryogenic treatment technology, researchers have prepared cellulose-based superabsorbent hydrogels with high mechanical strength, pH sensitive swelling properties and good bio-adhesiveness. The incorporation of cellulose into the polymer network provides the possibility to use the cryogels as excipients for the Biopharmaceutics Classification System (BCS) Class 1 preparation of drug delivery such as metronidazole.³¹ From this research, we are convinced that cellulose-based superabsorbent hydrogels will be a promising drug delivery system in the near future.

Electron beam irradiation techniques were also applied to the synthesis of the cellulose-based superabsorbent hydrogels.³² The resulting macroporous, sponge-like cellulose-based superabsorbent hydrogels, which were cross-linked by strong intermolecular hydrogen bonds, showed many promising features for effective wound dressings such as the ability to absorb exudates, an optimal environment for water vapor transmission for wound healing, and excellent biocompatibility.

Except for irradiation crosslink, interactions between the polymers is assumed to be another route to get cellulose-based superabsorbent hydrogels. Similarly, a cellulose-based superabsorbent hydrogel was fabricated by the interaction between cellulose and carboxymethyl cellulose sodium in the alkaline/urea aqueous medium.

In addition, Zhang et al. reported a stable, cellulose-based superabsorbent hydrogel based on bamboo, which formed through dialysis of the alkaline bamboo cellulose suspension against water followed by a short period of ultra-sonication.³³ The electrostatic repulsion between negatively charged –COO⁻ groups on the cellulose fibers generated during the oxidation process was assumed to be the driving force for the formation of the hydrogel. Compared with most methods for preparing cellulose hydrogels, which require complex and difficult dissolution processes usually with harmful solvents, the physical approach proposed here was environmentally friendly and quite effective.

2.3 Newly emerging approaches

Other than the traditional superabsorbent hydrogel synthesis method, rapid contact technology between solid and liquid interfaces is both more facile and faster. The entire process is shown in Fig. 2. Researchers used 2% agarose solution as a template gel-core and obtained cellulose solution by an adapted NaOH/urea solution system at low temperature. After loading 10 wt% acetic acid on the template gel-core, the solid–liquid interface contact was performed by immersing the template gel-core into the cellulose solution to prepare the first cellulose layer. Furthermore, the multi-layered cellulose-based superabsorbent hydrogels were fabricated by repeating the soaking process.³⁴ Cellulose-based superabsorbent hydrogels can also be prepared from native celluloses (cotton cellulose) dissolved in lithium chloride and N-methyl-2-pyrrolidinone (LiCl/NMP) by esterification cross-linking with 1,2,3,4-butanetetracarboxylic dianhydride (BTCA).³⁵ Subsequently, the conversion of un-reacted carboxyl groups to sodium carboxylates by the addition of aqueous NaOH was performed to enhance the water affinity of the hydrogels. It was confirmed that the absorbency of cellulose-based superabsorbent hydrogels was enhanced as the average degree of polymerization (DP) of the starting cellulose increased. Using cotton cellulose with a high DP of about 2400 produced a superabsorbent hydrogel with an absorbency of 720 times its dry weight, which exceeded the absorbency of commercial cross-linked sodium polyacrylate superabsorbent hydrogel (SPA). The hydrogels exhibited good biodegradability with a maximum degradation of 95% within 7 days.

Fig. 2 Physical synthesis method of the fast solid–liquid interface contact technique.³⁴ Reprinted with permission from He M., Zhao Y., Duan J., et al., Fast contact of solid–liquid interface created high strength multi-layered cellulose hydrogels with controllable size [J]. ACS Applied Materials & Interfaces, 2014, 6(3): 1872–1878. Copyright 2014 American Chemical Society.

Some physical methods are also developed to make a contribution to the family of the cellulose-based superabsorbent hydrogels. Isobe et al.³⁶ prepared cellulose-based superabsorbent hydrogels from LiOH/urea solvent with alcoholic coagulation, and some adsorption measurements were conducted for the surface and structural properties of cellulose-based superabsorbent hydrogels prepared from an alkali/urea solvent. In addition, highly aligned and covalently cross-linked hydrogel microfibers were obtained by the electrospinning technique, which provides a safe approach to fabricate nanoscale to microscale fibers.³⁷ The resulting cellulose-based superabsorbent hydrogel microfibers show great potential use in active biological tissues such as the replacement of damaged muscle tissue.

Apart from the most common cross-linker, NMBA, Senna et al. chose ethylenediamine tetra-acetic dianhydride (EDTAD) as a cross-linker in the preparation of superabsorbent hydrogels from cellulose acetate with a degree of substitution (DS) of 2.5. The reaction process can be described as simultaneous crosslinking and grafting of EDTAD and it occurred by the formation of diester and monoester linkages.³⁸

3. Cellulose derived from different resources

According to this survey, the resources of the cellulose in the cellulose-based superabsorbent hydrogels are various, which include the native cellulose of natural plants, cellulose derivatives, functionally modified cellulose, and bacterial cellulose. All of these resources used in synthetic systems are expected to achieve superabsorbent hydrogels with ideal properties.

3.1 Native cellulose

Native cellulose has many advantages such as repetition of usage, biodegradability, and especially good salt-resistance and anti-mildew resistance compared with starch.³⁹ Many plants in nature can provide cellulose, including wheat straw, cotton stalk flax and mulberry branches.⁴⁰ Cellulose, which mainly act as the supporting materials in the plant cells in biological for its stiffness and water absorbency. Common native cellulose resources used in the preparation of cellulose-based superabsorbent hydrogels are shown in Fig. 3.

Fig. 3 Common native cellulose resources used in preparation of cellulose-based superabsorbent hydrogels.

Liang et al.⁴¹ adapted wheat straw to furnish the fabrication of superabsorbent hydrogels. To better use the wheat straw and minimize its negative impact on the environment, Liu et al. prepared semi-interpenetrating polymer networks (semi-IPNs) cellulose-based superabsorbent hydrogels composed of wheat straw cellulose-g-poly(potassium acrylate) (WSC-g-PKA) networks and linear polyvinyl alcohol (PVA) by polymerization in the presence of a redox initiating system.¹⁴ The results showed that the semi-IPN cellulose-based superabsorbent hydrogels prepared under optimized synthesis conditions gave the best water absorption of 266.82 g g⁻¹ in distilled water and 34.32 g g⁻¹ in 0.9 wt% NaCl solution.

Currently, cotton stalks are mostly burned on the ground because they harbor diseases that could affect future cotton crops. However, cotton stalks are abundant, cheap, biodegradable and annually renewable, and some attempts have been made to study the potential of utilizing cotton stalks. For example, the modified maleylated cotton stalk was used to prepare superabsorbent hydrogels by Sawut et al.⁴² The modified cotton stalk cellulose has better hydrophilicity and is easier to graft monomer than cellulose. The maximum water absorbency of the cellulose-based superabsorbent obtained was 1125 g g⁻¹ in distilled water and 126 g g⁻¹ in 0.9 wt% aqueous NaCl solution. Compared to cellulose from other sources, flax cellulose has a longer molecular chain, which means that it has more active groups on a single molecular chain, has better hydrophilicity, and is easier to modify. Wu et al.⁴³ successfully prepared a new, low-cost, and eco-friendly cellulose-based superabsorbent hydrogel from flax yarn waste. Their results showed that, under optimized conditions, the water absorbencies of the superabsorbent hydrogels obtained were 875 g g⁻¹ for distilled water, 490 g g⁻¹ for rainwater, and 90 g g⁻¹ for 0.9 wt% aqueous NaCl solution.

Nguyen et al. made a cost-effective and scalable recipe for fabricating biodegradable cellulose aerogels from available waste paper. The product is highly absorbent, absorbing 18–20 times its weight in liquid. Coating the aerogel with methyltrimethoxysilane improves its hydrophobicity without affecting its absorbency.⁴⁴ Mechanically, the aerogel is flexible yet strong, making a wide range of applications possible. In addition, cotton and viscose waste textiles⁴⁵ were also included in the native cellulose family to synthesize cellulose-based superabsorbent hydrogels.

To the best of our knowledge, a new type of native cellulose origin is bacterial cellulose.⁴⁶ Bacterial cellulose (BC) has chemical structure, crystallinity and mechanical strength similar to plant cellulose, while the absorption capacity of BC is greater than that of plant cellulose,⁴⁷ which has led to the utilization of BC in the absorbing hydrogel field. For example, Halake et al.⁹ used exactly the cellulose produced by the bacteria to reach their goal.

3.2 Cellulose derivatives

Quantities of cellulose derivatives such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose and hydroxyethyl cellulose have been exploited to prepare cellulose-based superabsorbent hydrogels.⁴⁸

Among all the superabsorbent hydrogels prepared with cellulose derivatives (Table 2), the superabsorbent hydrogels involving carboxymethyl cellulose have the highest equilibrium water absorbency and swelling rate in distilled water and saline solution. Yang et al. prepared injectable polysaccharide superabsorbent hydrogels⁴⁹ to permit its possible use in drug delivery vehicles or tissue engineering matrices with the help of carboxymethyl cellulose. Moreover, Eyholzer et al.⁵⁰ fabricated bio-composite superabsorbent hydrogels for the replacement of the native human nucleus pulposus (NP) in intervertebral disks in the presence of the carboxymethylated, nanofibrillated cellulose powder. Cellulose microfibers, nanowhiskers, and nanofibers have been successfully used as reinforcing fillers in a series of synthetic and natural superabsorbent hydrogels. The main reason for this reinforcement by cellulose nanofibers is due to their high aspect ratio of around 20–50, low density of 1.56 g cm⁻³, high elastic modulus estimated at 145 GPa, and strength, which is reported to be 7500 MPa.⁵¹ Aouada et al.⁵² reported a simple, fast, and low cost strategy for the synthesis of micro- and nano-composite superabsorbent hydrogels by adding cellulose nanofibers as reinforcing agents, which are obtained by acid hydrolysis. It was found that the incorporation of cellulose nanofibers affected the crystallinity of superabsorbent hydrogels, thus contributing to improvement in mechanical and hydrophilic properties of superabsorbent hydrogels. It was also observed that cellulose nanoparticles improved the mechanical properties of superabsorbent hydrogels without negatively impacting their thermal and hydrophilic properties.

In addition, Hong et al. extracted the cellulose nanofibrils from sustainable natural sources and they proved that the hydrogel moduli may be tuned by appropriate choice of divalent or trivalent cations (Ca²⁺, Zn²⁺, Cu²⁺, Al³⁺, and Fe³⁺).⁵³ To provide valuable knowledge for designing high-performance nanocomposite superabsorbent hydrogels with cellulose as a raw material, Yang et al. used two sources of cellulose nanocrystals (CNCs) with different aspect ratios to model the reinforcement process. It could be achieved that the values of aspect ratios and nonpermanent interactions between the fillers and matrix dominate the reinforcement.⁵⁴

Table 2 Summary of the main cellulose derivatives and its corresponding superabsorbent hydrogels

Cellulose derivatives	Corresponding superabsorbent hydrogels preparation methods	Applications	Ref.
Carboxymethyl cellulose	Solution polymerization, in situ polymerization	Biomedical and agriculture	19, 20, 48, 49, 57, 62, 82 and 85
Methyl cellulose	Solution polymerization, in situ polymerization	Release fertilizer	48, 64 and 82
Hydroxyethyl cellulose	Solution polymerization, cryogenic treatment	Smart materials	31 and 48
Hydroxypropyl methyl cellulose	Solution polymerization, inverse-phase suspension polymerization	Controlled release	22 and 48
Cellulose acetate	Chemical cross-linkage	Drug carrier system	55 and 74

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Cellulose acetate (CA), a well-known derivative of cellulose, is produced either by heterogeneous or homogeneous acetylation of cellulose. Senna et al. described a detailed synthesis process of cellulose-based superabsorbent hydrogels using cellulose acetate.⁵⁵

Except for the common etherification product of cellulose, hydrazide or the aldehyde functionalized⁵⁶ product of cellulose were also reported recently as contributing to the construction of cellulose-based superabsorbent hydrogels. For instance, quaternized cellulose⁵⁷ was cross-linked with carboxymethyl cellulose in NaOH aqueous solution in the presence of epichlorohydrin (ECH).

4. Application fields

A number of cellulose-based superabsorbent hydrogel products have been either available commercially or are in the process of development. In addition, many patents for cellulose-based superabsorbent hydrogels have been granted for various possible applications. Most of these are used in agricultural and horticulture, personal health care field, water treatment, biomedical fields and in stimuli-response smart behavior applications. In addition, many promising applications such as protective barriers for volatile organic compounds spilled in the environment and as absorbents for waste oil⁵⁸ have also been explored.

4.1 Agriculture and horticulture

As is well known, aridness is still a threat for many countries, particularly in Africa, South America and the west of Asia. To improve the soil conditions in these areas, Li et al. applied the superabsorbent hydrogels as a type of soil additive; they examined the changes of water content, soil microbial activity, biomass and the crop yield between the original soil and the modified soil. It turned out that the addition of the superabsorbent hydrogels to the soil not only leaves no detectable adverse effects, but also benefits the soil physical properties and crop yield.⁵⁹ Research conducted by Demitri et al.⁶⁰ might have a revolutionary impact on the optimization of water resources management in agriculture. The proposed cellulose-based superabsorbent hydrogels allowed efficient storage and a sustained release of water to the soil and plant roots, showing potential as a water reservoir in agriculture.

Pesticides, the most cost-effective means of pest and weed control in agriculture, are also recognized as a source of potential adverse environmental impact. Superabsorbent hydrogels based on the cellulose series used as carriers for pesticides are of special interest in terms of both economic and sustainable development. Encapsulating herbicides into cellulose-based superabsorbent hydrogels could be used to decrease the release rate of these herbicides.⁶¹

For example, to minimize the hazardous influence of the herbicide acetochlor's potential toxicity to non-target organisms in the farmland, Li et al. developed controlled-release formulations of acetochlor, which provides an improvement in the safety to the user and non-target organisms and a reduction of the herbicide application rates and of leaching into soils. Using CMC gel and different types of clay, controlled-release formulations of the herbicide acetochlor were prepared. The performance of inorganic clays in dried gel formulations on slowing the release of acetochlor is related to their sorption capacities while organic clay did not lead to the slowest release. In addition, according to the parameters of an empirical equation used to fit herbicide release data, the release of acetochlor from clay/CMC gel formulations is controlled by a diffusion mechanism.⁶²

Laftah et al.⁶³ evaluated the effect of polymer hydrogels composite (PHGC) based on cotton microfiber on sandy soil holding capacity, urea leaching loss rate (ULLR), and okra plant growth. Their results showed that cotton microfiber has a prominent effect on the swelling rate, re-swelling capacity, and biodegradability of PHGC. Okra plant growth and ULLR were positively affected by PHGC and the best leaching loss rate of 33.3% was observed for the lowest urea loaded sample. Furthermore, Bortolin et al.⁶⁴ proved that PAAm/methyl cellulose/montmorillonite superabsorbent hydrogels imparted synergistic effects for the slow release of fertilizers. Their results revealed that the cellulose-based hydrogels effectively slow the loss of nitrogen via volatilization of ammonia.

4.2 Personal health care

Superabsorbent hydrogels are widely used in the field of hygiene, e.g., disposable diapers and female napkins, for their ability to absorb and retain large amounts of secreted fluids such as urine and blood. It was reported that the first generation commercial superabsorbent hydrogel was produced in Japan in 1978 as a component of female napkins. Since then it rapidly extended its market all over the world due to the ability to retain the secreted liquids under pressure. Therefore, superabsorbent hydrogels have caused a huge revolution in the personal health care industry.⁴ At present, superabsorbent hydrogels that are contained in sanitary napkins are also primarily polymerized by acrylic acid (AA) or acrylamide (AM), which are costly, poorly degradable and environmentally unfriendly. Liu et al. provided a novel tactic by the incorporation of flax yarn waste into superabsorbent hydrogels for sanitary napkin applications.⁶⁵ Their results showed that the product exhibits excellent biodegradability, superabsorbency and retention ability for artificial blood solution compared to those of the currently marketed sanitary napkin products.

Although more convenient, suitable, and comfortable disposable health care products^66–68 have been extensively developed in modern times, biodegradable health care products have not either been industrialized or been commercially available. In view of the foregoing, the key technique for converting the cellulose-based superabsorbent hydrogels into the core layer of healthcare products needs to be addressed.

4.3 Water treatments

Rapid industrial development leads to a series of problems for the environment, for example, water contamination. A number of technologies have been developed for water treatments, mainly including adsorption, chemical oxidation, and pressurized membrane-based separation. Owing to increased energy consumption and the latent secondary pollution caused by traditional materials, researchers have shifted their attention to cellulose-based superabsorbent hydrogels.

To deal with the polluted streams by heavy metals such as Pb²⁺, Zhou et al. prepared novel magnetic hydrogel beads, which blended chitosan with amine-functionalized magnetite nanoparticles, carboxylated cellulose nanofibrils (CCNFs), and poly(vinyl alcohol) by an instantaneous gelation method. These new magnetic hydrogel beads can absorb Pb²⁺ in sewage quickly and effectively with a high value of 171.0 mg g⁻¹, which can be attributed to numerous carboxylate groups on the CCNFs and abundant hydroxyl and amino groups on the chitosan.⁶⁹ Tripathy et al. have investigated the five metal ion (i.e. Cu²⁺, Ni²⁺, Zn²⁺, Pb²⁺ and Hg²⁺) sorption behavior of cellulose-based superabsorbent hydrogels. Sorption results showed that the values of the five percent ion uptake were 13.8, 11.5, 9.8, 9.0 and 8.7 at the maximum values, separately.¹² Their results also showed that the sorption percent values increase directly as the graft ratio increases, indicating that the sorption sites are increasing.¹³ In addition, cyanoethyl cellulose-based superabsorbent hydrogels were obtained to apply for the adsorption of copper(II) ions from aqueous solutions. The authors believed that metal-ion removal depends on the protonation and deprotonation properties of acidic and basic groups, namely, pH value.⁷⁰

Apart from pollution from metal ions, frequent oil spills and increasing oil pollution from industrial wastewater have already been other sources of water contamination. In order to reach energy-efficient and cost-effective separation of water from oil, Rohrbach et al. created a nanocellulose-based filter by a dipping and drying process of coating the filter with a layer of nanofibrillated cellulose-based superabsorbent hydrogel. The filter's efficiency can reach 99.1%.⁷¹ The water oil separation process is shown in Fig. 4.

Fig. 4 Schematic of water/oil separation using a regular cellulose paper with a layer of coated cellulose-based superabsorbent hydrogel. Reproduced from ref. 71 with permission from The Royal Society of Chemistry.

Despite the aforementioned research, the need for developing new strategies in water treatment using cellulose-based superabsorbent hydrogels will become more overwhelming in the future.

4.4 Biomedical

Cellulose-based superabsorbent hydrogels are also widely used in the biomedical field, for instance, in drug delivery, tissue engineering, cell bioreactors, and micropatterning neural cell cultures. He et al. fabricated the onion-like, multi-layered tubular cellulose-based superabsorbent hydrogels for the first time. Cell toxicity experiment results indicated that the L929 cell can survive and proliferate in the larger interior space of the multi-layer cellulose-based superabsorbent hydrogels, showing great potential application in the biomedical field.³⁴

The injectable cellulose nanocrystals (CNC)-reinforced superabsorbent hydrogels prepared by Yang et al.⁴⁹ could maintain their original shape for more than 60 days when immersed in purified water or 10 mM PBS and exhibit excellent storage modulus. Moreover, CHO–CNC-reinforced superabsorbent hydrogel is more elastic, more dimensionally stable, and facilitates higher nanoparticle loadings compared to hydrogels with unmodified CNCs without sacrificing mechanical strength. The cytotoxicity test showed that CNC-reinforced injectable hydrogels were of potential interest for various biomedical applications such as drug delivery vehicles or tissue engineering matrices.

As is well known, in the wound treatment field, wound dressings with good hydrophilicity and microorganism inhibition qualities are rarely achieved simultaneously. To obtain the ideal materials for wound dressings, researchers explored the use of different treatments to modify the viscose fiber to its non-woven form. Using alkali treatment or oxygen plasma treatment, high hydrophilicity was achieved.⁷² It turns out that the introduction of silver chloride nanoparticles into the cellulose matrix markedly improved the antimicrobial activity, which can be ascribed to the broad spectrum antibacterial quality of silver, and the hydrophilicity of the wound dressing was also improved to a degree in relation to the untreated viscose fiber. Compared to the “alkaline treatment followed oxygen plasma treatment” two-step procedure, the one-step ammonium plasma treatment significantly improved hydrophilicity, but could not provide the desired antimicrobial activity on all the bacteria used, such as S. Aureus, E. Coli, E. Faecalis and P. Aeruginosa, which means that the one-step approach may have a limited antimicrobial activity in clinical application. However, the one-step ammonium plasma treatment for modifying the viscose fiber provided a new outlook to prove the potential feasibility and developments toward clinical application and commercial production.

Lin et al. have proved that cellulose-based superabsorbent hydrogels were used as the drug carrier for in vitro release of doxorubicin and exhibited the behavior of prolonged drug release with special release kinetics.⁷³ To extend the application of the cellulose-based superabsorbent hydrogels, Eyholzer et al. prepared biocomposite superabsorbent hydrogels with carboxymethylated nanofibrillated cellulose (c-NFC) powder by UV polymerization of N-vinyl-2-pyrrolidone with Tween 20 trimethacrylate as a cross-linking agent for the replacement of the native human nucleus pulposus (NP) in intervertebral disks. Among the tested samples, the biocomposite superabsorbent hydrogels containing 0.4% v/v of c-NFC with a DS of 0.17 show the closest behavior to native NP, which could be a breakthrough in treating symptomatic intervertebral disk degeneration.⁵⁰ The entire process is shown in Fig. 5. Furthermore, the cellulose-based superabsorbent hydrogels have played a vital role in veterinary practice. Oliveira et al. adapted cellulose acetate and 1,2,4,5-benzenotetracarboxylic dianhydride to synthesize and assess controlled release systems, which are usually designed to protect patients from unfavourable environments, provide them with more comfort, prevent side effects and improve efficiency through structural modifications of the drug carrier system.⁷⁴ Joshi et al.⁷⁵ have revealed that their cellulose-based superabsorbent hydrogels' reversibility with temperature in physiological salt fluids such as simulated gastric and intestinal fluids have a better insight into the oral drug delivery system.⁷⁶ Some studies on the anticancer drugs docetaxel, paclitaxel, and etoposide have already been done by Jackson et al.⁷⁷

Fig. 5 Biocomposite cellulose-based superabsorbent hydrogels promised for the replacement of the human nucleus pulposus in intervertebral disks.⁵⁰ Reprinted with permission from Eyholzer C., Borges de Couraca A., Duc F., et al., Biocomposite hydrogels with carboxymethylated, nanofibrillated cellulose powder for replacement of the nucleus pulposus [J]. Biomacromolecules, 2011, 12(5): 1419–1427. Copyright 2011 American Chemical Society.

There has also been considerable interest in utilizing cross-linked-CMC as tablet disintegrants. The cellulose-based superabsorbent hydrogel in its powder form is mixed with other excipients and compressed to a tablet. Tablets containing cellulose-based superabsorbent hydrogels may soften at high humidity and may add instability concerns to the moisture-sensitive drugs.⁷⁸ Rheometry tests finished by Ngwuluka et al. have shown that their hybrid hydrogel product may be a suitable polymeric material for achieving controlled zero-order drug delivery.⁷⁹ Furthermore, cellulose-based superabsorbent hydrogels have also made a good contribution in non-immediate release devices.⁸⁰

Appel et al.⁸¹ investigated systematically the release mechanism/model of the physically cross-linked superabsorbent hydrogels by cross-link dynamics. It was determined that the cargo (containing the drugs) release processes from the cellulose-based superabsorbent hydrogels could be directly correlated with the dynamics of the physical interactions responsible for cross-linking and corresponding time-dependent mesh size.

Mechanically, cellulose-based superabsorbent hydrogels can be designed to have elastic and loss moduli similar to those of soft tissues, enabling their effective use in tissue engineering applications or as biological lubricants. Patenaude et al.⁸² combined a series of synthetic oligomers and carbohydrate polymers, such as methylcellulose, carboxymethyl cellulose, and dextran, to create in situ gelling, hydrazone cross-linked hydrogels using a double-barreled syringe. In this way, one property can (in many cases) be selectively modified while keeping other properties constant, providing a highly adaptable method of engineering injectable, rapidly gelling hydrogels for potential in vivo applications.

In the “smart” materials family, cellulose-based superabsorbent hydrogels tends to have wider applications in the biomedical field. Herein, we focus on the pH-responsive, salt-responsive and thermal-responsive behavior of cellulose-based superabsorbent hydrogels. With the development of cellulose derivatives, mainly cellulose ether, some stimuli-responsive cellulose-based superabsorbent hydrogels have been developed from MC, HPC, HPMC, and CMC by chemical or physical methods.

A type of nanocomposite hydrogel was synthesized on the basis of poly(acrylamide-co-acrylate) and cellulose nanowhiskers by Spagnol et al.,⁸³ which showed sensitivity to pH variation (2–12). Such on–off switching behavior as reversible swelling–deswelling has been reported⁸⁴ and has been seen as a good candidate for some technological applications. In the research of Wang et al., the hydrogels of CMC-g-poly(AA-co-AMPS) showed better reversible pH sensitivity in the pH 2.0 and 7.0 solutions, which makes the hydrogels available as candidates for drug delivery systems.⁸⁵

Subsequently, Hebeish et al.⁸⁶ synthesized the smart cellulose-based superabsorbent hydrogels with sensitive response to the environmental temperature stimulus and researchers verified its potentially promising application, particularly in the pharmaceutical field. In addition, Hu et al.⁸⁷ prepared cellulose-based superabsorbent hydrogels, which exhibited smart swelling and shrinking behaviors in NaCl and CaCl₂ aqueous solution, showing salt-responsive adsorption behaviors in different media.

5. Outlook

This review seeks to describe the research progress in superabsorbent hydrogels based on cellulose from different angles over the past decades. Cellulose-based superabsorbent hydrogels have many favorable properties such as hydrophilicity, biodegradability, biocompatibility, transparency, low cost, and non-toxicity. Therefore, cellulose-based superabsorbent hydrogels have wide applications in agriculture, horticulture, personal health care, water treatment, and the biomedical field. However, some new fields, such as oil plugging agents and electrochemistry, need to be expanded. We emphasized various methodologies, materials and achievements for these particular materials and provided an overview of their applications as functional materials on a large scale. However, with the development of living standards, more demands will be imposed. A mature conventional product could not meet all the requirements.

Therefore, the future scientific research on cellulose-based superabsorbent hydrogels needs to be designed to meet the demands for different properties and exhibit new performance such as in the fields of electronics, catalysis, and chemical and biomedical sensors. From the point of view of industrial applications, the superabsorbent hydrogels based on cellulose will surely result in new expanded fields with improved performance in terms of good mechanical strength, biocompatibility, biodegradation, non-toxicity, and anti-mildew performance. Therefore, the study on the preparation of the cellulose-based superabsorbent hydrogels needs to be developed. Moreover, cellulose is an environmentally-friendly, low-cost material, which will form an available substitute for petroleum-based materials in the near future. Thus, we increasingly tread a green area via replacing synthetics with bio-based materials, cellulose and its derivatives. In addition, new materials and methods need to be found and used for cellulose-based superabsorbent hydrogels in order to endow them with unique properties. Moreover, the preparation mechanism of cellulose-based superabsorbent hydrogels originating from an interdisciplinary angle needs to be further researched and the swelling kinetics of cellulose-based superabsorbent hydrogels in different media requires deeper investigation because more theoretical studies will lead to a better understanding and facilitate experimental trials and then large-scale application.

With continuous research in cellulose-based superabsorbent hydrogels, the properties of materials with cellulose will become excellent and the development prospects will be much brighter. It is hoped that this review will be helpful in this important field.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21376145), the Program for New century excellent talents in university (NCET-13-0855) and the Key Scientific Research Group of Shaanxi Province (2013KCT-08).

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