Cellulosic Hydrogels: A Greener Solution of Sustainability
Hydrogels are insoluble three-dimensional cross-linked polymeric network that swells in presence of water and other fluids. They can hold plenty of water compared to its own mass. The absence of dissolution attraction toward water is due to hydrophilic nature of the polymeric chain. Hydrophilicity arises because of holding hydrophilic functional groups in the chain. Highest portion of the world production of hydrogels is petrochemical based which is neither renewable nor biocompatible. In spite of some drawbacks like nondegradability, synthetic hydrogels are superior to natural one in water absorbency, diversification in chemicals, and longer service life. Taking into consideration sustainability factor, scientists are interested in preparation of hydrogels from renewable cellulosic sources. As cellulose possesses intrinsic nature of degradability, biocompatibility, and nontoxicity, also available in nature, and some cellulose derivatives show smart behavior, cellulose-based hydrogels can be an alternative to synthetic petrochemical-derived hydrogels. Numerous research articles concerning the synthesis and utilization of hydrogels in different fields have been published, and still restless labor is giving for the betterment of the product quality. It is a crying need to make available and adequate information on synthesis and characterization of cellulosic hydrogels for individual researchers. For this the specific aim of this paper is to accumulate some crucial information which will cover synthesis, detailed classification, characterization, and technological feasibility of application about hydrogels of renewable source. As of consequence, the research on hydrogel concerning current environmental issues will reach to its target of making the greener solution of sustainability. In addition, recent trend of hydrogel research is also discussed in this review.
KeywordsCellulose Cellulosic hydrogel Cellulose derivatives Hydrophilic Renewable Biocompatible Sustainability
Hydrogels are gel-like materials with hydrophilic functional groups in polymeric chain and capable of holding large volume of hydrofluids compared to its own mass. It can also be defined as hydrogels are water-absorbing natural or synthetic polymeric substance which swells in water and retain a significant amount of water within the structure without dissolving in water [1, 2, 3]. Hydrogels attract water due to polar functional groups on the skeleton of macromolecule and inhibit dissolving due to cross-linking. Both types of cross-linking, chemical or physical, can exist in the macromolecular chain. After absorption, generally hydrogels swell until the thermodynamic force of swelling is totally counterbalanced by the elastic, retroactive force exerted by the cross-links. The volume of water taken up by macromolecule depends on the structure of the polymer network itself and on the environmental conditions, such as the temperature, pH, and ionic strength of the water solution in contact with the polymer [4, 5]. The most important variable that affects the diffusive, mechanical, optical, acoustic, and surface properties of the hydrogel for given environmental conditions is the volume or mass swelling ratio of the hydrogel.
Hydrogels are of mainly two categories based on their natural or synthetic origin: biopolymer-based and synthetic. Hydrogels prepared at the early stage are non-biodegradable and originated from nonrenewable petroleum based. Among various biopolymers, cellulose is largely available in nature and shows hydrophilic nature and good mechanical properties because of enormous hydroxyl groups. The mechanical strength and its insolubility in water and in the major part of solvents depend mainly on the complex system of hydrogen bonds between hydroxyl groups. Due to plentiful hydrophilic functional groups, it gives possibility to cellulose as promising material for hydrogel preparation. Generally natural polymers, especially polysaccharide-based hydrogels, find application in many fields (agriculture, tissue engineering, drug delivery, biosensors, etc.) and make scientists interested about the matter due to some unique qualities like eco-friendly, renewable, and low cost. Among various polysaccharides, few have been investigated on hydrogel formulations, such as chitosan [6, 7], starch [8, 9], cellulose , alginate , carrageenan , and gellan gum .
Manufacture of hydrogels from cellulose generally is accomplished in two steps [4, 6, 7, 11], (i) dissolving of cellulose fibers or powder and (ii) cross-linking (chemical and/or physical) of the chains, in order to obtain a three-dimensional network of hydrophilic polymer chains, which is able to absorb and retain a significant amount of water. It is also reported that cellulose-based hydrogels were prepared and used as a novel porous bioabsorbent by graft copolymerization for absorption of heavy metal ions from aqueous solutions. Therefore, the diversified application of cellulose-based hydrogels is flourishing gradually. After the invention of first hydrogels by Otto Wichterle in the 1950s , from then scientists are trying to improve the qualities and to make novel superabsorbent which will fulfill the required expectations. The modern application of cellulose and its derivatives includes environment-friendly and economic nonaqueous gel polymer electrolytes for lithium and sodium ion batteries [14, 15, 16], aqueous electric double-layer capacitors, dye-sensitized solar cells , and starting materials for proton-exchange membranes (PEMs) in PEM fuel cells (FCs) .
It is obvious that cellulose and cellulose derivatives will not replace petroleum-based superabsorbent materials completely, but incorporation or use of the materials will bring some good qualities in hydrogels like biodegradability, biocompatibility, nontoxicity, cost-effectiveness, etc. As we know cellulose is the main constituent of plant and at the end use it decomposes. Moreover, the cellulose-based hydrogels are reusable, and processes involve less wastage of chemicals and not releasing greenhouse gases to environment, so the process can be mentioned as greener technology.
The aim of the review paper is discussion from cellulose-based published literature of hydrogels and focuses the types, use, and world consumption scenario, synthesis, controlling factors of reaction, etc. of cellulose-based hydrogels which will be an informative and fruitful tool to solve the sustainability problem. After discussing characterization techniques, some well-established and innovative applications of synthetic and cellulose and its derivatives hydrogels both are also discussed, with some suggestions for future developments.
2 History and Market Size of Hydrogels
In 1938, the first water absorbent was synthesized by thermal polymerization of acrylic acid and divinyl benzene in aqueous medium . Later in 1950, Otto Wichterle  synthesized first generation hydrogels which were based on poly(hydroxyethylmethacrylate) (PHEMA) and used in contact lenses. Though the swelling capacity of hydrogels from same type of monomers was only 40–50%, it was a revolution in ophthalmology .
The first commercial synthetic-type superabsorbent polymer (SAP) hydrogel was starch-graft-polyacrylonitrile (SPAN), and its hydrolyzed product (HSPAN) was developed in the 1970s at the Northern Regional Research Laboratory of the US Department of Agriculture . The product did not get commercial success due to expenses and inherent structural disadvantage (lack of sufficient gel strength) of this product. Japan started their first commercial production of hydrogel in 1978 for use in feminine napkins and later in 1980 Germany and France started using in baby diaper. Japan started manufacturing diaper in 1983 with 4–5 g SAP, hydrogel in every single piece of diaper. Within very short time, other countries in Asia, USA, and Europe also started manufacturing diaper with hydrogels. Practically SAP, hydrogels brought a drastic change in the concept of diaper item to replace fluff cellulosic materials. Finally, the diaper became thinner by the use of hydrogels. Actually within one decade, huge development occurred in personal health-care product.
From an online data analysis, it has been found that the worldwide hydrogel production and consumption are increasing. In this case, China is the biggest market in the Asia-Pacific region, and the USA is the largest end user and producer of SAP in the North American region. Both countries are the largest consumers in their respective regions and are expected to compete with each other to dominate the market by 2020, with advanced SAP materials for end users. Current market size of superabsorbent polymer was estimated at 2.07 million tons in 2014 and is likely to exceed 3.1 million tons by 2023 growing with an expected compound annual growth rate (CAGR) of over 5.5%. It has also estimated that the worth of global market value of SAP will reach USD 7.96 billion by 2020.
Renewable raw materials such as starch, cellulose, natural gums, and chitin have been used in the manufacture of biobased SAP. Biobased superabsorbent polymers provide environmentally sustainable alternatives to fossil-based materials. They also offer effective moisture retention and absorbency for applications such as baby diapering, packaging, feminine hygiene, and adult incontinence products. It is also reported that the Archer Daniels Midland Company (ADM) has launched newest generation of biobased product such as BIOSAP (the company patented the technology of preparing using modified starch with similar technique of acrylic-based SAP preparation) – Lysorp 218 and Lysorp 220.
Few companies are catering to large part of the demand of global superabsorbent polymer market share. The key companies involved in this market which bear largest share include the BASF, Nippon Shokubai, Evonik, Sumitomo, and LG Chemical. Some other prominent companies also have significant share of market which include the Formosa Plastics, KAO Corporation, SDP Global Corporation, Yixing Danson Technology, and Songwon Industrial Corporation Limited.
3 Classification of Hydrogels
Classification based on source: Hydrogels are of mainly two groups according to their origin, i.e., natural and synthetic. Depending on the natural sources, this category includes collagen, fibrin, hyaluronic acid, Matrigel, and derivatives of natural materials such as chitosan, alginate, and silk fibers. On the other hand, hydrogels prepared from synthetic sources are mainly petrochemical based. As biocompatible hydrogel (i.e., biobased) carries extra emphasis according to the title, the author finds responsible to give brief discussion on biobased hydrogel in Sect. 3.1.
- (ii)Hydrogels based on polymeric composition: The methods of preparation of hydrogels alter the types of product. The use of number of monomers gives hydrogels with different properties. Types of hydrogels based on polymeric composition are  described below:
Hydrogels from homopolymer: Polymeric network is formed from single monomer, and it is the fundamental structure on which the whole chain grows. The growing chain can also be cross-linked to alter properties by various polymerization techniques.
Hydrogels of copolymers: Hydrogels are formed by two or more different monomer species, among them at least one hydrophilic component arranged in a random, block, or alternating arrangement along the chain of the polymer network.
Hydrogels of interpenetrating polymeric network (IPN): This important class of hydrogels is prepared from two independent cross-linked synthetic and/or natural polymer components, arranged in a network form. In semi-IPN hydrogel, one component is of cross-linked polymer, and the other is a non-cross-linked polymer.
Classification based on crystallinity: Hydrogels are polymeric materials and gain various crystal structures during manufacture on the basis of technique applied. They are of mainly crystalline, amorphous, and combination of the previous two semicrystalline.
Classification based on type of cross-linking: Cross-linking is a common practice in hydrogel preparation. Two types of cross-linking are observed in hydrogels, chemical or physical. As a result, two types of networks are formed. In chemical cross-linking, the junctions formed are permanent, while in physical cross-linking, networks formed are fragile in nature. Transient junctions arise either from polymer chain entanglements or physical interactions such as ionic interactions, hydrogen bonds, or hydrophobic interactions.
Classification based on sizes: Hydrogels can be given many expected shapes according to requirements and polymerization technique applied. The common shapes are film, microsphere, rounded, matrix, etc.
- (vi)Classification based on ionic particle: Some hydrogels contain charged particles inside the polymeric chain and show conductive property. They can be divided into four classes depending on electrical charge available on the chain:
Anionic or cationic
Amphoteric electrolyte (ampholytic) containing both acidic and basic groups
Zwitterionic, containing both anionic and cationic groups in each structural repeating unit
- (vii)Classification based on mechanism during the drug release: Hydrogels that are formulated for using in drug delivery system and their mechanism should be in controlled manner. It is of four types:
Release systems controlled by diffusion.
Release systems controlled by swelling.
Release systems controlled by chemical.
Release systems controlled by stimuli.
3.1 Biobased Hydrogels
Biobased hydrogels are mainly synthesized from natural sources which include mainly polysaccharides (e.g., cellulose, starch, chitosan, and natural gums such as xanthan, guar, and alginates) and protein (e.g., gelatin). Polysaccharide-based hydrogels are prepared either by graft copolymerization of monomers in presence of cross-linkers or direct cross-linking. Both processes give hybrid hydrogels. The first commercial polysaccharide-based hybrid hydrogel was starch-g-PAN, obtained by direct cross-linking . The product showed path for conversion of other polysaccharides into hydrogels.
Hydrogels can also be formed from chitosan and its derivatives. Physical entanglement happens during curing at the temperature between 5 °C and 60 °C and within minutes to weeks time duration . Derivatives of chitosan give better hydrogel product as it is soluble in water or acid. The hydrogel formation takes place either physical or by chemical cross-linking, and obtained hydrogel is thermoset and pH sensitive . Sometimes chemical cross-linking is preferable due to stable structure and better swelling properties [25, 26]. By adding some synthetic part with natural polymer, composite or hybrid hydrogels with specific properties is obtained .
Not a good number but few works have been reported on hydrogels prepared from protein sources. In this regard, proteins from fish, soybean, and collagen are the three members which are practiced most frequently. It requires to modify the protein to convert it into hydrogel network . Modified fish protein-based hydrogels showed the swelling capacity of 540 g/g in deionized water and assumed to be dependent on pH and ionic strength of swelling media and suggested for water absorption under load such as diapers .
Gelatin, collagen-based protein, and hydrolyzed collagen have been used for hydrogel preparation by graft polymerization of AA/AM, and the formed hydrogel is gelatin-g-poly (NaAA-co-AM). The hydrogel was high sensitive to pH .
Hydrogel preparation has been practiced with various natural sources due to sustainable factors, such as availability, low cost, easily processable, and biocompatibility. Cellulose is the best option for the researchers due to inheritance of the mentioned properties.
4 Factors Influencing Cellulose to Be Perfect Alternative for Sustainability
4.1 Unique Structure of Cellulose and Biodegradability
Cellulose is a polymer of glucose and also the main constituent of plants and natural plant fibers. It is found most abundantly in nature. Besides plants, some bacteria can also synthesize as extracellular product inside their body. Both the cellulose found in plant and bacteria are chemically identical but different in physical properties and molecular structure. The glucose units which are common for bacterial cellulose (BC) and plant cellulose (PC) are held together by 1,4-β-glycosidic linkages and responsible for higher crystallinity (usually in the range 40–60% for PC and above 60% for BC). Because of this, they are insoluble or partially soluble in water and other common solvents. BC can be synthesized and obtained fibers are nanosized, and about two orders of magnitude are smaller than PC fibers. For this BC cellulose shows a peculiar, ultrafine fiber network with high water-holding capacity and superior tensile strength compared to PC. Compared to PC, BC is found in pure form, and usually PC is associated with other biogenic compounds, such as lignin and pectin. As a result, to make in use, PC requires purification and modification. Modification via chemical change of cellulose is common which usually involves esterification or etherification of the hydroxyl groups. The chemical modification generally forms cellulose derivatives, named cellulosics. Cellulosics are easy to process and have numerous consumable applications. Cellulose and its derivatives are eco-friendly as they degrade by some bacteria and fungi present in air, water, and soil (Fig. 1b) . The degradation process of cellulosic materials has been investigated by many researchers. Degradation process leads to decrease in molecular weight, lower mechanical strength, and lower degree of crystallinity and improved water solubility . The abundance and degradation nature of cellulosic materials have trigger down the use of them in biomedical application . As humans are unable to synthesize cellulose, they can’t digest cellulose. The degradation of cellulose can be expressed as.
4.2 Potentiality of Cellulose and Its Derivatives for Hydrogel Synthesis
Synthesis of cellulose-based hydrogels involves either physical or chemical stabilization of aqueous solutions of cellulosics. During synthesis, addition of natural or synthetic monomers or polymers gives composite hydrogels with specific properties [41, 42]. The polymer network can be formed in both physical and chemical means. In physical network formation, hydrophobic association of polymer chains takes place at low temperature. At higher temperature, macromolecules gradually lose their water of hydration, entangled with one another, and hydrogel network is formed. Physically cross-linked hydrogels have some drawbacks like degrade in uncontrollable manner, and reversibly  for this, they cannot be applied in many fields (e.g., biomedical application). The ill points of physically cross-linked hydrogels of cellulose derivatives (MC and HPMC) can be replenished by inducing the formation of chemical, irreversible cross-links among the cellulose chains. The degree of cross-linking which affects many properties (diffusive, mechanical, and degradation) can be controlled in required extent during synthesis. In some cases, chemical modifications of the cellulose backbone might be performed before cross-linking, in order to obtain stable hydrogels with given properties . Some cross-linking agents are toxic in nature. Taking into consideration environmental and health safety concerns, radiation cross-linking of polymers, based on gamma radiation or electron beams, has been receiving increasing attention in the last couple of years. The irradiation technique is governed by irradiation dose as well as by the cellulose concentration in solution [45, 46, 47]. As radiation treatment does not involve additional chemical reagents, is easily controllable, gives some benefits to biomedical applications, and allows the simultaneous sterilization of the product, the researchers are more interested to apply in hydrogel preparation.
4.3 Biocompatibility of Cellulose-Based Hydrogels Over Synthetic Hydrogels
Hydrogels manufactured from nonrenewable petrochemical sources are available in the market, and consumers are familiar with the products. With unconscious mind of the consumers after the end use, the thrown debris with hydrogels are making the environment more intolerable. The reason behind this is nondegradable nature of currently available hydrogels. If the petrochemical-based hydrogels are degraded by any means, the degraded products are gaseous and toxic to both human being and animals including environment. As cellulosics are more environment friendly, the hydrogels originated from them should also be biodegradable in nature . In addition to biocompatible factor, the world petroleum source is limited compared to renewable cellulose and its derivative sources. It has been largely investigated by many researchers that cellulose and its derivative-based hydrogels degrade by cellulose enzyme, which is unable toward noncellulosic hydrogels . This suggests cellulosic hydrogels can be used in biomedical application. Cellulose and its derivatives have many widespread perspectives and contribute positive attitude toward enhanced absorption performance of hydrogels. It has also been reported that introduction of cellulose not only increases the biodegradability but also absorption (up to 2500 g/g)  and other performances.
5 Methods of Preparation of Cellulose-Based Hydrogels
Attachment of polymer chains via chemical reaction
Generation of main chain free radicals using ionizing radiation which can recombine as cross-link junctions
Physical interactions such as entanglements, electrostatics, and crystallite formation within chains
Considering environmental protection and to establish the thought that partially or complete replacement of synthetics by greener alternatives, cellulose gaining the prime importance to synthesis hydrogels . The most important polysaccharides (chitin, cellulose, starch, and natural gums) are the cheapest and most abundant, and renewable organic materials are available. Among them cellulose is most abundant in nature. Generally two types of chemical reactions are performed for the synthesis of cellulose-based hydrogels: (a) in the presence of a cross-linker graft copolymerization of suitable vinyl monomer(s) on polysaccharide (cellulose) and (b) polysaccharides which are directly cross-linked.
To obtain hydrogel through graft copolymerization, generally a polysaccharide (cellulose) interacts with initiator by either of the two separate ways. In first case, the neighboring OHs on the saccharide units and the initiator (commonly Ce4+) interact to form redox pair-based complexes. Subsequently these complexes are dissociated to produce carbon radicals on the polysaccharide substrate via homogeneous cleavage of the saccharide C–C bonds. Created free radicals initiate the graft polymerization of the vinyl monomers and cross-linker on the substrate. On the other hand, in the second case, an initiator such as persulfate may abstract hydrogen radicals from the ~OHs of the polysaccharide to produce the initiating radicals (oxygen radicals) on the polysaccharide backbone (Fig. 3a). As these are thermal initiators, this reaction is more temperature sensitive compared to other methods.
In case of direct cross-linking of polysaccharides (cellulose), various cross-linking agents were employed like polyvinyl compounds (e.g., divinyl sulfone , DVS) or polyfunctional compounds (e.g., glycerol, epichlorohydrin, and glyoxal) [22, 50]. Most recently POCl3 and citric acid have been used to synthesize natural cellulosic hydrogels [51, 52]. The overall polymerization technique was shown schematically in Fig. 3b.
6 Characterization of Hydrogels Through Analytical Technique
Physical and chemical characterizations of hydrogels both are essential for academic and industrial applications. The following tests are conducted to characterize the hydrogels. Some experiments related to physical characterization are as follows.
6.1 Free-Absorbency Capacity
Absorption capacity is a common but fundamental property of hydrogel. When the term swelling and absorbency are used without specifying its conditions, it implies uptake of distilled water, while the sample is freely swollen, i.e., without applying load on the test sample. Several simple methods for the free absorbency test are available. These methods are dependent mainly on the amount of the available sample, the amount of water absorbed by the sample, and the method’s precision and accuracy. Among various methods mostly followed are tea bag method, centrifuge method, and sieve method. Tea bag method is simple and fast, but sieve method requires a large amount of sample (1–2 g). On the other hand, the measured values in centrifuge method are often more accurate, reliable, and lower than those obtained from the tea bag method values due to removal of interparticle liquid. For convenience, only tea bag method is discussed below.
6.1.1 Tea Bag Method
Centrifuge method is more accurate than the tea bag method and is occasionally reported in patents and data sheets . Sieve method requires a large amount of sample (1–2 g) and is also called filtering and rubbing method .
6.2 Absorbency Under Load
6.3 Wicking Capacity and Rate
Assuming a steady absorption for the duration of 60 s, an estimation of wicking rate (g/g.s) of the hydrogel may be obtained by dividing the WC value by 60.
6.4 Swelling Rate
Swelling rate is also an important parameter for determining quality of manufactured hydrogel. It can be calculated in a number of ways. Among them two common methods are vortex method and swelling-time profile . Vortex method is the most simple and rapid method for calculating swelling rate of hydrogel sample and is often employed in R&D and technical laboratories . For a conceptual understanding, swelling-time profile is described below.
6.4.1 Swelling-Time Profile
6.5 Swollen Gel Strength
From a practical viewpoint in specified area of application, the mechanical strength or modulus of swollen SAPs (hydrogels) is important. Rotational rheometry can be an important tool to quantify the swollen gel strength of hydrogels with conventional shape, i.e., sugar-like particles . The method works by using parallel plate geometry (plate diameter of 25 mm, gap of 3 mm) at 25 °C. The strain suggested for the experiment is to be in the linear viscoelastic (LVE) range, where the G′ and G″ are independent of the strain amplitude. Frequency test sweeps are done after a strain sweep test, and the test conditions are selected to ensure that the test is really carried out in the LVE range. In the determination of LVE, approximately 100–150 mg of dried SAP with average particle sizes of 180 μm is dispersed in 200 mL of distilled water for 30 min to reach maximum swelling. The swollen gel particles are made free of excess water and then placed on the parallel plate of rheometer, and the rheological properties are evaluated.
6.6 Soluble Fraction
6.7 Residual Monomer
Residual monomer content is of very significant in case of hydrogel materials, particularly SAPs used in hygiene product where the residual acrylic acid should be in safe level. To quantify residual monomer, high-performance liquid chromatography (HPLC) is often taken as a preferred method. Orthophosphoric acid solution is usually used as an extractant for this method. The total residual monomer is removed in form of either acid or salt from the hydrogel network and is measured in the next step. At lower pH, the acrylic salt is converted to acrylic acid both the extracting and the eluting media, i.e., mobile phase (pH < 3) . Isocratic mode at a 1.8 mL/min flow rate and ambient temperature on an analytical column (e.g., 250 × 4.6 mm, 5 μm) are used for separation. An aqueous 0.01% orthophosphoric acid is used as the mobile phase . UV-Vis absorbance can also be used to serve the purpose, in that case over the 190–400 nm range is registered and the wavelength used is 200 nm.
6.8 Ionic Sensitivity
For better performance, hydrogels with lower f are usually preferred, as higher f value indicates the higher absorbency loss of the sample swollen in salt solutions. Negative values of f are rare but happen, which reveal that the absorbency is not decreased, but it is increased in salt solutions. It is reported that the hydrogels with betaine structures exhibit such surprising behavior .
In addition to physical characterization, the synthesized product requires structural confirmation so that their application becomes flawless. Few relevant instrumental analyses give reliable data for the product. The commonly used instrumental techniques are briefly discussed.
6.9 FTIR Analysis
The observation of functional groups present in samples within the frequency range from 400 to 4000 cm−1 is generally used. Finely ground samples are mixed with KBr and pressed to form a KBr pellet. The FTIR spectra of sample (generally a characteristic peak of fuctional groups at specified wavelength) is observed by the method of transmission. This method supplies sufficient data by forming relevant peak to ensure about the functional groups that formed during reaction.
6.10 TGA and DSC
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are two reliable tools for the thermal analysis of the product. During decomposition, exothermic or endothermic nature of the hydrogel sample can be examined. TGA determines selected characteristics of materials that exhibit either by mass loss or gain due to decomposition, oxidation, or loss of volatiles (such as moisture). On the other hand, DSC measures the difference in the amount of heat required to increase the temperature of a sample and reference as a function of temperature.
6.11 SEM, TEM, and AFM
Surface morphology of the prepared hydrogels can be observed by SEM analysis. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are also two helpful tools for the study of surface morphology with high resolution. In case of SEM, the micrographs were taken at a definite magnification using (kV) accelerating voltage to observe the surface construction.
On the other hand, TEM is capable of imaging at a significantly higher resolution than light microscopes and can capture fine detail – even as small as a single column of atoms. TEM is also used to observe modulations in chemical identity, crystal orientation, electronic structure, and sample-induced electron phase shift as well as the regular absorption-based imaging. AFM serves three information about the product; force measurement determines physical properties like stiffness, image formation of the three-dimensional shape (topography) of a sample surface at a high resolution, and manipulation of forces used to change the properties of the sample in a controlled way.
6.12 XRD Analysis
X-ray diffraction (XRD) method is used to study the structure, composition, and physical properties of materials. It is a rapid analytical technique for phase identification of a crystalline material and can provide information on unit cell dimensions. XRD gives the idea of crystalline and amorphous region of the sample. One can predict about the nature of the product, how it has been transformed from one phase to another.
All diffraction methods rely on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are collected for analysis of specimen. An important factor of all diffraction is the angle between the incident and diffracted rays.
6.13 Biodegradability Test
Decomposition data carries immense importance for cellulose-based product. As it is mentioned earlier, biocompatible products are those when thrown to the environment decomposed by microorganism and leave nothing harmful for human being or animals. The two mostly exercised degradation tests are soil burial test and direct microbial studies. In the case of soil burial test, the test specimen is buried in soil, and after definite time interval, the sample is washed and weighted. The result is expressed as percent weight loss for specific time duration. At the same time, direct microbial studies also determine weight loss of the product, but the decomposition is carried out in presence of direct contact of cultured microorganism.
7 Application of Cellulose-Based Hydrogels and Their Biocompatibility
It is a proven truth that synthetic hydrogels have some better qualities, but when biodegradability or eco-friendliness of a hydrogel is required or recommended, cellulosics are prime choice hydrogel precursor materials, due to their low cost, the large availability and biocompatibility of cellulose, and the stimuli-responsive properties. Current research has unveiled the fact that cellulose-based hydrogels cannot be complete replacement of synthetic hydrogels but would be the most potential competitor. Some of the recent and conventional uses of cellulose-based hydrogels have been discussed below.
7.1 Hydrogels for Personal Hygiene Products
The recent trend is to use acrylate-based superabsorbent hydrogels in personal hygiene product to absorb body fluids. These products keep individuals dry and ensure healthy skin and personal comfort. It is observed that the majority of parents in developed countries, along with thousands of hospitals and day care centers around the world, use disposable diapers containing a superabsorbent polymer (SAP) for the infant. The use of SAP material in training pants and adult incontinence products is increasing day by day worldwide. In recent years, a number of papers have been published in the favor of the use of superabsorbent materials in personal care products and their safety and effectiveness [66, 67]. The leakage of diapers is very significant to raise the risk of fecal contamination and spread of illness in day care play areas . The SAP helps control the spread of germs keeping the skin dry and preventing diaper rash. Now it is a true fact through several medical studies that disposable diapers play an important role in reducing the risk of spread of gastrointestinal illnesses and are significantly more effective than double cloth diapers and plastic overpants . Although Harper  and Harmon  separately patented their superabsorbents in 1966, superabsorbent was used for the first time in diaper industry by the Unicharm in 1982 in Japan, following its use in sanitary napkins. The superabsorbent materials introduced a new generation of high-performance diapers . It made the diapers not only thinner but also improved retention performance compared to cloth diaper which helped reduce leakage and diaper rash . In this regard, premium diapers reduced leakage values and the average weight by below 2% and about 50%, respectively. Considering environmental and economic issue, the reduction of packaging cost was advantageous. For better understanding the current ecological impact of disposable diapers and other similar articles, it is substantial to provide a brief estimate of diaper consumption. In a published article, it has been shown that a child till the age of 30 months uses approximately six disposable diapers a day. If a diaper has an average volume of 500 cc, so one child produces on average 3000 cc of garbage a day, i.e., 1.092 cubic meters per year. As it is considered that there are about 50,000 diaper users per million population, every day it would be necessary to remove 1500 cubic meters of diapers from a city of one million inhabitants. Taking into account the benefit of recycling, different attempts have been made to recycle disposable diapers, napkins, hospital bed sheets, sanitary towels, and other similar products . The structure of all these cellulose-containing products is almost the same: an envelope of non-woven tissue, a plastic cover material, and an absorbent fluff of wood pulp cellulose, mixed in most cases with SAP. The main objective of diaper recycling is to recover separately the cellulose, which is biodegradable and recyclable, the plastic cover material, and the SAP, both of which are not biodegradable but might be recycled for other uses. The difficulties during recycling process have prompted the parallel development of biodegradable diapers, i.e., possessing a biodegradable SAP. An alternative solution has been found to replace SAP by cellulose-based hydrogels, which are totally biodegradable. Sodium carboxymethylcellulose (NaCMC)- and hydroxyethyl cellulose (HEC)-based novel hydrogels cross-linked with divinyl sulfone (DVS) possess swelling capabilities comparable with those displayed by SAP and high water retention capacities under centrifugal loads. Cellulose-based hydrogels show some advantages over current SAP that encompasses their biodegradability and environment-friendly nature. In this respect, recent innovations and total success of the cellulose-based hydrogels described above depend on the implementation of an eco-friendly production process  and the use of nontoxic cross-linking agents [75, 76]. Radiation technique for cross-linking instead of using chemicals might be a valuable alternative in the development of novel environmentally friendly superabsorbents.
7.2 Water Conservation in Agriculture
Due to continuous deficiency in surface water, the interest of using superabsorbent hydrogels in agriculture is increasing. Another important objective is to reduce water consumption and optimize water resources in agriculture and cultivation. Research articles have been published in the promotion of using a novel material to replace human habit and culture toward water so that it can be benefited to save and not make an excess to waste. It is well known to all that during the swelling process of a hydrogel, the material turns from solid to a gel-like material, which is able to store large volume of water even under significant compression. Consequently the swollen hydrogel can slowly release up-taken water through a diffusion-driven mechanism, if a gradient of humidity exists between the inside and the outside of the material. In addition up-taken water and other ingredients can be released in a controlled and sustained manner by means of diffusion. In arid and desert regions of the world, where scarcity of water resources is a burning issue, the xerogel (i.e., dry hydrogel), in the form of powder or granules, is spread in the area close to plant roots. The hydrogel can also be loaded with nutrients and/or plant pharmaceuticals. At the time of cultivation, water or nutrients mixed water is sprayed; the water is absorbed by the hydrogel, which then releases to the soil as needed, thus keeping the soil humid over long periods of time. The use of hydrogels in cultivation allows a high saving of water, which is not lost soon after the watering due to evaporation and drainage. Such distribution of the water resources available for cultivation can be used for other applications. Another observation of using hydrogels is that the swelling with water itself on the soil increases volume of dry hydrogels, and this increased volume of hydrogels changes the dimension, resulting increasing soil porosity and providing a better oxygenation to the plant roots. The aeration mechanism also suggests that large-granule hydrogels are likely to yield better results than fine-granule ones, if properly mixed with the soil. A number of commercially available products are able to absorb, retain, and release water to the soil. It has also been proven that all these products are effective in water conservation due to their extremely high water retention capacity for cultivation. It is also reported that overdosage of such products is potentially dangerous and should be avoided. Yet enough research studies have not been carried out to determine appropriate amounts of hydrogels and application rates for different environmental conditions and different plant species . It is worth mentioning that being acrylate-based, most commercial products are not biodegradable. Cellulose-based hydrogels can be the perfect alternatives to the recent trend of acrylate-based superabsorbent hydrogels. It is reported that Sannino and coworkers recently developed a novel class of totally biodegradable and biocompatible microporous cellulose-based superabsorbent hydrogels [9, 78, 79, 80], and such products are able to absorb up to 1 l of water per gram of dry material, without releasing it under compression. It is possible that hydrogel can be prepared both in form of powder and of a bulky material with a definite shape and a strong memory of its shape after swelling. The hydrogel can be entrapped with small molecules, such as nutrients, to be released under a controlled kinetic. Among the various applications of this material, a study has been performed on its specific use as water conservation in agriculture. Sorption capacity of hydrogel has been tested at different ionic strength of the swelling medium, with the expected target to simulate as much as possible the hydrogel-soil interaction. A pilot-scale production plant has been developed to prove the feasibility and the efficacy of the proposed technology and to produce the amount of hydrogel necessary for some studies being carried out in experimental greenhouses in the south of Italy, where the scarcity of water is a common problem. Early experimental results show great achievement. The soil with small quantities of the hydrogels is able to remain humid for periods more than four times longer compared to the soil watered without the presence of the hydrogel. When mixing the soil with a fine-granule gel, air flow is limited, and a layer of substrate/gel mixture might form, which further limits the flows of air and water within the soil. With a large-granular gel, a better air flow through the soil is yielded, resulting in higher oxygenation to plant roots. As the added cellulose-based hydrogels are biodegradable in nature and will decompose in soil at the end of its action, it will not make any adverse effect to soil or environment.
7.3 Body Water Retainers
Cellulose and cellulose derivatives are biocompatible, and hydrogels originated from them also show this property. In addition, they display versatile properties which suggest their use in biomedical applications. As hydrogels swell in water, cellulose-based hydrogels can be used as devices for the removal of excess water from the body, in the treatment of some pathological conditions, such as renal failure and diuretic-resistant edemas. In time of treatment, the hydrogel in powder form is taken up orally and absorbs water in its passage through the intestine, where the pH is about 6–7, without previously swelling in the acid environment of the stomach. The hydrogel is then expelled through the fecal way, thus performing its function without interfering with body functions. Cellulose hydrogels, based on NaCMC and HEC, have been investigated for such applications  due to pH sensitivity. This type of application is in research level, sufficient data are not available, but petroleum-based hydrogels might show toxic effect compared to cellulosic hydrogels.
7.4 Stomach Bulking Agents
From the findings of the World Health Organization, at least one in three of the world’s adult population is overweight and almost one in ten is obese. Moreover, there are over 20 million children under age 5 who are overweight. Obesity and overweight are responsible for several chronic diseases, such as type 2 diabetes, cardiovascular disease, sleep apnea, hypertension, stroke, and certain forms of cancer. It is also found that overweight or obese often has a dramatic impact on the psychological well-being, reducing the overall quality of life [82, 83, 84]. The treatment of overweight and obesity generally includes supervised diet or dietary supplements and is combined with adequate physical exercise. In the most cases, dietary supplements are claimed to act either by binding fats and so reducing fat absorption by human body or by directly reducing the appetite, which is done by absorbing liquids and swell inside the stomach, thus giving a sense of fullness [85, 86]. The first pathway is reported for chitosan-based products and the next for different natural fibers and herbal products. The natural fibers and herbal products use natural fillers or bulking agents that are very interesting for its great potential of reducing the amount of food intake by reducing the available space in the stomach. Though there is no clear evidence of the effectiveness of currently available bulking agents in promoting weight loss, neither in the short term nor in the long term, whereas their adverse effects, one should be taken into account the harmful effect of the products. Some hydrogels have required properties so that they can serve the purpose of using a stomach bulking agent. It is reported that novel cellulose-based hydrogels, obtained by cross-linking aqueous mixtures of NaCMC and HEC, have been shown to be appealing for the production of dietary bulking agents . The polyanionic nature of the NaCMC hydrogels provides higher swelling capabilities at neutral pHs rather than at acid ones; the swelling ratio obtained at acid pHs might still be significant for use of the hydrogel as stomach filler where the environment is acidic. Indeed cellulose-based hydrogels obtained from nontoxic cross-linking agents are particularly attractive for this kind of application .
7.5 Devices for Controlled Drug Delivery
Among various cellulose derivatives, cellulose ethers (e.g., hydroxypropyl methylcellulose (HPMC)) have long been used in the pharmaceutical industry as excipients in many drug formulations . These materials work as active ingredient carrier; they swell in presence of body fluids and release the drug molecule later. In the progress of swelling at certain period, the drug dissolves in water and diffuses out from the polymer network. The rate of drug release from hydrogel depends on the water content of the swollen hydrogel, as well as on its network parameters, i.e., degree of cross-linking and mesh size [89, 90]. In recent time not only the swelling tablets but also more sophisticated hydrogel-based devices have also been developed for controlled drug delivery. In some cases, they are not only taking less time but also can function at a specified site. The loading of the drug in a hydrogel is performed either after cross-linking or simultaneously during network formation. Smart hydrogels modify the mesh size of the hydrogel network by responding in physiologically relevant variables, such as pH, temperature, and ionic strength, and are particularly useful to control the time- and space-release profile of the drug. Cellulose-based polyelectrolyte hydrogels (e.g., hydrogels containing NaCMC) are particularly suitable for the application of oral drug delivery which is usually based on the strong pH variations encountered when transitioning from the stomach to the intestine. Anionic hydrogels based on carboxymethyl cellulose have been investigated recently for colon-targeted drug delivery . The most recent practice of controlled release through a hydrogel matrix dealt with the delivery of proteins, growth factors, and genes to specific sites, the need for which has been prompted by tissue engineering strategies. The direct delivery of drugs or proteins to different body sites requires the hydrogel biodegradation, in order to avoid foreign body reactions and further surgical removal. Injectable hydrogel formulations are particularly appealing and currently under investigation. In this case, the cross-linking reaction has to be performed under mild conditions in order not to denature the loaded molecule. Injectable formulations of hydrogels based on HPMC have been developed to deliver both biomolecules and exogenous cells in vivo . The microenvironment resulting from degradation of the polymer should be mild. Change is the mesh size of the network, which determines the free space available for diffusion and thus regulates the diffusion of molecules (e.g., drugs) through the network itself. From the above discussion, it is mentionable that petroleum-based hydrogels may include toxic effect inside the body and may remain as part of the body which requires surgery. So cellulose-based hydrogels are suitable than petroleum-based nondegradable and non-biocompatible hydrogels.
7.6 Scaffolds for Regenerative Medicine
Regenerative medicine is a branch of translational research in tissue engineering and molecular biology which deals with the induced regeneration of cells, tissues, and organs in vivo, by means of a scaffolding material or template that guides and supports the cells during the synthesis of new tissues. Cellulose-based hydrogels have large water content capacity, are biocompatible, and possess rubbery mechanical properties close to those of soft tissues and usually allow the incorporation of cells and bioactive molecules during the gelling . During regeneration, adhesion problem of cells to highly hydrophilic surfaces can be easily modified with extracellular matrix (ECM) domains of hydrogels, which promote cell adhesion as well as specific cell functions . This property gave opportunity to scientist to consider hydrogels for the design of biomimetic scaffolds for tissue regeneration. In the last decade, the excellent biocompatibility and good mechanical properties of cellulose and its derivatives have received increasing attention, as biomaterials for the design of tissue engineering scaffolds. It has been published that cellulose and its derivatives can be applied as scaffold material for the treatment of severe skin burns and in the regeneration of cardiac, vascular, neural, cartilage, and bone tissues [95, 96, 97, 98]. A few independent investigations show that cellulose-based hydrogels are potentially useful for inducing the regeneration of bone, cartilage, and neural tissues. From the various research, it has been pointed out that as the final product of cellulose degradation is glucose, which is a nutrient for cells, there are the advantages in using cellulose rather than other synthetic or natural polymers for tissue engineering applications. Independent investigations have reported that novel biomimetic hydrogels, based on cross-linking cellulose derivatives with hyaluronic acid (HA), show potential as scaffolds for regenerative medicine, with a tunable degradation rate. The degradation rate is dependent on the amount of degradable sites as well as the degree of cross-linking of the network, which affects enzyme diffusivity through the mesh size. The degradation nature of the cross-linking agent used is particularly important, especially in cases where reactive groups of the cross-linker are incorporated into the hydrogel network and might then be released upon degradation. Recently hydrogels based on HEC, NaCMC, and HA have been cross-linked with a water-soluble carbodiimide, which is both nontoxic and a “zero-length” cross-linker. It is also reported that the carbodiimide washed out from the polymer network after the synthesis; thus cellulose-based hydrogels cross-linked with carbodiimide show potential for a tunable biodegradation rate, even without containing HA . Although biodegradation behavior of hydrogels cross-linked with carbodiimide has not investigated yet. Another important factor that affects the development of regenerative templates is the scaffold porosity, which enhances the attachment, infiltration, and survival of cells within the scaffold. Several types of novel manufacturing techniques are being investigated to develop porous hydrogels and might be of great value in enhancing the regenerative potential of cellulose-based hydrogels [100, 101, 102, 103].
7.7 Wound Dressings
Some commercially available hydrogel for wound dressings containing carboxymethyl cellulose (CMC) or sodium CMC (NaCMC) are given with producer company. These are mostly available in two forms, either as sheets or as amorphous gels. Products with silver ions show antimicrobial activity
Hydrogel wound dressing (producer)
IntraSiteTM gel (Smith and Nephew)
Purilon gelTM (ColoPlast)
Aquacel AgTM (ConvaTec)
Silver ions (1.2%)
SilvercelTM (Johnson & Johnson)
Silver ions (8%)
7.8 Plastic Surgery
Hydrogel is such a field of research where multidisciplines have been involved and taking the fruit for the application of their own field.
8 Prospects of Cellulose-Based Hydrogels
Cellulose and cellulose derivatives are pioneer biocompatible materials for hydrogel synthesis. Compared to cellulose, other natural sources have scarcity all over the world, and in some cases, there is huge difference in costing. Again as much derivatives of cellulose have been practiced, but others from rest of the sources still remain unworked. Though synthetic hydrogels have better performance in some instances, non-biodegradation is the motivational factor for cellulose-based hydrogel research. Again synthesis of hybrid hydrogels through combination of some synthetic part with cellulose minimizes the property compromising factor. Research have reached in such a position that cellulose-based hydrogels have gained almost same capability of absorbency and in some published article inform better performance than a synthetic one .
Hydrogel is an interesting and attractive research topic to the scientists for the last one decade. In this review, authors tried to give an idea of synthesis, application, characterization, and the development of cellulose-based hydrogels from simple polymeric network to smart materials. Another core objective was to establish cellulose and its derivatives as the eligible alternative for the protection of environment. To meet definite demand in applied field, many researchers have gained success in preparation of their hydrogel network. Biodegradability and biocompatibility are of the prime importance where there is no room for compromise. Petrochemical-based acrylic hydrogels are dominating due to some competitive reasons, but considering environmental sustainability hydrogels from biomaterials is drawing more attention to the researchers as well as consumers. Though cellulose-based hydrogels possess typically higher cost and less performance than the synthetic one and most of the volume of world production is consuming in hygienic uses, i.e., disposable diapers (as baby or adult diapers, feminine napkins, etc.), currently cellulose-based hydrogels are applying in numerous fields like contact lenses, hygiene products, wound dressing, biomedical, tissue engineering and drug delivery. Other than hygienic products, other commercial hydrogels are still limited. Much improvements have been made in the properties of hydrogels, but further improvements need to be made to improve the applicability of hydrogels. Though there are many patents on hydrogels, only a few reached in the market. In spite of the development of hydrogels, there are some drawbacks which should be overcome. Especially in hydrogels in drug delivery and tissue engineering, more progress is expected. Considering high cost and other hindrances, the necessity of preparation of renewable source-based hydrogels seems more obvious. The motivation for making greener world will show light to the researchers in the further development of hydrogels in the near future.
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