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Cellulosic Hydrogels: A Greener Solution of Sustainability

  • Md. Ibrahim H. MondalEmail author
  • Md. Obaidul Haque
Living reference work entry
Part of the Polymers and Polymeric Composites: A Reference Series book series (POPOC)

Abstract

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.

Keywords

Cellulose Cellulosic hydrogel Cellulose derivatives Hydrophilic Renewable Biocompatible Sustainability 

1 Introduction

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 [9], alginate [10], carrageenan [11], and gellan gum [12].

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 [13], 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 [17], and starting materials for proton-exchange membranes (PEMs) in PEM fuel cells (FCs) [18].

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 [5]. Later in 1950, Otto Wichterle [13] 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 [19].

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 [20]. 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

Hydrogels can be classified from various perspectives and depend on many factors. Properties of hydrogels are governed by the sources from where they are prepared. One of the specific aims of this classification is to make an understanding about the nature of hydrogels of biocompatibility, on the basis of sources from where they are originated:
  1. (i)

    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.

     
  2. (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 [21] 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.

     
  3. (iii)

    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.

     
  4. (iv)

    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.

     
  5. (v)

    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.

     
  6. (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:
    1. (a)

      Nonionic

       
    2. (b)

      Anionic or cationic

       
    3. (c)

      Amphoteric electrolyte (ampholytic) containing both acidic and basic groups

       
    4. (d)

      Zwitterionic, containing both anionic and cationic groups in each structural repeating unit

       
     
  7. (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:
    1. (a)

      Release systems controlled by diffusion.

       
    2. (b)

      Release systems controlled by swelling.

       
    3. (c)

      Release systems controlled by chemical.

       
    4. (d)

      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 [22]. 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 [23]. 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 [24]. 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 [27].

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 [28]. 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 [29].

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 [30].

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

In principle, a product is said to be safe for sustainability when renewable, biobased materials are incorporated or made from such materials. Another perspective of sustainability is that the product goal is achieved with reduction in material consumption. It is well known that cellulose is renewable and biodegradable, and its derivatives are also highly biodegradable. Previously discussed requirements can be fulfilled by abundant renewable resource cellulose and its derivatives (Fig. 1a). Many researchers have been intentionally designed and tested to reach a suitable degree of biodegradability [25, 31]. Cost of materials is a key factor for consumer products, but conserving environment is more important. Hydrogels from nonrenewable sources might be cheap, but lack of biodegradability gives scientists motivation for development of cellulose-based and combination of cellulose and acrylic, hybrid hydrogels [32].
Fig. 1

(a) Indicates renewability of cellulose as it is the common ingredient of plant and (b) degradation of wood log by microorganism [36, 37]

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) [33]. 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 [34]. The abundance and degradation nature of cellulosic materials have trigger down the use of them in biomedical application [35]. As humans are unable to synthesize cellulose, they can’t digest cellulose. The degradation of cellulose can be expressed as.

$$ \mathrm{Cellulose}\underset{\mathrm{Hydrolysis}}{\overset{\mathrm{Cellulase}}{\to }}\mathrm{Cellobiose}\underset{\mathrm{hydrolysis}}{\overset{\mathrm{Cellobiase}}{\to }}\mathrm{Glucose} $$
$$ \mathrm{Glucose}\overset{\mathrm{Oxidation}}{\to}\mathrm{Organic}\ \mathrm{Acids}\overset{\mathrm{Oxidation}}{\to }{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O} $$

4.2 Potentiality of Cellulose and Its Derivatives for Hydrogel Synthesis

The outer surface of cellulose is crystalline regions which can interact with water but unable to imbibe any water [38]. The amorphous region of cellulose and hemicellulose can absorb water and swell [39]. Derivatization of cellulose makes it more biodegradable. The reaction of the hydroxyl groups of cellulose with organic species, such as methyl and ethyl units, gives most water-soluble cellulose derivatives via etherification. Cellulose derivatives with given solubility and viscosity in water solutions can be obtained by controlling the degree of substitution (as the average number of etherified hydroxyl groups in a glucose unit). The most widely used cellulose derivatives are cellulose ethers, such as methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), and sodium carboxymethyl cellulose (NaCMC). Figure 2 represents the chemical structure of cellulose and its derivatives. Many researchers have synthesized hydrogels from the abovementioned cellulose derivatives [25, 31]. NaCMC, among the other cellulose derivatives, is polyelectrolyte in nature, shows sensitivity to pH and ionic strength variations, and thus is a “smart” cellulose derivative. Due to a Donnan-type effect [40], the polyelectrolyte nature of NaCMC assists NaCMC-based hydrogels a double effect on the swelling capability.
Fig. 2

(a) Repeating unit of cellulose, also termed “cellobiose.” (b) Repeating unit of cellulose derivatives. The substituent group “R” is indicated for methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), and sodium carboxymethylcellulose (NaCMC)

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 [43] 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 [44]. 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 [33]. 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 [34]. 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) [48] and other performances.

5 Methods of Preparation of Cellulose-Based Hydrogels

In a very simple sense, hydrogels are hydrophilic polymeric network cross-linked in some fashion to produce elastic structures. As a result, any suitable technique can be used to create a cross-linked polymer to produce a hydrogel. Among various techniques, copolymerization/cross-linking free-radical polymerizations are commonly used to produce hydrogels by reacting hydrophilic monomers with multifunctional cross-linkers. By using any of the polymerization techniques (bulk, solution and suspension polymerization), the formation of the gels is possible. The integral components of whole process of hydrogel formation are monomers, initiators, and cross-linking agents. In addition, diluents like water or other aqueous solutions can be used to control the heat of polymerization. In the final stage, the end product requires washing to remove impurities left from the preparation process like non-reacted monomer, initiators, cross-linkers, and unwanted products produced via side reactions showed in Fig. 3b as a general scheme for preparation. In this respect, water-soluble monomers from both natural and synthetic origins are polymerized and then cross-linked to form hydrogels in a number of ways:
  1. 1.

    Attachment of polymer chains via chemical reaction

     
  2. 2.

    Generation of main chain free radicals using ionizing radiation which can recombine as cross-link junctions

     
  3. 3.

    Physical interactions such as entanglements, electrostatics, and crystallite formation within chains

     
Fig. 3

(a) General scheme for hydrogel preparation and (b) schematic diagram for hydrogel preparation

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 [49]. 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

For limited amounts of samples, this method is the most convenient, fast, and suitable (M0 = 0.1–0.3 g, sample weight) [53, 54, 55]. The hydrogel sample is placed into a tea bag (acrylic/polyester gauze with fine meshes), and the bag is dipped in an excess amount of water or saline solution for 1 h to reach the equilibrium swelling. Then excess solution is removed by hanging the bag until no liquid is dropped off. The tea bag is weighed (M1), and the swelling capacity is calculated by Eq. (1). The method’s precision has been determined to be around ±3.5%.
$$ \mathrm{Se}=\left({\mathrm{M}}_1-{\mathrm{M}}_0\right)/{\mathrm{M}}_0, $$
(1)
where Se is the equilibrium swelling capacity

Centrifuge method is more accurate than the tea bag method and is occasionally reported in patents and data sheets [20]. Sieve method requires a large amount of sample (1–2 g) and is also called filtering and rubbing method [56].

6.2 Absorbency Under Load

The absorbency under load (AUL) is also another representation of absorption data which is more reliable and authentic. Because of this, usually it is given in the patent literature and technical data sheet by industrial hydrogel (SAP) manufacturers [57]. The AUL is generally measured under some specified conditions but without specifying its swelling media. The process measures an uptake of 0.9% NaCl solution, while the test sample is pressurized by some loads (often specified to be pressures 0.3, 0.6, and 0.9 psi). For testing, a typical AUL tester (Fig. 4) is a simple but finely made device including a macro-porous sintered glass filter plate (porosity # 0, d = 80 mm, h = 7 mm) placed in a petri dish (d = 118 mm, h = 12 mm). During experiment, the weighed dried hydrogel sample (0.90 ± 0.01 g) is uniformly placed on the surface of polyester gauze located on the sintered glass. To apply load, a cylindrical solid load (Teflon, d = 60 mm, variable height) is put on the dry hydrogel particles, while it can be freely slipped in a glass cylinder (d = 60 mm, h = 50 mm). For the calculation of AUL, desired load (applied pressure 0.3, 0.6, or 0.9 psi) is placed on the hydrogel sample (Fig. 4), and saline solution (0.9% NaCl) is added up to the height of the sintered glass filter. To prevent surface evaporation and probable change in the saline concentration, the whole set is covered. The swollen particles are weighed again, and AUL is calculated after 60 min, using Eq. (2):
$$ {\mathrm{AUL}}_{\left(\mathrm{g}/\mathrm{g}\right)}=\frac{{\mathrm{M}}_2-{\mathrm{M}}_1}{{\mathrm{M}}_1} $$
(2)
where M1 and M2 denote the weight of dry and swollen SAP, respectively. The AUL indicates the swollen gel strength of SAP materials [58].
Fig. 4

(a) A typical AUL tester picture and (b) various parts [59]

6.3 Wicking Capacity and Rate

The wicking capacity (WC) of SAP (hydrogels) materials is a measurement of capillary action. To quantify this, a simple test has been suggested by pioneering researchers Fanta and Doane [60] with conventional physical appearance, i.e., sugar-like particle. The test is accomplished as hydrogel sample (M1 = 0.050 ± 0.0005 g) is added to a folded (fluted) filter paper cone prepared from an accurately tared circle of 9 cm Whatman 54 paper. To settle the sample into the tip, the cone was lightly tapped, and the tip of the cone is then held for 60 s in a 9 cm Petri dish containing 25 mL of water. Within a minute, water wicks up the entire length of the paper, and excess water is allowed to drain from the paper by contacting the tip for 60 s with a circle of dry filter paper on a square of absorbent towel. For calculation, weight of wet paper plus swollen polymer is determined (A), and the absorbency of the sample in g/g is then determined after correcting for the weight of dry paper and the amount of water absorbed under identical conditions by the paper alone in the absence of sample (Eq. 3). Repetition for 3–5 times each test is preferred and the results are averaged.
$$ \mathrm{WC}=\left(\mathrm{A}-\mathrm{B}-{\mathrm{M}}_1\right)/{\mathrm{M}}_1 $$
(3)
where B is weight of wet paper without polymer.

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 [61]. For a conceptual understanding, swelling-time profile is described below.

6.4.1 Swelling-Time Profile

Actually this is a graphical representation of swelling versus time and is obtained via separating swelling measurements of sample absorbed desired fluid at consecutive time intervals. For swelling measurement, any of the methods (tea bag, centrifuge, or sieve methods) can be used for the measurements depending on the amount of the available sample and the desired precision. This profile is helpful to the study of swelling kinetics of hydrogels. In a measurement, several 2 L Erlenmeyer flasks containing distilled water or desired solution are labeled, and SAP sample (e.g., 1.0 g, 50–60 mesh) is poured into each flask and is dispersed with mild stirring. The absorbency of the sample is measured by the abovementioned method at consecutive time intervals (e.g., 15, 30, 45, 60, 90, 120, 180, 300, 600, 1800 s). A typical profile is shown in Fig. 5.
Fig. 5

Representative curve for swelling kinetics of a hybrid SAP sample in distilled water [61]

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 [62]. 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

Soluble fraction (sol) for a hydrogel is an important criterion. It is a sum of all water-soluble species including non-cross-linked oligomers, β-hydroxypropionic acid (HPA), and non-reacted starting materials such as residual monomers. The amount is simply measured by extraction of hydrogel sample in distilled water (because of this, the sol is frequently referred to “extractable”). For calculation, a certain amount of the hydrogel sample (e.g., 0.10 g) is poured into excess amount of water and dispersed with mild magnetic stirring to reach equilibrium swelling (0.5–3 h depending on the sample particle size), filtering the sample after swelling and dried in oven. The weight loss of the sample is the soluble fraction. The gel content which is also important for a synthesized hydrogel can also be obtained by simple Eq. (4). The equation indicates that gel content may be taken as an actual yield of the cross-linking polymerization.
$$ \mathrm{Sol}\ \left(\%\right)+\mathrm{Gel}\ \left(\%\right)=100 $$
(4)

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) [63]. 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 [64]. 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

It is a measure of sensitivity of hydrogel materials toward the kind of aqueous fluid absorbed. To quantify a dimensionless swelling, factor, f, is defined as follows (Eq. 5) [65]:
$$ f=1-\left(\mathrm{absorption}\ \mathrm{in}\ \mathrm{a}\ \mathrm{given}\ \mathrm{fluid}/\mathrm{absorption}\ \mathrm{in}\ \mathrm{distilled}\ \mathrm{water}\right) $$
(5)

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 [53].

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 [68]. 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 [69]. Although Harper [70] and Harmon [71] 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 [72]. 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 [73]. 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 [74] 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 [77]. 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 [81] 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 [87]. 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 [52].

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 [88]. 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 [91]. 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 [92]. 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 [93]. 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 [94]. 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 [99]. 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

To release people from pain, expressing wound dressing is a must during wound healing progression, and application of hydrogels in this field is a recent practice. Protecting the wound from infection is important particularly in cases of chronic wounds (e.g., ulcers), appropriate wound dressing is essential. In such cases, a moist environment encourages rapid healing, and hydrogels are suitable for the development of wound dressings, either as sheets or in amorphous form. Cross-linked amorphous hydrogels with low viscosity may be packaged in tubes or in foil packets to serve the purpose. The gels are reinforced with a gauze or a polymeric mesh to allow an easy removal and prevent gel liquefaction. Accurate moisture balance in the wound bed is necessary, and hydrogels should be designed to maintain moisture by hydrating the wound surface or absorbing the wound exudates. Hydrogels provide nonadherent dressings which result easy removal without any damage to the wound bed, and transparency of it is a further advantage in this application to monitor easily. A number of hydrogel dressings have been patented so far and are available in the market, based on synthetic or natural polymers or a combination of them. The recent patented hydrogels are in situ forming gels (e.g., based on sprayable formulations and consisting nanoparticles with antimicrobial agents and those of radiation cross-linked. In this regard, bacterial cellulose (BC) has been widely investigated for wound healing due to its purity and high water retention capacity, and they are available in the market. Some commercially available hydrogel dressing contains NaCMC, in combination with propylene glycol, which works as a humectant and a preservative (Table 1). It should be mentioned that the products developed are usually indicated for the treatment of specific wounds and often require the use of secondary dressings. Thus, currently a number of investigations have shown that the development of cellulose-based novel hydrogels cross-linked with hyaluronic acid for wound dressings with improved performance [75]. Wound dressing requires nontoxic, anti-allergic, transparent, and indicative for specified wounds which can be met by cellulose-based hydrogels, as in some cases cellulosic hydrogels give better options to the consumers than hydrogels from nonrenewable sources.
Table 1

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)

Composition

IntraSiteTM gel (Smith and Nephew)

Water

Propylene glycol

NaCMC

GranuGELTM (ConvaTec)

Water

NaCMC

Propylene glycol

Pectin

Purilon gelTM (ColoPlast)

Water

CMC

Calcium alginate

Aquacel AgTM (ConvaTec)

NaCMC

Silver ions (1.2%)

SilvercelTM (Johnson & Johnson)

Calcium alginate

CMC

Silver ions (8%)

7.8 Plastic Surgery

As hydrogels resemble very close to body tissue, attempts were made to introduce hydrogels like new materials for plastic reconstruction (Fig. 6). In this case, hyaluronic acid (HA) considered the panacea to resolve the problem as one notable company MacrolaneTM was using HA in tissue-filling applications to enhance breast size and shape and offer a more biocompatible alternative to standard and aggressive silicone prosthesis [104].
Fig. 6

Hydrogels in different forms of use

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 [48].

9 Conclusion

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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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Polymer and Textile Research Laboratory, Department of Applied Chemistry and Chemical EngineeringUniversity of RajshahiRajshahiBangladesh

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