Effect of Irradiation for Producing the Conductive and Smart Hydrogels
This review presents the past and current efforts with a brief description on the featured properties of conductive and smart hydrogel fabricated from biopolymers and natural ones for different applications. Many endeavors have been exerted during the past 10 years for developing new smart hydrogels. This review mainly focuses on the effect of different irradiation methods for improving the properties of smart hydrogels. As the hydrogels with single component have low mechanical strength, recent trends have offered composite or hybrid hydrogel membranes to achieve the best properties. So this chapter provides the reader good information about the irradiation effects on producing the smart conductive hydrogels and perspective on further potential developments.
KeywordsHydrogel Conductive Smart Irradiation Cellulose
Hydrogels are cross-linked polymeric networks that have the ability to swell when suspended in water. These absorbent polymers have a wide range of applications, such as sustained-release drug delivery systems, contact lenses, biosensors, personal care products, and medical, pharmaceutical, and agricultural fields. Hydrogels are macromolecular networks able to absorb and release water solutions in a reversible manner, in response to specific environmental stimuli. Such stimuli-sensitive behavior makes hydrogels appealing for the design of “smart” devices, applicable in a variety of technological fields. However, the functional hydrogel displays poor biocompatibility and biodegradability due to petrochemical products as the raw material. Therefore, using renewable resources as raw materials to prepare hydrogels is getting more and more attention. Temperature- and pH-sensitive hydrogels were prepared successfully by using natural polymers and their derivatives, such as chitosan, sodium alginate, dextran, cellulose, etc., as base materials. Cellulose and its derivatives have demonstrated to be versatile materials with unique chemical structure which provides a good platform for the construction of hydrogel networks with distinctive properties as respects swelling ability and sensibility to external stimuli. Indeed, the high density of free hydroxyl groups in the cellulose structure makes them become a solid substrate that can undergo functionalization, allowing the production of new materials for novel advanced applications. From the last decades, there is a growing demand for conductive devices with high performance due to the fast development of electronic industry. Lightweight, high conductivity, and flexibility as well as environmental friendliness are some requirements for these products. Considering the extensive possibilities to use the new materials, this chapter introduces the information on the intelligent conductive cellulose hydrogels, which are able to respond to environmental changes by modification of their characteristics, and finally presents their possible applications in different fields. From this reason, the present chapter covers the applications of conductive cellulose-based hydrogels in different fields of industry such as the pharmaceutical and biomedical area. Moreover, a series of initiated techniques have been proposed for the production of cellulose-based hydrogels, such as chemical initiation, chemical cross-linking, UV-curing technique, and microwave, plasma, and gamma-ray irradiation. So this chapter provides the reader good information about the irradiation effects on producing the smart conductive hydrogels and perspective on further potential developments.
Hydrogels are three-dimensional, hydrophilic, polymeric networks capable of absorbing large amounts of water or biological fluids. Due to their high water content, porosity, and soft consistency, they closely simulate natural living tissue, more so than any other class of synthetic biomaterials. Hydrogels may be chemically stable, or they may degrade and eventually disintegrate and dissolve. Hydrogels are called “reversible” or “physical” gels if molecular entanglements and/or secondary forces such as ionic, H-bonding, or hydrophobic forces play the main role in forming the network .
The chemical cross-linking, physical entanglement, hydrogen bonds, and ionic bonds are responsible to achieve the network of hydrogels. They can be obtained from the synthetic and natural polymers and depend on various parameters, including the preparation method, charge, as well as mechanical and structural characteristics. It has been reported that the swelling of hydrogels is a complex process comprising of a number of steps. In the first step, the polar hydrophilic groups of the hydrogel matrix are hydrated by water, which appears in the form of primary bound water. In the second step, the water also interacts with the exposed hydrophobic groups, which appear in the form of secondary bound water. The primary bound water and the secondary bound water both form the total bound water. In the third step, the osmotic driving force of the network toward infinite dilution is resisted by the physical or chemical cross-links, so additional water is absorbed. The water absorbed into the equilibrium swelling is called the bulk water or the free water, which fills the spaces between the network or chains and the center of the larger pores. The amount of water absorbed by a hydrogel depends on the temperature and the specific interaction between the water molecules and the polymer chains, which can be explained by the Flory-Huggins theory .
It is worth noting that natural polymers have better biocompatibility and less latent toxic effect than most synthetic polymer hydrogels, so pure natural polymer hydrogels would be more suitable for biomaterials. Indeed, polysaccharide-based hydrogels behave as smart materials and offer a variety of properties that can be exploited in several applications. According to those abovementioned, cellulose represents the most abundant renewable and biodegradable polymeric material, being considered as the main constituent of plants and natural fibers. Also, cellulose is an environmentally friendly alternative to conventional materials and exhibits properties that make them very attractive in many applications. Nowadays, cellulose derivative-based hydrogel has gained a great popularity in agriculture and pharmaceutical industry and become more and more important in these fields, owing to the production of the new derivatives with extended applications [4, 5].
On the other hand, intelligent hydrogels, which exhibit sensitive responses to environment, such as temperature, pH, salt, electric field, and chemical environment, are widely applied in sensors, drug release, tissue engineering, and other fields. Various initiated techniques have been proposed for the preparation of smart hydrogels in the past few decades which include chemical initiation, UV-curing technique, microwave or gamma-ray irradiation, etc., but most of the chemical technologies had residual monomers and initiators and in some cases high energy consumption .
2.1 Cellulose Hydrogels
Most of the hydrogels are made from synthetic hydrophilic polymers such as poly (acrylic acid) or its copolymer with poly (acrylamide), but due to their biodegradability and low cost, the demand for using natural hydrogels such as starch, cellulose, chitosan, and alginate is continuously increasing. Cellulose is the most abundant natural polymer and a very promising raw material with low cost for the preparation of its various derivatives .
Cellulose is the most abundant naturally occurring polymer of glucose, found as the main constituent of plants and natural fibers such as cotton and linen. Some bacteria (e.g., Acetobacter xylinum) are also able to synthesize cellulose. Microbial or bacterial cellulose (BC) is chemically identical to plant cellulose (PC), although possessing different macromolecular structures and physical properties. In both BC and PC, the glucose units are held together by 1,4-β-glucosidic linkages, which account for the high crystallinity of cellulose (usually in the range 40–60% for PC and above 60% for BC) and its insolubility in water and other common solvents. However, BC biosynthesis yields nano-sized fibers, which are about two orders of magnitude smaller than PC fibers. BC cellulose thus shows a peculiar, ultrafine fiber network with high water holding capacity and superior tensile strength compared to PC. Moreover, BC is totally pure, unlike PC which is usually associated with other biogenic compounds, such as lignin and pectin.
Most water-soluble cellulose derivatives are obtained via etherification of cellulose, which involves the reaction of the hydroxyl groups of cellulose with organic species, such as methyl and ethyl units.
The degree of substitution, defined as the average number of etherified hydroxyl groups in a glucose unit, can be controlled to a certain extent, in order to obtain cellulose derivatives with given solubility and viscosity in water solutions. Cellulose-based hydrogels, either reversible or stable, can be formed by properly cross-linking aqueous solutions of cellulose ethers, such as methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), and sodium carboxymethyl cellulose (NaCMC). Among the abovementioned cellulose ethers, only NaCMC is a polyelectrolyte and thus a “smart” cellulose derivative which shows sensitivity to pH and ionic strength variations [8, 9, 10].
2.2 Application of Cellulose Hydrogels
2.2.1 Superabsorbents for Personal Hygiene Products
A number of studies have been published in recent years documenting the advantages resulting from the use of superabsorbent materials in personal care products and their safety and effectiveness. In addition to keeping skin dry and preventing diaper rash, the SAP helps control the spread of germs in group care settings. The basic idea 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 complexity of such a process has prompted the parallel development of biodegradable diapers, i.e., possessing a biodegradable plastic cover, which, however, still contain the nondegradable acrylate-based SAP. An alternative solution to the problem of SAP recycling has been recently suggested and has been envisaged in the use of cellulose-based hydrogels, which are totally biodegradable. Novel hydrogels, based on sodium carboxymethyl cellulose (NaCMC) and hydroxyethyl cellulose (HEC) cross-linked with DVS, possess swelling capabilities comparable with those displayed by SAP and high water retention capacities under centrifugal loads. Cellulose radiation cross-linking, which does not require the use of further chemicals, might be of value in the development of novel environmentally friendly superabsorbents .
2.2.2 Water Reservoirs in Agriculture
There is an increasing interest in using superabsorbent hydrogels in agriculture. A few research studies have been carried out to determine appropriate hydrogel amounts and application rates for different environmental conditions and different plant species. It is worth noting that, being acrylate based, most commercial products are not biodegradable. Cellulose-based hydrogels fit perfectly in the current trend to develop environmentally friendly alternatives to acrylate-based superabsorbent hydrogels. Sannino and coworkers recently developed a novel class of totally biodegradable and biocompatible microporous cellulose-based superabsorbent hydrogels. Such hydrogels are able to absorb up to 1 l of water per gram of dry material, without releasing it under compression .
2.2.3 Body Water Retainers
Due to the intrinsic biocompatibility of cellulose, together with the biocompatibility and the versatile properties displayed by hydrogels in biomedical applications, cellulose-based hydrogels are appealing materials for a number of applications in vivo. For example, hydrogels hold promise as devices for the removal of excess water from the body, in the treatment of some pathological conditions, such as renal failure .
2.2.4 Devices for Controlled Drug Delivery
Cellulose ethers have long been used in the pharmaceutical industry as excipients in many drug device formulations. Their use in solid tablets allows a swelling-driven release of the drug as physiological fluids come into contact with the tablet itself. The cellulose ether on the tablet surface (e.g., HPMC) starts to swell, forming chain entanglements and a physical hydrogel. As swelling proceeds from the swollen surface to the glassy core of the tablet, the drug progressively dissolves in water and diffuses out from the polymer network. The rate of drug release 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. Controlled release through oral drug delivery is usually based on the strong pH variations encountered when transitioning from the stomach to the intestine .
2.2.5 Scaffolds for Regenerative Medicine
Due to their large water content, hydrogels are highly biocompatible, possess rubbery mechanical properties close to those of soft tissues, and usually allow the incorporation of cells and bioactive molecules during the gelling. In the last decade, the use of cellulose and its derivatives as biomaterials for the design of tissue engineering scaffolds has received increasing attention, due to the excellent biocompatibility of cellulose and its good mechanical properties. Nonetheless, it is well known that a bio-durable material or a too slow degradation may lead to undesired biological responses (e.g., a foreign body reaction) in the long term, which limit the possible applications of cellulose in regenerative medicine. The mechanical stiffness of the hydrogel is also dependent on the degree of cross-linking and should be designed according to the tissues being addressed. Furthermore, the carbodiimide-mediated cross-linking reaction holds promise for the functionalization of cellulose with several biomolecules, able to promote specific cell functions, due to the ability of the carbodiimide to cross-link various polypeptides. This opens a wide range of possibilities for the design of biomimetic, cellulose-based hydrogel scaffolds for tissue engineering .
2.2.6 Wound Dressings
Appropriate wound dressings are designed to promote healing while protecting the wound from infection. Various types of hydrogel dressings have been patented so far and are commercially available, based on synthetic or natural polymers, or a combination of them. Due to its purity and high water retention capacity, bacterial cellulose (BC) has been widely investigated for wound healing, and a series of BC-based wound dressings are currently marketed. To the best of our knowledge, gel-forming cellulose derivatives, such as NaCMC, are included in the formulation of some commercially available hydrogel dressings, usually in combination with propylene glycol, which works as a humectant and a preservative .
3 Smart Hydrogels
Intelligent hydrogels, which exhibit sensitive responses to environment, such as temperature, pH, salt, electric field, and chemical environment, are widely applied in sensors, drug release, tissue engineering, and others.
3.1 Temperature- and pH-Sensitive Hydrogels
The most important kind of intelligent hydrogels has been paid special attention. Conventional preparations for temperature- and pH-sensitive hydrogels are radical polymerization, interpenetrating to introduce thermosensitive or pH-sensitive monomers, such as N-isopropyl acrylamide (NIPAAm) and acrylic acid (AA), and then the temperature- and pH-sensitive hydrogels with three-dimensional network are formed under the effect of cross-linking agents. However, the functional hydrogel displays poor biocompatibility and biodegradability due to petrochemical products as the raw material. Therefore, using renewable resources as raw materials to prepare hydrogels is getting more and more attention. Temperature- and pH-sensitive hydrogels were prepared successfully by using natural polymers and their derivatives, such as chitosan, sodium alginate, dextran, and cellulose as base materials, and all showed good performance. Moreover, hemicellulose, one of the three major components in plant biomass, exists in all cytoderm of plants (constituting about 20–30% of the biomass). Statistical information shows hemicellulose is one of the most abundant and cheapest renewable resources. But as a result of the complexity and diversity of its structure, the application in intelligent hydrogels was rarely reported [5, 6].
3.1.1 pH-Responsive Hydrogels
Some researchers reported polymeric hydrogels with ionic pendant groups that can accept or donate protons in response to an environmental pH change. In a pH-responsive hydrogel at a specific pH, the degree of ionization known as pKa or pKb is dramatically changed. This rapid change in the net charge of the ionized pendant group causes a sudden volume transition by generating electrostatic repulsive forces between the ionized groups, which creates a large osmotic swelling force. There are two types of pH-responsive hydrogels: anionic and cationic hydrogels. Anionic hydrogels have pendent groups such as carboxylic or sulfonic acid, where deprotonation occurs when the environmental pH is above the pKa leading to the ionization of the pendent groups, which, in turn, increases the swelling of the hydrogel. On the other hand, cationic hydrogels contain pendent groups such as amine groups, where ionization takes place below the pKb, which increases swelling due to the increased electrostatic repulsions [5, 6].
3.1.2 Temperature-Responsive Hydrogels
Temperature-sensitive hydrogels are defined by their ability to swell and shrink when the temperature changes in the surrounding fluid, which means the swelling and deswelling behavior mostly depends on the surrounding temperature. Temperature-responsive hydrogels can be classified as positive or negative temperature-responsive systems [5, 6].
188.8.131.52 Positive Temperature Hydrogels
Positive temperature hydrogels are known by the upper critical solution temperature (UCST).
This means that when the temperature is below the UCST, the hydrogels contract and release solvents or fluids from the matrix (dehydration). At temperatures higher than the UCST, swelling takes place. In view of the above, it can be concluded that these types of hydrogels are retrogressive at negative temperatures. Positive temperature hydrogels shrink at low temperatures because of the formation of a complex structure by the hydrogen bonds. The structure dissociates at a high temperature due to the breaking of the hydrogen bonds, and the gel will swell to the maximum possible extent rapidly above the UCST. There are a lot of polymers and copolymers that are positively temperature dependent, such as poly (AAm-co-BMA) acrylic acid (AA) and N-isopropyl acrylamide (NIPAAm) as monomers and N,N-methylene double acrylamide (MBA) as cross-linking agents [5, 6].
184.108.40.206 Negative Temperature-PHGs
This kind of hydrogel has a critical parameter called low critical solution temperature (LCST), which means that the hydrogels will shrink when the temperature increases above the LCST and will show a swelling behavior when lower than the LCST. The LCST is the most important parameter for negative temperature-sensitive hydrogels and can be changed in different ways, such as by mixing a small amount of ionic copolymer in the gels or by changing the solvent composition. In general, the LCST of a polymer with more hydrophobic constituent shifts to lower temperatures. By changing the ratio of hydrophobic to hydrophilic content of the structure of hydrogels, the LCST will be changed. Such hydrogels have two parts: the first is the hydrophilic part, CONH, and the second is hydrophobic part, R. At temperatures lower than the LCST, water or fluid interacts with the hydrophilic part by forming hydrogen bonds. Because of these hydrogen bonds, the dissolution and swelling will improve. As the temperature increases to greater than the LCST, the hydrophobic interaction with the hydrophobic part will be stronger, while at same time, the hydrogen bonds will become weaker. Therefore, shrinking of sample will occur due to the interpolymer chain association, and the absorbed fluid will go out through a deswelling process. An example is the PVP/PNIPAAm-based negatively thermosensitive drug release hydrogel [5, 6].
3.2 Stimuli-Responsive Hydrogels
Stimuli-responsive hydrogels respond to environmental stimuli and experience unexpected changes in their growth actions, network structure, mechanical strength, and permeability, hence called environmentally sensitive, smart hydrogels. Physical stimuli include light, pressure, temperature, electric fields, magnetic fields, mechanical stress, and the intensity of various energy sources, which change molecular interactions at critical onset points. Chemical stimuli include pH, ionic factors, and chemical agents, which change the interactions between polymer chains and solvents and between polymer chains at the molecular level.
Another class, which is called dual-responsive hydrogels, results from a combination of two stimuli-responsive mechanisms in one hydrogel system. Polyacrylic acid-co-polyvinyl sulfonic acid is an example of a dual-responsive polymer system. A biochemical stimulus involves the responses to ligand, enzyme, antigen, and other biochemical agents. So, stimuli-responsive hydrogels are attractive biomaterials for pharmaceutical, biomedical, and biotechnology applications [5, 6].
4 Conductive Hydrogels
Electroconductive hydrogels (ECHs) are polymeric blends or components that combine integrally conductive electroactive polymers (CEPs) with vastly hydrated hydrogels. Electroconductive hydrogels belong to the general class of multifunctional smart materials. As an emergent class, these materials seek to creatively combine the inherent properties of constituent materials, to give rise to technologically relevant properties for devices and systems as a bio-recognition membrane layer in various biosensors. In one instance, an electroconductive hydrogel that was synthesized from a poly(HEMA)-based hydrogel and poly(aniline) was fashioned into a biosensor by the incorporation of the recombinant cytochrome P450-2D6. This electroconductive hydrogel was subsequently fully characterized for its electrical, switching, and optical properties and demonstrated faster switching than its purely CEP counterpart. In another instance, an electroconductive hydrogel fashioned from poly (hydroxyethyl methacrylate) [poly(HEMA)] and polypyrrole (PPy) was investigated for its potential application in the clinically important biomedical diagnostic biosensors, by the incorporation of analyte-specific enzymes. Among the various devices for which electroconductive hydrogel polymers were investigated are neural prosthetic and recording devices, electro-stimulated drug release devices, and implantable electrochemical biosensors. In all cases, these polymeric materials, which are both electronically and ionically conductive, provided a non-cytotoxic interface between the device and native living tissue or cell culture medium .
Conducting polymer hydrogels (CPHs) represent a unique class of materials that synergize the advantageous features of both the hydrogels and organic conductors. Conducting polymer hydrogels provide an excellent interface between the electronic-transporting (electrode) and the ionic-transporting phases (electrolyte), between natural and synthetic biological systems and between soft and hard materials. As a result, conducting polymeric hydrogels demonstrated promising results for a broad range of recent applications, ranging from energy storage devices, such as biofuel cells and super capacitors, to molecular and bioelectronics and medical electrodes. CPHs have been shown to provide excellent process ability and can be easily cast into thin films and any desired shapes at its gelation stage. CPHs can also be ink-jet printed or screen printed into micro-patterns. Hydrogels, based on conducting polymers, combine the several characteristics of polymeric hydrogels with the electrical and optical characteristic of metals or semiconductors, thus offering an array of features, such as intrinsic 3D microstructured conducting frameworks that promote the transport of charges, ions, and molecules .
5 Application of Smart and Conductive Hydrogels
Polyaniline (PANi), polypyrrole (PPy), and polythiophene (PTh) structures are some suitable raw materials. The synthetic routes toward conducting polymeric hydrogels include synthesizing a conducting polymer/monomer within a three-dimensional network of hydrogel. Through these methods, nonconductive hydrogels can be converted into conducting hydrogels via in situ polymerization technique of the electrically conductive polymers (ECPs) into the preformed hydrogel three-dimensional network.
Tang et al. developed poly(acrylateaniline)-based conducting hydrogels by employing in situ polymerization procedure [12, 13]. The first step includes the preparation of cross-linked polyacrylamide hydrogel in powder form, followed by in situ polymerization of absorbed aniline in the swollen hydrogel powder by employing potassium persulfate as an initiator. Furthermore, conducting polymer hydrogels (CPHs) have been prepared by various methods, such as the use of electrically conductive polymer nanoparticles encapsulated into the three-dimensional hydrogel network. Recently, a few novel conducting hydrogels were developed based on only one material, such kind of materials (e.g., polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh), etc.) as the continuous phase (conducting polymeric hydrogels).
Conducting polymer hydrogels (CPHs) have been applied as potential candidates in chemical mimicry of neural networks, implantable electrochemical biosensors, electro-stimulated drug release, etc. CPHs have been proposed as potential conductive flexible electrodes for super capacitor applications, and also it can be used for bioelectronics and energy storage devices and as glucose enzyme biosensors with high sensing speed and sensitivity applications. CPHs have promising applications in lithium-ion battery technology due to their excellent electronic and electrochemical properties. For instance, CPHs can be used to address the challenges faced by next-generation high-capacity alloy-based anodes, such as silicon and germanium. CPHs are a special class of polymeric hydrogels with potential advanced application in bioactive electrode coating, actuators, and tissue engineering field. Another important application has been found in biosensors, which integrate biological sensing elements, such as enzymes, antibodies, nucleic acids, cells, etc. with an electronic transducer equipped with an electronic amplifier.
CPHs possess a number of advantages, such as providing improved electrode interface between the electronic and ionic transport phases, a possibility of casting into different, complex, and flexible shapes and the possibility for the preparation of micro-patterns by ink-jet printing or spray coating .
To assist in identifying the utility of novel materials in drug delivery applications, one study investigated the use of bacterial cellulose (BC), a natural biopolymer, in the synthesis of hydrogels for drug delivery systems. The results of swelling and in vitro drug release studies revealed the hydrogels to be both thermo and pH responsive. Such thermo and pH responsiveness, in addition to their morphological characteristics, suggests that these BC/AA hydrogels are promising candidates as controlled drug delivery systems .
In the other research, graphene oxide reinforced regenerated cellulose/polyvinyl alcohol (GO–RCE/PVA) ternary hydrogels have been successfully prepared via a repeated freezing and thawing method in NaOH/urea aqueous solution. The effect of GO content on the mechanical properties, swelling behavior, and water content of composite hydrogels was investigated. With the addition of 1.0 wt% GO, the tensile strength was increased by 40.4% from 0.52 MPa to 0.73 MPa, accompanied by the increase of the elongation at break (from 103% to 238%). Meanwhile, GO–RCE/PVA ternary hydrogels performed the excellent pH sensitivity, and the higher pH leaded to higher swelling ratio. With 0.8 wt% GO loading, the swelling ratio of GO–RCE/PVA ternary hydrogel was improved from 150% (pH = 2) to 310% (pH = 14). In addition, a slight increase in the water content of the ternary hydrogel was achieved with increasing concentrations of GO. It is believed that this novel ternary hydrogel is a promising material in the application of biomedical engineering and intelligent devices .
In the other research, conductive hydrogel composed of microcrystalline cellulose (MCC) and polypyrrole (PPy) was prepared in ionic liquid; and hydrogels showed relatively high electrical conductivity, up to 7.83 × 10−3 S/cm .
Lignosulfonate (Lig), a biopolymer derivative, is a good candidate to be used as super capacitor electrode material because of its electroactive components. The methoxy and phenolic functional groups in Lig can convert to quinone groups through the redox process and function for energy storage. However, it is difficult to directly utilize Lig’s electro activity, because of its insulativity. So a composite of Lig/graphene hydrogel (Lig-GH) has been fabricated from a mixture of Lig and graphene oxide (GO) via convenient hydrothermal process by Xiong et al. The results disclosed that the Lig-GH prepared at a mass ratio of Rm = 3:4 and temperature of 180 °C exhibited excellent electrochemical performance. The as-prepared hydrogel possessed high specific capacitance of 549.5 F g−1 at a current density of 1 A g−1 in 0.1 M HClO4 electrolyte. It also showed excellent cycling stability: 83.7% capacitance retention after 1000 cycles at 20 A g−1. This work provides an innovative strategy to prepare high-performance Lig-GH super capacitor electrodes material by introducing renewable and inexpensive biopolymers .
Chang et al. prepared superabsorbent hydrogels from carboxymethyl cellulose sodium (CMC) and cellulose in the NaOH/urea aqueous system by using epichlorohydrin (ECH) as cross-linker. The prepared hydrogels exhibited smart swelling and shrinking in NaCl or CaCl2 aqueous solution, as well as the release behavior of bovine serum albumin (BSA) that could be controlled by changing CMC content .
New stimuli-responsive hydrogels have been invented based on inclusion of cellulose nano whiskers (CNW)-polyacrylamide (PAAm) copolymer in poly N-isopropyl acrylamide (PNIPAm) semi interpenetrating network (IPN) hydrogel. These hydrogels exhibit the highest equilibrium swelling ratio (ESR) in acidic medium (pH 4). Meanwhile they perform good swelling behavior and hydrophilicity at a temperature of 32 °C .
In the other research, electrically conductive hemicellulose hydrogels (ECHHs) have been successfully synthesized by a straightforward and robust approach by introducing the conductive aniline tetramer (AT) into hydrophilic, nontoxic, and biocompatible hemicellulose networks. By increasing the AT content, the SRs were tuned from 548% to 228% [20, 21].
In the other research work, a highly conductive nanocomposite was made with multiwalled carbon nanotubes (MWCNTs) and chitin nanofibers (ChNFs). It was found that the resultant ChNF/MWCNT gel-film exposed much more MWCNT areas forming denser structure due to the shrinking of ChNFs after the gelation treatment. Compared with ChNF/MWCNT film, the one treated under hydrogel system (ChNF/MWCNT gel-film) exhibited almost twice higher conductivity (9.3 S/cm for 50 wt% MWCNTs in gel-film, whereas 4.7 S/cm for 50 wt% MWCNTs in film) .
In many revolutions of the cellulose-based conductive materials, the most significant one was the developed material could be either directly used as a working electrode or used as the underlying substrate for the electrochemical deposition metal electrodes. An in situ polymerization of aniline monomer onto the porous structured cellulose scaffolds has been carried out by Tian et al. and then electrodeposition of Ag nanoparticles on the obtained conductive composites directly by using it as electrode. The Ag nanoparticles were deposited homogeneously on the matrix of polyaniline (PANI)/cellulose gels. The conductivity of PANI/cellulose nanocomposite gels containing Ag nanoparticles was increased to 0.94 S C m−1, which was higher than that of pure PANI/cellulose composites (3.45 × 10−2 S/Cm). Furthermore, it could be used as electrode for the super capacitors, and the highest specific capacitance of the cellulose/PANI/Ag aerogels was 217 F g−1. This approach offered a facile method for improving the electronic conductivity of native polymer nano-hybrids and suggested a new strategy for fabricating nanostructured polymer nano-hybrids for application in energy storage .
The other research showed that the covalent incorporation of CNT into acrylate hydrogels composed of AAm and polyethylene glycol dimethacrylate (PEDGMA) as plasticizing monomer and cross-linker is a valuable strategy for the preparation of electroresponsive drug delivery systems suitable as topical devices (e.g., wound dressings) .
6 Irradiation Methods
The network of hydrogels, particularly those based on natural polymers, can be formed by different methods as chemical initiation or ionizing radiation from one component or two components by copolymerization, interpolymer complexes, and semi-interpenetrating polymerization. The irradiation methods are glow discharge, gamma-ray irradiation, electron beam irradiation, microwave, and ultrasonication. Aforementioned in the previous section, the pretreatment mechanism of each process is different according to the method applied. Physically cross-linked hydrogels usually show good biocompatibility but poor mechanical strength and stability. On the other hand, chemically cross-linked hydrogels usually exhibit enhanced mechanical strength and better stability but suffer from potentially harmful side reactions. Ionizing irradiation presents several advantages as it can occur without the need to add chemical initiators/cross-linking agent with subsequent separation of side reaction products. Moreover, the final products can be sterilized during hydrogel formation. Ionizing irradiation of polysaccharides leads to the chain scission and/or cross-linking reaction as a function of different parameters such as irradiation dose, irradiation phase, polymer concentration in aqueous solution phase, and the presence of oxygen. Usually, ionizing irradiation of polysaccharides at solid and non-concentrated solution states causes the breakage down of the chemical bonds between the repeated units of these polymers with consequent formation of lower-molecular-weight fragments. Moreover, the radiation dose can be easily controlled, and the experimental condition is simple for mass production of products. In addition, the product is free from undesirable chemical impurities such as residues from initiators, retarders, and/or accelerators for initiation and manipulation of the cross-linking reaction in chemical cross-linking methods.
6.1 Glow Discharge
The glow discharge owes its name to the fact that the plasma is luminous. This luminosity is produced because the electron energy and number density are high enough to generate visible light by recombination and excitation collisions. Glow discharges are used in a large number of applications. The light-emitting character of glow discharge has several applications such as in the light industry (the classical electrical discharge tube used in fluorescence lamps, neon discharge tube for advertisements, etc.), as the pump source for gas lasers, and as flat plasma display panels for the new generation of flat, large-area television screens. Besides, there are other important applications such as those in the microelectronic industry and in the material processing technology. These include surface treatment, etching of surfaces (for the fabrication of integrated circuits, etc.), plasma polymerization, plasma modification of polymers, and the deposition of thin protective coatings. Other forms of glow discharge for industrial applications are DC parallel plate plasma reactors, electron bombardment plasma sources, etc. In the simplest case, glow discharge is formed by applying a potential difference (of a few 100 V to a few kV) between two electrodes that are inserted in a cell or chamber filled with gas (an inert gas or a reactive gas) at a pressure ranging from a few mTorr to atmospheric pressure. Due to the potential difference, the electrons are accelerated away from the cathode and increase the collisions with the gas atoms and molecules. The collisions may produce processes such as excitation, ionization, dissociation, etc. The excitation collisions create excited species, which can decay to lower levels by the emission of light, and this is responsible for the characteristic name of the “glow” discharge. The ions are accelerated toward the cathode, and they release secondary electrons when bombarding at the cathode surface. These secondary electrons are accelerated away from the cathode, and they can give rise to more ionization collisions. Ionization collisions create ion-electron pairs, and this ion-electron multiplication process makes glow discharge a self-sustained plasma .
6.2 Gamma-Ray Irradiation
Gamma ray is a high-energy ionizing radiation in electromagnetic spectrum that easily penetrates most materials. This irradiation is extremely large high-frequency waves and largely depends on the radiation source. This technology is commonly applied in radiotherapy as a tracer in food and medical apparatus sterilization. Recently, the utilization of this technology has gain great attention especially in a biomass and biofiber pretreatment for liquid biofuel production. Radioactive nuclides such as cobalt-60 and cesium-137 are the common radioactive used in this pretreatment. The main goal of this irradiation is to decrease intra- and intermolecular order in cellulose due to the breakdown of the intermolecular hydrogen bonds. In this process, the radiation will travel from the seal source and penetrates (bombard) the biomass and biofiber. The energy carried by gamma radiation is transferred to the biomass component by collision of radiation, resulting to the loss of electron by the atom and leading to the ionization. Under exposure to radiation, the biomass component mainly cellulose macromolecules undergoes scission, and various short- and long-lived radicals are formed. Also, the content of fragments with a low degree of polymerization generated from the process gradually increases, leading to the alteration of biomass structure, thus providing ease of access for subsequent process such as enzymatic saccharification process. The potential of gamma irradiation technology in biomass and biofiber pretreatment has been studied on various types of biomass, for instance, jute fiber, poplar sawdust, wheat straw, and cotton cellulose. There were only scanty studies on gamma irradiation pretreatment on tropical biomass and biofiber that have also been reported. A study on gamma irradiation of empty fruit bunches (EFB) indicated that the pretreatment has reduced the lignin and increased the cellulose content in the EFB. Scanning electron microscopy-EDX (SEMEDX) analysis showed that there is a significant change on the carbon and oxygen content in the EFB biomass. Typically, untreated EFB contains high carbon and low oxygen content, while the study found a decrease of carbon (9% increment) and decrease of oxygen content (16% decrease), indicating the reduction of lignin content in the EFB. A comparison of gamma-ray irradiation pretreatment on soft- and hardwood has also been carried out using different levels of dosage ranges between 10 and 100 kGy. The study found that the most suitable condition for softwood was at 40 kGy, while higher dosage is required to pretreat hardwood (90 kGy). The study also concluded that gamma-ray pretreatment process is species dependent, wherein higher dosage is needed to disrupt hardwood cell structure compared to softwood .
6.3 Electron Beam Irradiation
Electron beam is one of the irradiation pretreatments used to pretreat biomass prior to enzymatic saccharification. This technology has been widely used in various applications such as welding, drilling, and surface treatment. For commercial use, the most important characteristics of an accelerator are its electron energy and average beam power. Therefore, industrial electron accelerators are usually classified according to their energy ranges, which are divided into low (80–300 keV), medium (300 keV–5 MeV), and high energy ranges (above 5 MeV). In the electron beam pretreatment, the biomass and biofiber are exposed to a highly charged stream irradiation pretreatment of tropical biomass and biofiber for biofuel production electron. The electron is emitted from an electron beam gun and accelerated by accelerator. In this pretreatment process, the electron energy can be controlled and modulated by varying the irradiation dose. The high-energy electrons emitted travel into biomass and biofiber component and transfer the energy within the materials. The heating process initiates chemical and thermal reaction in the biomass including cellulose depolymerization and production of carbonyl group, resulting from the oxidation of the biomass. Cross-linking of biomass component has also been reported to occur when the biomass is exposed to irradiation beam. Also, reduction of the biomass mechanical strength has been observed from the biomass exposed to electron beam. This could be due to the disruption of hydrogen bond between cellulose chains making it less crystalline and more amorphous .
6.4 Microwave Irradiation
Microwave is electromagnetic waves between the frequency range of 0.3–300 GHz, and most of the microwave systems used for industrial and domestic purposes range between 0.9 GHz and 2.45 GHz. Microwave radiation is a radiating wave movement and takes a straight-line path type of energy. This radiation does not require any medium to travel through and could penetrate nonmetal materials such as plastic and glass. Microwaves can affect the material thermally and nonthermally. Thermally, microwaves heat the material by the interaction of the molecules of material with electromagnetic field produced by microwave energy. Nonthermally, microwaves affect and interact with the polar molecules and ions in the materials causing physical, chemical, and biological reactions .
Another irradiation pretreatment that is widely used to pretreat biomass and biofiber for biofuel production is ultrasonication. This process can be performed either using probe-type ultrasonication or an ultrasonic bath. In this process, ultrasonic waves can be generated via piezoelectric or magnetostrictive transducers in the frequency range of 20–1000 kHz, in which the waves induced provide pressure difference in the medium. The pressure wave that travels through the liquid medium has high-pressure (compression) and low-pressure (rarefaction) regions. The rarefaction of the cycle can stretch the liquid molecules apart and create cavities also known as bubbles. As the wave cycles through the liquid, the bubbles expand and contract with the rarefaction and compression of the wave, respectively, drawing more liquid molecules into the bubbles as they grow. The bubbles that either continue to expand and then float to the surface are subjected to coalescence due to the forces or collapse during compression of the wave. This collapse is almost adiabatic and can result in localized temperatures of around 5000 K and pressures of 1000 atm. The collapse results in the formation of radicals through dissociation of the molecules within and around the bubbles, luminescence due to excited molecules formed losing energy, and micro jets shooting out of the bubbles of speeds in the realms of hundreds of km per hour .
7 Glow Discharge Effects on Hydrogels
Various initiated techniques have been proposed for the preparation of hydrogels in the past few decades which include chemical initiation, UV-curing technique, microwave or gamma-ray irradiation, etc., but all these trigger technologies had residual monomers and initiators, high energy consumption, etc. Glow discharge electrolysis plasma (GDEP), which belongs to the nonequilibrium plasma, exhibits the characteristics of non-faraday because it produces numerous highly active particles in plasma electrolysis, such as HO•, H•, HO2•, eaq−, and H2O2.
Glow discharge electrolysis plasma (GDEP) is emerging as a potential alternative to conventional technologies for the formation of hydrogels due to its low costs, high efficiency, easy operation, and mild reaction conditions. GDEP is a kind of non-faradaic electrochemical process and exhibits the characteristics of non-faraday, which produces numerous highly active particles in plasma electrolysis, such as HO•, H•, HO2•, eaq−, and H2O2•. The yield of HO is more than 12 mol per mol electron of electricity in the process of glow discharge electrolysis, which suggests that GDEP is a rich source of free radical in aqueous solution and can be applied to induce some unusual chemical reactions by taking place of chemical initiator in solution. In recent years, a series of adsorbing composites were prepared by GDEP technology and showed some excellent properties. However, there were few studies directly on the preparation of cellulose-based hydrogels with multi-stimulus response properties by GDEP technology. Novel ionic hydrogels have been prepared successfully from cellulose in the NaOH/urea aqueous system by a glow discharge electrolysis plasma (GDEP) technique by Zhang and his coworkers. The results showed that the swelling behavior and the network structure of the ionic hydrogels could be controlled by changing discharge voltage or discharge time, whereas we obtained the maximum absorbency of 898 g g−1 for distilled water at 570 V and 90 s. Shrinkage of the network hydrogels took place at higher or lower pH. Relative to the Na+ buffer solution, hydrogels were more sensitive to Zn2+ and Fe3+ buffer solutions and showed network shrinkage and lower swelling ratio. In Na+ ionic solution, the swelling ratios of hydrogels are higher and more different. This work provided a new pathway for preparation of cellulose-based hydrogels with environmental friendliness, high water absorption capacity, and rapid and multiple responses to pH and ions, which may allow their use in the biomaterial area .
8 Gamma Irradiation of Hydrogels
In recent years there has been a growing interest in hydrogel systems prepared by gamma-ray irradiation. Gamma-ray irradiation is an available technique for the modification of the chemical and physical properties of polymeric materials. The gamma-ray irradiation method has advantages, such as relatively simple manipulation without the need of any extra agents for polymerization and cross-linking; by contrast, the thermal activation method requires radical initiators and cross-linkers. For these reasons, gamma-ray irradiation method is useful for preparing hydrogels for medical applications, for which even a small contamination is undesirable, and is often used to sterilize biomedical devices for medical and veterinary applications. Natural polysaccharides such as hyaluronic acid (HA) have been extensively studied in medical applications since they provide intrinsic biological activity when used as basis for biomaterials. Zhao and his coworkers prepared hyaluronic acid (HA) and chondroitin sulfate (CS) and the synthetic polymer, poly (vinyl alcohol) (PVA), hydrogels using gamma irradiation. All HA/CS/PVA hydrogels showed relatively high water contents of greater than 90%. These hydrogels reached an equilibrium swelling state within 24 h .
Plungpongpan et al. prepared hydrogels from polymer blend between poly (N-vinyl pyrrolidone) (PVP) and methyl hydroxyethyl cellulose (MHEC) via gamma irradiation. The cross-linking structures in the hydrogels were induced by varying the irradiation doses from 10 to 40 kGy. The gel fraction and the swelling ratio of hydrogels were characterized. The results showed that the swelling ratio of hydrogels increases with increasing the irradiation dose .
In similar research work, thermo- and pH-sensitive Ag-P(NiPAAm/IA) hydrogel nanocomposites were prepared by in situ reduction of Ag+ ions with gamma irradiation .
Also, Yang et al. prepared inorganic/organic hybrid poly (N-isopropylacrylamide) (PNIPAM) hydrogels based on polyhedral oligomeric silsesquioxanes (POSS) via gamma-ray irradiation in one step. The swelling and deswelling behavior of PNIPAM hydrogels indicated that POSS-containing hydrogels had a good water absorption capability and rapid water release capability. Thus, these thermal-responsive POSS-containing hydrogels would have some potential applications such as biological fields .
In similar research, gamma irradiation synthesis of silver/poly(vinyl alcohol) (Ag/PVA) hydrogels has been carried out by exposing the mixture of PVA and AgNO3 in double-distilled water to gamma irradiation for 25 kGy dose. The swelling properties of the Ag/PVA hydrogel were compared with the PVA hydrogel by Swaroop et al. Significant increase in the swelling percentage of Ag/PVA hydrogel was observed in comparison to PVA hydrogel .
Application of polymers cross-linked by gamma irradiation on cutaneous wounds has resulted in the improvement of healing. Chitosan (CH)- and poloxamer 407 (P407)-based hydrogels confer different advantages in wound management. To combine the properties of both compounds, a gamma-irradiated mixture of 0.75/25% (w/w) CH and P407, respectively, has been obtained. Its thermo-reversibility and gelation properties at a low temperature allow for easy application to the wound, while the ability to swell in the presence of human serum permits the control of excessive exudate for skin wound application .
In similar research, poly(vinyl pyrrolidone) (PVP), poly(ethylene glycol) (PEG), and agar have been used for preparing wound dressing under gamma irradiation .
If a biomaterial is to be implanted in the body, it must be subjected to a sterilization procedure which often involves gamma irradiation. Magda et al. reported results for the effects of gamma irradiation on the glucose response of a hydrogel with glucose-binding boronic acid moieties. This “smart” hydrogel is of a type suitable for use in nonenzymatic glucose sensors. Exposure to gamma rays reduces the glucose-response sensitivity by over 50%, possibly due to the formation of additional hydrogel cross-links .
In the other research, interpenetrating network based on poly(acrylamide-aniline)-grafted gum ghatti has been synthesized under gamma irradiation using N,N′-methylenebis-acrylamide (MBA). The optimum grafting was observed when the reaction mixture containing 10 ml solvent, 0.324 × 10−1 molL−1 MBA, and 1.08 molL−1 monomer was exposed to 1.5 kGy dose of gamma radiation. The electrical conductivity of doped IPN structures increased with an increased concentration of HCl. However, a high concentration of HCl led to a decrease in electrical conductivity, which may be due to the over protonation of PANI chains in the cross-linked networks .
A network of covalently cross-linked wax hydrogel bearing acidic and amide groups using gamma irradiation has been successfully developed by Ghobashy et al. He found that it has a promising application in dye removal .
9 Effect of Electron Beam on Hydrogels
Electron beam cross-linking is a clean and safe technology, especially for biomedical application, since it does not require any external initiators and cross-linkers. The irradiation-induced cross-linking technique provides sterilization and hydrogel cross-linking in a single step. The physical properties of hydrogels produced in this manner are dependent on the degree of cross-linking and polymer composition. This method also provides an alternative to the use of chemical initiators and cross-linkers, which can be harmful and difficult to remove. Recently bacterial cellulose/acrylic acid (BC/AA) hydrogels synthesized by electron beam irradiation and investigate its wound healing potential in an animal model. Vivo experiments indicated that hydrogels promoted faster wound healing, enhanced epithelialization, and accelerated fibroblast proliferation compared to that in the control group. These results suggest that BC/AA hydrogels are promising materials for burn dressings [39, 40].
In the other research, electron beam irradiation has been applied to prepare a chemically cross-linked hydrogel based on tyramine conjugated gum tragacanth. The gel content of the prepared hydrogels was in the range of 75–85%. Equilibrium swelling degree of the hydrogels decreased from 51 to 14 with increasing polymer concentration and irradiation dose. Moisture retention capability of the hydrogels after 5 h incubation at 37 °C was in the range of 45–52 that is comparable with that of commercial hydrogels. The cytotoxicity analysis showed the good biocompatibility of hydrogels. These results indicated that electron beam irradiation is a promising method to prepare chemically cross-linked tyramine conjugated gum tragacanth hydrogels for biomedical applications. Also, the versatility of electron beam irradiation for cross-linking of a variety of polymers possessing tyramine groups was demonstrated .
In the other research, Choi and his coworkers prepared eggshell membrane (ESM)-based hydrogel by incorporating with polyvinyl alcohol (PVA) via an electron beam irradiation technique. Hen egg shell membrane (ESM) is an abundant natural protein resource and can be readily obtained almost anywhere as a waste. ESM is a thin, highly collagenized fibrous membrane formed by types I, V, and X collagen, making up 88–96% of its dry weight. It has been used as a biomaterial, particularly as a matrix for absorption of heavy metal ions, a template for the formation of ordered tube networks, a platform for enzyme immobilization, and a scaffold material for tissue engineering. The internal 3D porous network structure, the high absorption capacity, and the possible microbial sterilization due to EBI are the main features of the introduced hydrogel, which make it a suitable candidate for biomedical applications such as wound dressing, drug delivery, tissue scaffolds, etc. More importantly, the preparation strategy could be considered as a cost-effective method which could eliminate the use of toxic chemicals to prepare hydrogel for biomedical applications. We hope, this method can be extended to fabricate a variety of polymer hydrogels for different applications .
In similar research, the temperature- and pH-responsive characters of hydrogels prepared from aqueous solutions containing 4.2% and 25% (w/v) carboxymethyl cellulose (CMC) and acrylic acid (AAc), respectively, under the effect of accelerated electrons was investigated. Even though the initial content of hydrogel solution is constant, the swelling in water and responsive characters was greatly dependent on electron beam irradiation dose. In this regard, the percentage swelling in water of the hydrogel prepared at 50 kGy is relatively higher than that prepared at 80 kGy. However, both hydrogels displayed super water-absorbing behavior at room temperature in the range of ∼3500–4000%. The swelling of CMC/AAc hydrogel prepared at 50 kGy was found to substantially increase with increasing pH values from 3 to 10; the hydrogel prepared at 80 kGy was found to display pH-responsive character below and above 7 .
Park and his coworkers found that the pure keratin (human hair and wool) aqueous solution was not gelled by EBI, while the aqueous keratin solutions blended with PVA were gelled at an EBI dose of more than 90 kGy. Furthermore, in the presence of PEI, the aqueous keratin solution blended with PVA could be gelled at a considerably lower EBI dose, even at 10 kGy .
Blend hydrogels based on aqueous solutions of plasticized starch and different ratios of cellulose acetate (CA) and carboxymethyl cellulose (CMC) were prepared by electron beam irradiation (EB). The blends before and after EB irradiation were characterized by thermos gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The physicochemical properties of blend hydrogels prepared by electron beam irradiation were improved compared to unirradiated blends .
10 Microwave Irradiation of Hydrogels
The microwave irradiation is one technique, which is used for preparation of the hydrogel in several researches. Microwave heating process has high temperatures for attack the solution with relatively short times and thus creates reactions faster than under conventional thermal conditions .
Microwave irradiation is a special heating energy, and it has some significant advantages over the conventional thermal methods in preparation of colloid particles. Omprakash used microwave irradiation followed by hydrothermal method for synthesizing ZSM-5 type of zeolite and found that zeolite synthesized by microwave irradiation method only took half the time for ZSM-5 crystallization of 100% crystallinity to that of conventional hydrothermal heating method. Hence, microwave treatment method is economic and time saving. Li et al. used microwave irradiation synthesis method for the preparation of thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels. He found that the PNIPAAm hydrogels synthesized using microwave irradiation had much higher swelling ratios at 10.0 °C below the (lower critical solution temperature) LCST [47, 48].
Cross-linked acrylamide-based thermosensitive hydrogels are interesting candidates for biomedical or pharmaceutical applications, such as drug release because they change volume and expel a significant amount of its inner solution when a transition is induced by external action. These kinds of materials based on smart hydrogels could also be used in chemical or mechanical actuators as well as environment sensors for technological applications. It has been shown that the in situ polymerization water-soluble monomers (aniline or pyrrole) inside a hydrogel produce nanocomposite with conductive and dielectric domains. The nanocomposite suffers a phase transition by externally changing the temperature of the gel or by microwave absorption. Upon microwave irradiation, the conductive polymer absorbs the radiation and heats up driving the phase transition of hydrogel which involves a clear volume decrease and inner solution expulsion. It is found that the amount of water released correlates well with the surface temperature of the materials upon microwave irradiation .
Li et al. studied the swelling and deswelling kinetics of poly(N-isopropylacrylamide) (PNIPAAm) hydrogels separately synthesized by means of microwave irradiation and normal water bath heating. As compared with the PN hydrogel synthesized by the conventional method, the PM hydrogel synthesized by microwave irradiation had larger swelling and deswelling rate constants as well as lower swelling/deswelling activation energy due to its higher surface area and larger pore sizes, and thus it had faster response behavior. Also he found that the use of microwave irradiation method to prepare the hydrogel not only made it more porous but also made its pores more uniform and deeper .
In the other study, microwave irradiation technique has been used to synthesize poly(acrylamide-co-2-hydroxyethyl methacrylate)/poly(vinyl alcohol) (P(AM-co-HEMA)/PVA which are separately synthesized by using one-pot polymerization and two-step polymerization techniques. The hydrogel prepared by one-pot polymerization technique exhibited the highest swelling ratio and reduced step of polymerization, compared with the hydrogels prepared by two-step polymerization technique. The hydrogel of 50%HEMA-PM showed the maximum mass swelling percentage (the mass swelling percentage has higher than 900%) .
To prepare a novel biodegradable hydrogel for use in tissue engineering, pHEMA hydrogel has been synthesized by microwave-assisted polymerization using 2-hydroxyethyl methacrylate (HEMA) as the raw material, potassium persulfate as the initiator, and PCLX as the cross-linking additive. Biodegradation studies showed 75% uniform bulk degradation of the hydrogel over a period of 17 days. In addition, the biodegradable hydrogel had no observable cytotoxicity toward L-929 fibroblast cells. Therefore, this novel bioresorbable pHEMA can play an important role as a scaffold for tissue engineering applications .
11 Ultrasonication of Hydrogels
The radiation synthesis is a kind of new technology which was highly effective and eco-friendly. And because of its high temperatures for attacking the solution with relatively short times and thus creating reactions faster than conventional thermal conditions, microwave irradiation has been used for preparation of the hydrogel in several researches. Also ultrasound has been investigated for the initiation of polymerization reactions to prepare polymer hydrogel since it can be used both for dispersion of monomer droplets and for generation of free radicals. The use of ultrasonic irradiations during the hydrogel synthesis not only can control the molecular weights but also can improve the swelling ratio. Wang et al. synthesized the hydrogels based on gelatin cross-linked with chitosan (CS) and polyvinyl pyrrolidone (PVP) using microwave and ultrasonic coupling technique. This interpenetrating polymer network (IPN) hydrogels were cross-linked by glutaraldehyde and 1,2-Epoxy-4-vinylcyclohexane. The results showed that the hydrogel prepared with microwave and ultrasonic exhibited the highest tensile strength (86.68 MPa), compared with the hydrogel prepared with traditional method and only microwave reactive field. The FT-IR and XRD results showed that the chemical reactions occurred between the eNH2 of chitosan and the eCOOH of gelatin and the introduction of ultrasound can improve the reaction rate. The hydrogel film gained in microwave and ultrasonic coupling field has the best combination properties. Therefore, the new microwave-ultrasonic coupling technique is the potential technology to prepare the new hydrogel due to less synthesis time .
In the other research, Wang et al. successfully prepared a series of (sodium lignosulphonate) SLS-based hydrogels by ultrasonic-assisted synthesis which is green, fast, and convenient relative to other methods. The swelling capacity of SLS-based hydrogels was measured in various experimental conditions like pH, SLS content, and different electrolytes. The results demonstrated that the water uptake capacity achieved the maximum when containing 1 wt% sodium lignosulphonate. It was worth mentioning that the maximum of swelling capabilities could reach 1328 g g−1 and 110 g g−1 in deionized water and 0.9% NaCl solution, respectively. It was found through the exploration that the swelling behavior in aqueous solution was consistent with the pseudo-second-order kinetic model. At the same time, SLS-based hydrogel was selected as an adsorbent to remove Ni2+ from an aqueous solution, and the maximum adsorption capacity reached about 293 mg g−1 under the optimal experimental condition. The adsorption capability of SLS-based hydrogel was influenced by pH, temperature, and adsorption time as well as initial concentration. The mechanism of adsorption was evaluated by fitting adsorption data to different kinetic models and adsorption isotherms, and the findings revealed that the adsorption behavior was endothermic and spontaneous as well as multi-molecular layer chemisorption .
Nowadays adsorption of dyes is carried out with superabsorbent hydrogels due to their high water absorption property. Lignin is a biodegradable, eco-friendly, low-cost, and renewable raw material and is strongly reactive due to its aromatic nature and functional groups (carboxyl and phenol groups), which can potentially be used for high performance as adsorbent. However, its adsorption amount and adsorption rate need to be further improved for practical applications. Ultrasound-irradiation has been investigated for the initiation of bulk polymerization reactions to produce polymer hydrogel since it can be used both for dispersion of monomer droplets and for generation of free radicals. The use of ultrasonic irradiations during the hydrogel synthesis not only can control the molecular weights which are attributed to the high shear gradients generated by cavitational events but also can improve the dispersion of the clay with enhancement in the mechanical properties and adsorption capacity of hydrogels. A novel hybrid hydrogel has been synthesized from grafting of acrylamide and N-isopropyl acrylamide onto lignin by incorporation of montmorillonite under ultrasonic irradiation (lignin-g-p(AM-co-NIPAM)/MMT). This hydrogel was employed for the removal of methylene blue from the aqueous solution, and it exhibited good swelling-deswelling property. The initial swelling kinetic was Fickian-type diffusion, and the whole swelling process was good fit for Schott’s pseudo-second-order model. The adsorption process was found to be highly pH and temperature dependent and followed the pseudo-second-order rate model .
In the other research, hen egg white lysozyme (LZM) cross-linked with ultrasonic-treated tragacanth (US-treated TGC) under mild Maillard reaction conditions. Since this gum is extensively used in food industry and application of LZM as a natural antimicrobial agent in different food systems is recommended and practiced in some countries, the results of this study indicate that a conjugated product of these two polymers combines different properties into one macromolecule and improves the property of each. These properties may make the conjugate an attractive food ingredient .
Ultrasonic radiation is used in a vast range of applications. More commonly this includes imaging (industrial and medical), physiotherapy, and welding of metals and plastics. High frequencies are commonly used for imaging, whereas low frequencies (less than 500 kHz) provide strong physical and chemical interaction with materials. The physical effects of low-frequency (high power) ultrasound include high shear rates, free radical production in solution, and heat generation. This has been utilized for fragmentation and dissolution of solids, biological techniques, the preparation of nanomaterials, various synthetic sonochemical reactions, thermal applications such as plastic welding, and environment remediation. Ultrasound has also been investigated for many polymer-based applications. These include polymer synthesis through the generation of free radicals, activation of free radical initiators, degradation of polymers in solution by high shear rates, and physical mixing of heterogeneous emulsion/suspension polymerization systems. Several acrylic hydrogels have been prepared via ultrasonic polymerization of water-soluble monomers and macromonomers by Cass et al. Ultrasound was used to create initiating radicals in viscous aqueous monomer solutions using the additives glycerol, sorbitol, or glucose in an open system at 37 °C.
Ultrasound was found to be an effective method for the polymerization of water-soluble vinyl monomers and for the production of hydrogels. This occurs rapidly in the absence of a chemical initiator. The water-soluble additive was essential for the preparation of hydrogel. We propose that its effect is attributed to the enhanced viscosity increasing the free radical production as well as reducing the polymer solubility and hence retarding depolymerization. The most effective additive for the preparation of hydrogels was glycerol. Such a technique may find application in the field of biomaterial synthesis to avoid problems associated with cytotoxic initiators. In combination with techniques such as high intensity focused ultrasound, the polymerization method described here may also allow the formation of hydrogels in vivo .
Without a doubt, there are a number of significant features of hydrogels that qualify and allow them to exhibit extraordinary characteristics, which enables them to be employed as essential tools for applications in almost all the fields, such as biomedical, agricultural, industrial, and environmental areas. From time to time, significant modifications are made to revolutionize the field of hydrogels for their comprehensive applications. Nowadays, smart and electrical conductive hydrogels play very important role in different branches of industry. Although extensive research is ongoing in order to provide new kind of hydrogels for future necessities, novel conceptual assimilation of hydrogel preparation such as different irradiation methods may lead to tailored properties, translating its innovative applications in the diversified fields. Above all, an economical way to improve the efficacy of the hydrogel system will be the most demandingly needed approach.
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