Nanocomposite Hydrogels Obtained by Gamma Irradiation
During the past decades hydrogels have gained considerable interest and reviewed from different points of view, because of their unique properties. The hydrogel 3D structure, porosity, swelling behavior, stability, gel strength, as well as biodegradability, nontoxicity, and biocompatibility are properties which are widely variable and easily adjusted, making them suitable for many versatile applications, especially in the field of medicine and biotechnology. Generally, hydrogels possess the huge potential to be used as a matrix for incorporation of different types of nanoparticles. Namely, hydrogels in the swollen state provide free space between cross-linked polymer chains, in which the nucleation and growth of nanoparticles occurs. In this way, the carrier-hydrogel system acts as a nanoreactor that also immobilizes nanoparticles and provides easy handling with obtained hydrogel nanocomposites. It is well known that the properties of nanocomposite materials are dependent on the method of synthesis. Among various techniques, the radiation-induced synthesis offers a number of advantages over the conventional physical and chemical methods. Radiolytic method is a highly suitable way for formation of three-dimensional polymer network, i.e., hydrogels, as well as for generation of nanoparticles in a solution (especially metal nanoparticles). This method provides fast, easy, and clean synthesis of hydrogel nanocomposites. Moreover, and probably the most important from the biomedical point of view, is the possibility of simultaneous formation of nanocomposite hydrogel and its sterilization in one technological step. Despite all the mentioned advantages of radiolytic method, there are not so many investigations related to nanocomposite materials based on nanoparticles incorporated in a hydrogel matrix.
KeywordsGamma irradiation Hydrogels Nanoparticles Nanocomposites
Nanotechnology is one of the fastest growing new areas in science and engineering. It is a cross-interdisciplinary area which involves the precise control and manipulation with atoms and molecules in order to create novel materials with unique unusual and/or enhanced properties. This technology requires detailed understanding of physical and chemical processes, across a range of disciplines, at the range below 100 nm. The main goal is production of new materials, devices, and systems tailored to meet the needs of a growing range of scientific and engineering applications. Moreover, the commercial application of such materials is probably one of the most important driving forces for so huge global activity in nanotechnology.
Polymer-based nanocomposites are being considered as versatile materials in many scientific applications leading to technological advancements. This is due to the fact that incorporation of the nanoparticles into polymer matrix significantly affects its optical, thermal, and electrical properties while retaining its inherent characteristics. It opens a new gateway in developing the materials for improved performance in many potential applications.
Polymers are considered to be a good host material for incorporation of inorganic nanoparticles. During the synthesis of nanoparticles, the atoms tend to coalesce into clusters, which grow into bigger particles or eventually into precipitates. The control of particle size can be achieved by the use of polymers such as capping agents. Functional groups of polymers ensure the anchoring of the molecule at the particle surface, while the polymeric chains protect particles from coalescing with other ones and thus inhibit at an early stage further coalescence through electrostatic repulsion or steric hindrance . On the other hand, incorporation of nanoparticles, due to their high surface to bulk ratio, can significantly affect the properties of the polymer matrix.
In the recent years, the novel and very attractive class of materials are the nanocomposites in which cross-linked polymer networks, i.e., hydrogels, are used as a carrier for the incorporation and organization of nanoparticles. The properties of such nanoscale devices can be tuned according to the required functions and applications. The “soft” mesoporous network of hydrogels is suitable matrix for incorporation of metal nanoparticles (Ag, Au) [2, 3, 4, 5], magnetic particles (Fe3O4) [6, 7], as well as semiconductor nanoparticles (CdS, PbS) [8, 9]. Most of the technologies used to incorporate nanoparticles into polymeric matrix involve either chemical methods such as reduction, synthesis of complex compounds, mixing of preformed particles with polymers, or complicated physical techniques. Recent research efforts are directed toward exploiting possibilities for the in situ synthesis of nanoparticles within polymeric network architectures in order to produce enhanced and new hybrid nanomaterials. Three-dimensional network hydrogels are more suitable as templates for the production of nanoparticles than conventional nonaqueous or polymeric systems especially for biomedical applications, considering their exceptional compatibility with biological molecules, cells, and tissues. Hydrogels in the swollen state provide free spaces within the network, which can also serve for nucleation and growth of nanoparticles. In this way, the carrier-hydrogel system acts as a nanoreactor that also immobilizes nanoparticles and provides easy handling [10, 11, 12, 13, 14].
Beyond various techniques, the gamma irradiation-induced synthesis offers many advantages over the conventional physical and chemical methods. Radiolytic method is highly suitable for formation of three-dimensional polymer network, i.e., hydrogels, as well as for generation of nanoparticles in a solution (especially metal nanoparticles). This method provides fast, easy, and clean synthesis of hydrogel nanocomposites. Moreover, and probably the most important from the biomedical point of view, is the possibility of simultaneous formation of nanocomposite hydrogel and its sterilization in one technological step.
Despite all the mentioned advantages of radiolytic method, there are not so many investigations and literature related to nanocomposite materials based on nanoparticles incorporated in a hydrogel matrix. Therefore, some achievements in the field of radiolytic synthesis of nanocomposite hydrogels and their properties will be presented in this chapter.
Hydrogels are two- or multicomponent systems consisting of a three-dimensional network of polymer and water that fills the space between macromolecular chains. Depending on the properties of the polymers used, as well as on the nature and density of the network joints, such structures in equilibrium can contain various amounts of water. Typically, in the swollen state, the mass fraction of water in the hydrogel is much higher than the mass of polymer. The hydrophilicity of the network is due to the presence of chemical groups such as hydroxyl (–OH), carboxyl (–COOH), secondary amide (–CONH–), primary amide (–CONH2), sulfonic (–SO3H), and others that can be found within the polymer backbone or as lateral chains . Nevertheless, it is also possible to produce hydrogels containing a significant portion of hydrophobic polymers, by blending or copolymerizing hydrophilic and hydrophobic polymers . Hydrogels are one of the most promising materials for biomedical applications and have several advantages for wound dressing, contact lenses, drug delivery systems, etc. because of their biocompatibility with blood, body fluids, and tissue .
Two classes of hydrogel can be defined: physical gels or pseudogels and chemical or permanent hydrogels. In physically cross-linked gels, the macromolecular chains are connected by transient junctions that arise from either polymer chain entanglements or physical interactions such as ionic interactions and hydrogen bonds. Such types of gels are nonpermanent, and usually they can be converted to polymer solutions by heating. On the other hand, in chemically cross-linked hydrogels, covalent bonds are present between different polymer chains (permanent junctions). Therefore, they are stable and cannot be dissolved in any solvents unless the covalent cross-link points are cleaved [18, 19]. While physically cross-linked hydrogels have the general advantages of forming gels without the need for chemical modification or the addition of cross-linking entities, they also have limitations. It is difficult to decouple variables such as gelation time, network pore size, chemical functionalization, and degradation time; this restricts the design flexibility of a physically cross-linked hydrogel because its strength is directly related to the chemical properties of the gel constituents. In contrast, chemical cross-linking results in a network with a relatively high mechanical strength and, depending on the nature of the chemical bonds in the building blocks and the cross-links, relatively long degradation times. Chemically cross-linked gels are also mechanically stable owing to the covalent bond in these gels.
A special class of hydrogels, which is called stimuli-responsive or intelligent or smart hydrogels, shows an active and significant response (undergo reversible volume phase transitions or gel-sol phase transitions) to small changes in the surrounding environment . Many physical and chemical stimuli have been applied to induce various responses of the smart hydrogel systems. The physical stimuli, which include temperature, solvent composition, light, pressure, sound, electric and magnetic fields, and mechanical stress, will affect and alter molecular interactions at critical onset points. Chemical stimuli, such as pH, ionic factors, and chemical agents, will change the interactions between polymer chains or between polymer chains and solvent at the molecular level. Recently, biochemical stimuli have been considered as another category that involves the responses to antigen, enzyme, ligand, and other biochemical agents. Some systems combine two stimuli-responsive mechanisms into one polymer system, the so-called dual-responsive polymer systems. Smart hydrogels have been used in diverse applications, such as drug delivery systems, in making actuators [21, 22] and valves [23, 24], in the immobilization of enzymes and cells [25, 26], in sensors [27, 28], and in concentrating dilute solutions in bioseparation .
3 Radiation Technology
In general, chemical or material engineering mostly applies high temperature and/or high-pressure processes for material synthesis/modification, and quite often a catalyst is required to speed up the reaction. On the other hand, radiation is the unique source of energy which can initiate chemical reactions at any temperature, including ambient, under any pressure, in any phase (gas, liquid, or solid), without use of catalysts .
Since the pioneering work of Wichterle and Lim in 1960 , the hydrogels have been of great interest, especially in the biomaterials field. In fact, hydrogels have been prepared by chemical methods for a long time. However, in recent years, irradiation techniques have been recognized as highly suitable tool to aid in the formation of hydrogels and being used increasingly around the world. This technology is convenient because the properties of hydrogels can be easily manipulated and adjusted by controlling the radiation dose and rate, and the process of synthesis is very reproducible . The radiation process has various advantages, such as easy process control, the possibility of joining hydrogel formation and sterilization in one technological step (especially important for biomedical application), and the lack of necessity for initiators and cross-linkers, which are possibly harmful and difficult to remove. The radiation technique is clean, because it does not require any extra substances, does not leave some unwanted residues, and does not need any further purification. Moreover, this method has a relatively low running cost. All these qualities make irradiation the method of choice in the synthesis of hydrogels.
Hydrogels can be obtained by radiation technique in a few ways, including irradiation of solid polymer, monomer (in bulk or in solution), or aqueous solution of polymer. Irradiation of hydrophilic polymer in a dry form is rarely applied, so it will not be discussed here. More frequently used method is irradiation of monomer solution. This way is possibly most convenient when the chosen monomer is easily available but its polymer is not. In this method polymerization takes place in the first stage, followed by cross-linking of the formed chains. In this case, particular care has to be taken when using this method for the formation of hydrogels for biomedical use, because many of the monomers used are harmful or even toxic (contrary to the corresponding polymers). After irradiation of such systems, all unreacted residues have been fully extracted afterward, in a separate operation.
Among all methods, the most convenient method of radiation-induced synthesis of hydrogels is the irradiation of polymers in aqueous solution. Such systems, containing neither monomer nor cross-linking agent, are easier to control and to study. Also, with the application of this method, lower number of usually unwanted processes occurs, and hydrogels formed in this way are suitable for biomedical use with no need of further purification (synthesis and sterilization in one technological step) .
3.1 Synthesis of Hydrogels
When monomer or polymer solution is subjected to ionizing radiation, reactive intermediates are formed. This can result from direct action of radiation on the monomer unit or polymer chains and from indirect effect, i.e., reaction of the intermediates generated in water with monomer or polymer molecules. Since the fraction of energy absorbed by each component of the monomer-water or polymer-water system is proportional to its electron fraction, which can be well approximated by the weight fraction, in dilute and moderately concentrated solutions, the indirect effect dominates .
It is well known that under the irradiation of aqueous solution, short-lived reactive intermediates of the water radiolysis, such as hydrated electrons (eaq−), hydroxyl radicals (OH•), and hydrogen atoms (H•), were formed first, and they react with the monomer units or polymer chains. Among these reactive species, hydroxyl radicals are the main species responsible for reactivity transfer from water to the monomer units or polymer chains. They abstract hydrogen atoms from monomer units or macromolecules, and thus monomer or polymer radicals are formed. In the case of monomer radicals, the processes of polymerization and cross-linking occur simultaneously, while the polymer radicals simply cross-linked to form polymer network [35, 36].
For the formation of hydrogels, the most important reaction of macroradicals is intermolecular cross-linking, i.e., recombination of radicals localized on two different macromolecules. Also, the other various reactions in systems can occur, and their importance and role in the competition with intermolecular cross-linking is not always fully recognized. These other reactions include reactions between two radicals, as intramolecular cross-linking as well as inter- and intramolecular disproportionation, and also processes involving one radical, as hydrogen transfer or chain scission. These processes do not result in joining the polymer chains; thus they do not lead to the formation of macroscopic gels .
Typical examples of simple polymers used for hydrogel formation by this method are poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP). As already mentioned, the irradiation of an oxygen-free aqueous solution induces the radiolysis of water and generation of primary radicals (Eq. 1) :
These PVA radicals may interact with one another by recombination, disproportionation, and chain scission (by β-fragmentation) . In the case of PVA, the degradation cannot be totally avoided, but its yield is low, and formation of polymer network occurs without any difficulty (G-value for intermolecular cross-linking is 0.48).
Similar situation occurs in the cross-linking process of PVP. The investigations of the reactivity of the primary products of radiolysis with polymer chains in the process of hydrogel formations have indicated that the rate of reactions of solvated electrons is relatively low, with the rate constant in the range of 106 dm3 mol−1 s−1. Comparing this value with constant rate of reactions of hydroxyl radicals, which was determined as higher than 108 dm3 mol−1 s−1, it can be concluded that those radicals are the main species of water radiolysis which react with polymer chains [32, 38]. In argon-saturated aqueous solution, OH• radicals attack PVP macromolecules by abstracting hydrogen atoms and thus producing macroradicals. It was suggested that the majority of macroradicals (about 70%) was localized on the main chain of a polymer and formed as a result of hydrogen abstraction at the methylidyne group. Moreover, hydrogen atom abstraction occurs also at methylene group adjacent to the nitrogen at pyrrolidone ring [43, 44]. The result of recombination of produced macroradicals is the formation of polymer network, i.e., hydrogel.
3.2 Synthesis of Nanoparticles Within Hydrogels
In general, the obtained polymer network possesses the huge hydrated space between cross-linked polymer chains and can be used as a matrix for formation of nanoparticles (NPs). In colloidal chemistry, the process of particle growth usually occurs through the Ostwald ripening mechanism. As a result, the particle size increases continuously during growth because the larger particles grow on account of dissolution of smaller ones . A convenient procedure to restrict their growth is the in situ synthesis of nanoparticles within the polymer matrix with an improved architecture, i.e., within the three-dimensional network of hydrophilic polymers. Thus formed nanoparticles are entrapped in polymer network, and as a result, a novel inorganic/organic hybrid cross-linked nanocomposite was obtained.
The preparation of metal nanoparticles has received increasing attention due to their unique properties. Noble metal particles such as silver and gold are of great significance due to their size-dependent optical properties [52, 53, 54]. Moreover, size effect was also observed for antibacterial activity of silver nanoparticles . A large number of synthetic procedures have been employed in order to synthesize metal nanoparticles and/or nanocomposites. It has been shown that morphology, particle size distribution, stability, and properties of metal nanoparticles as well as corresponding nanocomposites are strongly dependent on the method of preparation and specific experimental conditions. However, the synthesis of nanoparticles of desired shape and uniform size distribution within the matrix remains challenging.
The radiolytic method is suitable for generation of metal particles, particularly silver, in solution. The radiolytically generated species, solvated electrons and secondary radicals, exhibit strong reducing potentials, and consequently metal ions are reduced at each encounter, while the control of particle size is commonly achieved by the use of capping agents such as polymers [56, 57, 58].
The ions of noble metals, as well as of many electronegative metals, can be reduced by exposing their aqueous solutions to γ-irradiation. The main reactive radicals among the primary products are solvated electrons (eaq−), hydroxyl radicals (OH•), and hydrogen atoms (H•). The solvated electrons (eaq−) and hydrogen atoms (H•) are strong reducing agents, while the hydroxyl radicals (OH•) are able to oxidize the ions or the atoms into a higher oxidation state and thus to counterbalance the reduction reactions. For this reason, an OH• radical scavenger is added in the solution. Among various possible molecules, the preferred choice is for solutes whose oxidation by OH• yields radicals that are unable to oxidize the metal ions but, in contrast, themselves exhibit strong reducing power, such as the radicals of secondary alcohols. Most commonly used scavenger is 2-propanol which converts OH• and H• radicals into 2-propanol radicals ((CH3)2C•OH) (Eqs. 3 and 4) :
At the argon-saturated solutions, for example, Ag+ ions are reduced into zero-valent Ag atoms (Ag0) with strongly reducing hydrated electrons (Eq. 5), 2-propanol (Eq. 6), and the polymeric radicals (Eq. 7), formed in the hydrated polymer network structure as primary and secondary reactive species during gamma irradiation:
The produced Ag0 atoms are homogeneously dispersed throughout the hydrogel network. Because the binding energy between two metal atoms is stronger than the atom-solvent or atom-ligand bond energy, these Ag0 atoms tend to dimerize (Eq. 8) when they encounter and/or associate with an excess of ions (Eq. 9) and by cascade of coalescence processes progressively grow (Eq. 10), yielding the formation of metal clusters with higher nuclearities:
The fast collision reactions of ions with atoms or clusters play very important role in the mechanism of cluster growth. Reduction processes of free and adsorbed Ag+ ions (may be reduced at any stage of the coalescence) are competitive, and they are controlled by formation rate of reduction radicals. Therefore, the formation of clusters by direct reduction, accompanying with collision, is dominant at higher dose rate, when the nanoparticles with smaller dimensions were obtained [59, 60].
Furthermore, besides the metal nanoparticles, the irradiation method can be used for synthesis of semiconductor nanoparticles . The mechanism of gamma irradiation-induced synthesis of semiconductor nanoparticles, in the presence of thiol, has already been known [61, 62, 63]. The thiol reacts with the solvated electrons (Eq. 11) and OH• radicals (Eq. 12) to form hydrogen sulfide anion (HS−) and thiyl radicals (CH3(CH2)11S•), respectively:
When the gamma irradiation occurs in acidic solution (pH ≈ 5), thiol also can react with H• atoms instead with solvated electrons (Eq. 11), but according to the rate constants, the reaction with solvated electrons is favored . The thiyl radicals produced via reaction (12) dimerize to produce disulfide ((CH3(CH2)11S)2). Finally, the PbS molecules formed through reaction (13) and then coalesce to form nanoparticle (Eq. 14):
The final size of PbS nanoparticles can be controlled by concentration ratio of thiol/lead ions, as well as by the dose rate and total absorbed dose.
4 Nanocomposite Hydrogels
4.1 Nanocomposite Hydrogels with Silver Nanoparticles
Silver nanostructures, due to their size-dependent properties, exhibit significantly improved catalytic and electronic properties, and distinctive shape-dependent optical properties [52, 64], making them highly attractive for the use in the development of modern technologies. Morphological anisotropy of nanoparticles induces anisotropic optical absorption properties, bringing difference in the color, as well as optical absorption spectra, associated with the collective oscillations of the conduction electrons . Silver, having the highest electrical and thermal conductivity among all metals, is an important material that has been used in various commercial applications [66, 67]. However, zero-charged nanocrystalline silver, i.e., silver nanoparticles (AgNPs), exhibits powerful antimicrobial capabilities and broad inhibitory biocidal spectra for variety of microbes, including bacteria, viruses, and eukaryotic microorganisms. Enhanced antibacterial properties of silver nanoparticles have been demonstrated both in vitro and in vivo [68, 69, 70].
From the practical point, nanocomposite systems with incorporated silver nanostructures are one of the most investigated new materials. Among all of them, special attention is paid to the nanocomposite hydrogels due to a wide range of applications. Hydrogels have the possibility to absorb large amounts of water and biological fluids; they are biocompatible, and therefore, they are often used in patient care and wound dressings . Hydrogels facilitate autolysis and may be beneficial in the management of ulcers containing necrotic tissue. For example, debridement with hydrogels is more effective than standard wound care for healing diabetic foot ulcers . A potential problem for the biomedical application of hydrogels is that microorganisms may grow in hydrogel materials because of their natural biocompatible properties. Therefore, the incorporation of antibacterial agents is required. However, the emergence of antibiotic-resistant bacteria as a result of the excessive use of antibiotics has led to a demand for newer antimicrobials. The increasing need to develop new formulations to solve this problem has led to considerable interest in the use of nanomaterials as new types of antimicrobials. It is well known that nanosilver has been proven to be probably the most effective antimicrobial agent. The multilevel antimicrobial mode of silver is particularly important for the treatment of wound infections in diabetic patients, which are usually polymicrobial, with the majority of infections being caused by aerobic Gram-positive cocci (predominantly Staphylococcus aureus and hemolytic streptococci) [72, 73].
The recent studies suggest that Ag nanoparticles exert toxicity to bacteria and other organisms not by direct particle-specific effect but by released Ag+ ions . When the pathways in the antibacterial activity and eukaryotic toxicity of Ag nanoparticles involve the Ag+ ions and its soluble complexes, Ag nanoparticles behave in analogy to a drug delivery system, in which the particle contains a concentrated inventory of an active species, the ions . Although the importance of Ag+ ions in the biological response to Ag nanoparticles is widely recognized, there is significant potential to improve nano-Ag technologies through hydrogel-controlled release formulations. Moreover, for the use of nano-Ag-loaded hydrogels in biomedicine, it should be possible to modulate the release of Ag+ ions, which could be delivered to the patient at a controlled rate. This would allow the achievement of adequate concentrations and prolonged effectiveness. In general, it is known that the release of a soluble drug entrapped in a hydrogel should be closely related to the swelling characteristics of the hydrogels. That is because the release occurs only after the fluid penetrates into the polymeric network and dissolves the drug; this is followed by diffusion along the aqueous pathways to the surface of the device .
Krstić et al.  investigated the nano-Ag/PVA hydrogel device, synthesized by in situ gamma irradiation method, with the aim of designing a hydrogel-controlled release system of Ag+ ions for antibacterial purposes. The in vitro Ag+ ion release study showed sustained and controlled release in a solution with a pH similar to the biological fluids, with the release profiles of silver similar with the other drugs. Therefore, the elements of the drug delivery paradigm were applied for the study of Ag+ ion release kinetics. It was found that the release of Ag+ ions was predominantly controlled by a diffusion process, i.e., by the mass transport rate due to a concentration gradient of Ag+ ions (Fickian diffusion). Moreover, the Ag+ ion release can be adjusted by the concentration of AgNPs as well as by their size. The investigated nano-Ag/PVA nanocomposite hydrogels showed antimicrobial activity against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli .
Silver shows bactericidal activity at concentrations as low as 0.035 ppm without toxic effects to mammalian cells . However, the typical minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against standard reference cultures and multidrug-resistant organisms are 0.78–6.25 and 12.5 ppm, respectively . A silver concentration of up to 1.2 ppm has no cytotoxic effect on fibroblasts in vitro, which play a critical role in wound healing. However, in general, cytotoxic effects seem to be dose-dependent . The maximum toxic concentration for human cells is around 10 ppm. However, some authors have suggested that for topical use, Ag nanoparticles induce apoptosis at concentrations of up to 250 μg/mL (ppm), which could favor scarless wound healing . Except for their use as antimicrobial topical dressings, nano-Ag/hydrogel systems could be applied for orthopedic implants, such as artificial cartilage, intervertebral discs, and artificial meniscus or tissue expander devices [80, 81].
When developing novel biomaterials, it is necessary to evaluate their biocompatibility and functionality, the latter of which is often related to certain biomechanical properties. Investigations of material functionality can be carried out by exposure of the material to in vivo conditions or to biomimetic simulations in vitro, followed by determination of the mechanical properties. In vitro conditions exhibit advantages in providing precise control and avoiding the complex in vivo environment. This type of investigation can be performed in biomimetic tissue engineering bioreactors. Bioreactor that mimics conditions in articular cartilage and combines dynamic compression and medium flow through the cultivated tissue  was used to examine the mechanical behavior of Ag/PVP nanocomposite hydrogels . Mechanical properties of investigated hydrogels varied from series to series, suggesting relative inhomogeneity of the PVP network and significant dependence of its properties to random processes of polymer cross-linking by radical polymerization and Ag+ ion reduction. Having in mind high compression moduli of human articular cartilage , PVP and Ag/PVP nanocomposite hydrogels have not exhibited mechanical properties necessary for the use as soft tissue implants, but they meet requirements needed for the mechanical properties of wound dressings .
Biocompatibility of Ag/PVP nanocomposites was investigated by cytotoxicity assays in peripheral blood mononuclear cell (PBMC) and human cervix carcinoma cell (HeLa) cultures . It was shown that the cytotoxicity of AgNPs released from Ag/PVP nanocomposites is dose-dependent, so that slight cytotoxicity is induced by Ag/PVP nanocomposites synthesized from 1 mmol dm−3 AgNO3 solution. Kinetics of silver release was examined under static conditions, continuous SBF perfusion (in perfusion bioreactors), and under dynamic compression coupled with SBF perfusion (in the biomimetic bioreactor simulating in vivo conditions in articular cartilage). Diffusion was the dominant mechanism of silver release in static conditions and under SBF perfusion, while a slight contribution of dynamic compression was observed in the biomimetic bioreactor. Silver release kinetics modeling provided estimation of the time allowed for PBMC to be safely exposed to Ag/PVP nanocomposites under static and perfusion conditions (e.g., as wound dressings) of about 3 days. In wound dressing applications, it is a reasonable time for dressing replacement. In addition, it is estimated that concentrations of silver released from an Ag/PVP nanocomposite implant in articular cartilage in a knee joint would be well below the levels that would cause even slight cytotoxicity as determined in this study .
Among stimuli-responsive or intelligent hydrogels, from a biomedical point of view, thermo- and pH-sensitive polymers are the most frequently studied, because the temperature and pH are two factors that are most often changed in a physiological, biological, and chemical systems. Many previous works have been focused on the thermosensitive polymers because of their fundamental and technological interest [85, 86]. Poly(N-isopropylacrylamide) (PNiPAAm) is one of the most investigated thermosensitive polymers with a sharp lower critical solution temperature (LCST) between 30°C and 35°C for PNiPAAm (depending on the detailed microstructure of the macromolecules) . The LCST represents the temperature at which the hydrophobic forces (due to interaction of the −NCH(CH3)2 groups), which lead to insolubility in an aqueous environment, are balanced by H-bonding with water, which maintains a polymer in solution. Below the LCST value, a hydration shell around the hydrophobic groups is formed by hydrogen bonds between the hydrophilic groups in the side chains and water, causing water uptake and swelling of the PNiPAAm. If the external temperature increases above the LCST, the scission of the hydrogen bonds occurs and hydrophobic interactions prevail, causing the leaching of water and collapsing of PNiPAAm, indicating the occurrence of phase separation and volume change. In general, it is well known that the volume phase transition temperature (VPTT) of PNiPAAm-type hydrogel can be adjusted very near to human body temperature (between 37 °C and 41 °C, which is the temperature range of body in response to some diseases) by copolymerization or utilization of additives, which makes them suitable for in vivo applications. The addition of hydrophilic ionic comonomers, such as itaconic acid (IA), maleic acid (MA), or methacrylic acid (MAA), may provide that thermosensitivity of PNiPAAm can be controlled by pH value of the external media [88, 89, 90, 91].
Spasojević et al. investigated dual-responsive, i.e., thermo- and pH-sensitive, Ag-P(NiPAAm/IA) nanocomposite hydrogel obtained by irradiation method . It was shown that increasing of IA content in polymer network increases equilibrium swelling degree value of hydrogels, but it is also increases the VPTT. For the P(NiPAAm/IA) hydrogels, VPTT increases in the range from 30 °C to 42.8 °C with increasing of IA content in network from 0 up to 4.5 wt%. The same trend can be observed also for Ag-P(NiPAAm/IA) nanocomposite hydrogels, but with somewhat lower values of VPTT, in the range from 30.5 °C to 38.5 °C. Such results indicate that the fine-tuning of VPTT values can be achieved by incorporation of AgNPs in copolymer network.
The investigated homopolymeric hydrogels did not show pH dependence of the swelling degree, as expected. In contrast, the swelling of the copolymeric hydrogels was strongly dependent on the pH value of the external medium. At low pH values, the swelling degree was low because the carboxylic groups in the side chains were not ionized and intermolecular complexation via H-bonds occurred. This complexation results in increased hydrophobicity of the network and lower SD values . As the degree of ionization increased above nominal pKa values of IA (pKa1 3.85 and pKa2 5.44) , the greater swelling degree was observed due to the three reasons: most of the H-bonds are broken, COO− ions are more hydrophilic than COOH groups, and the electrostatic repulsion between the COO− ions pushes the network chains apart. The most pronounced pH sensitivity was observed for the samples with highest IA content. Moreover, the incorporation of AgNPs induced slightly decrease in swelling capacity, in comparison with corresponding hydrogels without AgNPs. This was probably caused by the restriction of the large-scale segmental motion of the polymer chains and partial occupation of free space by AgNPs [4, 45]. Furthermore, by increasing of IA content, copolymeric hydrogels become weaker, and such mechanical behavior is in agreement with obtained network parameters. On the other hand, incorporation of AgNPs into polymer matrix improves mechanical properties, especially for systems with a higher IA content. As expected, investigated Ag-P(NiPAAm/IA) hydrogel nanocomposites showed good antibacterial potentials against both Gram-positive and Gram-negative bacteria .
4.2 Nanocomposite Hydrogels with Semiconductor Nanoparticles
Semiconductor nanoparticles or quantum dots, such as lead sulfide (PbS), cadmium sulfide (CdS), and zinc sulfide (ZnS), have specific optical and electronic properties because of their quantum size and dielectric confinement effects. These properties are tunable by varying the size and the morphology of the particles. Nowadays, the challenge is to control these two factors in order to reach the best conditions, permitting the synthesis of adequate materials for advanced technologies.
Among semiconductor nanoparticles, PbS has been used in several areas such as light-emitting diodes, infrared detectors, optic fibers, infrared lasers, solar energy panels, window coatings, and environment as Pb2+ sensors [94, 95]. This wide range of applications is due to its interesting physical properties. The particularly narrow bandgap of PbS gives the possibility to tune the optical absorption in a large domain by reducing the size to the nanometric scale. But, the challenge is not only to synthesize extremely small PbS nanoparticles but also monodispersed ones. Several methods have been used to produce PbS nanoparticles in different environments such as zeolites, glasses, polymers, and inverse micelles or in colloidal state. The radiolytic method has been proven to be an adequate tool to synthesize monodispersed and size-controlled semiconductor nanoparticles. The particle size can be tailored by controlling the irradiation dose. In this way, PbS, CdS, and ZnS particles have been synthesized in the presence of mercaptoethanol [61, 62, 63].
Recent research describes a simple and effective method for preparation of PbS/PVA hydrogel nanocomposite . Namely, the radiolytic in situ synthesis of PbS nanoparticles within the PVA hydrogel, previously cross-linked also by gamma irradiation, was achieved. The obtained PbS/PVA nanocomposite hydrogel shows the optical properties characteristic for semiconductor nanoparticles (quantization effects). The incorporated PbS nanoparticles are around 5 nm in diameter, with the bandgap energy of 1.84 eV. Incorporation of these nanoparticles into PVA hydrogel induces the slightly decreasing of swelling capacity of PbS/PVA nanocomposite hydrogel compared with PVA hydrogel. This probably occurs because the incorporated PbS nanoparticles can act as some sort of additional junction point in polymer network [96, 97]. Therefore, the thermal stability of PbS/PVA nanocomposite hydrogel is slightly enhanced.
This class of nanomaterials can be used for the fabrication of novel photonic materials and “solid-state” solar devices where the spacing between nanoparticles can be tuned for optimum photovoltaic efficiency. In particular, in situ growth of PbS nanoparticles in PVA 3D hydrogel matrix could produce soft 3D material nanocomposites with optical nonlinearity (metamaterials), opening perspectives for its application in special optical devices requiring small bandgap semiconductors embedded almost in water (optical properties of PVA are similar to water) [98, 99].
4.3 Nanocomposite Hydrogels with Magnetic Nanoparticles
The combination of magnetic nanoparticles with polymers in order to obtain colloidal or composite stable systems had attracted much interest. Magnetic nanoparticles are investigated in different areas because they are used in a wide range of applications in science, industry, and medicine. In particular, magnetic nanoparticles are used in data storage, medical drug delivery, hyperthermia, and bioseparation . Magnetic field sensitive gels (ferrogels) are new promising class of nanocomposite hydrogels. Zrinyi et al. developed magnetic field sensitive gels in which magnetic particles of colloidal size are dispersed and incorporated into the gels . These ferrogels combine the magnetic properties of magnetic fillers and the elastic properties of gels. When the gels were placed into a spatially nonuniform magnetic field, forces act on the magnetic particles, and as result of strong interaction between magnetic particles and polymer chains, they all move together as a single unit. The coupling of hydrogels and magnetic particles has potential application in soft actuators such as artificial muscles . On the other hand, recently the use of magnetic sensitive hydrogels has been explored for hyperthermia applications. The polymer networks have properties which make the hydrogels suitable for applications in controlled drug delivery systems, while the magnetic particles, with ferromagnetic or superparamagnetic properties, are used for magnetic hyperthermia .
In order to obtain magnetic sensitive hydrogels, magnetic nanoparticles can be incorporated in polymer network by loading coprecipitation technique as well as by radiation reduction technique . In the both case, the hydrogel matrix was firstly obtained by irradiation-induced cross-linking of polymer chains. The synthesized quasi-spherical magnetite (Fe3O4) nanoparticles were well dispersed in the polymer matrix, with the average particle size around 30 nm. Investigated ferrogels possessed good loading capacity toward doxorubicin hydrochloride (model drug), while the releasing behavior was highly dependent on the pH values of the normal and tumor cells. The anticancer drug release was also affected by the difference between normal tissue that contains tightly connected endothelial cells which prevents the diffusion of the nanomedicines outside the blood vessel and tumor tissue which contains large fenestration between the endothelial cells allowing the nanomedicines to reach the matrix and the tumor cells . These results may offer a suitable way for the preparation of anticancer drug carriers for tumor combination therapy aiming to increase anticancer activity and lower toxicity.
Moreover, ferrogels can be prepared by irradiation-induced cross-linking of polymer chains in the presence of magnetic nanoparticles . In this case, the magnetite nanoparticles were synthesized by coprecipitation method, dispersed in the PVA solution, and exposed to the gamma irradiation to obtain nanocomposite hydrogel. This study shows that irradiation cross-linking of polymer in the presence of particles doesn’t interrupt processes of intra- and/or intermolecular cross-linking of PVA chains, giving PVA/Fe3O4 ferrogel.
In general, nanocomposite hydrogels can be defined as cross-linked polymer network, swelled by water, with entrapped nanoparticles. The incorporated nanoparticles add unique physicochemical properties to polymer hydrogels such as responsiveness to mechanical, optical, thermal, sound, magnetic, electric stimulation, etc. These unique properties lead to applications in such different fields, such as in the electronics, optics, sensors, actuators, and microfluidics sectors, as well as catalysis, separation devices, drug delivery systems, wound dressings, soft tissue implant, and many other biomedicine and biotechnological areas. The huge success of hydrogel systems as biomaterials lies in their resemblance to living tissue because of their high water content which minimizes the frictional irritation of surrounding tissue. Additional advantages of those materials are their nontoxicity, non-irritability, and chemical stability.
Nanocomposite hydrogels can be synthesized by different chemical and physical methods. Among them, irradiation-induced synthesis offers a number of advantages. This method provides fast synthesis and easiness of process control. Moreover, it is green and eco-friendly synthesis because there is no need to use the initiators, cross-linkers, and reducing agents; all reactions in the system are initiated by the product of water radiolysis. Probably the most important, from the biomedical point of view, is the possibility of simultaneous nanocomposite hydrogel synthesis and their sterilization in one technological step.
Despite all the mentioned advantages of irradiation-induced synthesis as well as the nanocomposite hydrogels prepared by this method, research in this area is not so much widespread.
6 Future Scope
In the field of nanocomposite hydrogels, as a relatively new class of hybrid materials, lies tremendous potential for future investigation and applications in many different fields. It is well known that using irradiation for synthesis and modification of nanocomposite hydrogels has numerous advantages, especially for production of biomaterials. On the other hand, it is still a long way to transfer research knowledge into production facilities, in order to extend the use of these materials. However, since the healthcare is one of the main priorities, it seems that research in this area will become even more important and the radiation technology will get an opportunity to show all possibilities.
This work was supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia (Project No. 45005).
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