Morphological Characterization of Hydrogels
Hydrogels are physically or chemically cross-linked polymer networks that are able to absorb large amounts of water. They can be classified into different categories depending on various parameters including the preparation method, the charge, and the mechanical and structural characteristics. The morphological structures are differed from hydrogel compositions to preparation method, fabrication techniques, type of hydrophobic substitutes, etc. This chapter addresses an overview of the morphological characterization of hydrogels and impact of these properties in various potential applications of hydrogels. In a first part, morphological characterizations of hydrogels directly prepared from native materials are described. In a second part, morphological characterizations of hydrogels prepared from different derivatives of native materials by physical as well as chemical cross-linking strategies are introduced. In a third part, morphological characterizations of composite type hydrogels including blending composites, polyelectrolyte complexes, and interpenetrating polymer networks (IPNs) are discussed. In a final part, morphological characterizations of inorganic nanoparticles incorporated hybrid hydrogels are described.
KeywordsSuperabsorbent hydrogels Hydrogel’s morphology Hybrid hydrogel Cellulose
Hydrogel is an insoluble polymeric substance that shows the ability to swell and preserve substantial amount of water in its three-dimensional network . In a different way, hydrogels are explained as polymeric arrangements those exhibit the capability of swelling in water and holding a considerable portion of water (>20%) in their 3D structure, without dissolving in water . Sometimes, to describe the polymeric cross-linked network, the hydrogels and gels are used interchangeably by the biomaterial scientists. Actually whether they are gels or hydrogels depend on their fluidity in steady-state, where the gels are more dilute cross-linked system than hydrogels . This cross-linking is responsible for their insolubility in water because of the ionic interaction and hydrogen bonding , and this degree of cross-linking in hydrogels generally determines their mechanical strength and physical integrity .
There are numerous ways to classify hydrogels. Nowadays, classification based on physical properties has drawn more importance due to their extensive uses in diversified fields and exclusive characteristics, such as capacity of diffusion and swelling. Based on their physical properties and mode of applications, hydrogels can be classified into three ways, namely, solid, semisolid, and liquids . Nevertheless, as they are fundamentally made of cross-linking network, based on hydrogels’ cross-linking, they can be classified into two groups: (a) physically cross-linked or self-assembled hydrogel and (b) chemically cross-linked hydrogel . In the category of physically cross-linked or self-assembled hydrogel preparation, various methods have been reported by various researchers, such as freeze-thawing , stereo complex formation (dissolving each product in water and mixing the solution) , ionic interaction (addition of di- or trivalent counter ions result in hydrogel systems) , hydrogen bonding interactions , and maturation (heat-induced aggregation) . On the other hand, chemically cross-linked hydrogel preparations have been documented numerous testimonies, such as chemical cross-linking , chemical grafting , radiation grafting , radical polymerization , condensation reaction , enzymatic reaction , and high-energy radiation .
Due to hydrogels mechanical strength, physical integrity, biocompatibility, degradability, functionality, flexibility, and adaptability, they have achieved extraordinary appreciation in various fields of engineering and technology. Mentionable sites of their applications are tissue engineering , therapeutic applications , drug delivery , cartilage tissue, soft tissue engineering, cell scaffold, regenerative medicine and cartilage repair , agricultural and horticultural engineering , water purification , antimicrobials , and bio catalysis .
The water sorption ability and swelling kinetics of a hydrogel depend on its porosity. Therefore, increment of porosity in hydrogels is regarded as the most important issue to most researchers. It can be achieved in hydrogels either by physical or chemical techniques. The chemical techniques may encompass phase-separation, foaming, lyophilization, solvent casting, particulate-leaching, etc. , while the physical techniques may include laser sintering  and laser-enhanced surface modification . To portray a complete picture of surface morphology and topography of numerous hydrogels is the prime target of this chapter. With this aim, scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), and atomic force microscopy (AFM) of several types of hydrogels are elucidated comprehensively.
2 Morphological Characterization of Cellulose-Based Hydrogel
Hydrogels, a three-dimensional polymer network able to absorb and release large amount of water without dissolution in a reversible manner, has become a behemoth in research field owing to its versatility in application ranging from agricultural water conservation  to cancer drug delivery system . Though most of the hydrogels prepared are from synthetic polymers such as those formed by cross-linking poly (vinyl alcohol) , poly (N-isopropylacrylamide) , poly (acrylic acid) , poly (amido-amine) , poly(ethylene glycol) , and polyacrylamide , in recent times, natural polymers, mainly cellulose-based hydrogels, have attracted major attention due to their biocompatibility, biodegradability, and low toxicity.
Cellulose, the most abundant natural polymer, has become an enticing proposition as the base material to develop hydrogels. Because of its biocompatibility and inexhaustible nature, cellulose and cellulose-based hydrogels have been unequivocally used in wide range of applications. Cellulose, due to the presence of hydroxyl groups in the structure, offers an easier way of functionalization which leads to formation of so many cellulose-based derivatives. Hydrogels prepared from cellulose and cellulose derivatives can be modified to meet the demands of diversified product demand .
Morphological properties of cellulose-based hydrogels are analyzed by scanning electron microscope (SEM). SEM reveals the porosity and nature of hydrogel structure. Moreover, the effect of modification on the size of the pore is divulged by SEM images. SEM sample is generally prepared by first swelling the hydrogels, then freezing in liquid nitrogen, and finally freeze-drying and sputtering with gold prior to the SEM observation [40, 41]. Atomic force microscope (AFM) helps to evaluate the topological attributes of the hydrogels, where AFM discloses the uniformity of surface roughness of the hydrogels .
2.1 Native Cellulose-Based Hydrogel
Native cellulose-based hydrogels, owing to the presence of many hydroxyl groups, have been prepared through physical cross-linking. One such native cellulose-based hydrogel was prepared by adding a cross-linking agent in cellulose solution. NaOH-urea was used as the solvent to dissolve cellulose for the preparation of cellulose hydrogel with epichlorohydrin as a cross-linker. This was a “one-step” method which used unsubstituted cellulose . Two different posttreatment methods were compared in morphological characterization. One was heating treatment, and the other one was freezing treatment .
Kentaro et al. reported fabrication of hydrogel from cellulose nanofibers by single alkaline treatment only. There was morphological difference in hydrogels prepared by this method depending on the concentration of alkali used .
2.2 Cellulose Derivative-Based Hydrogel
Biocompatible cellulose derivatives, such as carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, and cellulose acetoacetate, have become an important competent in hydrogel fabrication. These hydrogels can be formed through physical-linking or chemical cross-linking [46, 47].
A superabsorbent hydrogel was prepared from hydroxyethyl cellulose (HEC), a cellulose derivative, and acrylic acid (AAc) by radiation-initiated cross-linking in aqueous solution . Hydroxyethyl cellulose (HEC), one of the most widely used cellulose derivatives, is known for its application as a stabilizer, coating agent, or thickener. Recently, it has generated interest in hydrogel synthesis. For modifying gel properties, a second polymer was introduced in the hydrogel system. Acrylic acid (AAc), a polymer with low cytotoxicity and good swelling properties, was incorporated into the hydrogel to improve gel properties . HEC of three different molar masses such as Mv = 90,000 named HEC90, Mv = 720,000 named HEC720, and Mv = 1,300,000 named HEC1300 were used .
On the other hand, HEC1300 gels showed a wide pore size with relatively smaller pores and homogeneous surface (Fig. 3e and f). The difference in pore size between current and previous gel could be due to higher crosslink density leading to substantial lower water uptake before freeze-drying. In HEC1300/AAc gels containing 5% AAc, a denser structure with smaller pores (Fig. 3g and h) can be located owing to the improved gelation in presence of monomer. Here, heterogeneity of the surface is more prominent compared to HEC90/AAc gels. This can be a result of high solution viscosity of high-molecular-mass HEC. As the mobility of acrylic acid is less impeded by the increased viscosity than the large polymer chains, homopolymerization becomes dominant which leads to rough network surface. High AAc content has no substantial effect on the gel properties, but it does impact homogeneity negatively .
Hongchen et al. reported a cellulose derivative-based smart hydrogel by combining cellulose acetoacetate (CAA) and cystamine dihydrochloride (CYS). This hydrogel exhibited good response to both pH and redox triggers . Stimuli-responsive hydrogels have become a new avenue of research because of their potential application in sensing , cell culture , drug release , tissue scaffolding , and 3D printing . Smart responsive hydrogels are capable of changing their size and shape in response to environmental stimuli such as temperature , enzymes , pH , light , and electric and magnetic field . The pH/redox dual-responsive cellulose hydrogels prepared from CAA and CYS combined a pH responsive dynamic covalent enamine moiety and a redox-sensitive disulfide moiety [61, 62]. They demonstrated capacity to work in physiological conditions which opens up possible application as smart sensors and targeted drug release.
SEM images were investigated to test morphological change of the hydrogel under physiological condition such as in phosphate-buffered saline (PBS) solution (pH = 7.4).
Jiaojiao et al. fabricated a stimuli responsive hydrogel, prepared from hydroxyethyl cellulose and lignosulfonate-graft-poly (acrylic acid), having semi-interpenetrating network (semi-IPN) structure . Lignin, the second most abundant natural polymer after cellulose, is generated in large amount during pulping process all over the world [65, 66, 67]. Lignosulfonate (LS), generally treated as a waste product in pulping industry, offers sufficient reactant functional groups to make it an attractive component in different applications [68, 69, 70]. Hydroxyethyl cellulose (HEC), a hydrogen-bond acceptor, was combined with acrylic acid (AAc), a hydrogen bond donor, and lignosulfonate in HEC solution by in situ polymerization. These hydrogels exhibited good mechanical properties and stimuli-responsive swelling properties which can open door for many potential new applications .
2.3 Cellulose-Biopolymer Composite Hydrogel
Cellulose and different biopolymers, such as gelatin, alginate, starch, and hyaluronate, have been blended to fabricate hydrogels with excellent properties applicable in various applications. Treesuppharat et al. successfully combined bacterial cellulose with gelatin to prepare hydrogel composite material useful in drug delivery systems . Gelatin, owing to its attractive properties like high water absorption, non-toxicity, and biodegradability, has become a notable contender in biopolymer-based hydrogels fabrication intended to be used in drug delivery systems [72, 73, 74]. Bacterial cellulose, one of the most effective reinforcement materials, has structure similar to plant-based cellulose but offers added advantages such as high specific surface area, high degree of polymerization, and high crystallinity. Presence of bacterial cellulose in cellulose-gelatin hydrogel imparts tensile strength and dimensional stability when put under externally applied forces [75, 76].
Linseed gum-cellulose hydrogels were developed by mixing cellulose and linseed gum in NaOH/urea solvent and epichlorohydrin was used as cross-linking agent . This hydrogel was prepared with water conservation as the intended application, as linseed gum possessed good water retention capability owing to its high swelling ratio and viscosity. But as linseed gum exhibits weak gel properties, it was combined with cellulose to impart toughness. Moreover, chlorohydrin was added to bring its superabsorbent property to the fabricated hydrogel . The linseed gum-cellulose composite solutions were prepared by mixing linseed gum (LG) and cellulose (C) solutions with ratio of 4:6, 3:7, 2:8, 1:9, and 0:10 (ratio of solid contents of linseed gum and cellulose) by weight, respectively. The composite hydrogels were named as G1 (LG-C ratio 4:6), G2 (LG-C ratio 3:7), G3 (LG-C ratio 2:8), G4 (LG-C ratio 1:9), and G5 (LG-C ratio 0:10) according to the ratio of linseed gum to cellulose.
2.4 Cellulose-Inorganic Nanoparticle Hybrid Hydrogel
Cellulose-inorganic nanoparticle hybrid hydrogels have garnered interest due to potential applications in optical, magnetic, electronic, and biomedical fields .Monireh et al. have conducted a study on superabsorbent hydrogel based on carboxymethyl cellulose (CMC) and graphene oxide nanoparticles. This hybrid hydrogel was prepared, with controlled drug delivery as intended application, by physically cross-linking cellulose and graphene oxide nanoparticles with FeCl3.6H2O. In contrast to pure polymer hydrogels, nanoparticle-incorporated hydrogels exhibited better results owing to their stronger physical, chemical, and biological properties [80, 81]. Graphene oxide (GO), due to biocompatibility, low toxicity, and amphiphilic nature owing to the functional groups such as hydroxyl, epoxide, and carboxyl on the surface, is an attractive candidate in drug delivery system, which also happens to greatly enhance the mechanical properties of CMC [82, 83, 84]. CMC, a biodegradable polymer with multiple carboxyl groups, exhibits good coordination with metal and thus forms excellent hybrid hydrogels .
Another hybrid hydrogel was prepared from hydroxypropyl cellulose (HPC), a cellulose derivative, and inorganic nanoparticle such as molybdenum disulfide (MoS2) with intended application as dye absorbent .
Hydroxypropyl cellulose, an important cellulose derivative due to its high water solubility, has been used to remove dyes from aqueous solution . But its relatively low absorption capacity has limited its application as absorbent [88, 89]. To counteract this limitation, incorporation of nanoparticle with large surface area and multiple functional group is often deployed [90, 91]. Moreover, addition of nanoparticle to the hydrogel system improves mechanical properties of cellulose-based hydrogels. Molybdenum disulfide (MoS2) has been used in the fabrication of hydrogels due to its two-dimensional structure, which is expected to improve absorption capacity of the hydrogels. In addition to this, MoS2 has been widely used as photocatalysts, which catalyzes the degradation of organic dyes facilitating its removal from aqueous system .
3 Morphological Characterization of Chitosan-Based Hydrogels
Chitosan is the deacetylated product of chitin (N-acetyl-d-glucosamine) which is the second most abundant natural biopolymers after cellulose. Recently chitosan and chitosan-based hydrogels have gained considerable attention for their unique properties like stability, biocompatibility, stimuli sensitivity, biodegradability, bacteriostatic effects, and mechanical strength to be used in numerous applications like in drug delivery, protein release, tissue engineering, dye removal, wastewater treatment, etc. [94, 95, 96].
Chitosan and chitosan-based hydrogels can be prepared by physical, chemical, or radiation-induced cross-linking, and the physical characteristics like pore size, distribution of pores, and wall thickness vary from one method to another. To tune these parameters for intended application, morphological characterization is essential. Morphological characterization reveals the shape, size, porosity, and size distribution of pores of the hydrogel which must be known to control or tailor these parameters for the desired application .
Morphological analysis of hydrogels is quite difficult due to the delicate nature of the system and chance of collapse of pore structure during dehydration. However a number of techniques are available for microstructural analysis of hydrogels like scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), confocal laser scanning fluorescence microscope (CLSM), fluorescence microscope (FM), etc., but the contrast and resolution of the microscope system are very important; lack of which can lead to poor images .
3.1 Scanning Electron Microscopy of Chitosan-Based Hydrogels
For morphological characterization of hydrogels by SEM, special sample preparations are required. To observe the surface morphology, after gelation, hydrogel samples are quickly frozen in refrigerator or in liquid nitrogen and further lyophilized with a freeze dryer system under vacuum at −50 °C to −70 °C for at least 48 h until all of the water is sublimed . The sample is then mounted on a metal stub usually made of aluminum with conductive tape and is coated with metal for conductance under vacuum by a sputter. Gold is used to coat the freeze-dried sample widely, but gold-palladium or platinum are also used [96, 100]. To observe the interior morphology, i.e., cross-sectional view of hydrogels, the freeze-dried hydrogels are usually fractured and then sputter-coated with metal .
When chitosan is chemically cross-linked to prepare hydrogel, the degree of cross-linking can affect the bulk and surface morphology of freeze-dried hydrogels. In case of hydrogel of chitosan prepared by chemically cross-linking with glutaraldehyde, the concentration of glutaraldehyde governs the size and distribution of pores. Increase in concentration of glutaraldehyde causes the growth of pore size and decreases their distribution by restricting the free movement of the polymer chains. As a result, the swelling ratio of the prepared hydrogel also decreases. As the pore size of hydrogel can significantly change gel swelling, it can also affect the properties of hydrogel like drug delivery behavior, enzyme activity, biocompatibility, etc., hence the importance of morphological analysis .
Polyacrylamide-chitosan hydrogel, a chitosan-based hydrogel, was found to have the ability to release drug in a sustained manner . The surface and cross-sectional morphology of the freeze-dried hydrogel was examined by SEM which revealed porous nature of the matrix with interconnected channel-like structures. The SEM images also helped to compare the nature of polyacrylamide-chitosan hydrogel over pure chitosan matrices which are fragile and exhibit uncontrollable porosity. The freeze-dried hydrogel showed a faster and extensive swelling which can be explained by the morphological analysis. As the hydrogel has interconnected pores, these channel-like structures can take up the water phase in the matrix by capillary action, and as a result swelling is achieved by fast diffusion of solvent in the matrix. Thus morphological characterization helps to explain various physical properties of hydrogel like hydrophilic nature, hardness, swelling behavior, etc.
The effect of constituent’s molecular weight on the structure of hydrogel can also be determined by analyzing SEM images. An injectable triple cross-linking network hydrogel prepared from chitosan and poly(ethylene glycol) diacrylate (PEGDA) was found to have a highly macroporous, sponge-like structure with average pore size of 20–60 μm . However, changing the molecular weight of one of the constituents PEGDA, it was found that the pore size became larger with the increase of molecular weight of PEGDA. This phenomenon arises because PEGDA with higher molecular weight has better solubility and helps it to stretch sufficiently to create a larger pore size network. Such porous structure of hydrogel is helpful for the delivery of macromolecular compounds as they can diffuse freely into the pores.
In nanocomposite hydrogels like chitosan-iron oxide coated graphene oxide (GIO) nanocomposite hydrogel prepared by gel casting technique, SEM images help to observe the nature of distribution of nanomaterials in the polymeric hydrogel matrix. From SEM images, the chitosan-GIO nanocomposite hydrogel was found to have a compact packing of hydrogel network macrostructures with surface roughness which is the unique morphology of hydrogel prepared by gel casting technique. It is also possible to tune the surface properties like hydrophobicity which can be changed by varying the loading percentage of iron oxide coated graphene oxide nanomaterial in the chitosan matrix by analyzing SEM images .
3.2 Laser Scanning Confocal Microscopy of Chitosan-Based Hydrogels
Although SEM is the most established method to observe the morphology of hydrogel because of its ability to provide structural information in sufficient detail, this technique suffers from some disadvantages of which collapse of pore structure during dehydration leading to volume shrinkage is the most serious one. In the native state, hydrogel contains substantial amount of water which must be removed prior to SEM examination that affects the morphology of hydrogel.
Two common methods used to dehydrate hydrogels prior to examination by SEM are freeze-drying and critical point drying . In case of freeze-drying, if the rate of freezing is too slow, then ice crystals are formed instead of vitreous ice which can cause damage to the sample. In case of critical point drying, the hydrogel may melt if the temperature required to reach supercritical conditions is above the glass transition point of the hydrogel resulting in distortion or destruction of pore structure. Moreover to examine the internal structure, freeze-dried hydrogel has to be fractured which causes further impairments.
Laser scanning confocal microscopy (LSCM) is an alternative method to investigate the morphology of hydrogel in its native state, i.e., hydrated state. It is a far simpler and more rapid technique for imaging hydrogels than SEM. LSCM allows to take images of hydrogel without fracturing or cutting the sample . In this method a series of images taken at successive intervals and by magnifying special region of interest can be superimposed to get detailed morphological information. With the help of application software that utilizes a series of successive LSCM images, the three-dimensional nature of hydrogel can also be observed.
The basic principle of LSCM includes labeling the hydrogel sample with a fluorescent dye and excitation of the sample by laser light. The emitted red-shifted light from the fluorescent dye is then detected and used to construct an image. As both labeling and imaging can be carried out in the hydrated state of hydrogel so no dehydration of hydrogel is required, as a result the aforementioned problems encountered in SEM can be avoided .
Typical sample preparation for LSCM involves soaking of hydrogel sample in aqueous solution of fluorescent dye, e.g., fluorescein isothiocyanate or rhodamine B isothiocyanate for about 24 h. To remove excess dye, the sample is rinsed off with distilled water for several times. The rinsing operation is carried out in the dark to prevent photobleaching of the dye, and after completion of washing, the sample is usually stored in dark at 4 °C .
4 Morphological Characterization of Collagen-Based Hydrogels
Collagen is the most abundant protein in mammals and the main structural component in the connective tissue of extracellular space. Degree of mineralization decides whether collagen tissue would be rigid (bone) or compliant (tendon). Cartilage is a special kind of collagen tissue which inclines from hardness to flexibility. According to Robert H. Bogue, collagen is not a simple anhydrous of gelatin but rather a polarized complex produced by chemical condensation . Collagen becoming a natural product has extensive use in wound healing , bone grafting , and regeneration of tissue by scaffolding .
The water sorption ability and swelling kinetics of a hydrogel depend on the porosity of hydrogels. For instance, to make available space for cell seeding, growth, and proliferation, optimum porosity of hydrogel for tissue regeneration is ranging from 5 μm in neovascularization to 100–350 μm in bone regeneration . Therefore, increment of porosity in hydrogels is regarded as the most important issue in most researchers. It can be achieved in collagen-based hydrogels either by physical or chemical techniques. The chemical techniques may encompass phase-separation, foaming, lyophilization, solvent casting, particulate-leaching, etc. , while the physical techniques may include laser sintering  and laser -enhanced surface modification .
4.1 Scanning Electron Microscopy of Collagen-Based Hydrogels
Clay-free composite hydrogels usually show higher porosity than its counterparts. It appeared in an investigation of Kabiri et al. that porogen has lost its efficiency in presence of clay while producing porous collagen-based composite incorporating poly (AA-co-potassium acrylate) and kaolin in collagen matrix . They investigated the morphology of superadsobent hydrogel composite with different porogens such acetone, sodium bicarbonate, and acetone-sodium bicarbonate in combined. It was observed that porosity was lowered considerably due to addition of kaolin .
4.2 Atomic Force Microscopy of Collagen-Based Hydrogels
Atomic force microscopy (AFM) is a powerful tool for surface analysis. To know the surface topography with nano or even atomic resolution AFM has been contributing widely since early 1980 . While the electron microscope provides a two-dimensional projection or image of a sample, the AFM delivers three-dimensional surface details. In addition, AFM does not require any extensive sample preparation (such as carbon or metal coatings) which is mandatory for SEM or TEM. Above all, high-resolution AFM conveys comparable information in consideration with SEM, TEM, and STM.
5 Morphological Characterization of Gelatin-Based Hydrogels
Gelatin is an assortment of peptide and proteins produced by partial hydrolysis of collagen extracted (by chemical denaturation) from the skin, bones, and connective tissues of animals such as domesticated chicken, cattle, pigs, and fish . During hydrolysis, the natural molecular bonds between individual collagen strands are broken down into a form that rearranges more easily. Its chemical composition is, in many aspects, closely similar to that of its parent collagen . Gelatin is generally consist of carbon, 6.8% hydrogen, 17% nitrogen, and 25.2% oxygen . Gelatin has extensive use in food, pharmaceutical, cosmetic, and photographic industries, as it has the distinctive functional properties. In the field of food industry, gelatin is regularly being used in bakery, dairy, beverages, and confectionary for gelling, emulsification, texturization, and stabilization . In the meantime, gelatin is widely being used in drugs encapsulation (both as hard and soft form), ointment filling, wound dressing, plasma expanding, and emulsification in pharmaceutical industries . Moreover, gelatin has been applied as emulsion layer, non-cult layer, and coating layer on the photographic materials .
5.1 Scanning Electron Microscopy of Gelatin-Based Hydrogels
As mentioned previously in this chapter, hydrogels can be prepared either using chemical method  or physical method . Interconnection of pores cannot be assured in the hydrogels which are generally produced by chemical methods, even the organization of pore size are complicated due to uneven evaporation during drying. Moreover, risk of toxicity and carcinogenicity due to existence of residual solvent discourages hydrogels produced by chemical method for application of public health .
In concern with physical well-being, physical approaches of hydrogel preparation are more attractive in opposition to chemical techniques. Physical method like femtosecond laser for modification is a one-step process with precision, noncontiguous to chemicals or solvents, and free from objectionable thermal damage . Daskalova et al. displayed the efficacy of application of femtosecond laser pulses for successful modification of surfaces of gelatin thin films, for creation of micro- and nanoscale structures . Laser used by them was a CPA Ti: sapphire laser (Femtopower Compact pro) emitting at 800 nm central wavelength, with pulse duration of 25 fs, at repetition rate of 1 KHz, and average output power of 800 mW. Condition used for the treatment was air and sample object was placed few mm away to avoid nonlinear optical effect of air. The size of the spot was 182 μm measured by shot diameter regression technique . The films of gelatin were irradiated with various fluences (a stream of particles crossing a unit area, generally represented by F with a unit of Joule per square cm) and various numbers of pulses (unitless and dimensionless parameter generally represented by N). Then the irradiated thin films of gelatin was observed utilizing SEM .
6 Morphological Characteristics of Synthetic Polymer Hydrogels
Hydrogels are the three-dimensional polymer networks that are swollen by trapping large amounts of water. Those networks are formed by molecular self-assembly through covalent, ionic, or hydrogen bonds . According to the sources, hydrogel can be classified into those formed from synthetic polymers and those formed from natural polymers. Both types of hydrogels have versatile applications in food products to medical purposes. Owing to the adjustable mechanical properties, ability for photopolymerization, and easy control of scaffold architecture and chemical compositions, the demand of synthetic polymer hydrogel has increased exponentially . Among the vast polymer application, only limited polymers have the ability to form hydrogel networks. Some of the synthetic polymer-based hydrogels have been reported such as poly(ethylene glycol) , poly(vinyl alcohol) , poly(amido-amine) , poly(N-isopropylacrylamide) , polyacrylamide , and poly(acrylic acid)  and their copolymers . The physical characteristics such as amount of void space, pore size, wall thickness, etc. vary from polymer to polymer, fillers as well as preparation method. In general, the pore size increases with an increasing swelling ratio of the hydrogels . The sampling for morphology analysis of synthetic polymer was similar to natural polymers.
Yu et al. prepared silver nanoparticles incorporated poly(vinyl alcohol)/poly(N-vinyl pyrrolidone) (PVA-PVP) hydrogels by repeated freezing-thawing treatment . The surface and cross-sectional morphologies of Ag/PVA-PVP composite hydrogels were investigated by SEM; both PVA-PVP and Ag/PVA-PVP hydrogels showed porous dimensional network structure. No distinguished difference was found among the hydrogels with different silver contents due to stable network structure within hydrogel and the strong interaction between the silver particles and the PVA and PVP molecules. Owing to excellent antibacterial ability, superb water retention ability and good oxygen transportation capability, Ag/PVA-PVP may be used a potential wound care dressing.
Hu et al. have prepared polyvinyl alcohol/carbon dot (PVA/C-dot) hydrogel by freeze-thaw method, and Ag nanoparticles was simply introduced to enhance the antimicrobial activity and enlarge their application potential in medical field .The morphology of this composite hydrogel was investigated by SEM and observed that nanoparticles were uniformly dispersed in PVA/C-dot-Ag hydrogel. The mapping image also showed that the Ag element was uniformly dispersed in PVA/C-dot-Ag hydrogel, indicating uniform dispersion of Ag nanoparticles in PVA/C-dot-Ag hydrogel.
Zhou and Li synthesized temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm)/poly(ethylene glycol)s (PEGs) hydrogels . The effect of molecular weight (2000–6000) and PEG content on the morphology of these hydrogels were analyzed with scanning electron microscopy. The micrograph revealed that the PEG-modified PNIPAAm hydrogels have more porous networks and the surfaces are looser than those of the conventional hydrogels. In addition, the average pore size of the PEG-modified hydrogels becomes larger with higher-molecular-weight PEGs and higher PEG content.
Li investigated the morphology of dry poly(acrylic acid-acrylamide-methacrylate)-amylose hydrogel by using field emission scanning microscopy (FE-SEM). The micrograph showed that the hydrophilic amylose homogeneously dispersed in poly(acrylic acid-acrylamide-methacrylate) hydrogel and gave large porous network structure. This highly porous (pore size ~ 100 μm) structure hydrogel may be suitable adsorbent for crystal violet .
The effects of saline water and buffer solutions in poly(vinyl alcohol) (PVA)/poly(acrylic acid) (PAA) hydrogels on morphology have been reported . The PVA/PAA hydrogels was hydrated in saline water at pH 2.8 and pH 5.8, and network structure was observed. A strong effect of the pH was observed in the pore size and inner structure of hydrogels. At pH 2.8, it displayed irregular pore shapes with variable sizes and at pH 5.8, expanded porous structure due to larger swelling was observed.
Morphological characterization of hydrogel reveals essential information regarding the surface of hydrogel as well as the size and shape of the pores in the structure of the hydrogel. Moreover, these characterization information are vital in determining the feasibility of the intended application of the synthesized hydrogel. Morphological properties like porosity of hydrogels, typically analyzed by scanning electron microscope images, are important parameters in deciding the effectiveness of hydrogels in application such as drug delivery system. Not only that, laser scanning confocal microscopy has been successfully used to analyze multilayered hydrogel with oriented structure and in cases where SEM is not viable due to risk of structural collapse during dehydration in SEM technique. As a result, these morphological techniques have been extensively used in designing hydrogels from both cellulose and other biopolymers.
Hydrogels have become a tremendously popular field in research fraternity. The scope it offers far outweighs the trivial disadvantages it brings in applications. Advantages, such as low cost, non-toxicity, hydrophilicity, biodegradability, transparency, and biocompatibility, which biopolymers, particularly cellulose-based hydrogels, bring in, are unmatched and drawing more attention toward this field. In fact, more and more avenues – drug delivery system, water purification adsorbent, chromatographic supports, and biosensors – are being ventured into to unfurl the true prospect of hydrogels. But there are still plethoras of opportunities available to look into for further investigation in the field of cellulose and other biopolymer-based hydrogels. Because of these unrivaled attributes and exponentially growing interest, cellulose-based hydrogel superabsorbents have the potential to become a sustainable and worthy replacement of synthetic polymer-based hydrogels.
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