Original data and documents: - Molecular structure of sodium alginate and acrylic acid; - Fabrication process of alginate-acrylic acid hydrogels via an ion-diffusion method; - Evaluatio
INTRODUCTION
General introduction of hydrogel
Hydrogel is a three-dimensional network structure that is water-loving and capable of absorbing a large amount of water or biofluids without dissolving in water [1] They can retain a significant amount of water without affecting their initial structure, which provides flexibility and swelling properties to the hydrogel structure With these characteristics, along with polar functional groups such as amines, amides, carboxylic acids, hydroxyl, and sulfonic acids, hydrogels meet the standards for various applications and hold great promise for the future This unique property is similar to living tissues due to their high water-holding capacity, absorption capabilities, and homogeneity Therefore, hydrogels have applications in the field of biomedical science, such as replacing load-bearing parts, cells, and tissues, developing supercapacitors, smart sensors, wound care, drug delivery, and distribution [2]
Due to their unique properties, such as biodegradability, biocompatibility, water affinity, superabsorbency, viscosity, softness, and smoothness, hydrogels play a crucial role in biomedical applications Additionally, hydrogels can react to various stimuli, such as temperature, electric field, magnetic field, biological molecules, and ion intensity Many hydrogels have the ability to prolong drug release because of their mucosal and biological adhesion properties, making them suitable candidates as drug carriers [3]
Alginate, extracted from brown algae, has a molecular structure made up of alternating blocks of α-L-Guluronic acid [G] and β-D-Mannuronic acid [M] The α-L-Guluronic acid [G] blocks are essential for ion binding, crucial in the gelation process of alginate They easily bind with ions like Ca 2+ to create Ca-Alginate hydrogels [4,5] Due to alginate's biocompatibility and biodegradability, Ca-Alginate hydrogels find widespread use in wound dressings, medical sponges, post-surgical recovery support materials, and gel-based drug delivery Hydrogels with responsive properties, like pH, temperature, or concentration- dependent changes, have been developed to enhance drug release, regulate healing processes, and provide supportive materials for recovery
Furthermore, hydrogels are also used in the agricultural industry to improve the quality and efficiency of crops They can retain water and nutrients, creating an ideal environment for plant growth and minimizing water loss during irrigation
Hydrogels also find applications in environmental technology, such as wastewater treatment and the adsorption of pollutants They have the ability to absorb both organic and inorganic substances, improving water quality and minimizing negative impacts on the environment
In conclusion, hydrogels are versatile and multifunctional materials with the ability to retain and absorb water in various environments With their unique characteristics, hydrogels have found widespread applications in the fields of healthcare, agriculture, and environmental technology, bringing numerous benefits and potential for improving quality of life and protecting the environment.
Topic importance
The significance of hydrogel lies in its wide range of applications and beneficial properties Here are some examples highlighting the importance of hydrogel
Biomedical Applications: Hydrogel plays a crucial role in the field of biomedicine It is widely used for drug delivery systems, tissue engineering, wound healing, and medical regeneration Hydrogel can encapsulate and release drugs in a controlled manner, providing targeted and sustained drug delivery It can also mimic the extracellular matrix (ECM) of tissues, promoting cell development and tissue regeneration [6]
Wound healing and dressing: Hydrogel is commonly used in wound dressings due to its ability to create a moist environment that accelerates the healing process They provide a protective barrier, prevent infection, and promote tissue regeneration Hydrogel dressings are non-adherent, easy to apply, and can absorb excess exudate from the wound [6]
Contact lenses and ophthalmology: Hydrogel-based contact lenses are widely used due to their high water content, which enhances comfort and oxygen permeability They provide a soft and flexible surface for the eyes, allowing for extended wear Additionally, hydrogel is also used in ophthalmology for drug delivery systems, corneal sensory tissue engineering, and intraocular lenses [6]
Agriculture and soil conservation: Hydrogel plays a significant role in agriculture, especially in arid and drought-prone regions They can absorb and retain water in the soil, reducing the frequency of irrigation and water loss Soil amendments based on hydrogel improve soil structure, nutrient retention, and seed germination, resulting in increased crop productivity and water efficiency [7]
Environmental applications: Hydrogel is used in various environmental applications such as wastewater treatment, soil remediation, and pollutant removal They can absorb and immobilize pollutants, heavy metals, and organic contaminants, helping to clean up polluted environments and improve water quality
Personal care products: Hydrogel is integrated into many personal care products, including moisturizers, face masks, and cosmetics They provide hydration and cooling effects and improve the delivery of active ingredients to the skin, enhancing the effectiveness of skincare
In 3D printing, hydrogel is used to fabricate complex structures and scaffolds for tissue engineering and medical regeneration They allow for precise control of the structure and mechanical properties of the printed constructs, enabling the production of patient-specific tissues and organ grafts.
Aims of this capstone project
In this study, we conducted research and development on a type of Ca-alginate– polyacrylic acid hydrogel to assess its tensile properties and Through the combination of their mechanical properties, the developed hydrogel exhibiting excellent mechanical properties was determined.
Research limitations
In this experiment, we produced various types of Ca-alginate–polyacrylic acid hydrogels We conducted research and experiments to fabricate these hydrogels, including varying the weight percentage of alginate (2wt%–5wt%), the molar concentration, immersion time in NaCl solution, and the molar concentration of the AAc solution We tested the tensile strength and evaluated the mechanical properties to select a suitable and stable process for the gel and compare its mechanical characteristics with those of the initial gel.
Approaching methods
- A survey on fabrications and applications of Ca-alginate hydrogels
- Synthesize all initial gels and develop their mechanical properties
- Conduct research to develop a method for synthesizing Ca-alginate–polyacrylic acid hydrogel
- Describe the molecular structure of sodium alginate
- Explain the process of creating a hydrogel based on calcium alginate using the diffusion method
- Discuss the polymer chain alignment method
- Evaluate the mechanical properties of the hydrogel through a tensile strength test
FABRICATING PHYSICAL ALGINATE HYDROGELS AND
Classification of Hydrogels
Hydrogels can be classified based on different factors, and their classification is summed up briefly in (Figure 1) The classification of hydrogels depends on the materials (polymers) involved, the source of the polymers, the crosslinking method, their response to stimuli, and their ionic charge Polymers involved in the hydrogels are natural, synthetic, or a combination of natural and synthetic polymers These polymers can form hydrogels as homopolymer hydrogels, copolymer hydrogels, block copolymer hydrogels, terpolymers, and so on Moreover, hydrogels are prepared by crosslinking polymers, and the crosslinking can be physical, chemical, or both simultaneously Crosslinking is formed in numerous ways as well, such as through simple mixing, solution casting, bulk polymerization, free radical polymerization, UV and gamma irradiation, and the interpenetrating network formation method Hydrogels can also be classified, based on ionic charge, as cationic, anionic, and neutral hydrogels The charge on the overall network depends on the charge on the polymer
Hydrogels are classified into chemical, physical, and hybrid types based on the crosslinking forces between polymeric chains Physical hydrogels form through non-covalent forces like hydrogen bonding, ionic interactions, and Van der Waals forces These hydrogels exhibit a reversible response to environmental changes due to relatively weak interactions,
6 making them disordered and mechanically weak when subjected to external stimuli They can dissolve in organic solvents and water upon heating
On the other hand, chemical hydrogels, or permanent hydrogels, involve covalent bonding between polymer chains They do not dissolve in the surrounding medium and lack the reversible response seen in physical hydrogels Chemical crosslinking, achieved through various methods using small molecules, forms strong covalent bonds This imparts excellent thermal, mechanical, chemical, and surface properties to the hydrogels The network structure of chemical hydrogels is maintained, even in a fully swollen state, due to the presence of strong covalent bonds
Chemical crosslinking mechanisms include condensation reactions or free radical mechanisms, the latter requiring crosslinkers containing at least two double bonds to bind with polymer chains on either side [9] This process ensures the stability and integrity of the hydrogel network, providing desirable characteristics for various applications.
Physical Crosslinked Hydrogels
Hydrogels need specific conditions to form: (a) strong interactions between polymer chains for a stable molecular network, and (b) a structure that allows water access and residence within the hydrogel Achieving these conditions can be done through non-covalent methods, such as electrostatic, hydrogen bonding, and hydrophobic forces among polymer chains Hydrogels formed by these interactions are purely physical gels, exhibiting high water sensitivity and thermo-reversibility [10] However, they have a short lifespan, typically lasting from a few days to a month in physiological media This makes physical gels advantageous for short-term drug release, and they are considered safe for clinical use as their gelation doesn't involve toxic covalent crosslinking molecules
Polysaccharide-based physical hydrogels, like chitosan, can be synthesized by combining polymers under suitable conditions For instance, ionic complexation of chitosan with small anionic molecules forms physical hydrogels Anions, like sulfates, phosphates, or citrates, bind to chitosan via its protonated amino group The properties of these hydrogels depend on factors such as the charge and size of anions, chitosan concentration, and its level of deacetylation However, chitosan's ability to form ionic complexes is reduced above pH 6, limiting its applications in physiological media [11]
Chemically Crosslinked Hydrogels
As a result of molecules aggregating to form physical gels, free chain loops and inhomogeneity—a hallmark of transient network imperfections—are created Compared to physical hydrogels, chemically crosslinked hydrogel networks are easier to manage since their synthesis and uses are not only reliant on pH It is possible to change the hydrogels' physical characteristics by chemical crosslinking Covalent crosslinking has often been used to control mechanical strength, swelling behavior, and biodegradability There are several methods for achieving covalent cross-linking These are covered in the section that follows
Polymers can be strengthened by crosslinking them with small molecules like formaldehyde, glutaraldehyde, genipin, diethyl squarate, ethyleneglycol diglycidyl ether (EGDE), and blocked diisocyanate, as shown in (Table 1) This covalent crosslinking improves the mechanical strength of polymers compared to physical crosslinking However, some crosslinkers may raise concerns about their biocompatibility To address this, hydrogels are created in situ by modifying polymers with reactive functional groups, eliminating the need for external crosslinking agents
Different hydrogels are generated according to specific functional groups, as indicated in (Table 2) For instance, Schiff bases are used to create chitosan hydrogels using pre- functionalized polysaccharides such as aldehyde hyaluronic acid and oxidized dextran [12] Enhancing the compressive modulus for cartilage tissue engineering, N-succinyl chitosan/aldehyde functionalized hyaluronic acid injectable composite hydrogels have also been created using a Schiff base process [13] Alginate and cellulose hydrogels are made in a similar way [14, 15]
Hydrogels can also be formed from amino group-containing polymers like chitosan through Michael addition reactions, where amino groups react with the vinyl group of other polymers Chitosan-polyethylene oxide (PEO) hydrogels have been created using this method, enhancing mucoadhesive properties with a short reaction time and mild reactivity towards biomolecules [16] Despite these advantages, drawbacks include multi-step preparation and purification processes, and there's a risk of polymers becoming cytotoxic after functionalization with reactive groups
Table 1 Crosslinking of polymers through small molecules [17]
Table 2 Crosslinking of polymers through reactive functional groups [17]
Comparing hydrogels with light-sensitive functional groups to chemical crosslinking techniques reveals advantages such as simplicity, speed of synthesis, and cost-effectiveness Lactose and azide were used as light-sensitive moieties by Ono et al to create UV light- irradiated chitosan hydrogels The azide group changed into a nitrene group during UV irradiation, and it quickly bound to the amino groups in chitosan to create a hydrogel [16] Pre-functionalization with photosensitive acrylates of chitosan and pluronic acid was used to create additional UV-irradiated chitosan hydrogels [18]
A new paper describes the use of UV light to facilitate step or chain-growth polymerization of methylacrylate-functionalized chitosan hydrogels, with dithiothreitol (DTT) serving as a chain-transfer agent 2-hydroxy-2-methylpropiophenone and a UV LED (365 nm) with different power densities (13.4 or 268 mW/cm2) were utilized in photo- initiation Chitosan hydrogels created by photo-initiated free radical polymerization (FRP) were designated with the letter RP; subscripts L or H denoted UV intensity thresholds of low (13.4 mW/cm2, 60 s) or high (268 mW/cm2, 3 s) for FRP initiation The usage of DTT during FRP was denoted by the subscript CT SP refers to hydrogels created by step polymerization using a thiol-ene reaction that is catalyzed by a base [19]
Figure 2: Synthetic routes for chitosan hydrogel preparation and schematic illustrations of the expected network structures of RPL (radical polymerization at low UV intensity) and RPH (left) radical polymerization at high UV intensity, or RPCT (chain-transfer radical polymerization) and SP (step polymerization) (right) Open circles represent the point at which the chain moves out of the plane Red and green linkages indicate inter- and intra-chain crosslinking, respectively [18]
In situ, light-sensitive polymers create hydrogels This method's hydrogel production has numerous significant limitations A light sensitizer and delayed irradiation are necessary for radiation crosslinking, which raises the local temperature and damages tissues and cells Table 3 displays the functional groupings and suggested mechanism of UV light sensitivity
Table 3 Crosslinking of polymers through small molecules and light-sensitive functional groups [20]
2.3.3 Crosslinking through Free Radical Mechanism
Hydrogels with chemical bonds are produced by starting the polymerization process of low molecular weight monomers with free radicals and a crosslinking agent Potassium persulfate (KPS), ammonium persulfate (APS), ferrous ammonium sulfate, ceric ammonium nitrate, 2-2'-azobisisobutyronitrile (AIBN), and benzoyl peroxide are examples of initiators that are used to start reactions This method creates chemically linked hydrogels by subjecting vinyl monomers to radical polymerization in the presence of a crosslinker
Moreover, natural polysaccharides are grafted with synthetic monomers via a free radical process Grafting is a generic method to improve the solubility of polymers in organic and watery solvents, as well as their complexation, chelating, and absorption-enhancing abilities Additionally, grafting improves a number of characteristics important for biomedical applications, including as mechanical strength, mucoadhesivity, biodegradability, and biocompatibility A large number of hydrogels described in the literature reveal natural polymers (chitosan, cellulose, starch, pectin, alginate, hyaluronic acid, dextran, carrageenan, and gums) that have been crosslinked with synthetic monomers to improve their inherent characteristics By grafting synthetic monomers, hydrogels generated from chitosan and its derivatives have been created, and their characteristics have been thoroughly studied Pourjavadi et al synthesized pH-sensitive chitosan-graft-poly (acrylamide) hydrogels by employing MBA and APS as crosslinkers and initiators, respectively The findings demonstrated that the swelling ratio is dependent on the amount of acrylamide present; alkaline hydrolysis of hydrogels and MBA further enhanced the swelling ratio Second-order
12 kinetics was shown to be followed by swelling kinetics [21] Mahdivinia et al investigated the effects of pH and salt solution on hydrogel swelling capabilities by utilizing KPS as an initiator and MBA as a crosslinker to construct chitosan-graft-poly (acrylamide-co-acrylic acid) hydrogels When alkaline hydrolysis was used to measure the swelling characteristics, it was found that non-hydrolyzed hydrogels with higher levels of acrylic acid displayed more swelling, whereas alkaline-hydrolyzed hydrogels with larger amounts of acrylamide had higher swelling ratios This research supported a prior study [22] Furthermore, employing KPS as an initiator and vinyltriethoxysilane as a crosslinker during copolymerization, the free radical mechanism is utilized to create carboxymethyl chitosan/acrylic acid hydrogels [23] Pourjavadi et al used MBA with APS, a free radical initiator, to create ultra porous kappa- carrageenan-g-poly (acrylic acid) in the air The composition of the reaction was changed to maximize the hydrogel production Significant swelling capacity has been seen in various salt solutions, particularly sodium chloride solution, as a result of the sulfate groups in carrageenan of super-absorbing hydrogels' anti-salt properties Furthermore, findings showed that air had no discernible impact on the swelling behavior of hydrogels [24] A hydrogel network structure was created by the copolymerization and crosslinking of acrylic acid and kappa carrageenan utilizing vinyltriethoxysilane as a silane crosslinker and KPS as an initiator [25] Elvira et al used the free radical process to create unique biodegradable hydrogels of acrylamide and acrylic acid in the presence of starch The hydrogels showed favorable qualities such swelling properties, biodegradability, and pH sensitivity [26] Using KPS and MBA as an initiator and a crosslinking agent, respectively, Bao et al developed superabsorbent hydrogels of sodium carboxymethyl cellulose-graft-poly (acrylamide-co- acrylic acid-co-2-acrylamido-2-methyl-1 propanesulfonic acid (AMPS) and montmorillonite (MMT) Because of MMT and carboxymethyl cellulose, these super porous hydrogels had porous networks with side chains that carried sulfate, carboxylate, and carbboxamide The hydrogels' swelling ratio showed a considerable sensitivity to external pH In the presence of salt solutions, the hydrogels exhibited swelling ratios as follows: K + > Na + > Ca 2+ > Mg 2+ Figure 3 [27] shows a potential method of hydrogel production
Figure 3: The plausible mechanism of carboxymethyl cellulose-g-poly (acrylamide –co - acrylic acid-co-2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)/montmorillonite (MMT)) hydrogel synthesis [27].
Interpenetrating Network (IPN) Hydrogel
Intertwining secondary polymers within crosslinked networks provides additional reinforcement An interpenetrating polymer network (IPN) is a polymer composed of two or more networks that are at least partly interlaced at the molecular scale, are not covalently bound to one another, and cannot be separated until chemical links are broken Stated differently, interpenetrating polymer networks (IPNs) are created when entangled polymer networks are allowed to expand in an aqueous solution in the presence of polymerizable monomers, and these monomers polymerize to form physically connected networks There are two ways in which an IPN might form: a semiinterpenetrating polymer network (semi-
IPN) is created when one polymer network is crosslinked and the second polymer network is physically connected to the crosslinked polymer However, if the second polymer is also crosslinked with the already crosslinked polymer network, then the network formed is called a full-interpenetrating polymer network (full-IPN) Numerous polymers form these types of networks in order to balance the deficiency of another polymer These polymers may be natural polysaccharides, proteins, or synthetic hydrophilic polymers containing CONH2, OH,
SO3H, COOH, NH2, and quaternary ammonium groups IPN hydrogels can be prepared via three different routes by combining natural polymers as well as synthetic polymers These routes are presented in Figure 4 [28]
Figure 4: Proposed reaction mechanism of interpenetrating network (IPN) formation: (a) simultaneous strategy; (b) sequential strategy; (c) selective crosslinking of a linear polymer entrapped in semi-IPN [28]
2.4.1 Semi-Interpenetrating Network (semi-IPN) Hydrogels
For effective biomedical applications, several attempts have been undertaken to manufacture semi-IPN hydrogels using synthetic polymers and polysaccharides Researchers have reported on hydrogels based on semi-IPN chitosan These hydrogels consist of chitosan mixed with cellulose and its derivatives, acrylamide-graft-dextran, and selective chitosan crosslinking using glutaraldehyde [29–33] The semi-IPN hydrogels of crosslinked chitosan entrapped in acrylamide-graft-hydroxyethyl cellulose were filled with diclofenac sodium It was discovered that the drug loading effectiveness varied between 50 and 66% Figure 5
15 illustrates the schematic of crosslinked chitosan entrapped in acrylamide-g-hydroxyethyl cellulose
Numerous alginate and synthetic polymer-based semi-IPN hydrogels have been studied These hydrogels featured unique characteristics such exceptional porosity, long-term drug administration, electrical sensitivity, and multi-responsiveness Semi-IPNs based on alginate and methacrylic acid shown a notable reaction to electrical fields Hydrogels were therefore recommended for application in electrically sensitive drug delivery systems, sensors, and artificial organ components [34]
Graft copolymerization of acrylic acid on cationic starch in the presence of poly (methacryloyloxyethyl ammonium chloride) has been used to create amphoteric semi-IPN hydrogels (PDMC) FTIR analysis was used to examine the salt connection between the COO− ions of the starch-graft poly (acrylic acid) and the quaternary ammonium groups of PDMC The starch-graft-poly (acrylic acid) hydrogel trapped PDMC, which did not wash out throughout the washing procedure The hydrogels exhibited significant sensitivity to pH; however, when the quantity of PDMC increased, the swelling ratios in the basic medium reduced as a result of the absence of carboxylic groups [35] Semi-IPN hydrogels are also formed by other polysaccharides, including cellulose, kappa carrageenan, hyaluronic acid, guar gum, and xanthan There were reports of semi-IPN hydrogels made of poly (N, N0- diethylacrylamide) with kappa carrageenan These hydrogels were highly temperature- sensitive [36]
Figure 5 The proposed reaction mechanism of a semi-IPN hydrogel of chitosan/acrylamide- g-hydroxyethyl cellulose [32]
2.4.2 Full-Interpenetrating Network (Full-IPN) Hydrogels
Selective crosslinking of a second polymer that hasn't been crosslinked before can generate a full-IPN hydrogel when a linear polymer—natural or synthetic—is trapped in the other matrix to form a semi-IPN hydrogel Because stable network creation and little phase separation can be achieved, full-IPN formation can triumph over thermodynamic incompatibility The crosslinked network architectures' interlocked components guarantee the durability of full-IPN polymers in both surface and bulk morphology When compared to typical hydrogels, full-IPN's comparatively compact hydrogel structures offer higher mechanical strength, variable physical characteristics, and enhanced drug loading efficiency The drug release rate and hydrogel-tissue interaction may be tailored by varying the surface chemistry and pore size of full-IPNs Many synthetic and natural polymers, including PVA and chitosan, can create full-IPNs By analyzing the strength and wear resistance of this novel material, we can investigate its special properties Fang and colleagues developed full-IPN hydrogels made of PNIPAM and chitosan These hydrogels were crosslinked with formaldehyde for chitosan and N, N0-methylenebisacrylamide for NIPAM They investigated the characteristics of hydrogels, such as phase transition behavior, swelling ratio measurements in water and in ethanol/water mixes, and the extraction of PNIPAM from crosslinked chitosan networks The results obtained revealed dissimilarities from the semi- IPN of chitosan/PNIPAM hydrogel Nevertheless, it was discovered that this hydrogel was just as thermosensitive as semi-IPN chitosan/PNIPAM hydrogel, exhibiting transparency at
30 0C but turning opaque above this point [37] Hydrophilic polymers like poly (vinyl alcohol) have prospective uses in the biomedical industry Two such uses include the removal of hazardous colors from wastewater and the absorption of heavy metal ions Using UV irradiation, Kim et al developed a full-IPN hydrogel based on chitosan/PVA and investigated its swelling ratio and free water contents The produced hydrogel swelled quickly and achieved equilibrium in less than an hour, according to the data As the chitosan concentration increased, so did the free water contents and swelling ratio Additionally, the hydrogels showed temperature- and pH-sensitive swelling behavior, which indicated important uses in the biomedical industry [38] Nonporous hydrogels often exhibit low drug loading efficiencies
17 and expand extremely slowly in the aqueous phase These hydrogels' disadvantages restrict their use in the biomedical industry In order to improve the mechanical strength, mucoadhesive force, and drug-loading efficiency of carboxymethyl chitosan/poly (acrylamide-co-acrylic acid) hydrogels, we thus developed extremely porous hydrogels Higher quantities of carboxymethyl chitosan, glutaraldehyde, and longer crosslinking times resulted in a reduction in the swelling ratio of these hydrogels Nonetheless, the development of a full-IPN structure significantly improved the hydrogels' mechanical strength, mucoadhesive force, and drug-loading effectiveness These hydrogels with full-IPN were biocompatible These hydrogel properties point to possible use in mucosal medication delivery systems [39]
Semi-IPN hydrogels including chitosan and polyacrylamide were created by Dragan et al., wherein the chitosan was ensnared within the polyacrylamide matrix Additionally, we created full-IPN hydrogels by using epichlorohydrin (ECH) to selectively crosslink chitosan
In addition to the crosslinking, which was done in an alkaline solution, the polyacrylamide matrix's amide group partly degraded to produce anionic sites The creation of a full-IPN structure and the partial hydrolysis of the amide group were both validated by FTIR analysis [40].
Practical application
Over the past twenty years, the development of diapers containing hydrogels, most of which contain different sodium polyacrylate formulas [41–43], has significantly reduced the number of skin diseases related to prolonged contact with the surface of disposable diapers According to a study by Gross and colleagues [42], nearly 95% of Western diapers are disposable Indeed, many chemicals used in the production of products, such as fragrances, leak-proof materials, and super absorbent polymers, are the main factors causing many diseases, such as chronic diaper rash and asthma Furthermore, the use of disposable diapers also leads to environmental pollution because their disposal is not easy
Another application of hydrogels includes a series of raw powders called polyacrylamide or potassium polyacrylate matrix, also known as plant gel Water gel crystals are used to absorb water and store water for plants As Chalker Scott from Washington State
University pointed out in her published works, Water gel crystals are often made from non- biodegradable materials containing monomers that can be toxic (arcylamide), so the potential risks when using these types are often much higher than the benefits of water retention for plants
Another application of hydrogel is as a bulking agent for treating urinary incontinence Smart injectable gel can be used in clinical practice, where hydrogel is used to tighten the urethra and reduce the condition of urinary incontinence in patients
2.5.4 Application to vaccines and immunity
Nap-GFFY (Naphthalene Acetic Acid-Glycine Phenylalanine Tyrosine) has been demonstrated as a short peptide for the formation of a gelator of nanofiber hydrogel This nanovector can strongly activate both humoral and cellular immune responses at rare equilibrium levels, which is crucial in the treatment and prevention of HIV Additionally, this hydrogel has shown good biocompatibility in vitro and in vivo The fiber-like nanostructure of this hydrogel is important for significantly improving immune responses compared to existing materials The high efficacy of this hydrogel can be attributed to its ability to condense DNA, promote DNA transformation, and enhance gene expression in laboratory settings Furthermore, it exhibits promising biocompatibility and no apparent toxicity
This nano-fiber peptide-based hydrogel is used as a carrier for HIV nano-DNA delivery, which may open up new possibilities for effective vaccination based on peptide-based nano- fiber hydrogel [44]
In recent years, countries have started to pay more attention to environmental issues and pollution Many governments have decided to adopt greener and safer policies for the environment Water pollution is one of the biggest issues, particularly affecting impoverished regions in Africa, Asia, and South America Due to their relationship with water, hydrogels can be used in two different ways to treat water
Firstly, the matrix can be used as a carrier to clean microorganisms Many interesting studies have been developed on this particular pathway by encapsulating microorganisms inside carrier materials [45] Chlorella and Spirulina are among the most commonly used ones These microorganisms have been employed to remove pollutant chemicals from water
19 sources The idea is to keep the bacteria within the network, thus protecting and controlling them while cleaning the purification area Both synthetic and natural hydrogels are utilized The most effective hydrogel seems to originate from alginate [45], or alternatively, carrageenan and agar [46]
The second interesting approach to addressing pollution is modifying hydrogels to capture and retain pollutants within their networks Many researchers have explored this method to capture metal ions A group of Iranian scientists published a paper discussing a new synthetic hydrogel for the removal of Pb (II) In summary, a hydrogel composite, polyethylene-g-poly (acrylic acid)-co-starch/organically modified montmorillonite (LLDPE- g-PAA-co-starch/OMMT), was developed and utilized as an adsorbent material for removing pollutants (Pb (II)) They immersed the hydrogel in a lead acetate solution and measured the adsorption using Atomic Absorption Spectrophotometry (AAS) They then performed a desorption stage and repeated the process in several cycles Electrostatic attraction, ion exchange, and iron release mechanisms were proposed to explain the metal adsorption observed in the experiments They reported that the adsorption equilibrium data of the hydrogel fit well with the Langmuir isotherm, and the adsorption capacity of 430 mg/g was comparable to other conventional adsorbents [47, 48]
Another feasible way to achieve interesting water filtering is explained in a paper by Yan et al., where the group performed etherification and consequent functionalization of chitosan beads in order to obtain carboxymethilated chitosan with enhanced adsorption of metal ions This has been proven to improve the selective adsorption of specific ions like Cu (II), Pu (II), and Mg (II) [48] It has interesting properties that can be exploited in dye removal applications, and that has been demonstrated to be possible by the magnetical doping of hydrogel microspheres with interpenetrated network (IPN) structures [49]
Hydrogels can be used as sensors to detect heavy metal ions, as demonstrated in the study by Wang et al [50] Furthermore, polyacrylamide gel finds application in the flood control device called WATER GEL BAG®, manufactured by TaiHei Co., Ltd
One of the major environmental challenges today that certainly needs to be addressed is oil spills in marine and other water bodies In recent years, the amount of oil released into the oceans worldwide has significantly increased Food processing, hydrocarbon industries,
20 and refining processes have contributed to the heightened risk of pollution It has been found that industrial wastewater can contain oil concentrations as high as 40,000 mg/L [51]
Various pollution remediation systems have been tested, such as organic bentonite [52, 53], palygorskite [54], aluminum magnesium phyllosilicate, and activated carbon In a study conducted in 2010, the researchers attempted to develop a hydrogel to retain water with oil- polluting molecules [55] They highlighted the promising aspect of hydrogel, specifically chitosan, due to its numerous amino and hydro groups that can react with vinyl monomers The authors clarified that grafting polyacrylamide onto polysaccharides improves the attraction and retention of suspended residual solid particles in water, referred to as flocculants The higher the cross-linking, the lower the retention ability, as the smaller distance between two nodes in the matrix network reduces swelling Furthermore, the initial concentration of oil waste in the environment is another crucial parameter, as its properties affect the adsorption kinetics In reality, higher concentrations correspond to faster swelling kinetics
This project affords an overview of the usage of hydrogel as a framework for bone regeneration, with a specific emphasis at the injection machine, membrane-guided bone regeneration, and the organic and mineralization features that mimic biological tissue Alginate hydrogel has been tested as a beneficial tool for producing bone and cartilage tissue [56] It is particularly biocompatible in human beings, and peptides can also be covalently bonded to its molecules Alginate is a herbal polymer found in seaweed and is generally used for gel formation
MATERIALS AND METHODS
Materials
Sodium alginate (Na-alginate, viscosity 80-120 cP) was obtained from FUJIFILM Wako Pure Chemical Corporation, Japan (Figure 6) Calcium chloride dihydrate (CaCl2) was purchased from Xilong Scientific Co., Ltd in China (Figure 7) All chemicals were purchased and used without further purification, and distilled water was used to prepare the gels (Figure 8) Sodium chloride was purchased from Xilong Scientific Co., Ltd in China (NaCl) was purchased from Xilong Scientific Co., Ltd in China (Figure 9), Ammonium persulfate was purchased from Xilong Scientific Co., Ltd in China (APS) (Figure 10), Thermo Scientific Pierce Tetramethylethylenediamine (TEMED) from Macklin China (Figure 11), Copper(II) chloride dihydrate CuCl2.2H2O (CuCl2) was purchased from Xilong Scientific Co., Ltd in China (Figure 12) All chemicals were measured using an analytical balance (Figure 13)
Figure 6 Sodium alginate (Na-alginate, viscosity 80-120 cP)
Figure 7 Calcium chloride dihydrate (CaCl2)
27 Figure 11 Thermo Scientific Pierce Tetramethylethylenediamine (TEMED)
Figure 12 Copper(II) chloride dihydrate CuCl2.2H2O (CuCl2)
Preparations of hydrogels
3.2.1 Fabrications of Ca-alginate hydrogels
Ca-alginate hydrogels [Ca-alginate/0.5/x], where 0.5 represents the concentration of
Ca 2+ solution and x (= 2, 3, 4, and 5 wt%) represents the initial weight percentage of alginate, were synthesized with different concentrations by adjusting the initial mass of alginate during the fabrication process (2, 3, 4, and 5 wt%) The procedure for preparing the hydrogels is as follows (Figure 10) The alginate solution (Figure 11) was prepared by dissolving alginate (2,
3, 4, and 5 wt%) in distilled water and allowing the mixture to self-dissolve for ~ 5 days A 0.5 M CaCl2 solution was prepared by dissolving CaCl2 powder in water to obtain the 0.5 M CaCl2 solution (Figure 12) Next, a gel mold was prepared using two glass plates separated by a 3-mm U-shaped silicone spacer The mold was securely fixed with clamps The alginate solution was poured into the mold until it filled half of the mold During the pouring process, air bubbles may form in the gel, so it is necessary to let them settle before continuing Once the air bubbles had dissipated, the 0.5 M CaCl2 solution was slowly poured from the top The
Ca 2+ ions would diffuse through the alginate, initiating the gelation process when the two solution mixtures met After the gel formation, the Ca-alginate hydrogel was immersed in a 0.5 M CaCl2 solution for one day to complete the crosslinking process Finally, the Ca-
29 alginate hydrogel was soaked in water to remove any residual salt and for preservation purposes
Figure 14 Schematic illustration of fabrication of isotropic Ca-alginate hydrogels
Figure 16 Fabrication of 0.5 M CaCl2 aq solution
3.2.2 Fabrications of Ca-alginate–polyacrylic acid hydrogels
Alginate-polyacrylic acid [Ca/Alg/PAA/0.5/x/y] hydrogel, where 0.5 represents the concentration of Ca 2+ solution; x (= 2, 3, 4, and 5 wt%) and y (= 2, 3, 4, and 5 M) represents the initial weight percentage of alginate and initial concentraion of acrylic acid, creation process: After gel formation (Figure 16), it will be immersed in a 1 M NaCl solution for 30 minutes Then, the gel will be rinsed with water for 15 minutes to remove excess Na + ions
As a result, the gel will become softer and expand compared to its initial state Next, the gel will be immersed in a 4 M AAc (Acrylic Acid) solution for one day to undergo the polymerization process and create a cross-linked network within the gel, making it thicker Then, 0.1% M APS salt (Ammonium Persulfate) will be added to the 4 M AAc solution The gel will be immersed in the mixture of AAc and APS for one hour This process also
32 contributes to the polymerization of the gel Subsequently, the gel will be removed and air- dried, followed by placement on a mold 50 àl of TEMED will be applied to the gel surface to accelerate the polymerization process The gel mold will be placed in an oven at 60 ºC for
3 hours to complete the polymerization process Once the polymerization is complete, the gel will be taken out and allowed to cool Then, the gel will be immersed in a CaCl2 solution for one day to reinforce the gel's structure Finally, the gel will be soaked in water for a day to remove any excess Ca 2+ ions
Figure 18 Fabrication of Ca/Alg/PAA/0.5/x/y hydrogel.
Water content measurement
The water content of each hydrogel was calculated from the weights of the hydrogels that were water-equilibrated (Wwet) and dried at 120°C by a heating equipment (Figure 14) for 24 hours (Wdry) as follows
Figure 19 Heating equipment for hydrogels.
Mechanical properties characterization
The mechanical properties of the Ca-alginate hydrogel are assessed through tensile strength tests conducted in the surrounding environment (temperature: ~26 °C, humidity:
~60 %) The tests are performed using a tensile strength testing machine (PT-1699vdo, PRO TEST, Taiwan) equipped with a 50 kg load cell sensor (Figure 20) To measure the dimensions of the Ca-alginate hydrogel samples, digital calipers are employed These hydrogel samples are then cut into rectangular shapes with a thickness of approximately 1.7 mm, a width of around 5 mm, and a length of about 50 mm Subsequently, the two ends of the gels are vertically clamped onto the testing machine, establishing an initial gauge length of approximately 8.5 mm between the clamps
During the tests, the hydrogel samples are stretched at a speed of about 42.5 mm/minute, determined by multiplying the initial gauge length by 5 (speed = initial gauge length × 5), until the gels rupture Multiple tests are conducted on each hydrogel sample, and 4-5 stable modulus values are chosen To enhance accuracy, the stress-strain curves are linearly adjusted within a deformation range of approximately 1% to about 2.5%
34 Figure 20 Tensile strength tester (PT-1699vdo, PRO TEST, Taiwan) and 50-kg load cell
FABRICATION OF THE HYDROGELS AND EVALUATION OF
Evaluation of mechanical properties of Ca-alginate hydrogels
As indicated in prior studies [69], the mechanical characteristics of gels are impacted by the weight percentage of alginate Therefore, in this study, we evaluated the mechanical properties of Ca-alginate hydrogel based on the weight percentage of alginate
Figure 21 Fabricating process of Ca-alginate 0.5 M hydrogels (a) Na-alginate solution was poured to the halfway level of the reaction mold; 0.5 M CaCl2 aq solution was introduced from the upper empty part of the reaction mold; Ca 2+ ions disseminated through the alginate solution; (b) gelation began and Ca-alginate hydrogels started to form when the alginate solution met Ca 2+ ions; (c) the Ca-alginate hydrogels were thoroughly washed with water for
2 days to get rid of non-crosslinked salts and polymers; and (d) the desired 0.5 M Ca 2+ physical hydrogels were obtained
The mechanical properties of the corresponding water-equilibrated Ca-alginate hydrogels were determined by tensile tests The Ca-alginate hydrogels were cut into rectangular shapes with dimensions of ~ 5 × ~ 50 mm (width × length) (Figure 22) The rectangular shape of the samples has been widely employed for testing gel materials
Figure 22 Tensile-tested isotropic Ca-alginate hydrogels with the dimension of ~ 5 × ~ 50 mm (width × length) (a) Ca-alginate/0.5/2 hydrogels, (b) Ca-alginate/0.5/3 hydrogels, (c) Ca- alginate/0.5/4 hydrogels, and (d) Ca-alginate/0.5/5 hydrogels
The mechanical properties of different Ca-alginate hydrogels are shown in (Figure 23) The results indicate that increasing the weight percentage of alginate (2wt%–5wt%) leads to an increase in the following parameters: Young's modulus (0.37 ± 0.13 Mpa, 0.48 ± 0.1 MPa, 0.71 ± 0.3 MPa, 0.96 ± 0.16 MPa), Tensile strength (0.24 ± 0.04 MPa, 0.48 ± 0.04 MPa, 0.57 ± 0.06 MPa, 0.67 ± 0.06 MPa), and work of extension (0.23 ± 0.01 MJ.m -3 , 0.57 ± 0.05 MJ.m -
3, 0.71 ± 0.14 MJ.m -3 , 0.94 ± 0.15 MJ.m -3 ), as shown in (Figure 22 abcd) On the other hand, the water content decreases as the weight percentage of alginate increases (2–5 wt%)
From these findings, we can infer that the mechanical strength of Ca-alginate hydrogel rises with higher alginate weight percentages, coupled with a reduction in water content
However, despite its various applications, Ca-alginate hydrogel has some drawbacks, such as weak mechanical properties, susceptibility to dissolution in high-ion concentration environments, and limited load-bearing capacity
Ca-alginate hydrogel has been extensively tested in various fields such as biomedical technology, agriculture, and environmental applications However, due to the weak natural mechanical properties of hydrogels, our research focuses on adding the polyacrylic acid network to improve their mechanical properties
Figure 23: Ca-alginate hydrogel (2 - 5 wt%) (a) Nominal stress – tensile strain curves recorded at different alginate’s concentrations of weight percent, and the (b) Young’s modulus, (c) tensile strength, and (d) tensile strain, (e) work of extension and (f) water content determined under each condition The error bars in plots (b) - (f) indicate the mean absolute deviations a e f c d b
Alginate-polyacrylic acid hydrogels
4.2.1 Study the soaking time in 1M NaCl
The Ca-alginate hydrogel has a high-water content (> 90 wt%), leading to weak mechanical properties To enhance the mechanical properties of the gel, we conducted several experiments on Ca/Alg/PAA/0.5/x/y hydrogel by varying the gel immersion time in a 1 M NaCl solution in 3 cases: 0, 30, and 60 minutes
Case 1 (No immersion in NaCl) (Figure 24): The gel is immersed in a 4 M AAc (Acrylic Acid) solution for one day As AAc is a monomer capable of polymerization and cross-linking the alginate chains in the hydrogel, the gel will shrink and form a denser network Next, 0.1% M APS (ammonium sulfate) is dissolved in the 4 M AAc solution, and the gel is immersed in this solution for one hour The gel is then removed and air-dried before being placed on a mold 50 àl of TEMED (Figure 24) is applied to the gel surface to accelerate the polymerization process The mold is placed in an oven at 60 o C for 3 hours to complete the polymerization process The gel is taken out of the mold and allowed to cool It is then immersed in a CaCl2 solution for one day to reinforce the gel's structure Finally, the gel is soaked in water for a day to remove excess Ca 2+ ions
The test results of the [Ca/Alg/PAA/0.5/3/4] hydrogel on the Tensile strength tester are as follows:
Figure 24: Survey on Ca/Alg/PAA/0.5/3/4 hydrogel under conditions without NaCl
Case 2 (immersed in a 1 M NaCl solution for 30 minutes) (Figure 25) The gel will undergo a 30-minute immersion in a 1 M NaCl solution, followed by a 15-minute soak in water to eliminate excess Na + ions This process causes the gel to swell and become softer compared to its initial state after NaCl immersion Next, we will proceed with the treatment steps as described in the procedure above The gel will be immersed in a 4 M AAc (acrylic acid) solution for one day to allow the polymerization process to occur and form a denser network within the gel Then, 0.1% M APS (ammonium sulfate) will be dissolved in the AAc solution, and the gel will be immersed in the AAc + APS mixture for one hour The gel will be taken out and allowed to air dry after one hour of immersion in the solution Afterwards, the gel will be placed on a mold, and 50àl of Temed will be applied to the gel surface to accelerate the polymerization process The gel mold will be placed in an oven at 60 o C for 3 hours to complete the polymerization process Once the polymerization process is finished, the gel will be taken out and cooled Subsequently, the gel will be immersed in a CaCl2 solution for one day to reinforce the gel's structure Finally, the gel will be immersed in water for one day to remove any excess Ca 2+ ions
After the gel processing was completed, we conducted a mechanical characterization of the [Ca/Alg/PAA/0.5/3/4] hydrogel using a tensile strength tester The obtained data includes:
Figure 25 Survey on Ca/Alg/PAA/0.5/3/4 hydrogel after soaking in NaCl for 30 minutes
Case 3 (immersed in a 1M NaCl solution for 60 minutes) (Figure 26): The procedure is similar to case 2, with the only difference being the immersion time in the NaCl solution, which is 60 minutes After the gel was completed, we used a tensile strength tester to evaluate the mechanical properties of the gel
Figure 26 Survey on Ca/Alg/PAA/0.5/3/4 hydrogel after soaking in NaCl for 60 minutes
Figure 27 (a) Nominal stress-tensile strain curves of Ca/Alg/PAA/0.5/3/4 hydrogel recorded at different treatment time in the 1 M NaCl solution, and the (b) Young’s modulus, (c) Tensile strength, and (d) Tensile strain, (e) Work of extension and (f) Water content of the resulting hydrogel The error bars in plots (b) - (f) indicate the mean absolute deviations
Based on the results and data from the three cases, we can observe that the gel immersed in NaCl for 30 minutes exhibits the following mechanical properties: Young's a b c d e f
43 modulus (stiffness) is 2.03 ± 0.25 (MPa), Work of extension is 2.12 ± 0.47 (MJ.m -3 ), and tensile strength is 1.06 ± 0.1 (MPa) These values are the highest and superior compared to the other two cases This indicates that the gel immersed in NaCl for 30 minutes has significantly improved mechanical properties, including Young's modulus, tensile strength, and work of extension
4.2.2 Survey of the effect of acrylic acid concentration (AAc)
In addition to investigating and testing the immersion time in NaCl solution, the molar concentration of AAc also significantly affects the gel's mechanical properties To achieve the best mechanical properties, we can conduct a study by varying the molar concentration of AAc (2 – 5 M) By changing the molar concentration and examining the mechanical properties, we can gather additional information about the correlation between the molar concentration of AAc and the mechanical properties of the gel, helping us optimize the process and adjust the AAc concentration to achieve the best properties for the gel
The procedure will be carried out similarly to the previous investigations, but this time we will vary the molar concentration of AAc and divide it into four different cases for examination Below is the procedure for each case:
Figure 28 Fabrication process of the Ca/Alg/PAA/0.5/2/2 hydrogel
Figure 29 Fabrication process of the Ca/Alg/PAA/0.5/2/3 hydrogel
Figure 30 Fabrication process of the Ca/Alg/PAA/0.5/2/4 hydrogel
Figure 31 Fabrication process of the Ca/Alg/PAA/0.5/2/5 hydrogel
Figure 32 The influence of the concentration of acrylic acid, (a) Nominal stress – tensile strain curves recorded at different alginate’s concentrations of weight percent, and the (b) Young’s modulus, (c) tensile strength, and (d) tensile strain, (e) work of extension and (f) a b c d e f
47 water content determined under each condition The error bars in plots (b)–(f) indicate the mean absolute deviations
From the data above, we can see that when increasing the mol concentration of AAc from (2 – 5 M), mechanical properties such as, Tensile strength, Tensile strain, and Work of extension all increase This indicates that the mol concentration of AAc significantly affects the mechanical properties of the gel, and the case with a mol concentration of 4 M has high mechanical properties among the cases studied However, based on the graph, we can see that at a concentration of 4 M, the gel maintains the most stable and suitable level for the alginate- polyacrylisc acid hydrogel manufacturing process
4.2.3 Survey of the effects of alginate
It is clear that factors such as the percentage of alginate mass, AAc mol concentration, and soaking time in NaCl solution significantly affect the mechanical properties of the gel Conducting multiple experiments and determining the optimal conditions for the gel is a reasonable method to achieve a gel with good mechanical properties
In this survey, we continue the experiments by varying the percentage of alginate mass and dividing it into four cases This can evaluate the influence of the percentage of alginate mass on the mechanical properties of the gel under different conditions
Drawing from prior surveys and numerical results, it is evident that the gel attains stable and optimal mechanical properties when immersed in a NaCl solution for 30 minutes and possesses an AAc mol concentration of 4 M
With this choice, we can continue the survey and experiments to see how the percentage of alginate mass affects the mechanical properties of the gel under these conditions
By changing the percentage of alginate mass (2 – 5 wt%), and collecting corresponding data, we can evaluate the influence of this factor on the characteristics of the gel
In this survey, we will continue the experiments by changing the percentage of alginate mass (2 – 5 wt%), and the procedures will be repeated
Case 1 (2 wt% alginate) obtained the following data:
Case 2 (3 wt% alginate) obtained the following data:
Case 3 (4 wt% alginate) obtained the following data:
Case 4 (5 wt% alginate) obtained the following data:
Figure 33 The influence of the percentage of alginate mass (a) Nominal stress – tensile strain curves recorded at different alginate’s concentrations of weight percent, and the (b) Young’s modulus, (c) tensile strength, and (d) tensile strain, (e) work of extension and (f) water content a b c d e f
50 determined under each condition The error bars in plots (b)–(f) indicate the mean absolute deviations
Based on the results of the 4 surveyed cases, when increasing the alginate content (2 -
5 wt%), the mechanical properties indices Young’s modulus, Tensile strength, and Work of extension increase This indicates that as the percentage of alginate by weight increases, the gel tends to have higher mechanical properties Young’s modulus increases from 2.06 (MPa) to 3.15 (MPa), Tensile strength increases from 1.08 (Mpa) to 1.38 (MPa), Work of extension increases from 1.57 (MJ.m -3 ) to 2.85 (MJ.m -3 ), the mechanical properties of the gel will increase as the percentage of alginate by weight increases and the water content will decrease
4.2.4 Survey of the influence of Cu 2+
CONCLUSIONS
Conclusions
The objective of this thesis is to research and develop alginate-polyacrylic acid-based hydrogels, with the aim of developing hydrogels and their mechanical properties
Implemented through experimental methods, analysis, and comparison, I successfully researched and identified the optimal composition for alginate-based hydrogels with excellent mechanical properties The hydrogels composed of: 5 wt% alginate, 1 M NaCl, 4 M AAc, 0.5
M CaCl2 And exhibited Young’s modulus: 3.15 ± 0.34 (MPa), tensile strength: 1.38 ± 0.17 (MPa), and work of extension: 2.85 ± 0.53 (MJ.m -3 ), respectively.
Future works
The next research efforts should focus on optimizing the hydrogel production process, refining their properties, and exploring a wide range of applications in various industries and medicine This includes studying variables such as composition, crosslinking methods, and post-processing techniques to enhance the mechanical, electrical, and biological properties of hydrogels By doing so, we can unlock their full potential for applications such as tissue engineering, drug delivery systems, wound healing, wearable electronics, and many other applications
[1] J Byeongmoon, A Gutowaska (2002), “Lessons from nature: stimuliresponsive polymers and their biomedical applications”, Trends Biotech., 20, p.305
[2] J Insu and M T I Mredha, "Biomimetic anisotropic hydrogels: Advanced fabrication strategies, extraordinary functionalities, and broad applications," Progress in Materials Science, vol 124, no 100870, pp 1-45, 2021
[3] Xavier Banquy, Fernando Suarez, Anteneh Argaw (2009), “Effect of mechanical properties of hydrogel nanoparticles on macrophage cell uptake”, Soft Matter, 5, pp.3984-
[4] Khajouei R.A., Keramat J., Hamdami N., Ursu A.-V., Delattre C., Laroche C., Gardarin C., Lecerf D., Desbrières J., Djelveh G., et al Extraction and Characterization of an
Alginate from the Iranian Brown Seaweed Nizimuddinia zanardini Int J Biol
[5] Hentati F., Ursu A.-V., Pierre G., Delattre C., Trica B., Abdelkafi S., Djelveh G., Dobre T., Michaud P Handbook of Algal Technologies and Phytochemicals CRC Press; Boca Raton, FL, USA: 2018 Production, Extraction and Characterization of Alginates from Seaweeds; pp 33–42
[6] I.V Yannas, E Lee, D.P Orgill, E.M Skrabut, G.F Murphy (1989), “Synthesis and characterization of a model extracellular matrix that induces partial regeneration of adult mammalian skin”, Proc Natl Acad Sci USA, 86, pp.933- 937
[7] G Umachitra, Bhaarathidhurai (2012), “Disposable baby diaper a threat to the health and environment”, J Environ Sci Eng., 54, pp.447-452
[8] Bashir, S., Hina, M., Iqbal, J., Rajpar, A H., Mujtaba, M A., Alghamdi, N A., & Ramesh, S (2020) Fundamental concepts of hydrogels: Synthesis, properties, and their applications Polymers, 3(11), 2702
[9] Costa-Júnior, E.S.; Barbosa-Stancioli, E.F.; Mansur, A.A.P.; Vasconcelos, W.L.; Mansur, H.S Preparation and characterization of chitosan/poly(vinyl alcohol) chemically crosslinked blends for biomedical applications Carbohydr Polym 2009, 76, 472–481
[10] Lin, J.; Zheng, S.Y.; Xiao, R.; Yin, J.; Wu, Z.L.; Zheng, Q.; Qian, J Constitutive behaviors of tough physical hydrogels with dynamic metal-coordinated bonds J Mech Phys Solids 2020, 139, 103935
[11] Brack, H.; Tirmizi, S.; Risen, W A spectroscopic and viscometric study of the metal ion-induced gelation of the biopolymer chitosan Polymer 1997, 38, 2351–2362
[12] Weng, L.; Chen, X.; Chen, W Rheological characterization of in situ crosslinkable hydrogels formulated from oxidized dextran and N-carboxyethyl chitosan
[13] Tan, H.; Chu, C.R.; Payne, K.A.; Marra, K.G Injectable in situ forming biodegradable chitosan–hyaluronic acid based hydrogels for cartilage tissue engineering Biomaterials
[14] Ito, T.; Yeo, Y.; Highley, C.B.; Bellas, E.; Kohane, D.S Dextran-based in situ cross- linked injectable hydrogels to prevent peritoneal adhesions Biomaterials 2007, 28, 3418–
[15] Lee, K.Y.; Alsberg, E.; Mooney, D.J Degradable and injectable poly (aldehyde guluronate) hydrogels for bone tissue engineering J Biomed Mater Res 2001, 56, 228–
[16] Ono, K.; Saito, Y.; Yura, H.; Ishikawa, K.; Kurita, A.; Akaike, T.; Ishihara, M
Photocrosslinkable chitosan as a biological adhesive J Biomed Mater Res 2000, 49, 289–
[17] Bashir, S., Hina, M., Iqbal, J., Rajpar, A H., Mujtaba, M A., Alghamdi, N A., & Ramesh, S (2020) Fundamental concepts of hydrogels: Synthesis, properties, and their applications Polymers, 12(11), 2702
[18] Yoo, H.S Photo-cross-linkable and thermo-responsive hydrogels containing chitosan and Pluronic for sustained release of human growth hormone (hGH) J Biomater Sci Polym Ed 2007, 18, 1429–1441
[19] Ahn, J.; Ryu, J.; Song, G.; Whang, M.; Kim, J Network structure and enzymatic degradation of chitosan hydrogels determined by crosslinking methods Carbohydr Polym
[20] Bashir, S., Hina, M., Iqbal, J., Rajpar, A H., Mujtaba, M A., Alghamdi, N A., & Ramesh, S (2020) Fundamental concepts of hydrogels: Synthesis, properties, and their applications Polymers, 14(11), 2702
[21] Pourjavadi, A.; Mahdavinia, G.R Superabsorbency, pH-sensitivity and swelling kinetics of partially hydrolyzed chitosan-g-poly (acrylamide) hydrogels Turk J Chem
[22] Mahdavinia, G.; Pourjavadi, A.; Hosseinzadeh, H.; Zohuriaan, M Modified chitosan 4 Superabsorbent hydrogels from poly (acrylic acid-co-acrylamide) grafted chitosan with salt- and pH-responsiveness properties Eur Polym J 2004, 40, 1399–1407
[23] Yasin, T.; Rasool, N.; Akhter, Z Synthesis of carboxymethyl-chitosan/acrylic acid hydrogel using silane crosslinker e-Polymers 2008, 8, 1636–1646
[24] Pourjavadi, A.; Harzandi, A.; Hosseinzadeh, H Modified carrageenan 3 Synthesis of a novel polysaccharidebased superabsorbent hydrogel via graft copolymerization of acrylic acid onto kappa-carrageenan in air Eur Polym J 2004, 40, 1363–1370
[25] Rasool, N.; Yasin, T.; Heng, J.Y.; Akhter, Z Synthesis and characterization of novel pH-, ionic strength and temperature-sensitive hydrogel for insulin delivery Polymer 2010,
[26] Elvira, C.; Mano, J.; San Roman, J.; Reis, R Starch-based biodegradable hydrogels with potential biomedical applications as drug delivery systems Biomaterials 2002, 23, 1955–1966
[27] Bao, Y.; Ma, J.; Li, N Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly (AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel Carbohydr
[28] Dragan, E.S Design and applications of interpenetrating polymer network hydrogels A review Chem Eng J 2014, 243, 572–590
[29] Cai, Z.; Kim, J Cellulose–chitosan interpenetrating polymer network for electro-active paper actuator J Appl Polym Sci 2009, 114, 288–297
[30] Rokhade, A.P.; Shelke, N.B.; Patil, S.A.; Aminabhavi, T.M Novel interpenetrating polymer network microspheres of chitosan and methylcellulose for controlled release of theophylline Carbohydr Polym 2007, 69, 678–687
[31] Ahmed, A.A.-K.; Naik, H.B.; Sherigara, B Synthesis and characterization of chitosan- based pH-sensitive semi-interpenetrating network microspheres for controlled release of diclofenac sodium Carbohydr Res 2009, 344, 699–706
[32] Angadi, S.C.; Manjeshwar, L.S.; Aminabhavi, T.M Interpenetrating polymer network blend microspheres of chitosan and hydroxyethyl cellulose for controlled release of isoniazid Int J Biol Macromol 2010, 47, 171–179
[33] Rokhade, A.P.; Patil, S.A.; Aminabhavi, T.M Synthesis and characterization of semi- interpenetrating polymer network microspheres of acrylamide grafted dextran and chitosan for controlled release of acyclovir Carbohydr Polym 2007, 67, 605–613
[34] Kim, S.J.; Yoon, S.G.; Lee, Y.H.; Kim, S.I Bending behavior of hydrogels composed of poly (methacrylic acid) and alginate by electrical stimulus Polym Int 2004, 53, 1456–
[35] Li, X.; Xu, S.; Wang, J.; Chen, X.; Feng, S Structure and characterization of amphoteric semi-IPN hydrogel based on cationic starch Carbohydr Polym 2009, 75, 688–
[36] Chen, J.; Liu, M.; Jin, S.; Liu, H Synthesis and characterization of κ-carrageenan/poly (N, N-diethylacrylamide) semi-interpenetrating polymer network hydrogels with rapid response to temperature Polym Adv Technol 2008, 19, 1656–1663
[37] Wang, M.; Fang, Y.; Hu, D Preparation and properties of chitosan-poly (N- isopropylacrylamide) full-IPN hydrogels React Funct Polym 2001, 48, 215–221
[38] Kim, S.J.; Park, S.J.; Kim, S.I Swelling behavior of interpenetrating polymer network hydrogels composed of poly (vinyl alcohol) and chitosan React Funct Polym 2003, 55, 53–59
[39] Yin, L.; Fei, L.; Cui, F.; Tang, C.; Yin, C Superporous hydrogels containing poly
(acrylic acid-coacrylamide)/O-carboxymethyl chitosan interpenetrating polymer networks Biomaterials 2007, 28, 1258–1266
[40] Dragan, E.S.; Perju, M.M.; Dinu, M.V Preparation and characterization of IPN composite hydrogels based on polyacrylamide and chitosan and their interaction with ionic dyes Carbohydr Polym 2012, 88, 270–281
[41] Tanaka T (1986) In: Encylopedia of Polymer Science and Engineering 2nd ed Vol 6: 514-536 Wiley
[42] Brandt KA, Goldman SA, Inglin TA US Patent RE32649 E Hydrogel-forming polymer compositions for use in absorbent structures 1988
[43] AssarssonPG, KingPA, YenSN(1975)USPatent3901236A.Disposable absorbent articles containing hydrogel composites having improved fluid absorption efficiencies and processes for preparation
[44] Zhang X, Chu X, Wang L, Wang H, Liang G, et al (2012) Rational design of a tetrameric protein to enhance interactions between selfassembled fibers gives molecular hydrogels Angew Chem Int Ed Engl 51: 4388-4392
[45] Reynolds WS, Dmochowski RR (2012) Urethral bulking: a urology perspective Urol Clin North Am 39: 279-287
[46] Moreno-Garrido I (2008) Microalgae immobilization: current techniques and uses Bioresour Technol 99: 3949-3964
[47] Joint I, Mühling M, Querellou J (2010) Culturing marine bacteria – an essential prerequisite for biodiscovery Microb Biotechnol 3: 564-575
[48] Irani M, Ismail H, Ahmad Z, Fan M (2015) Synthesis of linear lowdensity polyethylene-g-poly (acrylic acid)-co-starch/organomontmorillonite hydrogel composite as an adsorbent for removal of Pb(??) from aqueous solutions J Environ Sci 27: 9-20
[49] Yan H, Dai J, Yang Z, Yang H, and Cheng R (2011) Enhanced and selective adsorption of copper(II) ions on surface carboxymethylated chitosan hydrogel beads ChemEng J 174: 586–594
[50] Ahmad H, Nurunnabi M, Rahman MM, Kumar K, et al (2014) Magnetically doped multi stimuli-responsive hydrogel microspheres with IPN structure and application in dye removal Colloids Surf PhysicochemEng Asp 459: 39-47
[51] Wang X, Ye G, Wang X (2014) Hydrogel diffraction gratings functionalized with crown ether for heavy metal ion detection Sens Actuators B Chem 193: 413–419
[52] Arcadio PS, Gregoria AS (2003) Physical–Chemical Treatment of Water and
Wastewater IWA Publishing, CRC Press
[53] Panpanit S, Visvanathan C (2001) The role of bentonite in UF flux enhancement mechanisms for oil/water emulsion J Membr Sci 184: 59-68
[54] Hadi M, Viraraghavan T (1999) Removal of oil from water by bentonite organo-clay In: Hazardous & Industrial Wastes - Proceedings of the Mid-Atlantic Industrial &
[55] Qiu Z, Zhang Y, Fang Y (1995) Removal of oil from concentrated wastewater by attapulgite and coagulant Water Qual Res J 30: 89-99,
[56] Tse JR, Engler AJ (2010) Preparation of hydrogel substrates with tunable mechanical properties Curr Protoc Cell Biol Chapter 10: Unit 10
[57] Zhu J (2010) Bioactive modification of poly(ethylene glycol) hydrogels for tissue engineering Biomaterials 31: 4639-4656
[58] Carmeliet P (2000) Mechanisms of angiogenesis and arteriogenesis Nat Med 6: 389-
[59] Silva AK, Richard C, Bessodes M, Scherman D, Merten OW (2009) Growth factor delivery approaches in hydrogels Biomacromolecules 10: 9-18
[60] Hubbell JA (2003) Materials as morphogenetic guides in tissue engineering Curr Opin Biotechnol 14: 551-558
[61] Schmoekel HG, Weber FE, Schense JC, Grọtz KW, Schawalder P, et al (2005) Bone repair with a form of BMP-2 engineered for incorporation into fibrin cell ingrowth matrices Biotechnol Bioeng 89: 253-262
[62] Martino MM, Tortelli F, Mochizuki M, Traub S, Ben-David D, et al (2011)
Engineering the growth factor microenvironment with fibronectin domains to promote wound and bone tissue healing Sci Transl Med 3: 100ra89
[63] Gurny R, Kaufmann B, Moeller M (2012) International Patent WO 2012013670 A1 Process for the esterification of hyaluronic acid with hydrophobic organic compounds
[64] Bertrand N, Leroux JC (2012) The journey of a drug-carrier in the body: an anatomo- physiological perspective J Control Release 161: 152-163
[65] Ashley GW, Henise J, Reid R, Santi DV (2013) Hydrogel drug delivery system with predictable and tunable drug release and degradation rates Proc Natl Acad Sci U S A 110: 2318-2323
[66] Marin MA, Mallepally RR, McHugh MA (2014) Silk fibroin aerogels for drug delivery applications J Supercrit Fluids 91: 84-89
[67] Patra C, Talukdar S, Novoyatleva T, Velagala SR, Mühlfeld C, et al (2012) Silk protein fibroin from Antheraea mylitta for cardiac tissue engineering Biomaterials 33: 2673-2680
[68] Park CH, Jeong L, Cho D, Kwon OH, Park WH (2013) Effect of methylcellulose on the formation and drug release behavior of silk fibroin hydrogel Carbohydr Polym 98: 1179-1185
[69] N Chirani, L Yahia, L Gritsch, F L Motta, S Chirani and S Fare, "History and Applications of Hydrogels," Journal of Biomedical Sciences, vol 4, no 2, pp 1- 23, 2015.