INTRODUCTION:

In recent years, cross Linking technology is amongst the most promising research domains focusing on the development of new tissue engineered scaffolds and various drug delivery matrices. The main aim of the cross linking is to enhance the biomechanical properties of the scaffolds, by forming a firm network in the polymeric matrix^1-2.

Cross Linking is a stabilization process in polymer chemistry which leads to multidimensional extension of polymeric chain resulting in network structure. A crosslink is a physical or chemical bond that links the functional groups of a polymer chain to that of another one through covalent bonding or supramolecular interactions such as ionic bonding, hydrogen bonding, etc ^3-4. Crosslinkers, not only ameliorate the mechanical performance of the polymer network, but also demonstrate no cytotoxicity. Furthermore, it has been reported that cross linking can modify the antigenic sites of natural materials and reduce their antigenicity. Cross-Linking method insolubilizes matrix in water and improves the thermal and mechanical stability of the matrix under physiological conditions Moreover, cross Linking can be tailored to modulate the release rate of the incorporated active agents⁵.

Cross Linking changes a liquid polymer into ‘solid’ or ‘gel’ by restricting the ability of movement. When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. A liquid polymer (where the chains are freely flowing) can be turned into a ‘solid’ or ‘gel’ by cross-Linking the chains together. Cross linking increases the molecular mass of a polymer. Cross-linked polymers are important because they are mechanically strong and resistant to heat, wear and attack by solvents. However, the drawback associated with cross-linked polymers is that they are relatively inflexible when it comes to their processing properties because they are insoluble and infusible⁶.

Hydrogels are polymeric or supramolecular networks that absorb large amounts of water without dissolving in the aqueous medium or precipitating into the solid phase. As such, hydrogels require a balance between the forces promoting solvation of the polymer chains and the forces driving their association. While the materials and methods for engineering hydrogels vary widely, their high-water content and favourable mechanical properties have attracted significant attention for applications in biomedical engineering as well as consumer products. A hydrogel is also called as the aqua gel and it is a three-dimensional network of macromolecular of the polymer. And the polymer chains that are the hydrophilic, sometimes it is to be found as the colloidal gels, and in which water should be in the dispersion medium. Three-dimensional network formed by hydrophilic polymer chains with the help of physical and chemical cross-Linking. In the chemical gel chains are connected by covalent bonds, but in physical gels they are connected together by a noncovalent bond, such as van der Waals interactions, hydrogen bonding, hydrophobic interactions, ionic interaction traces of crystallinity and multiple helices or by molecular entanglements⁶.

The adverse effects of chemical cross-Linking can be avoided by the process of physical cross-Linking using radiation or electron beam method.Radiation cross-Linking is more advantageous as the amount of cross-Linking can be controlled by the amount of dose used and is an energy efficient and cleaner process with no unwanted residuals in the products.Cross-Linking in binders, electrolytes, and electrode materials increases resistance to dissolution, improves stability, and supports an amorphous nature to increase ion conductivity.Cross-Linking can disrupt ordering and restrictpolymer mobility to ensure an open, amorphous structureeven in the swelled state⁷.

Cross-link polymer is an important engineering material due to their excellent stability than same polymer without cross-Linking. Different types of solid-supports such as organic polymer, inorganic nanoparticle (silica, metal oxide, or mixed metal oxide nanoparticles), and organic-inorganic framework are available for immobilization of catalyst, reagent, substrate, enzyme, and protecting groups. However, each type of solid-support has own merits and demerits. Over the last two decades, cross-linked polymers are potentially used in different fields such as synthesis, extraction, tissue engineering, drug delivery, pharmaceutical, and in biomedical applications because polymer properties can be significantly improved by cross-Linking^5-7. The properties such as mechanical strength, thermostability, glass transition temperature, swelling, permeability, rigidity, and stiffness can be improved to a remarkable extent⁸.

Cross-Linked Polymers^9-11:

Cross-linked polymers have many interesting properties which make them very attractive materials. By cross-Linking, the structure of a polymer solution can be fixed. The resulting polymer networks (or gels) show elastic behaviour and, depending on the system, good mechanical properties. Polymer networks are able to swell by up taking water or organic solvents. Inducing a phase transition in cross-linked responsive polymers, the change of chain conformation leads to the change of the properties of the macroscopic network (elasticity, swelling behaviour, turbidity).

The major concern of cross-link polymer is the less reactivity because large number of functional groups buried into the polymer matrix. However, this concern can be eliminated to a remarkable extent using core-shell polymer. Thus, cross-link polymer with improved surface area, porosity, swelling, and thermal properties is the demand of solid-phase chemistry.

Over the last two decades, natural and synthetic polymers were potentially used for various applications. Both types of polymers have their own merits and demerits.

Natural polymers have more solubility, no strength, and low thermal stability. Therefore, to improve these properties is an essential for wide polymer applicability. Undoubtedly, synthetic cross-link polymers improve these polymers. In-situ cross-Linking is a process of direct cross-Linking of functional monomer with cross-linker to obtain macromolecular chain of polymer whereas post-cross-Linking is a process of cross-Linking after polymerization. Polymer cross-Linking may be reversible or irreversible depending upon the nature of the cross-Linking. Cross Linking of polymers undergo Phase Transition from liquid to solid at a critical extent of reaction. This Phenomenon is called gelation. The polymer is said to be at the gel point (GP) if its steady shear viscosity is infinite and its equilibrium modulus is Zero. Several processes may contribute to this transition besides the connecting of molecular strands by chemical cross Linking: Physical entanglements between the macromolecular strands, vitrification as the glass transition temperature rises with increasing extent of reaction, Phase separation of the reaction components or products and crystallization.

Methods of Cross Linking^12-14:

Various cross linking methods can be employed for fabricating scaffolds or designing drug delivery matrices, depending on the type and nature of the biopolymer. Through the cross Linking process, the active functional groups of polymer chains react chemically or physically with the cross Linking agents and form a three-dimensional network.

Depending upon the nature of the polymer, different techniques may be used to cause cross Linking. Cross-Linking may occur through polymerization of monomers having functionalities more than two (by condensation) or by covalent bonding between polymeric chain through irradiation, sulphur vulcanization or chemical reactions by adding different chemicals in conjunction with heating and, sometimes, pressure. Monomer unit containing two or more functionality (double bond or functional group) called as a cross-Linking agent or a cross-linker. Cross-linker may contain two, three, or four, cross-Linking sites. In all cases, the chemical structure of the polymer is altered through the cross-Linking process.

Types of cross-Linking methods:^15-18

Chemical Cross-Linking:

Chemical cross-Linking is a function of primary forces like covalent bond formation. Chemical cross-Linking is the cross-Linking by free-radical, condensation (cationic/anionic), UV radiation, or small-molecule cross-Linking.Chemical processes, which require a chemical initiator (peroxide or silane) to induce links in the polymer chain. Chemical cross-Linking is irreversible and cannot be reversed. Chemical cross-Linking is much stronger and stable towards heat, mechanical, or any other action. Chemical Cross-Linking is activated by a chemical substance, which could be peroxide—the method is called peroxide initiated cross-Linking. The resulting PE is also called PEX-A in the European standards. A silane—the method is called cross-Linking via silane or moisture-based vinyl silane cross-Linking. The resulting PE is also called PEX-C in the European standards.

Radiation processes:

It involves exposure to ionizing radiation from either radioactive sources or highly accelerated electrons, to liberate free radicals for cross-Linking.

Ultra-violet Radiation:

In this method, the degree of cross-Linking is controlled by high-energy ionising radiation, such as gamma or x-ray, and their radiation dose in UV radiation cross-Linking. UV polymerization is the safest and cleanest method of polymerization since it does not degrade the polymer characteristics. This form of polymerization does not require any chemical additions such as an initiator; solvent, protective colloid, or surfactant. As a result, polymer retains their biocompatibility. Recently, it was seen that the UV radiation technique for the polymerization of poly (N-isopropylacrylamide) to obtain a hydrogel. In another, study¹⁸ it was seen that the polymerization of poly (vinyl alcohol) using UV radiation in the presence of terephthalic aldehyde. The time required for photopolymerization is considerably low which is in the range of few second to few minutes, while thermal polymerization can take several hours. However, use of radiation dose depends upon the application area of the cross-linked polymer. UV radiation is a less expensive method of obtaining cross-linked polymer than free radical or condensation polymerization, and it can be done at room temperature.

Free-radical Polymerization:

To create chemically cross-link polymers, suspension, emulsion, and dispersion polymerization processes are commonly utilised, with suspension polymerization being the preferred method. This type of polymer is very stable and can be classified as either degradable or non-degradable. In the presence of an initiator and heat, free-radical polymerization takes place. This process is used to polymerize acrylic acid and acrylate-based monomers. In the past, poly (styrene-co-divinylbenzene) was widely used as a solid-support.

Condensation Polymerization:

The polymers obtained by this technique may be degradable or non-degradable depending upon the bridging groups formed during polymerization. This type of polymerization is carried out in the presence of catalyst, heat, or both. In 2015, Wei et al. reported the silsesquioxane-based polymer containing hydroxyl functionality which was obtained by condensation polymerization. Besides ester, ether, amide, and imine-based polymer can be obtained by condensation polymerization.

Small-Molecule Cross-Linking:

In addition to multifunctional (bi, tri, or tetra) cross-Linking agent, some small-molecule also acts as a cross-linker. Small-molecules like glutaraldehyde, formaldehyde, osmium tetroxide, potassium dichromate, and potassium permanganate were potentially used to obtain cross-linked polymer. This type of method is generally used for the polymers containing hydroxyl, carboxylic acid, and amine-based functionality. This is also widely used technique like addition or condensation polymerization. The polymers obtained by this method have the non-degradable property and this type of cross-Linking cannot be reversed by heat, light, acid, or base.

Peroxide Processes:

The basic process entails the high-temperature breakdown of a peroxide and the formation of carbon bonds along the PE chain; cross-Linking takes place in the molten state. The Engel method was created in the late 1960s in Central Europe by German scientist Thomas Engel. Peroxide-based procedures come in a variety of forms. A granular combination of PE, peroxide, and stabilisers is sintered together under high pressure in the Engel process, and cross-Linking takes place during extrusion through a long-heated die. This method is commonly employed in the manufacturing of HDPE.PE is combined with peroxide, extruded, and cross-linked in a salt bath at high temperature in the Pont a'Mousson process, yielding a low or medium density PE. Instead, in the Daoplas process, the peroxide is added after extrusion and activated at high temperature and pressure in downstream equipment (the extruder)¹⁶. A carbon-based compound with a minimum of two oxygen atoms bound together (–O–O–) is known as an organic peroxide. R1 O O R2 2 is the generic formula.

There are also peroxides with a second –O–O– bond and three R-groups. Several diverse families of alkyl, aryl, and acyl peroxides and peroxyketals have been found as a result of the chemical structure of such R-groups. Alkyl peroxides produce the most reactive free radicals and are the most commonly utilised cross-Linking agents.

Physical Cross-Linking:

Physical cross-Linking is a weak compared to chemical cross-Linking.Physical cross-Linking is a function of secondary forces, such as ionic, hydrophobic, hydrogen bonding (intermolecular/intramolecular) interaction, stereo complexation, supramolecular chemistry, and these are well-known cross-Linking methods. Temperature, pressure, light, electricity, magnetic fields, stress, and pH changes can all be used to reverse physical cross-Linking. In comparison to other physical approaches, ionic interaction is the most powerful interaction of physical cross-Linking.

Biological Cross-Linking:

Biological cross-Linking is a new cross-Linking technology that uses biomolecules such complementary oligonucleotides, oppositely charged peptides, and heparin growth factor to achieve biological cross-Linking.The oligonucleotides such as oligo(cholesteryl) methacrylaten-PEG-oligo(cholesteryl)methacrylate) n-(OC5ma-PEO-C5MA) and oligo (10-cholesteryloxy decyl methacrylate) n-PEG-oligo (10-cholesteryloxy decyl methacrylate) n-OC10MA-PEO-OC10MA) were used. Further, peptide containing opposite charge can be used to obtain biological cross-linked polymers. The interaction between oppositely charged peptides and polymers is attractive, resulting in biological cross-Linking. The polymer could not be dissolved in an aqueous medium or an organic solvent using this method. However, unlike chemical cross-Linking, this type of cross-Linking is not as strong. Temperature, pressure, light, electricity, magnetic fields, stress, and pH changes can all be used to undo biological cross-Linking. Unlike chemical cross-Linking, this approach has yet to be established for industrial use.

Advantages and disadvantages of the Cross Linking methods:^19-22

Advantages:

Minimum tissue reaction after Cross Linking process, Less toxic for cells than chemical agents, Unlike many chemical agents, enzymes are most active under mild aqueous reaction conditions, Cross-Linking process can often be controlled by modifying temperatures, pH, or ionic strength, Forming very strong bonds.

Disadvantage:

May alter the properties of the materials, Needs more time for Cross-Linking, Lack of control over the reaction kinetics of Cross-Linking.

Evaluation Parameters:^22-24

Chemical Analysis:

This analysis consists basically of the evaluation of chemical modifications induced in PE by Cross-Linking methods.

Gel Content:

The most important parameter to measure is the gel content (fraction or gel %), which indicates the Cross-Linking degree. It is generally measured, followingthe ASTM D 2765 standard, as the percentage ofthe original weight of a sample after extraction for 24 hr in boiling toluene (or xylene) and successive dryingin a vacuum oven at 90^oC. Cross-Linking of 70% can be obtained with exposure to a radiation dose of 90 kGy, or 50 hr curing in hot water with silane. However, the main difference between the two methods is that in EBR the cross-Linking degree can be easily increased by increasing the dose. Instead, in the silane method, lengthening curing has no significant effect; an increase in the cross-Linking degree requires the addition of an increased percentage ofsilane to the compound.

Swelling Ratio:

Another important parameter is the swelling ratio. Polymer swelling is an important parameter in deciding the polymer-solvent compatibility during polymer applications. Polymer swelling is a function of degree of cross-Linking, surface area, and porosity. Higher the polymer cross-Linking, lower is the polymer swelling and vice-versa for lower crosslink polymer. This is mainly because of lower cross-link polymers have longer chain length and easy to expand. In contrast, in higher cross-link polymer, chain length is smaller and difficult to swell. Surface area and porosity of a polymer are turnable by changing physico-chemical parameters. High surface area and porosity also encourages the polymer swelling.

Fourier Transformation Infrared Spectroscopy (FTIR):

The chemical structure of a molecule can alsobe analysed by Fourier transform infrared spectroscopy. Many radiolytic products are visible in the infrared Spectrumand can be detected by an infrared detector likethe mercury–cadmium–telluride detector. The absorbanceis proportional to the concentration of the chemical species active at the selected frequency. Again, formedical implants a trace element analysis is performedto ensure absence, or allowed percentages, of somesubstances like titanium, calcium, chlorine, etc.

Mechanical Analysis:

The mechanical properties, such as tensile, compression, shear, and fatigue strengths are fundamental for theproduct’s lifetime. Each product must be tested byconsidering the effective operative stresses.

Thermal Analysis:

Several methods may be used to analyse the materialbehaviour under controlled temperature variations.

Differential scanning calorimetry (DSC):

It is the most utilized thermal analysis technique. It consists of observingand recording (thermogram) exothermic andendothermic phenomena that occur in a sample sealedin an aluminium sample chamber under temperature variations.

Non-destructive Evaluation:

Cross-linked polyethylene may include impurities andvoids from which the major causes of prematurefailures (SCG in pipes, treeing in electric cables, andcracking in medical implants) can originate. Thus, non-destructive evaluation with effective techniquesthat can discover defects at the incipient stage, beforethe component is put in operation, is of vital importance.

Conventional ultrasound (up to 10 MHz), whichis the current technique for detection of flaws in metalpiping and vessels, is limited by the attenuated nature of polymers.

A scanning acoustic microscope withoperating frequency up to 100–150MHz has beenfound to be more effective. Another technique that has proved to be practicalfor the evaluation of PEX is photothermal radiometry, commonly known as locking thermography, which isthe multiplexed version.

Applications²⁵:

1. Cross-linked polyethylene is currently employed in civil engineering, electric-electronic fields, medicine, and the packaging industry, and as technology evolves it may be used in many other fields.

2. Application in tissue engineering Alginate has been incorporated in various scaffolds for different regeneration applications.

3. Developed a hybrid cross-linked paste of alginate and liposomes as an oral mucoadhesive delivery system for oral cancer therapy.

4. Application in sustained released delivery drug delivery system and designing scaffolds in tissue engineering.

5. Each field has specific requirements and so a specific material must be fabricated with compliance to existing standards (i.e., ASTM F648 for Surgical Implants, ASTM F876 for PEX Tubing, ASTM F877 for PEX Hot and Cold-Water Distribution Systems, ASTM D3555 for Wire and Cable Insulation, and so forth).

6. The thermomechanical properties of PEX can potentially be adjusted by choosing the right component constituents, cross-Linking amount, and manufacturing sequence.

7. Cross-linked chitosan hydrogels have become more important in medication delivery and tissue systems research. Chitosan modifications improve the polymer's intrinsic qualities, such as biocompatibility, chemical versatility, biodegradability, and low toxicity. These modifications can be tailored to a specific application, for example, Cross Linking chitosan with cross linking agents such as glutaraldehyde or sodium tripolyphosphate has proven to be a convenient and effective method of improving the physical and chemical properties for practical applications.

CONCLUSION:

In the present review, various methods to Crosslinked Polymer were discussed. Moreover, Present review help to know the Significance and importance of Cross Linking process of polymers with Advantages and Disadvantages of Crosslinked polymers and also beneficial for understanding the different methods for Cross Linking polymers with their Evaluation Parameters. Applications of Crosslinked polymers are also discussed in present review.

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Received on 15.09.2021 Modified on 23.12.2021

Res. J. Pharma. Dosage Forms and Tech.2022; 14(2):183-188.

DOI: 10.52711/0975-4377.2022.00029

Hydrogel	Cross Linking agent	Applications
Poly Vinyl Alcohol	Sodium borate/boric acid	Packaging
Polyvinyl alcohol	Glyoxal	Adhesives Plastic films for packaging and water-soluble plastic bags Binders Fuel-resistant hoses
Starch	Glyoxal	Paper industry
Cellulose	Glyoxal	Textile industry
Protein and gelatin	Glyoxal	Food packaging
Polyethylene	Silane	Wires, cables, pipes heat shrinkable tubes
Agarose and chitosan	Oxidized dextrins	Tissue engineering applications
Chitosan	Glutraldehyde	Scaffold of hepatocyte
Guar gum	Epichlorohydrin	Biomedical application
Gellan gum	Endogen polyamine spermidine	Drug delivery
Glycol chitosan	Oxidized alginate	Drug delivery
Hydroxamated alginates	Zinc	Drug delivery
Alignate bead	Zinc	Drug delivery
Scleroglucan	Borax	Drug delivery
Poly(acrylic-co-vinylsulfonic) acid	Ethylene glycol dimethacrylate (EGDMA)	Drug delivery
Polyacrylamide	N, N′-methylenebisacrylamide	Dehydrating agent
Polyacrylamide/guar gum graft copolymer	Glutaraldehyde	Sorbent material for chromium ion (Cr (VI)