Polymer gels are polymeric compounds formed from cross-linked monomer subunits and are capable of swelling in solvents. The swelling properties and the transition between the two sol-gel states of the polymer system, which are influenced by factors such as pH, temperature and ionic content in the solvent, have long been interested and applied by researchers. in medical and engineering innovations.

GEL Formation

The actual gels are classified according to the strength of the cross-linked intramolecular polymers. Common rowing bonds are classified into 2 types: chemical and physical. Chemical bonds are usually covalent bonds while physical bonds are usually hydrogen and ionic bonds, or simply entanglement of polymer fibers. Among them: free radical cross-linking polymerization (FCC) is the most common method in gel formation.

The polymer gel formation process is divided into three main stages: slow transformation, gelation and glass transition. The slow transformation phase is the formation of a basic polymer fiber using a monomer and an initiator. Theoretically, a decrease in the concentration of the reactant should lead to a decrease in the polymer rate. However, in stage 2 (the gelation phase), the reaction rate gradually increases with the amount of reaction already taking place (Trommsdorf Norrish Effect); resulting in enhanced polymerization of free electrons and increased viscosity of the reaction system. The glazing stage occurs when the temperature of the system is lower than the glazing temperature of the polymer, helping to form a strong polymer structure.

Figure 1. Transition between two sol-gel states

For physical gels, the gel can be subjected to heating of the system at this stage, which will cause the polymer to be formed to have a flexible structure, called a weak gel. The gelation process when cooling the system is called sol-gel transition, whereas when heating the system, the reverse gelation will take place is called gel-sol transition. The gel-sol transition is also influenced by solvent composition, pH change, ionic composition and electromagnetic influence.

A strong physical bond is a physical bond between polymer fibers, while a weak physical bond is the formation of a “reversible” bond. In contrast, chemical gelation involves the formation of covalent bonds, so the result is always strong gel formation.

Figure 2. The bonds that form the polymer gel


Polymer heating or cooling:

A good example of a group of natural gels is carrageenan, which is made from red algae, which can change its gel state according to temperature and the presence of a gelling agent: K+ ions. The participation of K+ ions helps to increase gel strength and elastic modulus of k-carrageenan gels according to conformation ordering mechanism (simply understood as the arrangement of monomer elements on the interactions that are determined). promoted by K+ ions) and subsequent aggregation. The essence of gelation is due to the formation of intramolecular physical crosslinked bonds of carrageenan which form (single) helices when cooled and the binding of positive K+, Na+ ions and sulfonic groups (3). -) as a bridge to pair the helix to form a stable gel system.

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Figure 3. K-arrageenan gel formation mechanism

Ionic bonding:

Another gel with strong potential for growth in the pharmaceutical and food industries is: Gel alginate, a negatively charged linear polysaccharide made of α-L-guluronic acid (G) and β-D-mannuronic acid (M). ) linked together via α-1,4-glycoside bonds.

The gelation of alginate is controlled through the use of positive ions that enlarge the “shield” that neutralize the electrostatic repulsion of the carboxyl units and form direct ionic bonds, which play a major role in the formation of the alginate. into an alginate gel. There are many types of ions that can be gelled with alginate, but only the Ca2+ ion is of interest due to its practical applications. Depending on the different Ca2+ concentrations, the gel formed exists in a heat-invertible or heat-invertible state (high Ca2+ concentration).

Figure 4. Alginate gel formation mechanism

The gelation mechanism of alginate with Ca2+ ions includes 3 steps: (a) Create egg-box between saccaride chains with Ca2+ ions as chelate bridges; (b) Formation of dimer bonds between egg-boxes; (c) Formation of lateral packing between dimerized egg-box pairs, side bonds are regulated by Ca2+ ions, Na+ ions, and hydrogen bonds between carboxy radicals on the paired G-chain .

Gel alginate is one of the most widely used natural gels in the pharmaceutical and food industries. The H+-alginate gel formed between alginate and gastric juice with the combination of Ca2+ ions to form Ca-alginate gel are the two main mechanisms in Gaviscon’s indications for the treatment of gastroesophageal reflux (Reckitt Benckiser). Ca-alginate is also used in encapsulation of unstable vitamins B1, B6, C, pectinase enzyme (an enzyme that hydrolyses pectin to help create clarity for juice solutions) or as an excipient to regulate the release of juices. Suitable for pharmaceuticals, suspensions or active ingredients to create gel layers for the treatment of ulcers caused by stomach ulcers.

Complex coacervation (complex flocculation):

Gel flocculation complexes can be formed through the combination of polyanion and polycation solutions. The basic principle is due to the pairing of opposite charges to form complexes that are soluble or insoluble depending on the concentration and pH of the solution system. For example, proteins at pH below the isoelectric point will be positively charged and tend to combine with negatively charged hydrophilic colloids (anionic hydrocolloids) to form multi-charged hydrophilic gel complexes (polyions). gel coagulation.

Figure 5. Ionotropic gelation occurs by the interaction of the negatively charged alginate groups (COO–) and the covalent double bond of Ca++ ions.

Hydrophilic gel binding and hydrophobic interactions:

Hydrophilic hydrogen bridges can be formed by lowering the pH of the polymer solution carrying the carboxyl group. This mechanism promotes the formation of rheological synergism, which means that mixing two polymers can result in a more gel-like system than the individual polymer components. For example, gelatin-agar, powder-CMC and hyaluronic acid (HA)-methylcellulose systems that can help form a gel-like structure by cross-linking can be used for injection.

Figure 6. Example of hydrophilic gel formed by hydrogen bonding

Polyacrylic acid (PAA) and poly(methacrylic acid) (PMAAc) form complexes with PEG by forming hydrogen bonds between the oxygen atom of PEG and the carboxylic group of PAA or PMAAc. Hydrogen bonds only form when the carboxyl groups are ionized, so it can be understood that pH plays a very important role in the process of creating and attracting water and swelling of the gel system. However, hydrogen bonding is also the weak point of the gel system, causing the gel system to tend to dissolve and disintegrate in the aqueous medium after a long time, so it can only be used for immediate drug release systems.

Reference source:

Redaelli, F., Sorbona, M., & Rossi, F. (2017). Synthesis and processing of hydrogels for medical applications. Print Bioresorbable polymers for biomedical applications (pp. 205-228). Woodhead Publishing.
Pekcan, Ö., & Kara, S. (2012). Gelation mechanisms. Modern Physics Letters B, 26(27), 1230019.
Hu, C., Lu, W., Mata, A., Nishinari, K., & Fang, Y. (2021). Ions-induced gelation of alginate: Mechanisms and applications. International Journal of Biological Macromolecules.