This article considers hydrogels and their properties, types, applications, and new research surrounding them.
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What Are Hydrogels?
A hydrogel is a three-dimensional network of hydrophilic polymers that can inflate in water. Hydrogel contains a high volume of water while being structurally stable, primarily due to the chemical or physical cross-linking of individual polymer chains. Hydrogels have a high degree of elasticity due to their high water content, and the hydrophilic groups (-NH2, -COOH, -OH, -CONH2, -CONH -, and -SO3H) contribute to the network's hydrophilicity.
Hydrogels undergo a significant volume phase change or gel-sol phase transition in reaction to specific physical and chemical stimuli (temperature, electric and magnetic fields, solvent composition, light intensity, and pressure). However, in most situations, such conformational shifts are reversible, and the hydrogels can revert to their initial configuration as soon as the trigger is withdrawn.
Hydrogels are synthesized using a monomer, a cross-linking chemical, and specific reaction conditions. Based on the methods of synthetization, hydrogels may be classified as a copolymer, semi-interpenetrating network (semi-IPN) homopolymer, and interpenetrating network (IPN).
Homopolymers have only one type of monomer in their structure and, based on the monomer's nature and the polymerization process, they may have a cross-linked structure. Copolymeric hydrogels are monomers, and at least one monomer in copolymeric hydrogels is hydrophilic.
Types of Hydrogels
Hydrogels are primarily classified as biochemical hydrogel, physical hydrogel, or chemical hydrogel and can also be classified based upon preparation, source, response, and physical properties. Physical hydrogels change in response to physical stimuli, chemical hydrogels utilize covalent bonding to change in response to stimuli, and biochemical hydrogels respond to enzymes or biochemical reactions.
Standard hydrogels are classified as pH-sensitive hydrogels, temperature-sensitive hydrogels, electro-sensitive hydrogels, and light-responsive hydrogels. In the presence of an applied electric field, electro-sensitive hydrogels shrink or swell.
When photo-responsive hydrogels are exposed to the appropriate wavelength of light, their characteristics change, due to light-induced structural alterations of specific functional groups in the polymer backbone or side chains. Temperature-sensitive hydrogels (thermogels) are aqueous monomer/polymer solutions that can form a gel when exposed to heat and have a slightly hydrophobic characteristic due to the presence of groups (methyl, ethyl, and propyl).
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Acidic or basic groups in pH-sensitive polymers respond to pH changes in their surroundings by gaining or losing protons.
Application of Hydrogels
Hydrogels are used in controlled drug delivery systems (DDS) to address the constraints of traditional drug formulations by delivering medications at specific rates over predetermined periods. Hydrogel also finds application in tissue engineering to replace or support artificial tissues.
Ophthalmology, particularly contact lenses, is an important discipline that uses synthetic hydrogels for bio applications. Hydrogels may also be employed as immobilization matrices for biosensing elements and provide ideal conditions for enzymes and other biomolecules to maintain their active and functional structure.
Recent Research on Hydrogels
Due to their outstanding solid-liquid interface, strong electric properties, and mechanical flexibility, electrically conducting polymer hydrogels offer tremendous promise for the predicted integration and constitute a viable material platform for developing flexible energy storage systems. Viscoelastic hydrogel is used for the treatment of spinal cord injury (SCI), which is a complex regenerative medical problem. Tough hydrogels are designed for tissue adhesive, tissue engineering, and soft actuator application.
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Professor Oren Scherman, a researcher at Cambridge University, developed a novel hydrogel that can achieve extreme compressibility under high pressures, which can be used in a wide range of industrial and bio-medical devices. In a paper published in the International Journal of Molecular Science, a unique breakthrough was investigated to develop Keratin-conjugated fibrinogen (KRT-FIB), build new keratin biomaterials, and analyze cell–biomaterial interactions.
Limitations and Future Scope
The major limitation of hydrogels is poor mechanical strength and difficulty in handling and loading. Hydrogels require secondary dressing materials for medical applications as they are non-adherent and there is some risk associated with the application of hydrogels in surgery. Certain hydrogels are expensive, which limits their scope of application; some hydrogels cause toxicity issues in biological applications due to the presence of cross-linking chemicals.
The future scope of hydrogels is in biological (implant materials, tissue engineering, and bio-printing) and environmental applications (hazardous pollutant removal) due to their biocompatibility and biodegradability. Furthermore, conducting hydrogels can be used to design and manufacture supercapacitors utilized in advanced electronics.
References and Further Reading
Morteza Bahram, Naimeh Mohseni and Mehdi Moghtader (August 24th 2016). An Introduction to Hydrogels and Some Recent Applications, Emerging Concepts in Analysis and Applications of Hydrogels, Sutapa Biswas Majee, IntechOpen, DOI: 10.5772/64301. Available from: https://www.intechopen.com/chapters/51535
Wenda Wang, Ravin Narain, Hongbo Zeng, Chapter 10 - Hydrogels, Editor(s): Ravin Narain, Polymer Science, and Nanotechnology, Elsevier, 2020, Pages 203-244, ISBN 9780128168066, https://www.sciencedirect.com/science/article/pii/B9780128168066000108
Baily, S. (2021, November 30). Manufacturing a Jelly-Like Ultra-Hard Hydrogel. AZoM.Com. https://www.azom.com/article.aspx?ArticleID=21012
Karomah, A. (2021, December 13). Keratin-Based Injectable Hydrogel to Promote Oral Tissue Regeneration. AZoM.Com. https://www.azom.com/news.aspx?newsID=57660