By Atif SuhailReviewed by Lexie CornerUpdated on Mar 20 2025Advances in materials science are transforming medicine, engineering, and environmental technology. Among the most promising innovations are hydrogels—highly absorbent polymer networks with applications in drug delivery, wound healing, and tissue engineering.
This article explores the structure and properties of hydrogels and how scientists are improving hydrogel technology to enhance performance in real-world applications.
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What Are Hydrogels?
Hydrogels are three-dimensional networks of hydrophilic polymers that absorb and retain large amounts of water while maintaining their structure. Their stability comes from chemical or physical cross-linking between polymer chains, allowing them to swell without dissolving.
The high water content of hydrogels gives them elasticity and flexibility, while their hydrophilic groups (-NH2, -COOH, -OH, -CONH2, -CONH -, and -SO3H) make them highly absorbent.
Hydrogels can undergo a significant volume phase change or gel-sol phase transition in response to physical and chemical stimuli, including temperature, electric and magnetic fields, solvent composition, light intensity, and pressure. In most cases, these conformational changes are reversible, meaning the hydrogel can return to its original state once the stimulus is removed.
Types of Hydrogels
Hydrogels are synthesized using a monomer, a cross-linking agent, and specific reaction conditions. Depending on their structure, they can be classified as homopolymer, copolymer, semi-interpenetrating networks (semi-IPN), or interpenetrating networks (IPN). Homopolymers contain only one type of monomer, while copolymeric hydrogels contain at least one hydrophilic monomer.
Manufacturing a Jelly-Like Ultra-Hard Hydrogel
Hydrogels can be classified based on composition, preparation method, response to stimuli, and source. The three main categories are biochemical, physical, and chemical hydrogels.
- Physical hydrogels respond to external stimuli such as temperature, electric and magnetic fields, or light.
- Chemical hydrogels rely on covalent bonding for stability and can change properties in response to specific triggers.
- Biochemical hydrogels react to biological signals, such as enzymatic activity or biochemical interactions.
Stimuli-responsive hydrogels are designed to react to specific environmental conditions.
- pH-sensitive hydrogels expand or contract depending on the acidity or alkalinity of their surroundings, as acidic or basic groups within the polymer structure gain or lose protons.
- Temperature-sensitive hydrogels, also known as thermogels, transition from a liquid to a gel when exposed to heat, often due to the presence of hydrophobic groups such as methyl, ethyl, or propyl.
- Electro-sensitive hydrogels shrink or swell in response to an applied electric field.
- Photo-responsive hydrogels change their characteristics when exposed to specific wavelengths of light.
In these cases, structural alterations occur in the polymer backbone or side chains, allowing the hydrogel to adapt to external conditions.
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Application of Hydrogels
Controlled Drug Delivery Systems
Hydrogels are widely used in drug delivery systems because they regulate the release of therapeutic agents, improving drug efficacy while reducing side effects. Their high water content, typically 70 to 99 %, makes them highly biocompatible and ideal for encapsulating hydrophilic drugs. By maintaining a hydrated environment, hydrogels help prevent drug denaturation and ensure sustained release.
The crosslinked polymer network of hydrogels can be tailored for specific mechanical properties, controlled degradation, and protection of bioactive molecules from enzymatic breakdown. Their mesh size regulates drug diffusion, enabling precise release control.
Functionalization through covalent bonding, electrostatic interactions, or hydrophobic associations further refines drug delivery. External factors such as pH, temperature, and mechanical forces can trigger drug release when needed.
Recent studies have demonstrated the potential of pH-responsive hydrogels in targeted drug delivery. One such hydrogel, developed by Lin X et al., was synthesized using a methacrylic acid copolymer (MAC) and polycaprolactone (PCL) through esterification. The MAC-g-PCL hydrogel remains intact at pH 1.2 but dissolves at pH 7.4, allowing for controlled drug release.
By incorporating the radioprotective agent amifostine, researchers developed a drug-loaded MAC-g-PCL-Ami hydrogel that enhances drug stability in the stomach and facilitates targeted delivery in the intestine. This approach could improve the bioavailability of drugs that degrade rapidly in acidic environments.
Tissue Engineering and Regenerative Medicine
Hydrogels are widely used in tissue engineering and regenerative medicine because they mimic the extracellular matrix (ECM), providing a three-dimensional scaffold that supports cell adhesion, proliferation, and differentiation. Their high water content helps maintain moisture, which is essential for cell migration and wound healing.
One of their main advantages is the ability to deliver therapeutic agents in a controlled manner. By encapsulating growth factors and drugs, hydrogels allow for localized and sustained release, improving tissue regeneration while minimizing systemic side effects. This makes them particularly useful in applications where precise control over drug delivery is necessary.
Hydrogels have been explored extensively in bone tissue engineering, where they are used as scaffolds to support bone regrowth. Studies have shown that loading hydrogel scaffolds with mesenchymal stem cells (MSCs) and bioactive nanoparticles can accelerate bone healing.
Recent work by Hussain et al. (2023) developed composite hydrogels based on GelMA-catechol, with an FeHAp coating to enhance bioactivity and mechanical strength. These modified hydrogels demonstrated improved durability and structural integrity, suggesting they could provide better support for bone regeneration in clinical applications.
New hydrogel can repair tears in human tissue
Ophthalmology and Contact Lenses
Hydrogels are widely used in ophthalmology, particularly in contact lenses and ocular drug delivery, due to their ability to retain moisture while remaining transparent and biocompatible. Their optical properties, including clarity and flexibility, depend on factors such as polymer composition, molecular weight, and water content, making them well-suited for vision-related applications.
One of the main challenges in ophthalmic drug delivery is overcoming the eye’s natural defense mechanisms, such as tear production and blinking, which rapidly remove conventional eye drops before the drug can take effect. Smart hydrogels, particularly thermosensitive variants, help address this issue by transitioning from liquid to gel upon contact with the eye’s surface. This prolongs drug retention, allowing for a more sustained release and improved bioavailability.
Recent innovations have explored nanozyme-based hydrogel eye drops, which have been shown to penetrate bacterial biofilms and provide enhanced resistance against drug-resistant pathogens.
Hydrogel formulations designed for corneal scarring and chemical burns act as protective ocular bandages, delivering therapeutic agents over extended periods to support tissue repair and reduce inflammation.
Biosensors and Wearable Technology
Hydrogels are increasingly used in biosensors and wearable technology, particularly in the development of wearable electrochemical biosensors (WEBSs) for non-invasive health monitoring. Their soft, biocompatible nature makes them well-suited for integrating with the skin, allowing for continuous detection of biomarkers in biofluids such as sweat, saliva, and tears.
A major application of hydrogel-based WEBSs is in glucose monitoring for diabetes management. These sensors enable continuous glucose detection without requiring invasive blood sampling. Conductive materials such as reduced graphene oxide (rGO) and polyaniline (PANI) are often incorporated into hydrogel matrices to improve electrical conductivity and mechanical stability.
Self-healing hydrogels have also been developed to improve sensor durability and functionality. By modifying hydrogels with nanospheres of cerium and manganese oxide, researchers have enhanced glucose oxidase immobilization and response rates while reducing sensor degradation.
In a study by Liang et al., a self-healing hydrogel was created using quaternized chitosan and oxidized dextran, incorporating cerium oxide and manganese oxide hollow nanospheres. These nanospheres were covalently immobilized within the hydrogel using EDC/NHS coupling, ensuring stability by preventing leaching and maintaining consistent sensing performance.
Energy Storage and Conducting Hydrogels
Hydrogels are being explored as materials for energy storage and conductive applications, particularly in supercapacitors, batteries, and flexible electronics. Their porous structure enhances ion transport, which is essential for efficient charge and discharge cycles in high-performance supercapacitors. By incorporating conductive materials such as graphene, polyaniline, or metal nanoparticles, hydrogels can achieve high electrical conductivity, making them suitable for use as electrodes and electrolytes in energy storage devices.
One of the key advantages of hydrogel-based supercapacitors is their mechanical flexibility, allowing them to be integrated into wearable electronics and biomedical implants without compromising durability or energy density. Additionally, self-healing and anti-freezing hydrogel electrolytes have been developed to improve the longevity and adaptability of these devices in various environmental conditions.
Recent research by O. Hu et al. focused on developing a high-performance organohydrogel electrolyte (PVA/P(AM-co-SBMA)/CaCl2) designed for flexible all-solid-state supercapacitors. By incorporating a zwitterionic double-network structure with CaCl2, the hydrogel demonstrated enhanced mechanical strength, adhesion, and water retention.
The resulting supercapacitor exhibited a wide potential window (0–2.1 V), long cycling stability over 7000 cycles, and high capacity retention of 82.4 %, maintaining electrochemical performance even under extreme deformations.
Water Purification and Environmental Applications
Practical Water Treatment Solution Using Hydrogel Tablets
Hydrogels are being explored for water purification due to their ability to absorb and retain pollutants while remaining easy to regenerate. Their porous structure enables the efficient removal of organic dyes, heavy metals, and emerging contaminants from wastewater. This makes them a cost-effective and sustainable solution for water treatment.
Recent developments have focused on improving hydrogel-based filtration. Membranes with embedded microgels provide selective permeability, self-cleaning properties, and biodegradability. Zhou et al. developed magnetic hydrogels by incorporating magnetic nanoparticles into the polymer matrix. These hydrogels can be easily separated and reused with external magnetic fields.
Advances in CO2-responsive hydrogel systems have introduced an on-off, selective, and recyclable adsorption mechanism. This eliminates the need for harsh chemical treatments. Hydrogels have also been applied for heavy metal removal, with double-network polymeric structures improving mechanical strength and adsorption efficiency for metals such as lead, cadmium, and copper.
In addition to filtration, hydrogel-based catalysts have been integrated with metal nanoparticles to break down persistent pollutants. These advancements highlight the potential of hydrogels in wastewater treatment. They offer a renewable and effective method for contaminant removal while reducing environmental impact.
Next Steps in Hydrogel Research and Development
Hydrogels have limitations that affect their practical use. Their mechanical strength is often low, making them difficult to handle and limiting their durability in certain applications. In wound care, many hydrogels require secondary dressings because they do not adhere well to tissue. In surgical settings, some formulations carry risks due to the presence of cross-linking chemicals.
Cost is another challenge. Some hydrogels are expensive to produce, restricting their use in large-scale applications. Maintaining stability over time is also an issue, as hydrogels need precise control over their physical and microbiological properties to ensure consistent performance.
Research is focused on addressing these limitations and expanding hydrogel applications. Efforts are being made to develop biodegradable polymers and more sustainable production methods to reduce environmental impact. Green chemistry approaches are also being explored to refine hydrogel synthesis and minimize unwanted residuals.
Future advancements will likely lead to hydrogels with tunable properties, such as adjustable composition, controlled drug release, and improved mechanical strength. These developments could make hydrogels more adaptable for personalized wound care and industrial applications. A combination of materials science, bioengineering, and nanotechnology will be key to enhancing their performance and broadening their use.
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References and Further Reading
Bahram, M., Mohseni, N. & Moghtader, M. (2016) ‘An Introduction to Hydrogels and Some Recent Applications’, Emerging Concepts in Analysis and Applications of Hydrogels, ed. S. Biswas Majee, IntechOpen. Available at: https://www.intechopen.com/chapters/51535
Ho, T.-C., Chang, C.-C., Chan, H.-P., Chung, T.-W., Shu, C.-W., Chuang, K.-P., Duh, T.-H., Yang, M.-H. & Tyan, Y.-C. (2022) ‘Hydrogels: Properties and Applications in Biomedicine’, Molecules, 27, p. 2902.
Hu, O., Lu, J., Weng, S., Hou, L., Zhang, X. & Jiang, X. (2022) ‘An Adhesive, Anti-Freezing, and Environment Stable Zwitterionic Organohydrogel for Flexible All-Solid-State Supercapacitor’, Polymer, 254, p. 125109.
Hussain, Z., Ullah, I., Liu, X., Mehmood, S., Wang, L., Ma, F., Ullah, S., Lu, Z., Wang, Z. & Pei, R. (2023) ‘GelMA-Catechol Coated FeHAp Nanorods Functionalized Nanofibrous Reinforced Bio-Instructive and Mechanically Robust Composite Hydrogel Scaffold for Bone Tissue Engineering’, Biomaterials Advances, 155, p. 213696.
Khan, M. U. A., Aslam, M. A., Abdullah, M. F. B., Al-Arjan, W. S., Stojanovic, G. M. & Hasan, A. (2024) ‘Hydrogels: Classifications, Fundamental Properties, Applications, and Scopes in Recent Advances in Tissue Engineering and Regenerative Medicine – a Comprehensive Review’, Arabian Journal of Chemistry, 17, p. 105968.
Kulbay, M., Wu, K. Y., Truong, D. & Tran, S. D. (2024) ‘Smart Molecules in Ophthalmology: Hydrogels as Responsive Systems for Ophthalmic Applications’, Smart Molecules, 2, p. e20230021.
Liang, Z., Zhang, J., Wu, C., Hu, X., Lu, Y., Wang, G., Yu, F., Zhang, X. & Wang, Y. (2020) ‘Flexible and Self-Healing Electrochemical Hydrogel Sensor with High Efficiency toward Glucose Monitoring’, Biosensors and Bioelectronics, 155, p. 112105.
Lin, X., Miao, L., Wang, X. & Tian, H. (2020) ‘Design and Evaluation of pH-Responsive Hydrogel for Oral Delivery of Amifostine and Study on Its Radioprotective Effects’, Colloids and Surfaces B: Biointerfaces, 195, p. 111200.
Ma, J., Luo, J., Liu, Y., Wei, Y., Cai, T., Yu, X., Liu, H., Liu, C. & Crittenden, J. C. (2018) ‘Pb(II), Cu(II) and Cd(II) Removal Using a Humic Substance-Based Double Network Hydrogel in Individual and Multicomponent Systems’, Journal of Materials Chemistry A, 6, pp. 20110-20120.
Markovic, M. D., Spasojevic, P. M., Pantic, O. J., Savic, S. I., Spasojevic Savkovic, M. M. & Panic, V. V. (2024) ‘Status and Future Scope of Hydrogels in Wound Healing’, Journal of Drug Delivery Science and Technology, 98, p. 105903.
Sharma, A. K., Sharma, R., Pani, B., Sarkar, A. & Tripathi, M. (2024) ‘Engineering the Future with Hydrogels: Advancements in Energy Storage Devices and Biomedical Technologies’, New Journal of Chemistry, 48, pp. 10347-10369.
Singh, V. K., Kumar, K., Singh, N., Tiwari, R. & Krishnamoorthi, S. (2022) ‘Swift Catalytic Reduction of Hazardous Pollutants by New Generation Microgels’, Soft Matter, 18, pp. 535-544.
Thirumalai, D., Santhamoorthy, M., Kim, S.-C. & Lim, H.-R. (2024) ‘Conductive Polymer-Based Hydrogels for Wearable Electrochemical Biosensors’, Gels, 10, p. 459.
Tran, V. V., Phung, V.-D. & Do, H. H. (2024) ‘Advances and Innovations in Hydrogel Particles for Sustainable Purification of Contaminants in Aqueous Solutions’, Chemical Engineering Journal, 486, p. 150324.
Wang, H., Song, F., Feng, J., Qi, X., Ma, L., Xie, L., Shi, W. & Zhou, Q. (2022) ‘Tannin Coordinated Nanozyme Composite-Based Hybrid Hydrogel Eye Drops for Prophylactic Treatment of Multidrug-Resistant Pseudomonas aeruginosa Keratitis’, Journal of Nanobiotechnology, 20, p. 445.
Wang, W., Narain, R. & Zeng, H. (2020) ‘Hydrogels’, in Narain, R. (ed.) Polymer Science and Nanotechnology, Elsevier, pp. 203-244. Available at: https://www.sciencedirect.com/science/article/pii/B9780128168066000108
Zhou, A., Chen, W., Liao, L., Xie, P., Zhang, T. C., Wu, X. & Feng, X. (2019) ‘Comparative Adsorption of Emerging Contaminants in Water by Functional Designed Magnetic Poly(N-Isopropylacrylamide)/Chitosan Hydrogels’, Science of The Total Environment, 671, pp. 377-387.
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