- Hydrogels are three-dimensional, hydrophilic polymer networks capable of absorbing and retaining large amounts of water or biological fluids while maintaining their structural integrity. Their remarkable water-absorbing capacity arises from the presence of hydrophilic functional groups, such as hydroxyl, carboxyl, amide, and sulfonic groups, within the polymer chains.
- Crosslinking—either physical (via hydrogen bonds, ionic interactions, or van der Waals forces) or chemical (via covalent bonds)—prevents the hydrogel from dissolving in water, instead allowing it to swell and form a soft, gel-like material.
- One of the defining features of hydrogels is their tunable physical and chemical properties. By adjusting polymer composition, degree of crosslinking, and environmental responsiveness, hydrogels can be engineered with controlled swelling behavior, mechanical strength, and biodegradability. Many hydrogels are stimuli-responsive, meaning they can undergo changes in volume, permeability, or shape in response to external triggers such as pH, temperature, light, ionic strength, or biomolecules. This adaptability has made hydrogels versatile in both fundamental research and applied science.
- In biomedical applications, hydrogels have attracted significant attention due to their biocompatibility, high water content, and similarity to natural extracellular matrix (ECM). They are widely used in drug delivery systems, where their porous structure allows for encapsulation and controlled release of therapeutic agents. Hydrogels can also serve as tissue engineering scaffolds, supporting cell growth and differentiation while gradually degrading to be replaced by natural tissue. Clinical applications include wound dressings, contact lenses, and injectable hydrogels for localized therapy or minimally invasive surgery. Advanced hydrogels are being explored for 3D bioprinting, where they act as bioinks to construct complex tissue-like structures.
- In industrial and environmental fields, hydrogels are valued for their absorbent and responsive properties. They are used in agriculture as soil conditioners and water-retaining agents to improve crop yield in arid regions. In personal care products, such as diapers and sanitary items, superabsorbent hydrogels made from polyacrylates are common. Hydrogels are also employed in biosensors, soft robotics, and actuators, where their ability to swell or contract in response to stimuli can be harnessed for mechanical or sensing functions.
- From a materials science perspective, hydrogels can be classified into several categories: natural polymer-based hydrogels (e.g., alginate, chitosan, hyaluronic acid, gelatin), synthetic hydrogels (e.g., polyacrylamide, polyethylene glycol, polyvinyl alcohol), and hybrid hydrogels that combine the advantages of both. Natural hydrogels offer excellent biocompatibility and biodegradability, while synthetic hydrogels provide superior mechanical strength, reproducibility, and tunability. Ongoing research is focused on developing smart hydrogels that can self-heal, respond dynamically to multiple stimuli, or deliver drugs with high precision.
