• Hydrogels are water-based, three-dimensional polymer networks capable of absorbing and retaining fluids.
• It can be also defined as a type of soft material formed by crosslinking hydrophilic polymers, capable of expanding in aqueous solvents and retaining water.
• Hydrogels can be molded into various shapes and its properties can be altered in response to external stimuli, making it useful in various applications such as drug delivery and tissue engineering.
• Many hydrogels are synthetic, but some are derived from natural materials.
• Key Features of Hydrogels:

1. 1. High water retention capacity.
   2. Biocompatibility (safe for biological use).
   3. Flexibility and responsiveness to external stimuli like temperature, pH, and pressure.

• Classification and Structure of Hydrogels

• • The classification of hydrogels depends on their source, composition, environmental stimuli, crosslinking, property, configuration, and ionic charge
  • Hydrogels are formed by the crosslinking of polymer chains.
  • The sources of hydrogels can be divided into natural, synthetic, or semi-synthetic polymers. 
    • Within the polymer source, hydrogels can be termed natural, synthetic, or semi-polymer hydrogels.
    • Naturally derived hydrogels (natural hydrogels) include cellulose, chitosan, collagen, alginate, agarose, hyaluronic acid, gelatin, and fibrin, etc.
    • They have inherent biocompatibility, bioactivity, and biodegradability, but relatively weak stability and mechanical strength.
    • Synthetic hydrogels are constructed by synthetic polymers. Those polymers are human-made polymers, which are prepared through the polymerization of a monomer, such as polyvinyl alcohol (PVA).
    • Semi-synthetic polymers are chemically modified natural polymers or a combination of natural and synthetic polymers as materials for the preparation of semi-hydrogels. An example of chemically modified natural polymers is methacryloyl-modified gelatin (GelMA).
  • Hydrogels can be amorphous, crystalline, or semi-crystalline based on their configuration.

Applications of Hydrogel:

• Agriculture:
  • Soil Moisture Retention: Hydrogels are used in agriculture to retain water in the soil, helping crops survive in dry conditions by slowly releasing moisture.
  • Controlled Release Fertilizers: Hydrogels are combined with fertilizers to provide slow and controlled nutrient release, improving plant growth and reducing fertilizer waste.
  • In addition to water management, hydrogels can also improve soil structure and nutrient availability. They can help loosen compacted soil, increase aeration, and improve the soil’s ability to hold nutrients. This can lead to healthier plants with better root development and increased yields
• Medical and Healthcare:
  • Wound Care: Hydrogels are commonly used in dressings for wounds and burns because they help maintain a moist environment that promotes faster healing and prevents infection.
  • Drug Delivery Systems: Hydrogels are used as drug delivery vehicles, allowing for controlled release of medication over time, especially for localized treatments like in cancer.
  • Contact Lenses: Hydrogels are used in soft contact lenses due to their flexibility and comfort, as the water content allows oxygen to pass through to the cornea.
  • Tissue Engineering:In tissue engineering, hydrogels act as scaffolds to support cell growth and tissue regeneration.
• Cosmetics and Personal Care:
  • Hydrogels are used in products like moisturizers, facial masks, and hydrating creams because of their ability to retain moisture and provide a soothing effect to the skin.
• Air Conditioning Systems: 
  • Hydrogels are used in passive cooling systems to absorb moisture and release it through evaporation, reducing energy consumption and enhancing cooling efficiency