Hydrogels: A Comprehensive Review of Structure, Properties, and Multifaceted Applications

Hydrogels, three-dimensional networks of hydrophilic polymers capable of absorbing and retaining significant amounts of water, have transitioned from laboratory curiosities to cornerstone materials in modern technology and medicine. Their unique biocompatibility, tunable physical and chemical properties, and stimuli-responsive nature have made them indispensable in fields ranging from drug delivery and tissue engineering to soft robotics and sustainable agriculture. This review provides a systematic overview of hydrogel classification, cross-linking mechanisms, and fundamental properties. We then delve into their advanced applications across biomedical and non-biomedical sectors, highlighting recent breakthroughs. Finally, we discuss the current challenges and future perspectives for the next generation of "smart" hydrogel systems. Keywords: Hydrogel, Cross-linking, Stimuli-responsive, Biomaterial, Drug Delivery, Tissue Engineering, Soft Robotics, Agriculture.