Like a solid, hydrogels do not flow. Like a liquid, small molecules diffuse through a hydrogel. Hydrogels are currently viewed as water-insoluble soft and wet materials, and they are usually composed of three-dimensional polymer network structure and a large amount of water (50 ~ 99%). It has potential applications in many fields, such as, drug delivery system, superabsorbent, biosensor, tissue engineering, wound dressing, and battery.