Natural polymers and the hydrogels prepared from them

Hydrogels are colloidal or gel-like substances made of three-dimensional polymer networks that contain more than 90% water. They are soft and wet materials comprised of solid and water contents enclosed in natural or synthetic polymeric networks. The hydrophilic nature of hydrogels is responsible for holding water molecules in their three-dimensional networks. The ability of such material to swell and shrink by absorbing and releasing water makes hydrogels suitable for several applications such as biomedicine, smart stimuli-responsive materials, tissue engineering, soft robots, and biomachines. This chapter deals with the synthesis, classification, properties, and applications of natural polymer-based hydrogels. Hydrogels from natural polymers such as cellulose, carrageenan, hyaluronic acid, polysaccharides (alginate, starch, agarose), chitosan, fibrin, and proteins (gelatin, collagen) are summarized in detail.