The present invention provides a method of producing a polymer hydrogel comprising the steps of: (1) preparing an aqueous solution of a water soluble polysaccharide derivative and a polycarboxylic acid； (2) optionally agitating the solution, for example, by stirring； (3) isolating a polysaccharide derivative/polycarboxylic acid composite from the solution； and (4) heating the polysaccharide derivative/polycarboxylic acid composite at a temperature of at least about 80 °C, thereby cross-linking the polysaccharide with the polycarboxylic acid. The invention also provides polymer hydrogels produced by the methods of the invention.
