a review of the properties of hydrogels

Hydrogels are three dimensional hydrophilic polymeric network with cross-linked bonds that swell in contact with water. with increase of the number of gels hydrophilic groups, the water storage capacity is enhanced, but they are not solved. in fact, the properties of hydrogels are interacting with their component parts, the polymer structure and the water. the polymeric network prevents the fluidization of hydrogels and water from the conversion of hydrogels to solids. in general, hydrogels have a semi-solid state and contract when water is lost.there are various categories for hydrogels, including the following types:1-based on polymer origin, which are classified into two categories (a): natural such as polysaccharides and (b): hybrid such as polyacrylamide.2- response to external environments that are divided in two types including (a): natural that absorb water when placed in aqueous medium, but do not exhibit changes in ambient temperature, electrical field and ph, and no change in the amount of absorption occurs. (b): responsive to the stimulus, that they are changed due to stimuli such as temperature, electric field application and ph, the amount of water absorption.3- network structure, which is divided into two groups (a): permanent hydrogels which have covalent bonds, such as poly methyl methacrylate (pmma) and (b): physical hydrogels produced by hydrogen, ionic or molecular bonds such as gelatin.