Source: Amber N. Barron, Ashlea Patterson, and Taylor D. Sparks, Department of Materials Science and Engineering, The University of Utah, Salt Lake City, UT Hydrogels are a versatile class of cross-linked polymers produced through relatively simple procedures and with generally inexpensive materials. They can be formed from solution and involve a polymer backbone formed from monomer reagents, an initiator which makes the polymer reactive and a crosslinking species which binds the polymer chains together. An important aspect of these materials is that they swell in the presence of water, but this response can be tuned further to enhance swelling as a function of salinity, pH, or other signals. As a final product, hydrogels can be used in aqueous or dry environments, with a range of useful properties such as flexibility, high absorbance, transparency and thermal insulation. They are commonly used for liquid absorbance, sensors, consumer products, and drug delivery.