Photoresist is a polymeric coating that changes its chemical properties when it is exposed to light.  Photoresists are used as one of the first steps in several processes such as photolithography, where a pattern is engraved onto the surface of a given material. The material is first coated with the photoresist and exposed to light. There are two types of photoresists, a positive and a negative. In a positive photoresist the areas that are exposed to light are solubilized by a chemical called the "developer." In a negative photoresist the opposite happens and the areas that are exposed to light become less soluble by the developer and therefore protects the irradiated area.  The material is further treated with another type of chemical reagent that etches away the substrate but those areas covered with the photoresist, "resist" the chemical treatment and are again protected. Some of the different chemical components of photoresists are polymethylmethacrylate (PMMA), SU-8 and polymethylglutarimide (PMGI).