Using thermoresponsive hydrogel nanoparticles, researchers at Georgia Institute of Technology in Atlanta have formed colloidal photonic crystals that are color tunable by temperature-controlled particle compression.The crystals are formed from a thermal cycling process that repeatedly transforms their structure from a hot liquid to a stable crystalline gel (see figure). Each cycle changes the overall water content in the crystals, allowing designers to specify their optic properties. While in the gelatinous phase, the crystals can be cast, molded or spin-coated onto a surface.Although they acknowledge that application of the crystals may be far off, the researchers said that the versatility of the materials may make them ideal for many photonics applications, including multiplexing/demultiplexing.