A photonic gel that rapidly alters color in response to external factors such as pressure, salt concentration, pH, humidity and temperature has been created by a group of researchers at MIT in Cambridge.Such a gel has a number of possible applications for sensors, including in food processing plants, where the material could be used to determine whether food has been maintained at the correct moisture level, the investigators reported. Other potential uses include electronically controlled tunable optically pumped lasers, photonic switches and multiband fibers.Photonic gel crystals demonstrate the tunability of materials made from alternating layers of hard and soft polymers. The soft polymers are swollen easily with liquid or vapor, causing the materials to reflect different colors of light based on the way their molecules are chemically tuned. Photo courtesy of MIT.The tunable gel was created with a self-assembling block of copolymer thin film made of alternating layers of polystyrene and poly-2-vinyl pyridine (P2VP). The thickness of the layers and their refractive indices determine which color light is reflected by the resulting gel. When the researchers maintained the thickness of the polystyrene layer and changed the thickness of the P2VP layer with water solutions having various concentrations of acid or salt, they could change the gel’s color in less than a second. The key to changing the thickness of the P2VP layer is to give the nitrogen on each segment of the P2VP block a positive charge, creating a polyelectrolyte chain that can swell to more than 1000 percent its volume in water.The polymer thin film is a one-dimensional periodic stack that can reflect light from the ultraviolet to the near-infrared region (350 to 1600 nm), yielding a color shift of 575 percent in the reflected wavelength.The scientists also are working on a gel that changes color in response to applied voltages. Their research is described in the Oct. 21 online edition of Nature Materials.