Ultrahigh‐Water‐Content Photonic Hydrogels with Large Electro‐Optic Responses in Visible to Near‐Infrared Region

Abstract The embedding of photonic crystals within stimuli‐responsive hydrogels has attracted tremendous interest because it provides new applications such as optical switches, displays, and sensors. However, the production of electrically tunable photonic hydrogels with a wide range of color tunability, fast electrical response, and high stability for repeated use is still challenging. Here, electrically tunable photonic hydrogels are fabricated using ultrahigh‐water‐content polyelectrolyte layered hydrogels composed of thousands of bilayer domain structures. The layered hydrogel exhibits versatile color tunability by applying an electric field parallel or perpendicular to the direction of the gel layers. The hydrogel exhibits homogeneous color tuning when a perpendicular electric field is applied, while it shows a large rainbow‐like electric‐optic response in the visible to near‐infrared region when a parallel electric field is applied. Additionally, by a simple method using patterned electrodes, a reflective display showing the designed characters is demonstrated and maintained for hours underwater without an external source of energy. Moreover, the electrically induced optical patterns can be erased, and the responsive photonic hydrogel shows excellent stability for repeated use. We anticipate that this study will provide the foundation for the development of responsive photonic hydrogels with new and large electro‐optical effects for future chromatic applications.