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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, among others. 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 a challenging task. Here, electrically tunable photonic hydrogels were fabricated using ultrahigh-water-content polyelectrolyte layered hydrogels composed of thousands of bilayer domain structures. The layered hydrogel developed in this work exhibited versatile color tunability by applying an electric field parallel or perpendicular to the direction of the gel layers. The hydrogel exhibited homogeneous color tuning when a perpendicular electric field was applied, while it showed a large rainbow-like electric-optic response in the visible to near-infrared region when a parallel electric field was applied. Additionally, by a simple method using patterned electrodes, a reflective display showing the designed characters was demonstrated and maintained for hours under water without an external source of energy. Moreover, the electrically induced optical pattern can be erased, and the responsive photonic hydrogel showed 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.

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, among others. 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 a challenging task. Here, electrically tunable photonic hydrogels were fabricated using ultrahigh-water-content polyelectrolyte layered hydrogels composed of thousands of bilayer domain structures. The layered hydrogel developed in this work exhibited versatile color tunability by applying an electric field parallel or perpendicular to the direction of the gel layers. The hydrogel exhibited homogeneous color tuning when a perpendicular electric field was applied, while it showed a large rainbow-like electric-optic response in the visible to near-infrared region when a parallel electric field was applied. Additionally, by a simple method using patterned electrodes, a reflective display showing the designed characters was demonstrated and maintained for hours under water without an external source of energy. Moreover, the electrically induced optical pattern can be erased, and the responsive photonic hydrogel showed 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.