(Invited) Angle-Independent Structurally Coloured Hydrogels
Authors: Takeoka Yukikazu 
Published: February 2020

Stimuli-sensitive soft materials composed of three-dimensional polymer networks or block copolymers with fine periodic dielectric structures of dimensions comparable to the wavelength of visible light can display bright structural colours[
1
-
5
]. The hues of the structural colours can reversibly change by varying the periodic dielectric structures with different stimuli. Extensive research has been performed in recent decades to develop such stimuli-sensitive structurally coloured soft materials for application to displays[
2
,
4
,
6
] and sensor systems[
5
,
7
]. In contrast to any other display and sensor technologies, these stimuli-sensitive structurally coloured soft materials can exhibit a wide range of colours on their own, without requiring colour filters and complicated control devices[
2
]. However, the colours produced by the periodic dielectric structures show distinct variations depending on the viewing and light illumination angles[
8
]. The iridescence originates from the Bragg reflection, resulting from the long-range order of the periodic structures. For potential applications to reflective full-colour displays and sensor systems with wide viewing angles, the angular dependence of the structural colour is a major issue. In this study, we report the development of thermally tunable, angle-independent, structurally coloured hydrogels. We demonstrate that the application of colloidal amorphous arrays as templates and the introduction of a small amount of black particles into the thermo-sensitive hydrogels are effective for obtaining tunable and angle-independent bright coloured systems.
Many examples of stimuli-sensitive structurally coloured soft materials with rapid response and hue changes have been reported; however, such systems without angle-independent structural colours have yet to be reported. The porous PNIPA hydrogels prepared using colloidal amorphous array templates can exhibit rapid changes in volume in response to temperature variations and can potentially display angle-independent structural colours. Our results showed that the microstructure of the prepared hydrogels using the colloidal amorphous arrays as templates is not suitable for displaying brightly saturated structural colours due to the large contribution from incoherent multiple scattering of light to the optical properties. The small amount of CB introduced into the porous hydrogels suppressed multiple scattering, enabling clearer observations of the structural colours. This method is easily applicable to many other existing stimuli-sensitive soft materials. We expect that such stimuli-sensitive soft materials displaying bright, angle-independent structural colours will have many useful applications, such as in (bio)chemical sensing technology and electronic paper.
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