Microfluidics-Assisted Assembly of Injectable Photonic Hydrogels toward Reflective Cooling

Development of fast curing and easy modeling of colloidal photonic crystals is highly desirable for various applications. Here, a novel type of injectable photonic hydrogel (IPH) is proposed to achieve self-healable structural color by integrating microfluidics-derived photonic supraballs with supramolecular hydrogels. The supramolecular hydrogel is engineered via incorporating beta-cyclodextrin/poly(2-hydroxypropyl acrylate-co-N-vinylimidazole) (CD/poly(HPA-co-VI)) with methacrylated gelatin (GelMA), and serves as a scaffold for colloidal crystal arrays. The photonic supraballs derived from the microfluidics techniques, exhibit excellent compatibility with the hydrogel scaffolds, leading to enhanced assembly efficiency. By virtue of hydrogen bonds and host-guest interactions, a series of self-healable photonic hydrogels (linear, planar, and spiral assemblies) can be facilely assembled. It is demonstrated that the spherical symmetry of the photonic supraballs endows them with identical optical responses independent of viewing angles. In addition, by taking the advantage of angle independent spectrum characteristics, the IPH presents beneficial effects in reflective cooling, which can achieve up to 17.4 degrees C in passive solar reflective cooling. The strategy represents an easy-to-perform platform for the construction of IPH, providing novel insights into macroscopic self-assembly toward thermal management applications.