Biomimetic Iridescent Skin: Robust Prototissues Spontaneously Assembled from Photonic Protocells

Abstract Cells in biology are usually organized into cohesive tissues. Here, a simple and scalable strategy is reported to connect cell‐like containers (protocells) into robust prototissues. The protocells are capsules with an aqueous core and a shell of the biopolymer alginate crosslinked by Ca 2+ . When placed in contact, the capsules spontaneously connect into arrays through Ca 2+ ‐mediated bridging of adjacent alginate shells. A similar mechanism operates in plant tissues, where cell‐walls are bridged by the biopolymer pectin and Ca 2+ . The capsule arrays are self‐standing centimeter‐scale solids (2D films or 3D shapes) with robust rheological properties. In an effort to endow these prototissues with biomimetic properties, this study focuses on color. Tissues like the chameleon skin exhibit structural color arising from arrays of uncolored nanocrystals in cells. Similarly, uniform silica nanoparticles (NPs) are embedded in the capsules and organized into periodic arrays (photonic crystals) by osmotic pressure from the external medium. Thereby, these ‘photonic capsules’ exhibit vivid colors that can be tuned via the NP size (200 to 300 nm). Assembly of these capsules then yields soft iridescent sheets that mimic the hierarchical architecture of the chameleon skin. This is the first demonstration of structurally colored prototissues assembled from protocells.