Bio‐Inspired Magnetically Tunable Structural Colors from Elliptical Self‐Assembled Block Copolymer Microparticles

Cephalopods achieve their vivid, dynamic structural coloration through reconfigurable lamellar reflectors, acting as a powerful inspiration for adaptive optical materials. However, synthetic systems that replicate this functionality typically require complex material architectures or multistep fabrication processes. Here, a straightforward strategy is presented for producing magnetically responsive photonic microparticles using linear poly(styrene)-b-poly(2-vinylpyridine) block copolymers. Self-assembly in emulsion droplets yields ellipsoids with stacked lamellar domains that exhibit structural colors spanning the entire visible spectrum. Blending block copolymers with different molecular weights enables precise, continuous control over lamellar periodicity and photonic bandgap spectral position, eliminating the need for chemical modification or the use of swelling agents. Notably, incorporating superparamagnetic iron oxide nanoparticles imparts rapid and reversible magnetic alignment of the microparticles. This orientation aligns the lamellar stacks perpendicular to the magnetic field, resulting in Bragg reflection and angle-dependent coloration. The reflected wavelength can be tuned by changing the viewing angle, which mimics the dynamic optical responses of biological iridophores. Compared to prior systems, our approach reduces synthetic complexity while maintaining high color intensity and angular tunability. Thus, this work introduces a bio-inspired materials concept that offers a simple route toward dynamically tunable optical materials for adaptive camouflage, smart coatings, and next-generation photonic devices.