Chameleons use a non-close-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in the full visible range. Inspired by the biological process uncovered in panther chameleons, we designed photonic films containing a non-close-packed face-centered-cubic array of silica particles embedded in an elastomer. The non-close-packed array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photocuring of the dispersion medium. The artificial skin exhibits a structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables tuning without experiencing significant rearrangement of particles, providing elastic deformation and reversible color change, as chameleons do. The simple fabrication procedure consists of film casting and UV irradiation, potentially enabling the continuous high-throughput production. The mechanochromic property of the photonic films enables the visualization of deformation or stress with colors, which is potentially beneficial for various applications, including mechanical sensors, sound-vision transformers, and color display.

Colloidal arrays show structural colors through wavelengthselective diffraction. The structural colors are dynamically tunable with mechanical deformation for a non-close-packed colloidal array embedded in an elastic matrix. However, such compositions usually render photonic materials transparent and structural color low saturated. In this work, we formulate colloidal inks to produce mechanochromic films and patterns that show consistent structural colors with high saturation. The inks are composed of a high-volume fraction of silica particles and a low fraction of polydopamine nanoparticles dispersed in an elastomer-forming resin. The silica particles have repulsive interparticle potential and form a non-closepacked array, whereas polydopamine nanoparticles are positioned in the interstitial areas. The colloidal arrays are captured in the elastomer by photopolymerization of the resin. As polydopamine nanoparticles reduce incoherent scattering and make the materials opaque, the structural color arisen from the colloidal array is pronounced and independent of the background. Moreover, the photonic materials show a dynamic and reversible change of structural color according to deformation. For large strains, the photonic effect is overwhelmed by absorption of polydopamine nanoparticles, rendering the materials dark brown. This unique mechanochromic property is used to make patterns that are reversibly color-tunable and hidable, which are appealing for user-interactive anti-counterfeiting and active camouflage.

suffer from low stability due to lack of matrix and commonly used film formats show orientation-dependent structural colors. [9,10] Colloidal crystals have been made into spherical granules using emulsion-templating methods, [11][12][13][14] which reduces the inaccuracy caused by the orientation-independent colors. However, spherical sensor ball composed of nonclose-packed array embedded in microgel has the low mechanical stability of the gel matrix, as well as the slow response, low color brightness, and narrow color shift. [14] The response of colloidal crystals can be accelerated by using stimuli-responsive colloids rather than responsive matrices since the rate of response has been shown to be diffusion-limited in hydrogel meshes. [15] At the same time, a high color brightness and wide color shift can be achieved by forming single crystals each composed of a close-packed array. In a close-packed array, any change in the volume of the array will cause a change in the lattice constant. Although such colloidal crystals are fragile, they can be protected from external stresses by encapsulating them with a stable solid membrane. Moreover, microsensors in a capsule format can be injected, suspended, and implanted in any target volume. Such capsules may be used to report microenvironmental conditions. These arrays when in suspensions may be used to provide the spatial distribution of the microenvironmental conditions in a manner similar to the sensor balls named "Dorothy" that were depicted to gather spatial information when suspended in a tornado in the film "Twister." [16] In the current work, we designed such capsule sensors by assembling temperature-responsive colloids to form hexagonal closepacked (hcp) crystallites via depletion attraction in the aqueous core of double-emulsion drops.Colloidal crystals formed by interparticle repulsion or entropy-driven crystallization in a spherical container have shown a uniform distribution of colloids in the volume, forming a single phase. [17][18][19] Therefore, a change in the volume of individual colloids in this case causes no change in average interparticle separation, but instead frequently leads to crystal melting, and the loss of structural colors. [20,21] To make colloidal crystals color-changeable, two phases-a close-packed colloidal crystal and colloid-free gas-should be made to coexist in the volume, with the gas phase accommodating the volume changes of the crystals. To implement the formation of the two phases in the core of capsules, in the current work we used depletion-induced phase separation [22][23][24][25] in the aqueous core Technologies to monitor microenvironmental conditions and its spatial distribution are in high demand, yet remain unmet need. Herein, photonic microsensors are designed in a capsule format that can be injected, suspended, and implanted in any target volume. Colorimetric sensors are loaded in the core of microcapsules by assembling core-shell colloids into crystallites through the depletion attraction. The shells of the colloids ar...

Colloidal crystals have been used for creating stimuli-responsive photonic materials. Here, macroporous hydrogels are designed, through a simple and reproducible protocol, that rapidly and reversibly switch between highly transparent and structurally colored states. The macroporous hydrogels are prepared by film-casting photocurable dispersions of silica particles in hydrogel-forming resins and selectively removing silica particles. The silica particles spontaneously form a nonclose-packed array due to repulsive interparticle interaction, which form the regular array of cavities after removal. However, the cavities are randomly collapsed by drying, losing a long-range order and rendering the materials highly transparent. When the hydrogels are swollen by either water, ethanol, or the mixture, the regular array is restored, which develops brilliant structural colors. This switching is completed in tens of seconds and repeatable without any hysteresis. The resonant wavelength depends on the composition of the water-ethanol mixture, where the dramatic shift occurs in one-component-rich mixtures due to the composition of the hydrogel. Micropatterns can be designed to have distinct domains of the macroporous hydrogels, which are transparent at the dried state and disclose encrypted graphics and unique reflectance spectra at the wet state. This class of solvent-responsive photonic hydrogels is potentially useful for alcohol sensors and user-interactive anti-counterfeiting materials.

Despite highly promising characteristics of three-dimensionally (3D) nanostructured catalysts for the oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolyzers (PEMWEs), universal design rules for maximizing their performance have not been explored. Here we show that woodpile (WP)-structured Ir, consisting of 3D-printed, highly-ordered Ir nanowire building blocks, improve OER mass activity markedly. The WP structure secures the electrochemically active surface area (ECSA) through enhanced utilization efficiency of the extended surface area of 3D WP catalysts. Moreover, systematic control of the 3D geometry combined with theoretical calculations and various electrochemical analyses reveals that facile transport of evolved O2 gas bubbles is an important contributor to the improved ECSA-specific activity. The 3D nanostructuring-based improvement of ECSA and ECSA-specific activity enables our well-controlled geometry to afford a 30-fold higher mass activity of the OER catalyst when used in a single-cell PEMWE than conventional nanoparticle-based catalysts.