3D Printing of Structural Colors

Micrographs of the 3D-printed model of the Eiffel Tower in different structural colors. (Source: SUTD)

A research group led by Joel Yang from the Singapore Uni­versity of Tech­nology and Design (SUTD) printed probably the smallest colorful 3D model of the Eiffel Tower. Impressively, no pigments or inks were used. Instead, the 3D-printed model of the Eiffel Tower, measuring less than half the width of a human hair at 39 micrometers, exhibits multiple colors due to the manner in which light interacts with the nano­structures that hold up the model. The 3D models are made of a finely printed mesh of transparent polymer, forming photonic crystals. These mostly hollow designs remarkably shrink down in size by about 5 times when heated to produce a wide range of colors.

Yang said: “There is great excitement in the research community to further develop sustainable sources of colors that aren’t extracted from animals or plants. What if the products that we make could derive its color by nano-texturing of the material that it itself is made of? Certain butter­flies and beetles have evolved to do this, perhaps we could learn to do this too.” Compared with pigments and dyes relying on chemical compo­sition, structural colors are high-resolution, permanent, and eco-friendly.

In nature, the coloration of some butter­flies, Pachy­rhynchus weevils, and many chameleons are notable examples of natural organisms employing photonic crystals to produce colorful patterns. Photonic crystal structures reflect vivid colors with hues dependent on their lattice constants. To reflect vivid colors, the lattice constants of a photonic crystal must be suffi­ciently small. For example, the lattice constant is only ~280 nm on butter­fly wings giving a blue hue of color. Due to the limi­tation on current 3D printing resolution, it is a challenge to print arbitrary colors and shapes in all three dimensions at this micro­scopic length scale.

To achieve the required dimension of lattice constants comparable to the butterfly scales, researchers from Yang’s group employed a coloring-by-shrinking method which introduces an additive heating step to shrink the photonic crystals printed using a commercial two-photon polymeri­zation litho­graphy system, i.e. the Nanoscribe GmbH Photonic Profes­sional GT. Yang added: “The challenge is in shrinking structures at these nanoscopic dimensions without having them coalesce into a blob. By patterning larger structures, and shrinking them later, we produced structures that could not have been printed directly with standard methods.” Indeed, the repeating lines of the woodpile structures were shrunk down to 280 nm, almost two times smaller than the machine speci­fications. As a bonus side-effect of shrinking, the refractive index of the cross-linked polymer increased in the heating process, which further benefits the gene­ration of colors. With the freedom of designing 3D photonic crystals that are shrunk to fit specific colors, this tech­nology would be broadly applicable to achieve compact optical components and integrated 3D photonic circuity operating in the visible region. (Source: SUTD)

Reference: Y. Liu et al.: Structural color three-dimensional printing by shrinking photonic crystals, Nat. Commun. 10, 4340 (2019); DOI: 10.1038/s41467-019-12360-w

Link: Engineering Product Development, Singapore University of Technology and Design, Singapore

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