Structural Colour Printing of 3D Microscale Objects by Shrinking Photonic Crystals

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Yejing Liu, Hao Wang, Jinfa Ho, Ryan C. Ng, Ray J. H. Ng, Valerian H. Hall-Chen, Eleen H. H. Koay, Zhaogang Dong, Hailong Liu, Cheng-Wei Qiu, Julia R. Greer, Joel K. W. Yang

Heat-shrinking induced colors of 3D printed woodpile photonic crystals. (a) Schematic of the fabrication process. Left: woodpile photonic crystal written in commercial IP-Dip resist by two-photon polymerization at dimensions well above the resolution limit of the printer to prevent structures from collapsing. Right: after heat treatment, the dimensions of the photonic crystal are reduced below the resolution limit of the printer, and colors are generated. The colors change with different degrees of shrinkage. (b) Composite optical micrographs of heat-treated woodpile photonic crystals with varying structural dimensions as viewed from the side. Micrographs of the 3D-printed model of the Eiffel Tower in structural blue (c) and structural red (d). (e) Oblique view of an Eiffel Tower printed with intentional gradient of colors. (f) Further down-scaled multi-color 3D print of the Eiffel Tower.

In a report recently published in Nature Communications, a research group led by Associate Professor Joel Yang from the Singapore University of Technology and Design (SUTD) printed probably the smallest colourful 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 colours 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 colours.

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