The direct ink writing of three-dimensional functional materials is detailed in the Feature Article by Lewis on p. 2193. The left side of the cover image displays schematic images that show the conversion of a direct-write polymer woodpile to a silicon hollow-woodpile structure. The 3 × 3 image matrix showcases the resulting silicon photonic crystal (center) surrounded by a higher-magnification view of a representative hollow silicon feature (ca. 1 μm in diameter). The figure was prepared by F. Garcia-Santamaria, G. M. Gratson, and P. V. Braun.
The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct-write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.