INSPIRED by the porous macrostructure of plants, researchers in the US have developed ceramic foam inks which can be used to 3D-print macro- and microporous materials.
Plants such as grass have a hollow tubular macrostructure and a porous microstructure, which have great strength and resistance. The team, with researchers from John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering, both at Harvard University, and MIT, wanted to develop a technique to reproduce a similarly strong structure in artificial materials. They believe their new 3D-printing technique could be used to produce lightweight structural materials, thermal insulation, catalyst supports, electrodes, or even scaffolds for growing tissue.
The ceramic foam ‘ink’ comprises three basic ingredients, alumina particles, water and air, with the air effectively being whipped into the alumina and water mixture. This ink can then be 3D printed into a variety of shapes including triangular and hexagonal honeycombs.
“Foam inks are interesting because you can digitally pattern cellular microstructures within larger cellular macrostructures,” said researcher Joseph Muth. “After the ink solidifies, the resulting structure consists of air surrounded by ceramic material on multiple length scales. As you incorporate porosity into the structure, you impart properties that it otherwise would not have.”
The team controls the porosity of the ink by controlling the microstructure of the ink foam, that is how much air is whipped into it and the size of the bubbles. This allows them to control the stiffness and density of the final printed structure. Once printed, the structure is dried in air for 24 hours, then heated to 70?C to remove residual water, before going through a pre-firing and firing/sintering process at temperatures up to 1,500?C.
“This process combines the best of both worlds. You get the microstructural control with foam processing and global architectural control with printing. Because we’re printing something that already contains a specific microstructure, we don’t have to pattern each individual piece. That allows us to make structures with specific hierarchy in a more controllable way than we could do before,” said Lorna Gibson, a materials science and engineering professor at MIT.
The researchers say that the foam ink could be made from other materials such as metals, polymers and other ceramics. Muth says that the printed materials can be multifunctional, with tunable material properties including mechanical, thermal and transport characteristics.
The paper will shortly be published in Proceedings of the National Academy of Sciences.
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