A new technique that allows metals to mimic the internal structure of wood could make nickel, and other metals, far far strong.

Led by James Pikul, Assistant Professor in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering, the new study looked at new ways to take metal and give it the porous structure that gives wood its strength. In the past, this has been done by finding ways to turn molten metal into foam, or using 3D printing with hundred-nanometer precision to build up wood-like metal bit by bit. The problem is that metal foam is crude by modern engineering standards, while the 3D printing process is slow and very hard to scale up from lab-bench scales.

“The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature,” Pikul says. “Cellular materials are porous; if you look at wood grain, that’s what you’re seeing  –  parts that are thick and dense and made to hold the structure, and parts that are porous and made to support biological functions, like transport to and from cells. Our structure is similar. We have areas that are thick and dense with strong metal struts, and areas that are porous with air gaps. We’re just operating at the length scales where the strength of struts approaches the theoretical maximum.”

According to the researchers, the key was to go to much smaller scales to produce much greater increases in strength. They manage this by suspending plastic spheres a few hundred nanometers wide in water, which is allowed to evaporate. As the water disappears, the spheres drop into a neat geometrical, crystalline pattern. This is then electroplated with a thin layer of chrome and the spaces between the spheres are filled with nickel. The plastic is then dissolved, and what’s left behind is an open network of metallic struts with 70 percent empty space – making it light enough to float in water.

The process is however very expensive, and so far has only produced some very small samples.