Fancy Erector Set? Nope. The elaborate fractal structure shown at left is many, many times smaller than that and is certainly not child's play. It’s the latest example of what Julia Greer, prof. of materials science and mechanics, calls a fractal nanotruss—nano because the structures are made up of members that are as thin as 5 nm; truss because they are carefully architected structures that might one day be used in structural engineering materials.
Greer's group has developed a three-step process for precisely building such complex structures. They first use a direct laser writing method called two-photon lithography to "write" a 3-D pattern in a polymer, allowing a laser beam to crosslink and harden the polymer wherever it is focused. At the end of the patterning step, the parts of the polymer that were exposed to the laser remain intact while the rest is dissolved away, revealing a 3-D scaffold. Next, the scientists coat the polymer scaffold with a continuous, very thin layer of a material—it can be a ceramic, metal, metallic glass, semiconductor, "just about anything," Greer says. In this case, they used alumina, or aluminum oxide, which is a brittle ceramic, to coat the scaffold. In the final step they etch out the polymer from within the structure, leaving a hollow architecture.
Taking advantage of some of the size effects that many materials display at the nanoscale, these nanotrusses can have unusual, desirable qualities. For example, intrinsically brittle materials, like ceramics, including the alumina shown, can be made deformable so that they can be crushed and still rebound to their original state without global failure.
"Having full control over the architecture gives us the ability to tune material properties to what was previously unattainable with conventional monolithic materials or with foams," says Greer. "For example, we can decouple strength from density and make materials that are both strong (and tough) as well as extremely lightweight. These structures can contain nearly 99% air yet can also be as strong as steel. Designing them into fractals allows us to incorporate hierarchical design into material architecture, which promises to have further beneficial properties."