Depending on who you ask, you may get slightly different definitions of what constitutes a metamaterial. The basic idea is that the engineered materials behave in ways that natural materials don’t. If you squeeze an object that deforms, for example, it will most likely expand in the direction perpendicular to compression, in the way that a marshmallow oozes out the sides of a s’more when squished between two graham crackers. But through clever engineering, researchers can design metamaterials that do just the opposite.
Naroa Sadaba, of the University of Washington in Seattle, her postdoctoral adviser Alshakim Nelson, and their colleagues have now shared their discovery of a unique metamaterial behavior observed in a protein-based polymer lattice. That structure can be crushed to just 20% of its size, be returned to its original size and shape, and then exhibit up to 2.5 times as much as its original strength. The 3D-printed lattices are built from a combination of a hydrogel and the globular protein bovine serum albumin, which is derived from cow’s blood. The researchers experimented with different lattice shapes and spacings.
In some cases, the structures exhibited a unique combination of two distinct material behaviors: shape memory and strain hardening. The lattices were already designed for shape memory—after being crushed, they are returned to their original shape by being submerged in water for 30 minutes. They can then be dried out again and retain the remembered shape. When the lattice was built from a unit cell that was a twisted, truncated octahedron—a shape that is especially good at evenly distributing stress throughout the material—that’s when the strain-hardening effect appeared. Being deformed made the material stronger.
Small-angle x-ray scattering measurements of the samples both before and after deformation, and after shape recovery, show that the long, curled-up fibers of the globular proteins unfurl during deformation but refold during shape recovery, as illustrated in the figure. And in some cases, something about how they refold makes the material stronger. More specific details of the mechanism have not been totally unraveled. “I think that the magnitude of the learning is really interesting,” says Lucas Meza, a mechanical engineer and member of the research team. “It was very surprising that we could make a material that was trainable at all.”
Because of the material’s biocompatibility, the researchers envision possible future use in biomedical applications like stents—but those uses are hypothetical at the moment. Now that the mechanism has been observed, the research team sees potential for maximizing desirable material properties by exploring the use of different protein structures, of which there are nearly unlimited varieties. That could include designed proteins with structures predicted using the AI technology that was recognized with the 2024 Nobel Prize in Chemistry (see “Protein designers and modelers share 2024 chemistry Nobel,” Physics Today online, 9 October 2024). To design proteins for new materials, the researchers are collaborating with David Baker, also of the University of Washington, who is being awarded half of that prize. (N. Sadaba et al., Proc. Natl. Acad. Sci. USA 121, e2407929121, 2024.)
