Scientists looking to nature for inspiration have long known that the whole of a structure is often greater than the parts. In bones, for instance, soft proteins and stiff but brittle minerals join in a complex matrix that distributes stresses to absorb impacts.
But it isn’t very easy for those knocking off nature’s best tricks to reproduce something like bone because the complex positioning of the different materials has been impossibly difficult to mimic. The hierarchical layering of soft collagen proteins with hard hydroxyapatite mineral is what gives the naturally occurring composite its great load-bearing qualities.
Now, though, an MIT team has revealed that they’ve gotten much closer.
They first investigated the molecular structure of natural bone to understand how the pieces fit together. Using that information, they developed a computer-optimized blueprint to lay out precise geometric patterns of soft and stiff polymers with properties similar to the biological material. Then they fed those instructions into a 3-D printer capable of extruding both polymers at once, which built the strong bonelike material layer by layer.
“The geometric patterns we used in the synthetic materials are based on those seen in natural materials like bone or nacre, but also include new designs that do not exist in nature,” said Markus Buehler, an associate engineering professor, in a university announcement. “As engineers we are no longer limited to the natural patterns. We can design our own, which may perform even better than the ones that already exist.”
Combining soft and hard
The advanced composite material they produced from this process is 22 times more fracture-resistant than its strongest constituent polymer part. They detailed their work in a recent edition of the journal Nature Communications.
"Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material’s deformation response," the team wrote in the paper’s abstract. “These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents."
“The possibilities seem endless, as we are just beginning to push the limits of the kind of geometric features and material combinations we can print,” he said. Buehler says the production process could be scaled up to 3-d print multicomponent materials in precise patterns configured for specific functions in structures, with entire buildings manufactured with optimized materials that incorporate electrical circuits, plumbing and energy harvesting.