Comparison of natural and synthetic nacre.
Comparison of natural and synthetic nacre.

Uniting strength and toughness in one material is challenging for materials scientists and engineers. But nature successfully combines different structural elements made out of hard and soft materials to achieve outstanding properties. Nacre, or mother-of-pearl, is a classic example of a material with hard and soft components that attains high stiffness, strength, and fracture toughness.

Researchers from Massachusetts Institute of Technology and Politecnico di Milano, Italy have used additive manufacturing to create artificial hybrid materials that mimic nacre’s outstanding properties [Gu et al., Journal of the Mechanical Behavior of Biomedical Materials (2017) doi: 10.1016/j.jmbbm.2017.05.007]. Markus J. Buehler and his team believe that the key to nacre’s remarkable properties is an overlooked microstructural feature known as a ‘mineral bridge’ (MB).

“The physical structure of nacre is often termed as a ‘brick-and-mortar’ structure in which stiff aragonite material (brick) is glued together with soft biopolymer (mortar),” explains Buehler. “But more detailed analysis of nacre reveals that the structure is better described as a ‘brick-bridge-mortar’ structure.”

The bridges between the bricks in nacre’s structure bring additional strength and stiffness to the material.

“Nacre is unique in that it achieves orders of magnitudes higher toughness than stiff aragonite material alone by incorporating 5% soft biopolymer material of meager mechanical characteristics,” point out Buehler.

This sort of amplification of physical properties is unprecedented in synthetic materials, he adds. But Buehler and his team wanted to determine precisely how the inclusion and number of MBs affects these properties. The researchers used 3D printing to print mineral bricks and bridges simultaneously in a soft polymer matrix with different proportions of bricks to mortar and number of MBs.

Too much mineral and the nacre-inspired composite is brittle. Too little mineral content, and the material is not stiff or rigid enough. But with just the right mineral content and number of MBs – around 70% and 9-12, respectively, according to the researchers’ findings – the material shows a marked improvement in both toughness and strength.

“Our results demonstrate the major impact of volume fraction on toughness-strength, making it possible to tune composite properties by tuning the size and content of structural features,” says Buehler.

The researchers believe that the presence of MBs deflects the trajectory of cracks as they move through the composite. Instead of zigzagging freely through the material, the MBs force the cracks to follow a block-wise pattern of deflection, which is associated with an increase in strength.

Along with other structural features, the researchers believe that their systematic approach will help drive the optimization of bio-inspired composite materials that can meet demanding engineering challenges.

“Applications could also include creating tailor-made materials for various structures such as airplanes, buildings, and vehicles,” adds Buehler.