Giant Panda eating bamboo.
Giant Panda eating bamboo.

Tooth enamel protects teeth by providing a hard surface resistant to wear and tear, withstanding impacts without breaking over the lifetime of an organism. The Giant Panda has particularly clever tooth enamel, according to researchers, which can recover its structure and geometry to counteract the early stages of damage [Liu et al., Acta Biomaterialia (2018),]. The team from the Institute of Metal Research, Chinese Academy of Science, the University of Science and Technology of China, Lanzhou University of Technology, and the University of California Berkeley believe their observations could be replicated in the tooth enamel of all vertebrates, including humans, and inspire the design of artificial durable ceramics.

“Tooth enamel possesses an exceptional durability and plays a critical role in the function of teeth, however, [it] exhibits a remarkably low resistance to the initiation of large-scale cracks comparable to geological minerals,” points out Robert O. Ritchie, who led the study.

The ingenious design of the Panda’s tooth enamel, which has to withstand a daily diet of bamboo – a material of remarkable strength and toughness, comprises parallel microscale prisms made up of vertically aligned nanoscale fibers of the mineral hydroxyapatite embedded in an organic-rich matrix. When there is an impact on the enamel, a variety of different deformation mechanisms take place to mitigate the growth of small cracks and prevent the formation of large cracks.

“The tooth enamel is capable of partially recovering its geometry and structure at nano- to microscale dimensions autonomously after deformation to counteract the early stage of damage,” explains first author Zengqian Liu. “[This] property results from the unique architecture of tooth enamel, specifically the vertical alignment of nano-scale mineral fibers and micro-scale prisms within a water-responsive organic-rich matrix.”

Hydration plays a key role in the process. The viscoeleasticity of the organic-rich matrix surrounding the mineral prisms and fibers facilitates self recovery, while the presence of water decreases the width of any cracks that do form, with only a minor cost in terms of hardness.

“Our findings identify a novel means by which the tooth enamel of vertebrates develops an exceptional durability to accomplish its functionality,” says Liu. “The self-recovery process represents a new source of durability that differs markedly from the conventional protocol of fracture mechanics.”

As the architecture of the Panda’s tooth enamel is essentially similar to other vertebrates, the researchers believe that this self-recovery behavior is likely to occur in tooth enamel in general.

“Our findings also offer inspiration for the development of artificial durable, self-recoverable ceramic materials,” says Ritchie.

The team is hoping to develop tooth enamel-inspired self-recoverable durable materials by introducing shape-memory polymers at the interfaces of ceramics.