The Mantis Shrimp has one of the fastest and hardest blows in the animal kingdom. His tentacles, which have been transformed into clubs, break even glass and have the force of a pistol bullet. Researchers have now found out why the clubs don’t break on this impact. Accordingly, in addition to the already known three-layer microstructure of the crab shell, a coating made of special nanocrystals ensures a combination of damping and hardness. According to the researchers, both together have so far been unmatched to this extent with artificial materials.
The approximately hand-sized mantis shrimp Odontodactylus scyllarus lives in the coral reefs of the tropical Pacific. In order to kill its prey, this crab has developed a special strategy: it kills it. If a small fish or other crustacean approaches it or if it sees a potentially tasty clam, Odontodactylus explosively hurls its thickened legs. They hit their prey faster than a boxer’s fist and penetrate even thick armor and shells. “The crab can generate forces of up to 1500 Newtons when striking and accelerate its club at around 10,000 times the acceleration of gravity,” explain Wei Huang from the University of California at Irvine and his colleagues. When it hits, the club gets a speed of up to 23 meters per second. This is one of the fastest moves in the animal kingdom. Nevertheless, the “boxing gloves” of the colorful crab can withstand thousands of such blows without being damaged.
Impact protection layer in the visor
Researchers uncovered the first information about how cancer does this a few years ago. According to this, the mantis shrimp mace consists of a three-layer material. The two inner layers are primarily made up of spirally wound, partially mineralized chitin fibers, which make the shell elastic and yet firm. Thanks to their twisted shape, they also prevent the spread of cracks. The outside of the club is covered with a mineral protective layer that is only as thin as a human hair, but absorbs a large part of the impact force. Previous studies have already shown that this layer consists of specially aligned crystals of the mineral hydroxyapatite, the material that also gives our teeth strength. But how exactly this layer manages to absorb the force of the blows has been unclear until now.
For their study, Huang and his colleagues first examined the nature of the protective layer, which is only around 70 micrometers thick, on the mace of a freshly molted mantis shrimp and an older specimen that is about to molt. As expected, the fresh surface of the club was smooth and undamaged. “In contrast, the analysis of the heavily used protective layer showed substantial signs of wear and tear,” the researchers report. “This suggests that some material from this protective layer is lost in the course of thousands of blows.” In the next step, electron microscope images confirmed that the protective layer actually consists largely of hydroxyapatite nanocrystals that are embedded in an organic matrix. Contrary to earlier assumptions, these crystals are in turn composed of several even smaller grains. Their edges are not flat against each other, but are slightly tilted against each other at an angle of 1.5 degrees, as the scientists found.
Edge breaking of the nanocrystals absorbs impact energy
Impact tests revealed the benefits of this special structure. To do this, the researchers had tiny spherical or pointed impactors hit the protective layer of the cancer clubs under the atomic force microscope. They were able to observe how the nanocrystals changed when they hit. “At relatively low loads, the particles deformed almost like a marshmallow and then bounced back into their old arrangement,” reports Huang’s colleague David Kisailus. The situation is different with impacts with high energy: “Then this layer stiffens and the interfaces of the nanocrystals break,” says the researcher. The organic components solidify due to the impact and at the same time the tiny crystal grains shift in such a way that their edges pulverize. This absorbs part of the energy without destroying the structure of the nanocrystalline layer. At the same time, this breaking on a very small scale reduces the depth of penetration of the shock wave by around half, as Huang and his team found.
“The combination of stiff inorganic and soft organic components in this interpenetrating network gives the layer impressive damping properties without losing any of its stiffness,” says Kisailus. This is a very rare combination of features that far surpasses the performance of most metals and technical ceramics. According to the researchers, the knowledge about the structure of the protective layer of the mantis shrimp opens up new possibilities to manufacture artificial materials with these properties. “Such materials have a wide range of applications – from impact and vibration-resistant coatings for buildings, body armor, aircraft or automobiles to erosion-resistant coatings for the turbines of wind turbines,” state Huang and his colleagues.
Source: Wei Huang (University of California, Irvine) et al., Nature Materials, doi: 10.1038 / s41563-020-0768-7