Mantis Shrimp: Weapon to Armor

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Last post, we covered the incredibly tough and highly impact resistant mother of pearl which keeps most mollusks safe from predators. But the mantis shrimp can break through these extremely hard structures with the speed of a .22 caliber bullet.

Researchers are interested in the animals due to the weapons they use to catch and consume prey. They believe that the appendages of the mantis shrimp could lead the way to lighter and tougher armors for soldiers and vehicles.

Mantis shrimp, or stomatopods, live in shallow coastal waters and are relatives of shrimp and lobsters. The stomatopods are classified as either a "smasher" or "spearer." Smashers have club like arms that they use to pummel the shells of the mollusks and crustaceans they eat, while spearers have spiny forearms used to catch fish, similar to how praying mantis catch insects.

Cavitation Bubble

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Researchers are particularly interested in the smashers due to the repeated high velocity impacts the animal's clubs endure. The mantis shrimp can employ their weapon with blinding quickness, reaching up to 10,400 g of acceleration (102,000 m/s² or 335,000 ft/s²) and speeds of 23 m/s from a standstill, which is about the same speed as a .22 caliber bullet. The club moves through the water so quickly that it creates cavitation bubbles between the club and the striking surface. When these bubble collapse, they produce an instantaneous force of around 800 Newtons (180 lbf).

Note the cavitation bubble that forms between the club and the clam

Between molts, each club of the shrimp endures up to 50,000 of these high speed, high force impacts. How do they take all of these impacts without breaking? The secret is in the material of the clubs and the way they are structured.

Scientists have discovered that clubs are comprised of three distinct parts. The outermost layer is made up of a crystallized form of hydroxyapatite, which is a mineral that imparts strength to human bones and teeth. Below this, there are sugars arranged in a flattened spiral pattern called a helicoid. The spirals are slightly offset such that no layer lines up perfectly with the one below, with the sugars acting as shock absorbers. Finally, the innermost region of the club consists of layers of chitin, what most insect exoskeletons are made of. These fibers wrap around the sides of the club, acting somewhat like the tape boxers wrap around their hands, and preventing expansion of the hydroxyapatite/sugar matrix when striking. All of these combined create an organic matrix which allows small cracks to form, but in many different directions. This keeps the cracks from propagating across the length of the club and prevents fracturing.

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Researchers have already built a fiberglass structure that mimics the helicoid pattern of the mantis shrimp's club. They shot at the material with a gun, it was bulletproof. If we can create lighter structures that use this same design, not only would we be able to create lighter body armor, but it could be used for many everyday items from automobile bodies to sports safety equipment.