The peacock mantis shrimp packs a powerful punch. The crustacean uses its hammerlike claws to smash through mollusk shells and even aquarium glass without getting injured. Now, a new study reveals what makes its claws so tough: a unique composition and structure that stops cracks in their tracks—one that could help engineers design lighter, stronger materials for military, medical, and other applications.
"This is an absolutely gorgeous study, a tour de force … that opens so many new windows for biologists and engineers," says Sheila Patek, a biologist at the University of Massachusetts, Amherst, who first measured the speed of the mantis shrimp's blows. "Plus, I really wanted to know all this stuff, and I'm glad someone did it."
Though mantis shrimp are relatively common, a lot about them isn't. The colorful crustaceans have remarkable vision, unusually resilient armor, and the fastest punch on earth. When they strike, they swing out their dactyl clubs, armlike appendages normally held close to their bodies, at 80 kilometers per hour, accelerating faster than a .22-caliber bullet. Mantis shrimp use this mechanism to smash their often hard-shelled prey, and can do so as many as 50,000 times between molts without destroying their clubs.
David Kisailus, a chemical engineer at the University of California, Riverside, and his team wondered how mantis shrimp could keep up their hitting without catastrophic damage to their clubs. So they dissected the clubs of 15 mantis shrimp (who just grow them back if they're removed). Using everything from scanning electron microscopes to x-rays, the team examined the club's inner structure and found a complex arrangement of layers.
The impact region features a highly crystallized form of the mineral hydroxyapatite, a key ingredient in human bones and teeth. Below that are more layers of hydroxyapatite, this time in an amorphous, noncrystallized form. The innermost region contains chitin, a less stiff material often found in the exoskeletons of crustaceans stacked in helices with hydroxyapatite filling in between the stacks. The differences in hardness, stiffness and orientation between the three layers allow small cracks to form but prevent them from growing or spreading, so the club stays intact, the researchers report online today in Science.
"It's counterintuitive: Mother Nature prevents catastrophic failures by allowing local failures," says Kisailus. "It's that architecture that makes them so strong."
The team aims to use that architecture to build lighter and more effective armor for soldiers, fortify cars and other vehicles, and even protect athletes from concussions. Kisailus and colleagues are already developing materials that mimic the structure of the mantis shrimp's club, and preliminary tests show the materials are bulletproof, he says.