Lethal shrimp weapons inspire harder composites

Move over, Sugar Ray. The title of fastest punch in the animal kingdom firmly belongs to the peacock mantis shrimp, whose club-like appendages reach the speed of a .22-caliber slug, shatter clam shells with ease, and can slice human fingers to the bone.

Able to withstand thousands of strikes without breaking, the marine crustacean’s “fists” have inspired a carbon-fiber composite material that is stronger and more durable than what is currently used by the commercial aircraft industry. Researchers created an architecture of carbon fibers to mimic the claw’s shock-absorbing interior and then used impact testing to judge its toughness versus other composites.

In the end, the mantis shrimp’s design reigned supreme, with less denting and greater residual strength after impact. Potential applications for such a material could include aircraft and automotive panels, and athletic helmets and military body armor. The study was published online Tuesday in the journal Acta Biomaterialia.

A peacock mantis shrimp looks like the praying mantis’s outlandish underwater cousin, with its loud rainbow shell and big, googly eyes. But this extremely aggressive predator doesn’t play around, smashing its way through crabs, mollusks and even the skulls of small fish.

“They push their prey up against a rock and start beating on it until their shells crack open,” said study author and materials scientist David Kisailus of the University of California at Riverside. “Fishermen refer to them as ‘thumb-splitters’ for a reason.”

In his lab, Kisailus keeps peacock mantis shrimps in plastic tanks because they have been known to break glass aquariums.

The ballistic mechanism of the clubbing appendages is unique to certain kinds of mantis shrimp and is not found in other crustaceans. Their ancestors had mouth parts that gradually became enlarged and – because some prey had armor – evolved into the hammer-like appendages we see today.

“The mantis shrimp has evolved this extreme weapon, which for its size is probably the most potent in the animal kingdom,” said biologist Roy Caldwell of the University of California at Berkeley, who was not involved in Kisailus’ research. They grow to about 2 to 7 inches long.

Over his 30 years of studying the creatures, he has been battered by his little subjects “many, many times.”

“I keep a file of my injuries,” said Caldwell. “While I haven’t lost any appendages, I have had deep and serious wounds.”

The peacock mantis shrimp stores energy within dense muscle like a bow that is pulled and then released, causing its club to pop out and knock the target. This happens 50 times faster than the blink of an eye – so swift that the club packs a second punch from a phenomenon called cavitation.

“The water gets squeezed out of the way and literally boils,” Caldwell said. “In a millisecond or so, the bubbles collapse, forming a shock wave that helps to fracture and destroy the target.”

Within each club is a scaffolding of fiber layers stacked in a corkscrew-like, or helicoidal, arrangement. The fibers are made of chitin, an organic polymer commonly found in insect and crustacean exoskeletons, while gaps between are filled with calcium carbonate and calcium phosphate.

Each layer of parallel chitin rods is horizontally rotated a small angle from the layer below it, which creates an architecture that can absorb an extreme amount of impact energy.

“When a small crack forms, it has to travel a very tortuous path – around and around the helicoid – in order to escape the entire club, but runs out of energy and stays in the club,” said Kisailus. Helicoidal architecture has also been observed in the exoskeletons of some beetles and crabs.

Kisailus and his colleagues created a carbon-fiber version of the peacock mantis shrimp’s helicoid, using epoxy as fill-in, and put it to the test against two other composites made from the same basic components. The first was a unidirectional structure in which fiber layers were simply stacked parallel to one another.

Then they used the aircraft standard, called quasi-isotropic, which is similar to helicoidal but using a larger layer rotation angle of 45 degrees. In comparison, the helicoidal angle used was roughly 15 degrees.

After dropping a weight vertically on to each sample using a control machine, the team assessed the damage. The unidirectional sample completely failed, splitting in two, while the quasi-isotropic sample was punctured through to the underside. The helicoidal sample showed some wear but overall was 49 percent less dented than the quasi-isotropic structure.

Using ultrasound waves and also confirming with computer simulation, the researchers found that the structure spread the damage laterally rather than vertically into the sample.

Mechanical engineer Francois Barthelat of McGill University, who was not involved in the study, believes this superior helicoidal composite could easily be mass produced.

“There are machines already to make this type of composite material, so what’s critical is the orientation of the fiber layer,” he said. “It would just be a matter of adapting the machines.”

Using design from mollusk shells, his lab recently created a bio-inspired glass that is 200 times tougher than regular glass. Although it sounds counterintuitive, introducing weaker regions within glass made it stronger.

“What is pervasive in natural materials is the idea that you have weak interfaces to guide the crack where you want it to go,” said Barthelat. “Once you use this, you can make amazing materials.”

Meeri Kim is a freelance science journalist based in Philadelphia.

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