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Energetic Science and Piranha-Proof Armor

December 16, 2013 - 4:51pm

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Arapaima gigas is an air-breathing fresh water fish in the Amazon Basin that swims with impunity through piranha-infested waters. | Photo courtesy of Jeff Kubina, National Geographic.

Arapaima gigas is an air-breathing fresh water fish in the Amazon Basin that swims with impunity through piranha-infested waters. | Photo courtesy of Jeff Kubina, National Geographic.

Wise souls steel themselves before heading out at this time of year. There's always a potential bite waiting: An icy road, a missed parking space, or the last-minute rush at a retail outlet.

In contrast, a species of fish -- which goes by the ungainly name of Arapaima gigas -- faces potential bites from ferocious predators every single day. Yet somehow the Arapaima survives, and even thrives, in the piranha-infested waters of the Amazon basin. So how does the fish go unscathed in an environment even more unruly than an outlet mall?

It's not for lack of anything to bite. Arapaima is one of the world's largest freshwater fish, growing up to nine feet long and weighing more than 400 pounds. It could make quite a meal for packs of piranhas, which have one of the strongest bites (relative to body size) of any known animal.

However, as a team of researchers led by Robert Ritchie and Marc Meyers using the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (Berkeley Lab) showed, Arapaima can shrug off such predators thanks to its incredible, adaptable armor. Understanding the unique structure of this armor may help researchers develop new tough and flexible materials for energy and other applications.

Arapaima's scales -- its outer layer -- consist of a tough, mineralized material, arranged in corrugated fashion. That makes it tough enough to resist bites, while flexible enough to allow for swimming (rather essential if you're a fish).

However, the real key is the inner layer. It's built largely from collagen fibers, a substance found in creatures ranging from humans to the Tyrannosaurus rex, and one that's a large component of human skin. X-rays from the ALS showed that the collagen fibers in Arapaima are arranged in parallel sheets, with each layer slightly twisted compared to those above and below. According to Ritchie, an authority on materials strength and toughness, it resembles a "Liberace-type spiral staircase." 

Unlike a conventional staircase, Arapaima's inner layer can twist and compress when under stress. As Ritchie noted, the scales reorient themselves -- in real-time -- to resist force. That increases both their strength and their ability to resist fracturing.

In fact, the inner and outer layers give Arapaima a skin that can withstand pressures of 1.7 million pounds per square inch. That's not only far tougher than anything piranhas can penetrate, it's also almost double the pressure hulls of nuclear submarines are built to shoulder.

Armor strong enough for a sub yet light and flexible enough for a fish offers many possibilities for security and defense. Ritchie believes its unique mechanism might one day be adapted to create an armor that's as effective as Kevlar but far lighter; one that police officers and soldiers could use to protect themselves against predators even worse than piranhas.

Admittedly, perhaps no armor will ever fully protect against the ferocious bites of circumstance -- especially in the throes of the holiday season. But science – and the world-class facilities of the Energy Department -- offers the chance. So perhaps someday, a group of science-minded Santa's helpers will create the perfect gift: A set of armor that protects against piranhas … as well as the frantic last-minute shoppers at the local mall.

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