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Beetle's toxic blasts trigger innovation

A beetle’s chemical warfare against marauding ants, birds and frogs has provided the inspiration for a European effort to design more efficient fire extinguishers, reliable pharmaceutical sprays and fuel-injection engines.

A beetle’s chemical warfare against marauding ants, birds and frogs has provided the inspiration for a European effort to design more efficient fire extinguishers, reliable pharmaceutical sprays and fuel-injection engines.

The bombardier beetle’s toxic blasts of boiling-hot poison could even provide the impetus for mini rocket boosters to keep a spacecraft on the right trajectory, according to Andy McIntosh, a professor of thermodynamics and combustion theory at University of Leeds in the United Kingdom.

Found mainly in Africa and Asia, the bombardier beetle owes its unusual defense system to a chemical concoction that mixes in an abdominal chamber and then explodes out through a kind of release valve in a series of high-pressure, rapid-fire squirts aimed directly at attacking predators. One chemical, hydroquinone, combines with hydrogen peroxide to generate tremendous heat, as well as water and the noxious irritant benzoquinone — but only in the presence of specific catalysts secreted by cells in the beetle’s thick-walled combustion chamber.

In the beetle, the process is governed by “flash evaporation” directed by inlet and exit valves on its chamber. When open, the inlet valve allows hydroquinone and hydrogen peroxide to enter and begin mixing. The pressurized solution soon exceeds its boiling point, fueled by the heat of the chemical reaction. Once a critical pressure has been reached, the chamber’s exit valve pops open, instantly dropping the pressure and letting the water evaporate in the form of a rapid steam explosion. Ejection of the mix and closure of the exit valve let in more chemicals via the inlet valve and the process repeats.

Explosive defense system
McIntosh first became fascinated with the insect’s explosive defense system when Cornell University entomologist Tom Eisner published a seminal 1999 study that used high-speed photography to document the beetle’s pinpoint precision and rapidly pulsing bursts of spray from a small chamber at the tip of its abdomen. “I could see that this involved combustion in nature,” McIntosh said. “That really got me going. I thought there must be something interesting going on here.”

After some initial computer modeling with grant money from the Engineering and Physical Sciences Research Council, McIntosh met entrepreneur Novid Beheshti of Swedish Biomimetics 3000 Ltd., an organization that funds biomimetic-related research.

The result of that partnership is a new cylindrical chamber that mimics the beetle’s best-known ability by sending surges of hot vapor arcing across McIntosh’s lab. Like the beetle’s tiny combustion chamber, which can shoot out 500 pulses per second at a distance of 8 inches, the half-inch-long experimental rig can shoot spray up to 13 feet — achieving the same size-to-distance ratio — albeit with only 25 to 30 blasts per second.

The artificial version also features a friendlier mix of water and steam. “The beetle squirts out hot water and steam but also quinones, which are toxic and nasty things,” McIntosh said. “You can smell the quinone and you can hear the ‘Phhhht.’ ”

Better fire extinguishers
McIntosh and Beheshti, who described their research in the April issue of the Institute of Physics publication Physics World, believe their success in replicating the mechanical process could point the way toward more versatile fire extinguishers in as little as five years.

With some simple adjustments, the pair found that their rig could emit droplets ranging from a fine mist useful for smothering a fire (similar to the mechanism for automatic fire-suppression systems in some vehicles and buildings), to larger droplets capable of fully extinguishing it. McIntosh said the versatility could be critically important for fighting fires in spacecraft, where rapid suppression would be paramount — and achievable with a micro-mist using very little water.

Cornell’s Eisner said he hadn’t seen McIntosh’s latest work, but agreed that a fire extinguisher could be a useful derivative of the beetle’s formidable talents. If engineered properly, he said, a sort of oscillating spray gun could deliver a steady, constant volume under substantial pressure.

Of the beetle’s natural spray gun, Eisner said, “it’s an oscillation that operates faster than muscles can contract. Once initiated, it proceeds automatically.” The result, he said, is a “pulsation rate that is second to none in nature.”

At the time of his study documenting that rate, Eisner said, he realized the beetle’s natural system shared several parallels with the German World War II V-1 flying bomb, called the “buzz bomb” because of the buzzing sound made by its pulsing jet engine. The bomb, he said, also featured an initial propulsion system based in part on a hydrogen peroxide reaction, akin to the beetle’s spray-triggering concoction.

Safety in numbers
More recently, the beetle has inspired fanciful brooches that squirt perfume. For more serious pharmaceutical applications, McIntosh and Beheshti believe a water-based combustion chamber could lead to improved inhalers and nebulizers that need not rely on the finicky mechanical springs used to trigger the spray in current versions.

Likewise, they reason that the new technology, licensed to Swedish Biomimetics 3000 and dubbed µMist, could be used in fuel-injection systems that would require less pressure and therefore use less energy. By producing smaller fuel droplets that burn more efficiently, the beetle-based injection system could further reduce energy costs. And the water-based system would be more environmentally friendly to boot, they contend. As a more distant application, McIntosh said, the beetle’s pulsing system could be mimicked for a guidance jet system to keep a spacecraft on target with precise pulses of fuel.

Matching all of the beetle’s natural talents could still be a mean feat. “If we can think of a mechanism, nature has already done it, and better,” Eisner said. He’s found, for example, that the beetle can discharge its toxic bursts more than 20 times in a row before depleting its glands. Not that such repeated blasts are usually necessary. “After firing once, he can walk through a crowd of ants, and they just literally part and let him pass,” he said.

The beetles also apparently enjoy considerable safety in numbers. In some places, Eisner said, a lifted rock may reveal 300 of them clustered together. “You can put your hand in there and you feel the heat,” as well as see the little puffs of vapor, he said. “Every time you work with the beetle you end up with something you can’t believe is true.”

Eisner, who has worked with bombardier beetles for four decades, said an enduring mystery is how they tolerate the spray that regularly coats them along with their attackers. He initially thought the insect had an adaptation in its exoskeleton — air pockets, perhaps, to insulate it from the heat and toxin. Physical analyses have revealed no obvious features, however. Nor do the beetle’s nerve endings appear to be shielded from feeling pain. “So I think it probably hurts like hell,” he said, “but the beetle considers it the lesser of two evils.”