Beetle's toxic blasts trigger innovation

Bryn Nelson Columnist

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.’ ”

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