Grasshoppers Explosives Smells

In a new study published this month in the journal Biosensors and Bioelectronics: X, a team of scientists at Washington University in St. Louis showed how they were able to hijack the olfactory system of the American grasshopper (Schistocerca americana) to both detect and discriminate between different explosive scents — all within a few hundred milliseconds of exposure. The researchers were also able to optimize their biorobotic sensing system that could detect the insects’ firing neurons and convey that information in a way that told them about the smells the insects were sensing.

An American grasshopper with an improved brain sensor implant. Image credit: Raman Lab / Washington University in St. Louis.

An American grasshopper with an improved brain sensor implant. Image credit: Raman Lab / Washington University in St. Louis.

“We didn’t know if they’d be able to smell or pinpoint the explosives because they don’t have any meaningful ecological significance,” said team leader Dr. Barani Raman, a researcher in the Department of Biomedical Engineering and the Department of Electrical and Systems Engineering at Washington University in St. Louis.

“It was possible that they didn’t care about any of the cues that were meaningful to us in this particular case.”

Dr. Raman and colleagues previously found that the American grasshopper olfactory system could be decoded as an ‘or-of-ands’ logical operation. This allowed the researchers to determine what a grasshopper was smelling in different contexts.

With this knowledge, they were able to look for similar patterns when they exposed grasshoppers to vapors from TNT, DNT, RDX, PETN and ammonium nitrate — a chemically diverse set of explosives.

“Most surprisingly, we could clearly see the neurons responded differently to TNT and DNT, as well as these other explosive chemical vapors,” Dr. Raman said.

“With that crucial piece of data we were ready to get to work. We were optimized.”

Now the authors knew that the grasshoppers could detect and discriminate between different explosives, but in order to seek out a bomb, a grasshopper would have to know from which direction the odor emanated. Enter the ‘odor box and grasshopper mobile.’

The explosive vapors were injected via a hole in the box where the grasshopper sat in a tiny vehicle.

As the insect was driven around and sniffed different concentrations of vapors, the scientists studied its odor-related brain activity. The signals in the bugs’ brains reflected those differences in vapor concentration.

The next step was to optimize the system for transmitting the grasshoppers’ brain activity. The researchers focused the breadth of their expertise on the tiny grasshopper.

In order to do the least harm to the grasshoppers, and to keep them stable in order to accurately record their neural activity, the authors came up with a new surgical procedure to attach electrodes that didn’t hinder the grasshoppers’ movement.

With their new instrumentation in place, the neuronal activity of a grasshopper exposed to an explosive smell was resolved into a discernible odor-specific pattern within 500 milliseconds.

“Now we can implant the electrodes, seal the grasshopper and transport them to mobile environments,” Dr. Raman said.

“One day, that environment might be one in which Homeland Security is searching for explosives.”

“The idea isn’t as strange as it might first sound.”

“This is not that different from in the old days, when coal miners used canaries. People use pigs for finding truffles.”

“It’s a similar approach — using a biological organism — this is just a bit more sophisticated.”

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Debajit Saha et al. Explosive sensing with insect-based biorobots. Biosensors and Bioelectronics: X, published online August 6, 2020; doi: 10.1016/j.biosx.2020.100050

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