It’s a moth, it’s a plane, it’s … the Smellicopter, a little drone that senses odors with an onboard moth antenna. Seriously, that’s the scientists’ real name for their creation, a system that monitors the electrical signals sent from the antenna of an actual moth, allowing the drone to lock onto the source of a scent and navigate toward it. And, seriously, this has a long-term purpose: to one day get drones equipped with moth antennae to sniff out objects like bombs.
It’s all thanks to the sacrifice of the hawk moth Manduca sexta, which is an extremely sensitive smeller, like other moths. When a moth picks up a scent, like that of a flower or a potential mate, the odors bind to proteins inside the antennae, and these proteins in turn activate neurons dedicated to specific chemicals. That means the antennae are producing electrical signals that researchers can tap into.
In order to create a sort of moth-drone cyborg, mechanical engineer Melanie Anderson of the University of Washington cold-anesthetized a hawk moth in a freezer before removing its antennae. Then she cut both ends off of a single antenna and attached each to an itty-bitty wire hooked up to an electrical circuit. “A lot like a heart monitor, which measures the electrical voltage that is produced by the heart when it beats, we measure the electrical signal produced by the antenna when it smells odor,” says Anderson, lead author on a recent paper in the journal Bioinspiration and Biomimetics describing the research. “And very similarly, the antenna will produce these spike-shaped pulses in response to patches of odor.”
Slap the rig on a small, open-source Crazyflie quadrotor and you’ve got yourself a hybrid animal-machine. The detached moth antenna keeps on living for up to four hours, giving the drone plenty of time to sniff out odors.
Anderson and her colleagues went a step further and programmed the Smellicopter to hunt for odors just like an actual moth would. If you’re able to smell an odor, there’s a good chance that the source is upwind from you. The same goes for insects like moths, who do something called cross-wind casting, in which they lock on to a presumably upwind source and fly toward it, and then shift their bodies left or right as needed to stay targeted on the odor. Anderson’s team trained the Smellicopter to do the same thing. “If the wind shifts, or you fly a little bit off-course, then you'll lose the odor,” says Anderson. “And so you cast cross-wind to try and pick back up that trail. And in that way, the Smellicopter gets closer and closer to the odor source.”
The researchers call this a “cast-and-surge” algorithm: The drone moves toward a scent—in the lab they used a mixture of flower compounds—and tacks left or right if it loses the odor, then surges forward once it locks on again. The drone is also equipped with laser sensors that allow it to detect and avoid obstacles while it’s sniffing around.
And, boy, does it work well: The researchers have found that the Smellicopter gets to the source of an odor 100 percent of the time. That’s due in large part to the extreme sensitivity of a moth’s antenna, which can detect minute odors not on the scale of parts per million, or billion, but trillion. A moth further increases its efficiency with physics: As it flaps its wings, it circulates air over its antennae, helping to sample more of an odor. Here, too, the researchers took inspiration from nature, using the quadrotor’s spinning blades to move more air over their borrowed antenna.
Sure, at the moment humanity may not have much use for a moth drone that sniffs out flowers, so the researchers are now exploring ways to use gene editing to create moths with antennae that sense odors like those associated with bombs. But could these Frankenmoths possibly be as sensitive to the scents of human-made materials as regular moths are to the pheromones of potential mates and the smell of flowers? That is, can the researchers retune a sense of smell that evolution has perfected for the moth over hundreds of millions of years of evolution?
“Theoretically, you could get more sensitivity,” says Anderson, “because the moth antenna can sense a variety of different chemicals, a lot like how we can smell a variety of different things.” Her lab’s idea would be to genetically engineer a moth antenna to be chock-full of the particular protein that’s involved with sensing a desired chemical. That would focus the antenna’s powers on one odor, not many.
One of the current limitations of the Smellicopter, though, is that while they max out at four hours of use, the disembodied moth antennae live on for only two hours, on average. The drones last for even less time—at most 10 minutes on a charge—so battery life is actually more of an issue. In terms of storage, the researchers are finding that the antennae last a week, if not longer, when refrigerated. That’s not a lot of time, but “it’s easier to disseminate and transport than coronavirus vaccines,” says University of Washington biologist and neuroscientist Thomas Daniel, coauthor on the new paper.
Perhaps the machines of the future, then, will utilize the best of both human engineering and evolutionary engineering; after all, we humans haven’t invented an odor sensor anywhere close to the sensitivity of the moth’s. (Although researchers have tried to make a robot that can smell as well as a dog to detect cancer.) “I think it is a powerful concept,” says University of Zurich roboticist Antonio Loquercio, who researches drone navigation but wasn’t involved in this new work. “Nature provides us plenty of examples of living organisms whose life depends on this capacity. This could have as well a strong impact on autonomous machines—not only drones—that could use odors to find, for example, survivors in the aftermath of an earthquake or could identify gas leaks in a man-made environment.”
And listen, sniffer dogs are arguably the best dogs with jobs, but they’ve got limitations. Clambering over rubble in search of earthquake survivors is dangerous work, both for dog and human handler. A moth drone could more safely navigate dangerous search scenarios. “Additionally,” says Anderson, “the drone is able to explore vertical airspace too, and could search out a gas leak in a large industrial facility that might be hard for a dog to navigate or a person with a handheld sensor to reach.”
So thank you, valiant moths, for your sacrifice. May you forever live on as Smellicopters.