Virtual reality has gotten really good over the last few years, but with a few exceptions, the “reality” part ends up getting restricted to sight and sound. You put on the headset, plug in the headphones, and that’s your experience. The rest of our senses usually get left out, most often for totally reasonable technical reasons. Like, yeah, you can totally feel touch and temperature sensations in VR as long as you’re willing to stick your hand in a giant mystery box and leave it there or (more recently) wear some expensive gloves attached to an even more expensive pair of robotic arms.
For bespoke, practical VR, that’s probably fine, but for the kind of really immersive VR that you’d want for gaming, it’s important to not feel constrained by the hardware that would be required—anything that’s going to add an extra sensory dimension ends up that much more stuff that has to be powered and is hanging off of you somewhere.
In order to replicate temperature sensations in VR, for example, the go-to hardware has been Peltier elements, which can do thermoelectric heating and cooling of whatever they’re attached to. They work well enough, but they’re power-hungry, making them impractical for long-term use while reliant on batteries. And other VR temperature solutions, like heat lamps, are even worse.
Researchers from the University of Chicago have come up with a much more power efficient way of generating different temperature sensations in VR, and they’ve done it by hacking into your face. By using very specific chemicals to access the trigeminal nerve in your nose, they can make you feel hot and cold through smells without realizing you’re smelling anything at all.
Your trigeminal nerve carries sensory information from your face to your brain. Image: Wikipedia
The trigeminal nerve connects your brain to most of your face, and it carries a bunch of sensory information, including both smell and temperature. The actual temperature-sensing mechanism comes from transient receptor potential (TRP) ion channels, and while we can skip over exactly how these work, the important thing to understand is that some of these TRP channels can get triggered by either skin temperature or certain kinds of aerosolized chemicals. You’ve almost certainly experienced this for yourself: When you smell peppermint, it feels cold, because the menthol in the peppermint is triggering a receptor in your trigeminal nerve called TRPM8 that responds to both the menthol and temperatures under 25 °C. On the other end of things, capsaicin (which you can find in hot peppers) triggers the TRPV1 receptor, which also responds to temperatures above 42 °C. That’s the key: One receptor that can be triggered by temperature or a chemical, but sends the same temperature sensory message to your brain. The researchers describe this as “a perceptual duality,” and if you aerosolize one of these chemicals and puff it up your nose, you’ll feel a temperature change.
Although smells can trigger temperature sensations, for virtual reality, that’s not going to do all that much for you if your VR user is always smelling something distinctive when you just want to convey a temperature. Fortunately, pure capsaicin doesn’t actually smell like anything by itself, so you can sniff it, have it trigger the TRPV1 receptor in your trigeminal nerve, feel a hot sensation in your face, but not detect any odor. “The temperature sensation was largely localized to the face and definitely tied to breathing,” first author Jas Brooks told us when we asked them to describe the warm sensation. “I didn’t smell anything, but I felt an increasing sense of warmth as though my face was being warmed by sunlight for a while.”
Cold is quite a bit trickier, because menthol has a very powerful and distinctive smell, so the researchers instead tried using eucalyptol to activate the TRPM8 receptor. Eucalyptol also has a detectable odor, but it’s much less distinctive, as Brooks describes: “For eucalyptol, the effect was even more intense: breathing in the puffs felt like drawing in cool, fresh air. Unfortunately, you could notice the green odor of eucalyptol, but it was harder to recognize it as compared to menthol (easily recognized as minty).” They added, “Both the warm/cool sensations we were able to elicit in VR were surprisingly immersive, but definitely not as intense as directly heating/cooling the air or skin.”
Brooks says that there are a few different ways of managing the odor that comes with eucalyptol. Ideally, they’d be able to find a chemical that works more like capsaicin does (without any detectable odor), and there are some candidates like icilin, a synthetic chemical that can trigger TRPM8 more than twice as powerfully as menthol does. There’s also an option to leverage the virtual reality itself to disguise odors by masking them with a more powerful odor that matches an event happening in VR.
If none of that stuff is good enough, the researchers have developed a sort of nuclear option, called pink smell. Pink smell is analogous to pink noise, which is similar to white noise, one practical use of which is to drown out other noises by producing static. White noise is essentially random noise, while pink noise is random noise that’s balanced so that there’s equal amount of energy in each octave (making it sound a bit deeper). Pink smell attempts to do for odors what pink noise does for sound, using a balanced noise of unidentifiable odors to drown out any other odors that the user might recognize. “It’s a confusing sensation,” Brooks says, “which is precisely what it is designed to achieve. You can immediately smell something, but it was hard for us (authors) to recognize what any odor was exactly because of the olfactory noise, even when we tried it with menthol.”
A diagram of the smell delivery system. Image: University of Chicago
The smell delivery system is small, weighing a little over 100 grams with batteries included. It uses just 0.25 W of power, between 20 and 50 times less than typical Peltier elements. “Power efficiency is everything,” says Brooks. “Ultimately, any kind of new modalities for VR/AR will only succeed if they are feasible in a mobile/untethered context. If we want to have thermal experiences that are portable, the device needs to be power efficient.” Three 1 mL vials of liquid last almost 6 hours of sustained temperature changes, with the device delivering an atomized puff toward the user’s nose every six seconds. Here’s a video of what it looks like in action:
This iteration of the device can create discrete levels of both hot and cold, and vary the intensity by changing the frequency of the puffs of chemicals, causing the user to inhale the compounds more or less often. Brooks tells us that there are other TRP channels that encode for different temperature levels that they’d like to explore, along with a wider variety of synthetic compounds that can produce stronger temperature sensations. The researchers are also thinking about whether there might be ways of activating these channels via electrical stimulation, and if so, whether they might be able to access TRP channels elsewhere in the body.
“Trigeminal-based Temperature Illusions,” by Jas Brooks, Steven Nagels, and Pedro Lopes from the University of Chicago, is available online here. And if you’re interested in how smell, taste, and temperature can be used in human-computer interaction, Brooks and other researchers will be presenting their human-computer interaction work in a free online symposium on Tuesday June 30.