In the past two years, the term “metaverse” has become popular, and it seems that this is something that can change the future of mankind. Regardless of the commercial prospects, some researchers are indeed working on combining virtual reality and digital life. One of the questions is: can you experience the same cold, hot, sour and sweet feeling in the virtual world? Chemical touch may tell us the answer.
”The alarm sounded suddenly, and he left the scene quickly. Just as he walked to the exit, there was a sudden explosion, and sparks splashed on him, making him feel stinging. When he came outside, there were snowflakes in the sky, and the chill hit him. He found that It turned out to be an alarm from the nuclear reaction room next door. He went to investigate, but found that his arm was numb and weak. After finally opening the door, he saw the heat wave in the room, and he was surrounded by a burning and suffocating feeling. However, when he took off the Put on the VR headset and it all stops.”
It was a simulated experience. Those sensory changes were the result of several chemicals coming into contact with the skin through a wearable device: numbness and stiffness in the arm because the lidocaine had come into contact with the skin; a heat wave because capsaicin was released to the face; the tingling sensation comes from sanshool, while the cooling sensation at the end is menthol.
This amazing wearable device uses a technology called “chemical haptics”. In fact, chemicals have long been used to understand the mechanisms of touch. In the 1990s, research on menthol and capsaicin helped us understand how the body responds to hot and cold stimuli. Now, Jasmine Lu of the University of Chicago and her colleagues are applying this knowledge to create a chemically induced “tactile sensation.”
Tactile, more than touch
Haptic technology creates a sense of touch without direct contact. It’s not a new word, devices with haptic technology are ubiquitous in our daily life. For example: the home button on the iPhone 13 is not a physical button, it cannot be pressed, but “pretends” to be pressed by sensing vibration—this may be the most common application of tactile technology around us.
The limitation of such devices is that most of them function through force or vibration, so they can only use one form of tactile mechanism – pressure, and our skin can sense much more than pressure. For example, if you touch an ice cube and a soldering iron, obviously, the two objects give you different sensations, which means that in addition to pressure, your skin feels other types of stimuli. The sense of touch is produced by the joint action of various receptors in the skin, so if all these sensations can be simulated and integrated into the same device, an extremely realistic sense of touch can be replicated.
The intention of chemical haptic technology is to use chemical substances to simulate the sensations of cold, heat, vibration, pressure, etc. in a “package”. Jas Brooks, who is in the same research team as Jasmine Lu, has previously designed a VR helmet, which can release chemicals such as menthol and capsaicin to the trigeminal nerve in the nasal cavity, and then the trigeminal nerve can convey the feeling of cold and heat to the human body. brain. Moreover, users not only think of cold and heat because they smell the smell of menthol or capsaicin, but also because the trigeminal nerve is clearly “telling” the brain: the current temperature of the environment It just changed.
2021 Nobel Laureate in Physiology or Medicine David Julius (left) and Arden Pataptian
However, using this device is not friendly enough. Pumping capsaicin directly into the nasal cavity can indeed convey the feeling of heat intuitively, but the stimulation of “spicy” is also very strong. So, Jasmine Lu wanted to find a way to bypass the nose and directly produce the sense of touch through the skin. So how do chemicals act on the skin to create sensations?
Inspiration from capsaicin and menthol
In the early 1990s, David Julius at the University of California wanted to find an alternative to opioid painkillers. But first, he needed to understand molecularly how the body perceives pain and how the signaling pathways for touch are enhanced.
He and his colleagues created a repository of millions of DNA fragments, each corresponding to genes in neurons that respond to pain, heat and touch. In 1997, they finally identified a receptor called TRPV1. TRPV1 is located on the cell membrane of nociceptive neurons, and when stimulated by capsaicin, TRPV1 opens ion channels in response. Interestingly, TRPV1 is also activated by thermal stimulation. When encountering a high temperature above 43°C, TRPV1 will release exactly the same electrical signal and transmit it to the brain-so the feelings of “spicy” and “hot” are very different. similar.
We often mention “sour, sweet, bitter, spicy” together, but in fact, if you think about it carefully, you will know that spicy is obviously different from the other three flavors. For example, if you rub your eyes with the hand that just cut a pepper, your eyes will feel hot, but if you rub your eyes with the hand that just touched a cake, your eyes will not feel sweet. This example reminds us that spiciness and heat are related in the same way, and spiciness is not a kind of taste, it is essentially a “heat sensation”.
David Julius and others continued to spend many years studying in detail the complex system of receptors, neurons, receptors, and ion channels that contribute to our endless and wonderful touch sensations. Ardem Patapoutian, another central figure in this study, used menthol as the research object to discover the mechanism of the skin’s “cold sensation”, and later discovered the pressure receptors on the skin. These works have allowed people to gradually understand what the sense of touch is. In 2021, David Julius and Ardem Patapoutian were awarded the Nobel Prize in Physiology and Medicine for their discovery of temperature and touch receptors.
Their work also inspired Jasmine Lu a lot, so she reformulated the experimental plan. Instead of creating a thermal environment, applying force or vibration to achieve haptic technology, she is trying to use chemicals to directly stimulate the skin and trigger receptors in the skin to mimic the sensations associated with touch.
The third “chemical sense”
We feel stimuli from chemicals because receptors on cells interact with the chemical molecules, which then respond and send signals to the brain. In most people’s inherent cognition, this process is mainly reflected in the sense of smell and taste. However, if your thoughts stop here, you are obviously underestimating the receptors.
The receptors for the sense of smell are in the nasal cavity, and the receptors for the sense of taste are on the tongue. In fact, you can imagine that even if there is no nose and tongue, there will be a third “chemical sense”. For example, when you chew peppercorns, not only will your mouth feel hot and spicy, but your lips will also feel like pins and needles. The mechanoreceptors are activated, as if a little vibrating motor is placed on the lips, so you feel a tingling sensation. It can be seen that the receptors that respond to chemical stimuli are not only present in the nose and tongue, and the sensations triggered by chemicals are not limited to smell and taste.
In 1912, American zoologist George H. Parker proposed the concept of “common chemical sensation”, which is used to describe the chemical sensitivity of animals (especially skin). Parker believes that it is widely present in the nervous system through animal experiments. properties, rather than properties of the taste and smell nerves. It was not until the 1990s that someone proposed to use the concept of “chemesthesis” instead of “common chemical sensation” to describe the third chemical sensation.
There are prospects, but there are also limitations
Now, Jasmine Lu’s team has applied this chemical haptic technology to VR. They created a wearable device in which chemicals are stored and pushed through tiny pumps through channels open to the skin, allowing the skin to touch and even absorb the reagents as they pass through, creating sensations. But can we completely replace real touch with this experience? If not, what is the difference between these two “tactile senses”?
At the 2021 User Interface Software and Technology conference, they released a video of a virtual reality scene in which a person wearing their wearable device is “Experience” a thrilling nuclear accident-that is, the scene at the beginning of this article.
However, not everyone is excited about this technology. At least in the eyes of practitioners in some fields, touch is an act of sociological significance. It is not only an action, but sometimes also symbolizes emotional connection. It cannot be summed up and replaced by cold, heat, pain and numbness simply and roughly.
These skeptical voices also revealed the limitations of this technology. Why do I feel relieved when petting a cat? The answer lies in the nerve fibers that mediate this type of touch. Different from the sense of touch (called discriminative touch, discriminative touch) that perceives cold or heat, pain or itching, the sense of touch when touching a cat is an affective touch (affective touch), which consists of a special nerve fiber—— CT fibers dominate. CT fibers respond only to slow, gentle touch, such as touch stimuli that move around 5 centimeters per second. Touching a cat is this type of stimulation. As the name suggests, it is called “emotional” touch because it can be connected with people’s psychological function and emotional experience. What Jasmine Lu’s chemical haptic system can simulate is discriminative touch, but it is powerless for emotional touch. This is indeed a limitation of chemical haptic technology, but even so, its prospects are still exciting.
Or we can say that the chemical tactile technology developed by Jasmine Lu’s team is a mutual achievement with the development of the VR field. In fact, people’s experience in virtual reality is already realistic enough, and it is for this reason that the “Metaverse” has become one of the hottest concepts at the moment. However, if we want to achieve a feeling comparable to the real world, we may still need In addition to vision and hearing, it simulates a richer and more diverse senses-to be exact, a complete and comprehensive, including discriminating touches such as cold, heat, pain, and numbness, as well as comfortable “emotional senses”. touch”.