We often think about our skin as a barrier that keeps our internal tissues protected from dangerous elements. However, it is also an extremely sensitive surface, which lets us learn about our external environment via feeling touch, temperature, etc. This is thanks to the complex structure of our skin which holds various receptors that respond to different external stimuli.
The skin is our largest haptic organ. So how might we engineer new haptic devices that better partner with our skin in producing realistic sensations?
Traditionally, haptic devices have used actuators such as motors, peltier elements, etc. for creating external haptic sensations on the skin by targeting these nerve receptors: motors create pressure on the skin to target mechanoreceptors; Peltier elements generate heat or cold to target thermoreceptors. While this type of thinking certainly advanced our field of haptics, it also left us only with one choice of external hardware actuator for each type of sensation. In our research, we’ve come to realize that every one of these skin receptors can also be actuated via a more indirect pathway: they can be chemically actuated — this is the key idea behind Chemical Haptics.
Using chemical ingredients to induce sensations has been widely used in food, cosmetics, and medicine. Some popular examples include warming/cooling products to reduce pain (VapoRub, TigerBalm, etc.) or the use of spices, such as Szechuan peppercorns, in dishes to create tingling/numbing sensations on the lips.
Building off these prior uses, we identified six chemical ingredients and explored their use for rendering haptics in an interactive VR experience. The result is a new type of haptic device, one that works by delivering chemicals to the skin. In turn, the user’s skin reacts and internally creates the haptic sensations. Our device, shown above, can create warming, cooling, stinging, tingling, and numbing.
As the first exploration into this new territory of chemically-induced haptics, we first conducted a study to explore the temporal profiles of each chemical, noting how sensations changed during and after application of the chemical on the skin. We also asked participants what they felt was the most dominant sensation. This study was done in isolation of any visual stimuli or interactive experience so that participants focused solely on what sensations arose.
Using the results of this study, we then designed five VR experiences for the sensations of warming, cooling, numbing, tingling, and stinging. We then validated our chemical haptics approach for VR by conducting a study where participants experienced VR scenes with and without chemical haptics. All participants preferred the experience with chemical haptics and also rated their experience with chemical haptics as more immersive than without.
Building a Chemical Haptics Device
What excites us about the chemical haptics approach is that the same device form factor (a chemical delivery device) can be used to achieve a wide variety of sensations by simply swapping out the chemical being applied. Additionally, these chemical liquids can be applied to almost any skin area, allowing for such a haptic device to reach areas traditionally less explored. We tried to emphasize these points when designing our device and ended up with two form factors — one for the cheeks and one for the forearm.
Underlying both designs are silicone channels that are open to the skin, allowing chemicals to be absorbed by the user, and a closed-loop design, allowing chemicals to be reused and recirculated.
Our cheek device is designed so that a central control unit (that contains the pumps, microcontroller, battery, reservoirs) is connected via tubes to silicone patches that are attached to the cheeks. In this device, we stimulate an area that is usually hard to reach when worn with a headset and even leverage the headset’s compression on the cheeks in the design. Moreover, this device is extremely flexible. While we stimulate the cheeks, those silicone patches could be moved and attached virtually anywhere on the skin, limited only by how far a tube could reach.
Our forearm device is designed to be fully self-contained. All components needed to deliver our chemicals to the skin are fully embedded in the sleeve. One could imagine a variation of this design to stimulate areas like the neck, waist, legs, etc. This design is highly wearable and feels like an extension of the body.
More Approaches to Chemical Haptics
We see our work as the first step for chemical haptics. Much more interesting chemical haptics experiences are on the horizon. We predict that more can be achieved, for example, by mixing compounds and seeing what other sensations arise. Additionally, the chemical formulations we used were not made with the intent for haptics. These were all chemical products that could easily be purchased from drug stores to ensure that they were skin-safe. However, with more testing and explorations, the development of chemical products made specifically for haptics might enable even richer sensations. We are making all of our device materials open source and are excited to see others remixing the device and further exploring the chemical haptics approach.
The Bigger Picture: Human Computer Integrated Devices
This work is inspired by our approach at the Human Computer Integration Lab: rather than add more electronics and mechanisms to externally create sensations, we engineer ways to add only the minimum technology to trick the user’s body to internally induce sensations. In the past, we’ve explored this through electrical muscle stimulation, as a replacement for traditional motors in force-feedback devices, and more recently, we’ve turned to chemosensory interfaces.
This work builds off our previous Trigeminal Temperature Illusions, led by my colleague and co-author Jas Brooks. In their CHI 2020 paper, they developed a method of rendering temperature also by chemically-induced haptics. In their work, users felt hot/cold sensations as they breathed in chemical ingredients in an aerosol form, such as capsaicin/menthol. With chemical haptics, we take this line of work one step further and explore both a wider range of sensations beyond just hot/cold, and explore how to trigger chemically induced sensations anywhere on the user’s skin.
We’re really excited by the potential for this approach to broaden the ways we approach haptics, particularly in ways that create novel sensations beyond just vibration and pressure that might be more deeply connected with our underlying biological mechanisms.
Engage with our paper more
Jasmine Lu, Ziwei Liu, Jas Brooks, Pedro Lopes. 2021. Chemical Haptics: Rendering Haptic Sensations via Topical Stimulants. In Proceedings of ACM Symposium on User Interface Software and Technology 2021 (UIST’2021). Association for Computing Machinery, New York, NY, USA. https://doi.org/10.1145/3472749.3474747
Project Website: https://lab.plopes.org/#chemical-haptics