Chemical Haptics: Rendering Haptic Sensations via Topical
Stimulants
Jasmine Lu

Ziwei Liu

University of Chicago
jasminelu@uchicago.edu

University of Chicago
ziweiliu@uchicago.edu

Jas Brooks

Pedro Lopes

University of Chicago
jasbrooks@uchicago.edu

University of Chicago
pedrolopes@uchicago.edu

Figure 1: We propose a novel haptics approach, which we call Chemical Haptics, based on delivering topical stimulants to theuser’s skin. Upon absorbing these stimulants, receptors in the user’s skin are chemically triggered, rendering distinct haptic
sensations. To explore our approach in interactive contexts, such as VR, we engineered a self-contained wearable that delivers
liquid stimulants. Here, it allows this VR user to feel four haptic sensations: (a) Sanshool creates tingling, which renders
electric sparks emitted from a łshort-circuitingž touchscreen on the arm; (b) Lidocaine creates numbing, which renders amalfunctioning arm interface by reducing tactile feedback as the user taps the buttons; (b) Menthol creates cooling, which
renders cold winter air on the face; finally, (d) Capsaicin creates warming, which renders the hot air on the face from a nuclear
reactor on the brink of meltdown.
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https://doi.org/10.1145/3472749.3474747

ABSTRACT
We propose a new class of haptic devices that provide haptic sensations by delivering liquid-stimulants to the user’s skin; we callthis chemical haptics. Upon absorbing these stimulants, which contain safe and small doses of key active ingredients, receptors in
the user’s skin are chemically triggered, rendering distinct haptic
sensations. We identified five chemicals that can render lasting haptic sensations: tingling (sanshool), numbing (lidocaine), stinging

UIST ’21, October 10ś14, 2021, Virtual Event, USA(cinnamaldehyde), warming (capsaicin), and cooling (menthol). To
enable the application of our novel approach in a variety of settings(such as VR), we engineered a self-contained wearable that can beworn anywhere on the user’s skin (e.g., face, arms, legs). Implemented as a soft silicone patch, our device uses micropumps to push
the liquid stimulants through channels that are open to the user’sskin, enabling topical stimulants to be absorbed by the skin as they
pass through. Our approach presents two unique benefits. First, it
enables sensations, such as numbing, not possible with existing
haptic devices. Second, our approach offers a new pathway, via theskin’s chemical receptors, for achieving multiple haptic sensations
using a single actuator, which would otherwise require combining
multiple actuators (e.g., Peltier, vibration motors, electro-tactile
stimulation). We evaluated our approach by means of two studies.
In our first study, we characterized the temporal profiles of sensations elicited by each chemical. Using these insights, we designed
five interactive VR experiences utilizing chemical haptics, and in
our second user study, participants rated these VR experiences withchemical haptics as more immersive than without. Finally, as thefirst work exploring the use of chemical haptics on the skin, weoffer recommendations to designers for how they may employ ourapproach for their interactive experiences.

KEYWORDS
chemicals, tactile haptics, fluidics, haptic feedback, virtual reality
ACM Reference Format:
Jasmine Lu, Ziwei Liu, Jas Brooks, and Pedro Lopes. 2021. Chemical Haptics:
Rendering Haptic Sensations via Topical Stimulants. In The 34th Annual
ACM Symposium on User Interface Software and Technology (UIST ’21), October 10ś14, 2021, Virtual Event, USA. ACM, New York, NY, USA, 19 pages.
https://doi.org/10.1145/3472749.3474747

1

INTRODUCTION

Physical touch is a critical aspect of our experience, essential tointeracting with our physical world. Thus, much emphasis has been
put on engineering haptic devices that render physical sensations[27, 52, 58, 59], which is especially important for immersive experiences such as in Virtual Reality (VR). One promising approach iswearable devices that deliver tactile sensations directly to the skin[6]. This is primarily achieved by stimulating mechanoreceptors inthe skin via actuators that physically create pressure [38], vibration[11], or skin stretch [35].
However, while these traditional cues (pressure, vibration,
stretch) are undoubtedly important for rendering realistic sensations on the skin, these are not the only sensations our skin canfeel. As such, in recent years, researchers started to explore how
to render other sensations that our skin can feel, especially thosewith more nuanced and affective connotations, such as tingling [6],itching [53], temperature [24, 51, 52, 55], and pain [40].
Unfortunately, two key factors limit the application of theseemergent haptic sensations that go beyond vibration/pressure: (1)most actuators capable of creating tingling/tickling/temperature
are large (e.g., ChillyChair [18] requires desktop-sized actuators,
Therminator [24] requires a large fluidics system) or consume a lotof power (e.g., Peltier elements [51, 52, 55]), rendering them unfeasible in a wearable form-factor; and, (2) most of these approaches are

Jasmine Lu et al.

primarily based on mechanical actuation, and as such are unable
to actuate many of the remaining skin receptors involved in thesetypes of sensations, such as thermoreceptors and nociceptors.
In this paper, we introduce a new way to create haptic effects
on the skin. We propose leveraging chemicals to create haptic sensations on the user’s skin, we call this Chemical Haptics. Withchemical haptics, the sensations are not created mechanically, but,instead, are created when the skin reacts to a particular chemical
ingredient and responds by generating a haptic sensation. For example, when our device delivers cinnamaldehyde to the skin, it createsa long-lasting stinging sensation. Much like this example, all our
haptic effects are possible because many of the receptors foundthroughout the skin can also be triggered chemically (in addition
to mechanically triggered). By means of our engineering, explorations, and user studies, we found five skin-safe ingredients thatcan create five haptic sensations when applied to the skin: tingling
via sanshool, numbing via lidocaine, warming via capsaicin, cooling
via menthol, and stinging via cinnamaldehyde.
To demonstrate how chemical haptics can be used for untethered haptic experiences, such as the VR experience depicted in
Figure 1, we engineered a self-contained wearable that can be worn
across multiple areas of the user’s skin. Implemented as a soft silicone patch, our device uses micropumps to push liquid stimulants
through channels that are open to the user’s skin, enabling stimulants to be absorbed by the skin as they pass through. In Figure
1, we illustrate how chemical haptics allows this VR user to feelfour distinct haptic sensations on the arm and cheeks: (a) sanshool
creates tingling, which renders electric sparks emitted from a łshortcircuitingž touchscreen on the arm; (b) lidocaine creates numbing,which renders a malfunctioning arm interface by reducing tactile
feedback as the user taps the buttons; (c) menthol creates cooling,which renders cold winter air on the face; finally, (d) capsaicin creates warming, which renders the hot air on the face from a nuclear
reactor on the brink of meltdown.
To shine light on this novel way to achieve haptics, we validated
our key hypotheses by means of two user studies. In our first study,
we investigated the temporal profile of these chemically induced
haptic sensations. We found that four chemicals provide distinct
haptic sensations (cinnamaldehyde as stinging, sanshool as tingling, menthol as cooling, and capsaicin as warming), and anotherchemical, lidocaine, rendered an ambiguous sensation, which wesuspected would benefit from a narrative and visual context to work
as haptics. We used these insights to further refine our selectionof chemicals and design five VR experiences. Then, in our second
study we found that chemical haptics increased immersion for allthese five experiences, including numbing via lidocaine.

2

WALKTHROUGH

To help readers understand the applicability of our device, we
demonstrate it in a virtual reality (VR) experience with haptic sensations that are all rendered via our chemical-based device. The
user is wearing two of our wearable devices: one device on thecheeks, between the user’s face and the VR headset, and a second
device worn as a forearm sleeve. These devices are self-contained(i.e., battery-powered and wireless), communicating with the VR
headset via Bluetooth. To deliver chemical stimulants to the skinChemical Haptics: Rendering Haptic Sensations via Topical Stimulants

our device uses pumps that push the stimulants through silicone
channels on top of the skin; these channels are open at their baseso that the liquids come in direct contact with the skin. Once a
stimulant is absorbed by the skin it creates a haptic sensation. The
chemicals used in this VR experience were sourced from off-theshelf, skin-safe, topical products (refer to Implementation for details). In this walkthrough, we demonstrate four of our chemicallyinduced haptic sensations: tingling (sanshool), numbing (lidocaine),
warming (capsaicin) and cooling (menthol).
At the start of this VR experience, users find themselves in a
nuclear power plant on the brink of meltdown, their objective is to
avert the disaster by shutting down the reactor.
Chemically-induced tingling to render electrical sparks.
After being alerted about the imminent meltdown, the user walks
down a corridor leading to the nuclear reactor core. The user opens
doors using an interface anchored on their arm (which depicts a map
and touch-buttons to open doors). Suddenly, after the user opens
one door, a large explosion is heard and electrical sparks cascade
down the walls as depicted in Figure 2 (a). When this happens, theuser’s control panel short-circuits as depicted in Figure 2 (b, c). Here,the user not only sees electrical sparks on their arm, but also feels
them tingling their forearm as depicted in Figure 2 (d). This haptic
sensation is not created by vibrations [11] or electrical-stimulation[34] but by delivering sanshool to the user’s forearmÐthe molecule hydroxy-alpha sanshool reacts with receptors in the skin to
induce a tingling sensation. This tingling sensation via sanshool
is commonly experienced when eating food containing Sichuan
peppercorns.

UIST ’21, October 10ś14, 2021, Virtual Event, USA

the electrical sparks. As the user exits the last door of the corridor,
they walk out to a snowy mountainous landscape (Figure 3). Here,our second device, renders the cold by passing a 10% solution ofmenthol across the cheeksÐmenthol is a chemical known to interact
with thermoreceptors and cause a cooling illusion (for instance,
it is why breath mints feel cold). As such the user slowly feels
an increasing cold sensation as they cross this wintery landscape.
After crossing it, the user enters the reactor’s core facilities, andthe cheek-device stops stimulating, letting the cooling sensation
slowly fade awayśsimilar to how our skin slowly warms up whenentering a warmer room.

Figure 3: (a,b,c) As the user steps outside into a wintery landscape, they feel a cooling sensation on their cheek. (d) Thisis rendered via menthol pumped through attached silicone
patches on their cheeks.

Figure 2: (a) While in VR, the user experiences an explosionthat triggers electric sparks everywhere, (b, c) including on
their arm interface. (d) As this happens, our device delivers
sanshool to the user’s skin, inducing a tingling sensation.
Chemically-induced cooling to render walking outside in
the cold. To get to the nuclear power plant, the user keeps opening
doors by touching their arm interface, which still functions despite

Chemically-induced numbing to render reduced haptic
sensation. Finally, as the user reaches the reactor room’s entrance,
which requires authentication from the user’s arm interface to enter, their VR arm interface starts to malfunction (Figure 4). Whilethe display is still operational, buttons appear łgrayed outž and thewhole arm becomes transparent and starts to łglitchž. As the userpresses buttons to try to open the final door, they feel that theirarm is numb. In other words, they feel less sensations on their arm
because the virtual arm is łmalfunctioningž. Our device renders
this unique haptic sensation (not achievable using existing haptics)
by delivering 5% lidocaine solution to the user’s arm. Lidocaine isa local anesthetic that blocks nerve signals, thus reducing touchsensation.
Chemically-induced warming to render rising temperatures. As the door to the reactor opens, hot steam slowly invades
the room. Here, not only does the user see the steam coming fromthe core but they also slowly feel the heat rising as our cheek-device
delivers a 0.025% capsaicin solution to the user’s face (Figure 5). Capsaicin is the active ingredient in chili peppers that triggers a senseof warmth when it binds with thermoreceptors in the skin (it is why

UIST ’21, October 10ś14, 2021, Virtual Event, USA

Jasmine Lu et al.

3

Figure 4: (a) The user notices their arm becomes holographic
and starts to glitch. (b,c) When the user tries to press a button, they feel their arm is numb to the touch; (d) This is rendered via lidocaine being applied to the area.
chili peppers feel hot). Finally, the user pulls an emergency leverand prevents the meltdown. As the temperature slowly returns
to normal, our device stops the capsaicin and the heat sensation
slowly fades away.

BENEFITS, CONTRIBUTIONS, AND
LIMITATIONS

Our key contribution is that we propose, explore, and engineer a
new approach based on using liquid stimulants to induce haptic
sensations on the skin.
Our approach provides three key benefits: (1) a new type of
haptic device that uses chemicals rather than the more traditional
mechanical approach (e.g., vibration motors or Peltiers), which we
believe can inspire future work; (2) increasing the haptic design
space with a unique numbing sensation, which existing haptic
devices cannot render; (3) while our chemically-induced haptic
sensations are, typically, slow-evolving they are also long-lasting;leveraging this, our device can create sensations lasting on the orderof minutes with relatively low powerÐrequiring only the power tomove the liquid stimulant (0.52W to move two topical stimulants),
rather than to create, for instance heat/cold, which require a lotof power (e.g., a typical Peltier uses 4-13W per Peltier [51, 55]);
moreover, it is the fact that we recycle the chemical in a closed
loop that decreases our power-consumption, i.e., the chemical will
either be absorbed by the skin or evaporate slowly (too slow to bemeasured in a few days).
Our approach is limited in that: (1) the absorption of a chemical via the skin (the haptic stimulus onset) is influenced by the
epidermal thickness, which might vary across individuals as wellas across locations in the body [63]; while we have confirmed thatour proposed set of chemicals induces haptic sensations, future
research might shine more light on how these factors impact skinabsorption; (2) likewise, chemical stimulation is a slow dissipating
phenomena, thus, the haptic stimulus offset is also slower whencompared to traditional haptic actuators. As such, this approach isbest suited for slow changing haptic events (e.g., how the sting ofa wound evolves over the first few minutes or rising heat) rather
than rapidly changing discrete haptic events; and (3) as the first
exploration into chemical haptics, we investigated only six ingredients that we found in over-the-counter medicine or food spices,
resulting in five sensations, but we expect that more sensations
will be possible as researchers build on this work by developing
custom-engineered chemicals for haptics.
Finally, we are not proposing to replace existing haptics with chemical haptics, but rather enlarging the set of haptic techniques by
also considering chemical stimulation of the user’s skin and understanding what effects it can create; our paper lays the groundworkfor this new technique.

4

RELATED WORKThe work presented in this paper builds primarily on the field of
haptics, especially on-skin haptics.

4.1
Figure 5: (a) In VR, the nuclear reactor glows red and is on
the brink of meltdown. (b) The user finds the manual override and pulls the lever. (c) Doing so averts the meltdown. (d)The temperature of the hot room is rendered by capsaicin
delivered to the cheeks.

Rendering vibration, pressure and skinstretch (traditional mechanical haptics)

As the skin is our largest sensory organ, it has received a great
deal of attention with regards to haptics research. When it comesto inducing sensations on the skin, wearable haptic devices (i.e.,
ungrounded) are generally preferred over stationary devices (i.e.,
grounded) since they allow to directly deliver tactile sensations
to the user’s skin [12]. This is primarily achieved by stimulating

Chemical Haptics: Rendering Haptic Sensations via Topical Stimulants

mechanoreceptors in the user’s skin using actuators that physicallycreate vibration [5], pressure [38], or skin stretch [31].
Vibration: One of the most popular and established approaches
to skin haptics is vibration. Small vibration motors have enabled alarge variety of wearable vibrotactile displays for haptic feedback[11, 38]. Depending on their frequency, these mechanical vibrations
are sensed by our skin using a combination of its mechanoreceptors,
such as the Meissner corpuscles and/or Pacinian corpuscles [12].
Pressure & skin stretch. The receptors that govern the perception of vibrations are not the only receptors that contributeto our tactile perception. Another significant area in skin haptics
includes pressure and skin-stretch. Many have engineered tactile
displays that deform the skin (i.e., stretching and dragging) [31] or
render a certain texture [35]. Most of the these displays target slow
adapting (SA) mechanoreceptors found in the skin, e.g., SA1 andSA2 afferents.
However, while these traditional haptic cues are undoubtedly
important for rendering realistic sensations on the skin, these are
not the only sensations our skin can feel. As such, in recent years,
researchers started to explore how to render other sensations that
our skin can feel, especially those with more nuanced and affective
connotations, such as light touch (tickling, gentle stroking, etc.),
tingling/itching, temperature, and pain.

4.2

Rendering affective types of touch(tingling, tickling, light-stroking, etc.)
mechanically and electrically

When it comes to rendering a łlight touchž sensation, which
is linked to affective sensations such as tickling or tingling, researchers tend to resort to different actuators rather than vibration
motors. One popular approach is to use actuators to stimulate theC-tactile neurons, which are rapidly adapting mechanoreceptors
present in the root of the hair-cells, that respond preferentially toslow, stroking touch [12]. To achieve this, researchers often employ
motors that move end-effectors along the user’s skin, stretchingand dragging to trigger the skin’s directional sensitivity [31, 44, 59].
Similarly, M-Hair and its wearable counterpart, MAGHair requires
a user to coat their body hair with magnetic paint, and then thesystem uses an X-Y gantry to move a magnet over the user’s body,which much like the aforementioned mechanical actuators, cause
the user’s hair to move, resulting in a light-stroke, which mimics atingling or tickling sensation [6, 7].
One alternative approach that also aims at stimulating the haircells is using electrostatic forces. For instance, ChillyChair induces
goosebumps by placing the user’s arm between two large metal
plates [18] and producing an electrostatic force between the twoplates which raises the forearm’s hair, causing a goosebump-likeexperience. Finally, electrical tactile stimulation has also been used
to induce a large range of sensations, including itching [53] andpain [40].

4.3

Rendering numbing using additional haptic
sensations that overpower tactile feedback

While most haptic interfaces focus on allowing users to feel additional sensations, our sensorial experiences are not always additive.

UIST ’21, October 10ś14, 2021, Virtual Event, USA

For instance, when our arms and hands are exposed to strong vibrations for long periods (e.g., when manipulating powerful handheld
tools such as a rock drill [9]), the tactile sensations in one’s fingertips are decreased; this is known as hand-arm vibration syndrome[54]. While this effect has not been used in interactive systems, it
has been used in HCI studies to minimize sensations. For instance,
researchers in electrical muscle stimulation (EMS) use vibrations tominimize the tingling sensations that can alert participants to the
EMS stimuli [36, 45]. Other researchers have explored mechanicalways to diminish tactile sensations. For instance, Ochiai et al. [48]utilized the squeeze air film effect, where surfaces feel smootherwhen a small ultrasonic vibration is applied, to interactively reduce
texture sensations from real world objects. Unlike these, our chemical pathway allows for a real numbing effect because it blocks thesensory pathways involved in processing sensation.

4.4

Rendering temperature changes using
thermoelectric elements

More recently, increased attention has been given to renderinga realistic sense of temperature [24, 51, 52, 55, 56]. One popular
approach is adding thermoelectric materials in contact with theuser’s skin. The most popular actuator for temperature haptics
is the Peltier, which is a thermoelectric material that creates atemperature between the side that is in contact with the user’s
skin and the opposite side when supplied with a strong electricalcurrent. For instance, ThermalBracelet [51] used Peltiers attached tothe user’s wrist to explore eyes-free feedback; ThermoVR [52] used
Peltiers added to the foam between the VR headset and the user’s
face; Season Traveler [56] used heating elements on the user’s neck;and, lastly, Thermotaxis [47] added Peltier elements on the user’s
ears.
Moreover, alternative methods to generate temperature changes
include hydraulics (i.e., moving cold or hot water by means oftubes that touch the user’s skin) [24, 26], gel packs [33], or resistive
heating [39, 62].
Finally, it is worth noting that while thermoelectric materials are
more mobile than air-based devices (e.g., A/C units or heat lamps[17]) they consume an enormous amount of power (4-13 W per
Peltier even considering only small Peltiers). In fact, the devices
such as ThermalBracelet [51], ThermoVR [52], Thermotaxis [47], or
resistive heating [39, 62] do not operate on battery because theirlifespan would be drastically reduced and impractical.
While we have a variety of methods for temperature, all of thesecome with an added technical challenge: how to create temperaturesensations on the user’s skin at low power. One recent promising
approach by Brooks et. al [8] used an illusion of temperature bystimulating the user’s trigeminal nerve, inside the nose, chemicallyrather than using temperature. Their system works by emitting
capsaicin or menthol, which the user breaths in as they inhale;these chemicals interact with the nose’s nerves and create a senseof temperature. We take inspiration in this approach but expand
it in two key aspects: (1) we explore chemical haptics in the skinrather than inside the nose; and (2) we propose achieving a rangeof sensations beyond just hot/cold. Moreover, while some of theseother devices realize temperature by moving liquids in tubes overthe skin, these liquids are not in contact with the skin. Instead, weUIST ’21, October 10ś14, 2021, Virtual Event, USA

Jasmine Lu et al.

Figure 6: An overview of the six chemicals we explored in chemical haptics and the touch receptors they interact with. Also,a non-exhaustive depiction of some traditional haptics approaches for actuating these receptors.
implemented open channels to enable skin absorption and tap into
chemically induced sensations.

4.5

Chemical interactions with skin neurons

While, to the best of our knowledge, our work is the first to articulate how to leverage chemical stimulation of the user’s skinto achieve a range of interactive haptic sensations, the idea thatchemical pathways interact with tactile innervated neurons is anactive area of neuroscience research. Much research has been dedicated to mapping out the role of each tactile receptor, including
those responsible for temperature changes (so called thermo-TRPs[5, 50]) and the receptors responsible for pain (so called nociceptors[13]). In the next section, we will discuss our approach of usingchemicals for haptics and explain the biological mechanisms thateach chemical interacts with. Moreover, in the next section, we will
also complement our chosen chemicals with the neuroscientificevidence for their skin interactions.

5

OUR APPROACH: CHEMICALLY-INDUCEDHAPTIC SENSATIONS

We propose to interactively deliver topical stimulants to the user’s
skin to induce haptic sensations. The first key challenge in our
approach is to expand the palette of sensations that our devices cangenerate. To tackle this, we surveyed the literature of neurophysiology, cosmetics, and haptics to find candidate key ingredients tocreate haptic sensations. While there are several chemicals thattrigger reactions on the human skin, there are also a few unsafe
chemicals. As such, our literature analysis was filtered by skinsafety. In the end, we selected six topical stimulants at a skin-safe
dosage, which we depict in Figure 6; these key active ingredientsare commonly found across topical health products, skin cosmetics,
and food products.
Many of these ingredients are used for the purpose of alleviating
pain by producing counterirritation that distracts from pain (e.g.,cooling gels with menthol), stimulating the release of a neurotransmitter making nerve fibers transmit less pain signals (e.g., theorizedmechanism of pain relief from hot patches with capsaicin), or numbing to reduce sensation (e.g., lidocaine patches), while others arecommon in gastronomy (e.g., Sichuan peppercorns, spices, andmint in drinks or food). While these have everyday uses (medical orfood), they are not seen as chemicals that trigger interactive haptic
sensationsÐthis is our focus. To help the readers understand the
choice of topical stimulants and the biological mechanisms thateach of these key ingredients triggers upon absorption by the skin,
we provide an overview of our chosen topical stimulants.

5.1

Chemical Haptics Ingredient #1: sanshool
creates a tingling sensation

Key active ingredient: Hydroxy-alpha sanshool is an organic
molecule found in Sichuan peppercorns or uziza dried fruits; its
chemical composition is C16 H25 NO2 . Sanshool is commonly associated with the tingling feeling Sichuan peppercorns leave in
one’s mouth and lips. Biological skin mechanism: Upon absorption by the lip skin, it reacts with sensory neurons, in particularwith nociceptors and light-touch receptors [25]. Moreover, it reacts
with mechanoreceptors in the lips, specifically, the tactile rapidly
adapting channels that detect vibrations, which is why it feels asa tingling sensation similar to a mechanical 50Hz stimulus froma vibration motor [25, 42]. Others studies also attribute the tingling to activation of both TRPV1 and TRPA1 simultaneously [41,43], which is why there is also a slight warming sensation. Unlike
these previous psychophysics studies, we use sanshool not on thelips but on other skin areas, aiming to create tingling sensations
beyond just intra-oral tingling. How we obtained it: We source
this ingredient by soaking store-bought Sichuan peppercorns in
Everclear (a grain-based alcohol). This method does not guarantee aChemical Haptics: Rendering Haptic Sensations via Topical Stimulants

precise percentage of the sanshool, but it is easily reproducible without chemical lab equipment; alternative routes to acquire precise
quantities are also available from chemical suppliers [30].

5.2

Chemical Haptics Ingredient #2: lidocaine
creates a numbing sensation

Key active ingredient: Lidocaine is an engineered ingredient,which is commonly used as a local anesthetic and often foundin the form of an off-the-shelf cream in drug stores; its chemicalcomposition is C14 H22 N2 O. Due to its useful numbing effect it isconsidered one of the essential medicines by the WHO [49]. Aswith any ingredient of this nature, Lidocaine exhibits side-effects
especially when applied intravenously, however, it rarely displays
side-effects when topically applied under safe dosage [22]. Biological skin mechanism: Upon absorption by the skin, Lidocaine
reduces tactile sensations by chemically binding to neuronal cell’schannels and preventing these neuronal cells from firing to thecentral nervous system [3, 28]. How we obtained it: We source
this ingredient from an off-the-shelf Ebanel spray, which contains
a 5% lidocaine solution as its key active ingredient.

5.3

Chemical Haptics Ingredient #3:
cinnamaldehyde creates an itchingsensation

Key active ingredient: Cinnamaldehyde is an organic compound,which occurs naturally in the bark of trees that produce cinnamon
and is what gives cinnamon its flavor and odor; its chemical composition is C9 H8 O. Biological skin mechanism: Upon absorption
by the skin, it reacts with the sensory nerve’s TRPA1 and induces
an itching sensation [29]. It has also been showed to promote fasterhealing of infected skin wounds when applied as a topical medicine[15]. How we obtained it: We source this ingredient from thechemical manufacturer HiMedia (product GRM3277); this is thenmixed at 5% with Everclear to ensure a skin-safe dosage.

5.4

Chemical Haptics Ingredient #4: capsaicincreates a warming sensation

Key active ingredient: Capsaicin is a colorless organic ingredientproduced in chili peppers (or plants from the genus Capsicum),which is also the key ingredient of many off-the-shelf medicinal
products (e.g., hot patches) as well as in food (hot peppers, paprika, etc.); its chemical composition is C18 H27 NO3 . Capsaicin is
the ingredient that causes all these products and foods, such as chili
peppers, to feel hot. Biological skin mechanism: Upon absorption by the skin, capsaicin reacts with sensory nerve’s TRPV1, thistemperature channel also reacts to hot temperatures above 42°C at
the epidermal layer [50, 65]. How we obtained it: We source thisingredient from the off-the-shelf PainBloc24 topical roll-on product, which contains a 0.25% solution of capsaicin as its key active
ingredient.

5.5

Chemical Haptics Ingredient #5: menthol
creates a cooling sensation

Key active ingredient: Menthol is a mostly colorless organic compound made synthetically or obtained from the oils of mint-plants;

UIST ’21, October 10ś14, 2021, Virtual Event, USA

its chemical composition is typically C10 H20 O. It is common infood/health products, where it is used as a tea or in off-the-shelfproducts (e.g., toothpastes or cold patches). Biological skin mechanism: Upon absorption by the skin, it interacts with the coldsensitive TRPM8 receptor, which reacts to both chemical stimulations from compounds like menthol and to cold temperatures
below 25°C at the epidermal layer [50]. This ability to chemicallytrigger the TRPM8 in the skin explains the well-known coolingsensation that menthol provides when inhaled or eaten [14]. Finally, while cooling is its main understood function, it also exhibits weak anesthetic properties (e.g., slightly reduces other sensations) and counterirritant qualities (e.g., assists in soothing anitch) [19]. How we obtained it: We source this ingredient fromthe manufacturer Nature’s Oil (product: 23-0038-000). Unlike manyof our other selected topical stimulants in our list, mint is typicallysolid at room temperature and requires a solvent to maintain aliquid form without needing to be heated; we mix it with Everclear
at 10%.

5.6

Chemical Haptics Ingredient #6: methyl
salicylate creates a mixed hot & cold
sensation

Key active ingredient: Methyl Salicylate is also commonly foundin many topical analgesics and can be found in wintergreen oil.
It is a colorless organic compound, produced by many speciesof plants, particularly wintergreen; its chemical composition isC6 H4 (OH)(CO2 CH3 ). It is often added in the production of mint
candies (as a flavoring agent) [32] or to fragrances. Biological skinmechanism: Upon absorption by the skin, it has been shown tostimulate as well as inhibit the TRPV1 channel (which also reacts
to the aforementioned capsaicin) [23, 50] as well as the noxious
cold TRPA1 channel [61]. Thus, it has previously been shown toelicit a combination of cool, warm, and burning sensations [23]. As
such, we chose it as we were interested in understanding how apotentially mixed receptor stimulation might affect a user’s haptic
sensations. How we obtained it: We source this ingredient fromthe off-the-shelf product, Zim’s MaxHeat (Kobayashi Laboratories,
product 54273-001-01), which contains a 20% methyl salicylate as
its key active ingredient.

6

HARDWARE IMPLEMENTATION

To help readers replicate our design, we now provide the necessary
technical details. Furthermore, to accelerate replication, we provide
all the code, chemical formulations, firmware, and schematics of
our implementation1 .
The key components of our haptic device are the channels implemented using silicone (this is where chemicals are flowing andwhere they contact the user’s skin), the pumps, and the electronics
that regulate the flow. Our wearable device uses its pumps to circulate the chemical stimulants in a closed-loop manner, i.e., pullingthem from their reservoirs, to the silicone channels, then to theopen section in contact with the user’s skin, then back into theclosed channels, and, finally, back to their respective reservoirs.
1 https://lab.plopes.org/#chemical-haptics

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Figure 7: We designed two versions of our device to deliver liquid stimulants to two locations ś the cheeks and the arm. Thesealso showcase two engineered solutions for delivering our chemicals to the skin: (a) a central control & pump unit connected
to silicone patch via tubing or (b) fully encased and self-contained silicone wearable, including microcontroller, pumps, andbattery.

Figure 8: (a) Material stack of our device, (b) close-up of the sleeve’s open channels, and (c) close-up of channels touching theskin.
Figure 7 shows two of our devices, one worn on the cheeks as amask, and one worn on the forearm. The mask device showcases
how we created a central control and pump device connected via
tubing to silicone patches adhered to the skin, enabling delivery
to difficult locations like the cheeks. The sleeve showcases how
we created a self-contained device generic enough to be used on
different limbs, such as arms or even legs.

6.1

Fluidic channel design

We used Dragon Skin FX-Pro silicone to implement our patcheswith channels open to the skin as seen in Figure 8. This open areaof the channels, which allows any chemical flowing through it tocontact directly with the user’s skin, measured 8.75 cm2 in the arm
device and 0.96 cm2 in the cheek device.
The sleeve’s reservoirs hold 1.2mL of liquid, enabling approximately 20 minutes of actuation (in our study 2, only 3 minutes
was enough for desired effect). While we have measured minimal
evaporation within the silicon (<20% in 24h), the silicone is nothermetically sealed, and long-term storing works best in sealedcontainers (which is what we used for studies).
For sealing the devices to prevent leakage of the chemicals, we
adhered the cheek patches using a waterproof skin adhesive tapeand rely on the compression of the Oculus headset to form a seal
with the user’s cheek skin. For our sleeve device, we rely on thetightness achieved from the silicone’s compression to ensure agood seal with the skin surface. While this method allows somevariability in forearm size, we still recommend measuring forearm
size prior to fabricating these patches to ensure a robust fit. We
also made a sleeve that closes via Velcro (study 2) allowing one
sleeve to be used across participants. With the mp6 pump, werecommend channels no wider than 3mm in width but otherwise
can take any form as our pumps can overcome even a 180 degreeturn.
As silicone can be cured to take many forms, our device can bemolded for many skin regions. In addition, silicone is a breathable
material, allowing for some minor airflow to the user’s skin [64].
Lastly, in our self-contained arm device, we also added compartments in the silicone for reservoirs, electronics, and battery.

6.2

Electronics and Pumps

At the core of our devices is a ATmega328 microcontroller, which
communicates with external applications (such as VR experiences
running in an Oculus Quest) via Bluetooth LE (chipset CC2540) andis responsible for the orchestrated control of two micro pumps andfour thermistors, which act as liquid sensors.
Pumping and absorption. For each channel, we use a MP6
piezoelectric micropump to push a topical stimulant from its reservoir to the channel. The fluidic circuits are closed, which allows us
to recycle the stimulant for a long time. We set the pump to pulse
at 10ms intervals establishing a flow rate of 0.07 ml/sec (4 ml/min).

Chemical Haptics: Rendering Haptic Sensations via Topical Stimulants

When pumping across a 119 mm2 of skin for 10 minutes, we measured an average absorption of 0.66mL (capsaicin: 1mL, menthol:
0.8mL, sanshool: 0.6mL, methyl salicylate: 0.8mL, cinnamaldehyde:
0.6 mL, lidocaine: 0.2mL, water: 0.48 mL, everclear: 0.8mL). This isa worst case as some liquid is inevitably lost in ways other thanskin absorption (e.g., stays in the channels).
Sensing the liquids. The thermistors are sensitive temperaturebased resistors, which makes them useful as liquid sensors. We
leverage the fact that a liquid moving by a thermistor (which is incontact with the liquid as it pokes into our channels) will introduce a change in temperature, since the fluids are more thermallyconductive than air (present when the channel is empty). We took
inspiration from the fact that this approach is used to sense water inlarge hydraulics systems (e.g., RS offers an industrial liquid sensing
system based on this principle [67]). We measure temperature byusing one thermistor in a voltage divider and sampling the resulting
output voltage with our microcontroller. This enables our device touse one thermistor at the start and end of the channel region thatis open to the user’s skin to sense if liquid is present. This information is all our device needs for its logic. As such, when the device
receives a haptic command, it starts cycling the liquid through thechannels. Conversely, when it receives a stop command, it will turnoff the pumps as soon as the thermistors indicate no liquid in the
open channel region. This enables the chemical to be stored for
future use without any contact with the user’s skin.
Power consumption. Our complete prototype uses only 0.52W
of power at full intensity (two channels). In contrast with otherdevices such as Peltier elements (commonly used for thermal feedback), which use at least 4-13W per small Peltier [51, 55], our prototype is approximately 10 times more efficient. Moreover, a device
that combined Peltier elements, vibro-motors, electrical stimulation,or other haptic actuators to induce feelings of temperature, tingling,itchiness, and numbing as this device delivers would likely be lessefficient than our design.
Latency. The end-to-end latency of our device is around 800ms.
This latency includes: the Bluetooth latency (∼40ms), microcontroller latency (<10ms), and the physical action of the micropump
as it pushes a liquid from its reservoir until its first contact withthe user’s skin on the open region of the channel (∼750ms).

6.3

Form-factor trade-offs

While our two wearable devices (one that stimulates the arm andanother that stimulates the cheeks) could be integrated as one,
we opted for separating these in order to showcase the versatility of each of our two designs, as each form-factor has inherent
advantages and limitations. First, the self-contained sleeve unit ishighly wearable but harder to adapt for different skin locations as it
requires the device to wrap around a limb and relies on the stretchability of the silicone to prevent leaks. Conversely, the centralized
unit, which we used to distribute chemicals to patches on the user’s
cheeks, enables more flexibility since it can deliver chemicals tomore challenging locations (as long as a patch can be adhered tothis location). However, this centralized approach is less wearable
as it relies on distributing the chemicals via tubes from the central
pump to the peripheral patchesÐthese tubing connections will limit
freedom of body movement.

UIST ’21, October 10ś14, 2021, Virtual Event, USA

Moreover, the choice of skin locations was also a factor in ourdesigns. We chose the cheeks because: (1) they are a strong candidate for ambient sensations that are often associated with the face(e.g., warming/cooling of the face), and (2) they illustrate the abilityof our system to enable haptics in hard-to-reach areas. Meanwhile,the forearm was chosen because (1) they depict an ideal candidate
for sensations related to arm UIs (which are popular in VR [4, 66]),and (2) our sleeve design can be adapted also to other body areas(e.g., legs, neck, and so forth).

7

USER STUDY 1: REVEALING THE HAPTICSENSATIONS INDUCED BY TOPICALSTIMULANTS (IN ISOLATION)

In our first study, we explored how the chemicals induced sensations
on the skin in isolation, aiming to gain a deeper understanding of
how chemical haptics can be utilized.
To assess this, we employed the Temporal-Check-All-That-Apply
design (TCATA [10]), which is often used to test sensory experiences where the effect of a product is tested over time, e.g., for
how a taste of a new ingredient develops over the first minutesof eating [2] or how a texture of a liquid develops after the first
sip [10, 37].
In this study, participants’ goal was to verbalize which sensations
they felt as the topical stimulant contacted their skin. Participants
were able to choose from a list of pre-existing descriptors. The
TCATA study design is extremely powerful because not only doesit capture which sensations participants feel but also how thesesensations develop over time (i.e., when they start feeling eachsensation, when they stop feeling it, etc.).
Our hypothesis was that each chemical would exhibit distinct
sensations. Furthermore, to ensure that these haptic sensations are
due to interactions with the chemicals, we introduced a baseline
topical stimulant (water) for comparison. Moreover, we did notreveal any ingredient labels or names to participants during thetrials (i.e., blind trials, participant had no knowledge of the stimulant
used in each trial). Our study was approved by our Institutional
Review Board (IRB20-0290).

7.1

Stimulants

We selected seven topical stimulants: capsaicin, methyl salicylate, menthol, lidocaine, sanshool, and cinnamaldehyde; and,lastly, water as a baseline. To ensure safety, the key ingredientswere sourced from: three topical products obtained as off-the-shelf(Zim’s Max Heat, PainBloc24, Ebanel Numbing Spray); one productmarked as safe for food consumption (Szechuan peppercorns soaked
in Everclear); and two liquid stimulants at concentrations deemedsafe in previous studies (Cinnamaldehyde at 5%, Menthol at 10%)[29, 60]. Additionally, due to safety concerns regarding COVID-19,
all studies were conducted remotely over video-conferencing withexperimental devices and (unlabeled) stimulants mailed directly to
our participants.

7.2

Apparatus

Participants were given: a manual version of our device and pipettes
pre-filled with stimulants for each study round (with anonymized
labels, such as łAž, łBž rather than łcapsaicinž, łmentholž) as seen

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Figure 9: (a) Overview of the materials included in one study kit that was mailed to a participant (b) Participant in study overZoom.
in Figure 9. The manual version of our device, which participants
wore throughout all trials on their non-dominant forearm, wasdesigned to be as similar as possible to our sleeve device, exceptthat it uses a manual method of actuation (i.e., participants squeezing the pipettes) rather than relying on pumps, to prevent any
technical setup (pumps, Bluetooth, battery replacement) from interfering with the study outcomes, which were focused on the elicited
sensations and not on the hardware.
To facilitate data collection according to the Temporal-CheckAll-That-Apply (TCATA) design, we implemented a web-interface
that allowed participants to indicate when they started/stopped
feeling a particular sensation. Moreover, it is worth noting that
TCATA allows a participant to indicate that they feel a particularsensation, but not label its associated intensity [10]. Moreover, forspeeding up replication of our work we provide our TCATA web interface as open-source1 . This interface of our TCATA experimental
software was shared with participants over Zoom so that participants could see experimenters check and uncheck sensations asverbally communicated. This interface also provided a visual andaudio metronome that served as a cue to remind participants tocompress the pipettes, which actuates the liquid back and forth
inside the silicone channels (in contact with the participant’s skin);but also reminded participants to tap their arm every ten seconds,
to assess any numbing/excitation effects.

7.3

Task and Procedure

Total trials. Each participant performed a total of 21 trials: 7 topicalstimulants (six chemical haptic stimulants and water as a baseline)x 3 repetitions.
Setup: Prior to each trial, participants were instructed by theexperimenter of which pipette to load in the haptic device (all labels were anonymized, i.e., łAž or łBž rather than łcapsaicin łorłmentholž). Loading a topical stimulant into the silicone patch was
simply done by pushing one pipette, pre-loaded with the stimulant,
and an empty pipette into two openings. During the trials, participants squeezed the pipettes in sequential order (i.e., first squeeze

the one with the liquid, then the other, repeat), which causes theliquid to move back and forth on top of their skin.
Task. A single trial was comprised of experiencing 10 minutesof stimulation with one topical stimulant; followed by 10 minutes
of no-stimulation (without applying any topical stimulant on theskin). During both the stimulation and no-stimulation phases, participants were asked to verbalize when a new sensation arose orvanished. The experimenter checked/unchecked each sensationaccording to the participant’s verbal indications, which generated
a timestamp for when the sensation started/endedÐthis is the key
behind the TCATA study design [10]. Furthermore, because weare also investigating numbing sensations, we asked participants
to also tap their non-dominant arm every five seconds, and after
each tap, verbalize if they feel any change (either suppression or
excitation) in tactile perception. At the end of the trial, they chosewhich sensation they felt as the most dominant.
Haptic descriptors: We used nine haptic descriptors inspired by[1, 16, 21, 23, 29, 43]. Descriptors included burning, painful, tingling,
warm, stinging, itching, pressure, cool, and nothing (by not choosingany). Furthermore, we added numbness as well as increased anddecreased sensation to encode for tactile sensitivity to touches.
Procedure. We randomized the order of the trials. Participants
were never aware of which topical was being applied. Once a trial
was done, participants were required to wait at least 30 minutes
before starting another trial.
Participants. Seven participants (four self-identifying as female,three as male, 26.29 ± 5.77 years old) were recruited from our city(including outside our research institution) and received a 50 USD
compensation. No participant reported any known allergies to ourchosen ingredients.

7.4

Results

We collected data from 147 trials (7 topical stimulants x 3 repetitionsx 7 participants), where each trial’s data provides us with: the dominant sensation (chosen by the participant at the end of each trial)and start/end timings for each sensation over time (derived fromChemical Haptics: Rendering Haptic Sensations via Topical Stimulants

UIST ’21, October 10ś14, 2021, Virtual Event, USA

Figure 10: Dominant sensations chosen by participants, as felt after 10 minutes of experiencing a topical stimulant on skinand 10 minutes without it. Note that there was no additional stimulus, e.g., no VR, interactive applications, etc.

Figure 11: For water, the most felt sensation were łcoolž, but rarely felt.
the TCATA design). To facilitate replication and future research,
we provide all our data in 1 .
Dominant sensation. Figure 10 depicts the most dominantsensations chosen by participants after each trial.
All stimulants, except water (baseline) and lidocaine, elicited aclear haptic sensation. Capsaicin was felt as predominantly warm(and, secondly, also burning); cinnamaldehyde was felt as predominantly stinging; menthol was felt as predominantly cool; methyl
salicylate was felt as predominantly cool; and sanshool was felt
as predominantly cool or warm. However, as we will see in our
temporal analysis, these dominant sensations forced a participant
to collapse a rich 20 minutes of varying sensations down to one descriptor; thus, it does not capture: (1) which sensation arose first, (2)how sensations develop over time; and (3) how sensations fade in
the absence of the stimulus. Moreover, as participants did not know
the stimulant, the cold sensation of the liquid reaching one’s skin at
first, always leaves a tactile impression, which led many to choosing łcoolž as a descriptor. But, as revealed in our temporal analysis
and comments, chemicals deemed as cool actually felt more diverseand unique than cool. To showcase this, we plotted how much a
sensation was experienced over time (in %), which was calculatedby the number of times a sensation was reported divided by total
trials with each chemical.

Water (baseline). In Figure 11, we start our analysis of thetemporal profiles with the baseline to demonstrate how its sensations were łblandž compared to all other stimulants. P1’s comment
illustrates what most participants felt with water: łI can feel theliquid moving but don’t feel (. . .) anything, it is probably warmed
up to my body heatž.
Menthol. In Figure 12, we depict the temporal profile of menthol.
To give the reader a sense of how varied and rich haptic sensations
with chemicals can be, contrast menthol’s temporal profile to water’s (baseline) profile. For menthol, the sensations felt most often
were łcoolž and, secondly, łtinglingž. Notably, łcoolž was felt almost
throughout the entire time. We also observed that cool sensations
arise much quicker than any other sensation, sometimes withinmere 10s of the application. Moreover, cool tens to fall over a longer
period of several minutes but decreases rapidly when not applied.
Participants comments clearly echoed these sensations, such asłthis is really blasting cold, wowž (P7) or łThis is a lot more intense(. . .) immediately feel coolž (P6). This suggests that menthol is astrong chemical haptics candidate for cool sensations.
Capsaicin. In Figure 13, we depict the temporal profile of capsaicin. the primarily sensations felt were łwarmž; closely followed
also by łburningž (which is also a łheatž-related descriptor), łstingingž, and łtinglingž. These sensations developed most linearly with

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Figure 12: For menthol most felt sensations were łcoolž and łtinglingž.

Figure 13: For capsaicin most felt sensations were łwarmž and łburningž.

Figure 14: For cinnamaldehyde most felt sensations were łstingingž and łpainful.ž
time and also reversed when the stimulus was removed. Theseobservations, alongside the most dominant sensation being choseas łwarmž point to capsaicin’s ability to target the hot temperature receptors. Moreover, participants’ comments echoed this, forexample, łThis one is super strong and long lasting. I’ll check burning because it is much more intense than warm nowž (P3) or łwith
device, it’s burning, when it was off it’s warmž (P1). This suggeststhat capsaicin is a promising chemical haptic candidate forwarm sensations.
Cinnamaldehyde. In Figure 14, we depict the temporal profile
of cinnamaldehyde. The sensations felt most often were łstingingž,and secondly łpainfulž or łtinglingž. Notably, łstingingž was felt
more often than any other sensation during stimulus, by allparticipants. Participants comments capture the clarity of this sensation łoh this is stingingž (P1) or łthis one is really intense. It feels
like I have a wound or something on my skinž (P2). This suggeststhat cinnamaldehyde is a strong chemical haptics candidate
for stinging sensations.
Methyl Salicylate. In Figure 15, we depict the temporal profile
of methyl salicylate. The sensation felt most often was łcoolž, butłwarmž, and łtinglingž were close seconds, as they were felt during
the application period in almost equal amounts to cool by the end of

Chemical Haptics: Rendering Haptic Sensations via Topical Stimulants

UIST ’21, October 10ś14, 2021, Virtual Event, USA

Figure 15: For methyl salicylate most felt sensations were łcoolž, łwarmž, łtinglingž.

Figure 16: For sanshool most felt sensations were łwarmž, łtinglingž, and łcoolž.
trials. A reminder that we chose this chemical precisely since neuroscience work had demonstrated its ambiguous actuating of boththe łhot and coldž receptors. Participants comments revealed thisambiguity, such as łit was cool, so that dominated (. . .) the initialintensity. But I felt the warm lasted longer, so that [is stronger]
time wisež (P7) or łthis is icy hot, I feel like it’s cool then warmž
(P3). This suggests that methyl salicylate is not a straightforward candidate for chemical haptics, due to its ambiguous
sensations.
Sanshool. In Figure 16, we depict the temporal profile of sanshool. The sensations felt most often were łwarmž,łtinglingž, andłcoolž. Notably, łwarmž was felt almost throughout the entire time
participants felt sensation and łtinglingž showed up consistently
when the stimulant was removed. After removal of the stimulus,
participants felt increasing levels of łtinglingž, possibly in contrast
to when it was applied. While it was chosen as cold or warm at theend of the trial by many participants it was also often chosen asłtinglingž during the time of the application. Moreover, participants’comments mentioned tingling more than warmth, for instance,łThis one is super light, but I think I feel a slight tinglež (P2) or łThis
is both warm and tingling, but I think tingling is slightly strongerž
(P3). This suggests that sanshool is a potential chemical haptic candidate for tingling or warm sensations (in our Study 2,
we showed that visuals from VR can resolve this to łtinglež).
Lidocaine. In Figure 17, we depict the temporal profile of lidocaine. The sensations felt most often were łcoolž, łtinglingž, andłdecreasing sensationž, which suggested some numbing was taking
place. In fact, the recorded sensations were more varied than water,suggesting that participants perceived richer sensations than in
the baseline. Participants comments suggested they felt numbing,such as łmy skin feels like rubberž (P6), "I don’t feel much, just a
numbing, (. . .) hard to say [as in to put it in words]ž (P7) or łI’m
feeling less touch, [. . .] yes, yeah, I feel numbž (P7, another repetition). Yet, most participants were not able to relate a decrease
in sensation to a numbing sensation. The challenge is that thissensation is not often occurring in everyday life, only occasionallywhen we receive analgesics. This result suggests that lidocaine
is a potential chemical haptic candidate for numbing sensations but might require additional cues to inform participants ofthe sensation, because a diminished sensation is hard to łfeelž.

7.5

Discussion & narrowing the set of final
chemical ingredients

This first study confirmed that our chemical stimulants display haptic sensations upon skin absorption. The strongest candidates werementhol (cool), capsaicin (warm) and cinnamaldehyde (stinging).
These were so remarkably strong in their haptic sensations thatthey were felt in this distinct way by most participants even without
any other stimulus (e.g., no visuals). Secondly, while more ambiguous, sanshool (tingling) and lidocaine (numbing) were promising inparticipants’ comments, which often indicated that these worked

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Figure 17: For lidocaine most felt sensations were łcoolž, łtinglingž, and łdecreasing sensationž.
as well. We decided to explore if the additional of visuals (from VR)
would assist participants in clarifying the sensations induced bythese chemicalsÐthis comprises our set of five chemicals we use in
the rest of the paper.

8

DEMONSTRATING FIVE CHEMICALHAPTIC SENSATIONS IN VIRTUALREALITY

Now, equipped with the set of chemical contenders for haptic sensations from our first study we designed five VR experiences, i.e.,
one VR experience per chemical. We present the design of these VR
experiences in detail, while in our second user study, we validatethat our chemical haptics adds immersion to each of these VR
experiences.
Electrical tingling with sanshool: lighting up the roomwith tesla coils. The user shoots electrical rays from a łportabletesla coilž on their arm as shown in Figure 18. Using these rays,
they light up lamps on the ceiling. When the ray shoots from theirarm, they feel as a tingling sensation, which our device renders byusing sanshool.
Stinging of a wound with cinnamaldehyde: finding the antidote to the poison. A flask of poison falls and drips poison onto
the user’s arm, depicted in Figure 19 (a). Thus, a wound develops,
and our chemical haptics device delivers cinnamaldehyde to thearm. This causes a strong stinging sensation. The user tries to findthe antidote amidst several flasks. When they finally find the antidote that cures the poison from affecting their arm, our device
stops cycling cinnamaldehyde over their skin and the sensation
fades away, as the antidote starts working.
Warming the face with capsaicin: exploring the nuclear
reactor overheating. In this VR experience users roam around a
nuclear reactor core, which is slowly heating due to a malfunction.
While the reactor heats up, users see and hear more steam coming
out of the core. Additionally, they feel this steam is hot as our device
passes a solution of 0.025% capsaicin in their cheeks; this scene isdirectly inspired from our walkthrough (Figure 5).
Cooling the face with menthol: catching snowflakes in
winter. In this VR experience, the users find themselves in a cold
forest, in the middle of a light snow fall, which is similar to the scene
depicted in Figure 3. Users can roam around the forest and collectFigure 18: (a, b, c) Users shoot electrical rays from their arm.
They use this power to turn on all the lamps in a dark warehouse. (d) The electrical sparks visuals are paired with sanshool stimulating the arm, which renders a tingling sensation.
points by catching falling snowflakes before they reach the ground;however, the longer the user stays out in the cold, the stronger
the cold sensation will be, which is delivered by our chemical haptics device by cycling a 10% menthol solution across the user’s
cheeks.
Numbing tactile sensations with lidocaine: defending
from the lasers. In this VR experience, users must defend themselves from incoming lasers from a rotating cannon by setting up
force fields around them, which they achieve via buttons on theirarm interface. However, when hit by a laser the arm interface starts
to visually malfunction and becomes transparent with its buttons
disabled, and our chemical haptic device starts cycling lidocaine.
This reduces haptic sensations on that arm. Then, the user needs toregenerate a new arm interface but this time on their dominant arm.

Chemical Haptics: Rendering Haptic Sensations via Topical Stimulants

UIST ’21, October 10ś14, 2021, Virtual Event, USA

9.1

Procedure

Participants tried all five VR experiences (from previous section,
each using one single chemical) with our device and without, ina counterbalanced order. Moreover, since the results of our firststudy suggested that water did not elicit distinct haptic sensations
compared to our set of chemical haptics, we opted to not include
water as a baseline in this study; this also allowed us to minimize
each participants’ study time and the overall complexity of the study
setup. The VR scenes were adjusted to ensure that the participants
experienced them for a minimum of three minutes, providing acomparable experience across participants. After each trial, we
asked participants to assess how immersive the scenes were using
a 7-point Likert scale and at the end of all trials we asked themwhich condition they preferred for each scene.

9.2

Figure 19: (a) In VR, users find themselves around flasks. A
bottle of poison topples and sprays their arm, inducing awound. (b) The user must find the antidote that will curethe wound. (c, d) However, most potions make the wound
worse. (e) The sensation of a stinging wound is rendered via
the application of cinnamaldehyde to the arm.

Participants

We recruited four participants (two self-identifying as female, two
as male, 30.5 ±5.5 years old). All participants had tried VR, but none
tried it in conjunction with haptics on skin. No participant reported
any allergies or sensitivity to the ingredients in our chemicals. With
consent of the participants, we videotaped their VR experience(by remotely controlling their Oculus Quest). Participants werecompensated for their time with 50 USD.

9.3

Results

Figure 21 depicts our main findings. We refrain from statistical analysis due to the low sample size, but we are confident these results

To achieve this, they need to tap several times on their malfunctioning arm interface, which displays a łregenerate on other armžbutton. This design allowed us to have users touch both arms, enabling them to feel the difference in tactile sensations. As revealed
by our first study, participants found it hard to understand the sensations from lidocaine because they are not arising but decreasing,a rare event in everyday life. As such, we hoped that this interaction
design, which offers a stronger visual explanation, might provide
lidocaine some opportunities for haptic effects (we validated thisin our second study and found that lidocaine did add immersion tothis scene).

9

USER STUDY: CHEMICAL HAPTICS IN USE
FOR VR

We conducted a user study to verify our hypothesis: our stimulants
will enable more immersive experiences in VR. During this study,
participants explored five distinct VR scenes with either five different stimulants or no stimulants as a baseline. Out of concern
of COVID-19, study participants were mailed a study kit and thestudy was conducted remotely over Zoom. These stimulants weredispensed using a desktop version of our device, i.e., same silicone
channels on the arm and cheeks but the pumps were laying on
the participants’ desks for simplicity (prevented participants fromtinkering with Bluetooth, LiPo batteries and fitting the electronics). The study was approved by our Institutional Review Board(IRB20-0290).

Figure 20: (a) In VR, users find themselves needing to defend
themselves against a laser cannon. (b) They try to use theirarm interface to generate the force fields, but it malfunctions, rendering their left arm holographic and glitching. (c,d) Instead, users must regenerate the controls to their rightarm to create the force fields to protect themselves. (e) Lidocaine stimulating the arm is used to render numbing for theholographic and glitching left arm.

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Figure 21: The results from Study 2: (a) immersion (1-7) per each interactive VR experience, and (b) interface preference.
indicate how our device added immersion to these VR experiences.
First, we observed an average immersion rating, across all trials (allVR experiences), of M=4.8 (SD=1.094) for chemical haptics, and of
M=2.3 (SD=1.238) for the baseline.
Immersion per-chemical. We analyze each VR application independently, which allows us to see how each individual chemical
improved immersion when compared to the baseline. First, in thełelectrical sparksž VR scene, participants rated their immersion as
M=4.4 (SD=0.8) with sanshool, and M=2.5 (SD=1.3) with the baseline; suggesting that it added immersion. Second, in the łwoundž
VR scene, participants rated their immersion as M=5.5 (SD=1.3)
with cinnamaldehyde, and as M=1.8 (SD=1.0) with the baseline;
suggesting that it added immersion. Third, in the łreactorž VR
scene, participants rated their immersion as M=4.5 (SD=0.6) withcapsaicin, and as M= 2.0 (SD=0.8) with the baseline; suggestingthat it added immersion. Fourth, in the łwinterž VR scene, participants rated their immersion as M=4.5 (SD=1.3) with menthol,and as M= 2.4 (SD=1.4) with the baseline; suggesting that it added
immersion. Last, in the łhologramž VR scene, participants rated
their immersion as M= 4.8 (SD=1.5) with lidocaine, and as M=3.0(SD=1.8) with the baseline; suggesting that it added immersion.
Again, the immersion of all experiences improved with our device.
This last result validated that numbing via chemical haptics adds
immersion.
Interface preference. Moreover, Figure 21 (b) depicts participants’ preferences of condition. All participants chose chemical
haptics for all scenes. While there is certainly a potential confoundfor a novelty effect, almost all participants did comment on negative
aspects of the complexity of setup or setup time but added at theend that łthis adds a whole new dimension to itž (P1 and similarly
P3).
Comments from participants. Participants comments provide a glimpse of their lived experienced with chemical haptics.
In the łwoundž VR experience, participants acknowledged stinging, for instance, łslight pain. . . and now it’s stinging. It comes inwavesž (P1), łthe chemical one, [. . .] even though it was painful it
was really immersive. It was really cool [not in temperature sense]ž
(P2), or łliterally looked exactly how it feltž (P4) or łoh it stingsa bit like the wound, wow... it does feel like a woundž (P3). In
the łelectrical sparksž VR experience, participants acknowledgedsome form of tingling, for instance, P2 stated łFelt [...] pulsing
kind of like in sync with the light flickering [. . .] and tingling too

a little bitž or P3 stated łit pulsates [. . .] that feels like electricaltinglež. In the łreactorž VR scene participants acknowledged sensations related to heat, for instance, łwarm, stinging, burning. . .
It was cooler. . . not in the sense of temperature but because it introduced a sense of urgency to the scenež (P2), ł[felt like a] spiceburnž (P1) or łthe sensations [from chemicals] made me feel more
like I was there with the steamž (P4). In the łwinterž VR scene,
participants reported feeling their arm as cold, for instance, łchillingž (P1) or łI can feel it’s coldž (P3). Finally, the łhologramž scene
evoked more numbing sensations than our previous study, where
lidocaine was applied in isolation from any explanation or visuals.
We observed comments such as łoh this is much better [than thebaseline, which was the previous trial], the weird [hologram] handalso feels weird, [. . .] it is feeling strange, I can’t put a name on
it, weird, like the [hologram] handž (P3), or łI don’t feel my armmuchž (P4).

10RECOMMENDATIONS FOR DESIGNING
WITH CHEMICAL HAPTICS

We now condense our recommendations, which we gathered fromour two user studies as well as our many pilot experiments anddesign sessions using chemical haptics.
1. When to use chemical haptics: We believe chemical haptics
is best suited for slow changing haptic events (e.g., how the stingof a wound evolves over the first few minutes or rising heat) rather
than rapidly changing discrete haptic events. While some chemicals are relatively faster than others, e.g., menthol, cinnamaldehyde
or capsaicin all act withing the first 30s of application, these arestill relatively slow events when considering fast-paced interactive applications. As such, for these types of applications, such asvideogames, we recommend using mostly these three sensations,which can be achieved quicker, but still for slowly evolving haptic
events (e.g., ambient temperatures, slowly evolving body conditions,such as the wound, tingling, and so forth). Prospective designers
can find inspiration in our VR experiences for how to use eachof these chemicals even in a game-like fast paced environment.
Lastly, for slowly evolving applications (e.g., a guided meditation
app that uses chemical haptics to lead users into slowly focusing
their awareness on different body parts) our approach is properly
paced.
2. Where to use chemical haptics: We recommend thatdesigners, wishing to explore chemical haptics, adjust theChemical Haptics: Rendering Haptic Sensations via Topical Stimulants

concentrations of their chosen chemicals for the body location.
This is important because epidermal thickness varies across different parts of our skin, for instance: the thickness of cheek skin isaround 38.8 micrometers while forearm is 60.9 micrometers [63]. As
such, thin skin regions, such as the cheeks, afford quick absorption,
which also provides opportunities for faster effects than that ofthicker skin regions, such as the arm or legs.
3. Which chemicals to use: In choosing a set of chemicals fora particular haptic design, we recommend that designers first prioritize chemicals by their dominant sensation, and only subsequently,
by their haptic complexity (variety of multiple sensations). The
results of our Study 1 can supply guidelines to choosing chemicals
based on their dominant sensation. As demonstrated by our Study 2,
when chosen by their dominant sensation, cinnamaldehyde (stinging), sanshool (tingling), menthol (cooling) and capsaicin (warming)
are clear choices. This study also illustrates one way that lidocaine
can be used, but it requires an additional łexplanationž to work,as this is not a readily recognizable sensation that occurs often indaily life (we describe how to design for numbing below).
4. How to ensure safety: We recommend that safety precautions should be taken at all times when handling or using thesechemicals. At high dosages, some chemicals might cause skin irritation and might exhibit variable effects across individuals [22, 29].
Additionally, epidermal thickness and other skin characteristics canvary across people [20, 57]. We minimized these risks by choosing
ingredients from off-the-shelf products (safe at their dosage). In ourstudies, we also made sure that participants did not have any skinallergies or sensitive skin before using our devices.
5. Haptics are contextual: We recommend that after choosing
a set of chemicals, haptic designers revisit this choice based on
their actual applications. As demonstrated by the comments werecorded from participants in our Study 2, the visual suggestions
provided by the VR experiences collapsed the variability of thechemical sensations that were recorded in isolation (from Study
1). For instance, while sanshool felt warm for some participants
when applied in isolation, it was perceived as tingling by the VR
participants when visually observing the electrical sparks on their
VR arm. This is similar to how if a VR wall displays electrical sparks,even a vibration actuator (which does not stimulate one’s skin withelectricity) might still feel realistic to many users [46]; in otherwords, haptics is contextual and chemical haptic designers shouldtake advantage of this to resolve any ambiguities.
6. Designing for reduced haptics (numbing): There is very
little literature in HCI that tackles designing for reduced sensations,especially those as strong as the numbing effect our lidocaine canachieve on skin. The key challenge in designing for lidocaine is that
this sensation is not often occurring in everyday life, only occasionally when we go to the dentist/doctors and receive analgesics
or when our arm łfalls asleepž (the latter is slightly different fromthe feeling of lidocaine as it also tickles). As such, we provide some
guidance for prospective designers using lidocaine to achieve thenumbing effect. The main insights are three-fold, (1) the situationmust involve touching the stimulated area, potentially multiple
times as one single event might not resolve the oddness of thesensation; (2) add a simple łexplanationž that users can build theirmental model on to understand the effect (e.g., in our VR scene thearm interface displayed a łarm malfunctioningž and appeared as

UIST ’21, October 10ś14, 2021, Virtual Event, USA

transparent, visually suggesting reduced touch); and (3) allow users
to compare the intensity of the sensation to a non-stimulated skinareaÐthis allows users to finally understand that the intensity oftouches in the two areas are very different, thus, allowing them toinfer their skin is numb; these three key insights are how we madethe lidocaine haptic effect from łnothingž (Study 1 results) to łI
don’t feel my arm muchž (Study 2 results). We recommend prospective designers to also take advantage of these three techniques aswell when designing for numbing.

11

CONCLUSIONSWe proposed, explored, and engineered a new class of haptic devices that provide haptic sensations by delivering liquid-stimulants
to the user’s skin, which we called chemical haptics. Our approachleverages the ability of a user’s skin to be chemically triggered
to render haptic sensations such as tingling, numbing, warming,
cooling, and stinging. We identified five promising chemical stimulants for use in our approach and engineered two wearable haptic
devices that pump these liquid stimulants through channels open
to the skin. Our devices demonstrate that chemical haptics can beachieved through wearable, self-contained devices and stimulate
various locations of skin.
In our first user study, we characterized the temporal profilesof sensations elicited by six potential chemicals. Using these insights, we designed five interactive VR experiences utilizing chemical haptics, one for each of our chosen chemical ingredients. Our
experiences centered around the haptic sensations of tingling (sanshool), numbing (lidocaine), stinging (cinnamaldehyde), warming(capsaicin), and cooling (menthol). In our second user study, we
validated that chemical haptics adds immersion to these five VR
experiences. Last, as the first work exploring the use of chemical
haptics on the skin, we condense recommendations to designers
for how they may employ our approach for their own interactive
experiences.
We believe our novel approach is unique to our community inthree ways: (1) generally speaking, it pushes the boundaries of haptics by emphasizing the role of sensations beyond just vibrationand pressure; (2) it offers a new pathway, via the skin’s chemical
receptors, for achieving multiple haptic sensations using a single
actuator, which would otherwise require combining multiple actuators (e.g., Peltier, vibration motors, electro-tactile stimulation); and,
finally, (3) it explores the use of a numbing agent, which was notpossible via existing haptic actuators. While our device is the firstto deliver a numbing haptic ingredient, the participants’ comments
in our user studies included a mix of references to actual łnumbingž
but also other ambiguous descriptions. Ultimately, we believe that
more research is still required to improve the quality of numbing
sensations; still, we hope this investigation inspires more research
into łnumbingž haptics.
Finally, we believe our work represents a first step towards empowering a larger set of HCI researchers to explore chemicallyactuated haptic sensations on the skin. Now that we know thatthe skin can be interfaced with liquid stimulants to create unique
haptic sensations, future research might want to investigate finer
details of these haptic sensations: transitions from one sensation toanother, new sensations by mixing compounds, direct comparison

UIST ’21, October 10ś14, 2021, Virtual Event, USAof these with other traditional approaches, and of course, using
custom-engineered chemicals.

ACKNOWLEDGMENTS
This work was supported by NSF grants 2047189 and 1746045. Any
opinions, findings, and conclusions or recommendations expressedin this material are those of the authors and do not necessarilyreflect the views of any funding agencies. We are grateful for the
feedback from our reviewers and device debugging support from
Romain Nith.

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