ThermalGrasp: Enabling Thermal Feedback
even while Grasping and Walking
Alex Mazursky*

Jas Brooks

Beza Desta

Pedro Lopes

University of Chicago

Figure 1: (a) We propose a new approach that enables thermal feedback devices to provide realistic temperature sensations while still allowing
users to grab or step on real objects—not only can virtual objects display a temperature, but users can also feel and grasp real objects, such as
props. Our approach is different from traditional thermal interfaces, in which Peltier devices are applied to the user’s hands to render thetemperature of virtual objects. Unfortunately, attaching a Peltier element and its cooling unit (fan and heatsink) directly to one’s palm preventsusers from also grasping real-world objects. Similarly, existing thermal interfaces cannot be applied to feet soles (as one would need to step on
the cooling unit). The result is that existing temperature interfaces are restricted mostly to virtual interactions (hands-free, no props). With
ThermalGrasp, (b) we explore a flexible thermal mechanism that allows the cooling unit to be moved away from the user’s palms or soles—
enabling them to grasp and step on objects.

ABSTRACT

1

Most thermal interfaces attach Peltier elements and their required
cooling systems (heatsinks and fans) directly to the palm or sole,preventing users from grasping or walking. To solve this problem,
we present ThermalGrasp, an engineering approach for wearable
thermal interfaces that enables users to grab and walk on real
objects with minimal obstruction. Our approach moves the thermal
device and cooling unit to areas not used in grasping or walking(e.g., dorsal hand/foot). We then use thin, compliant materials to
conduct heat to/from the palm or sole. Unlike traditional Peltierswith heatsinks, our thin materials enable grasping and walking onreal objects while enjoying thermal feedback. Using our approach,a user can, for example, grasp a passive prop (e.g., a stick that acts
as a torch in VR), yet feel its thermal state (e.g., hot due to its
flame). In our user studies, ThermalGrasp struck a useful balance
between thermal and haptic realism. We believe that ThermalGrasp
is a first step towards not forcing users to choose between either
feeling thermal feedback or being able to engage withgrasping/walking in interactive experiences.

Haptic feedback improves user experience in virtual reality (VR)and mixed reality (MR) by providing physical sensations thatcomplement the immersive audiovisuals. Haptics research has led
to a remarkable range of wearable haptic devices for delivering
tactile (e.g., vibration [1], [2], [3], pressure [4], skin stretch [5], [6])and force feedback (e.g., exoskeletons [7], [8], [9], [10] or electricalmuscle stimulation [11], [12], [13]).
More recently, researchers explored stimulating senses beyond
forces and tactile cues, such as providing thermal feedback [14],[15]. At the same time, researchers also demonstrated thecompelling use of real-world props as proxies for virtual objects
and virtual terrains because they provide cheap yet hyper-realistic
haptic feedback [16], [17], [18].
Unfortunately, wearable thermal actuators and props are at odds.
This is because typical thermal actuators are inherently bulky or
fragile; thus, applying wearable thermal actuators to the user’s
hands or feet prevents users from grasping or stepping on physicalobjects (e.g., props or terrains). For example, some recent thermal
devices create hot and cold sensations by pumping water throughflexible tubes worn by the user [19], [20], [21]. While the tubes’
flexibility allows them to conform better to the body, this is also
their limitation: any kink in the tubing cuts off the liquid flow andhalts the thermal sensations, which happens when a tube iscompressed, like if gripped or stepped on.

Keywords: thermal feedback, haptic feedback, virtual reality,
wearables.
*e-mail: alexmazursky@uchicago.edu

INTRODUCTIONAs such, the most popular approach to wearable thermal
feedback is still the Peltier element [22]—a thermoelectric material
that, when powered, heats up on one side while cooling down on
the other. This makes them appealing for wearables because of theirsimple electrical control and lack of moving parts or fluids. Morerecently, even emergent flexible Peltier materials have begun
development [23] (price is still prohibitive at 100x the cost of arigid Peltier, and their flexibility is limited to <12 repeated folds orvery small angles). Despite the benefits of Peltiers, to sustain both
hot and cold sensations (especially lasting more than a few seconds,
e.g., rendering temperature of objects and terrains), Peltierelements, both rigid and flexible, require additional cooling fromheatsinks and fans to remove excess heat from the hot side [24].
Without these bulky cooling units, Peltier elements quickly
saturate, i.e., heat leaks from the hot side to the cold side—cascading into a positive feedback loop of undesirable heat.
This requirement for cooling units prevents Peltier elements frombeing applied to body parts that contact real objects, such as theuser’s palms or soles. In fact, interactive systems based on attaching
Peltiers and their heatsinks to users’ palms and soles will not allow
users to grasp objects nor walk on top of surfaces.
To tackle this challenge, we introduce ThermalGrasp, anengineering approach for wearable thermal interfaces that enables
users to grab and walk on real objects with minimal obstruction. To
realize this, our approach moves the Peltier and its cooling unit toareas not used in grasping or walking like the top of the foot or backof hand. We then use thin, compliant materials to conduct thePeltier’s heating or cooling to the palm or sole. Unlike existing
approaches, our thin materials enable grasping and walking on real
objects while enjoying thermal feedback. For example, Figure 1
depicts a user walking in a VR desert at dusk, feeling both the
sand’s texture and the associated (virtual) decrease in temperature.
2

OUR APPROACH: THERMALGRASP

ThermalGrasp devices consist of three components, depicted in
Figure 2: (1) a thermal element (typically a Peltier but others are
possible), (2) a thermally-conductive yet flexible material (e.g.,heat pipes, sheet metal, etc.), and (3) insulation (e.g., foam, plastics,etc.). The thermally-conductive materials contact the surface of thethermal element (in this case, a Peltier element) and channel theheat flow. Thus, when the Peltier element warms up, heat conducts
through the conductive material. By wrapping the channel ininsulation, we improve the heat transfer efficiency. The user feels
heat at the correct location because we expose the target area’s skinto the conductor, while insulating all its other surfaces. Similarly,
when the Peltier is cooled, heat is conducted away from the user,
giving the sensation of cooling.

bleeds into the cold side, cancelling out the cooling effect [29].
While brief temperature changes can be effective for feedback like
notifications, they cannot provide realistic temperatures needed torender the sense of walking on a gradually cooling floor in VR (e.g.,
stepping on desert sand that cools as the sun sets, which wedemonstrate in Study 2). Again, even flexible Peltiers require added
cooling to hold cold temperatures [30], [31].
As depicted in Figure 3, our technique balances thermal realism
and haptic realism from props. While placing a Peltier directly on
the palm/sole maximizes thermal realism (sensations are at thepalm/sole), one cannot step on or grasp objects with cooling units
directly on the palm/sole. Conversely, one could try to place thePeltier element on the backside of this limb to keep the palm/sole
free (as in [32] or tactile devices that leave the palm free [33], [34]);
however, the thermal realism is now drastically reduced because
the sensation is felt at wrong location (opposite-side of palm/sole).
Instead, our approach delivers thermal feedback at the palm/solewhile only minimally impeding upon real-world interactions,
thereby optimizing for both thermal and real-world haptic realism.

Figure 3: ThermalGrasp balances the realism of virtual temperatures
and the realism of haptic props by conducting temperatures from
Peltiers using flexible conductive channels, which enables to step or
grasp at the application point.

To strike this balance, our approach relocates the bulky yetnecessary cooling hardware away from body parts involved withgrasping or walking, such as the palms of our hands and the solesof our feet. Then, using robust yet flexible & thin thermal
conductors it transfers thermal feedback to the center of thepalm/sole. These flexible materials can withstand large forces andwrap around the body—enabling thermal feedback while graspingor walking.
Finally, we tend to think of the devices that we engineered not as
end-products but as artifacts of the ThermalGrasp approach—these
depict some instantiations of our approach. In the process ofcreating these devices, we created four additional variations using
different types of conductors, which are depicted in Figure 4.
Namely, conduction using a copper mesh, a copper netting, acopper tape, and a silicone doped with liquid metal (EGaIn). Whilein our early technical pilots we found these to have reduced thermal
efficiency compared to our final devices using thin flexible copper
sheets, these can still be used or serve as a source of inspiration for
future thermal devices that also allow grasping and walking.

Figure 2: The key technical elements that comprise our approach,i.e., flexible thermal conductors to transfer heat.

ThermalGrasp’s approach is different from the traditional
approach (which we map in Figure 3)—placing Peltiers directly on
the user’s palm/soles to deliver thermal feedback (e.g., [25], [26],[27], [28]—to cite a few palm devices). Unfortunately, without
cooling hardware (heatsinks and/or fans), Peltiers cannot provide
sustained cooling for more than a few seconds before residual heat

Figure 4: We also explored other conductors: (a) copper net, (b)copper mesh, (c) copper tape, (d) silicone doped with liquid metal.

3

RELATED WORKThe work presented in this paper builds on the field of wearable
haptics, with emphasis on devices that allow users to also feel realworld sensations and devices that render thermal feedback. Finally,
we succinctly review prop-based passive haptics in VR since these
are the key application domain for our technique.
3.1

Wearable thermal feedback = attaching thermal
actuators to target area
Researchers have been exploring how to miniaturize thermal
actuators into wearable form factors. The most common approachto this challenge has been commercially-available thermoelectric
Peltier elements. When electrical energy is supplied to a Peltier, it
acts as a heat pump, transferring heat from one side of the device
to the other—one side cools while the other heats, depending on thecurrent’s direction. This simple electrical control has made Peltierelements easy to deploy in wearable form factors, as shown by
ThermoVR [35], LiquidReality [36], and TherModule [37].
However, while Peltier elements are inexpensive and easy tocontrol, they pose disadvantages when applied directly on theuser’s hands and feet: (1) Peltier elements tend to be thick (~4-6mm
typically); (2) Peltier elements are typically rigid (they do notconform well to the body); and more importantly (3) Peltierelements tend to require additional cooling (e.g., large heatsinks
and fans). While some wearables have used Peltiers without fansand heatsinks, these are limited to quick notifications [14], [15],[22], and cannot sustain realistic temperatures that are desirable forVR. This is because without cooling, Peltiers can only reliably
perform cooling for a few seconds before residual heat from the hotside bleeds into the cold side, which cancels the cooling effect [29].
As such, most interfaces that aim to reproduce the thermal
properties of touching an object use dedicated cooling hardware[38], [39]. This limitation has prevented thermal feedback frombeing applied, for instance, to the sole.
While recent research has led to the advancement and initialcommercialization of flexible Peltier elements, these too requirefans/heatsinks to deal with their residual heat during/after cooling.
Take the Asahi Rubber DK-TEM es-02 [40] flexible Peltierelement: its thermal performance was still characterized whileattached to a fan and heatsink (3x the area of the Peltier). Moreover,flexible Peltiers remain both expensive and fragile. For example,[40] currently costs $240 a unit (100x a normal Peltier) and has amaximum bending angle of just 115 degrees (i.e., 50 mm bend
diameter), which is smaller than even simple human joints, likeclosing the palm (which folds over itself when making a fist [41])or curvatures like the arch of the foot (bending angle of 152 degrees
while standing [42]).
As an alternative, other types of thermal elements have been
explored, such as nichrome wire and conductive fabric [43], [44],[45], which provide resistive heating in a thin form factor but
cannot create cooling sensations, thus missing half of our spectrumof thermoception. Some research has instead examined pumping
hot and cold fluids for feedback [21], [46], [47]. For example,
HydroRing [20] delivered temperature-controlled liquids to thefingerpad. Similarly, Therminator [19] developed a hydraulicsystem for thermal feedback on the arm in VR. Finally,
ThermAirGlove [48] presented a pneumatic glove for grasping
virtual objects to feel their thermal properties. Thus, while theseapproaches address the rigidity downside of Peltier elements, theycome with their own challenges: (1) putting moderate forces on a
tube will stop the liquid flow (e.g., if one steps on it, it halts thethermal feedback); and (2) temperature-controlled fluid tanks
have yet to be made wearable. Faced with the power consumption
challenges, others have instead turned to using liquid chemicals to
induce illusions of temperature change by chemically stimulating
skin receptors to trigger pseudo-temperature changes [49], [50]—still, these chemical approaches are at an infancy (e.g., slow andcoarse) compared to expressive and time-tested thermal feedback
from Peltier elements and other devices.
In addition to each approach’s individual limitations, all theseapproaches share a conceptual limit: they all apply some form of
thick thermal actuator (be it a Peltier or a tube) directly to the areaof the user’s skin at the target they wish to stimulate. Here, we
observe a conflict with our original goal: to provide thermal
sensations yet minimize obstruction to the target area. The closest
approach to tackle this is generating phantom heat sensations at the
midpoint between two thermal elements [51], [52]; while we findthis approach promising and draw inspiration from it, it is limitedin that, (1) the thermal sensations may be confusing—in fact, tomeasure the illusion, the “author instructed the participants toindicate the strength of the sensation perceived on their fingerpad
regardless of their perception on the finger side” [52]—users will
feel a spatial mismatch since the sides are hotter than the illusion
point, and (2) the thermal actuators need to be close to the pointof the illusion (e.g., 14 mm; so close that they only add actuators
to the side of the fingers in [52], not larger body parts).
Solving this challenge of enabling thermal feedback in
conjunction with grasping and walking has, to the best of ourknowledge, never been addressed. As such, we looked for
inspiration in how researchers have approached this challenge inother haptic modalities, namely those that attempt to provide tactile
sensations while keeping the user’s hands free.
3.2

Wearable strategies towards haptic feedback
while preserving real world sensations
This tension in preserving haptic sensations while interacting with
objects is not unique to thermal feedback; in fact, recent researchin tactile feedback has focused on engineering devices thatminimize encumbering the sensitive areas we use for manipulation,
especially the hands [53], [54], [55], [56]. We cluster thesestrategies, including examples of thermal instantiations, in three
categories: (1) relocated actuators; (2) foldable actuators; and (3)thin actuators.
Relocated actuators. One way to preserve tactile sensations is
to move the actuator away from the area that will contact anobject—this approach is often referred to as relocated haptics [57].
The key concept is to still deliver a haptic sensation but not in thelocation where it is expected to happen. For example, Ando et al.
[33] and Haplets [34] placed vibrotactile actuators on the fingernail
to render feedback that should be felt on the fingerpads. Anotherexample, Tasbi [58], moved the actuators even further from thehand by placing them on the wrist. This idea of relocation has been
applied to thermal feedback. ThermoFeet placed Peltiers on thedorsal side of the foot to provide directional cues [59]. Altered
Touch placed its Peltier element on the nail to give thermal
feedback while leaving the fingerpad free [32]. Similarly Sato et al.
placed its Peltiers on the sides of the fingerpad [52]. As a result,these haptic devices excel in minimizing tactile obstructions(hand/sole free to grasp/walk) but sacrifice this for realism, i.e., thesensation occurs in a location where it is not expected—in fact, tomeasure this illusion of thermal feedback created from actuators
away from the fingerpad, in [56] the “author instructed theparticipants to indicate the strength of the sensation perceived on
their fingerpad regardless of their perception on the finger side”.
Foldable actuators. A second approach is to use an actuator thatprovides the haptic effect only when it is necessary but then tucks
away while not in use. Examples include a wrist-mounted actuatorthat taps the palm on-demand [60] and a nail-mounted actuator thattaps or warms up the user’s fingerpad on-demand [61].
Unfortunately, these approaches are only suited for touching either
virtual or real objects, but do not allow users to touch both at theFigure 5: The user immersed in our VR desert survival experience interacts with haptic props such as sand, logs, grass, and water while feelingthermal feedback.

same time—this folding approach does not support augmenting aphysical object (such as a VR prop) with thermal sensations.
Thin actuators. The third strategy is to balance virtual and real
feedback by placing an actuator on the skin area that interacts with
objects but making it as thin as possible, so that it impedes tactile
sensations as little as possible (also referred to as “feel-through”[62], [63]). Examples include an electrotactile device forstimulating the fingerpad via a thin film [63], a latex ring forpresenting pressure, vibration, and temperature in mixed reality[20], and hand-worn air bladders that can be in/de-flated on demand[61]. Using this approach, the user does not have full tactile acuity
but can still feel objects through the thin actuator [62], while also
enjoying haptic effects. With respect to thermal feedback, thisapproach has only been explored by recent flexible Peltier devices[23], [64], [65], which again still require cooling to givesustained/realistic feedback, have limited flexibility, and remain
expensive [40] (see section 3.1). ThermalGrasp is directly inspiredby the latter by proposing a technical approach that places a thinthermal conductor that allows the user to feel thermal feedback andstill grasp or walk.
3.3

The importance of physical cues even in VR (i.e.,
props as highly realistic haptic feedback)In recent years, the use of physical props as proxies for virtual
objects has grown popular because they provide cheap yet hyperrealistic haptic feedback [16], [17], [18]. Key to the rise of haptic
props is that they can be low-tech; they don’t necessarily require
expensive actuation or wearables and can be made from readilyavailable materials (wood, plastic, etc.), yet they can provide
sensations very close to their real-world counterparts, such as theweight of a bat [66] or tool [67]. Often, these sensations cannot be
produced by common haptic actuators, such as the vibrotactile
motors in VR controllers [66]. For example, Franzluebbers et al.
found that replacing VR controllers with a golf club prop waspreferred by participants and even led to improved VR golf
performance [68]. In our work, we strive to allow thermal interfaces
and prop-based experiences (VR experiences that rely heavily on
grasping and walking) to finally work together.
4

WALKTHROUGH

To help readers understand how ThermalGrasp can present thermal
feedback while enabling grasping and walking, we demonstrate it
in a prop-based VR desert experience; the user’s goal is to survive
the night by staying warm. They experience this via physical props,
such as tracked handheld props (e.g., wooden logs) or terrain props
upon which they can walk (e.g., sand, artificial grass, and a puddleof water). Users wear four ThermalGrasp devices, one on each handand foot (see Figure 5a).

Figure 5ab depicts that as the sun sets, the user feels the sandbeneath their feet getting cold —using ThermalGrasp, the user
walks on the real sand while feeling the cooling sensation from thevirtual world—this is an example of our how approach improves
thermal realism (as shown in Study 2). Walking on sand is possible
because the footworn Peltiers and their required cooling units on
top of the user’s feet (dorsal side) conduct heat to the soles of thefeet via thin, flexible copper sheets.
Now, users must survive the desert night by finding a heat source.
Figure 5c-d depicts the user collecting wooden logs into a bonfire—
their hands grab haptic props (e.g., the wooden logs) and feel the
prop’s realistic texture. The user even steps into shallow water tograb a log, which would not be safe with Peltiers on the bottom of
the feet, as these could short-circuit.
As the user places logs in the fire, the wind blows out the bonfire,
scattering the burning logs. The user must now find a new heatsource. They search throughout the terrain, while feeling cold on
their soles and palms. Finally, they find a scattered log still burningand repurpose it as a torch, as shown in Figure 5e. Upon grabbing
the torch, they feel the prop as warmer because the thermal device
on the grasping hand heats up due to the virtual flame—an exampleof how ThermalGrasp allows users to feel the realism of the haptic
prop alongside the accompanying thermal sensation. Finally, as the
sun rises, the user feels the sand increase in temperature, indicating
they survived the desert night.
5

BENEFITS, CONTRIBUTIONS AND LIMITATIONSThe key contribution of our work is a new method for providing
thermal feedback (both hot and cold) in a form that still allows forinteracting with real-world objects, like grasping props or walking
on terrains. The benefit is that this approach (1) relocates bulky
yet necessary cooling hardware to unobtrusive locations, suchas the back of the hands and top of the feet and as a result, (2) allows
users to feel hot and cold sensations and still grasp, walk on,
and feel real-world objects, which is relevant to many interactive
domains, such as prop-based VR or AR.
Our approach is not without limitations. First, our approach
naturally requires more energy than adding a heat source directlyon the target area to reach the same temperature; this is because we
conduct the energy over a distance, which requires a thermal
gradient where the temperature at the source is more extreme thanat the skin. Similarly, because it takes time for the heat to transfer
over the distance from the heat source to the target area, ourapproach is slower than directly placing a heat source on the target
area (e.g., a Peltier which can heat and cool at 3⁰C/sec [15] vs. ours
which is <8x slower at cooling and <6x slower at heating as shownin our device characterization). While our primary goal isdeveloping thermal devices that enable walking and grasping, wemitigate this lag as much as possible by using heat pipes with higheffective conductivities and insulation to reduce losses. That said,
we characterized the time to generate a feelable sensation in Study
1 and found that it fits several applications well where slower
thermal feedback is realistic (such as radiative heat from fire,gradual temperature changes as the sun sets and rises, and soforth—such as our application for Study 2). This is consistent withfindings from prior work on slower thermal feedback [69].
However, these are limitations as much as they are tradeoffs, as ourapproach balances thermal realism with the ability to grasp andwalk on surfaces. Finally, because our approach is designed aroundfeeling real-world objects, it is important to note that these objects
also have their own temperatures, which interact with thetemperatures presented by ThermalGrasp, which may influence
perception.
6

IMPLEMENTATION

To help readers replicate our approach, we now provide thenecessary technical details. Furthermore, we provide all code and
CAD files of our designs to accelerate replication so that others may
create devices based on this approach1.
6.1 Materials
Our ThermalGrasp devices consist of three basic components:
hot/cold sources, conductors, and insulators.
Hot/cold sources. For hot and cold sources, we use Peltierelements to be able to render both hot and cold (though ourprinciples of guided conduction would hold for guiding heat fromhot/cold liquids, etc.). Moreover, to achieve these realistic,
sustained cold sensations, Peltiers require cooling units—we too
use a heatsink and fan per Peltier.
Conductors. We use thermally-conductive materials to transfer
heat between the thermal element and the user. There are threefactors that affect heat transfer in our conductors: (1) materials’
conductivities (higher is better); (2) source-sink distance (shorter isbetter); (3) cross-section (larger is better). Thus, we aim to leverage
all factors in our final design to minimize power requirements andimprove performance.
We explored various metals and thermally-conductive polymers
as conductive channels [70], [71]. Notably, copper boasts a highthermal conductivity (k=~400W/(m·K)) among metals while being
low-cost. Thus, we centered our explorations around copper.
Specifically, copper heat pipes are the key to enabling ourtechnique to work across long distances. Commonly found within
computer cooling systems, copper heat pipes consist of a sealed
copper tube containing water and wicking material. When one endof the heat pipe is heated, the water vaporizes and naturally travels
to the cold end due to the pressure gradient, where it then condenses
back into a liquid, releasing the latent heat. The water then returns
to the hot end of the pipe via capillary action where the cycle
repeats. Notably, this phase change is entirely passive and selfcontained (no pumps or fluid in/out)—this gives copper heat pipes
an effective thermal conductivity of ~500x that of an equivalent
piece of solid copper [72]. While heat pipes are not intrinsically
flexible or thin, they are malleable and can be bent into custom
shapes. However, heat pipes alone are not sufficient for condensing
the bulkiness of Peltier elements into minimally obtrusive thermal
actuators. We use them to transfer the heat as far as possible without
impeding the user and then transfer the heat into a thinnerconductor, such as copper sheet metal. To ensure effective heattransfer at all material interfaces, we add thermal paste.

1lab.plopes.org/#ThermalGrasp

3D files, evaluation scripts)(software, firmware, schematics,

Insulators. Insulation is critical: (1) insulation around thechannels reduces losses to the environment as heat travels throughthe conductive materials, thus boosting our device’s efficiency, and(2) insulation prevents the user from feeling thermal sensations at
undesired locations. For example, if we wish to cool the palm, we
can insulate the conductors at all points except for the desired pointof contact with the skin. Our devices use two types of insulators:
rigid plastics (such as PLA, k=~0.13W/(m·K)) for giving structure
to the devices and soft foams (such as neoprene, k=~0.05W/(m·K))
which leverage closed air cells as an effective insulator for
comfortably interfacing with the user [73], [74].
6.2 Fabrication
We fabricated devices for the palm of the hand and the sole of thefoot, as shown in Figure 6. In both designs, we 3D print a formfitting shell that attaches to the dorsal side of the hands and feet.
Peltier elements are placed within the shell and springs are used topress the Peltier’s surface to the heat pipes, along with a layer ofthermal paste for improved heat transfer. The heat pipes connect tosheet metal that wraps around to the opposite side of the hand/foot.
To ensure good contact with the skin, the tightness of the device
can be adjusted by sliding the heat pipes up/down in their slots.
Additionally, we explored three strategies (all possible) to attach
our devices to the body (in order of increasing contact reliability):(1) spring-loading the flexible sheet metal against the foot by
enforcing a tight bend radius; (2) adhering the conductor to the skinwith skin-safe glue; and (3) wrapping fishing line from the tip ofthe conductor around the body and anchoring it to the 3D-printed
shell. For reliability and ease of donning/doffing, we evaluate
designs using the fishing line approach.
Specifically, each design uses the following components: Peltierelement (foot: TEC12706, hand: CP60231H), flat copper heat pipes(foot: 70W 11.2 x 3.5 x 100mm, hand: 60W 8.3 x 2.5 x 70mm),
copper sheet metal (0.2mm thickness), soft neoprene foam (3.2mm
thickness, easily compressible), heatsinks (foot: 40 x 40 x 12mm,
hand: 20 x 20 x 10mm), fans (foot: 24V, hand: 12V), and 3Dprinted shells1. The foot-worn device weighs 145g and the handworn device weighs 34g.

Figure 6: (a) Exploded view and fabricated devices for the (b) hands
and (c) feet.

6.3 Sensing, electronics, and PI-controllers
All our wearables feature temperature sensors (100kΩ NTC 3950
thermistors) at the point where the Peltier contacts the heat pipe andwhere the sheet metal contacts the user—allowing it to monitor thethermal gradient in real-time.
All devices share the same hardware. The Peltier elements are
driven by a VNH3SP30 motor controller, which is controlled by an
ATmega2560 microcontroller. To combat saturating the Peltierelements in the cooling condition, we use two separate PI tunings

for heating (P=75; I=3) and cooling (P=200; I=8). Each controller
receives only the temperature at the flexible sheet metal, while thePeltier temperature is used as a safety feature. To achieve rapid
temperature change, we use bang-bang control when outside thegoal temperature by more than 1⁰C in cooling and 2⁰C in heating,and the PI controllers when within range of the goal temperature.
The fans are triggered programmatically via a MOSFET(RFP30N06LE) when the temperature exceeds the goal. At peak
power, one of these devices consumes 25W, which we power via alarge LiPo battery worn by the user in a slim backpack.
6.4 Characterizing the performance of our devices
To characterize the performance of our devices created with the
ThermalGrasp approach, we tested the heating and cooling
performance while wearing the devices.
Temperature stability. We tested the heating and cooling
performance on the example of the device for the sole of the foot,as shown in Figure 7. First, to determine the cooling performance,
we drove the worn foot device at full power (25W) for five minutes.
As shown, the temperature at the point of contact with the footdecreased from 25⁰C to 20⁰C. In the following five minutes, ourcontroller drove the Peltier element to heat and maintain 40⁰C.
Finally, to demonstrate the system’s stability, we drove the Peltier
for five more minutes to cool from 40⁰C back to room temperature.
Thermal camera images (FLIR C3) were taken of the device’s
externals to determine the effectiveness of the neoprene insulation.
As shown, temperature change was observed at the surface of theinsulation (22⁰C at end of cooling, 33⁰C at end of heating).
Considering the Peltier’s temperature was 16⁰C and 52⁰C at thesetimes, heat leakage is expected due to the high thermal gradient.

Figure 7: Performance of our ThermalRedirect device for thermal
feedback on the sole.

Rate of change. Figure 8 shows temperature over time where
our devices contacted the skin. Each device was driven at fullpower (25W, starting from Peltier at room temperature) for 35
seconds. Moreover, to assist the reader in interpreting thisperformance characterization, we also show the results of our Study
1, in which we calculated the point at which the average participant
felt a temperature change (hand heating and cooling: 11.3 sec, 12.3
sec; foot heating and cooling: 25.2 sec, 16.1 sec).

Figure 8: Heating and cooling curves for our (a) hand and (b) footworn devices.

It is important to note that while this approach is not as fast as a
Peltier directly applied to the skin (i.e., these rates are less than
3⁰C/sec exhibited in related works [15]), they are consistent withprior works in which speed is not the goal, yet effective feedback isachieved (e.g., [69]). Instead, our approach generates perceivabletemperature changes while still allowing us to grasp and walk on
surfaces. Despite this strategic tradeoff, Study 2 demonstrates thevalue of this approach for conveying realism in VR.
7

USER STUDIES

We conducted two user studies to validate our technique and its
implications in interactive applications, specifically in the case of
VR. Our studies were approved by our institution’s ethics
committee (IRB ID anonymous for review).
In our Study 1, we evaluated ThermalGrasp’s performanceby first determining if our devices could effectively transfer thermal
feedback to the desired location (by sketching the border of
sensation over a blank anatomical diagram [75]). Then, wecharacterized how long it took participants to notice temperaturechanges while wearing our devices on their hands and feet. The
result of this study demonstrated that our approach effectivelytransfers hot and cold sensations to the user at the target location(comparable to placing a Peltier directly on the target) and isperceivable within an interactive timeframe.
In our Study 2, we compared the participants’ sense of realism in
a VR task involving both real-world and thermal sensations. We
found that ThermalGrasp led to improved realism and sensory
engagement in VR.
7.1

Study 1: evaluating ThermalGrasp’s thermal
performance
Our first study focused on characterizing ThermalGrasp’s ability toprovide thermal feedback. This study assessed (a) the areas ofperceived stimulation for both Peltier devices placed on the top andbottom of the foot, along with our foot-worn ThermalGrasp device;and (b) the time it took for the temperature change to beperceivable. Both metrics were evaluated using standardized study
designs from psychophysics literature [75], [76].
7.1.1 Study 1a: location of thermal stimulation
We hypothesized that ThermalGrasp provides stimulation closer tothe target area compared to placing a Peltier on the dorsal side ofthe limb but underperforms compared to a directly Peltier on thetarget area.
Location = foot. We chose to evaluate the foot (rather than thehand) because it represents the more challenging case for existing
approaches: (1) virtually no thermal devices can be applied on thesoles, since the user cannot step on top of Peltiers and their coolingsystems or would otherwise stop the thermal flow for tubes withhot/cold fluids; and (2) the distance from top of the foot to the soleis greater than that of the hand, which pushes ThermalGrasp to its
limits more than if we had tested the hand.
Conditions. Participants experienced three interface conditions(in randomized order, across all participants): (1) direct-peltier(wearing a traditional Peltier along with heatsinks and fans applied
directly to the target area of the sole, the same TEC12706 Peltier as
incorporated into the ThermalGrasp design), (2) dorsal-peltier (aPeltier applied along with heatsinks and fans to the top of the foot),and (3) our ThermalGrasp device (a Peltier on top of the foot, heattransferred via a thin conductor that wrapped from the medial sideof the foot to the sole). Thermal contact areas were held constantacross all three interface conditions (56 x 27.5mm).
Participants. We recruited eight participants (two identified as
female, five as male, one as nonbinary, with an average age of 24.5
years old, SD=1.73). No participants had prior injuries on their feet.
Participants received $20.

Apparatus. Participants sat on a chair in front of theexperimenter. They could comfortably rest their leg on a stool.
Participants were blindfolded so that they could not see the stimulus
applied to the foot, nor the thermal device. Participants experienced
a cold (20⁰C) and hot (40⁰C) temperature for each condition. The
temperature over time was PID-controlled to follow the same curve
for all conditions.
Procedure. When participants felt the 20⁰C and the 40⁰C
stimulus, they were asked to indicate the area where they felt
thermal sensations for each stimulus and the point where thesensation was most intense. Participants were provided with adiagram of the foot (sole and dorsal side) and could draw freely;
this is a standard method used by prior works [75]. Finally,
participants rated the intensity of the thermal sensation (1-7 scale).
Results. Figure 8a depicts our main findings regarding theparticipants’ thermal acuity. For both hot and cold stimuli, dorsalpeltier resulted in the area the furthest away from the sole, which
implies that dorsal-peltier doesn’t present thermal feedback to thesole of the foot. Conversely, direct-peltier exhibited the smallest
area spread and this area was the closest to the target area (morethermal acuity). While these were expected, it was unknown how
ThermalGrasp would compare to either of the baselines. We foundthat ThermalGrasp sat between these conditions, exhibiting lessthermal acuity than direct-peltier, but more than dorsal-peltier.
This validates our hypothesis. While all participants reported
temperature change on the sole during the ThermalGrasp condition,
five participants reported sensation on the side of the foot during
heating, indicating some heat bleeding through the insulation.
Finally, we found no statistical difference between the intensity
ratings across all conditions (p=0.24 for hot; p=0.15 for cold
stimuli), which suggests that our apparatus was functioning
robustly (all conditions followed the same temperature curves as
dictated by our PID controller).

Figure 9: (a) Data (all participants) indicate that participants felt both
hot and cold sensations in the desired location of the sole of the footin the ThermalGrasp condition. (b) Time to notice the change intemperature for both our foot and hand device for heating andcooling. Error bars show standard deviation.

7.1.2 Study 1b: time to perceive thermal stimulation
Having determined that ThermalGrasp could accurately present
thermal feedback at the desired locations, we now aimed to find
how long it takes participants to experience its temperature
feedback.
Conditions. Participants experienced ThermalGrasp on either
their dominant hand or foot (counterbalanced).
Participants. We recruited eight participants (three identified as
female, five as male, with an average age of 25.1 years old,
SD=2.35). No participants had prior injuries on their feet.
Participants received $20.
Apparatus. Participants comfortably rested their foot on
artificial grass while holding a wooden dowel rod.
Trials. We used two types of trials: (1) real temperature, in which
our device was actuated to maximum heat/cool, and (2) placebo
temperature, in which our device was not actuated and stayed atskin temperature. Participants experienced a total of 24 trials (4xrepetitions of 3x sensations (cooling/heating/placebo), on 2x bodyparts (hand/foot))
Procedure. Participants were notified when a trial began. Afterthe experimenter started the trial, a random delay (0-10 sec) was
implemented before power was supplied to the device’s Peltier.
Participants were instructed to confirm using a keypad upon feelinga change in temperature. We recorded the time difference frompowering on to the time participants indicated the temperaturechanged. If participants could not perceive a change in temperature,they were instructed not to press any keys. If a participant did notindicate a change in temperature, the trial would conclude after 45
seconds. Importantly, we purposely included placebo trials inwhich no stimulation was presented to ensure that participants were
not just pressing confirm without no real sensation. Finally,
between trials, we waited for the device to return to skintemperature (measured at the sheet-metal) and room temperature(measured at the Peltier).
Results. Figure 8b depicts how long it took participants to noticea change in temperature. For the hand, we found it took on average
11.3 sec (SD=2.9) to notice heating and 12.3 sec (SD=3.3) to noticecooling. For the foot, we found it took on average 25.2 sec(SD=7.3) to notice heating and 16.1 sec (SD=3.2) to notice cooling.
Discussion. We found that participants could feel sensations inunder 30 seconds despite transferring the heat over a distance.
While this delayed feedback is a limitation for certain applications,
it is also exactly what enables being able to walk and grasp while
wearing thermal devices. Using these findings, we hypothesizedthat for interactive applications in which both real-world andthermal sensations are required, our technique will provide users
with an improved experience. In our next study, we investigatedthis with the example of prop-based VR.
7.2 Study 2: Realism in VR
While our first study examined the psychophysics aspects of ourapproach, our second study focused on observing our approach in
an interactive application. Specifically, we assess the extent towhich our approach (which allows feeling or walking on props andfeeling virtual thermal sensations) influences the sense of realism
in prop-based VR experiences.
Our main hypothesis for this study was that ThermalGrasp
would feel more realistic than a baseline without our devices.
Moreover, we were also interested in a range of additional measures
typical in studies that evaluate realism of VR experiences, such as
immersion (extent to which the experience is inclusive, extensive,
surrounding and vivid [77]), enjoyment (taking pleasure in theexperience), and sensory engagement (the range of sensory
modalities experienced).
Conditions. In this study, participants experienced two interface
conditions in counterbalanced order, across all participants: (1)ThermalGrasp (four devices, one on each foot and hand) and (2)a no thermal feedback baseline.
Participants. We recruited eight participants (four identified as
female, three as male, one as nonbinary, average age of 21.8 years
old, SD=2.22). None reported injuries on their feet or hands.
Participants received $20 for their time.
Task. The VR task mirrored the experience presented in our
Walkthrough, including all the interactions depicted in Figure 5.
The goal for the participants was to find a way to survive the nightin this VR desert by building a fire to keep warm. Participants
explored this VR experience by walking on the physical terrain
shown in Figure 9. The prop-based terrain measured 2.5m x 2.8m
with regions of sand, artificial grass, and shallow water. The room
was maintained at 22.8⁰C. In our experimental condition,
participants wore four ThermalGrasp devices, one on each handand foot. In all conditions, they also wore a wireless HTC VIVE
headset, and a backpack with a battery and our devices’ controllers,which communicated using Serial over the VIVE’s wireless link.
Additionally, participants wore a VIVE tracker on each wrist andankle to track hands and feet. The remaining physical props(wooden logs) were tracked using VIVE trackers.

Figure 10: (a) Physical prop-based terrain. (b) Wearable study setup.

Procedure. As participants explored the VR desert wearing
ThermalGrasp, their location in the VR world triggered thermal
effects. Figure 10 presents the timeline of events alongside thetemperatures that participants could experience. The experience’s
timeline was the same for both conditions, except for the thermal
sensations. These timings were designed to ensure a comparable
experience across participants and work realistically with ourdevices’ speed. Experimenters added the physical log props into theterrain periodically outside of the participant’s field of view, indifferent locations across conditions. When a participant picked up
the log and placed it in the fire, the experimenter “froze” the virtual
log’s position in the fire and proceeded to move the physical prop
to a new location to be collected again. After each trial, participants
were asked to report their sense of realism, immersion, sensory
engagement, and enjoyment on a 7-point Likert scale, as well as
elaborate upon their ratings and the sensations they experienced.

Figure 11: Timeline of events and temperatures (for the
ThermalGrasp condition).

Figure 11 presents our main findings. We analyzed our data usinga paired T-test (two-tailed). Specifically, ThermalGrasp (M=5.63,
SE=0.17) was perceived as more realistic (p=0.017) than thebaseline (M=4.63, SE=0.30). Similarly, ThermalGrasp (M=5.63,
SE=0.35) was perceived as more engaging (p=0.009) to the senses
than the baseline (M=4.63, SE=0.58).

Figure 12: Participants’ ratings for both conditions. Errors bars show
standard error.

No significant difference was found between the baseline andThermalGrasp in terms of immersion (p=0.099; baseline:
M=5.38, SE=0.35; ThermalGrasp: M=5.75, SE=0.23) andenjoyment (p=0.090; baseline: M=5.25, SE=0.42; ThermalGrasp:

M=5.88, SE=0.37). Finally, five of eight participants reported thatthey preferred the condition in which they were instrumented with
our ThermalGrasp devices.
Qualitative feedback. We analyzed transcriptions of all
responses to our open-ended questionnaire. We identified four
topics: (1) thermal referral, (2) behavioral change, (3) presenceand (4) preserved grasping and walking.
Thermal referral. Six (out of eight) participants associatedthermal feedback to objects or events in the scene (P1, P2, P5, P6,
P7, P8). For instance, P4 reported the temperature of the physicalsand being warmer than the grass, even though the sand and grasswere at room temperature; it was our device that warmed the feet
only when standing on sand. Moreover, participants reported
missing the thermal feedback when absent: “[baseline led to] aslight disconnect” (P1); “[this experience is] a sensual
experimentation and you lose an entire sense [in baseline]” (P7); or“[baseline] felt less like [I was] interacting with the world” (P3).
Behavioral change. We observed behavioral changes in responseto the thermal feedback. Notably, heat altered half the participants’relationship to the campfire and its surrounding space (P5, P6, P7,
P8). For example, P6 stated “I’ve got to hold this [torch] so that I’m
not near the part that’s on fire”. P5 also stated that the alignment
between the heat and VR situation “feels like something ishappening, so you’re more encouraged to actually stand by the fire
and wait.”
Presence. Some participants mentioned their sense of presence
increasing based on thermal feedback. P5 remarked that, without
thermal feedback, the experience felt like “spectatorship or aloading screen more so than an immersive experience.” P3
remarked, “[ThermalGrasp] felt like I was actually taking up spacein the environment (…) even though it might seem like a smallthing, having that physical feedback […] really affects theexperience.”
Preserved grasping and walking. Qualitative feedback affirmedthat our approach preserves some tactile sensations along withgrasping and walking. In both conditions, six participants (out ofeight) described the tactile regions in detail (P2, P4, P5, P6, P7, P8).
Seven participants (out of eight) reported the textures were
unaffected by the thermal devices (P1, P2, P3, P4, P6, P7, P8). P8
detailed their experience by stating, “everything was the same
besides adding the heat (…) when I went to pick up the sticks, I
didn’t have the same feeling on my palm because the device was in
the way.” Notably, no participant reported any difficulty grabbing
or manipulating the tracked prop, no participants reported
temperature change felt on the dorsal side of their hands/feet, andno participants reported any discomfort during the experience.
8

DISCUSSIONIn Study 1a, we found that ThermalGrasp accurately presented
thermal feedback at the desired locations; in fact, with similar
performance to adding Peltiers directly on target—however,
ThermalGrasp works even while grasping or walking. Conversely,in Study 1b we found that, as expected, ThermalGrasp is slower
than directly applying a Peltier to the target, since it requires totransfer heat over distances. Remarkably, in Study 2, we found that
this tradeoff between speed and realism can be beneficial, as ourparticipants felt that ThermalGrasp was more realistic and engaging
in a VR experience that made use of walking and grasping props.
We see ThermalGrasp as not an end-product, but as an approachto make thermal feedback with props more seamless. Transferring
heat over distances has weaknesses for interaction, namely, the rateof temperature change, which may explain why immersion was not
greater in Study 2’s ThermalGrasp condition. As such, future work
can enable wider use cases by improving feedback speed. Several
potential avenues to achieve this include:Increased Peltier/power density. Adding a second Peltier to thedesign (i.e., sandwiching both sides of the heat pipe) or using higher
power Peltier elements will lead to a greater thermal gradient
between the Peltiers and the skin, thus increasing the heat transfer.
Spatially-divided hot and cold stimuli. Traditional Peltiers
encounter a similar need for rapid temperature changes, especiallyin scenarios simulating the transition between touching hot andcold surfaces. To overcome the lag of individual elements, acommon strategy is to arrange Peltiers in a 2x2 grid with dedicatedPeltiers for heating and for cooling. This arrangement enables rapid
perceived temperature changes by exploiting two characteristics ofhuman thermal perception: spatial summation and the adapting
temperature [78] and has been implemented in various thermal
devices [15], [35]. ThermalGrasp can incorporate this arrangement
by using multiple Peltiers, each with their own conductive paths
meeting in a grid on the skin, to increase the speed of perceived
temperature change.
Advanced materials. The bottleneck in our presented hardware isthe copper sheet metal. While the heat pipe component transfers
heat especially fast, the copper sheet metal has orders of magnitude
lower effective thermal conductivity yet is necessary to present thetemperature in a thin and conformal interface against the user’sskin. Advances in thermal materials may alleviate this bottleneck,such as thin and flexible heat pipes [79], [80], [81].
Beyond this limitation, our work underscores the value ofconsidering thermal latency trade-offs, as our Study 2 foundenhanced realism even with this non-instantaneous thermal
feedback. Thus, our approach is apt for scenarios expecting noninstantaneous thermal feedback (our Walkthrough/Study 2 usedslow thermal feedback, e.g., the sun rising or the flames growing).
In our study, we occupied participants (e.g., collecting logs, firegrowing visuals) to allow the device time to heat/cool. This strategy
is used in haptic displays that require seconds to minutes to actuate,
e.g., pneumatic [82] or motor-based [83]. We emphasize that ourapproach enables an experience that (despite some limitations) isotherwise not possible with traditional approaches: overlaying
thermal sensations while grasping/walking on props.
9

ACKNOWLEDGEMENTS
This work was supported by NSF grant 2047189 and the Sloan
Foundation. Additionally, this work was partially supported by the
University of Chicago MRSEC, which is funded by the NSF underaward number DMR-2011854. Any opinions, findings,
conclusions, or recommendations expressed in this material arethose of the authors and do not necessarily reflect the views of any
funding agencies.
Moreover, we are grateful for the feedback from our reviewers.
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CONCLUSION

We proposed, engineered, and validated ThermalGrasp, anapproach for wearable thermal interfaces that enables users to grab
and walk on real objects with minimal obstruction. Our approachmoves the thermal device and cooling unit to areas not used ingrasping (e.g., back of hand) or walking (e.g., top of foot). We thenuse thin, compliant materials to conduct the device’s heating or
cooling to the palm of the hand or sole of the foot. Unlike traditional
actuators and heatsinks, our thin materials uniquely enable grasping
and walking on real objects while enjoying thermal feedback. We
demonstrated that our technique can be applied to VR experiencesthat heavily rely on props or tool manipulation.
We believe that ThermalGrasp points to a direction in whichinteractive devices aim to harmonize as many senses as possible,
while minimizing obstruction to real-world manipulation. Here, we
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end-products, but as artifacts of the ThermalGrasp approach, which
may serve to inspire the creation of new thermal devices that enableinteractions with real-world objects. As such, we plan to opensource detailed fabrication, hardware schematics and code to aid
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