FeetThrough: Electrotactile Foot Interface that Preserves
Real-World Sensations
Keigo Ushiyama

Pedro Lopes

University of Chicago
ushiyama@kaji-lab.jp

University of Chicago
pedrolopes@uchicago.edu

Figure 1: We argue that foot haptic interfaces should optimize for what users feel during output and prioritize letting users
feel the terrain under their feet. Feeling ground features with our soles is critical for balance on uneven terrains and stairs. In
this paper, we demonstrate that electrotactile stimulation not only achieves an improved “feel-through” of terrains compared
to traditional vibrotactile foot interfaces but also lets users feel the output more clearly (lower two-point discriminationthreshold).

ABSTRACT
Haptic interfaces have been extended to the feet to enhance footbased activities, such as guidance while walking or stepping on
virtual textures. Most feet haptics use mechanical actuators, namely
vibration motors. However, we argue that vibration motors are notthe ideal actuators for all feet haptics. Instead, we demonstrate thatelectrotactile stimulation provides qualities that make it a powerfulfeet-haptic interface: (1) Users wearing electrotactile can not only
feel the stimulation but can also better feel the terrain under theirfeet—this is critical as our feet are also responsible for the balanceon uneven terrains and stairs—electrotactile achieves this improved“feel-through” effect because it is thinner than vibrotactile actuators,
at 0.1 mm in our prototype; (2) While a single vibrotactile actuator
will also vibrate surrounding skin areas, we found improved twopoint discrimination thresholds for electrotactile; (3) Electrotactile
can be applied directly to soles, insoles or socks, enabling new
applications such as barefoot interactive experiences or without
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https://doi.org/10.1145/3586183.3606808

requiring users to have custom-shoes with built-in vibration motors.
Finally, we demonstrate applications in which electrotactile feetinterfaces allow users to feel not only virtual information but also
the real terrain under their shoes, such as a VR experience whereusers walk on ground props and a tactile navigation system thataugments the ground with virtual tactile paving to assist pedestrians
in low-vision situations.

CCS CONCEPTS• Human-centered computing → Human computer interaction(HCI); Interaction devices; Haptic devices.

KEYWORDS
Haptics, electrotactile, foot, feel-through, VR, AR
ACM Reference Format:
Keigo Ushiyama and Pedro Lopes. 2023. FeetThrough: Electrotactile FootInterface that Preserves Real-World Sensations. In The 36th Annual ACM
Symposium on User Interface Software and Technology (UIST ’23), October
29–November 01, 2023, San Francisco, CA, USA. ACM, New York, NY, USA,
11 pages. https://doi.org/10.1145/3586183.3606808

1

INTRODUCTION

Haptic interfaces have been extended to the user’s feet in orderto enhance foot-based activities, such as guidance while walking[4, 38, 56], feeling virtual textures [50, 52, 57], pose correction[7, 8], and more. Most contemporary haptic interfaces for the feet

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

consist of mechanical actuators, particularly vibration motors. Thisis understandable since this type of haptic actuator has provenuseful for many hand-based haptic interfaces, such as vibration
gloves [1] or finger haptics [41], and is relatively easy to embed
inside shoe insoles [37, 62], even including in larger arrays [38, 56].
However, we argue that vibration motors are not the ideal actuators for all feet haptics. While vibration motors will excel atapplications that render textures to the user’s feet (e.g., [50, 62]),
we argue, and demonstrate in this paper, that this is not the case forany applications in which the user not only need to care about feeling the output from the haptic interface but also feeling the terrain
under their feet. Our feet soles play a considerable role in our senseof balance in uneven terrains, stairs, and other ground obstacles[27, 51]. This has especially been linked to the tactile senses in oursoles, as denoted by Viseux in their comprehensive review “(. . .)cutaneous afferents in the foot contribute to our ability to standupright” [58]. In fact, the author goes further and denotes: “(. . .)decline in sensitivity is frequently associated with poorer posturalcontrol and increased risk of falls” [58]—we argue that designingmost haptic foot interfaces using thick mechanical actuators risks
bringing users into this situation as the thickness of the actuator
will limit the tactile perception, in other words, the actuator “filtersout” the ground’s tactile features (Figure 1).
To tackle this, we propose a turn in haptic interfaces for the feetby exploring electrotactile stimulation. In this paper, we demonstrate, through a series of user studies, three key advantages ofelectrotactile foot stimulation over vibrotactile stimulation. In particular: (1) Unlike with vibration motors, a user wearing our electrotactile FeetThrough prototype can not only feel the haptic stimulation (e.g., shapes for guidance while walking) but can also betterfeel the terrain under their feet, which was validated in our Study
2—electrotactile electrode arrays achieve this “feel-through” effectbetter because they are inherently thinner (0.1 mm in our FeetThrough prototype) than an array made with mechanical actuators.
(2) While in vibrotactile stimulation, a single actuator will also vibrate surrounding areas (crosstalk), we found this is not the casewith electrotactile stimulation to the feet and found that it provides
superior two-point discrimination thresholds, which was validated
our Study 1; (3) We also found that, due to its flexibility, electrotactile arrays are easier to apply to several interactive contexts,
including some cases in which vibration motors are mostly impractical. Such as barefoot interactive experiences—users would stepon the vibration motor’s case and find their entire body’s weight
distributed into a few uncomfortable, high-pressure points. Moreover, electrotactile interfaces can be applied directly to the soles (orin a sock form factor) and thus perform without customized shoes,
as is the case with embedding vibration motors in the insole.
Finally, we demonstrate applications in which electrotactile feetinterfaces allow users to feel not only virtual information but also
the real terrain under their shoes, such as a VR experience whereusers walk on ground props and a tactile navigation system thataugments the ground with virtual tactile paving to assist pedestrians
who are visually impaired.

Keigo Ushiyama and Pedro Lopes

2

RELATED WORKThe work presented in this paper builds primarily on the field of
haptics, with particular emphasis on tactile haptic interfaces forthe feet. Moreover, since our primary goal is engineering a tactile
feet interface that allows users to still feel the terrain under theirfeet, we take inspiration from research exploring this goal for thefingerpad. Finally, as our approach uses electrotactile stimulationon the sole, we succinctly review the field of electrotactile, which
has explored chiefly other skin areas, especially fingerpads.

2.1

Majority of tactile foot haptic interfaces use
vibrations

In recent years, haptic interfaces have been extended to the feetto enhance foot-based activities, such as navigation [4, 25, 38, 47,
56, 63], information (e.g., safety information [34], awareness ofsurroundings [20, 32], sensory substitution [6], music rhythm [42]),
foot I/O devices [3, 44], language communication [25], walking[19, 33, 37, 62, 65, 67], and rendering the feel of virtual surfaces[40, 50, 52, 55, 57].
While the choice of actuators for feet haptics differs based on
the intended purposes, most feet haptic devices rely on mechanicalstimulation, especially using vibration motors—usually eccentricrotating mass motors or linear resonant actuators. While othermechanical actuators, such as brakes [54], inflatables [14], or fluids[49], have been used in foot stimulation, these are less popularthan vibrotactile actuators for feet. This popularity is likely a consequence of the small size & ease of use of vibration motors, aswell as the widespread use for tactile stimulation of fingers andhands—the largest area of haptics research.
Implementing feet haptics with vibration. The canonicalimplementation of the most vibration-based feet haptics is embedding vibration motors under the sole or inside insoles. Some devices
have vibrators installed in shoes rather than under the sole. GymSoles [7, 8] used eight vibrators around the foot to realize posture
feedback for assisting squats and dead-lifts. Pace-sync shoe [60]and devices for helping training [5, 9] used vibrators placed on an
instep or an ankle to notify the timing and training states—these
employ vibration as a notification. In such instances, providing
haptic feedback to areas other than the soles is sufficient for notifications or symbolic information. Yet, our soles have the same typeof skin receptors as our hands [51] and, as such, pose an excellent
target for stimulation with a higher number of points (e.g., as inmost haptic displays for realism).
While vibrators are well suited to simulate stepping on virtual
ground textures, it is challenging to present static information or
shapes because vibrations are also propagated via the housing of
the device and via the surrounding skin. Hill et al. engineered afoot device using multiple vibrators for language communication.
However, they needed to adopt a sequential stimulation because
vibrations can be confused if presented simultaneously [15]. Thisissue can be seen in other works and was typically handled bydriving vibrators sequentially [6, 28, 56] or by spacing vibrators
further away to minimize inter-vibrator crosstalk [45].
More importantly, we argue that conventional wearable footinterfaces usually cover the sole with rigid devices and do notconsider the importance of letting users feel the terrain. One example

FeetThroughtechnique that avoids direct placement of vibrators in the soles uses
a toenail vibration illusion [17, 46] (inspired by an approach of
fingernail vibration illusion [2]). However, the region that can be
stimulated is very limited, and the resolution is insufficient to depict
shapes and more.

2.2

Feel-through haptic devices for the finger

The problem of haptic actuators blocking real-world sensations has
sparked recent interest for researchers that work in hand-based
interactions for AR/MR because, in these contexts, users need not
only to touch virtual objects (e.g., AR buttons) but physical objects(e.g., tools or props). A canonical example is Tacttoo [61], whichattaches electrodes to the user’s fingerpad and can simultaneously
preserve some macro-features of the touched objects. As demonstrated in the experiments of Like a Second Skin [39], it is possible
to “feel through,” to some extent, while one’s skin is covered by adevice made from a thin film—in fact, the thinner the film, the less
it obstructs feeling the real world. Similarly, Hydroring [13] also
aimed to preserve the haptic perception of the physical world, butinstead, wrapped the user’s finger in a soft actuator where liquids
can flow through—its softness allows users to still feel through thedevice to some extent. We take inspiration from these works that
promote feel-through for the fingerpad but explore how to achieve
this in the case of foot haptics.

2.3

Importance of feeling physical terrain
under our feet

Sensory feedback from the sole plays a crucial role in gait control,balance, and maneuvering around uneven terrains and other groundobstacles [27, 51]. This has especially been linked to the tactile
information acquired by the skin of our soles as we walk around[58]. Strzalkowski et al. make a similar argument for the role oftactile sensations originating at the sole in walking movements
and gait patterns “(. . .) feedback associated with standing balance
are conveyed by cutaneous afferents into the CNS [central nervous
system], where it interacts with descending motor commands” [51].
Viseux also highlights that when our ability to feel foot sensations
deteriorates, which occurs naturally with age with a number ofneurological disorders, it “(...) is frequently associated with poorer
postural control and increased risk of falls” [58]. We argue that
this key role of our soles should be a guiding factor in developing
new haptic interfaces for the feet—we stress the importance ofengineering devices that preserve real-world sensations.

2.4

Electrotactile stimulation

Many researchers have explored electrical stimulation to realize
haptics without mechanical parts. Moreover, because electrodes
can be typically made smaller than a mechanical actuator (which
requires physical displacement and that requires space), these electrotactile arrays have also been heavily explored for higher spatial
resolution (e.g., [24, 35]).
Electrotactile stimulation has been chiefly used for hands and fingers, such as for textures [12, 64], delivering sensory substitution
for prostheses [11, 30], or touch information in virtual environments [59]. While most of the electrotactile stimulation has been
applied on the fingers and hands, some researchers in medicine &UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

neuroscience have also confirmed its feasibility to stimulate thesoles. These researchers used electrotactile stimulation of the soles
as either a way to investigate the role of afferents from the sole inwalking [66] or to quantify the feet’s tactile receptors [10]—the latter measurements by Frahm et al. found that sensations at the heel
were milder than at other sites and that the foot arch was a moresensitive area. Solomonow et al. reported a two-point electricalstimulation discrimination threshold on the sole’s arch at 7.67 mm[48]. We take inspiration from this but advance it by investigatingthese thresholds at three sole regions since they did not survey it.

3

CONTRIBUTION, BENEFITS, AND
LIMITATIONS

Our key contribution is the concept that foot haptic interfaces canbe designed to render haptic sensations and let users feel the terrain
under their feet. For this, we proposed a turn from the more traditional vibration-based foot haptics to electrotactile actuation. Bymeans of two studies, our FeetThrough prototype and its applications, we demonstrate three key benefits of this approach to feethaptics: (1) Users wearing electrotactile can not only feel the stimulation but can also better feel shapes under their feet—this is criticalas our feet are also responsible for balance on uneven terrains andstairs—electrotactile achieves this improved “feel-through” effectbecause it is thinner than vibrotactile actuators, at 0.1 mm in our
prototype; (2) While a single vibrotactile actuator will also vibrate
surrounding skin areas, we found improved two-point discrimination thresholds for electrotactile; (3) Electrotactile can be applied
directly to soles, insoles or socks, enabling new applications suchas barefoot interactive experiences or without requiring users tohave custom-shoes with built-in vibration motors.
Electrotactile is not without limitations: (1) electrotactile has notbeen shown to rival vibration regarding texture rendering; and (2) as
any other technique based on electrical stimulation, it requires theattachment of electrodes to the skin and calibration. However, we
argue that the benefits can outweigh some limitations as our studies
demonstrate that participants could better discriminate electrotactile stimulation than vibrations and better feel shapes under theirfeet. We think of this work as a first step into prioritizing letting
users feel the ground beneath their soles with haptic feedback.

4

IMPLEMENTATION

To help readers replicate our FeetThrough prototype, we now provide the necessary technical details. Additionally, all source codes
and materials are made publicly available.1

4.1

Electrode layout

We developed a flexible electrode array with 60 electrodes thatcover the sole from the ball to the heel (Figure 2).
While one can apply this technique to any region of the foot, we
opted not to extend the electrodes to the toes for two reasons: (1)since our goal is to balance feeling the terrain under one’s foot andhaptic stimulation, we opted to let the toes be free and available toadjust the sole while stepping—toes have a crucial role in walking[16]; moreover, (2) toes are joints, unlike the fairly smooth and
1 http://lab.plopes.org/#FeetThroughUIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

Figure 2: A prototype of electrodes and stimulation devices.

Keigo Ushiyama and Pedro Lopes

driven by an existing electrical stimulator. We utilized the medicallycompliant stimulator RehaStim (often used in electrical interfaces,
e.g., [18]), but any other electrical stimulator is compatible (e.g.,[22]). Our multiplexer is responsible for selecting which electrodechannels output the stimulator’s waveforms. Note that our stimulator is not a strict 1:N, in which one input is distributed to only
one of N possible outputs. Instead, we implemented our stimulator to allow one input to be distributed to any number of the N
output channels, i.e., it can deliver the input to multiple channels
simultaneously. In total our multiplexer can deliver stimulation to
60 channels for one foot.

continuous surface of the sole, which would make the electrodeattachment more difficult. Future work might certainly explore
extending our device to toes to investigate new variations.
As for our electrode design, electrodes are 8 mm in diameterand separated by 15 mm (center-to-center). This diameter is basedon the sole’s receptive fields of skin receptors (SA I and FA I) [51].
This relatively larger diameter than the canonical electrodes forfingers worked to avoid any pain that could be induced due to small
electrodes [21, 30].

4.2

Electrode arrays on a flexPCB

To keep our electrode arrays sturdy enough that users can step onthem and remain as flexible and thin as possible to feel the macrofeatures of terrain, we opted for fabricating these as flexible PCBs
(flexPCB). While other techniques are possible, such as screenprinting (e.g., as used in [61]), copper tape (e.g., as used in [18,26]) or even the traditional pre-gelled electrodes (e.g., as used in[21, 23, 48]), the flexPCB substrate is very strong (e.g., hard to rip)compared to these prior techniques, while still being relatively thin
at 0.1 mm, striking a balance in terms of being thin and durable. The
substrate used in our flexible PCBs was made from polyimide, andthe electrodes were copper coated with immersion gold (ENIG)—
these materials are commonly used for electrotactile displays [11]or even featured in intra-oral electrotactile displays [53].

4.3

Stimulation parameters

We used an AC square waveform (biphasic stimulation) with a pulse
width of 250 𝜇s and a 100 𝜇s gap between two phases. Stimulation
frequency was set to 50 Hz for studies. These pulse width and frequency were decided based on previous work [23, 30]. Furthermore,
Solomonow et al. [48] reported the lowest two-point discrimination threshold was observed at around 50 Hz. We opted for AC
stimulation since the skin of the sole is thicker than many otherparts (e.g., fingers), and AC stimulation is known to be effective
for tactile sensations in these conditions [30], which we confirmed
in our early pilots. The stimulation intensity was always set peruser to ensure pain-free operation. To illustrate its magnitude, theaverage amplitude for participants was 8.6 mA (SD=5.5 mA) in
Study 1 (maximum intensity, no pain) and 5.0 mA (SD=1.7 mA) in
Study 2 (noticeable sensation threshold).

4.4

Stimulation multiplexer circuitry

Figure 3 depicts the multiplexer circuit that we engineered to select which electrodes are utilized. Our electrotactile array can be

Figure 3: Our multiplexer circuit for 60 channels.
At every electrode, we added a half-bridge circuit with two
TLP176 MOSFET high-voltage photorelays, which allows our device to select whether this electrode is connected to the negativeor positive side of the stimulator —thus, all channels can be freely
routed. To control all 60 photorelays, we utilize 16 SN74HC595
8-bit shift registers, which are controlled by our microcontroller,an ESP32 DevKit-C. Moreover, our device needs 1 ms at maximum
to reroute all stimulation channels. Finally, we manufactured twocomplete devices in our AR/VR applications (Figure 14 and Figure15) to stimulate both feet.

4.5

Electrical stimulation method

As typical in electrotactile stimulation, we use time-division multiplexing via fast-switching (1 ms) of the stimulated point to present
concurrent stimulations [23]. We connected only the surrounding
electrodes of the positive electrode (the stimulation point) to thenegative channel to concentrate the current around the stimulation
point. Figure 4 illustrates an example of a stimulation pattern inStudy 1. Stimulation is delivered to each of the two points sequentially by time-multiplexing. Our stimulation frequency is set at 50
Hz and a maximum of 12 simultaneous points to account for thefull round-trip latency of the multiplexer (1 ms to change the stimulation point, 600 𝜇s of stimulation, rotate by 60 points). For example,
our device can stimulate up to 12 points for 50 Hz. However, we
can also stimulate all 60 points simultaneously, instead, at 10 Hz.

FeetThroughUIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

Figure 4: We use time-division multiplexing for multi-point
stimulation.

4.6

Alternatives for wearing our device

In Figure 5, we illustrate several approaches for attaching our electrode array to the user’s soles, arranged in order of increasing easeof use. In this paper, we explored and studied the attachment depicted in Figure 5 (a), which uses skin-safe medical tapes, resulting
in a strong coupling between electrodes and skin. However, otheralternatives can be explored depending on the design goal. Figure
5 (b) depicts how the electrodes can also be attached to the soleusing custom double-sided tapes with holes for the electrodes. It is
worth noting that while the aforementioned (a) and (b) approaches
are less convenient since they require an attachment, they allow
for unique applications not possible before, i.e., feet haptics whilebarefoot, which we later take advantage of in two of our applications. Further, two methods can provide additional ease of use: (c)stitching electrodes in a sock and (d) lining the insole of the shoewith electrodes while users are barefoot—these latter methods allow
users to put/remove the device rapidly and effortlessly. Finally, it isworth noting that further investigations are necessary to explore
all these alternative approaches.

Figure 5: Four possible attachment techniques.

Figure 6: Key hardware attached to the user’s feet: one of
flexPCB with 60 electrodes for each foot, a multiplexer unit
for each electrode array, tracking for any VR application,and four pressure sensors (FSR) worn at the sole for balance
detection.

5

USER STUDIES

We conducted two user studies to validate the benefits of switchingfrom vibrotactile to electrotactile foot stimulation. To the best of
our knowledge, our studies are the first to explore electrotactile
stimulation while stepping on electrodes and physical shapes. In
our first study, we assessed whether electrotactile stimulation
was easier to spatially distinguish than vibrotactile stimulation
through the canonical two-point discrimination study. We foundthat participants could distinguish electrotactile stimulations at
half the distance needed for vibrotactile (effectively double theperceived resolution, i.e., half two-point discrimination). In our
second study, we measured participants’ ability to discriminate
two simultaneous shapes while they stepped on them, one shaperendered via stimulation and one physical shape. We found that byusing electrotactile stimulation, participants could better identify
both shapes.

Application-specific implementations

5.1

Study 1: Two-point discrimination while
standing

Figure 6 illustrates the key hardware components attached to theuser’s feet, not including headsets used for AR/VR. To track footpositions in VR, we employed HTC VIVE trackers. For AR navigation, we used HoloLens 2 to estimate coarse foot positions based onhead position (provided by HoloLens’ room tracking). For posture
feedback, we attached four FSR-based pressure sensors to each solecorner.
In VR/AR applications, the stimulator and a laptop are wornin a backpack. AR and VR applications run on a separate computer, and stimulation commands are transmitted to the laptop via
OSC. Finally, the pressure sensor values are sampled via the same
microcontroller used for multiplexing.

The two-point discrimination threshold is one of the canonical criteria when investigating tactile sensations. While previous studies
have investigated two-point discrimination thresholds for vibrotactile and electrotactile stimuli, there has been a lack of investigation
into these thresholds while standing on the actuators (i.e., pressing
electrodes/vibration motors). Such investigation is needed to design
vibrotactile/electrotactile devices that provide interactive experiences. Regarding electrotactile stimulus, although Solomonow et al.
[48] measured the threshold on the middle of the sole, it is crucial
to explore discrimination thresholds at other regions of the feet(e.g., ball and heel), which contact more with the floor or objects
than the middle of the sole. Thus, we investigated discrimination

4.7

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

thresholds across the entire sole via a standard staircase design tocompare the discrimination capabilities of electrical and vibratory
stimulation on the sole while standing on the haptic feedback devices. This study was approved by our Institutional Review Board(IRB21-1229).
Hypothesis. We posited that the two-point discriminationthreshold for electrotactile would be lower since electrical stimulation does not propagate omnidirectionally inside the body as
vibration does.
Conditions. Participants experienced two stimulation conditions: electrotactile and vibrotactile. Moreover, we measured
the two-point discrimination in three-foot areas since the sole isheterogeneous [51]. Thus, per condition, participants experienced
stimulation to three sole segments: ball (the skin area just below
the toes), arch (middle of the foot), and the heel. The order (twostimulation conditions × three regions) was randomized per participant.
Apparatus. Figure 7 depicts the apparatus at the example of theball location; these devices were applied to all three locations, one
location at a time.

Keigo Ushiyama and Pedro Lopes

Hz for vibrotactile stimulus. However, LRAs have a fixed resonant
frequency, and their performance decreases when actuated outsideof it. Thus, we actuate our LRAs at 175 Hz, as close as possible to[31]. Finally, electrodes and vibration motors were arranged witha 2 mm gap because we found slight alternations in the electrodegap changed the vividness and comfort of the sensation, possibly
caused by the thick skin of the sole and the difference in current
density [21, 30].
Calibration. Before any stimulation, the number of electrodes/vibrators used for each region in the measurement was
adjusted based on the foot size of the participant. The electrodes orvibrators were attached to the sole with flexible tapes (to minimize
the propagation of vibration), and participants stepped on a silicone
pad to ensure the attachment and further minimize the propagationof vibration (Figure 8). Before the trials, the intensities of the electrical stimulation at each point were calibrated (maximum intensity
without any pain sensations nor any discomfort).

Figure 8: The study setup. Participants stand on the (a) electrodes or (b) vibrators and a silicone pad.
Figure 7: Apparatus used for Study 1 (shown at the ball).
In the electrotactile condition, we used our device as described in
Implementation, with a three-by-ten electrode array, with an 8 mm
diameter and 2 mm gap, as depicted in Figure 7 (a). The electrode
diameter was selected based on the receptive field of skin receptors[51]. Note that only the electrodes in the center row were used as
the positive electrode of the electrical waveform. In contrast, theelectrodes on the top and bottom rows were used as ground (see Implementation). Similarly, in the vibrotactile condition, ten vibrators
were arranged in line with 2 mm gaps, as depicted in Figure 7 (b).
We utilized a 10 mm diameter LRA (C10-100, Precision Microdrives)
based on prior work [56], driven at its resonant frequency at 175
Hz and at nominal voltage, to output peak performance. Vibrators
were driven by a multiplexer similar to the one we engineered forelectrotactile stimulation but using 2N3904 NPN transistors, which
were, in turn, connected to LTC1660 digital-to-analog converters.
Any stimulation, either vibration or electrotactile, was applied for
500 ms.
In early pilots, the authors determined the stimulation frequency
and configuration (center-to-center distance and diameter) thatelicited the most vivid sensations for both electrotactile and vibrotactile interfaces—enabling a fair comparison for both conditions(i.e., both selected at their top performance). For the frequency, we
employed 50 Hz electrotactile stimulation and 175 Hz vibrotactile
stimulation, which were selected based on prior work. Solomonow
et al. [48] reported the best discrimination ability at 50 Hz for electrotactile stimulus, while Kowalzik et al. [31] reported it at 200

Study procedure. We utilized the canonical 1-up and 2-downstaircase method to measure two-point discrimination thresholds.
One descending series was carried out in each condition from themost distant set of stimulation points, which are on the left andright edges of the sole. Participants answered whether they felt two
unique stimulation points, and the staircase proceeded accordingly
to their answers until three reversals were observed. As typical ofthis design, the average value of these reversal points was the outcome of a staircase run (calculated distance was center-to-center).
Since multiple stimulation point sets share equal distances (e.g.,with distance=2, sets include (#1, #3), (#2, #4), (#3, #5), etc.), electrodes and vibrators were randomly selected for each trial to randomize stimulation sites and mitigate memorization effects, making
the task harder & more robust.
Participants. We recruited twelve participants (eight identified
as male, three as female, one did not identify, average age = 24.9
years, SD=3.3, all right-foot dominant) from our local institution.
This study took about 30 minutes to complete. Participants received$10 as compensation.
Result. The main result is depicted in Figure 9. The average
thresholds of electrical stimulation were 30 mm (ball region), 32 mm(arch region), 26 mm (heel region), and 29 mm (average). Instead,in the vibration condition, most participants could not discriminate
vibratory stimulation well—we later illustrate this in detail by computing the normalized to each participant’s sole width. A Two-way
repeated measures ANOVA was conducted and the assumption of
sphericity was not violated. The analysis revealed significant differences with a 5% significance level in the main effects (stimulation

FeetThroughcondition: F (1, 11) = 106.19, p < 0.001, region condition: F (2, 22) =8.90, p = 0.001) and the interaction effect (F (2, 22) = 3.67, p = 0.042).
Post-hoc analysis with the Bonferroni method found significant
differences between vibration and electrical stimulation for eachregion (ball: p < 0.001, arch: p < 0.001, heel: p < 0.001).

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USAalso differ from those reported by Solomonow et al., found to be∼8 mm [48]. Just as in the previous case, this is also because our
setup and conditions differ from theirs. In addition to that, the discrimination threshold for electrotactile differs based on stimulation
methods [30], and Solomonow et al. utilized concentric electrodes
while we adopted the traditional matrix of electrodes typical inmost interactive uses of electrotactile.

5.2

Figure 9: Two-point discrimination thresholds for each region while standing on actuators. Error bars mean standard
errors.
Qualitative feedback. Participants voiced qualitative feedback,which we transcribed throughout all trials. Eight participants (outof 12) verbally commented that they felt that the electrotactile
stimulation induced precise point sensations, and they felt confidentof the stimulus location. Moreover, eight participants (out of 12)
voiced difficulties locating positions via vibrotactile; to quote one
participant, “vibration feels spread out.”
Discussion. The result of the electrotactile was significantly
better than the vibrotactile. The discrimination thresholds were
nearly consistent in three regions, allowing each sole region to be
stimulated with the same electrode density despite the nonuniform
distribution of skin receptors in the sole [51]. We also conducted
two-way repeated measures ANOVA with normalized data (i.e.,
normalizing each participant’s two-point threshold with their own
foot width), and the assumption of sphericity was not violated.
Both main effects are also statistically significant (stimuli: F (1, 11)= 95.499, p < 0.001; region: F (2, 22) = 3.721, p = 0.041), but theinteraction effects were not significant (F (2, 22) = 0.732, p = 0.492).
This is because most participants did not discriminate between thetwo points during the vibration condition (despite the maximum
distance). Altogether, these results suggest that our hypothesis was
supported.
Finally, the results of our vibrotactile condition were different
from the data reported by Kowalzik et al. [31], in which the thresholds ranged from 15-34 mm. This is likely because our study setup
and conditions differ dramatically from theirs. Kowalzik et al. used
two very fine wires (1 mm) to transmit vibration to the sole ofparticipants lying on a bed—not actively stepping; this propagatesvibrations even more, worsening perception and, thus, the threshold increases. Secondly, the results of our electrotactile condition

Study 2: Feeling-through while standing on
virtual & real

In Study 2, participants were asked to identify shapes presented
physically and virtually on the sole simultaneously. Although previous studies [50, 62] examined the modulation of ground stiffness,the impact of actuator choice (the traditional vibrotactile vs. electrotactile) on the perception of physical haptic cues has not been
investigated, which is also essential as users will be feeling not only
on virtual information but also the actual ground under their feet.
This study was conducted to (1) explore how well participants canrecognize information presented by vibrotactile or electrotactile
stimulation and (2) investigate how electrodes and vibration motors
interfere with the feel-through perception while stimulations arepresented.
Hypothesis. We hypothesized that participants would recognize
both physical and virtual shapes under electrical stimulation better
than the vibration due to the thin form factor of the electrodes
compared to vibration arrays.
Apparatus. We used the same stimulators from Study 1. The
electrode and vibration arrays were upgraded to a matrix arrangement to present 2D shapes. We used the design from Velázquez et al.
[56], in which 10 mm vibrators were placed in a four-by-four matrix
with 7 mm interspaces. The size of the electrodes was 10 mm (thesame as vibrators). Electrical stimulation was delivered in the same
way as in Study 1. Moreover, to improve vibration transmission to
the skin, we attached small protruding drops on top of the vibrators[56]. To compare and measure the impact of two stimulation methods (electrotactile vs. vibrotactile) on the perception, this study was
conducted with the barefoot condition (i.e., without the impact of aparticular shoe material).
Shapes. Four types of 3d-printed physical shapes were prepared,
as depicted in Figure 10. We decided to use a subset of the shapes
used in Velázquez et al.’s design [56], namely: circle, right “arrow,”left “arrow,” and vertical line, since these can be used for simple
haptic navigation. The size of the physical shapes was 50 × 50
mm, the same size as the actuator matrix, and the height was 5
mm. Finally, the virtual shapes (rendered by either vibration orelectrotactile) were the same (same location, shape, and size).

Figure 10: (a) Physical and (b) virtual shapes—a subset ofshapes from the identification study by Velázquez et al. [56].
Study procedure. As common in these study designs (e.g., [43],prior to stimulation trials, a practice round was conducted, in which

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USAparticipants stepped on the practice shape without any stimulation
and answered what the shape was from our shape list. This was
repeated until participants reached a 90% correct answer ratio—
now, the trials began. During the trials, participants stepped ona physical shape placed by an experimenter when they heard afirst beep and lifted their foot when they listened to a second beep.
Simultaneously, while pressing against the shape, they felt thestimulation that presented the virtual shape.
Participants. We recruited twelve participants (five identified
as male, six as female, one did not identify, average age=24.8 years,
SD=3.4, all right-foot dominant) from our local institution. Seven ofthem also participated in Study 1. This study took about 1.5 hours
to complete. Participants were compensated with $10 for their time.
Results. The correct answer ratio of physical and virtual shapes
is shown in Figure 11. Importantly, note that the chance level ofgetting both shapes correct is only 6.25 % (a difficult task since it
has 16 possible combinations of the four shapes). The chance levelfor correcting one of the shapes (e.g., physical or virtual) is 25%(four shapes). Overall, participants were above the chance level on
all conditions. We conducted two-way repeated measures ANOVA.
The assumption of sphericity was violated only for the interaction
effect (p < 0.001, Greenhouse-Geisser 𝜖 = 0.562). The main effects ofstimulation and shape conditions were significant (stimulation: F (1,11) = 37.046, p < 0.001; shape type: F (2, 22) = 73.548, p < 0.001). On
the other hand, the interaction effect was not significant (F (1.12,
12.37) = 0.349, p = 0.59, computed with Greenhouse-Geisser correction); thus, we did not conduct a post-hoc analysis. Both significant
main effects indicate that vibration was considerably less effective than electrical stimulation for identifying physical and virtualshapes. The results revealed that with electrotactile, participants
doubled their recognition ability for recognizing two simultaneous
shapes with an average accuracy of 47% (SD=13%), compared to amuch lower ability with vibrotactile (M-26% (SD=11%)).

Figure 11: The correct ratio of physical and virtual shapes
for each stimulation. The ratio of correctly answering bothshapes is also shown. The error bars represent standard errors.
Moreover, the confusion matrices of physical and virtual shapes
under each condition are also shown in Figure 12. These follow the

Keigo Ushiyama and Pedro Lopes

same trend as the results above, depicting more accuracy in shaperecognition via electrotactile.

Figure 12: Confusion matrix of both shapes in both conditions.
Qualitative feedback. Participants voiced qualitative feedback,which we transcribed throughout all trials. Ten participants (out of12) commented that they found it easier to feel virtual shapes via
electrotactile than vibrotactile stimulation. In fact, five participants(out of 12) even went as far as to state that vibration did not “feellike any specific shape” (as one put it), leading them to “empty
guesses.”
Discussion. Overall, these results confirm our hypothesis. However, it is also worth noting that we observed the limits of electrotactile, with some shapes being more easily confused with others(namely those with more segments or acute angles). Moreover,some participants verbally noted they could not recognize the virtual shapes under the vibration condition. On the other hand, mostparticipants recognized virtual shapes under the electrotactile condition. This inferior performance of the vibration condition is consistent with the findings of Velázquez et al. [56]. They noted that
integrating many actuators would be pointless if the foot cannot
accurately discern which one is vibrating and concluded (like many)
that they should be operated sequentially to deliver information
using vibrotactile effectively [15, 45]. In contrast, electrotactile
stimulation can produce distinguishable shape sensations without
relying on sequential actuation.

6

ENVISIONED USE-CASES

Having validated that switching from the vibrotactile to electrotactile allowed users to better feel the terrain under their feet, we
envision three use cases in which our FeetThrough device might provide interactive benefits compared to traditional vibrotactile: (1) VR
feedback while stepping on ground props, (2) feedback for posturalcontrol in yoga, and (3) as a haptic cue for walking navigation.

FeetThrough

6.1

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

Use-case 1: Feeling Virtual Sensations andFoot Props

Figure 13 depicts a sequence of a barefoot VR experience with agrass prop, where users can feel virtual sensations and ground props
under the sole. This experience is challenging for existing vibrationbased tactile interfaces for feet because users cannot avoid feelingthe vibrotactile actuators. As an example, we developed a jungle
survival simulator. In this experience, users must find a tool tosurvive barefoot in a jungle. Figure 13 (a) depicts the user walking
in the grass prop. While they do not see the virtual twig on the
ground, as it is hidden under foliage, they still feel it when stepping
on it. They feel it via electrotactile feedback while simultaneously
feeling the real grass prop under their feet, as depicted in Figure 13(b). Finally, they use the virtual twig to obtain an apple from a tree.

Figure 14: Postural control via electrotactile while barefoot.

6.3

Use-case 3: AR Navigation while Walking

In Figure 15, we demonstrate how our device provides haptic cuesthat guide users, including perhaps those in low-vision situations,to a room inside of a building while still allowing them to feel
physical tactile paving under their feet.

Figure 13: Using FeetThrough to feel both virtual sensations
from stepping on VR objects as well as real ground props.

6.2

Use-case 2: Posture Feedback for Yoga

Figure 14 illustrates how our device can enhance barefoot activities, such as yoga. Yoga involves maintaining balance in various
poses, which can be challenging. In this application, we attachedpressure sensors are attached to the corners of the sole, as depicted
in Figure 14 (b) When this user leans too much to their right side,(c) the pressure sensor values on the right side will increase; and, in
response, (c) electrotactile stimulation will be delivered to the side
experiencing higher pressure, alerting users that they are putting
too much weight there.

Figure 15: Electrotactile for haptic guidance in real terrains.
These tactile paving surfaces provide haptic information to users
with low vision—unfortunately, they are not ubiquitous. Our interface, paired with a HoloLens 2 and a preloaded building map
for spatial tracking, allows us to digitally “extend” the concept of

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

tactile paving by rendering the haptic guidance via the electrotactileinterface. Figure 15 (a) depicts the user walking over (real) tactile
paving. Note that, as depicted in Figure 15 (b), the user is feelingsimultaneously the bumps on the tactile paving and a haptic arrow
indicating to “continue forward.” However, as depicted in Figure
15 (c), while tactile paving is not ubiquitous in this building, ourdevice extends its reach and continues providing users with tactile
sensations even outside the paving, allowing them to find theirway.

7

FUTURE WORK & LIMITATIONS

Study limitations. As with any study, ours is not without limitations. While we required participants to stand and step with theirfull weight on the shapes, we did not require them to walk aroundor run. The studies were conducted without walking (yet with active stepping) to minimize fatigue, enable a focused examinationof the discrimination threshold, and measure the interference of
haptics with the perception of real objects. To effectively utilize
FeetThrough in mobile scenarios, such as in use-cases 1 & 3, further
investigation on the effects of walking and running (dynamics) is
required. We expect that running would decrease the accuracy ofboth vibrotactile and electrotactile, as stronger ground vibrations
need to be accounted for.
Moreover, to keep the duration and difficulty of the task reasonable, we opted for four simple shapes rather than a larger number.
However, it is worth noting that shapes were presented simultaneously across two modalities (physical and haptic shapes), resulting
in a fairly difficult task. We recommend that researchers take theselimitations into account when building upon our results.
Towards everyday use. Our prototypes require manual attachment and individual calibration, which are open challenges inelectrode-based devices [36]. Moving towards everyday usage of
FeetThrough, one can envision electrodes integrated via conductive threads into socks (textile electrodes [29]) and automating thecalibration process [30].

8CONCLUSION

We proposed an electrotactile foot interface designed to enable users
to experience not only haptic feedback but also the physical terrain
under their feet. We identified three advantages of our approachcompared to vibrators: (1) users can simultaneously feel physicalobjects beneath their soles and distinguish between haptic feedback
and real-world sensations; (2) electrotactile stimulation has half
the discrimination threshold of vibrotactile stimulation, enablingusers to better differentiate multiple stimuli than with vibration;and (3) our FeetThrough prototype, at 0.1 mm thick, is thinnerthan vibrators, allowing for barefoot experiences. These benefits
were validated through two user studies and illustrated with three
additional applications.

ACKNOWLEDGMENTS
We would like to acknowledge the insightful feedback of our colleague Yudai Tanaka in the conceptualization of one of the studies& guidance in manuscript formatting, and the invaluable help of
Beza Desta, Alan Shen, and Noor Amin with video and photos. This
work was supported by JSPS KAKENHI Grant JP22J21190.

Keigo Ushiyama and Pedro Lopes

REFERENCES[1] Adilkhanov, A. et al. 2022. Haptic Devices: Wearability-Based Taxonomy and
Literature Review. IEEE Access. 10, (2022), 91923–91947. DOI:https://doi.org/10.
1109/ACCESS.2022.3202986.
[2] Ando, H. et al. 2002. SmartFinger: nail-mounted tactile display. ACM SIGGRAPH
2002 conference abstracts and applications (New York, NY, USA, 21 2002), 78.
[3] Anlauff, J. et al. 2018. Feel-a-bump: Haptic feedback for foot-based angular menu
selection. 2018 IEEE Haptics Symposium (HAPTICS) (Mar. 2018), 175–179.
[4] Anlauff, J. et al. 2017. VibeWalk: Foot-based tactons during walking and quiet
stance. 2017 IEEE World Haptics Conference (WHC) (Jun. 2017), 647–652.
[5] Bial, D. et al. 2012. Improving Cyclists Training with Tactile Feedback on Feet.
Haptic and Audio Interaction Design (Berlin, Heidelberg, 2012), 41–50.
[6] Brusa, T. et al. 2022. FeetBack – Redirecting touch sensation from a prosthetic
hand to the human foot. Frontiers in Neuroscience. 16, (Oct. 2022), 1–12. DOI:https://doi.org/10.3389/fnins.2022.1019880.
[7] Elvitigala, D.S. et al. 2020. GymSoles++: Using Smart Wearbales to Improve Body
Posture when Performing Squats and Dead-Lifts. Proceedings of the Augmented
Humans International Conference (Kaiserslautern Germany, Mar. 2020), 1–3.
[8] Elvitigala, D.S. et al. 2019. GymSoles: Improving Squats and Dead-Lifts by Visualizing the User’s Center of Pressure. Proceedings of the 2019 CHI Conference on
Human Factors in Computing Systems (Glasgow Scotland Uk, May 2019), 1–12.
[9] Feeken, C. et al. 2016. ClimbingAssist: direct vibro-tactile feedback on climbing
technique. Proceedings of the 2016 ACM International Joint Conference on Pervasive
and Ubiquitous Computing: Adjunct (New York, NY, USA, Sep. 2016), 57–60.
[10] Frahm, K.S. et al. 2013. Experimental and model-based analysis of differences inperception of cutaneous electrical stimulation across the sole of the foot. Medical& Biological Engineering & Computing. 51, 9 (Sep. 2013), 999–1009. DOI:https://doi.org/10.1007/s11517-013-1079-9.
[11] Franceschi, M. et al. 2017. A System for Electrotactile Feedback Using Electronic
Skin and Flexible Matrix Electrodes: Experimental Evaluation. IEEE Transactions
on Haptics. 10, 2 (Apr. 2017), 162–172. DOI:https://doi.org/10.1109/TOH.2016.
2618377.
[12] Germani, M. et al. 2013. Electro-tactile device for material texture simulation. The
International Journal of Advanced Manufacturing Technology. 68, 9 (Oct. 2013),
2185–2203. DOI:https://doi.org/10.1007/s00170-013-4832-1.
[13] Han, T. et al. 2018. HydroRing: Supporting Mixed Reality Haptics Using Liquid
Flow. The 31st Annual ACM Symposium on User Interface Software and Technology- UIST ’18 (Berlin, Germany, 2018), 913–925.
[14] Hayakawa, Y. et al. 2022. Development and application of silicone outer shell-type
pneumatic soft actuators. J. Robot. Mechatron. 34, 2 (Apr. 2022), 444–453.
[15] Hill, E. et al. 2014. Haptic foot interface for language communication. Proceedingsof the 5th Augmented Human International Conference (Kobe Japan, Mar. 2014),
1–4.
[16] Hughes, J. et al. 1990. The importance of the toes in walking. The Journal of
Bone & Joint Surgery British Volume. 72-B, 2 (Mar. 1990), 245–251. DOI:https://doi.org/10.1302/0301-620X.72B2.2312564.
[17] Iijima, Y. et al. 2019. Enhancement of range of creation of foot sole tactile illusion
by vibration stimulation of the foot instep. 2019 IEEE World Haptics Conference(WHC) (Jul. 2019), 31–36.
[18] Jingu, A. et al. 2023. LipIO: Enabling Lips as both Input and Output Surface.
Proceedings of the 2023 CHI Conference on Human Factors in Computing Systems(New York, NY, USA, Apr. 2023), 1–14.
[19] Jirattigalachote, W. et al. 2011. Virtual pebble: A haptic state display for pedestrians. 2011 RO-MAN (Jul. 2011), 401–406.
[20] Jones, B. et al. 2020. FeetBack: Augmenting Robotic Telepresence with Haptic
Feedback on the Feet. Proceedings of the 2020 International Conference on Multimodal Interaction (Virtual Event Netherlands, Oct. 2020), 194–203.
[21] Kaczmarek, K.A. et al. 1991. Electrotactile and vibrotactile displays for sensory
substitution systems. IEEE Transactions on Biomedical Engineering. 38, 1 (Jan.
1991), 1–16. DOI:https://doi.org/10.1109/10.68204.
[22] Kajimoto, H. 2021. Electro-Tactile Display Kit for Fingertip. 2021 IEEE World
Haptics Conference (WHC) (Jul. 2021), 587–587.
[23] Kajimoto, H. 2016. Electro-tactile Display: Principle and Hardware. Pervasive
Haptics: Science, Design, and Application. H. Kajimoto et al., eds. Springer Japan.
79–96.
[24] Kajimoto, H. et al. 2014. HamsaTouch: tactile vision substitution with smartphone and electro-tactile display. CHI ’14 Extended Abstracts on Human Factors
in Computing Systems (New York, NY, USA, Apr. 2014), 1273–1278.
[25] Kammoun, S. et al. 2015. Sole based tactile information display for visually impaired pedestrian navigation. Proceedings of the 8th ACM International Conference
on Pervasive Technologies Related to Assistive Environments (New York, NY, USA,
Jul. 2015), 1–4.
[26] Kao, H.-L. (Cindy) et al. 2016. DuoSkin: rapidly prototyping on-skin user interfaces using skin-friendly materials. Proceedings of the 2016 ACM International
Symposium on Wearable Computers (New York, NY, USA, Sep. 2016), 16–23.

FeetThrough[27] Kikuchi, T. 2013. Contribution of senses of foot for identification of shape, elasticity and tilt angle of ground. 2013 23rd International Conference on Artificial
Reality and Telexistence (ICAT) (Dec. 2013), 61–66.
[28] Kim, H. et al. 2006. Vibrotactile Display for Driving Safety Information. 2006 IEEE
Intelligent Transportation Systems Conference (Sep. 2006), 573–577.
[29] Knibbe, J. et al. 2021. Skill-Sleeves: Designing Electrode Garments for Wearability.
Proceedings of the Fifteenth International Conference on Tangible, Embedded, andEmbodied Interaction (New York, NY, USA, Feb. 2021), 1–16.
[30] Kourtesis, P. et al. 2022. Electrotactile Feedback Applications for Hand and Arm
Interactions: A Systematic Review, Meta-Analysis, and Future Directions. IEEE
Transactions on Haptics. 15, 3 (Jul. 2022), 479–496. DOI:https://doi.org/10.1109/
TOH.2022.3189866.
[31] Kowalzik, R. et al. 1996. Two-point Discrimination of Vibratory Perception on theSole of the Human Foot. Foot & Ankle International. 17, 10 (Oct. 1996), 629–634.
DOI:https://doi.org/10.1177/107110079601701008.
[32] Kruijff, E. et al. 2022. First insights in perception of feet and lower-body stimuli
for proximity and collision feedback in 3D user interfaces. Frontiers in Virtual
Reality. 3, (2022).
[33] Kruijff, E. et al. 2016. On Your Feet!: Enhancing Vection in Leaning-Based Interfaces through Multisensory Stimuli. Proceedings of the 2016 Symposium on Spatial
User Interaction (Tokyo Japan, Oct. 2016), 149–158.
[34] Lee, J. et al. 2007. Preliminary User Studies on Tactile Interface for Telematics
Services. 2007 IEEE Intelligent Transportation Systems Conference (Sep. 2007),
929–933.
[35] Lin, W. et al. 2022. Super-resolution wearable electrotactile rendering system.
Science Advances. 8, 36 (Sep. 2022), eabp8738. DOI:https://doi.org/10.1126/sciadv.
abp8738.
[36] Lopes, P. et al. 2018. Adding Force Feedback to Mixed Reality Experiences andGames using Electrical Muscle Stimulation. Proceedings of the 2018 CHI Conferenceon Human Factors in Computing Systems (New York, NY, USA, Apr. 2018), 1–13.
[37] Matthies, D. et al. 2014. ShoeSoleSense: demonstrating a wearable foot interface
for locomotion in virtual environments. CHI ’14 Extended Abstracts on Human
Factors in Computing Systems (Toronto Ontario Canada, Apr. 2014), 183–184.
[38] Meier, A. et al. 2015. Exploring vibrotactile feedback on the body and foot for the
purpose of pedestrian navigation. Proceedings of the 2nd international Workshop
on Sensor-based Activity Recognition and Interaction (New York, NY, USA, Jun.
2015), 1–11.
[39] Nittala, A.S. et al. 2019. Like A Second Skin: Understanding How Epidermal
Devices Affect Human Tactile Perception. Proceedings of the 2019 CHI Conferenceon Human Factors in Computing Systems - CHI ’19 (Glasgow, Scotland Uk, 2019),1–16.
[40] Nordahl, R. et al. 2012. Enhancing realism in virtual environments by simulating
the audio-haptic sensation of walking on ground surfaces. 2012 IEEE Virtual
Reality Workshops (VRW) (Mar. 2012), 73–74.
[41] Pacchierotti, C. et al. 2017. Wearable Haptic Systems for the Fingertip and theHand: Taxonomy, Review, and Perspectives. IEEE Transactions on Haptics. 10, 4(Oct. 2017), 580–600. DOI:https://doi.org/10.1109/TOH.2017.2689006.
[42] Papetti, S. et al. 2011. Rhythm’n’Shoes: a Wearable Foot Tapping Interface with
Audio-Tactile Feedback. NIME (2011), 473–476.
[43] Roudaut, A. et al. 2013. Gesture output: eyes-free output using a force feedback touch surface. Proceedings of the SIGCHI Conference on Human Factors in
Computing Systems (New York, NY, USA, Apr. 2013), 2547–2556.
[44] Rovers, A.F. and van Essen, H.A. 2005. FootIO - design and evaluation of adevice to enable foot interaction over a computer network. First Joint Eurohaptics
Conference and Symposium on Haptic Interfaces for Virtual Environment andTeleoperator Systems. World Haptics Conference (Mar. 2005), 521–522.
[45] Rovers, A.F. and van Essen, H.A. 2006. Guidelines for haptic interpersonal communication applications: an exploration of foot interaction styles. Virtual Reality.
9, 2 (Mar. 2006), 177–191. DOI:https://doi.org/10.1007/s10055-005-0016-0.
[46] Sakai, K. et al. 2017. Characteristics of illusory vibration in the toe pad inducedby vibration applied to toenail and toe force to surface. 2017 IEEE World Haptics
Conference (WHC) (Jun. 2017), 281–286.
[47] Schirmer, M. et al. 2015. Shoe me the Way: A Shoe-Based Tactile Interface forEyes-Free Urban Navigation. Proceedings of the 17th International Conference on
Human-Computer Interaction with Mobile Devices and Services (New York, NY,

UIST ’23, October 29–November 01, 2023, San Francisco, CA, USA

USA, Aug. 2015), 327–336.
[48] Solomonow, M. et al. 1977. Electrotactile two-point discrimination as a functionof frequency, body site, laterality, and stimulation codes. Annals of Biomedical
Engineering. 5, 1 (Mar. 1977), 47–60. DOI:https://doi.org/10.1007/BF02409338.
[49] Son, H. et al. 2019. RealWalk: Haptic Shoes Using Actuated MR Fluid for Walking
in VR. 2019 IEEE World Haptics Conference (WHC) (Tokyo, Japan, Jul. 2019), 241–
246.
[50] Strohmeier, P. et al. 2020. bARefoot: Generating Virtual Materials using Motion
Coupled Vibration in Shoes. Proceedings of the 33rd Annual ACM Symposium on
User Interface Software and Technology (Virtual Event USA, Oct. 2020), 579–593.
[51] Strzalkowski, N.D.J. et al. 2018. Cutaneous afferent innervation of the human footsole: what can we learn from single-unit recordings? Journal of Neurophysiology.
120, 3 (Sep. 2018), 1233–1246. DOI:https://doi.org/10.1152/jn.00848.2017.
[52] Takeuchi, Y. 2010. Gilded gait: reshaping the urban experience with augmented
footsteps. Proceedings of the 23nd annual ACM symposium on User interface
software and technology (New York, NY, USA, Oct. 2010), 185–188.
[53] Tang, H. and Beebe, D.J. 2006. An oral tactile interface for blind navigation. IEEE
Transactions on Neural Systems and Rehabilitation Engineering. 14, 1 (Mar. 2006),
116–123. DOI:https://doi.org/10.1109/TNSRE.2005.862696.
[54] Tsao, C.-A. et al. 2022. FrictShoes: Providing Multilevel Nonuniform Friction
Feedback on Shoes in VR. IEEE Transactions on Visualization and Computer
Graphics. 28, 5 (May 2022), 2026–2036. DOI:https://doi.org/10.1109/TVCG.2022.
3150492.
[55] Turchet, L. et al. 2013. Haptic Feedback for Enhancing Realism of Walking Simulations. IEEE Transactions on Haptics. 6, 1 (2013), 35–45. DOI:https://doi.org/10.
1109/TOH.2012.51.
[56] Velázquez, R. et al. 2012. Insights into the Capabilities of Tactile-Foot Perception.
International Journal of Advanced Robotic Systems. 9, 5 (Nov. 2012), 179. DOI:https://doi.org/10.5772/52653.
[57] Visell, Y. et al. 2008. A Vibrotactile Device for Display of Virtual Ground Materials
in Walking. Haptics: Perception, Devices and Scenarios. M. Ferre, ed. Springer Berlin
Heidelberg. 420–426.
[58] Viseux, F.J.F. 2020. The sensory role of the sole of the foot: Review and update on
clinical perspectives. Neurophysiologie Clinique. 50, 1 (Feb. 2020), 55–68. DOI:https://doi.org/10.1016/j.neucli.2019.12.003.
[59] Vizcay, S. et al. 2022. Design and Evaluation of Electrotactile Rendering Effects
for Finger-Based Interactions in Virtual Reality. Proceedings of the 28th ACM
Symposium on Virtual Reality Software and Technology (New York, NY, USA, Nov.
2022), 1–11.
[60] Watanabe, J. and Ando, H. 2010. Pace-sync shoes: intuitive walking-pace guidance
based on cyclic vibro-tactile stimulation for the foot. Virtual Reality. 14, 3 (Sep.
2010), 213–219. DOI:https://doi.org/10.1007/s10055-009-0137-y.
[61] Withana, A. et al. 2018. Tacttoo: A Thin and Feel-Through Tattoo for On-Skin
Tactile Output. Proceedings of the 31st Annual ACM Symposium on User Interface
Software and Technology (Berlin Germany, Oct. 2018), 365–378.
[62] Wittchen, D. et al. 2023. Designing Interactive Shoes for Tactile Augmented
Reality. Proceedings of the Augmented Humans International Conference 2023 (Mar.
2023), 1–14.
[63] Xu, Q. et al. 2016. Design and Evaluation of Vibrating Footwear for Navigation
Assistance to Visually Impaired People. 2016 IEEE International Conference on
Internet of Things (iThings) and IEEE Green Computing and Communications(GreenCom) and IEEE Cyber, Physical and Social Computing (CPSCom) and IEEE
Smart Data (SmartData) (Dec. 2016), 305–310.
[64] Yoshimoto, S. et al. 2015. Material Roughness Modulation via Electrotactile Augmentation. IEEE Transactions on Haptics. 8, 2 (Apr. 2015), 199–208. DOI:https://doi.org/10.1109/TOH.2015.2412942.
[65] Zanotto, D. et al. 2014. SoleSound: Towards a novel portable system for audiotactile underfoot feedback. 5th IEEE RAS/EMBS International Conference on
Biomedical Robotics and Biomechatronics (Aug. 2014), 193–198.
[66] Zehr, E.P. et al. 2014. Cutaneous stimulation of discrete regions of the sole duringlocomotion produces “sensory steering” of the foot. BMC Sports Science, Medicine
and Rehabilitation. 6, 1 (Aug. 2014), 33. DOI:https://doi.org/10.1186/2052-1847-633.
[67] Ziat, M. et al. 2022. Walking on paintings: Assessment of passive haptic feedback
to enhance the immersive experience. Frontiers in Virtual Reality. 3, (2022).