Full-hand Electro-Tactile Feedback without Obstructing Palmar
Side of Hand
Yudai Tanaka

Alan Shen

University of Chicago
yudaitanaka@uchicago.edu

University of Chicago
alans@uchicago.edu

Andy Kong

Pedro Lopes

University of Chicago
andykongresearch@gmail.com

University of Chicago
pedrolopes@uchicago.edu

Figure 1: (a, b) We uncover an alternative technique for electro-tactile stimulation that creates touch sensations in 11 distinct
locations on the palmar (i.e., front) side of the hand without placing electrodes at the locations where sensations are felt. (c,d) While previous approaches to electro-tactile stimulation showed great promise, they also came with a major limitation aselectrodes attached to the palmar side of the hand impair the user’s tactile acuity and dexterity; these approaches do not scale
to full-hand interactions nor support interactions with physical props. Instead, we explored placing electrodes in strategic
locations at the back of the hand and wrist, which enables the electrical currents to pass through the median/ulnar nerve, thus
providing (e, f) tactile feedback to the palmar side of the hand while keeping it free to interact with physical objects.

ABSTRACT
We present a technique to render tactile feedback to the palmar sideof the hand while keeping it unobstructed and, thus, preserving
manual dexterity during interactions with physical objects. We
implement this by applying electro-tactile stimulation only to theback of the hand and to the wrist. In our approach, there are noelectrodes on the palmar side, yet that is where tactile sensations
are felt. While we place electrodes outside the user’s palm, we doso in strategic locations that conduct the electrical currents to themedian/ulnar nerves, causing tactile sensations on the palmar sideof the hand. In our user studies, we demonstrated that our approachrenders tactile sensations to 11 diferent locations on the palmarside while keeping users’ palms free for dexterous manipulations.
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CHI ’23, April 23–28, 2023, Hamburg, Germany© 2023 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ACM ISBN 978-1-4503-9421-5/23/04. . . $15.00
https://doi.org/10.1145/3544548.3581382

Our approach enables new applications such as tactile notifcations
during dexterous activities or VR experiences that rely heavily on
physical props.

CCS CONCEPTS• Hardware → Communication hardware, interfaces and storage;
Tactile and hand-based interfaces; Haptic devices.

KEYWORDS
Electro-tactile, Haptics, Mixed Reality, Virtual Reality
ACM Reference Format:
Yudai Tanaka, Alan Shen, Andy Kong, and Pedro Lopes. 2023. Full-handElectro-Tactile Feedback without Obstructing Palmar Side of Hand. In Proceedings of the 2023 CHI Conference on Human Factors in Computing Systems(CHI ’23), April 23–28, 2023, Hamburg, Germany. ACM, New York, NY, USA,
15 pages. https://doi.org/10.1145/3544548.3581382

1

INTRODUCTION

A driving vision for HCI has been to bring all our senses into
interactive experiences—as Sutherland put it “[the interface] should
serve as many senses as possible” [65]. This also implies that haptic
devices should not be limited to fngerpads but extend to the whole

CHI ’23, April 23–28, 2023, Hamburg, Germany

hand, which is our primary means of interaction. In fact, a recent
study by Maunsbach et al. has shown that even an interaction as
mundane as “pressing a button” is improved if the haptic feedback“extends to the whole hand” [46]. To this end, our community has
developed many successful haptic devices that can render contact,texture, or force to many locations of the user’s palm [2, 14, 17, 22,
26, 35, 70]. These devices achieve their spatial fdelity by attaching
many actuators directly to the part of the user’s hand that they
are meant to stimulate. For instance, the recent Meta glove [30]features tactile actuators placed directly at ffteen locations on theuser’s palmar side of the hand: fngerpads, fnger segments, andpalm—this provides realistic hand sensations.
However, this existing approach to realizing full-hand tactile interfaces comes with a major limitation. Placing the actuators directly at the user’s palmar side prevents using these tactile
haptic devices beyond purely virtual interactions [52, 67]. In otherwords, these actuators prevent the user’s palms from feeling haptic feedback from the physical object they are grasping, and more
dramatically, limit their dexterity to grab or fnely manipulate thisobject—this prevents the usage of these tactile devices in many
interactive situations, such as mixed reality.
To circumvent this key issue, researchers have been engineering
actuators that minimize the encumberment in three technical ways.
The frst is to switch to thin haptic actuators that allow the user tofeel-through physical objects to some degree [25, 71]. However, theuser still feels these thin actuators between their fngerpads andthe physical world, resulting in a decreased sensation of textured
surfaces [52]. A second alternative is to use foldable actuators thatkeep the user’s fngerpad or palms free [41, 67]. These devices fold
the actuators away from the palm or the fngerpad when the users
are not interacting with virtual objects. However, these are still
limited in size (i.e., folding is done by mechanical actuators, whichstill impair dexterity as these occupy much of the sides and backof fngers/hands) and in the application domain (i.e., do not allow
for tactile augmentation of real objects). Finally, a third approach,from which we take inspiration, is to place the haptic actuators
away from the fngerpad but drive them in a way that causes thesensation to occur at the fngerpad. Researchers have applied thisconcept to creating sensations only at the fngerpads, by using
vibrotactile actuators on the nail [5, 56] / back-of-the-fnger [40],or using electro-tactile stimulation via electrodes on the middle
and proximal phalanges of the index fnger [74, 75]—unfortunately,
these electrodes are still on the palmar side. Even a single electrodeon any part of the palm would exacerbate manual dexterity. Thisis because manual activities involve the whole hand [60], and its
dexterity builds on the sensory image of the entire palmar side
synthesizing each part’s sensation [33, 63]. Therefore, we strive tokeep the palmar side entirely unobstructed.
To address this, we propose applying electro-tactile stimulation
only to the back of the hand and the wrist—without electrodes on thepalmar side, yet that is where sensations are felt (Figure 1b). We
discovered that this is possible by strategically placing electrodes,
building on a neurological concept called referred sensations. To
fnd a suitable electrode arrangement, we tackled the following
challenges: (1) fnding the anatomically suitable location to placeelectrodes (we found that the back of the hand is superior to thearm, due to the separation of nerves that conduct sensations to each

Yudai Tanaka et al.

fnger); (2) fnding a stimulation intensity that creates sensations
primarily on the palmar side, rather than the back side (we leverage
the hand’s dorsal vs. palmar tactile asymmetry); (3) optimizing thestimulation polarities (anodic or cathodic) to increase the numberof stimulated locations. In our frst user study, we validated that theresulting electrode arrangement rendered tactile sensations in 11
distinct locations on the palmar side of the hand. To the best of ourknowledge, this is an unprecedented number of referred sensationsin any related research feld including neurology, rehabilitation,and prosthetics.
Finally, enabling haptic feedback to the palm while keeping thepalm free opens applications where users can feel virtual tactile
feedback while simultaneously touching and grasping physical objects, which we validated in our second study and demonstratedusing three applications: (1) integrating physical props in a VR
bouldering experience (Figure 1e), (2) providing tactile notifcations
while DJing, and (3) feeling both virtual & physical models in mixed
reality.

2

RELATED WORK

Our work is built primarily on (1) electrical stimulation for tactile
feedback (known as electro-tactile) and (2) haptic devices designed
purposefully with the goal of keeping the user’s hand unobstructed.

2.1Electro-tactile Stimulation

While most haptic devices use vibration or pressure via mechanical actuators [11, 12, 14, 42], they do not easily scale down. Thus,
researchers keep exploring ways for further miniaturization, andsome have turned to electrical stimulation.
By stimulating nerves with electrical currents, an interface can
induce tactile sensations, this is referred to as electro-tactile stimulation. The application of this technique has historical roots datingback to 1970, when researchers utilized electro-tactile stimulation
for blind users [64]. In the late 1990s, Kajimoto et al. proposed atactile display based on the stimulation [38]. Since then, electrotactile stimulation rapidly grew as a haptic actuator, and by now, anumber of systems build on it for diferent purposes, e.g., simulating
material textures [4, 21, 73], hand skill training [68], or extending
mobile surfaces [37]. For providing tactile feedback to the whole
hand, Kajimoto et al. proposed a cylindrical handheld device whose
surface is an array of stimulation electrodes [38]. Abbass et al. extended this concept further by proposing a haptic glove with a layerof arrayed electrodes for rendering tactile sensations to the wholehand [2]. Pamungkas et al., also explored whole-hand feedback via
electro-tactile stimulation, to add realism in VR, by stimulating theuser’s entire hand, at once, via a conductive glove [53]—this conductive glove is fully closed, as such, it also applies stimulation to
the back side of the hand. Unfortunately, all these devices obstruct
the hand with electrodes, impeding the user’s ability to interact
with the physical world.

2.2

Haptic Devices that Preserve the Hand’s
Tactile Acuity

Most haptic devices are designed to render sensations for touchingvirtual objects. Thus, the traditional way to realize haptics was toplace actuators directly at the target location where the sensation

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

is meant to be evoked [14]. However, with the rise of AR (and evenprop-based VR), users need to move back and forth between VR andthe real world, or even interact with augmented physical objects
or VR props. Unfortunately, the traditional approach (actuatorsplaced on targets) impedes the user’s ability to feel and dexterously
manipulate real objects.
Many researchers have realized this tension and tackled it via
diferent routes. We present these grouped into fve categories: (1)foldable actuators; (2) thin actuators; (3) relocated actuators; (4)non-contact actuation; and (5) remote actuation (also known as
referred sensations)—since our approach falls in this conceptual
category, we discuss this approach in further detail in Section 2.3.
Foldable actuators. One approach that keeps the user’s palmarside of the hand free is to leverage an actuator that provides thehaptic efect on-demand but then folds away while not in use. For
example, Touch & Fold [67] allows users to feel their fngerpads
touching virtual objects, while also quickly reaching to grab tools(the device responds and tucks away prior to the user grabbing
the tool). Haptic Pivot [41] takes this approach further and into thewhole palmar side, by moving a ball-shaped end-efector from theforearm to the palm. However, these devices display two striking
limitations: (1) they are only suited for touching either virtual orreal objects, but do not allow users to touch both at the same time;and (2) these approaches are mechanical in nature, which implies
that they require substantial time to activate (340ms for [41] and92ms for [67]) and end up larger and more encumbering than flms
or electrodes.
Thin actuators. A strategy is to balance tactile feedback onreal and virtual objects by placing a thin actuator on the skin. For
example, Tacttoo [71] is an electro-tactile device built on a thin flmof electrodes that stimulates the fngerpad while allowing the user
to feel physical objects through the flm to some extent. Similarly,
HydroRing [25] is a soft ring that renders pressure, vibration, andtemperature in mixed reality, while still being soft enough thatusers can feel slightly through it. While these approaches are very
promising, the user still feels the impairment of the thin actuators
every time they touch physical objects, impacting their sensationof textured surfaces [52].
Relocated actuators. Another way to preserve tactile sensations is to move the actuator completely from the target area andplace it somewhere else—this is often denoted as relocated haptics[5, 50]. The idea is to still deliver a sensation but not at the “correct”
location. For example, Ando et al. [5] and Haplets [56], utilize vibrotactile actuators on the fngernail to render sensations that should,
normally, be felt at the fngerpads. Some approaches go further by
substituting feedback to fngerpads by applying mechanical pressures to the wrist [55, 58], or the forearm [50, 51]. The resulting
haptic devices engineered from this principle, excel in minimizing
any encumberment to the target area but sacrifce realism for this—because the tactile sensation occurs in a location where the usermight not have expected it.
Non-contact actuation. A conceptually diferent strategy is
to install actuators in the environment, rather than in the user’s
skin—thus, rendering tactile feedback in a non-contact manner. One
canonical example of this is to cause skin displacement via focusedultrasound [9, 28]. Similarly, researchers also employed air fow torender this type of tactile feedback [24, 61]. However, this requires

CHI ’23, April 23–28, 2023, Hamburg, Germany

a line of sight to the user’s skin, which prohibits their application in
scenarios where the hands move dexterously in various orientations.
Moreover, this approach’s interactive volume is still limited.

2.3

Referred sensations: creating tactile
sensations in the target by stimulating adiferent location

The ffth approach, which we draw inspiration from, is to create
sensations in a target skin area, but using actuators attached toanother patch of skin. Unlike relocated actuators, this approachstimulates the intended target (not in an unrelated skin patch). Thishas been achieved in two diferent ways: (1) using constructive
interference of vibrations; and, more important to our approach, (2)via electrical stimulation of the nerves to create sensations “further
away”.
Remote tactile sensations via constructive interferenceof vibrations. Dandu et al. have demonstrated that vibrotactile
stimuli directly applied to the fngertip can cause tactile sensations in the middle phalanx, the proximal phalanx (fnger base),or the whole fnger via constructive interference of the vibrations
propagating the fnger [15, 16]. While this shows the fundamental
insights and the potential of the approach, the device itself covers
up the fngerpad.
Referred sensations via electrical stimulation. Referred sensations are defned as “somatosensory feelings that are perceived
to emanate from a body part other than, but in association with,the body part being stimulated” [47]. This has been primarily explored in prosthetics to provide tactile feedback to the prosthetic
hands or feet of amputees [10, 43, 54] . Recently, neuroscientists
started investigating this phenomenon in subjects with intact limbs,
and reported tactile sensations were evoked on the palmar sideof the hand through electrical stimulation at the forearm [20, 59],
elbow [19], and arm [69]. Specifcally, Alonzo et al. showed stimulating the lower palm could evoke sensations in the fngers [3].
Moreover, this can also be leveraged for interactive devices. Yoshimoto et al. leverage it by attaching a pair of electrodes to the rootof the fnger, on the palmar side, to cause a tactile sensation at thefngerpad [74, 75]—we depicted this important approach in our
Figure 1 (c). While we think this is a step in the right direction
and take inspiration from it, we take this further by removing all
the electrodes from the palmar side, removing the encumbermentof this critical tactile area. Finally, while these approaches support
fngerpad haptics, they have not been shown to scale to more partsof the hand. Our work presents the frst actuation technique thatscales to many locations of the palmar side of the hand.

3

OVERCOMING THE CHALLENGES TO
CREATE TACTILE SENSATIONS AT
MULTIPLE POINTS IN THE FRONT OF THE
HAND WHILE KEEPING IT ENTIRELYUNOBSTRUCTED

We tackled two, seemingly, incompatible goals: (1) rendering tactile sensations on multiple locations of the hand’s palmar side,
while simultaneously (2) preserving the tactile acuity of the

CHI ’23, April 23–28, 2023, Hamburg, Germany

same locations where we create sensations. We tackled this by electrically stimulating the palmar digital nerves (namely, the medianand ulnar) via electrodes attached to the dorsal (i.e., back) side of
the hand and the wrist. In this section, we illustrate key challenges
that led us to fnd our fnal electrode arrangement to realize thisback-to-front tactile stimulation.
All the following fndings in this section were informed by six
months of pilot experiments as well as consultation with an external scholar who is an expert on tactile stimulation in the feld of
neuroscience (see Acknowledgments).

3.1

Decoupling stimulation points from thepalmar side of the hand by evoking referredsensations

Our approach builds on the fact that electrical currents can cause
sensations in mechanoreceptors, even in those distant from thestimulated point [48]—the aforementioned concept denoted as referred sensations [3, 19, 20, 59]. This implies that, in theory, it would
be possible to cause sensations on the palmar side of the hand by
selectively stimulating nerves anywhere between the hand andthe brain (e.g., forearm, elbow, arm). To realize our goal, the frst
step was to fnd suitable stimulation points that cause sensations
at multiple points on the palmar side, including for all individual
fngers.

Yudai Tanaka et al.

stimulation from evoking sensations in individual fngers without
interference across fngers [59, 69].
However, as shown in Figure 2 (a), the nerves start branching out
for the fngers after the wrist. Thus, we proposed stimulating theback of the hand, where the nerves are separated, becoming easiertargets for electrical stimulation; to the best of our knowledge, thisis the only way to achieve per-fnger referred sensations without
interference.

3.3

Now, one could expect a naïve electrode arrangement for the back of
the hand: applying two electrodes (positive and negative) directly
atop a target location. Applying this to our technique, one canimagine attaching two electrodes next to each other on the dorsalside of a target fnger, hoping that they stimulate the palmar-side
mechanoreceptors (Figure 2b). Unfortunately, this is where we fnd
our second technical challenge—the current fow between
these electrodes is too shallow to stimulate palmar digital nerves
and it only stimulates the dorsal side. This is because the closer twoelectrodes are together, the shallower the current penetrates into
the tissue [36].
However, as prior work in electrical stimulation has demonstrated, increasing the distance between the electrodes allows thecurrent fow to also stimulate deeper regions [36]. Indeed, adopting
this, we discovered that the stimulation with a large inter-electrodedistance, as depicted in Figure 2 (c), did generate sensations on thepalmar side of the hand.

3.4

Figure 2: (a) The digital (i.e., fnger) nerves branch out forindividual fngers after the wrist. (b, c) An inter-electrodedistance must be large to stimulate palmar digital nerves;
otherwise, the current fow only stimulates the back side
(dorsal digital nerves).

3.2

Challenge #1: stimulation from the forearmor wrist does not realize sensations inindividual fngers

Figure 2 (a) shows the two main nerves directly connected to themechanoreceptors on the palmar side of the hand. As illustrated,both nerves go through the forearm and then the wrist. At thewrist, these nerves connect directly to the palm. This allows us to
immediately stimulate the whole palm via electrodes at the wrist,
as with prior work leveraging referred sensations to create similarsensations. Theoretically, one would expect to stay at the wrist (oreven forearm) and be able to create sensations in the individual
fngers. Unfortunately, this is where we found our frst technicalchallenge—this is not possible as the nerves are too close toeach other (bundled) at the wrist. This prevents (non-implanted)

Challenge #2: electrodes applied naïvely tothe back of the hand do not stimulate thefront of the hand

Challenge #3: stimulation from the backside creates sensations on both sides

Now, the mechanism described so far should not be sufcient forrealizing our goal. As depicted in Figure 2 (c), the current is stimulating both the dorsal side of the fnger (undesired) as well as thepalmar side of the fnger (desired). Applying this idea naïvely would
suggest a mixed sensation on both sides—our third technicalchallenge. To make the sensation on the palmar side more dominant, we leverage the asymmetry in tactile sensitivity between thepalmar and dorsal side of the hand, which is depicted in Figure 3 (a).
In fact, prior work has shown that the palmar side has ∼60 times
more mechanoreceptors (≈18000) than the dorsal side (≈300)[8, 32, 45]—this is understandable since the palmar side plays themost important role in dexterous manipulations.
Now, the density of mechanoreceptors is correlated to the sensory threshold to the electro-tactile stimulation [36, 44]. As such,
this asymmetry makes the dorsal side less sensitive to stimulationcompared to the palmar side. Using this, we can fnd a stimulation intensity that surpasses the sensory threshold of the palmarside while staying sub-threshold on the dorsal side. With this, wefound it to be feasible to primarily stimulate the palmar side of thehand by adjusting the intensity of electro-tactile stimulation to thisrange. (Note that we used the range of stimulation intensity cappedby 4 mA, which is substantially below the threshold for muscle
contraction via the back-of-the-hand stimulation [66].)

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

Figure 3: (a) Tactile threshold for electrical stimulation issignifcantly lower on the palmar side than on the dorsal
side. (b, c) Utilizing both polarities (anodic and cathodic) forthe electrode attached to the fngertip enables stimulationof the fngerpad.

3.5

could also create referred sensations in a segment of the ring oreven the pinky fnger, which is entirely undesired.
The reason behind this interference is anatomical; Figure 4 (a)depicts how the median and ulnar nerves divide the left and rightregions of the hand, i.e., the thumb, index, and middle fngers sense
via the median nerve whereas the ring and pinky fngers sense via
the ulnar nerve. If an electrical current traverse the two nerves,
it can cause sensations to be felt across multiple fngers. To prevent this interference, we implemented two ground electrodes that
respectively serve the median and ulnar nerves. By using one of
the two grounds at a time depending on which nerve the target
fnger belongs to, we found that the currents primarily stimulated
the correct fnger.

Challenge #4: stimulating the back side of
the fnger results in ofset sensations on thefront side

Putting all our solutions so far together, we can posit the electrodearrangement depicted in Figure 3 (b) to create sensations in eachfnger segment, i.e., distal phalanx (i.e., fngerpad), middle phalanx,
and proximal phalanx. Note that these electrodes are paired with aground electrode attached further away on the back of the hand.
We found this arrangement did not perform well for two reasons.
Firstly, we found that the proximal-phalanx electrode is not able tocause a tactile sensation on the palmar side—this can be explainedby the presence of a tendon layer (central slip) under the electrodelocation [27]. Instead, we found that the middle-phalanx electrodecreates sensations on the palmar side of the proximal phalanx,
which revealed a more fundamental issue, our fourth technicalchallenge—electrodes on the back of the fnger create ofset
sensations on the palmar side of the fnger, as depicted in Figure 3(b). Importantly, this revealed that none of these electrodes createdsensations at the fngerpad, i.e., the distal-phalanx segment.
To tackle this, we resorted to a known practice demonstrated
in prior work: cathodic stimulation advances the location of thesensation in the fngertip direction [36, 74]—yet, so far, all our
stimulation followed the conventional anodic current fow. As such,
we applied this polarity inversion to the distal-phalanx electrodeand confrmed that the electrode is able to stimulate the fngerpad
from the back of the fnger, resulting in the electrode arrangement
depicted in Figure 3 (c). Note that our device uses the distal-phalanx
electrode for stimulating the fngerpad and the middle-phalanx
targets by switching between the cathodic and anodic stimulation(see the Implementation section for details). Finally, the thumb has
only one electrode on the distal phalanx as it has one joint less.

3.6

CHI ’23, April 23–28, 2023, Hamburg, Germany

Challenge #5: stimulating the back side with
one ground electrode creates inter-fngerinterference

Finally, we observed that the resulting electrode confguration so
far not only caused sensations in the target fnger, but also unwanted tactile feedback in other fngers. This is our ffth technicalchallenge—fngers do not individually code tactile sensations.
For instance, while stimulating a segment of the middle fnger, it

Figure 4: We implemented (a) two ground electrodes to avoid
median/ulnar interference. (b) Our fnal electrode arrangement.
Combining our fve solutions, described in this section, weestablished our fnal electrode arrangement, shown in Figure 4 (b),which can create tactile sensations at up to 15 locations (one on
the palm, two on the thumb, three on each of the index, middle,
ring, and pinky fngers). Tying this up with our Study 1’s results,
we discourage the use of the middle segments, which we found to
exhibit more spatial variance (blurred sensations). Choosing locations where we found lesser spatial variance (focused sensations),
our approach totals 11 locations of the user’s palmar side, without
placing a single electrode on the palm.

4

CONTRIBUTION, BENEFITS, AND
LIMITATIONS

Our key contribution is a method to render electro-tactile feedback
to the palmar side of the hand while keeping it unobstructed. The
benefts of our approach include: (1) Renders tactile sensations
on the palmar side of the hand, while maximizing the hand’s
dexterity during interactions with physical objects; (2) Scales wellto full-hand interactions—in fact, we know of very few wearable
techniques that are capable of 11 distinct tactile stimulation points
on the palmar side of the hand, and none that realize this without
physically attaching actuators to the palmar side; and, (3) Applicable in a range of domains, from VR experiences that rely heavily
on prop manipulation, AR experiences that use physical tools, ormobile applications that render tactile feedback during dexterous
activities.

CHI ’23, April 23–28, 2023, Hamburg, Germany

Yudai Tanaka et al.

Figure 5: (a) Out wrist-worn electro-tactile stimulator. (b) Electronics schematics of our custom electrical stimulator.
Our approach is not without limitations. First, as with mostelectrical-stimulation techniques, it requires per-participant calibration of intensity and electrode orientation within fnger segments.
Second, while we found that most of the primary sensations werefelt on the palmar side of the hand (93.3%), some primary sensations
could still be felt on the back of the hand (6.7%). Third, while our approach keeps the palmar side free and preserves some tactile acuity,
it still uses electrodes on the dorsal side, which might restrict someof the user’s movements. Fourth, to make our technique readily
available for any stimulator and existing electro-tactile implementations, we used the traditional square waveform for stimulation,
hence there is ample space for increasing the quality of perceived
sensation by exploring custom waveforms [6]. Fifth, our study population was limited to adults between 20 and 28 years old. Finally,
as with existing electro-tactile stimulation, ours is unable to render continuous pressure, which would have been benefcial forincreased realism in our VR bouldering application. Others havealso argued for the importance of rendering pressure [13]—we agree
and acknowledge that this remains an open challenge in electrotactile stimulation.

5

IMPLEMENTATION

To help readers replicate our design, we now provide the necessary
technical details. Furthermore, to accelerate replication, we provide
all the source code, frmware, and schematics of our implementation1 .

5.1

Electronics: custom 10-channel electricalstimulator

As shown in Figure 5 (a), we implemented a wrist-worn electrotactile stimulator that can output currents to 10 polarity-switchable
channels (i.e., cathodic/anodic stimulation). It measures 3.4 cm ×4.6 cm × 4.2 cm and weighs 34 g.
Circuit design. As depicted in Figure 5 (b), we power our stimulator using a 3.7 V LiPo battery, converted to 5 V by a step-up
1 https://lab.plopes.org/#boh-electro-tactile

regulator (U1V11F5). This 5V powers the microcontroller (Seeeduino XIAO) and the Bluetooth module (HC-06). To generate the72V voltage supply for electrical stimulation we added a 5V/72V
DC-DC converter (NMT0572SC).
Generating electro-tactile impulses. When our microcontroller receives a serial message from an application via Bluetooth,it responds by generating an analog signal using its DAC; this 0-3.3V
signal controls the stimulation intensity. Using a 10-bit resolution
DAC allows our system to perform precise increments to fne-tune
the tactile thresholds, which is a key requirement for back-of-handtactile stimulation. We feed this DAC output into a dual op-amp(LMV358) and a FET (BSS87) to output a load-independent current.
Then, two transistors (FCX705) duplicate the current source, outputting it to an analog-switch IC (HV2701). We use it to direct thestimulation to a target pair of electrodes with a specifc polarity:
SW1-4 forms an h-bridge that dictates the stimulation polarity,
while the states of SW5-16 decide which pair of electrodes output the stimuli. For instance, when stimulating the fngerpad of
the index fnger, the IC confgures SW1,3,6,15 to ON and all otherswitches to OFF—a cathodic stimulation between channel-2 andbase-1 electrodes.
Electrodes. We utilized of-the-shelf pre-gelled electrodes from
Omron, sized as follows: 1.1cm2 rectangular electrodes for the fngers; 2.5cm2 rectangular electrodes for the wrist; and 5cm2 electrodes for the bases.
Stimuli. For the fngerpads, we deliver cathodic currents to theelectrodes at the distal phalanges. For the proximal phalanges andthe palm, the device outputs anodic currents to the middle-phalanx
and the wrist electrodes respectively.
Application-specifc haptic efects. Based on the literature[21, 36, 39, 75] and our initial pilots, we used the following parameters for haptic efects shown in the Application section: (1) forpressing MR buttons, we output a single 50-ms pulse; (2) for othertouching or grasping interactions (e.g., touching a teddy bear or
DJing), we used 100 Hz pulses with 400-�s pulse width; and (3) for
rendering the roughness of the bouldering pegs, we used 40 Hz
pulses with 800-�s.

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

Temporal Multiplexing. To mitigate interference across multiple channels, our switching IC only opens a current path for one
target at a time. Still, as this IC can toggle between channels in 10
�s [29], our device completes a cycle of sequentially single-pulse
to all 11 targets within 10ms. Since the temporal discriminationthreshold of tactile stimuli is about 50ms [57], this means our device
can stimulate multiple targets concurrently in the user’s perception,
up to 100 Hz.
Safety Measures. To ensure a safe operation, the variable resistor is adjusted to set the maximum current to 4 mA. As a fail-safe,
we also added a current limiting diode (E-452) that shuts of currents
over 4.5 mA.

5.2

Stimulus. We used the traditional square wave stimulation withthe current polarities described in Implementation (i.e., cathodic forthe distal-phalanges and anodic for all others).
Ensuring no bias from calibration. The calibration process
typical of electrical stimulation (i.e., attach electrodes, stimulate, ask
perceived touch location, move/rotate electrodes, and repeat until
the sensation is on the expected location) would bias participants’
responses during actual trials as they would expect the sensation to
be at the location they reported during calibration. Thus, instead of
running calibration before trials, we conducted trials with multiple
electrode adjustments and, per participant, used the data from thebest (i.e., calibrated) electrode adjustment.

Tracking, display & application-specifc
implementations

Our MR/VR applications run via a Quest 2 headset and Unity3D.
Moreover, the headset also tracks the user’s hands. For the VR
bouldering simulator, we used a VR hand physics simulator [34] tomake the virtual hands compliant when touching virtual objects. For
the MR clay modeling application, we adopted Quest’s PassThroughAPI [49] (we also explored the Hololens 2, but found that it could
not provide robust hand tracking while touching physical objects).
For our DJ application, we implemented its backend using
Max/MSP and the front-end uses MIXXX (DJ application that supports Digital Vinyl System, allowing DJs to play digital audio via
turntables). To decode the timecode information stored on the digital vinyl record we utilized xwax, a decoder to extract the current
speed and position of a record. Our implementation compares theseto the metadata saved for the records (which DJs save using MIXXX,
including the original BPM, cue points, and so forth). We use thisto determine when to render an electro-tactile cue, for example,
when the current record position matches any saved cue point.

6

CHI ’23, April 23–28, 2023, Hamburg, Germany

USER STUDY #1: LOCALIZING THE
PERCEIVED TACTILE SENSATIONIn our frst study, we measured where the participants perceived
tactile feedback on their hands. We stimulated all 15 possible locations via electrodes on the back of the hand. In each trial, we
stimulated one location, then we asked participants to denote theskin area where they felt the stimulation and its strongest point—
this study design is based on traditional psychophysics methods
employed also by prior work to investigate perceived locations ofelectro-tactile stimuli [59, 69, 74, 75]. Our study was approved by
our Institutional Review Board (IRB21-1229).
Participants. We recruited ten participants from our institution (7 identifed as male, 3 as female, average age = 23.9 years,
SD = 2.5). All participants were right-handed. The participants
were compensated with $30 USD.
Apparatus. Participants sat at a desk with their non-dominanthands resting on a cushioned arm stand with our stimulator andelectrodes connected. To ensure good conductivity, we applied
conductive gel on electrodes. We provided an iPad and Apple pencil
so that they could draw (with their dominant hand) to indicate theperceived sensation area on our GUI, which depicted both a palmarside and dorsal side of a hand model.

Figure 6: To not bias participants’ responses in locating theperceived sensation, we included all these electrode adjustments as trials.
Electrode adjustment (only within a fnger segment). As
described in our pilot experiments (see Section 3), we found that our
technique worked with fxed locations, i.e., electrodes are placed
within the fnger segments. However, while coarse location calibration is not needed, as with most electrical-stimulation techniques,
it requires minor adjustment of the electrode position and orientation within the fnger segments. Thus, we used the following
electrode adjustments, depicted in Figure 6, determined from pilot
experiments: (a) fve adjustments for the distal phalanges and (b)six adjustments for the middle phalanges. We found the wrist andthe ground electrodes do not need adjustment.
Study procedure. Each participant performed 75 trials in arandomized order: all 15 targets in all aforementioned electrode adjustments. At the beginning of each trial, we calibrated the intensity
by increasing the current amount by 0.1 mA steps while ensuring
it was pain-free (the maximum current limit was set to 4mA). We
stopped at the intensity where participants noticed any tactile sensation, i.e., we did not ask where the sensation was felt. We used
this intensity + 0.1mA to ensure participants could clearly perceive
the stimulation, yet still operated pain-free (after calibrating all
participants, we found an average intensity of 1.85 mA, SD=0.8).
Then, after a random waiting period, our device output ten 50-ms
pulses spaced by one second. During stimulation, our interface reminded participants to move their hand between two poses: restingon the desk and staying in mid-air; this allowed participants tofeel stimuli in both skin-contact and non-contact conditions. Finally,
the participant indicated the point where the sensation was thestrongest and the area where the sensation occurred on our GUI. At
the end of the trials for each target, we arranged the responses by
the distance from the strongest point to the center of the target area.
Then, to analyze data based on calibrated electrode adjustments,
we selected the response with the smallest distance.

6.1

Results

Palmar vs. dorsal side. Figure 7 depicts our key fndings. We
found that, in aggregate, 93.3% of points where the strongest

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Yudai Tanaka et al.

Figure 7: (a) The 15 target locations on the palmar side of the hand to elicit tactile sensation via our back-of-hand electro-tactile
stimulation. (b) The overall ratio of the tactile sensation elicited on the palmar vs. dorsal side. (1-15) Overlayed raw data for allparticipants; black points correspond to strongest points; white points are averages; colored shades depict the area indication.
sensation was felt occurred on the palmar side, despite theelectrodes being attached to the dorsal side and the wrist. Moreover,
we found that the stimulated area was 82.5% on the palmar side
(SD=13.41 %). In fact, the majority of unwanted (dorsal) sensation
was reported only for the middle and pinky fngers. Conversely,

for all participants, the strongest points for the index, middle, andpalm were always on the palmar side. On average participants felt
strongest points on the palmar side for 14 out of 15 targets (SD =1.63; minimum = 11; maximum = 15).

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

Location accuracy. Figure 7 (1-15) also depicts the average location (white dot), per target, where sensations were felt. To further
analyze these in detail, we depict in Figure 8 (a), all 15 locations:each ellipse depicts the center of the average location and the horizontal/vertical standard deviations of all strongest points on thepalmar side for each target. Overall, the distal-phalanx (fngerpad)
targets had smaller deviations than the others. Moreover, as depicted in Figure 8 (b), we found a clear spatial separation (i.e., thestandard deviations did not overlap with each other) between fnger
segments when we exclude the middle-phalanx ones which tended
to overlap primarily with the proximal phalanges (Figure 8a).

CHI ’23, April 23–28, 2023, Hamburg, Germany

participants felt the strongest tactile sensation on the dorsal side;(2) in 11 trials, participants felt the sensation on the palmar side
but of from the target fnger segment.

7

Our unencumbering tactile feedback enables a variety of applications, including some not possible before, for instance: (1) VR
climbing with haptic props; (2) DJ’ing with tactile notifcations; and(3) modeling clay in mixed reality.

7.1

Figure 8: The aggregated plots of the means and the standard deviations of the strongest points across all participants.
For each ellipse, the center point depicts the mean, and thehorizontal radius and the vertical radius represent the horizontal and vertical standard deviations respectively. (a) The
version with all 15 targets. We found that sensations at the
middle phalanges overlap with those at the proximal phalanges. Therefore, (b) we concluded that 11 individual targets
are robust with our approach.
Study interpretation. Overall, we found out that our approachcreates tactile sensations mostly on the user’s palmar side of thehand, not the dorsal side. Moreover, even if we discard the middle phalanges where sensations were more blurred (higher spatial
variance than other regions), we still found that our approach canrender an unprecedented number of distinctive points—the 11 tactile locations depicted in Figure 8 (b). Yet, we do acknowledge that:(1) in rare individual cases (10 out of 150 trials, 6.7% of total trials),

APPLICATIONS: KEEPING THE PALMAR
SIDE FREE ENABLES NEW INTERACTIVE
EXPERIENCES

Adding Physical Props to VR Bouldering
Simulator

Using physical props that stand in for virtual objects is a popular
technique to yield higher haptic realism in virtual environments[7, 31]. However, typically, this approach is employed when users
are not wearing any haptic wearable devices, such as vibration
gloves, because most haptic devices would impair the user’s ability
to manipulate and feel the rich tactile feedback from the props. We
demonstrate how our technique can alleviate this as it allows users
to feel both physical props as well virtual feedback. As shown in
Figure 9 (a), the user is in a VR bouldering simulator and before
climbing the wall, they put chalk onto their hands. Our device does
not encumber the user’s palms while rubbing the real chalk, whichacts as a physical prop to enhance immersion.
While climbing, they feel electro-tactile stimulation at the locations where their hands touch the bouldering pegs—in fact, Figure 9(b) shows how the user’s thumb and pinky are not touching the peg
and thus do not experience feedback; this depicts how our approachis selective at the level of individual fnger segments. Note that ourdevice only presents the tactile aspect of the virtual boulderingpegs, unable to render force feedback (e.g., continuous pressure) as
this is a limitation of contemporary electro-tactile stimulation (seeLimitations).
Moreover, this VR climbing experience is furnished with anotherprop, a climbing rope attached to a pulley in the ceiling. As depicted
in Figure 9 (d), the user can grab the rope and feel its texture, or even
apply force on it, without any encumbrance from our electrodes.

Figure 9: Our VR bouldering includes grabbing virtual pegs to climb but also manipulating real props (e.g., chalk and a rope).

CHI ’23, April 23–28, 2023, Hamburg, Germany

Yudai Tanaka et al.

Figure 10: Our electro-tactile feedback allows this DJ to receive tactile notifcations while still using their fngerpads to mix.

7.2

Tactile Notifcation in Dexterous Activities(e.g., DJing)

In our DJ application, our tactile feedback approach supports a DJ’s
performance without encumbering their hands’ dexterity, i.e., placing the vinyl onto the turntable, manipulating the mixer, “scratching” the record, etc. As depicted in Figure 10 (a), the user is DJ’ing,
attempting to mix the two turntable decks (A and B) together, by
matching their tempo. DJs that use Digital Vinyl Systems (e.g., thepopular Serato [1], which allows playing digital audio using traditional turntables) rely on visual information in their laptops for
mixing. Instead, our DJ uses electro-tactile notifcations.
First, the DJ adjusts the tempo of deck B by moving theturntable’s pitch slider. When the tempo is in sync with that ofdeck A, the tactile feedback to the fngers holding the slider notifes
the user, as depicted in Figure 10 (b). Now that deck B is at thecorrect tempo, they need to fnd the cue point to fade deck B into
the main mix. Again, DJs that use Digital Vinyl Systems must lookat their laptop screen and move the record until it hits the desiredcue point. Instead, our DJ can focus on the turntables, relying onelectro-tactile notifcations. As depicted in Figure 10 (c), the user
moves deck B’s record back and forth, searching for the cue point,
which our system renders as an electro-tactile sensation under thefngerpads that hold the record—this indicates the physical cue
point. Finally, DJs need to release the record at the cue point, on
time. As shown in Figure 10 (d), our DJ waits for electro-tactile
feedback to the proximal phalanges (close to palm) that indicatesto release deck B. Now both tunes are being played in sync and the
DJ fades deck A using the mixer’s fader, as depicted in Figure 10(e). Note they can perform this without encumberment from ourelectrodes.

7.3

Interactively Guided Clay Modeling in
Mixed Reality

We demonstrate the usage of our system for guiding the user as
they model physical shapes in mixed reality (MR) using real clay.
This is inspired by prior work on interactive painting [18] andsculpting [76], but takes these ideas further by allowing users tophysically model directly with their hands, rather than throughhandheld tools. Figure 11 (a) depicts the user pushing MR buttons
to browse through 3D models to choose which model they want

Figure 11: MR clay application alongside virtual 3D models(superimposed the virtual content for the sake of visual clarity).
the system to assist them with—as they press buttons, they feelelectro-tactile feedback to confrm the actions and add realism to
the interactions.
The user now chooses the bear model and attempts to clay a copyof it. To understand the model’s geometry, the user can touch, grasp,
move, and rotate the virtual bear with their whole hand. As they
do so, our system renders each of the 11 possible palm segmentsthat are in contact with the 3D model, e.g., feeling on the wholehand as they grab the model as shown in Figure 11 (b). Then, as
depicted in Figure 11 (c), the user molds clay, putting force on theirpalm and fngerpads—since our electrodes are attached to the backof the hand and the wrist, they provide minimal encumberment.
Finally, to give the fnishing touches to the clay bear’s head, theuser aligns the physical and real model and uses our electro-tactile
stimulation to feel the places where the VR model difers from theclay model, as depicted in Figure 11 (d).

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

8

USER STUDY #2: USER’S EXPERIENCE IN
APPLICATIONS INVOLVING OBJECTMANIPULATION

While in Study #1, we validated that our approach was able to cause
sensations at 11 diferent points on the palmar side of the hand,
now we turned to understanding how much our device preserved
dexterity and tactile acuity while rendering electro-tactile feedback.
The motivation of the study was inspired by prior work that also
investigated how wearable devices afected the user’s tactile acuity [25, 67, 71]. Following the study design employed by [67], we
asked participants to interact with real-world objects while using
our device and interviewed them regarding whether our device
preserved or encumbered their interactions with these real-world
objects. The study was approved by our Institutional Review Board(IRB21-1229).

8.1

Study Design

Participants. We recruited eight participants (three identifed as
male, fve as female, average age = 22.3 years, SD = 2.1) from ourinstitution; four had partaken in our frst user study; one was lefthanded. Moreover, with the participants’ consent, we videotaped
and transcribed the study. Participants received $30 USD as compensation.
Tasks. The participants experienced our MR clay modeling andVR bouldering simulator. We modifed our MR clay modeling to ft
the study format by removing the browsing feature and, instead,adding two buttons that could respectively reset the 3D model’s position to its origin; and fx the 3D model in place. When participants
were satisfed with their clay model, they proceeded by choosing
a textured surface to place their “clay bear” on (Velcro, sandpaper,and silk, which acted as props for the foor of an MR forest, desert,
and snowfeld). We also added a 5-inch virtual button at the top of
the VR bouldering wall to conclude the experience.
Interactions. We designed these tasks to require the participants
to touch, grab, rub, and knead physical objects while enjoying tactile
feedback from our system. To elaborate: (1) both tasks required theparticipant to put on/of the Quest headset by themselves, which
involves rotating a mechanical knob to adjust the tightness; (2) VRbouldering application demanded that the participant rubbed the
chalk onto both hands prior to the frst wall; (3) it also required
them to grab the rope and apply force to pull it down; (4) the clay
modeling involved kneading, molding, and grabbing the clay; (5) italso demanded the participant to feel the diferent textured samples(e.g., sandpaper).
Apparatus. The participants wore a Quest headset and a USB
tethered version of our stimulator device with the electrodes andthe cables. While, in the MR task, participants wore the full set oftwelve electrodes on their dominant hand, in the VR boulderingtask, we evenly distributed these electrodes so that they could
feel tactile feedback on both hands (fve tactile points per hand:
fngerpads of the thumb/index/middle fngers, the thumb’s proximal
phalanx, and the palm). We also provided participants with physicalprops, including play-doh, with three texture samples (i.e., Velcro,
sandpaper, and silk), a bowl with bouldering chalk, and a one-inchthick rope.

CHI ’23, April 23–28, 2023, Hamburg, Germany

Procedure. The participants performed the two tasks in a counterbalanced order. Prior to each task, we calibrated the stimulation
intensity and adjusted the attachment for all electrodes. Unlike ourfrst study, we verbally asked the participants which part of thehand they felt tactile stimuli to adjust the electrode’s position. We
set the intensity one step above the minimum intensity required to
evoke the sensation (see User Study 1 procedure for details). During
the trials, we encouraged the participants to "think out loud" andvoice any of their own thoughts about the experience.
Interview. We followed each task with a semi-structured interview where we asked the participants about their experiences with
virtual widgets and manipulating physical props. We started with
two questions, common to both tasks: (1) “Could you describe how
you felt tactile feedback while interacting with virtual props?”; (2)“Could you describe how much our device, including its electrodes
and cables, encumbered putting on/of the headset?”. For the frst
question, after a response, we also asked about their experience
with individual props (i.e., the virtual bear and the buttons for themixed-reality task, the bouldering holds, and the fnish button for
the VR task). Then, we asked two additional questions that wereunique to each task. For the MR task: (3) “Could you describe howmuch our device encumbered touching, molding, and moving theclay?”, and (4) “Could you describe how much our device encumbered feeling diferent textures?”. For the VR task: (5) “Could you
describe how much our device encumbered chalking your hands?”;(6) “Could you describe how much our device encumbered grabbing the rope?”. Finally, after both tasks, we concluded the study
by asking (7) "Is there any other aspect of your experience that you
would like to share with us?".

8.2

Qualitative Feedback

Discrete tactile feedback when touching MR buttons. Six participants directly described that they enjoyed the tactile feedback
when pressing the MR buttons, e.g., "like a nice detent" (P2), "likephysical resistance that I was supposed to feel when pushing the
button" (P3), or "I liked the button, felt more like pressing something
than touching " (P6).
Continuous tactile feedback when touching VR/MR objects. Seven participants directly mentioned that tactile feedback
to the 3D “bear” model was helpful during the interaction; the comments included, "it reinforced my understanding of the shape" (P1),or "I felt the size of the bear, when I was putting my fngers aroundthe head, I could tell how large it was" (P6). P5 described how tactile
feedback to the proximal phalanges positively shaped their experience: "I was pretty amazed that I felt on these areas [pointing theproximal-phalanx segments] because that’s the part where the mostpoints of contact occur when I’m actually grabbing." For the “fnish”
button in the bouldering simulator, four participants commented
that it contributed to their experience; the comments included “I
felt the sensation all over my palm, which made me think that I
was actually touching something physical” (P2), and “I felt exactly
what I expected to feel” (P7). Moreover, six participants stated thattactile feedback to the bouldering pegs shaped their experience;the comments included “I enjoyed the continuous feeling on mypalm and fngers (. . .) it felt like (. . .) holding something tangible”(P1), “[while grabbing the pegs] I felt exact locations where I was

CHI ’23, April 23–28, 2023, Hamburg, Germany

Yudai Tanaka et al.

Figure 12: (a) Shaping the clay mold by rolling, (b) squeezing, or (c) pinching (d) Overlaying clay on the virtual model, feelingthe haptics of the physical and virtual models concurrently. (e) Touching and feeling the alignment of the entire model in thesame way.

Figure 13: (a) putting the chalk. (b) rubbing the chalk against the palms. (c) grabbing the rope and pulling it down. (d) exiting
the physical (the rope) part of the experience by grabbing the virtual peg. (e) putting on/of the headset, rotating the adjustingknob.
supposed to feel (pointing the fngerpads and the center of thepalm)” (P5), or “[it] made the experience more realistic” (P7).
Perceived sensation of the stimuli. While describing theirexperiences with the virtual feedback, four participants expressed
how the stimulation felt: “it felt like vibration, while I was grasping
the bear” (P1); “it was tingl[ier] than what [a bouldering hold]was supposed to feel” (P6); “it felt like short, light pressure [whilepressing the button]” (P7); and “the feeling itself was warm andtingly” (P8). Both P6 and P8 added that they got used to the “tingling”
feeling over time. For instance, P8 stated, "it was a little surprising
at frst but at some point, I got used to it". Next, we turn to theparticipants’ qualitative feedback regarding the interactions withthe physical objects.
Kneading the clay. Figure 12 depicts how participants manipulated the clay in a variety of ways: (a) rolling; (b) squeezing, or (c)pinching. In terms of the encumberment from our device (including
the cables and the electrodes), six out of eight participants reported
it did not encumber them. Their comments included “they were
not in the way (. . .) actions were all about my palm and the cables and the electrodes were on the back of my hand” (P7). For theremaining two participants, they reported some encumberments
to their manual actions by stating: “[while rolling the clay], I was
more aware of the electrodes, so I rolled less frequently compared
to not having anything” (P4) and “I was trying not to move as fast
as natural [while wearing the device]” (P5).

Overlaying the virtual and physical objects. Three participants commented on how our device allowed them to compare
virtual vs. physical objects also via its tactile feedback, as depicted
in Figure 12 (d, e). P4 stated that “I put the clay [right at the same
position as the virtual bear] to check if my bear was proportional(. . .) I felt the bear and my clay at the same time”; similarly, P5
liked the experience itself of touching the virtual bear augmented
with the physical clay: “I liked that the play-doh added the pressure
when I touched the head of the [virtual] bear”.
Feeling diferent textures. None of the participants mentionedthat our device encumbered them while feeling the texture sheets,
which they used as an AR prop to place their clay bear on (Figure12f). P1 and P5 supported their responses by adding: “since it’s at
the back of the hand (. . .) there were no hurdles for (. . .) palm or
the fngertips” (P1) and “not at all encumbering because (. . .) I knew
how secure they were (. . .) if I say nothing on my hand is 10/10,
this was 9/10” (P5).
Chalking their hands before VR bouldering. All but two
participants noted that the device did not directly encumber while
putting the chalk (Figure 13 a, b), the comments on encumberment
included, “I was a little worried accidentally breaking the wires”(P3), or “I was aware of the stuf [back of the hand] and didn’t want
to disrupt it” (P5).
Grabbing the real rope in VR. None of our participants mentioned encumberments while interacting with the rope, which is
depicted in Figure 13 (c, d). They described that they could feel

Full-hand Electro-Tactile Feedback without Obstructing Palmar Side of Hand

its texture while pulling it down: “it made me forget about theelectrodes (. . .) I perceived the roughness of the rope without fault”(P1), or “I was able to grab the rope without noticing the cables (. . .)[the rope’s texture] felt natural” (P8).
Putting on/of the headset. Lastly, in both tasks, while theparticipants were able to put on/of the headset without any serious
problems, four participants mentioned our device getting in theway of adjusting the tightness by rotating the rear knob: “the wires
were a little bit annoying when I put my hand back of the headset
to rotate the knob” (P5).
Study conclusions. Taken together, qualitative feedback fromour participants suggests that our device was able to preserve theparticipants’ dexterity and tactile acuity on the palmar side of theirhand for most interactions that included either virtual or physicalobject manipulation. We also noted that while we focused primarily
on our electrode arrangement we did not specifcally research ways
to keep the cables out of the way. As such, users mentioned that theyfelt some minor encumberment by the electrode cables. However,this did not prevent them from really completing tasks involving ahigh degree of precise fnger movements, such as modeling clay or
putting on a VR headset by themselves.

9

FUTURE WORKThe focus of our paper was formalizing and evaluating the feasibilityof stimulating the palmar side exclusively via electrodes attachedto the back of the hand and the wrist, as such, there is room forfollow-up research and exploration.
Understanding evoked sensations. While we observed someparticipants’ comments regarding evoked sensations in Study 2,
there is still room for more refned evaluations. First, further characterization of the evoked sensation is important. One possible way
is to follow the methods employed in prior work [6, 62], and collectdata from participants regarding how their perceived sensationand intensity compare to physical stimuli (e.g., vibration). Second,since our approach is built on referred sensation, its operational
principle is inherently diferent from typical electro-tactile methods
that stimulate on top of the mechanoreceptors. As such, a follow-up
comparison between ours and the typical electro-tactile stimulation is possible, i.e., evaluating whether the typical method evokes
clearer sensations or not; and how much they encumber manual
dexterity compared to ours. Lastly, further research is needed tomeasure how the stimuli might interact with haptic sensations fromphysical objects or the environment (e.g., sense of temperature).
Exploring texture augmentation. As demonstrated in our
MR application, our approach allows the user to feel both physicaland virtual objects concurrently. Follow-up work might choose to
explore if this allows for altering the texture of physical objects
such as stifness and roughness by modulating the stimuli in correspondence with the hand motion; while Yoshimoto et al. [75]modulated the perceived roughness of materials at the fngertip,
our approach might further extend these illusions to more areas of
the user’s palmar side of the hand.
Measuring the stimuli at the target receptors. As with any
non-invasive electrical stimulation based on electrodes, our stimuli
can be afected by the impedance and capacitance of the body tissue
while passing through it. A precise measurement of the stimuli at

CHI ’23, April 23–28, 2023, Hamburg, Germany

the target receptors would be benefcial to optimize the sensation,for instance, by using implanted electrodes [23].
Miniaturizing electrodes. As we observed in the participants’comments, while our approach kept the palmar side free, the electrodes on the dorsal side still afect the users’ movements. We expectthat recent advances in fabrication (e.g., custom screen-printed electrodes [71, 72]) can further miniaturize the hardware and address
this issue.

10

CONCLUSIONSWe proposed and validated a method to render tactile feedback to
the palmar side of the hand while keeping it unobstructed—thus,
maximizing the hand’s dexterity during interactions with physicalobjects. We implemented this by applying electro-tactile stimulation only to the back of the hand and to the wrist, in contrast to the
traditional approaches that attach electrodes to the palmar side. Yet,as we demonstrated in our frst user study, tactile sensations occur
at multiple locations on the palmar side of the hand. Moreover, wedemonstrated exemplary applications that our technique allows us
to build, such as VR experiences that rely heavily on manipulating
physical props or feeling tactile notifcations during dexterous activities. Finally, in our second user study, we investigated participants’experiences while using our device in two applications and foundthat our approach allowed participants to use their full hands tomanipulate physical objects, feel the object’s textures, and even
apply force with minimal encumberment, all while still enjoying
tactile feedback when touching any virtual or augmented objects.

ACKNOWLEDGMENTS
We would like to thank Prof. Sliman Bensmaia for his insightful
feedback and expert consultation on the neurological principles
underlying our technique. This work was supported by NSF grants
2047189. Any opinions, fndings, and conclusions, or recommendations expressed in this material are those of the authors and do
not necessarily refect the views of any funding agencies. We are
grateful for the feedback from our reviewers.

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