Haptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity
Shan-Yuan Teng

Aryan Gupta

Pedro Lopes

University of Chicago
tengshanyuan@uchicago.edu

University of Chicago
aryan05g@gmail.com

University of Chicago
pedrolopes@uchicago.edu

Figure 1: (a) To improve the usability & minimize encumberment of tactile devices for the fingerpads, researchers moved awayfrom thick actuators (e.g., vibration motors) and, instead, focused on thin devices—one successful example is electrotactile
stimulation. These can be engineered to be thin, which allows the user to still feel some sensations even though their fingerpads
are covered by the electrode film (e.g., compliance or macro features). However, we argue this is not enough and on should
also balance how much a tactile device impairs feeling the real world vs. how accurately it delivers virtual sensations. Thus,
we propose & evaluate how adding holes to electrotactile devices results in: (1) improved perception of tactile features; and(2) improved force control in grasping tasks (b) Our approach significantly improves the haptic users’ abilities in dexterous
activities, including manipulating tools, in mixed reality.

ABSTRACTCCS CONCEPTS

Feeling haptics with our fingerpads is how we achieve manual tasks(e.g., operate a needle or press buttons). Following this, researchstarted adding actuators atop the users’ fingerpads to render haptic
feedback for interactive virtual environments. Recently, many have
moved away from thick actuators (e.g., vibration motors) and turned
to electrode-films with electrotactile stimulation—allowing users
to still feel some sensations through the devices when touching
physical objects (e.g., compliance or some macro features). However, we argue & demonstrate that thin devices are not enough tomaximize the user’s dexterity. We evaluate how adding small holes
to electrotactile films can allow direct contact and thus increase
haptic permeability, resulting in: (1) improved perception of tactile
features; and (2) improved force control in grasping tasks. Finally,
we observed participants in interactive experiences and found that
holes can preserve dexterity with physical tasks while still benefiting from haptic feedback.

• Haptic devices; Mixed / augmented reality;

This work is licensed under a Creative Commons Attribution International 4.0 License.
CHI ’24, May 11–16, 2024, Honolulu, HI, USA© 2024 Copyright held by the owner/author(s).
ACM ISBN 979-8-4007-0330-0/24/05
https://doi.org/10.1145/3613904.3642156

KEYWORDS
Haptics, Augmented Reality
ACM Reference Format:
Shan-Yuan Teng, Aryan Gupta, and Pedro Lopes. 2024. Haptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity. In Proceedings of the
CHI Conference on Human Factors in Computing Systems (CHI ’24), May
11–16, 2024, Honolulu, HI, USA. ACM, New York, NY, USA, 12 pages. https://doi.org/10.1145/3613904.3642156

1

INTRODUCTION

Feeling haptic cues using our fingerpads is how we achieve precise
tasks in the physical world (e.g., pick up a match, press buttons, feel
fabrics, and more). Inspired by the crucial role of fingerpad tactile
cues, haptic devices have been developed to enable users to also
more accurately interact with virtual environments. These devices
typically provide precise haptic feedback by attaching mechanicalor electrical actuators directly on our fingerpads, which allows thehaptic device stimulate the touch on virtual interfaces (e.g., VR[25, 30, 36, 41, 54], AR/MR [11, 45]).
In recent years, to improve the usability & minimize encumberment of these tactile haptic devices for the fingerpads, many
researchers have moved away from thick actuators (e.g., vibration
motors) and, instead, focused on tactile haptics via thin devices

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Shan-Yuan Teng et al.

[11, 51]—one successful example of this is electrotactile stimulation[23, 51].
Electrotactile devices can be engineered to be thin (<100 𝜇m),
which allows the user to still feel some sensations despite the factthat their fingerpads are covered by the electrode film, e.g., pressure, compliance and even some macro features—unfortunately,
researchers also found evidence that even thin films impair haptic
perception [33].
We argue that minimizing the thickness of these haptic actuators
alone is not enough. Our field needs to be equipped with more
approaches to balance how much a tactile device impairs feeling thereal world vs. how accurate it can deliver virtual sensations. Thus,
we propose & evaluate how adding holes to electrotactile devices
enables haptic permeability, i.e., allowing one’s skin to directly makecontact with the real-world via the holes.
In our studies, we found that, for the generic case of uniformly
distributed holes (Figure 1), haptic permeability resulted in: (1)improved perception of tactile features by 17% (e.g., orientation oftactile gratings, as found in our Study 1); and (2) improved force
control in grasping tasks by 34% (e.g., less grasp force needed while
grasping, as found in our Study 2). Finally, in our Study 3, we foundthat even in its most extreme design option (i.e., replacing electrodes with holes, which decreases the device’s output resolution),
was useful to participants since it was easier to perform dexterous
assembly tasks in mixed reality.
Our three studies validate how haptic permeability enables users
to retain more dexterity, which ultimately opens the design space for
haptic devices—in other words, if users can manipulate small objects& feel fine textures, we can open-up haptics to new interactive
domains heavily involving tool-use.

Most tactile devices were engineered using rigid & thick mechanical actuators—the most popular are vibration-motors. While theseoffer advantages in low-cost, control simplicity, and haptic capabilities, their inflexibility (mechanical rigidity) limits their applications.
As such, in the last decades, much attention has been devoted toengineering devices that conform better to the user’s body (e.g.,
on-skin devices, epidermal devices) [15, 34]. This led to soft andthin actuators [5], including microfluidic actuators [11, 12], magnetic actuators [31, 58], dielectric elastomers [27, 57], piezoelectric actuation [17, 62]. While promising, they are still mechanicaldevices, thus inheriting their limitations (e.g., a device with moving parts takes more space than one without). Thus, researchers
have explored creating tactile sensations without moving parts, bystimulating the mechanoreceptors with electricity—electrotactile
stimulation [21, 23]. Given that electrodes can be made smaller
than a mechanical actuator (which requires physical displacement
and that requires empty space), electrotactile devices can be madeinto arrays suitable for the fingerpad [1, 18, 21, 22, 28]. Thus, electrotactile has been mainly used as a device for fingerpads, to deliver
textures [10], shapes [40], softness [55] or touch information in
virtual environments [51, 53].

2

2.3

RELATED WORKThis paper builds on the field of haptics, with particular emphasis on
tactile haptics for the fingerpad. Given our goal of uncovering new
ways to feel real-world surfaces, we focus on electrotactile interfaces, which are built from thin films, most suitable for interactions
involving feeling virtual & real objects. Thus, we succinctly review
the field of electrotactile stimulation (complete review can be foundat [28]), and turn our attention to how researchers have decreased
the thickness of films to improve the sensation felt through them,
despite they cover the fingerpads completely—this is where we takeinspiration for proposing haptic permeability (via holes).

2.1

The role of touch in our interactions

Texture Discrimination. Texture discrimination at the fingerpad
is important for tasks such as recognizing different fabrics, or factory workers detecting defects. Naturally, materials that we touch
stimulate many of the receptors [20] (e.g., vibrates, deforms theskin, conducts heat) and, temporally, this forms the sensation of
texture [38]. Moreover, spatial acuity in a patch of skin [19] (i.e.,
how well we perceive tactile sensations in a location) is critical.
Fingerpads are one of the most sensitive parts and can detect stimuli
within the distance of 1-2 mm [37].
Grip Control. Our ability to grasp objects with our fingers
without slipping relies heavily on tactile sensitivity to sense friction[42, 50] (i.e., sense micro-deformation of the skin). The ridges on

the fingerpads (which form the fingerprint) and the sweat glands
form a dynamic regulated system that can adjust the moisture andmaximize the friction of the skin [2, 24, 59]. Altogether, these help
us grasp with optimal force (by using the least energy).

2.2

Towards less encumbering tactile interfaces:
actuators that conform to skins (e.g., soft,
films, etc.)

Improving tactile interfaces by using
thinner films (i.e., feel-through)Recently, researchers argued that it is not only important to render
haptics, but also need to preserve haptic sensations from the realworld [39, 43–45, 51], as they are critical for achieving precise
manual tasks (e.g., in MR). Making devices thin and soft is one waythat researchers explored preserving cues from the real world. For
instance, HydroRing [11] is a ring worn on the fingerpad that uses
pumped water for haptics. Its relatively low thickness results in an
unobtrusive device that still lets users feel some tactile sensations
from the real world. Similarly, Tacttoo [51] is an electrotactile device
based on layers of temporary tattoo papers (35 𝜇m) [51]. This canalso be taken a step thinner, as shown by Ji, et al. in their 18 𝜇m
thick dielectric elastomer [16]. Note that devices with low rigidity
can preserve very more tactile sensitivity, but they do so by trading
this off with fragility (i.e., prone to breaking when interacting withphysical surfaces, such as rough textures) [33].

2.4

Tactile sensitivity is impaired by covering
the fingerpads (e.g., films, gloves, etc.)

Unfortunately, despite the improvement brought the reducing the
thickness of a film, researchers confirmed that even thin films impair tactile perception and affect grip control. For instance, wearing
thin gloves such as the ones typically used in dental work has beenshown to decreased force perception and dampen vibrations [8, 60].

Haptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Figure 2: (a) Holes on the film allow direct contact with fingerpad, revealing papillary ridges and can directly transmit tactile
signal to receptors. (b) Ink test on tattoo paper with holes. (c) Ink test on polyimide paper with holes.
Similarly, it has also been shown that the required grasping force
increases in lifting and holding task while wearing gloves [26].
Specifically, thin films obstruct static tactile perception which is
required for optimal grasp efficiency [4]. Most relevant to electrotactile research, Nittala, et al. conducted studies with various
thickness of films, ranging from 2.5 𝜇m to 177 𝜇m, and found thateven the thinnest film covering the fingerpad would increase thedetection threshold when discriminating the roughness of a surface(144% compared to bare finger) [33].
As such, we argue that making electrotactile devices thin is not
the only route to improve tactile sensations. Inspired by this and toadvance this research question, we turn our attention to creating
actual haptic permeability.

3

HOLES: A NEW APPROACH TO PRESERVEDEXTERITY & HAPTICS FROM THE REALWORLD

We propose & explore a new dimension that designers can incorporate on haptic-devices based on films: haptic permeability—adding
holes (i.e., any type of cutout of the electrode film) through which
the skin can contact the physical environment, instead of always
being mediated by the film; this is depicted in Figure 2.
Direct skin contact. Figure 2 illustrates the principle behindadding holes. In this ink-test, a finger was coated with red ink. In
this case, instead of wearing an existing thin-film device (e.g., [51])that would cover the entire area of their fingerpad, the user wears
a film modified with our approach. The film is the same (samethickness/material as [51]), except we added 2 mm holes at equalintervals (4 mm center-to-center) throughout. The ink-test reveals
how the skin is able to contact the paper—even the fingerpad ridges
are visible in the red ink, which implies that the reverse is also true,i.e., small features that the user is touching are in direct contact
with the skin.
Tactile receptors. While 2 mm of opening might sound small,it contains ∼eight mechanoreceptors [19] and ∼five ridges. As humans are sensitive to a stimulus on one single ridge [14], even a

proposal as simple as a mm-sized hole might prove powerful at
improving tactile perception. The reason is that the exposed receptors can gather more information. This yields our first hypothesis:
exposing parts of skin directly via holes would lead to higher haptic
sensitivity, which we validated in our Study 1 (i.e., holes increased
tactile recognition by 17%).
Grip control. This seemingly simple ink-test reveals a second
key benefit of adding holes to haptic devices. Again, we turn thereader’s attention to the fingerpad ridges shown in the ink-test.
Ridges contribute to slip control and help us grasp firmly without
exerting excessive force [59]. This yields our second hypothesis:
exposing parts of skin helps regulate grip and thus decrease theminimal force to grasp an object, which we validated in our Study
2 (i.e., holes decreased grasping force by 34%).
Structural stability of adding holes. The most naïve approachto adding holes would be to turn all unused space (i.e., space that is
not an electrode or a conductive trace) into a hole. However, cuttingout too much space on the film can have structural limitations. First,
these films are adhesive (i.e., the larger the surface area the stronger
they bond to the skin), thus loosing adhesive risks the device coming
off the user’s finger. Second, the less material envelopes electrodes,
the more likely is that this electrode will get caught in some material
or feature that the user is touching, e.g., as a user operates a tool it
can get caught, crinkle, fold or get ripped out. Moreover, while holes
can be of different shapes, circular holes minimize aforementioned
issues as they (1) are less likely to get caught in materials since
there are no sharp angles (as there would be in a square or otherpolygonal shapes); and, (2) have less impact to the overall bonding
force of the adhesive, since circular holes are supported on all
sides, thus electrodes are less likely to get dislodged; (3) are shown
to be the most stable hole shapes [52], which would make ourthin devices more durable for longer use. In our third study, inwhich participants interacted with objects with high friction (e.g.,screwdriver) and small parts (e.g., wires) , no device was caught or
dislodged.

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Shan-Yuan Teng et al.

Figure 3: Four design options to implement haptic permeability on an electrotactile device.
Applicability & design options. Figure 3 illustrates how ourconcept can be applied in a number of ways, depending on both
the goal of the haptics application (i.e., how much it requires prioritizing feeling real vs. virtual cues) and engineering constrains(e.g., resolution, unused space, layout of the conductive traces).
We envision four ways a designer can use our approach, presented in ascending order of how much one might expect theywould tradeoff the resolution of the target electrotactile device:(1) swapping unused space for holes—no impact to virtual rendering & gains permeability to feel physical cues; (2) rearranging thetraces to add holes—low/no impact to virtual rendering & gains
permeability to feel physical cues; (3) rearranging the electrodes
to add holes—medium impact to virtual rendering & gains permeability to feel physical cues; and (4) replacing electrodes forholes—purposefully trades-off some virtual rendering for more permeability to feel physical cues. Importantly, it is worth noting that
we present these design options to illustrate how one might explore
holes in electrotactile devices, but our investigation only measured
the benefits of swapping unused space for holes (design option 1)
and replacing electrodes for holes (design option 4) in the genericcase of a uniformly distributed layout.
1. Swapping unused space for holes: the most straightforward & readily available option is trading holes for unused spaceon the device. This is useful for electrotactile films with sparse electrode density (e.g., similar to [23]), which leave plenty of space ofholes to be added—this is typical of applications that do not require
high-resolution.
2. Rearranging the traces to add holes: traces can be rearranged to make space for holes by either rearranging the these topass through to the back of the film using a via [48], or rearranging
the traces to optimize empty space. The limitation of using multiple
layers is it leads to thicker films, (e.g., 150 𝜇m for double layers [9]).
3. Rearranging the electrodes to add holes: the electrodepositions can be altered so that the electrode-to-electrode distanceallows for holes—this option might fit applications where denselypacked electrodes are not required, yet their number is important(e.g., rendering information of directions [47, 49]).
4. Replacing electrodes for holes: in our most interesting
design option one trades-off electrodes for holes, which trades-off
some virtual rendering for permeability to feel physical cues, which
we validated in our Study 3.
Additional factors. There are several additional factors thatmight influence haptic permeability when adding holes, including:the size of the hole, the shape of the hole, the layout of the holes, andthe material of the film. As the first paper to propose & investigate
adding holes to electrotactile devices, we focused only on studying
the generic case of uniformly distributed holes on a film. While ourfindings for the case of uniformly distributed holes already provide
generalizable insights to other devices (see Section 8), future studies
should follow up on our findings to explore how these additionalparameters affect tactile performance.

4

STUDY OVERVIEW

We conducted three user studies to inform, measure & understandthe impact that adding holes to electrotactile haptic devices has on
dexterity & tactile perception. In all our studies we compared thinfilms applied to the fingerpad against same films with added holes.
Study 1: we assessed tactile perception, using a standardized task(orientation of grooves in a material); we found that in the holes
condition participants were able to better perceive orientation by
17% than while their fingerpads were covered (no holes). Study
2: we assessed grip control, using a standardized grip-force test
(force needed to grasp without slipping); we found that participants
were able to grasp 34% less force than while covered. Study 3:
participants experienced a mixed reality tutorial involving assemblyof a physical toy truck, with virtual instructions accompanied byhaptic feedback. We found that participants found it easier toaccomplish the task and felt more real-world feedback with theholes condition than when covered.
All our user studies were approved by our Institutional Review
Board (IRB23-1225).

5

STUDY #1: HOLES IMPROVE TACTILEPERCEPTION

We aimed to understand whether adding holes to thin-film haptic devices would affect the feel-through perception of physicalobjects. As the first study to investigate this, we opted for the
aforementioned generic case of uniformly distributed holes added
to the film used in Tacttoo [51]. To evaluate the impact of adding
holes, we conducted a tactile perception study using the standard
grating orientation test [6, 61], in which participants are presented
with gratings (grooves on a material) and are asked to guess theorientation of the grating by touch alone.
Hypothesis. Our hypothesis was that adding holes, which exposes parts of skin directly in contact with the touch surface, would

Haptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

lead to higher haptic sensitivity (i.e., higher accuracy at determining
the orientation of the grating).

5.1

Study Design

Task. Participants were asked to touch (eyes-free) a sample withgrooves running in four directions and report which direction was
felt—this is a popular test used in psychophysics research to measure tactile perception [6]. As we are interested in fine texturesthat could be affected by thin film covering, we chose a grating
spacing 1.5 mm. Note that at 1.5 mm of spacing, four directions
and eyes-free, this is a fairly challenging task that requires highsensitivity; thus, we pretested our participants at 70% accuracy.
Grating. We 3D-printed a grating with ridges of 0.65 mm × 0.65
mm (height, width) spaced by 1.5 mm.
Pretest. To qualify for our main experiment, recruited participants performed the task (grating orientation, 20 trials of randomized patterns from four directions) with their bare fingerpad—when
scoring equal or above 70%, they would continue to the holes vs.
covered conditions.
Participants. 12 participants qualified (6 females, 6 males;
average-age=27.0 years old, SD=4.4; all participants were righthanded). Participants were compensated with $10 USD voucher.
Haptic film. We fabricated a 30 𝜇m thin-film inspired by [51],comprised of stacked layers of tattoo paper and cut using a paper
plotter (Cricut). The resulting film was attached to the participants’dominant fingerpad.
Interface conditions. Participants performed the gratingrecognition task in two interface conditions: holes (thin film withholes added to it, 2 mm diameter with 4 mm center-to-center spacing) and covered (thin film).
Procedure. Participants performed 20 trials per condition (order
was counterbalanced across participants). In total, we collected 480
trials across all participants (i.e., 4 directions × 5 repetitions × 2
conditions × 12 participants).

Figure 4: (a) Tactile sensitivity task (grating orientation); (b)Results from tactile sensitivity task.
needed to lift & hold objects without these slipping—a popular task
in psychophysics to measure grip control [13, 26].
Hypothesis. We hypothesized that since adding holes exposes
skin, namely fingerpad ridges which contribute to friction and grip
control [59], it would allow participants to enact better grip control
when compared to the covered condition, resulting in less force
required to lift objects without slipping.

6.1

Study Design

Figure 4 depicts our findings regarding tactile discrimination.
We found a statistically significant difference between all conditions (paired t-test). Our main finding is that the holes condition (M=74.6%; SD=11.6%) improved tactile recognition by ∼17%
(p<0.0001, F(11)= 10.1637) compared to the covered condition(M=57.5%; SD=12%). As expected, the accuracy of the bare-finger
result was the highest (M=80%; SD=8.3%), which was unsurprising
given that we set the threshold of our pretest above 70%. Taken
together, these results confirm our hypothesis that adding holes
improved the tactile recognition significantly when compared to acovered film.

Task. (1) Grasp a 300g weight on a rail; (2) lift it 5 cm upwards;
and, (3) hold it for 5 seconds.
Apparatus. We fabricated a 30 𝜇m thin-film (same as in Study
1). We used a 1kg (max) load cell, sampled using HX711 and an
Arduino board to measure the grasping force. The grasping handle
was made from acrylic.
Interface conditions. Participants performed the lift-and-holdtask in three interface conditions: bare-finger; holes (thin film withholes added to it, 2 mm diameter with 4 mm center-to-center spacing) and covered (thin film).
Procedure. Participants performed six lifting trials per condition. As the participants may be adjusting their grasping forces in
the initial trials, we only measured the force for the last three trials.
Force was measured at the middle of the 5-second hold by averaging
10 readings of the load cell. The condition order was counterbalanced across all participants. A total of 108 measurements (3 trials
× 3 conditions × 12 participants) were collected.
Participants. 12 right-handed participants (6 females, 6 males;
average age was 28.2 years old, SD=7.3 years) were recruited fromour neighborhoods. Participants were compensated with $10 USD
voucher.

6

6.2

5.2

Results

STUDY #2: HOLES IMPROVE GRIPCONTROL REQUIRED TO GRASP WITHOUTSLIPPING

We set out to understand what the impact of adding holes to atactile haptic device on the fingerpad might have in terms of grip
control—a dexterous skill that depends on one’s ability to sense
friction [50] and control it by adding pressing force. We assessed itusing a standardized grip-force test, which measured the grip force

Results

Figure 5 shows our findings regarding grip control. We tested
the data with one-way ANOVA and found a significant difference(F(2, 105)=75.06, p=0). Tukey’s HSD Test for multiple comparisonsfound significant difference between all condition pairs (p=0 for all).
Our main finding is that the holes condition (M=4.06 N; SD=1.32)
improved grip control by reducing the required amount of force by∼34% compared to the covered condition (M=6.13 N; SD=2.18). AsCHI ’24, May 11–16, 2024, Honolulu, HI, USA

Shan-Yuan Teng et al.

but likely the advantage given by the holes exposing more skin and,thus, providing more tactile information to the fingerpads.
Limitations. We acknowledge that our results are specific tothe case where the electrotactile films are of lower friction than
the fingerpad’s skin. While this seems to be the case for most electrotactile devices (i.e., including [51] which we used as a material
for this study), films with a larger coefficient of friction, require
further investigation.

7

Figure 5: (a) Study apparatus of lift-and-hold task. (b) Force
results for the grip task.
expected, the force required with the bare-finger condition was thelowest (M=1.79 N; SD=0.55), which was unsurprising given thatparticipants use the whole fingerpad for optimal grip control.

6.3

Study interpretation & limitations

We denote that there were two possible factors at play. First, holes
expose skin, which might lead to more tactile cues felt at the fingerpad, resulting in more precise grip control. Second, the friction of
the film’s material may contribute to the results, since a low coefficient of friction between film and skin could require more force toprevent slipping. Thus, to explore which of these is likely the main
factor, we measured the friction of the film with and without holes.
Measuring friction of the materials. We conducted a technical evaluation, in which we created an artificial fingerpad (SmoothOn KX Flex 40, a material known to simulate a fingerpad [7]), andmeasured its static coefficient of friction on an acrylic plate using a
mass-pulley tribometer [46]. We measured the coefficient of static
friction for three conditions: artificial fingerpad (simulating barefinger), artificial fingerpad covered with the holes film, and artificial
fingerpad completely covered with a film—we conducted five measurements per condition. For the condition in which the artificial
finger was covered with the holes film, we ensured that the artificial
finger was contacting with the acrylic through the holes by using
the ink test shown in Figure 2. We found the following coefficientsof static friction for: artificial fingerpad at 1.77 (SD=0.13); holes at
0.86 (SD=0.08); and, covered at 0.84 (SD=0.08).
Interpretation. The coefficient of friction for the artificial fingerpad (M=1.77) was 2.1 times larger than that of the covered condition(M=0.84), indicating that adding a film to the artificial fingerpad
decreased friction. So naturally, one expects to see a higher force
when grasping via the films, which we indeed observed in Study 2.
However, the coefficient of friction of the artificial finger covered
with holes film (M=0.86) is remarkably similar to that of the completely covered film (M=0.84), with a difference smaller than 3%.
That being said, the forces we observed in these conditions differed
by a much larger margin than 3%; in fact, participants’ grasping
forces for holes (M=4.06N) were 34% less than covered (M=6.13N).
This suggests the major factor that is likely to explain the grip
improvement is not the 3% of increased friction of the added holes,

STUDY #3: HOLES IMPROVE DEXTERITYDURING MIXED REALITY ASSEMBLY TASKS

Finally, we aimed at understanding how holes added to tactile devices affect actual interactions in Mixed Reality (MR), where theusers interact not only with virtual interfaces but also with physicalobjects. Specifically, we focus on our extreme design option, shownin Figure 3(4), where holes even replace some electrodes. Following
this, we created an MR task that required manual dexterity to complete: assemble a toy truck following an interactive MR assembly
guide. Since our objective was to focus on participants’ observed
dexterity, we video-taped the study (with their prior consent) andconducted interviews to assess their experience.

7.1

Study Design

Conditions. Participants experienced our MR application in two
conditions: covered and holes. The order was counterbalanced
across all participants.
Tactile devices. As the study requires robust devices that canhandle dexterous manipulations with lots of friction, pulling &pushing, we opted for polyimide films, a common material used inelectrotactile devices [18, 47]; in contrast to tattoo paper which is
known to be easily scrubbed off during interactions (as reportedby [33]; we also confirmed in our early pilots that when using thescrewdriver, the tattoo paper rubbed off on the thumb and/or index).
Our device was thus comprised of flexible polyimide with copper
traces (cut with a Cricut). The final device measured 150 𝜇m inthickness. In the covered condition it consisted of 16 electrodes (2mm diameter) with 4 mm spacing. In the holes condition, we used
the same layout except holes replaced the electrodes alternatively,
for a total of 8 holes and 8 electrodes. According to our ink test,
see Figure 2 (c), in order to provide a similar contact area as with
our previous studies (2 mm diameter of skin contact) we chose to
cut out holes in 3 mm diameter.
Active & passive devices. Achieving electrotactile actuation in
all ten fingers is challenging (and not done in almost any prior work).
Thus, for the sake of setup-simplicity (and to avoid wiring thatmight contribute to decreased dexterity), we only asked participants
to wear one active device on their dominant index finger, and wear
nine passive devices on all other digits (i.e., same material, but not
wired to our multiplexer & stimulator).
Stimulation design. In our MR experience, users encountered
three different haptic sensations on their dominant index finger:(1) feeling the shape of large buttons; (2) feeling the shape of small
buttons; (3) feeling the truck vibrating if its motor was running.
The electrodes actuated for each of these haptic effects are depicted
in Figure 6. Note that, since our holes condition traded-off someelectrodes for holes, the haptic stimulation becomes distorted inHaptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Figure 6: (a) Study setup. (b) Covered device and their stimulation patterns (c) holes device and their stimulation patterns.
this condition—a purposeful trade-off we wanted to explore in thisstudy. For example, when touching the outline of a large squarebutton, the electrodes are actuated in a hexagonal pattern, due
to the missing electrodes swapped for holes. As for stimulation
intensity, we kept it equalized (per-participant) for both conditions.
Haptic apparatus. We utilize a medical-compliant stimulator
to generate electrotactile (Rehamove 3). To multiplex all 16 electrodes, we utilize a custom-made multiplexer (similar to [47]). For
every electrode, we employ a half-bridge circuit with two photorelays (TLP176), allowing us to route this target electrode to either
negative or positive side of the medical-stimulator. We control
the photorelays via shift registers (SN74HC595) using an ESP32
microcontroller. This setup requires ∼1 ms to cycle through all stimulation channels. The final circuit was worn on the participants’
wrist.
MR apparatus. An MR headset (Microsoft HoloLens 2) wasused to render virtual graphics and track fingers. Our custom-madeMR application displayed a series of assembly instructions on how
to build a truck from a real toy model. This included buttons totouch that advanced to the next instruction, adjusting buttons forvirtual truck cover, and feeling the car vibrate as its virtual motor
started running. Once any of these interactions was detected, anOpen Sound Control (OSC) packet was sent wirelessly to the PC
which controlled the haptic stimulation.
Props. Participants assembled a toy truck (51 mm in widthwithout the wheels, 120 mm in length, modeled after a 1955 Chevy
Stepside Pick-up). The top half of this truck was made from metal,
and the bottom part was plastic. Wheels were 38 mm in diameter(𝜙), 18 mm thickness, and connected via a metal axle (𝜙 2 mm). All
screws were 30 mm long hex-screws (𝜙 3 mm). The truck featured
an antenna (𝜙 0.8 mm wire, length 28 mm). Participants were also
handed a screwdriver (model F145-419-1, aluminum-construction,
𝜙 12 mm at grasping handle, length 150 mm).
Task. Participants were asked to “assemble the toy truck following the MR instructions”. These were comprised of step-by-stepguides, depicted in Figure 7. To navigate the instructions, participants tapped on mid-air buttons. Every virtual interaction was
accompanied by haptic feedback. The task involved five phases:(1) operate the screwdriver: pick up the screw and use the screwdriver to assemble the top & bottom parts of the truck; (2) assemble
the wheels: push the four wheels onto the truck’s axle; (3) assemble the antenna: pick up the thin antenna and insert it onto
the truck; (4) adjust the virtual truck cover: tap on MR buttons
to adjust the size of the virtual cover to match the truck’s volume
accordingly; and (5) start the truck’s engine: touch the top of thefinished truck to start a simulation of virtual roads accompanying
by an engine vibration (and sound).
Participants. Eight participants were recruited (four females,
four males; two left-handed), with average age 24.3 years old(SD=2.3). A voucher of $10 USD was given to each participant
as compensation.
Procedure. After the truck was assembled, participants wereasked to rate realism (1: not realistic; 7: very realistic); “how difficult
was the whole task wearing the devices” (1: easy; 7: difficult), and“how much texture/material can you feel” (1: nothing; 7 fully). We
also collected comments at the end of the study and conducted ashort brainstorming & discussion with each participant. The wholestudy took ∼40 minutes.
Hypothesis. We hypothesized that holes would allow participants: (H1) to perform tasks easier (i.e., lower perceived taskdifficulty); (H2) to feel more of the materials & textures under
their fingerpads (i.e., higher perceived feel-through); (H3) feel lessrealistic virtual interactions, due to the trade-off between holes
and usable electrodes for electrotactile output (i.e., lower realism
scores).

7.2

Results

All participants were able to assemble their own trucks, in both conditions. We present participants feedback organized by (1) assembly(2) virtual sensations. Finally, we present the overall comments
about the experiences.

Figure 7: Steps of the MR assembly.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Shan-Yuan Teng et al.

Figure 8: Results of participants’ ratings.

Figure 9: Some exemplary interactions from participants observed in this study.
Reported dexterity & realism. Figure 8 depicts perceived
task-difficulty, feel-through, and realism of virtual interactions. We
found a statistically significant difference between perceived taskdifficulty (paired t-test; p<0.01; F(7)=3.99) across conditions. Thissuggests that participants found it easier to perform manipulations
with the holes condition (M=2.9; SD=1.2) than with the covered(M=4.1; SD=0.8)—which supports our H1. Moreover, we found a
statistically significant difference between perceived feel-through(paired t-test; p<0.01; F(7)=4.78) across conditions. This suggeststhat participants felt more textures/materials with the holes condition (M=4.0; SD=1.2) than with the covered (M=2.3; SD=0.7)—which
supports our H2. However, we did not find statistically significant difference between realism of buttons (paired t-test; p=0.1970;
F(7)=1.43) or perceived realism of vibrations (paired t-test; p=0.2753;
F(7)=1.18) across conditions—which did not support our H3.
Observed dexterity. With this study being focused on observing participants, we annotated videos and depicted, in Figure 9,
some illustrative examples where the same participant experienced
visible differences between manipulating a tool across conditions.
For instance, Figure 9 (a) shows P7 successfully manipulating ascrew with the holes device, but letting it slip with covered. Similarly, Figure 9 (b) shows P5 successfully using the screwdriverwith holes, but letting it slip with covered (they even dropped asecond time after picking it up). Moreover, Figure 9 (c) shows P4
assembling the wheels while holding the truck with holes device but
needing to rest the truck on the table for extra stability when doing
this with the covered (later confirmed during interview). Finally,
Figure 9 (d) shows P1 inserting the antenna onto the truck in thefirst try with the holes device, which they could not do with thecovered device, causing the antenna to be bent (and later needing
to straighten it, which also was difficult).
Difficulties grasping with cover. Six (out of eight) participants
reported it felt easier to use the screwdriver with holes device: 6(P1, P3, P4, P5, P6, P7) than with the covered. In fact, of these six,
three dropped the screwdriver while wearing the covered device(P5, P6, P7)—none dropped it with holes. Indeed, these observed
grip difficulties seem to be correlated with the size of the tool, since
four dropped screws several times while wearing the covered device(P2, P3, P4, P5)—none with holes. Two (out of eight) participantscommented specifically on why they think it was easier to use
the holes, quoting “the holder felt slippery with [covered]” (P1,and similarly P3). Conversely, larger objects seem to be easier tomanipulate in either condition. For instance, all participants foundit straightforward to assemble the wheels. That being said, withthese larger objects, some participants voiced their preferences.
For instance, P4 stated that “[with holes] I can feel the truck when

Haptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity

grabbing it and it’s easier to orient the truck [to insert the wheel,proceeds to visually demonstrate it]”. P2, P4 & P10 reported thatthe truck itself was also hard to grasp with the covered device, butwith holes device “I had a better grip.” (P2, similarly P4 & P10).
Limits of both devices. Again, the smaller the feature, themost problems arose. For the assembly of the antenna, six (out ofeight) participants found it difficult to do with either device (P1,P2, P4, P5, P6, P8). Still, participants commented on differences
between both. For instance, P1 stated “I couldn’t pick it up theantenna [with covered device] and it fell off the table onto my pants(. . .) I could do it with the [holes]” and P4 stated “hard to pinch,
can’t feel [the] antenna, need another hand (. . .) but with [holes
device] it’s so easy to pick up, I don’t need two hands”.
Tactile sensitivity: Seven (out of eight) participants reported
they felt more through the holes device, especially feeling the texture of the tires (only P8 stated these felt similar for both conditions).
To this end, P1 stated “[with covered] I couldn’t even feel any texture. It feels like my fingers bashing into things”, P3 stated “[withcovered] I could only feel I was holding something”. In fact, twoparticipants compared the devices directly in their own words:
P4 stated that the devices provide “totally different feelings, with[holes] is easier to feel texture and grasp stuff”. Finally, P8 stated“for [covered] I feel I have tapes on the fingers, for [holes], it feels
like wearing thinner gloves”.
Feel-through. When it came to the different feel-through afforded by each device, only one participant (out of eight) denoted
they could still feel specific sensations through the covered, while all
eight participants stated they could feel specific sensations throughthe holes. Namely, regarding feel-through the covered device, P7
stated “[I] can still feel rubbery [texture] but hard to feel the patterns (. . .) I can barely tell there’s a pattern (. . .) I can tell onlypressing down very hard”. Conversely, with the holes participants
accounts suggested they were able to feel more. For instance, P2(similarly P5) stated “[with holes] I could feel a lot more (. . .) I canreally feel the rubber and it passes [the] little things on the tires(. . .) I am able to feel texture easier”, or P7 who stated “[with holes]
I can feel (. . .) rubbery (. . .) doesn’t feel as sticky”. Moreover, twoparticipants stated they perceived the truck’s surface through theholes, P1 stated “[with holes] I could feel it’s cold like metal (. . .)with [covered] I couldn’t feel as much” and P6 “[with holes] I could
feel it’s not completely smooth material, I could feel slight friction,
like the paint on the metal”.
Sweating. Two participants commented on sweat generated
under the cover. P1 noted that “it’s sweatier wearing the fully
covered one” and P4 was surprised at noticeable sweat when taking
off covered devices.
Virtual realism. When asked about the haptics from the buttons (e.g., the larger big “plus” vs. small “minus” buttons when
scaling the virtual cover to fit the truck’s real size), all participants reported to feel difference between them in both conditions.
However, differences started to emerge when asked if they could
distinguish the intensity of the stimulation between both buttons(i.e., smaller button provided a weaker stimulus) or the shape of the
buttons. Here, we observed the covered device providing superior
benefits, for instance, five participants (P1, P5, P6, P7, P8) foundit easier to distinguish intensities, and two (P1, P2) found it easier
to distinguish shapes. Exemplary comments included, P1 stating

CHI ’24, May 11–16, 2024, Honolulu, HI, USA“minus button [’s intensity] was less as strong and more localized”(P1) or, P2 stating “I could feel the two buttons stimulate different
parts of the fingers”. While participants were still able to do thisas well with the holes device, they commented this required more
attention, for instance, P4 stated “I had to pay attention to feel thedifference”. Moreover, while we did not see a statistically significant
difference in reported realism, their comments suggest otherwise.
With three participants (P1, P3, P6) specifically stating they foundthe virtual feedback more realistic with the covered, as expected
per our H3. For instance, P1 stated “[with covered] really felt likethe engine rolling”, “way stronger” (P3), “even more vibration” (P6).
Conversely, the holes device had a range of mixed options regarding
realism, with P7 stating “decently realistic”, while P8 stated that“[vibration] is weaker and irregular”. Finally, P4 found haptics via
holes as more realistic, stating “felt like it’s coming from the object”.
Envisioned use-cases. Finally, we asked participants what theywould see themselves using our holes-based device for. The majorityof participants provided examples that blended real-world with
virtual assistance, some of the most unique included: “education,in chemistry labs, feel objects that you cannot touch, like mercury”(P7); “feel my pet’s fur despite wearing haptics” (P4); “feedback forwhen you hold the badminton racket too tight” (P8, and similarly
P6 for Ping-Pong); “indoor design, like set up virtual furniture in
my real room” (P3); and, “typing in VR and then grabbing my coffee,
having the friction to do so” (P5, also P1 & P2 had similar uses for
VR/MR typing).
Study Limitations: While we were not able to statisticallyvalidate our third hypothesis (i.e., trading off holes for electrodes
results in decreased realism), we observed participants’ accounts
that might support it. We believe this was in part due to two limitations of our study. First, like any study of this nature, participants
were not experienced with electrotactile and will need long-term
acquaintance to develop an ability to recognize high-resolution
sensations (e.g., 12 points of stimulation with covered vs. six withholes, when feeling the large button). Secondly, since our goal was
to focus on physical manipulation, our MR task did not feature a
lot of virtual interactions with the full array—these might explain
why we did not quantitatively measure the realism trade-off. Finally, we only tested polyimide, commonly used in electrotactile
devices—leaving other variants (e.g., materials with higher friction)
for future investigation.

8

DISCUSSION OF HOWRESEARCHERS/DESIGNERS CAN APPLYOUR FINDINGS TO THEIR DEVICES
8.1 How our approach can improve haptic
permeability of existing electrotactile
devices
While we only explored the generic case of uniformly distributed
holes (leaving other parameters, such as hole size, for future investigations), our findings can already be immediately applied tostate-of-the-art devices, namely Tacttoo [51]. This device consistsof eight electrodes, leaving enough space in between to place (2mm) holes once its traces are rerouted, which we depict in Figure
10 (a). Further, we expect that many more electrotactile devices

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Shan-Yuan Teng et al.

Figure 10: Simulating the addition of holes (blue circles) to existing electrotactile devices. (a) [51]; (b) [1]; (c) [55].
can be improved by utilizing our design options. For example, therouting of [1] can be changed to the backside, allowing spaces forholes, as depicted in Figure 10 (b). Our approach is likely to bebeneficial beyond the fingerpad, for instance it should extend to thewhole hand. For electrotactile arrays with high resolution [55], researchers can explore how trading-off electrodes with holes might
improve tactile sensitivity, as depicted in Figure 10 (c).

8.2

How our approach can improve haptic
permeability of future electrotactile devices

Future devices can be designed with haptic permeability in mind
from the start. According to the expected usage scenario, designers
can assess how much tactile sensations should be preserved, andhow much resolution of virtual haptic should be needed, and weightin technical limitations (e.g., material thickness, traces thickness).
In the study, we uniformly laid out the holes on the film. However,the layout of the holes can be tailored for specific applications (e.g.,holes that allow for better grip on a tool with a specific shape).
Following this, researchers can determine a suitable hole size forthe desired application (here, they can utilize our ink test to see
fingerprint ridges revealed by holes, see Figure 2). We hope thatvariations on these parameters might lead to new designs that canbest balance the virtual haptics without obstructing many sensations from the real world.

8.3

sensing [35] might also take advantage of holes, but may require(in some cases) altering the sensing layout.

8.4

Exploring haptic permeability beyond holes

Finally, because we focused on improving tactile aspects of interaction (e.g., tactile sensitivity or grip friction), we implemented our
haptic permeability by adding holes. Yet, it is entirely possible that
other forms of permeability exist. For instance, our design withholes also can achieve air permeability, similar to [29, 56], allowing the user to feel the wind and also let sweat evaporate (as we
observed in the user study, noticeable sweat accumulates with covered devices). Similarly, if a tactile device will be used in a context
where it is paramount that users can feel liquids on surfaces (e.g.,
cooking, repairs, etc.), one might engineer a device for liquid permeability—e.g., purposefully engineering the device from a porous
material that allow fluids to passthrough. Likewise, if a tactile interface will be used in an interactive context where it is paramountthat users feel the temperature of surfaces (e.g., cooking, factories,etc.), one might engineer a device for thermal permeability—e.g.,
purposefully engineering the device from a highly conductive thermal material that rapidly transmits thermal energy to the user’s
skin.
These examples illustrate how our notion of haptic design canchange once we embrace the idea of haptic permeability, which we
humbly hope will inspire researchers to follow this direction.

Haptic permeability beyond electrotactile

While our exploration was centered around the haptic permeabilityof electrotactile interfaces, we envision that our approach can beapplied to other film-based devices that do not use electrical stimulation, including actuators or sensors. While these are exciting newopportunities for haptic permeability, we warrant caution when
extrapolating our findings beyond electrotactile as these might firstrequire further studies.
Film-based mechanical actuators: Other on-skin actuators
might benefit from our approach of adding holes. For instance,
dielectric-elastomer actuators [27]; yet, note that holes should notinterfere with the mounting points for moving parts. Second, miniature piezo actuators [17] could potentially be improved with holes;
however, when adding holes one should pay attention to possible
changes in resonance frequency. Finally, adding holes to fabricbased actuators [3] might provide some benefits but might impact
parameters such as fabric stretchability.
Film-based sensors. Conversely, our approach might even be
beneficial in the case of on-skin sensors [34]. For instance, EMG
based devices [32], might benefit from adding holes between/inelectrodes; however, it is worth noting that holes might impact
the sensitivity of the electrode. Similarly, on-skin capacitive touch

9

CONCLUSION

To improve the usability & minimize encumberment of tactile haptic
devices for the fingerpads, many researchers have moved awayfrom thick actuators (e.g., arrays of vibration motors) and, instead,focused on thin devices—e.g., electrotactile. While these can beengineered to be thin, not all of the important haptic cues can passthrough the device’s film, which our three studies confirmed. To
improve the feel-through of electrotactile devices, we proposed& evaluated adding holes. By means of this, we argue that haptic
permeability—the amount to which a haptic interface lets the user
feel real-world sensations—is a understudied aspect that needs tobe brough to the foreground. We argue that the next generationof haptics interfaces will need to consider not only how accurately
they deliver virtual sensations but also how they balance this withallowing to feel the real world.

ACKNOWLEDGMENTS
We thank Keigo Ushiyama for the support of electrotactile hardware
and Akifumi Takahashi for its software driver. In addition, weare grateful for the constructive and insightful feedback from theHaptic Permeability:
Adding Holes to Tactile Devices Improves Dexterity

reviewers. This research was supported by NSF 2047189 and Alfred
P. Sloan Foundation.

SPECIAL ACKNOWLEDGMENTS
We would like to acknowledge that the ACM CHI 2024 conference
has decided to take place in Hawaii, offering extremely limited
options for remote participation. While some authors have chosen to present their work in person or bring its physical prototype
to the CHI community, they have done so with compassion towards Native Hawaiians and with a heavy heart. We urge that
the ACM’s process for conference selection should more carefully
consider the impact our conferences have on local communities.
We especially want to acknowledge Prof. Josiah Hester, a Känaka
Maoli Professor in Computing, who organized comprehensive resources that educated us about the negative impacts of over-tourism
and climate degradation in Hawaii1 . Finally, the decision of someauthors to participate in this conference does not represent theviews of other members of our lab, who have chosen not to engage
with this edition of the CHI conference due to its impact on Native
Hawaiians.

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