Altering Perceived Softness of Real Rigid Objects by Restricting
Fingerpad Deformation
Yujie Tao

University of Chicago
yujiet@uchicago.edu

Shan-Yuan Teng

University of Chicago
tengshanyuan@uchicago.edu

Pedro Lopes

University of Chicago
pedrolopes@uchicago.edu

Figure 1: (a) We propose a haptic device that can make rigid objects feel softer. The device works by restricting fingerpad
deformation with a motor pulling a hollow frame around the fingerpad. What is unique about our approach is that it leaves
the center of the fingerpad free, so the users can still feel the objects they are touching; it is different from typical haptic
devices, which cover the fingerpad and only render virtual haptics. We explore our device to alter the softness of (b) rigid
protrusions to serve as buttons, (c) part of a rigid 3D printed object, or (d) make the same VR prop changes between soft andhard state.

ABSTRACT
We propose a haptic device that alters the perceived softness ofreal rigid objects without requiring to instrument the objects. Instead, our haptic device works by restricting the user’s fingerpad
lateral deformation via a hollow frame that squeezes the sides ofthe fingerpad. This causes the fingerpad to become bulgier thanit originally was—when users touch an object’s surface with their
now-restricted fingerpad, they feel the object to be softer than it
is. To illustrate the extent of softness illusion induced by our device, touching the tip of a wooden chopstick will feel as soft as arubber eraser. Our haptic device operates by pulling the hollow
frame using a motor. Unlike most wearable haptic devices, which
cover up the user’s fingerpad to create force sensations, our device creates softness while leaving the center of the fingerpad free,which allows the users to feel most of the object they are interacting with. This makes our device a unique contribution to altering
the softness of everyday objects, creating “buttons” by softening
protrusions of existing appliances or tangibles, or even, altering thePermission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citationon the first page. Copyrights for components of this work owned by others than theauthor(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or
republish, to post on servers or to redistribute to lists, requires prior specific permission
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UIST ’21, October 10–14, 2021, Virtual Event, USA© 2021 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ACM ISBN 978-1-4503-8635-7/21/10. . . $15.00
https://doi.org/10.1145/3472749.3474800

softness of handheld props for VR. Finally, we validated our device
through two studies: (1) a psychophysics study showed that thedevice brings down the perceived softness of any object between
50A-90A to around 40A (on Shore A hardness scale); and (2) a user
study demonstrated that participants preferred our device for interactive applications that leverage haptic props, such as making
a VR prop feel softer or making a rigid 3D printed remote control
feel softer on its button.

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

KEYWORDS
haptic illusion, compliance, wearable haptics, tactile, VR, props, AR,
soft, rigid
ACM Reference Format:
Yujie Tao, Shan-Yuan Teng, and Pedro Lopes. 2021. Altering Perceived
Softness of Real Rigid Objects by Restricting Fingerpad Deformation. In
The 34th Annual ACM Symposium on User Interface Software and Technology(UIST ’21), October 10–14, 2021, Virtual Event, USA. ACM, New York, NY,
USA, 12 pages. https://doi.org/10.1145/3472749.3474800

1

INTRODUCTION

Touch is crucial to our interactions with the physical world. As such,many researchers have devoted time to engineering haptic devices
to simulate realistic physical effects [2, 3, 25, 26]. One particularUIST ’21, October 10–14, 2021, Virtual Event, USA

sensation that many interactive systems try to approximate is thesensation of softness (e.g., pushing a rubber surface), which denotes
the material’s ability to “easily give way under pressure” due to
its mechanical compliance. To interactively simulate the softness
sensation, researchers have generally taken two approaches: (1)instrumenting physical objects by adding actuators [23] or, simply
using physical props [19, 40]; and, (2) attaching actuators to theuser’s fingerpad to simulate the softness of virtual objects.
The advantage of physical props is that they are realistic because they are truly compliant. However, modifying the feeling ofan existing physical object is not trivial. Prior work has explored
instrumenting the object’s material (e.g., texture, compliance, andthermal properties) and geometry properties (e.g., size and shape)to provide physical objects a sense of changing compliance/softness.
For instance, Kildal et al. [23] altered the users’ softness perception toward a rigid box by providing a subtle vibration when users
squeeze the box. Other approaches such as PneUI [49], used shapechanging interfaces to create soft tangibles materials. While physical props offer realistic sensations, this approach requires us to
instrument every single object the users might interact with toachieve ubiquitous interactive softness.
On the other end of the spectrum, researchers engineer haptic
actuators that cover the user’s fingerpad and allow them to feel“virtual” softness sensations, i.e., touching virtual objects that feel
soft (but the objects are not physically present). For example, Quek
et. al. [34] created a skin stretch stylus that leverages artificial skinstretch together with augmented force feedback to increase the perceived stiffness of a virtual surface. Many other works rendered theillusion of softness using vibrations [10, 11, 46], stretching fabrics[17], or electrical muscle stimulation [51]. These approaches tend
to be either hand-held (users grab onto handles) or wearables, but
in both cases, their actuators are directly attached to the fingerpad.
As such, with these haptic devices, users cannot feel the texture of
real objects. In other words, these devices are typically limited tosimulating virtual softness.
Thus, the open challenge we try to address is: can we alter thesoftness of everyday rigid objects, without instrumenting them? To
tackle this, we propose a haptic device that alters the perceived softness of rigid objects by restricting the user’s fingerpad deformation
via a hollow frame that squeezes the sides of the fingerpad. In particular, because the fingerpad is restricted by our devices’ frame, it
is not allowed to deform laterally as it normally would. This causes
the fingerpad to bulge outward from its initial position (describedin full in Section 3)—thus, when users touch an object’s surface
with their now-restricted fingerpad, the spread of the contact area
between their finger and the object changes. As a result, users feelthe object to be softer than it is. Figure 1 illustrates the extent
of experiencing the softness illusion induced by our device, where
touching the tip of a wooden chopstick will feel as soft as a rubber
eraser.
To render the softness illusion interactively (i.e., to turn softness
on/off for certain parts of a rigid object), we engineered a haptic device that restricts the fingerpad’s deformation by pulling the hollow
frame against it using a motor. Unlike most wearable haptic devices,which cover up the user’s fingerpad to create sensations, our device
creates softness while leaving the center of the user’s fingerpad free,allowing users to feel most of the object they are interacting with.

Yujie Tao et al.

This makes our device a unique contribution to altering the softnessof everyday objects. For instance, we demonstrate a range of its
applications, such as: creating “buttons” by softening protrusionsof 3D printed materials (Figure 1(b)), or even, altering the softnessof props for VR (Figure 1(d)).
Finally, we validated our device through two studies: (1) a psychophysics study showed that the device reduces the perceived
softness of rigid materials between 50A-90A to around 40A (onShore A hardness scale); and (2) a user study demonstrated thatparticipants preferred our device for interactive applications thatleverage haptic props, such as making a VR prop feel softer or
making a rigid 3D printed gamepad feel softer on its button.

2

RELATED WORK

Our work builds upon wearable haptic devices for haptic augmented
reality [21], softness perception, and softness rendering techniques.

2.1

Perception of softness“Softness” and “hardness” are subjective concepts that arise fromour evoked perceptions while handling compliant materials [43].
Discriminating a material’s compliance allows us to better manipulate and classify objects [43]; thus it is seen as a critical sensationthat many interactive systems seek to emulate.
The most common way we assess an object’s softness is bysqueezing it with our fingerpads. During this, two types of haptic
information are sensed, which assist in discriminating the object’s
softness: kinesthetic and cutaneous cues [43]. Kinesthetic cuesarise from the ratio between the force applied by the user and thedisplacement experienced by the finger as it penetrates the soft
object. Simultaneously, the contact between the object and theuser’s skin also stimulates our skin’s mechanoreceptors, arising as
cutaneous (or tactile) cues [22]. It is using these cutaneous cuesthat we sense the object’s surface properties, such as its texture
and, more importantly for our case, its contact area, which priorwork has established as critical for judging softness [1, 7, 14].
Haptic devices have used both these cues to alter the perceived
softness, such as creating force feedback, tactile feedback, and evenpseudo-haptic feedback. We will now describe how researchers
leveraged these cues to emulate softness while touching both virtualand real objects.

2.2

Rendering softness while touching virtual
objects

To create an illusion of touching a soft virtual object, researchers in
virtual reality (VR) typically construct haptic devices that feature
actuators attached to handheld controllers or directly to the user’s
fingerpad.
One way to achieve this is to leverage the kinesthetic cues is byusing force-feedback actuators that push against the user’s finger
or body. This effectively alters the force-displacement relationshipwhen the user squeezes or stretches an object. For instance, CLAW
[12] is a haptic VR controller with a rotating arm that moved theuser’s index finger according to the amount of object penetration
and rendered normal forces based on the stiffness of the object.
Similarly, CapstanCrunch [41] is a VR controller that utilized brakemechanisms to render compliance between thumb and index finger.

Altering Perceived Softness of Real Rigid Objects by Restricting Fingerpad Deformation

Beyond finger-level softness/compliance, some researchers also
explored stiffness rendering between hands, such as by connecting
and dynamically braking to lock two controllers [45].
While the force feedback technique creates softness sensations,these kinesthetic cues alone are not sufficient for providing discriminable softness levels [5]. To improve softness discrimination,
researchers also leveraged cutaneous cues by applying tactile feedback. Prior work attached vibrotactile motors to fingertip wearable
devices [10] as well as VR controllers [24] to simulate the compliance of virtual objects. Beyond finger touching, the vibration was
also used to generate a feeling of compliance from the floor, as wewalk on it [46, 48]. While vibration-based actuators are promisingfor softness illusions, they also do not provide all the tactile cues. In
particular, they do not provide a directional compliance feeling. To
compensate for the missing directional information, researchers investigated adding skin stretch [34, 35] and skin deformation [38, 39]
to improve stiffness discrimination. Many other techniques havealso been explored to render softness for virtual objects, including
modulating fabrics stiffness state [6, 9, 17], tilting plates [50], andeven electrical muscle stimulation [51].
While these haptic devices render virtual softness, they are notsuitable for interactions with physical objects (e.g., in augmented
reality, with tangibles, or with everyday objects). Unfortunately,
to realize these tactile and kinesthetic fingerpad cues, researchers
apply the actuators on the user’s fingerpad, thereby limiting users’ability to feel real objects. In other words, these haptic devices are
only for virtual haptic interactions.

2.3

Rendering softness while touching real
objects

On the other side of the spectrum, researchers have explored ways
to alter the perception of softness on real objects. While one canconsider shape displays or other soft-actuators a way to manipulate
softness [44], and these certainly have their value in achieving this,
we focus on techniques that manipulate softness on everyday rigid
objects.
One popular way that researchers have explored to change thesoftness of a rigid object is by means of pseudo-haptics illusions (i.e.,
illusions that trick the tactile senses without using tactile actuators).
These create a sense of softness by leveraging the multisensory
integration between visual and tactile sensations that happens in
the human brain [16]. For instance, using mixed reality, Hirano et al.
[20], superimposed a computer-generated image of a deformation(an indentation on the object) onto real objects. This illusion tricks
the user into feeling that the object is deforming by mere visualsuggestion. Similarly, Punpongsanon et al. [33] used a projectionbased method to add a visual deformation effect around the contact
area between the user’s finger and the object surface to visuallysuggest more softness than the object exhibits. Beyond visual cues,
researchers have also demonstrated that audio cues as indicatorsof softness [4, 8]. We take inspiration from the principles behindthese illusions but, instead, explore an illusion that works without
the need for projectors or mixed reality equipment.
Since pseudo-haptics does not provide any actual tactile experience, researchers also explored combining tactile cues with visuohaptic illusions. Choi et al. [11] devised a hand-held device, with a

UIST ’21, October 10–14, 2021, Virtual Event, USA

voice coil actuator transmitting active transient vibrations to theindex fingertip, to alter the perceived softness of passive haptic
proxy.
Another approach is to develop rigid objects that might approximate everyday objects (such as generic tangible blocks) that can also
display softness sensations. For instance, Kildal proposed Kooboh[23], a rigid tangible user interface that is able to display compliance when squeezed. Its key principle is to use a pressure sensor
that measures the user’s pressing force and a vibrotactile actuatorthat responds with vibrations typical of compliant objects. This
technique has also been adapted for stylus interactions, in which it
enables the user to feel that the writing surface becomes softer [22].
Moreover, also using vibrations, PseudoBend [18] creates a senseof object deformation during bending of an instrumented proxy.
While all of the aforementioned techniques are promising, they
are, unfortunately, also dramatically limited: (1) some require instrumenting the objects; (2) others require instrumenting the user’s
fingerpads, preventing users from any natural haptic exploration of
their surroundings; and (3) some rely on custom-engineered generictangibles, which have their benefits but are not everyday objects.

2.4

Rendering softness for real objects without
object-side instrumentations

We drew inspiration from the work of de Tinguy et al. [47], whoengineered a haptic device that simulates stiffness on everyday
objects. Their device is based on a different principle: it pulls a fabricbelt against the finger, not on the fingerpad but on the proximal
finger phalanx, which generates pressure and gives the user the
impression that the object is pushing harder. As a result, a soft object
will appear stiffer than it is. Moreover, in a follow-up work [36],these authors found that releasing the fabric belt at the momentof touch can also make an object feel softer than it is. Our device
offers a different take on this, which is based on restricting thedeformation of the user’s fingerpad, rather than applying pressure
to the user’s proximal finger phalanx. One difference between our
device and their approach is that [36] and [47] require pressurechanges to signal a change in stiffness, while our technique works
as long as the fingerpad is restricted (no changes needed, even if
the restriction force is statically applied, which we validated in our
first study). Finally, it is worth noting that our approach is currently
one-way (softness), while the fabric belt approach works in both
directions.

3

WORKING PRINCIPLE OF ALTERINGSOFTNESS BY RESTRICTING FINGERPAD
DEFORMATION

Our work builds upon prior findings in softness perception andcontact area. Haptic discrimination of softness is determined byboth kinesthetic and cutaneous cues. Tiest et.al. [5] specificallyquantified that the cutaneous cues, produced by the contact between
the object and the user’s skin, provide the majority (nearly 90%) of
information regarding an object’s compliance. Other researchers[1, 7, 14] also validated the role of the contact area, which is the
key factor we leverage in our device to induce softness. Moreover,
Moscatelli et.al [31] found that contact area modulation is even able
to induce illusory displacement. In this section, we will illustrate

UIST ’21, October 10–14, 2021, Virtual Event, USA

Yujie Tao et al.

Figure 2: Depiction of contact area when interacting with a rigid object: (a) the finger in its natural shape prior to contacting
a hard object; (b) the finger deforms laterally as it pushes against the object; the object does not conform to the fingerpad,
leaving the contact area as Arigid . Depiction of contact area when interacting with a soft object: (c) the finger in its natural
shape prior to contacting a soft object; (d) the finger also deforms laterally as it pushes against the object; however, since theobject is soft, it conforms to the fingerpad, adding more contact area between the finger and object (Asoft > Arigid ).
how our technique modulates the contact area between finger andobject, which leads to a change in softness sensation.
First, as depicted in Figure 2 (a) and (b), we examine what happens when a finger comes in contact with a rigid object: the fingerpad experiences pressure on a small area [15] and the fingerpad
tends to move outwards due to the increased pressure. Because theobject is rigid, its surface does not conform to the fingerpad. We
denote the resulting contact area as Arigid in Figure 2(b). The resulting pressure distribution over this skin area stimulates the skinmechanoreceptors, providing cutaneous information regarding theobject’s compliance.
Now, in Figure 2(c) and (d) we examine what happens whena finger contacts a soft object: the fingerpad perceives a wider
contact area because the material itself is more compliant andconforms mechanically to the fingerpad. We denote the contact
area as Asoft , shown in Figure 2(d), and the pressure spreads more
evenly through this area [15]. Much like in a rigid press, fingerpad
fat moves outwards due to the pressure. Moreover, the resulting
pressure distribution on the skin stimulates the skin mechanoreceptors, which provides cutaneous information regarding the object’s
compliance.

Finally, let us examine what happens when the fingerpad is
restricted in its lateral deformation (e.g., two rigid structures apply
a constant force on the sides). As shown in Figure 3(a), the restricted
fingerpad is about to touch a rigid object. The restriction force
causes the fingerpad to be bulgier than it is in a normal state. Thus,
when the finger contacts a rigid object, as depicted in Figure 3(b),this new lateral force from the restricting structures leads to a smallpart of the fingerpad to contact the side of the object. This results
in a larger contact area, which we denote as Arigid ’, and, as we willmeasure empirically, it is very similar to Asoft . In other words, thecontact area and, thus, the cutaneous information from its pressure
is similar to that of a soft object.
In Figure 4, we depict a typical contact area test using color dye.
The pressing force was normal to the object and controlled at 150g
for each trial using a load cell. As shown in Figure 4(c), pressing
a rigid cylinder (hardness of 90A, 7mm radius, normal touch on
top side) via our fingerpad with deformation restriction results ina larger contact area than pressing the same object without the
restriction (Figure 4(b)). This validates that the contact area changes
with the restriction. In fact, the contact area of pressing the rigid
cylinder with our haptic device is most similar to the contact area oftouching, with a bare fingerpad, a same-sized cylinder made froma softer material (40A), as shown in Figure 4(d).

4Figure 3: Depiction of contact area when interacting with arigid object while having one’s finger pad restricted to prevent large lateral deformations: (a) the restriction alreadydeforms the fingerpad vertically prior to contact with the object and gentle side pressure is converted to bulginess of thepad; (b) when pressing an object, the restricted fingerpad has
a larger contact area with the objects (Arigid ’ > Arigid ), compared with touching the rigid stimuli with a bare fingerpad.
This larger area is caused by the restriction.

BENEFITS, CONTRIBUTIONS AND
LIMITATIONS

Our key contribution is a haptic device that alters the perceived softness of rigid objects, without the need to instrument these objects.
Our device induces a softness illusion while leaves the center ofthe finger pad free; this enables users to feel the original texture of
the objects while experiencing the same object switching between
a hard and soft state. Our device opens possibilities in softness
modification for everyday objects, 3D printed objects, props, oreven small protrusions of existing appliances. Finally, we believe
our work contributes to the importance of designing and engineering haptic devices that leave the fingerpad free, so that users canalso feel the exciting haptics of our real world, not just virtualworlds.
Of course, our work is not without limitations: (1) Our device only works for objects/object parts that are smaller than thefingerpad areas, which limits its application to smaller regions orAltering Perceived Softness of Real Rigid Objects by Restricting Fingerpad Deformation

UIST ’21, October 10–14, 2021, Virtual Event, USA

Figure 4: Visualization of contact area when touching cylindrical rubber stimuli of different hardness (touch was normal toobject’s top surface and controlled at 150 g for each trial using a load cell). Yellow paint covered the top of the object, whileblue paint covered the side of the object. The color left on the fingerpad indicates the contact area between fingerpad andobject. (a) simple objects used in this contact area test; (b) contact area of touching a rigid stimulus (90 A) with a bare finger
pad; (c) contact area of touching the rigid stimulus (90A) with fingerpad deformation restriction; (d) contact area of touchinga soft stimulus (40A) with a bare fingerpad.

Figure 5: Our self-contained haptic device (a) viewed from the side, (b) viewed from the top, and (c) electronic details of our
PCB.
protrusions on existing objects; however, this still leaves ample
opportunities for designers to make use of this softness illusion
in VR props, 3D printed objects and even small detents on realappliances. (2) Currently, the device allows for a simple switch on
the softness state of an object. We expect that researchers building
on our work might find ways to modulate this to more degrees ofsoftness. (3) The softness illusion induced by our device works only
down to objects around a hardness of 40A; in other words, at somepoint, it cannot render an object that is already soft, even softer. (4)While our device is unique in its design that leaves the center ofthe fingerpad free, its frame around the user’s fingerpad can still beobtrusive, especially for applications with high dexterity on large
surfaces. (5) Like any device that is finger-mounted, the vibrationfrom the operation of the DC motor can be occasionally felt, despite being faint, as such it introduces additional tactile cues thatare superfluous to the tactile experience. Still, the benefits of ourdevice and mechanical design might inspire future work to tackle
these limitations. (6) Finally, the number of participants employedin our first study is relatively low (N=5), which suggests that more
replications should follow.

5

IMPLEMENTATION

We engineered a self-contained wearable device, depicted in
Figure 5. To help readers replicate our design, we now provide thenecessary technical details. Furthermore, to accelerate replication,
we provide all the source code, firmware, 3D files, and schematics
of our implementation1 .
Mechanical design. Our device actuates the hollow frame
around the fingerpad using a pulley controlled by a small DC motor(136:1 Sub-Micro Planetary Gearmotor 0.55 kg-cm, Pololu), which
is housed by a 3D printed casing at the first phalanx of the finger.
When the motor is actuated, it pulls the frame towards the fingernail, gently squeezing the fingerpad—this restricts the fingerpad,
which is key to enabling the softness illusion.
Electronic design. Figure 5(c) depicts the electronics schematicof our device. Our 16.8 ×10.3 mm PCB houses at its core a nRF52811
microcontroller with on-board Bluetooth Low Energy (Nordic Semiconductor). To decrease its footprint, we used a ceramic chip antenna (W3008C, Pulse Larsen), instead of the traditional zig-zag
PCB antennas. We power the entire device using a 40 mAh LiPobattery.
1 http://lab.plopes.org/#altersoftness (link to PCB, schematics, 3D files, source code,
VR applications, etc.).

UIST ’21, October 10–14, 2021, Virtual Event, USA

Force sensing. To measure the force that the frame applies
on the fingerpad so that we can interactively toggle fingerpad deformation restriction on and off during use, we attached a force
sensor (FSR, Taidacent) between the finger and the device. A thinsilicone pad (2mm, 20 Shore A hardness) is layered on top of the
FSR to ensure good contact with the skin. Then, we use a PID(proportional–integral–derivative) controller to adjust the motor
to achieve a consistent restrictive force level of 60 g.
Wearability. Inspired by Pacchierotti et al.’s taxonomy [32], we
also believe that it is important to emphasize the wearability of oursystem. The device has a total dimension of 55L × 16W × 25H mm
and a weight of 5.03g.

Yujie Tao et al.

connected directly via USB, to exclude any confound caused bywireless latencies.
For the stimuli, i.e., the objects that participants touched, we used
10 objects of varying hardness, all within the Shore A hardness
scale, specifically: 2A, 10A, 20 A, 30A, 40A, 50A, 61A, 70A, 80A, 90A.
This scale captures a “wide range of hardness from extra soft, to soft,
to medium soft, to medium hard, and to hard materials” [42]. Eachtest object was a rubber rectangular prism (6mm x 6mm x 7mm).
These objects are all part of a pre-calibrated hardness durometer kit
from both VTSYIQI (precision ±2 HA per object) and Smooth-On
vendors.

7.2
6

OVERVIEW OF USER STUDIES

We conducted two user studies. Our first study consisted of a softness perception experiment, based on a psychophysics design employed specifically to measure the perceived softness [5, 27–30].
The result of this first study established that our device brings down
perceived softness of objects between 50-90A to around 40A (onthe Shore A hardness scale). While this first study focused on thepsychophysics aspects of the softness illusion, our second study
concerned observing in use device in two interactive applications:(1) a VR application, in which our approach allows the same passive
prop to exhibit both a soft and hard state; as well as (2) a simple
video game played by pressing a button of a 3D printed remotecontrol, in which the rigid button was made to feel softer using
our approach. The result of our second study was that our haptic
device was, overall, the preferred interface with regards to haptic
realism across both applications.

7

STUDY#1: RESTRICTED FINGERPAD
DEFORMATION MAKES HARD OBJECTSFEEL SOFTER

Our first study focused on validating the softness illusion inducedby our device. In this study, participants were asked to touch objectsof varying hardness and determine the softness of objects touchedwith a finger wearing our haptic device. This study is based on a
traditional psychophysics study employed to measure perceived
softness [5, 27–30]. Each trial of this experiment consisted of a 1-up
1-down adaptive staircase procedure.
Our main hypothesis (H1) is that restricting the deformation of thefingerpad would alter the perceived softness of touched rigid objects;
in other words, we expect that objects touched while wearing our
haptic device would appear softer than when touched with the barefingerpad.
This study was approved by our Institutional Review Board(IRB21-0397).

7.1

Apparatus

Figure 6 depicts the setup used in our first experiment. Participants
sat on a chair in front of the experiment desk. They could rest
their forearms comfortably on the table to manipulate objects. A
cardboard stand blocked the participants’ view of the actual objects,
ensuring that no visuals would confound the softness perception.
Participants wore our haptic device (see Implementation for details)

Study procedure

Stair-case design. Our study is modeled after psychophysics studies employed to measure perceived softness [5, 27–30]. The objective was to find what level of hardness/softness does our device
induce. We used a 1-up 1-down adaptive staircase design. In eachtrial of the staircase, participants touched two sample objects: (1) atest object touched by one of their index fingers, which wears our
haptic device; and (2) a reference object touched by the index fingerof the opposite hand, which is not instrumented with any haptic
device (bare fingerpad). Per trial, participants touched the reference
and test object at the same time for five seconds and responded
whether the test object felt softer. As in traditional staircase study
designs, if the participants responded “yes,” i.e., the test object felt
softer, the hardness of the next reference stimulus was decreasedby 1 (i.e., they were presented with a softer reference object); conversely, if the participants responded “no,” i.e., the test object did
not feel softer), the hardness of the next reference stimulus wasincreased by 1 (i.e., they were presented with a harder reference
object).
Staircase starting conditions. Participants started each staircase procedure by comparing the test object to the softest stimuli
in the kit (2A). The test objects were: 40A, 50A, 61A, 70A, 80A, 90A.
This test object scale includes two “medium soft” objects (40A, 50
A), two “medium hard” objects (61A, 70A), and two “hard” objects(80A, 90A) based on Shore A hardness scale [42].
Staircase stopping conditions. Each staircase continued until
five reversals were reached. The final discrimination value wasobtained by averaging the last three reversals [13].
Repetitions and counterbalancing. Because we asked the participants to use their two hands (one instrumented with our device
and another without), we repeated each staircase twice, swapping
which hand wore the haptic device. The order of these was counterbalanced across all participants.
Procedure duration: In total, each participant completed 12
staircases (six test objects × counterbalancing of which hand wore
the haptic device), which took around 60 minutes to complete.
Participants were able to take short breaks in between staircases as
desired.

7.3

Qualification roundWe conducted a simple experiment to ensure our participants haveenough haptic discrimination to identify the difference between ourbase stimuli. We asked participants to order all the stimuli (the 10
objects previously described) in ascending hardness. If a participant

Altering Perceived Softness of Real Rigid Objects by Restricting Fingerpad Deformation

UIST ’21, October 10–14, 2021, Virtual Event, USA

Figure 6: Depiction of (a) the experimental apparatus used in our study 1; (b) a participant touching two different samples tojudge softness.

Figure 7: Study 1 result. Grey markers visualize the physical softness of test objects participants touch in each trial. Orange
markers indicate the average of perceived softness across all five participants (10 hands) and the standard deviation.
was not able to rank correctly in five trials, we did not include theircomparison data. We conducted the qualification round after all the
study tasks were completed to avoid the participants memorizing
the test stimuli softness.

7.4

Participants

We recruited five participants (two self-identified as female, three
self-identified as male, all right-handed; with an average of 24.4
years old, SD=1.02). No participants reported prior injuries on theirhands. Participants received a compensation of $30 for their time.

7.5

Results

All five participants passed the haptic discrimination qualification
test, and no data was excluded. Figure 7 shows the main findings,
which validated that our device made objects feel softer than they
were.
The average perceived softness was found to be around 38A, considering all test objects. For test stimuli with the physical softness of
40A, 50A, 61A, 70A, 80A, and 90A, our device brings the perceived
softness down to 34±12A, 36±10A, 36±15A, 37±15A, 45±19A, and39±17A. Except for the 40A object, all other stimuli have their
physical softness at least a standard deviation above the perceived
softness. It includes all the test stimuli classified as “medium hard”

and “hard” (61A, 70A, 80A, 90A), and one “medium soft” object(50A), based on the durometer shore hardness scale [42].
The above results suggest that our main hypothesis is supported,
i.e., restricting finger pad deformation makes rigid objects feel softer
than they physically are. Furthermore, we see our device played arole in a medium soft stimulus that is closer to the hard object end(50A), but not the stimulus closer to the soft object end (40A).

8STUDY#2: OUR SOFTNESS DEVICE ADDSREALISM TO HAPTIC PROPS IN
INTERACTIVE APPLICATIONS

Our first study validated the softness illusion in a controlled psychophysics experiment using abstract objects (i.e., rubber stimuli
from a durometer kit). Our second study focused on the device’s
effectiveness when participants touch real objects serving as props
in interactive applications. We wanted to validate how the softness
illusion induced by our device adds benefit to two applications:(1) a VR shopping application, where the users are presented with
two VR pencils with different cap erasers. Here, our haptic props
are a pair of chopsticks and yet, our device allows the user to feelthat one of the chopstick tips is soft, which corresponds to thesofter cap eraser in VR; and (2) playing a jump ’n’ run game using

UIST ’21, October 10–14, 2021, Virtual Event, USA

Yujie Tao et al.

Figure 8: Study 2 applications. (a) chopstick as a prop for VR pencil rubbers; (b) 3D printed gamepad for jump-and-run game.
a 3D-printed remote control, in which the button on the remotecontrol is touch-sensitive, inspired by the work from Savage et. al[37]; our haptic device can add the sense of softness when the user
pushes the rigid buttons.
Our hypothesis is that by wearing our haptic device, participants
would experience more realistic haptic feedback while touching theeveryday props or 3D-printed objects in the interactive applications.
This study was approved by our Institutional Review Board(IRB21-0397).

8.1

Apparatus and applications

Figure 8 depicts the setup used in our experiment. We utilized thesame haptic device as in our previous experiment. Participants
sat on a chair in front of a desk. They could rest their forearms
comfortably on the table while interacting with the VR scene or withthe 3D-printed remote control. Unlike in our previous experiment,
the participants’ view was not blocked or restricted in any way(except by the HMD in the case of the VR application).
VR application. Our application is a VR shopping experience,in which the participants touched two pencils’ cap erasers with their
dominant index finger to compare the softness before purchasing.
Visually, they would see one of the cap erasers deform upon touch,
indicating that the eraser was soft. In reality, we used chopsticks
as VR props. For the device condition, we actuated the device on
the index finger when the user’s fingers were approaching the“softer” cap eraser and deactivated it when they moved away. The
deformation of the eraser and the actuation timing of the device
were both calculated based on distance between fingertip and theeraser top, detected via Oculus’ hand tracking.
Video game application. For the video game, we asked users toplay a jump ’n’ run game using a custom 3D printed remote control.
The controller button was touch-sensitive enabled by capacitive
sensing. Participants were asked to put their thumb finger on a
resting area on the controller when not pressing the “jump” button.
Our device actuated when the participants move their fingers fromthe resting area to the button area and deactivates when they moved
the fingers back. In this application, participants wore the haptic
device on their thumb, which allowed as to explore the device onother fingers as well.

8.2

Study procedure and interface conditions

Participants experienced each application twice, one for each interface condition: with haptic feedback from our device and without(baseline). The order of applications and the order of conditions incounterbalanced across all participants. For each trial, they experienced the application for three minutes.
After each trial (i.e., playing one) participants were asked torate how realistic were the pencil top eraser or button pushing felt
on a Likert scale (1 to 7), with 1 being “not realistic” and 7 being“realistic.” At the end of both trials for one application, we asked
participants which interface condition they preferred with regards
to the haptic realism. At the end of the study, we conducted aninterview to understand participants’ general experience.

8.3

Participants

We invited 10 participants for this study (three self-identified as female, seven self-identified as male, all right-handed; with an averageof 23.2 years old, SD=2.48). Five participants previously participatedin study #1; none of them reported injures on their hands. Eachparticipant received a compensation of $30 for their time.

8.4

Results

Figure 9 depicts the results of our study. Overall, we found out that
our haptic device provided more realistic haptic feedback for the
gamepad and was the preferred interface by the participants inboth conditions.
We analyzed our data using two-way repeated measure ANOVA
and found a significant difference in rated realism of the props between two interface conditions (with and without the device F (1, 9)= 15.47, p < 0.005). This suggests that our hypothesis is supported,
i.e., participants experienced a more realistic haptic feedback whentouching haptic props with our device (M = 4.6, SD =1.2) comparedto without the device (M = 3.2, SD =1.7). Moreover, we found nointeraction effects between the two interface conditions and theapplication types (p = 0.34). We further conducted Tukey multiple
comparisons to understand realism rating in individual applications
regarding interface conditions. It shows a statistically significant

Altering Perceived Softness of Real Rigid Objects by Restricting Fingerpad Deformation

Figure 9: Realism and preference across all participants for:(a) the gamepad interaction, and (b) VR prop interaction.
difference between the interface conditions in the video game application (p < 0.05). Button pushing is more realistic under our haptic
device condition. Seven out of ten participants claimed that they
prefer our haptic device regarding haptic realism after trying out
both conditions. For the VR application, while there is no statistically significant difference between the two interface conditions (p= 0.09) in the realism rating, their comments, and their preference
toward our device (8 out of 10 participants preferred our device),suggested they might have found it beneficial as well.

8.5

Qualitative feedback

Feedback on video game application. When asked about participants’ experience in interacting with the 3D printed remote control
without the haptic device, almost all participants mentioned thatthe button felt unrealistic as it was rigid and could not be pushed
down. For example, P1 stated “it’s weird to press something hard;
I didn’t feel how hard I need to push on it to make it work”. P8
and P5 added that the button “has no feedback” (P8) and “doesn’tfeel like interactive, only a solid object” (P5). The three out of ten(P2, P3, P9) that preferred this condition, were hesitant in choosinga favorite for this application. In particular, P2 and P3 stated thatthey did not feel a significant difference in the two conditions towarrant a favorite, while P9 felt the softness difference but also felt
additional feedback from the motor vibration, which they disliked.
Conversely, with our haptic device, participants felt the button
pushing experience was more realistic. In fact, a majority preferred
our haptic device to the baseline condition (seven participants outof 10). For example, P7 commented that “the button is clickable.
It feels like moving away from a real button, which has a spring.”
Similarly, P8 commented “there is a changing resistance from thebutton.” P10 and P1 emphasized the reaction from the button, saying

UIST ’21, October 10–14, 2021, Virtual Event, USA

the button “reacted to what I did [pressed]” (P10) and “felt response
to my input” (P1). Both P10 and P5 also brought up the feelingof “deformation sensation.” P9 related the experience to pushing abutton on iPhone and commented that “feel nice about something isbeing pushed to make the action happen.” Again, it is worth notingthat this button is rigid, and all these evoked haptic sensations arose
due to our device.
Feedback on VR application. Participants had mixed feelings
regarding touching the pencil eraser with their bare fingerpad, i.e.,
without the device. The majority of the participants felt a mismatch
between the visual and haptic experiences. For example, P9 explained that it was “just visual response on softness deformation,
nothing on my hand; both [props] felt the same as rigid”. Some participants (P4, P6, P10), however, were tricked by the visuals, which
indeed deformed the virtual rubber. P4, for example, stated theyfelt a haptic difference by “just seeing the visuals,” which is typical of these kinds of pseudo-haptic experiences elicited by visual
modalities.
Conversely, while wearing our device to touch the rigid haptic
props, the majority of participants (eight out of ten) reported a closer
match between haptic and visual experience when touching theeraser. P8, for example, said “when I rub the finger on the chopstick[prop], it does feel bouncier.” P5 also added that “physically what I
felt match with the visual bounciness.” Even for participants who
were tricked by visual cues, the device still played an effect. P4, P6,
and P10 all stated that the softness level rendered by the device iscloser to the visual softness as compared to the no-device condition.
Last, there are two (out of ten) participants (P2, P3) who reported
that they did not feel a strong sensation difference and thus did notchoose our haptic device as a preferred interface.

9

DEMONSTRATING HOW TO APPLY OURSOFTNESS DEVICE TO INTERACTIVESYSTEMSThe applications of our softness device can be categorized withthree main scenarios: (1) augmenting haptic props in VR, (2)adding interactivity to everyday appliances, and (3) altering softness for 3D printed objects. We achieve all use cases without further
instrumentation on the object side. Beyond video game application
and VR pencil eraser application demonstrated in user study 2(Figure 8), we implement more applications for each category.

9.1

Application #1: Augmenting haptic props
in virtual reality

Our softness device enables rigid haptic props to display both “soft”
and “rigid” states when users touch it. We demonstrated its effectiveness in the VR shopping application in user study 2. To further
showcase its usage, we created a simple “pat the Batman rubberduck” scene (shown in Figure 10), in which the users would be able
to feel different rubber softness when the finger taps on the earsof the Batman rubber duck. One of them would feel soft as ourdevice is on and the other would feel hard with our device off. The
haptic prop we leverage is a power plug common to households.
The modification of perceived softness of existing objects without
further instrumentation would allow users to better leverage daily
objects as props in VR to provide a haptic experience.

UIST ’21, October 10–14, 2021, Virtual Event, USA

Yujie Tao et al.

Figure 10: Allowing VR props to switch softness: (a) one ear of the rubber duck feels soft, since our device restricts the fingerpad
as the user presses the prop (here, a simple power plug). Conversely, (b) with our device off, the other ear feels hard.

9.2

Application #2: Adding interactivity toeveryday appliances

Beyond VR, our haptic device can add interactivity to daily objects
in our physical environment. In Figure 11, we demonstrate an application in which we repurpose the screws on a 3D printer as keys
for playing melodies (we implement this using simple capacitive
sensing). When touched with a bare fingerpad (or when our device
is off), the screw feels rigid. However, when our device is active,
the same screw feels as soft as the pad of a rubbery button.

Figure 11: Shows application of (a) adding interactivity to 3D
printer, where (b) the screws on the 3D printer feel like soft
buttons.

9.3

Application #3: Altering softness for 3D
printed objects

Finally, we generalize the usage of our softness device to 3D
printed objects that users can fabricate on their own. As depicted

in Figure 12(a), we depict how our device can soften a “button” on
a 3D-printed remote-control/gamepad (demonstrated in study #2)or a 3D printed phone case. When touched via a bare fingerpad (orwhen our device is off), any of these protrusions on the 3D printed
objects are relatively rigid. However, when our device restricts thefingerpad, these protrusions feel softer (almost like “clickable”).
Thus, in these examples, our device enables a richer haptic experience when the users interact with these 3D printed devices. For
example, in the smartphone case example, our softness device canaugment experiences such as playing games or taking photos.

Figure 12: Examples of altering softness of regions on fully
rigid 3D printed objects (made from PLA).
Moreover, we also expect the “softened” 3D printed parts might
be helpful for designers in the prototyping stage. As shown in
Figure 12(b), when designers evaluate their 3D printed prototype(i.e., a teddy bear), they can wear the device and experience “howit would feel like” if they switch specific parts (i.e., the teddy bear’sAltering Perceived Softness of Real Rigid Objects by Restricting Fingerpad Deformation

heart) to a softer material. It requires no extra fabrication from thedesigner side.

10

CONCLUSION

In this work, we proposed a haptic device that alters the perceived
softness of rigid objects by restricting fingerpad deformation. Whatis unique about our proposal is that it allows for simple (on/off)interactive softness modifications of a rigid object without the needto instrument objects themselves. Our haptic device pulls a hollow
frame around the fingerpad and thus changes the contact area
experienced by the fingerpad when the user touches a small rigid
object. Our first user study validated that the now augmented fingerpad would perceive test stimuli softer than it is. It brings objects
from 50-90 A to an average of 40 A softness level. Unlike previous
work, our device leaves the center of the fingerpad free, whichenables users to feel the objects they interact with.
Then, in our second study, we demonstrated that our device isa preferred interface with regard to haptic realism in interactive
applications that utilize haptic props, such as making VR props
softer and altering the softness of a 3D printed remote control on
its button. Last but not least, we demonstrate with more applicationsof this haptic device, in the context of augmenting haptic props
in virtual reality, adding interactivity to everyday appliances, andaltering softness for 3D printed objects.
Moreover, we firmly believe our work contributes to the importance of designing and engineering haptic devices that leave thefingerpad free, so that users can also feel the exciting haptics of ourreal world, not just virtual worlds.
Finally, we expect that researchers building on our hardware,
which we will make available to the community1 , will extend our
findings in several new ways, such as exploring other regions of
the skin and body where this might be applicable, modulating moredegrees of softness and so forth. Some exciting future directions
include varying the restriction force provided by the device andaltering the size of the hollow frame.

ACKNOWLEDGMENTS
We would like to thank Romain Nith for his support in PCB design
and Prof. Yon Visell for his valuable feedback. This work was support by NSF grant 2047189. Any opinions, findings, and conclusions
or recommendations expressed in this material are those of theauthors and do not necessarily reflect the views of any funding
agencies.

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