HandMorph: a Passive Exoskeleton that Miniaturizes Grasp
Jun Nishida1

Soichiro Matsuda2 Hiroshi Matsui3 Shan-Yuan Teng1
Ziwei Liu1 Kenji Suzuki2 Pedro Lopes1
2 University of Tsukuba, Japan
3 Ruhr-University Bochum, Germany
1 University of Chicago, USA

Figure 1. (a) We engineered a passive exoskeleton, HandMorph, that approximates the experience of a smaller grasping range. HandMorph uses
mechanical links to transmit motion and haptic feedback from the wearer’s fingers to a smaller hand. (b) In our first user study, we found thatparticipants perceived objects as larger when wearing HandMorph, which suggests that their size perception was transformed. (c) HandMorph achieves
this transformation in the user’s real environment. As such, it can, for instance, allow a product designer to change their grasp into that of a child while
evaluating a toy. In fact, our second study found that participants felt more confident in their toy designs when using HandMorph to validate ergonomics.

ABSTRACTINTRODUCTION

We engineered an exoskeleton, which we call HandMorph,that approximates the experience of having a smaller grasping range. It uses mechanical links to transmit motion fromthe wearer’s fingers to a smaller hand with five anatomicallycorrect fingers. The result is that HandMorph miniaturizes awearer’s grasping range while transmitting haptic feedback.

Embodied interaction tells us that “how we understand theworld, ourselves, and interaction comes from our location
in a physical and social world” [14, 15]. Many researchers
characterize it as moving interactions away from a screen
and into the real world, which has been a central tenet ofmany schools of thought in HCI, such as tangibles [32] orsomaesthetics [26, 29].

Unlike other size-illusions based on virtual reality, HandMorph achieves this in the user’s real environment, preserving
the user’s physical and social contexts. As such, our device
can be integrated into the user’s workflow, e.g., to allow product designers to momentarily change their grasping range intothat of a child while evaluating a toy prototype.
In our first user study, we found that participants perceived
objects as larger when wearing HandMorph, which suggeststhat their size perception was successfully transformed. In
our second user study, we assessed the experience of using
HandMorph in designing a simple toy trumpet for children.
We found that participants felt more confident in their toy
design when using HandMorph to validate its ergonomics.
Author Keywords

Exoskeleton; Haptics; Perception; Embodied Design
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http://dx.doi.org/10.1145/3379337.3415875

In fact, one of the key challenges in embodiment is to design
devices that allow a user to temporarily feel the bodily experience of another person. For example, researchers have shownthat embodying a virtual reality (VR) avatar of a different size
can change one’s perception of size or distances [5, 41, 70] andeven one’s behaviour [18, 85]. Unfortunately, while previous
embodiment devices are powerful, they only work in VR and
not in the user’s social and physical environment.
As such, researchers have turned to wearable devices thatenable users to experience the sense of having a different body,while in the real world. Examples include wearables thatallow a user to see the world through the height of a smaller
person via a waist mounted camera and a AR headset [21, 54].
While these wearables allow the user to look from the eyes ofa smaller person, they do not allow the user to interact with
objects through the hand of the smaller person.
To tackle this, we propose a wearable, depicted in Figure 1,
that transforms the user’s grasping and reaching range into thatof a person with a smaller hand. To achieve this, we engineereda passive hand-exoskeleton, which we call HandMorph, that
achieves its grasp transformation via mechanical linkages thattransmit movements from the user’s fingers to a set of five
anatomically-correct small rubber fingers.

Moreover, we purposely designed HandMorph using mechanical linkages to also transfer haptic information to the user, suchas pressure, which is key to enabling dexterous manipulation.
We found via two user studies, that: (1) when using HandMorph participants perceived that objects were larger, suggesting that their grasping range was transformed into that ofsomeone with a smaller hand; and, (2) participants felt more
confident in their designs when designing a toy trumpet forchildren using HandMorph to validate the toy’s ergonomics.
While we acknowledge that a grasp-illusion could perhaps berealized in virtual reality, we believe that achieving it in thereal world affords a wider range of applications, includinginteractions with real objects that promote a more embodied
design process [15, 29].
RELATED WORKThe work presented in this paper builds primarily on body
representation/ownership illusions from the field of haptics.
Plasticity in body representation

Our brain maintains an updated map of our body representation, which holds spatial information of our limbs’ shape, dimensions, and positions [45, 49, 50]. Our sensory experiences,such as touch and visual feedback, contribute to constructing
and updating our body representation at every instant [47, 48].
It has been shown that our body representation is plastic and
can even be extended to include objects that are not originallypart of our body [24, 31, 36, 47]. A canonical example ofthis plasticity is the Rubber Hand Illusion [8], in which a user
feels that a dummy rubber hand [20] or a smaller doll [74,75, 76] has become the physical embodiment of their own
body. The illusion relies on the user seeing the rubber handbe passively touched while simultaneously feeling that touchon their own hand. Similarly, it has been shown that during
active tool use, such as when manipulating a simple stick, ourbody representation is enlarged [9, 47, 49, 52, 67].
Illusions using virtual avatars

Virtual reality (VR) has been leveraged to transform body
representation. For instance, VR was used to create an illusory
ownership of a virtual child’s body [5, 70], to transform user’s
hands into smaller virtual hands [40, 42, 57], to see oneself
with a different virtual skin tone [46, 59], and to create thesensation of a taller body [85] or scaled arms [34, 41]. Thesedemonstrate that changing (the virtual) bodies in a virtualenvironment is effective in changing one’s perceptions, actions,and behaviors [18, 85]. Therefore, many postulate that theapplication of these experiences may help users gain empathy
toward different people [6, 66]. However, all these illusions
can only be realized in closed-off virtual environments using
virtual avatars. We believe that by pursuing these illusions ina real-world environment, we can allow users to also benefit
from interactions with people and surrounding objects.
Changing body ability through wearable devices

With recent advancements in wearable actuators, it is possible
to change one’s body abilities into those of another person,for instance, a user can: simulate and feel the effects of age

on their own movement range [55, 61, 63] or, even, feel a
different body [7, 13, 22, 83]. These devices are different fromthe aforementioned VR approaches in that they are fusing
visual and haptic sensations in the real world. These devices
are most used to provide a more empathic understanding toother people’s conditions [7, 13, 55].
Two canonical examples of transformations of the user’s bodyability in real time are: AGNES, is a passive wearable suit that
uses: elastic bands to limit the user’s range of motion, gloves
to suppress tactile feedback and a pair of goggles that tint theuser’s vision; the device allows the user to feel the abilities
of elderly people inducing motor and visual impairments [10,37]; and, CHILDHOOD, a wearable system that shifts down auser’s visual perspective in real-time using a combination of a
waist mounted camera and a headset has proposed [54]. Whilethe latter allows a user to see the world from the perspectiveof a smaller person, it does not allow the user to grasp objects
as if they had the hand of a smaller person. To achieve thistransformation of one’s grasp, exoskeletons are required.
Exoskeletons

Exoskeletons are the primary device for haptic feedback in
VR, tele-existence and rehabilitation. “Active” exoskeletonspossess sufficient actuation power (typically by means of large
motors or pneumatic actuators) to move the user’s body. One
popular interactive application for exoskeletons is force feedback in VR [1, 16, 19, 73]. Exoskeletons are also used forremotely controlling robotic hands [53] or for hand rehabilitation [3, 11, 23, 77]. Moreover, supernumerary robotic limbs(e.g., attaching a third hand) [27, 28, 38, 39, 64, 65, 84] oractive prostheses [35, 58] have also been explored as means totransform physical hand ability. While active exoskeletons areuseful in applications that require force feedback or restoring
loss motor function, they also come at the expense of bulky
form factors and decoupled haptic sensations, e.g., when using
an exoskeleton to control an external robotic hand, one cannot
feel what the robotic hand feels without adding sensors to theexternal hand and actuators to one’s hand.
In fact, studies have shown that the lack of haptic feedback
or its delay affect the sense of control and ownership toward
external limbs [2, 17, 68]. As such, researchers have turned
to "passive" mechanisms that provide realtime, and congruent
haptic feedback [78, 79]. Furthermore, passive exoskeletons
are generally safer than their actuated counterparts. We takethese aspects into account in our system’s exoskeleton design.
Body transformations using passive exoskeletons

Passive exoskeletons have been used to transform ability, suchas by giving users a different body size [51, 72]. Anotherexample is a hand-exoskeleton [56] that achieves a smaller
hand; this is an earlier prototype related to ours. However, theauthors did not evaluate whether the prototype was successful
at creating the perceptual transformation of the user’s grasping range through a controlled laboratory experiment. First,
our present work differs by the validation of this phenomena.
Second, our prototype outperforms the previous one [56] inthree ways: (1) we utilized a four-bar linkage, which transmits
the finger motion at the last edge of the finger segments; (2)we reduced friction on the small hand by splitting the fingers
into their constituent segments, rather than connecting them
via a rigid link (see Implementation); and, lastly, (3) we added
a rubber palm and a linkage for the thumb finger; altogether,enabling a usable and dexterous experience.
OUR APPROACH: SMALLER HAND EXPERIENCE

Figure 2 depicts how HandMorph approximates the experienceof a smaller hand by changing the wearer’s grasping andreaching range, while still allowing the user act in their realworld environment. Our main focus was investigating thetransformation of grasping range, rather than reaching range,
as such, our studies predominantly focused on this first goal.
Grasping Range

Reaching Range

user's fingers
hapticfeedback

link mechanism
motionsmaller fingers

restriction
wire

Figure 2. Transformation of the user’s grasping and reaching range.

HandMorph transforms the user’s grasping range by meansof its mechanical linkages, which transmit both movements
and force from the user’s own fingers to their respective small
rubber fingers. By doing so, this mechanism changes the scaleof the user’s grasping motion, requiring the wearer to fully
open their hand when trying to pick up an object that theynormally would have grasped with a half open hand. This
implementation provides a more realistic somatosensory experience (touch, pressure and proprioception) than just restricting
the range of motion of the fingers using wires. This is important since it has been shown that one’s perceived grasping
ability changes one’s overall perception [12, 44, 81, 82]. In
fact, internal representation of our hands affects the perceived
size of objects we interact with as well as the distances inour surroundings [43]; this is thought to happen because ourbrains utilize our hands as a "perceptual ruler" to measure theworld [60]. Leveraging this fact, we expect that wearing ourdevice will transform the user’s perception of the real-world.
As such, in our study, we hypothesized and confirmed that
real-world objects felt larger while wearing HandMorph.
Lastly, HandMorph also features a wire attached from theuser’s belt loop to the exoskeleton. As the user reaches out
with their hand, this wire restricts their arm’s reach. While
this is simplistic, it effectively limits the user’s reaching range.
CONTRIBUTION, BENEFITS AND LIMITATIONS

Our key contribution was engineering a passive handexoskeleton that modifies the user’s grasping range. Our user
study confirmed that participants perceived objects as larger
than they were, when wearing our exoskeleton; this suggests
their perception of their own grasping range was transformed.

Our approach has three benefits: (1) real-time and congruent
haptics: unlike active exoskeletons which require sensors andactuators, our passive-hand allows users to experience realtime haptics (e.g., pressure) without any perceptual delay–adirect benefit of our link mechanism. (2) real-world: unlike
illusions of body transformation that can only take place inVR, our device enables the hand’s perceptual transformation
to happen anytime, anywhere, preserving user’s physical andsocial contexts. This allows it to be used in tasks such as
prototyping toys, exploring usability of products for people ofall sizes, and so forth. Lastly, (3) safe and walk-up-use: as
our device reproduces movements from the user’s fingers to thepassive-hand’s fingers without using powered actuators, it isfairly easy and safe to use even without any explanation (refer
to our exploratory sessions for examples of this)—making ourdevice ideal for real-world activities such as product design(e.g., designing and evaluating toys for children).
Our device is not without its limitations. First, as with any
passive exoskeleton, its range of transformation and the glove
dimensions are fixed by its mechanism. Therefore, while
the current range works (see our studies), changing it would
require re-manufacturing. In fact, in our studies, we observed
a few participants whose fingers did not fit ergonomically in
the glove; however, we believe that future versions of thisdesign will easily mitigate inter-subject variability by either
customizing each glove a particular user or using variablelength linkages. Moreover, users see a cover that hides theirown fingers. We acknowledge that this might diminish theeffect of our perceptual transformation. However, solving thiswould require either using VR, which in turn loses all thetangible benefits of operating in a real-world, or integration
with complex computer vision systems that would projectback the environment onto the user’s hands, rendering them“invisible” (e.g., [30]).
IMPLEMENTATION

To help readers replicate our design, we now provide the necessary technical details. To accelerate replication, we provide
the 3D models of our device1 . Our exoskeleton, depicted in
Figure 3, is divided into four components: (1) finger sockets
to attach the exoskeleton to the user’s hand (link Si ); (2) linkmechanism to transmit the grasping motion (link Bi , Bi, j , Ci );(3) flexible silicone skins for smaller fingers and a palm tosimulate skin-like haptic feedback; and, (4) 40 cm wire witha carabiner to restrict the range of motion of the user’s arm.
Our device is inspired by previous work [56] but solves manyof the challenges that the previous implementation exhibited.
In particular: (1) it allows for movement in all five fingers;(2) our linkages (described below at greater length) are basedon a four-bar linkage, which drastically reduces friction at
the last edge of the finger segments; and, (3) our small handis not rigid but instead is comprised of several rubber finger
segments; this greatly reduces friction on the mechanism while
increasing friction between the smaller hand and the objects it
contacts with, thus increasing the dexterity of our device.

1 https://lab.plopes.org/#handmorph

Finger Socket
θ3
S3

θ2
S2

b32

Angle Adjuster

S1
b21

J0

B1

B2

B3

Pin Slot

θ1

C2

C1

C3

Silicon Sheet

Ball Joint

Figure 3. (a) Link model that transmits the finger motion to the smaller rubber fingers (b) CAD model of the link mechanism. The rubber fingers areachieved with plastic fingers and silicon sheets (c) Implementation of the exoskeleton. The rubber finger and the palm was created by using molds.

Mechanics

HandMorph transmits motion via a mechanical link based on
two four-bar mechanisms, depicted in Figure 3(a). This linkconsists of three finger sockets (S1 , S2 , S3 ) with pin slots, threelinks that act as smaller fingers (C1 ,C2 ,C3 ), five bridge links(B1 , B2 , B3 , b21 , b32 ), and a joint J0 on the palm.
Figure 3(a) shows that when the distal interphalangeal joint
is bent, the fingertip pushes down the bridge link B3 , whichtransfers the motion to the smaller finger link C3 . Thus, theuser’s finger position is replicated by the smaller hand. Since
the distance between each socket Si will decrease as fingers
bent, a pin slot was attached to the sockets Si to translate
movement from each joint to the bridge links b21 , b32 (thegreen ellipse in Figure 3(a)). The length of this pin slot is thecritical factor that determines the grasping range. In order tocalculate its length, we measured the angular displacement ofeach finger joint using an OptiTrack V100 R2 motion capturesystem for a full grasping motion (all five fingers). Then, bymeans of a simulation in Autodesk Fusion 360, we obtained
the minimum pin length required to achieve this motion range.

Figure 4. The thumb mechanism achieves (a) the extension and (b) theflexion while preserving unconstrained thumb movements.

Figure 3(b) shows our link mechanism for one finger. Joint J0
was achieved with a ball joint with two degrees-of-freedom(DOF) and a short linkage that connects the C1 link to theball joint. Each small link Ci has a finger shaped appendage
coated with a silicone sheet surface. A palm attachment witha stretchable band is used to hold the four finger link mechanisms. The thumb mechanism is independent from these, toallow for unconstrained thumb movements, and anchors to asmall pad around the user’s thumb base, as shown in Figure 4.
Manufacturing

The entire exoskeleton was manufactured from polylactic acidplastic in a Ultimaker 3 3D printer. The fingers were coatedwith a sheet of liquid rubber 2mm (Dragon Skin FX-Pro) witha skin color pigment, resulting in the soft look depicted in
Figure 3(c). The exoskeleton’s total weight is 171g.
Scaling factor between user’s hand and HandMorph

To determine the movement scaling factor (i.e., how muchthe user’s finger movement is amplified or reduced by our
linkages), we measured the joint angles from an open handto a closed hand pose, for the index finger mechanism with
one participant. From these, we calculated an angular scaling
factor by dividing the HandMorph’s finger movement by theuser’s finger movement. This simple test revealed that: thedistal interphalangeal joint amplifies movement with a factorof 3.97; the proximal interphalangeal joint slightly reduces
movement (0.74); and the metacarpophalangeal joint has almost no impact on movement scaling (0.95). Note that the
four-fold amplification of our first joint is intentional, as itenables users to easily grasp objects given that any rubber
fingers lack the level of friction and dexterity of actual hands.
OVERVIEW OF USER STUDIES

We conducted two user studies to validate HandMorph: (1)size perception experiment, in which we found that thegrasped objects were perceived to be larger; and, (2) simple toy design session, in which we examined participants’experiences with HandMorph; we found that they felt more
confident in their simple toy designs when using HandMorphto test their design’s ergonomics. Lastly, we also took HandMorph outside the lab and observed that visitors were able touse it without instructions.
STUDY 1: INVESTIGATING SIZE PERCEPTION

The objective of this study was to understand how our HandMorph impacted size perception of objects participants interacted with. This measurement provides direct insightsof whether the user’s grasping experience was transformed.
Therefore, our study was a size perception experiment modelled after [80].
Our hypothesis was that if HandMorph transformed a participant’s grasp into that of a smaller hand, then objects would feellarger than they actually were. This prediction was grounded
in previous studies that reported that when participants embodied a smaller body in VR [5, 70] or by means of the rubber
hand illusion [74, 75, 76], objects would to be perceived aslarger. However, since HandMorph does not require VR, weutilized real objects. Our study was approved by our Institutional Review Board (IRB19-1431).

Apparatus

Participants

Figure 5 depicts the setup used in our experiment. Participants wore an earlier model of our exoskeleton glove on their
dominant hand, which did not feature the thumb mechanism.
To constrain the thumb movement into the adequate range
we featured a restriction wire. The exoskeleton was covered
with a white cloth to encourage participants into observing thesmall exoskeleton-fingers rather than their own fingers.

We recruited 12 healthy right-handed participants from ourlocal organization (six self-identified as female; six as male;mean age = 23.9 years old, SD=2.54). Participants were compensated with 10 USD for their time. No participant had
previously tried an exoskeleton.
Figure 6 shows our main findings, i.e., the result of the perceived object size with/without the exoskeleton.

Link Cover

100

1

2

30cm

52mm
42mm

Figure 5. Experimental setup used in our grasping range experiment.

Participants were asked to interact with five plastic balls ofdifferent diameters, ranging from 32mm to 72mm (in steps of10mm). There were two positions on the table, 30cm apart,marked with "1" (start position) and "2" (end position).
Trial Design

In each trial, participants were asked to: (1) pick up a ball,(2) move it from start to end position, (3) move it back tostart position, and (4) squeeze it three times. Then, the experimenter took off the exoskeleton hand. Following this, we
asked participants to demonstrate the perceived size of theball they just interacted with by posing their index finger andthumb, with their eyes closed, as if they were holding the ball;this allowed us to focus on their kinesthetic experience. The
experimenter measured this distance using a digital caliper
with sub-millimeter precision.
Procedure

Participants experienced the aforementioned task in two conditions: with HandMorph and with their OwnHand. The
order of these two conditions was counterbalanced across all
participants. In each trial, participants were provided one newplastic ball at random, each ball was shown three times (repetitions). Only one ball was at the table at all times of the study.
In total, each participant performed a total of 360 trials (fiveball diameters × three repetitions × two conditions).
At the end of all conditions, participants were asked three
Likert scale questions regarding their experience including
perceived grasping range (Q1 ), the feeling of control (Q2 ) andownership (Q3 ) toward the exoskeleton. We asked participants
to fill in a Likert scale, as shown in Figure 7; this questionnaire
was based on that of [8, 54]. To examine body representationin detail, we also asked them to choose or draw their hand representation, as shown in Figure 8. They were asked to answer
these questions on a 7-point scale for Q1 (-3 = decreased, 3= increased) and a 6-point scale for Q2,3 (1 = disagree, 6 =agree).

Reported Diameter [mm]

32mm

72mm
62mm

Results90

OwnHand

HandMorph

80
70
60
50
40
30
20
10
0

32mm

42mm

52mm

62mm

72mm

Figure 6. Participants’ perceived object size in both conditions. We
found that objects were perceived to be larger when using HandMorph.

A two-way repeated measures ANOVA revealed a significant difference in the main effect between the two interface conditions (F(1, 110) = 25.7, p < 0.001). This suggests that our main hypothesis was confirmed, i.e., when using HandMorph participants perceived objects to be larger(M = 54.7mm, SD = 20.0mm) compared to when using their
OwnHand (M = 47.6mm, SD = 16.2mm). Furthermore, we
found no interaction between the two interface conditions andthe ball’s diameter ( p = 0.39).
Qualitative findings

At end of the study, we asked the participants about theirexperience using three Likert scale questions and a multiplechoice question regarding their perceived body representation.
Grasping range

As depicted in Figure 7, the questionnaire results indicatethat participants felt that their grasping range decreased whenusing HandMorph (Q1 ; score=−1.77, SD=1.25). Participantsalso reported a clear sense of agency while the controlling
HandMorph (Q2 ; score=4.31, SD=1.32), while their sense ofownership was relatively low (Q3 ; score=2.77, SD=1.19).
Decreased (-3)

-2

-1

0

1

2

(3) Increased

How your grasping ability
has been changed whenusing the exoskeleton?
Disagree (1)

2

3

4

5

(6) Agree

I felt I was controllingthe exoskeleton's fingers
I felt the exoskeleton's
fingers were my fingers
average(×), median( | ), IQR, outlier(�) and total range of variation

Figure 7. Participants’ experience with regards their grasping range,
agency and ownership while wearing our exoskeleton.

Hand representation

Impact of ownership and perceived ability

Figure 8 depicts the six hand representations presented toparticipants in our multiple choice question: (a) nothing haschanged; (b) my entire hand became smaller; (c) four fingers
became smaller; (d) four fingers became thicker; (e) Had additional fingers; and, (f) participants could draw their own
representation if none of the above were fitting. Figure 8 also
depicts our results: four participants perceived as if their handbecame smaller; six participants perceived as if their four fingers became thicker; and, one participant drew in which theirfour fingers became thicker and smaller (Figure 8(f)). This
insight provides further indicates that, indeed, participants’hand representation was changed by HandMorph.

One participant mentioned that a failure to grasp a ball ina single trial lowered their sense of agency and ownershiptowards our exoskeleton. We acknowledge that an outcome
bias could affect the score [81], i.e., failure in grasping could
decrease the sense of ownership.

7
6
5
4
3
2
1
0

Moreover, prior research suggested that participants exhibit
a higher sense of ownership for larger virtual hands than forsmaller virtual hands [4], which could also contribute to thelower sense of ownership observed by our participants whenusing our smaller hand.
STUDY 2: EXPERIENCING HANDMORPH IN DESIGN

While in our first study we focused on a psychophysical experiment to understand the impact on perception, our second study investigated whether HandMorph could assist in
evaluating/designing products for people with smaller hands.
Therefore, we asked participants to improve the design of asimple toy trumpet to be used by smaller persons, such as children (ages 3-10). Our study was approved by our Institutional
Review Board (IRB19-1431).
Apparatus

Figure 8. Participants’ perceived hand representation. We found that 11(out of 12) participants reported their hand felt smaller or the distance
between the four fingers and the thumb felt shorter.

Discussion

Altogether, our quantitative and qualitative results suggest thatparticipants perceived their hands were smaller while using
HandMorph to grab the objects. Still, we believe a few otherfactors should be discussed in light of our findings.
Perceived object size

We believe the main factor driving the perception of objects being enlarged while using our exoskeleton-hand was somatosensation, i.e., sense of touch and proprioception. We find thisto be the case since, while using our exoskeleton, participants
could not see their fingers. Furthermore, one could suspect
the perception of an enlarged object came from visual comparisons between the object and the small rubber fingers. However, we do not believe this to be the case, since we asked
participants to judge the size of the object blindfolded; precisely to understand the impact of proprioception over visuals.
Still, visual information might affect size perception as well.
Characterizing precisely the impact of each sensory modality
might require further examination. Furthermore, combining
the smaller grasp enabled by our exoskeleton with the lower
visual perspective of prior work [54], one could potentiallyachieve a change in both size and distance perception.
Limitations of our device

Additionally, discrepancies between participant’s skin and theexoskeleton’s silicone skin, the unaltered size of the thumb(which we addressed in our subsequent prototype), and thechanges in tactile feedback via the link mechanism could also
contribute to a lower sense of ownership.

Our study apparatus is depicted in Figure 9. Participants were
provided with modeling clay and a set of typical modelingtools (e.g., a spatula, a ruler, etc.) to quickly fabricate their toy
designs. A 3D printed toy trumpet was provided as an initialdesign, as depicted in Figure 10(a).
Web Camera
Recording PC
Initial Design
Specification Sheetwith a Ruler
Exoskeleton

Soft Clay
SpatulaFigure 9. Experiment Setup: Participants designed a small toy trumpet,using clay, meant for children (ages 3-10). To assist them, we provided:a specification sheet with hand dimensions or an average child and our
exoskeleton.

Additionally, participants were presented with two tools to
assist them in ensuring that the resulting prototype was ergonomic for children: (1) HandMorph: our prototype withall five fingers (Figure 1); and, (2) SpecSheet: a US-Letter
size specification sheet, which is depicted in Figure 10(b) andprovides full ergonomic details with the average dimensionsof the hand and fingers of children (ages 3-10). The provided
SpecSheet was adapted from the hand dimension diagramsin Tiley et al.’s human factors for design textbook [71]. This
specification illustrated the average dimensions (ages 3-10)
for hand (length, breadth, and grip radius) and fingers (length,
thickness, breadth, diameter), which were obtained from previous studies [25].

Disagree (1)70GRIP WIDTH
68 / 2.69’’

REACH DEDUCTION
31 / 1.22’’
97 / 3.82’’

40 / 1.57’’
CROTCH

CROTCH

HAND LENGTH
108 / 4.25’’

THUMB THICKNESS
17 / 0.67’’

GRIP WIDTH
43 / 1.69’’

INDEX FINGER
PUSH POINT

THUMB
GRIP
POINT

GRIP LENGTH
67 / 2.64’’

PALM LENGTH
45 / 1.77’’

CROTCH

52 / 2.05’’

6.5
30

77 / 3.03’’

FINGER TIP TO CROTCH
63 / 2.48’’

FINGER
THICKNESS
12 / 0.47’’
FINGER WIDTH
13 / 0.51’’

88 / 3.46’’

60

HAND WIDTH
70 / 2.76’’

15
120
Dimensions
Millimeter / Inch[mm]

Figure 10. Apparatus: (a) initial design of the toy trumpet, and (b) specification sheet in which its graphic design is adapted from an actual book.

2

3

4

5

6

(7) Agree

2

3

4

5

6

(7) Agree

Using the specsheet, I could
understand how my final productwill be handled by a smaller person
Using the exoskeleton, I could
understand how my final productwill be handled by a smaller person
Disagree (1)I felt the specsheet was usefulfor gaining confidence in myfinal design
I felt the exoskeleton was usefulfor gaining confidence in my final
designaverage(×), median( | ), IQR, outlier(�) and total range of variation

Figure 11. Participants’ reports of their experience and confidence intheir designs while wearing HandMorph or using SpecSheet.

Procedure

Prior to the start of the trials, we asked participants to get acquainted with HandMorph for one minute, during this time we
asked them to interact with the balls from Study 1. Likewise,
we walked them through the information on the SpecSheet.
Then, participants were given a maximum of 30 minutes to(1) evaluate and improve the accessibility of the toy trumpet
provided by the experimenter, and (2) present their designusing modeling clay. Prior to the start of the trial, we informed
participants that their resulting trumpet was targeted at children (ages 3-10) and that they could use both the exoskeleton
and the specification sheet anytime.
Participants

We recruited eight new right-handed participants from ourinstitution (four self-identified as female; four as male;mean=25.6 years old, SD=4.3). Participants were not professional designers but had all taken design classes in college.
Participants were compensated with 10 USD for their time.
No participant had previously tried any exoskeleton. With
their consent, we video-recorded all trials for analysis.

Video-observations and Interviews

We found that participants used HandMorph an average offive times (SD=3.77) and SpecSheet on average only once
(SD=1.29). The average single usage time was 68s (SD=53.2;
includes time to put it on/off) for HandMorph and 79s(SD=79.9) for SpecSheet; suggesting that HandMorph wasused for quicker and shorter evaluations, while the SpecSheet
was used once for a longer period of time.
We observed that five participants (out of eight) wore HandMorph right at the start to evaluate the initial design’s ergonomics, as shown in Figure 12-(1). P6 was the most extreme
case as they wore HandMorph throughout the complete session, including while modeling. When interviewed at the end,
two participants remarked on using HandMorph to feel that theinitial design was "too big for grasp or pick up" (P1 and similarly P3); P5 added "[I] felt pain [from stretching] my palm
when tried to grab". Conversely, three participants started byconsulting the SpecSheet; one of these (P7), spent two minutes
studying the SpecSheet but ended up using HandMorph forrest of the session, depicted in Figure 12-(4).

Results

Overall, we found out that: (1) participants felt that HandMorph provided them more confidence in their designs; (2)participants used HandMorph, on average, five times more
often than the SpecSheet, in quicker design bursts, mostly fortesting ergonomics; and, (3) participants improved five out ofsix flaws in the initial design.
Questionnaire

Figure 11 depicts the main findings from our questionnaire.
Regarding how much using the tools improved judgements on
the ergonomics of their designs, participants rated the HandMorph on average as 6.0 (out of 7; SD=1.8) and the SpecSheet on average as 3.9 (out of 7; SD=2.3); this suggests thatparticipants felt empowered by HandMorph to enable better
ergonomic judgements. Furthermore, regarding how much using the tools improved confidence in their designs, participants
rated the HandMorph on average as 5.75 (out of 7; SD=2.1)
and the SpecSheet on average as 3.0 (out of 7; SD=2.1); thissuggests participants felt that the HandMorph provided themmore confidence in their designs.

Figure 12. Participants used HandMorph for assessing ergonomics.

When asked about what drove participants to use HandMorph:P1 added "[evaluating] body size and button placement so thatthe instrument can be held comfortably, and letting [my] fingers move quickly and easily"; P3 stated "grab [it] to emulate
how [a] smaller hand would feel"; and, P4 added, "verifying
positions [that I] can reach, grab and use comfortably", asshown in Figure 12-(2). To our surprise, three participants
used HandMorph to actually model the clay, creating design
strategies such as marking finger shape and distance on theclay with HandMorph (Figure 12-(3)), or squeezing the clay todecrease the toy’s diameter until they felt a comfortable grasp.

When asked what drove participants to use the SpecSheet: P3
stated, "approximate the dimension of the whole design"; P4stated, "to consider potential size"; P2 stated, "I looked at this[SpecSheet] at first because I knew it describes the dimensions
precisely. But [later] I found that my early design based on it[SpecSheet] didn’t feel right". Furthermore, P2, P5, and P6
experienced some difficulties while calculating appropriate
length from the SpecSheet; this is normal as these humanfactor specifications do not illustrate all possible hand poses.

Dimensions of the Trumpet Elements
150

120
105

Design Outcomes

Figure 13 shows participants’ toys made from modeling-clay.
While there is no "perfect solution" for this toy trumpet (i.e., asthere are infinite possible designs), there were six challenges
in improving the initial design: (1) reduce the length of themain shaft; (2) reduce the grasp radius of the main shaft; (3)reduce the spacing between pistons; (4) reduce the piston’sheight; (5) increase the piston’s cap diameter for an easiertargeting; and, (6) add a finger support (where trumpeters rest
their thumb and/or pinkie for stability). Guidelines for sizing
products denote that graspable parts of a toy for ages above 3
must relate to the user’s hand size [62], which for a 3-10 year
old child was found to be on average 116.84mm (length) and
53.34mm (breadth) [25, 33]. Participants were unaware of thislist or given any clues of what to fix in the initial design.

e.g.

90
75



60
45
30
15
0

When asked to compare the tools: P1 added, "the exoskeleton
helped me feel out major issues in the design’s usability, andwas really useful for testing how the instrument would feel";
P5 added regarding the SpecSheet, "numbers didn’t help verymuch"; and, lastly, P7 stated "I relied on [HandMorph] to help
me to evaluate the design, test it and to see how confident I
am for this to be handled by a smaller hand".

Participant’s Design
Original Dimension

135

[mm]

4
Body
Length

Body
Diameter

PistonDistance

PistonHeightPistonDiameterFingerSupporter

Figure 14. Dimensions of original vs. participant’s toy design. Participants successfully adjusted the dimensions of most design elements considering the toy’s target audience (children between 3-10 years old).

Our findings regarding number of uses and duration per use,suggest that HandMorph may be seen as complimentary rather
than mutually exclusive to existing design tools. Participants
used HandMorph many times but in short design bursts, while
they tended to use the SpecSheet in longer design iterations.
EXPLORATORY SESSIONS OUTSIDE THE LAB

In addition to our studies, we observed participants’ interactions with HandMorph in demonstrations we did at a museum
and a local supermarket, depicted in Figure 15. While thesesessions did not carry the rigor on our controlled experiments,
they allowed us to gather insights of how people reacted tothe device in a looser setting without precise instructions. To
incite visitors to interact with our device, we set up a tablewith several objects on display, such as toy cars, eating, anddrinking utensils. Then, as visitors approached our table, they
were offered our HandMorph device prior to interacting withthese objects. Most interactions took around 3 to 5 minutes. In
these sessions, we counted over 500 visitors, with the majority
at the museum. We did not record demographics since this was
not a controlled experiment but visitors were all seemingly
adults.

Figure 13. Participants’ toy trumpets made from modeling-clay.

Finally, Figure 14 depicts the adjustments that participants
made compared to the initial design. We found that all participants corrected five (out of six) flaws by reducing (1) main
shaft length; (2) main shaft radius; (3) piston spacing; (4) piston’s height; and, by increasing (5) the piston’s cap diameter.
Lastly, P1/2/3/8 addressed (6) by adding a finger support.
Discussion

Taken altogether, our results suggest that participants embraced HandMorph as a tool for quick iterations, mostly totest ergonomics. Participants used it several times, without
mentioning any frustration regarding putting it on or off. Participants used it with surprising dexterity, even modeling theclay while wearing our device. This suggests that HandMorphcan be used as a design tool without a long learning-curve.

Figure 15. Observations at our exploratory sessions at a museum and alocal supermarket (taken with consent of visitors).

Many visitors used HandMorph to manipulate toys and eating utensils, immediately after they wore the exoskeleton, forthe first time, with virtually no training. We observed thatmost visitors quickly understood how to manipulate it, making in fact very few mistakes. This might suggest that theexoskeleton was relatively easy to use.
Several visitors noted, without being prompted, that they felt
aware of the difficulties someone with a smaller hand mightexperience. For instance, one visitor mentioned that they felt
difficulties in grasping a water bottle with our exoskeleton
hand because the bottle was so large. They suggested thatgiven that these bottles are practically ubiquitous and are also
used by children, they could have a small handle, similar
to a cup. Moreover, after interacting with the toy cars, many
visitors discussed with their fellow visitors (friends or families)
about how some of these objects felt excessively large forchildren’s hands while others could be easily picked up andheld by children. In the same vein, a supermarket patron used
our device and tried to pick up items around the fruit andvegetable section and brought them to a cashier. This visitor
stated the difficulty in reaching even a simple apple, noting
how a smaller person must have a hard time with these objects
at the supermarket.
We also observed unique conversation and attitude change
between the user and their fellow visitors (friends or families) such as speaking in a child-like manner or performing
grasping reflex, which can be seen in infants, with other fellow
visitors. Furthermore, many visitors expressed surprise about
the hand size difference when they see and compare both theirunrestricted grasp vs. the hand that wears HandMorph.
APPLICATIONS FOR DESIGNERS

While our key contribution is the validation of the transformation of one’s grasping range into that of someone with asmaller hand, we think there is a range of interesting avenues
to apply our findings; we depict two examples in Figure 16.
We believe the main potential of our device lies in feeling theergonomics of devices designed for people with bodies different from that of the designer. This is especially relevant in thecontext of designing for children, smaller people, or individuals with motor impairments that affect dexterity. Our device
might assist a designer to gain sensitivity to challenges facedby the target users. Figure 16 (a) depicts our device assisting a
product designer in evaluating the usability of eating utensils.
Figure 16 (b) shows our device being used to reconsider theplacement items in a physical space (e.g., a classroom or museum), in a more inclusive manner towards people with shortstature. Lastly, we believe HandMorph might be useful for
visually-impaired designers since they heavily rely on theirsomatosensory experience [69].

and verified an illusion that transforms one’s grasping range
into that of a someone with a smaller hand. HandMorphachieves this using mechanical links that transmit motion fromthe wearer’s fingers to a smaller soft hand with five fingers.
As such, HandMorph miniaturizes a wearer’s grasping range
while transmitting haptic feedback.
Unlike other size- or grasp-illusions based on virtual reality,
HandMorph achieves this in the user’s real environment while
preserving the user’s physical and social contexts. As such,our device can be integrated into the user’s workflow, e.g., toallow product designers to momentarily change their grasp
into that of a child while evaluating a toy prototype.
We conducted two user studies to validate HandMorph. In our
first user study, we found out that the participants’ perceived
objects as being larger when wearing HandMorph, which suggests that their size perception was transformed as if having asmaller hand. In our second user study, we assessed the experience of using HandMorph as an assistive tool for designinga simple toy trumpet for children. We found that participants
felt more confident in their toy design when using HandMorphto validate its ergonomics. We believe that, unlike consulting
human factors ergonomic charts, the somatosensory feedback
felt while using HandMorph helped participants to understandthe usability of their designs in a more embodied manner.
Lastly, we find that our device is unique in that it enables
the wearer, especially designers, to gain embodied knowledgeof the challenges of those with a smaller grasp: children,
individuals of short stature or with dwarfism, and so forth.
ACKNOWLEDGMENTS

This work was supported by Grant-in-Aid for JSPS Research
Fellow (JP16J03777), Scientific Research on Innovative Areas(JP18H04182) and the BIG Ideas Generator at the Universityof Chicago. We thank Hikaru Takatori and Kosuke Sato forsupporting the demo session.
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