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Soft Wearable Skin-Stretch Device forHaptic Feedback Using Twisted and
Coiled Polymer Actuators
Jean-Baptiste Chossat , Daniel K. Y. Chen, Yong-Lae Park , and Peter B. Shull
Abstract—Soft and integrated design can enable wearable hapticdevices to augment natural human taction. This paper proposes anovel, soft, haptic finger-worn wearable device based on compliantand adhesive silicone skin and lightweight twisted and coiledpolymer (TCP) actuators using ultra high molecular weightpolyethylene (UHMWPE) fibers to provide lateral skin stretchsensations. Recently, silicone elastomers have been used inwearable sensors and in haptic applications for their highcompliance or adhesion. TCP actuators have also demonstratedhigh power to weight ratios, large stroke length, simplemechanism, and inherent softness. Lateral skin stretch is sensitiveto small motions and has been used for intuitive proprioceptivefeedback applications. We combined these characteristics todesign and manufacture a wearable, functional haptic prototype.Prototype performance was evaluated using an optical trackingsystem, a force gauge test bench, and compared to vibrotactilehaptic feedback in a experiment with 14 healthy participants.Results showed that participant mean reaction times werecomparable to those of a vibrotactile feedback system, though taskcompletion times were longer. This paper is the first to employTCP actuators for haptic stimulation and could serve as afoundation for future applications involving soft wearable hapticsin gaming, health, and virtual reality.
Index Terms—Haptics, TCP Actuators, Silicone, Soft Robotics.
I. INTRODUCTION
USE of haptics as a communication strategy is well
accepted and employed by a growing number of every-
day devices [1]. The skin, our body’s largest organ [2], is a
highly innervated sensory organ responsible for a large part of
what is considered our sense of touch. Touch is essential to
many dexterous tasks [3], [4], and its loss can be debilitat-
ing [5]. Correspondingly, tasks requiring high degrees of
hand-eye coordination strongly benefit from recreating or
supplementing the sense of touch through haptic feedback [4],
[6]. Since it relies on different organs and separate neural
pathways, the sense of touch has qualitative differences com-
pared to other senses. These differences can also be harnessed
by haptic technologies with goals, such as reducing the user’s
cognitive load, averting sensory overload, or eliciting different
emotions [7]–[9].
Consumer-based wearable haptic devices, such as cell-
phones and video game controllers, primarily rely on simple
vibration feedback while aiming at being worn with minimal
user impairment (using the definition proposed in [10]).
Although efforts have been made to create richer modali-
ties [9], vibration stimulation is still generally used for simple
notification purposes. Alternatively, grounded haptic devices
have been used to provide meaningful feedback during highly
dexterous and risky manipulation tasks, such as teleoperated
surgeries [6], [11].
This discrepancy highlights a fundamental difference between
grounded and wearable haptic devices. Grounded devices can
generate high forces and measure motions accurately while
transmitting contact reaction forces to the ground, thus creating
haptic feedback. However, cost, size, weight, and limited work-
space are limiting factors that can prevent wider adoption. Two
key aspects toward increased haptic device performance and
adoption are:
1) Expressiveness: Since perceptual importance of haptic
feedback changes with the tasks performed by the user,
in order to be relevant in a large number of situations,
the haptic device should aim at creating a wide array of
different haptic feedback [10].
2) Wearability: Such a device would be worn by the user,
it should be compact, lightweight, and comfortable, as
well as minimally impair user motion [10].
Recent research has focused on creating expressive haptic
devices; however, the design of wearable devices imposes
constraints on the type of haptic feedback and, ultimately, on
the device expressiveness. As a consequence, both the expres-
sivity and the design of a device must be considered equally
important [10].
Conventional materials used for the device structure or
actuation are typically metal and hard plastics. When in con-
tact with the user, hard materials are at best uncomfortable
and at worst harmful. To avoid this issue, exoskeletons are
Manuscript received March 19, 2019; revised August 30, 2019; acceptedSeptember 17, 2019. Date of publication September 23, 2019; date of currentversion December 12, 2019. This work was supported by the National NaturalScience Foundation of China (51950410602). This paper was recommended forpublication by Associate Editor E. Steinbach upon evaluation of the reviewers’comments. (Corresponding author: Peter B. Shull.)
J.-B. Chossat, D. K. Y. Chen, and P. B. Shull are with the State Key Labora-tory of Mechanical System and Vibration, Shanghai Jiao Tong University,Shanghai 200240, China (e-mail: [email protected]; [email protected]; [email protected]).
Y.-L. Park is with the Department of Mechanical and Aerospace Engineering,Seoul NationalUniversity, Seoul 08826, SouthKorea (e-mail: [email protected]).
Digital Object Identifier 10.1109/TOH.2019.2943154
IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019 521
1939-1412 � 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
usually designed to contour to the user’s body, and use softer
materials, such as fabric straps, where they make contacts
with the user’s body. Even when significant efforts are
invested in designing miniature devices [12], [13], such a
design often results in somewhat bulky devices that compro-
mise the user natural interaction with the world. One alternate
possibility is to use a large number of small actuators arranged
on a flexible structure, each actuator being individually hard.
Because the actuators are small, the device is able to conform
to the body. For example, many gloves have been equipped
with large numbers of small vibration actuators [14], [15].
Another, is the use of cables to transmit forces while relocat-
ing the actuators to a more suitable position on the user [16].
These designs, although less cumbersome, still require the
transmission of significant forces and, as a consequence usu-
ally cover and anchors over a large area of the skin. Although
these devices can generate haptic sensations, they also prevent
users from directly touching objects, and therefore diminish
the overall user tactile experience of the world. Soft materials,
such as silicone elastomers, have been used in the past to cre-
ate wearable artificial skins which have little impact on user
comfort and motions [17]. Soft silicone has also been used to
create skin-safe distributed adhesion in the context of hap-
tics [18]. Twisted and coiled polymer (TCP) artificial
muscles [19], [20] have demonstrated a high power to weight
ratio in a compact and flexible form factor. They have been
used as an alternative to conventional actuation for applica-
tions such as orthotics [21], [22] or robotics [23]–[25] and soft
robotics [26], [27]. The transmission of forces capable of limb
motion assistance though a soft and minimally intrusive struc-
ture is challenging. However, skin stretch as a cutaneous
haptic feedback modality is promising for applications, such
as physical rehabilitation and training, while also requiring the
actuators to produce lower forces over smaller distances. As a
consequence, the design of a soft haptic device for skin strain
is both relevant and innovative [28]–[30].
The goal of this paper is to present the development and
implementation of these different smart materials for the
design of a soft wearable haptic silicone skin (shown in
Fig. 1), and to demonstrate the prototype potential for wear-
able haptic feedback in an interactive movement application.
II. DEVICE DESIGN MOTIVATION
A. Haptic Feedback
Haptic feedback devices are usually classified in two main
categories: cutaneous and kinesthetic [10]. While cutaneous
sensations are elicited by skin deformation, kinesthetic sensa-
tions refer to the sense of the body motion. This distinction is
essential to the understanding and classification of wearable
haptic devices and of their respective applications.
Wearable haptic devices that target kinesthetic feedback
usually take the shape of exoskeletons. As Pacchierotti et al.
describes [10], these haptic devices must be anchored at a dis-
tance from the target area to create strong feedback. As a conse-
quence, kinesthetic sensations are usually achieved using long
reaching, rigid devices. Although wearable, these devices are
often quite bulky. Placed on the targeted area, cutaneous haptic
feedback devices are usually more compact, as best exempli-
fied by the ubiquitous vibrations feedback present in video
game controllers and cell phones. Vibration-based haptic feed-
back has been studied in depth due to the availability of com-
pact eccentric mass vibration motors [14], [15] and, more
recently, linear resonant actuators and piezo-electric actua-
tors [31]. These are very good at transmitting sensations of
caress and textures [9], [32]. However, other less explored cuta-
neous sensations such as normal indentation, lateral stretch or
relative tangential motion also exist [10]. These other types of
cutaneous haptic stimuli can be used to intuitively convey dif-
ferent feedback. For example, lateral skin stretch has been used
as a means of intuitive proprioceptive feedback [29] or to evoke
illusory movements in the index finger [28].
Since mechanoreceptor density and type vary [2], choosing
a relevant skin surface for haptic feedback is of great impor-
tance. Our hands are our main tactile exploration tool, which
means that they possess a high density of mechanoreceptors.
Recent literature suggests that only a few millimeters of skin
stretch are necessary to create haptic feedback [28]. Our hands
are also vital in our daily interaction with the world. For both
these reasons, they provide a ground truth for wearable devi-
ces that aim at eliciting haptic sensations while creating little
tactile occlusion. In this paper, we propose to elicit lateral skin
stretch and create a skin stretch pull sensation as an intuitive
tactile cue for the user.
B. Twisted and Coiled Polymer Actuators
Previously proposed cutaneous haptic feedback actuators
have advantages and flaws. Pneumatic systems using air jets
Fig. 1. Soft wearable prototype ready for skin stretch feedback on the userindex finger.
522 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
require compressors and pneumatic tubing [33], while pin
arrays are usually complex and use non-compliant and hard
materials [34], [35]. Dielectric elastomer actuators have also
been used to create cutaneous feedback through soft haptic
devices [36], [37]. Although promising, these devices require
high voltages and complex electronics while only generating
small forces.
The recent introduction of TCP actuators [19], [20] has
shown great promise in varied research fields, such as soft
robotics and prosthetics [22], [38]. These manufactured artifi-
cial muscles have a unique form factor and inherent softness
making them suitable for integration in clothes [21] and wear-
able devices [39]. Other advantages of TCP actuators include
high power to weight ratio, self sensing [40], large contractile
capabilities [19], scalability [41] and simplicity of actu-
ation [39], [42]. The TCP actuators numerous qualities moti-
vated us to use them as soft artificial muscles in a soft
wearable haptic feedback device.
These actuators are made of polymeric fibers, usually char-
acterized by a negative linear thermal coefficient. Thanks to
preliminary twisting and coiling, these fibers contract along
their length when heated [19]. Although different fibers have
been used for the manufacturing of TCP actuators, nylon is
the current fiber material of choice for TCP actuators. Nylon
TCP actuators can attain large contractions at temperatures
(up to 240� C) that may not be safe for a wearable device [19].
Although linear low density polyethylene (LLDPE) has been
proposed as an efficient alternative capable of large contrac-
tions at lower temperatures [21], [43], because of their lower
crystallinity LLDPE fibers are also mechanically weaker. As a
consequence, we decided to use ultra high molecular weight
polyethylene (UHMWPE) which is stronger and contracts in a
more efficient manner than nylon at lower temperatures (maxi-
mum temperature of up to 130 C) [19], [44].
C. Soft Wearable Silicone Skin
Apart from lightweight actuation, device wearability is also
of high importance [10]. This characteristic, relevant to any
worn haptic device, depends on both adhesion from the device
on the user and compliance of the device according to the
user’s motion.
Although it may seem a trivial problem at first, maintaining
good contacts between the user and the device is one of the
key factor for creating consistent and strong haptic feedback.
Many devices are attached by surrounding the target part of
the body. Gloves and straps are perfect examples of such strat-
egies. Although using these fastening elements might be
unavoidable in some cases, they should be minimized as they
can only diminish the overall user ability to move and to feel
the world. Glove-based haptic devices may be fitted with a
large number of haptic elements [14], [15], but may, in many
aspects, impair the user’s most important tactile body part.
Another point to consider is that fabric may not be the best
material to use for all tactile sensations. In our case, lateral
skin stretch relies heavily on close mechanical contact and
friction between the skin and the device. Although fabric
satisfies the requirements of proximity, it usually fails to
adhere and communicate skin stretch.
The second challenge encountered when designing haptic
devices is that, although human bodies are fairly soft, conven-
tional actuation uses rigid materials. As a consequence, even
though the device may be wearable, it may feel uncomfort-
able, or even hinder the user’s natural motion and grasping
capabilities. Smaller and lighter mechanisms can mitigate this
issue, but ultimately using soft materials may be the most
promising avenue. Silicone elastomer has been used in the
field of soft sensing for their extreme compliance and have
been molded into soft sensing artificial skins that can undergo
large deformations [17]. Furthermore, previous wearable hap-
tic devices have also used such materials as a soft adhesive
layer [18].
To solve the issues of both adhesion and compliance, we
developed a soft elastomeric adhesive skin as the haptic
device structure. This skin would be very compliant, light-
weight, adhere to the user, secure the TCP actuators, and as
added benefit, provide good thermal isolation to the user.
III. PROTOTYPE DESIGN
A. TCP Actuators
The TCP actuators were manufactured using UHMWPE
braided fibers (0.20 mm diameter, Nantong Jin Hong Environ-
ment New Material Co., Ltd.) as summarized in Fig. 2. The
UHMWPE fibers were twisted and coiled along with two sin-
gle nickel-chromium (NiCr) wires (0.05 mm diameter, Hui-
lide) using a weight of 100 g (� 31:2 MPa). The NiCr wires
were also suspended from the motor shaft, but only straight-
ened using a weight of approximately 1.4 g (3 paper clips) and
were left free to untwist. Because the NiCr wires were so thin,
they didn’t meaningfully impact the UHMWPE fiber coiling
process. First, the fibers and NiCr wires were twisted at a very
low speed1 (120 rpm, 1100 turn/m) as we experimentally
found that rotating at lower speed allowed the NiCr wires to
wrap around the UHMWPE in a slightly more regular and
Fig. 2. Schematic of the TCP actuator manufacturing process. a) Initial fiberssetup, b) twisting and coiling of the fibers, and c) annealing and thermal settingthrough Joule effect heating.
CHOSSAT et al.: SOFT WEARABLE SKIN-STRETCH DEVICE FOR HAPTIC FEEDBACK USING TWISTED AND... 523
dense manner. Finally, the UHMWPE fibers were twisted and
coiled (autocoiling) at a slightly higher speed1 (250 rpm,
� 550 turn/m). Overall, slow rotational speeds were used dur-
ing the manufacturing process since it has been shown to
impact actuator performances [45]. Once the UHMWPE fiber
was twisted and coiled, the NiCr wires were knotted to the
actuator end. The actuators were then annealed using the
embedded NiCr wires, and using the same 100 g weight
(� 31:2 MPa). No mandrel was used since actuator compact-
ness was preferred. Based on the actuator’s initial coiled
length, the actuator was annealed in four consecutive, five
minute long time periods, with the annealing power ranging
from 0.06 W/cm to 0.10 W/cm each period. Each period was
separated from the next one by a cooling phase of 50 seconds.
Over the course of the annealing process, the fiber elongated
by approximately 24% of it’s initial coiled length. Other meth-
ods for actuator heating and cooling have been proposed
(air [46] and water [47]). The control of a fluid thermal proper-
ties in a compact wearable device is a complex problem that is
beyond the scope of this paper. A simple and compact heating
method is to place a Joule heating element, such as the NiCr
wire, in contact with the polymeric fiber. Alternative
manufacturing processes for Joule heating capable fibers such
as conductive paint [42], carbon nanotubes [19], or silver
coated fibers [25], [38], are also available. However, they typi-
cally either restrain the user to use consumer grade fibers or
require specialized equipment, increasing the cost and com-
plexity in the manufacturing process.
Thanks to this manufacturing process, the actuators obtain a
very predictable resistance and are manufactured using only
cheap and readily available materials. This process is analo-
gous, although slightly different to other methods proposed
previously in literature [39], [40], [48]. The finished TCP actu-
ator had a resistivity r � 750 V/m, and an external diameter
of 0.55 mm. The TCP actuator coil index was C ¼ 2:75,where C is the average coil diameter divided by the filament
diameter (see Fig. 3b)).
Once annealed, our TCP actuators demonstrated a consis-
tent nondestructive contraction of 5.2% when heated using
0.08 W/cm and attached to a 100 g weight. The actuators were
capable of contraction using up to 0.12 W/cm constant linear
power, corresponding to approximately 110�C as measured
with a thermal camera (Ti 400, Fluke), but produced reduced
relative contraction. Above these temperatures, the UHMWPE
starts melting and the actuators are permanently damaged. Up
to 16% of actuator contraction has been previously achieved
using UHMWPE fibers [19], but through more complex
manufacturing or less wearable heating methods previously
mentioned. Isometric tests were conducted to evaluate the TCP
actuators pull force. As shown in Fig. 4, a single 10 cm TCP
actuator, pre-loaded at 500 mN to remove slack, was mounted
on test bench equipped with a 3 kg force gauge, HX711 analog-
to-digital converter module, and Arduino Mega 2560 micro-
controller development board. The actuator produces up to
650 mN of pull force when heated using 0.08 W/cm of linear
power. Although their contraction is modest, when compared
with TCP actuators made from commercially available conduc-
tive nylon fibers [25] the UHMWPE TCP actuators produce a
much larger force in isometric conditions (� þ440%) and have
a smaller diameter (� �25%) while only requiring a slightly
superior amount of power (� þ60%).
B. Soft Silicone Skin
Our device uses a two layer platinum-cure silicone rubber
structure for the artificial skin, the first silicone layer ensures
adhesion while the second layer eases skin handling and
strengthen the prototype, the full manufacturing process is
illustrated in Fig. 5.
The first layer was made by hand mixing an extremely soft
(Shore hardness 000-35, elongation at break > 1000%), skin-
Fig. 3. a) Overview of the device, b) detail of the TCP actuators, andc) detail of the TCP actuator and electrical wire crimped together by analuminium cylinder.
Fig. 4. Single 10 cm TCP actuator isometric maximal pull force using differ-ent linear power for heat generation.
1 We used an Arduino development board (Arduino Mega), a motor con-trol module (L298 N dual motor control module), and a small DC motor.
524 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
certified silicone rubber (Ecoflex Gel, Smooth-On, 1 Part A,
1 Part B) with an additive (Slacker, Smooth-On, 2 Parts), mak-
ing the silicone softer and more adhesive. This mix was then
poured in a 1 mm depth, finger shaped, 3D printed mold
(Objet 30, Stratasys) previously sprayed with a release agent
(Mann, Ease Release 200). After the first silicone rubber layer
had cured, a second mold was added onto the first. This
allowed the deposition of a second 1 mm thick silicone rubber
layer (Ecoflex 00-10, Smooth-On) of similar shape and close
mechanical characteristics (Shore hardness 00-10, elongation
at break = 800%). This manufacturing process is slightly dif-
ferent but analogous to that of previously published work [18].
While the second silicone rubber layer was curing, four
rows of 3D printed (Objet 30, Stratasys) small low-profile
thread-through pins were placed on the skin’s first layer,
through the curing silicone, in two columns, starting from the
proximal and toward the distal end of the soft artificial skin.
This process allowed for good bonding between the silicone
elastomer and the 3D printed parts. The pins were positioned
along the edge of the skin in order to minimize the TCP
actuator’s change in length due to finger flexion. Their role is
to hold the TCP actuators in place while minimizing friction
and maximizing air contact. This increases heat dissipation,
improves the device serviceability, and prevents contact
between actuators. Given our simple electrical open-loop elec-
trical circuit, short circuits lead to the overheating and destruc-
tion of the actuators and it was vital to prevent them. Previous
to placing the two most proximal pins, these were glued (Sil-
Poxy silicone rubber adhesive, Smooth-On) on a simple piece
of office paper, mechanically coupling and anchoring them to
a large portion of the artificial skin. The last, most distal, pin
is of circular shape and designed to allow two of the TCP
actuators to loop back to each side of the finger, while the third
actuator was threaded from one side to the other.
C. Prototype Assembly
The TCP actuators were threaded through the pins on the
skin’s surface as the skin lay flat, and were pulled to remove
any slack (see Fig. 3a)). Due to the low melting point of
UHMWPE, the actuator cannot be soldered on, a small steel
cylinder of 1.2 mm of diameter was used to crimp together the
TCP actuator and conductive wires (Fig. 3c)), providing a reli-
able electrical contact and preventing the TCP actuator from
slipping through the holding pins. Heat shrink tubing was then
placed at the connections and tightened at a low temperature
using a heat gun.
We empirically determined that as much as 60 cm of actua-
tor fiber can be placed from one side of the finger to the other.
Although a single actuator will contract over a larger absolute
distance when compared to several smaller actuators of the
same total length, a single actuator contraction strength is also
lower. Using multiple shorter actuators requires additional
connections but also offers benefits in modularity and in safety
as they require lower voltage when driven in parallel. As a
consequence of these considerations, we decided to use three
separate actuators, each with length, resistance, and maximal
contraction of approximately 20 cm, 150 ohm and 1 cm,
respectively. The completed prototype weighs about 14 g, of
which 0.1 g were due to the actuators, 3.5 g the electrical wires
and connections, and 10.4 g the artificial skin. The device can
be worn for extended period of times, and is naturally adhe-
sive, and will remain so until covered by particles. It can be
worn many times and does not require special arrangements
before or after being worn.
TCP actuator contraction and heating is regulated using
a microcontroller (Arduino Mega) generated logic level
pulsed-width modulation (PWM) signal sent to a transistor
(FQP30N06 L, Fairchild Semiconductors). The transistor
controls two serially connected 30 V channels of a variable
DC power supply (UNI-T UTP3303).
D. Prototype Softness
Since the device aims to be wearable, the overall softness of
the device is a very important characteristic to assess. How-
ever, measuring the prototype overall softness is a non-trivial
endeavour. Indeed, the user finger flexion, responsible for the
device deformation during use, does not equally impact all the
parts of the device and is not uniform. As a consequence, we
first separately measured the strain that each component of
device undergoes during the course of a full finger bending
motion. To evaluate the different prototype component’s
strain, a non stretchable fiber was placed and marked on the
prototype before and after finger flexion. From the proximal to
the distal end of the silicone skin, we measured the silicone
skin elongation to be about 20 mm. The TCP actuators
Fig. 5. Schematic of the soft silicone artificial skin manufacturing process. a) First “sticky” layer molding, b) second stronger layer molding, c) placing of thethread-through pins while second silicone layer cure, d) threading the TCP actuators, and e) finished prototype with crimped TCP actuators and copper wires.
CHOSSAT et al.: SOFT WEARABLE SKIN-STRETCH DEVICE FOR HAPTIC FEEDBACK USING TWISTED AND... 525
maximal elongation was evaluated, respectively, at 6.5 mm
and 3 mm per 10 cm for central and lateral TCP actuators.
The softness of the device was then evaluated using a man-
ual strain test bench equipped with a 3 kg force gauge, HX711
analog-to-digital converter module, and Arduino Mega 2560
micro-controller development board. Fig. 6, shows the mea-
sured pressure and forces for each component, as well as com-
posite curve showing a linear fit of the pressure and forces
during a full finger flexion. The values were captured using a
single 10 cm TCP actuators pre-loaded to about 500 mN to
remove slack, and in accordance with the literature [41]. The
measured pressure and force were doubled and quadrupled to
respectively match the center and side prototype actuators
total length. The silicone skin pressure was computed based
on the the unstressed skin cross section area, while the TCP
actuators pressure are based on the most distal pin contact sur-
face with the user (4 mm radius disk), evaluating for the
device perceived rigidity in a worst case scenario where the
TCP actuators passive pull force is directed toward this single
pin. Although the silicone skin did exhibit a mostly elastic
behavior, the actuators showed significant hysteresis. This is
consistent with previous literature results and can be mitigated
by appropriate control algorithms [25]. Human skin Young
moduli under stretch ranges from 1.5 to 1 MPa. Based on the
Fig. 6c), our prototype is 5 to 7.5 times softer than human skin
under similar stretch [49], [50].
E. Haptic Stimuli
A direct consequence of using almost exclusively soft mate-
rials is that the design of a clear and meaningful haptic signal
is not straightforward but must instead be carefully studied.
1) Device Operation: As the three TCP actuators simulta-
neously contract, the most distal pin of the prototype is pulled
backwards. As the pin moves, it pulls the artificial silicone
skin whose adhesion transfers to the user skin, creating the
haptic stimuli. As the TCP actuators cool, the natural elasticity
of both the user skin and of the device artificial skin passively
pulls the pin back to it’s original place.
2) Stimuli Design & Optimization: To design the haptic stim-
uli, we evaluated the motion of the prototype pins created by
the interplay between the TCP actuators and the silicone skin
using an optical tracking system (SIMI Reality Motion Sys-
tems, Unterschleissheim, Germany). Since skin mechanics
involved in haptic stimuli is beyond the scope of this paper,
the experiment was performed as the device was attached on
the surface of a table, only evaluating the prototype character-
istics. Markers were placed on top of the thread-through pins,
allowing the system to track their motion as the actuators con-
tracted (see Fig. 7, Fig. 8, and Fig. 9). Three important TCP
actuator parameters regulating the haptic stimuli were identi-
fied: friction, contraction speed and contraction amplitude.
a) Friction: Depending on the manufacturing process and
on the thread-through pin design, friction may occur between
the pins and the TCP actuators. As the TCP actuators contract,
this friction distributes the TCP contraction forces to all the
Fig. 6. Experimental pressure and force relationship to strain. Strain valuesestimated for a full index bending motion. a) and b) presents the prototype sili-cone skin and TCP actuators absolute values while c) is a normalized compos-ite graph generated by adding each of the device element force and pressure,and linearly scaling over the full index motion.
Fig. 7. a) Markers relative motion using continuous 0.08 W/cm linear powerheating (data averaged and second order fit) and schematics illustrating thedevice motion during b) the actuators contraction phase and c) the siliconeskin relaxation phase.
Fig. 8. First marker motion during and after three TCP actuators overpower-ing experiments. Data points and second order fit.
526 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
pins (see Fig. 7). Although this increases the skin surface on
which the haptic feedback is applied, it also shares the TCP
actuator contraction between all the pins. By minimizing the
number of pins moving, one can combine the pull from all
TCP actuators in one location, and therefore create and control
the haptic stimuli in a much easier manner. We decided to
concentrate the pull in the most distal pin, allowing for the use
of long TCP actuators as well as high mechanoreceptor den-
sity in that part of the skin.
In order to mitigate motion from the most proximal pins,
these were glued (Sil-Poxy, SmoothOn) on large paper insert
embedded in the silicone elastomer, as described in Section
III-B, thus limiting their motion. As can be seen in Fig. 7,
although this strategy was successful in almost eliminating
any movement from the intermediary pins and concentrating
the motion in the prototype most distal pin, it did not
completely remove all motion from pins eight and nine.
b) Contraction speed: Skin strain velocity has been shown
to be of major impact on mechanoreceptors discharges
rates [51]. As a consequence, TCP actuator contraction speed
must be considered as an important factor for skin strain haptic
feedback. The linear power limit used to contract the TCP
actuators can be exceeded for a short duration of time, grant-
ing a swift rise in the UHMWPE fiber temperature. By using
near maximum voltage of our setup (60 V), we were able to
overpower the actuators up to a peak linear power of 0.8 W/
cm per actuator. The measured peak speed on the most distal
pin was between 15 to 21 mm/s (see Fig. 8 and Fig. 9).
c) Contraction amplitude: As demonstrated in Fig. 8, the
rapid contraction of the TCP actuators for 100 ms generates
up to 2.1 mm of pin movement which is enough to create a
perceivable lateral skin stretch. Although further contraction
from the TCP actuators is possible, a conservative approach to
TCP actuation is required, since TCP cooling requires a much
longer time, and that the haptic stimuli may be triggered many
times in a row by the user during the experiment. As a conse-
quence, we aimed at using only the necessary constant linear
power to maintain the pin in position after the initial 100 ms
of TCP actuators overpowering. As showed in Fig. 9, about
0.03 W/cm of constant linear power was required.
IV. HAPTIC USER EXPERIMENT
Using the haptic skin, we sought to determine whether users
could locate static one-dimensional virtual targets relying
solely on haptic feedback, and if so, how long it would take
them. We also wanted to determine how the haptic skin per-
formed compared with vibration feedback. Fourteen partici-
pants with no known neurological conditions were recruited
for this study (10 males and 4 females, 21 to 36 years of age).
Two participants had previous experiences with the haptic
skin. In addition to determining whether or not our haptic skin
could guide participants in a real-time haptic feedback task,
we also tested the two hypotheses:
1) Is there a statistically significant difference in the reac-
tion time of a participant using the haptic skin compared
with the reaction time of 274.3 ms? (274.3 ms being the
mean time observed by Forster et al. for a tactile stimu-
lus [52]). The H0: mean reaction time = 274.3, with a =
0.05.
2) Is there a statistically significant difference in the means
of the completion times of the haptic task using the hap-
tic skin and a standard vibration motor? The H0: mean
completion time of haptic skin - mean completion time
of vibration motor = 0, with a = 0.05.
A. Experimental Set Up
A Vicon Motion Capture system (Oxford Metrics Group,
Oxford, UK) was used in this study for capturing the marker
translations (sampled at 100 Hz) which was attached to the tip
of the participants index finger on the same hand the haptic
skin was applied to. A MATLAB program was used to acquire
the marker position in real-time from the Vicon system whilst
simultaneously acting as a TCP client, sending feedback (on
or off) commands to a custom windows application which
acted as the TCP server. The TCP server then sent the corre-
sponding commands to the Arduino development board via a
serial COM port which controlled the switching of three tran-
sistors (FQP30N06 L, Fairchild Semiconductors) using PWM.
We used the linear power values determined in the previous
section (0.8 W/cm for overpowering, 0.03 W/cm for constant
contraction) but restrained the overpowering phase to once
every five seconds in order to prevent actuator overheating. A
similar set up was used during trials using vibration feedback,
whereby instead of the Arduino switching the transistors, they
turned on or off an eccentric rotating mass motor (9 mm diam-
eter) with an input of 2 V (40 mA) which was taped to the pal-
mar surface of the distal phalanx as shown in Fig. 10.
B. Procedure
1) Participants Goal: Participants were seated in the center
of a motion capture laboratory. The direction they were facing
was calibrated to be the Vicon x-axis coordinate, causing the
x-axis to be parallel to the participant median plane, while the
origin of the coordinate was placed near them. A reflective
marker (10 mm diameter) was placed on the nail of their index
finger, tracking the finger translation along the x-axis. The
haptic skin was placed on their index finger with the most dis-
tal pin just behind their nail, and adhered toward the back of
their dominant hand. Participants wore a blindfold throughout
the duration of the experiment to prevent any possibility of
Fig. 9. First marker motion during TCP overpowering phase (0.8 W/cm) andlower linear power phase (0.03, 0.04, and 0.06 W/cm). Second order fit.
CHOSSAT et al.: SOFT WEARABLE SKIN-STRETCH DEVICE FOR HAPTIC FEEDBACK USING TWISTED AND... 527
visual feedback from occurring. The participants were asked
to explore the space directly in front of them, thus following
the x-axis using their finger equipped with the haptic device.
They were told the goal of the task was to position their finger
just before the static virtual wall. The virtual wall was a 2D
plane, perpendicular to the direction the participant was facing
(x-axis) stretching infinitely in the y and z axes. Haptic feed-
back was activated as the users finger moved past the wall,
and stayed active as long as the distance of the marker in the
x-axis was greater than the nominal target distance of the wall
from the origin. Otherwise, the haptic feedback remained inac-
tive. The participants were told that the goal of the experiment
was for their finger to be in the tolerance region just in front of
the wall and to remain there for two seconds. When they had
successfully reached and remained inside the target area, the
participants were told that the trial was completed. If nothing
was said, then the participants were told to assume they were
out of the tolerance region. A timeout period of five minutes
was used, if the participants were unable to complete the task
within that time, the trial was deemed “not complete”. Fig. 12
illustrates the experiment’s procedure as a block diagram.
2) Virtual Wall Tolerance Region: Noting that it is almost
impossible for participants to maintain a perfectly static pose
in free space for the time period required for the completion
condition of the user study, we sought to determine the natural
variance of the participants position in the x-direction while
they attempted to maintain this static pose. A pilot experiment
with just the reflective marker on the index finger was per-
formed with ten participants (nine of which were part of the
main study) to determine a suitable tolerance based on the
marker trajectory variation when the participants tried to
maintain a static pose. Participants were told to point their fin-
ger away from their body, along the x-axis, to reach the middle
of their full arm extension and to hold the position for five sec-
onds. The standard deviation of their “static” finger position in
the x-axis during the five seconds was pooled (n = 10). The
upper limit of the 95% confidence interval (CI) of the distribu-
tion (s = 2.09 mm) was used as a conservative s for when
one’s finger is maintaining a “static” pose. Using the sample
s, the 95% CI for an arbitrary marker position “x” when one
is attempting to maintain a “static” pose is x � 2s. The toler-
ance region thus has a total length of 4s = 8.36 mm. For the
given nominal target distance (250 mm), the actual target is
therefore the nominal target minus 4.18 mm (2s), howeveronly the nominal target will be referred to in this paper.
3) Experiment: A crossover study was implemented in which
half the participants were chosen randomly to begin the study
using the haptic skin, and other half with vibration feedback. A
practice target position of 100 mm in front of the participant
was set, and participants were given one to two minutes to
experience the haptic skin feedback as they extended their arm
and that their index finger moved back and forth through the
target. After the practice trial, each participant performed the
haptic experiment with the target distance set at 250 mm. Par-
ticipants were not given any other instructions as to how they
should move. The same experiment was then performed using
the other type of feedback, there was at least a five minute break
between the testing of the two feedback modalities. Fig. 11
shows a typical marker trajectory during the haptic experiment,
illustrating the nominal target, feedback zone and tolerance
region. Temporal event T1 correspond to the haptic feedback
starting, event T2 marks the user motion change, and event T3
shows the user entry in the virtual wall tolerance region.
C. Results
1) Reaction Time: Using the collected Vicon marker trajec-
tories, we investigated if there was a difference in the mean
reaction time in the participants responding to the feedback.
Reaction time is defined as the time difference from the
moment participants first passed the target distance and the
time for them to reverse their trajectory in the x-axis after per-
ceiving the skin stretch feedback (T2 - T1, from Fig. 11). The
bootstrapped sample mean of the reaction times was
321.7 � 48.3 ms (mean � 2*SE). After performing a one sam-
ple bootstrapped (n = 10,000) hypothesis test we conclude that
there is no statistically significant difference by failing to
reject the H0: mean reaction time = 274.3 (p = 0.0556).
Fig. 10. a) Participant during the haptic experiment with the haptic skin onhis dominant hand and close up on the participant hand b) during vibrationfeedback and c) during skin stretch feedback.
Fig. 11. Typical x-axis Vicon marker trajectory during the hapticexperiment.
528 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
2) Completion Time: The results show that every participant
in this study was able to complete the task before the five min-
ute timeout period. Fig. 13 shows participants were able to
complete the task much quicker using vibration feedback with
mean differences in completion times of 21.22 seconds. After
performing a two sample bootstrapped (n = 10,000) hypothesis
test we reject the H0: mean completion time of haptic skin -
mean completion time of vibration motor = 0 (p = 0.0112),
and conclude the mean difference in completion times are sta-
tistically significant.
V. DISCUSSION
A. Soft Silicone Skin
The design of this soft adhesive silicone skin is relevant to
wearable haptics as it allows the user to safely and easily cover
relevant parts of the skin without occluding tactile perceptions
from other parts of the skin. It is also reusable, and does not
require using additives or other chemicals prior to use, which
makes the device operation very convenient. We expect this
type of adhesive skin to be used for soft wearable haptics and
soft wearable sensing systems. However, the reader should
note that the difference in hardness and limited bonding
between silicone elastomer and some hard materials (plastics,
metal) can make some of the parts of this device subject to
delamination when large deformation occur. Good mitigation
strategies involve using silicone elastomers of increasingly
high hardness (Sil-Poxy), creating composite material struc-
tures (paper, textile), using as few hard components as possi-
ble, and generally handling this type of device with care
during the removal phase.
The overall weight of the device might be further reduced
by creating openings in the skin and reducing the skin surface.
However, this should be done carefully as the device must
stay robust enough to endure removal from the user. Skin
thickness is another axis of improvement. Using the aforemen-
tioned silicone polymers, our manufacture experience has
shown that the optimal skin thickness is about 1 millimetre for
each layer. Thinner silicone layers either lack in adhesion or
in mechanical strength.
The silicone skin showed no significant changes in softness
or adhesion during and after the experiment, and after being
worn more than a hundred times over a period of several
month. However, aging is known to affect silicone mechanical
properties and anecdotal evidence from previous experience
suggests the idea that these devices adhesion may diminish
after several years.
B. TCP Actuators
Compared to heavy and bulky conventional electromagnetic
actuators, the TCP actuators we manufactured are also flexible
and lightweight (� 0.7% of total device weight). Another ben-
efit of using thin, low coil index TCP actuators is that they can
be used for manufacture in a very similar manner as threads
would. They can be braided, weaved [41], knotted, or threaded
to achieve contractions along various directions, and have
demonstrated self-sensing capabilities [40]. One of TCP
actuator’s most important challenge is their safety when used
in wearable devices. With the notable exception of Hiraoka
et al. [21], recent papers on TCP actuators have not targeted
wearable applications, and as a consequence, have not focused
on this issue. We proposed the use of thin, lower temperature,
UHMWPE actuators that have little thermal inertia, as well as
a thermally insulating silicone artificial skin. Because of their
multiple advantages and new opportunities in design they
Fig. 12. Block diagram showing the procedure of the user study. Blue, red, and green dashed boxes indicate the start, unsuccessful, and successful end conditions,respectively.
CHOSSAT et al.: SOFT WEARABLE SKIN-STRETCH DEVICE FOR HAPTIC FEEDBACK USING TWISTED AND... 529
provide, with proper care, we expect to see TCP actuators
being used in the creation of wearable haptic devices.
The TCP actuators used in this work demonstrated a maxi-
mum contraction of about 5.2%. However, not only was this
enough to create simple haptic feedback, but the device’s abso-
lute maximum contraction can also be easily increased by
embedding longer actuators. As a consequence, the prototype’s
limitations are mostly due to the actuators’ thermal characteris-
tics. Improved cooling will lead to better actuation bandwidth,
and better or different haptic sensations for the user. Applying
an even electrically and thermally conductive layer at the sur-
face of the fiber might help in that regard [42]. Since we used
simple open loop actuation, a second avenue for the improve-
ment of the device haptic capabilities is closed loop con-
trol [25]. Due to their high weight to force ratio, large stroke,
rapid contraction, flexible nature, and thanks to the aforemen-
tioned numerous improvement avenues, we expect to see TCP
actuators being used in future wearable haptic devices, espe-
cially for applications related to haptic skin stretch.
C. Haptic Experiment
In order for us to determine how suitable our haptic skin is
for a real-time haptic feedback task; we measured the time it
took for participants to react to the cue from the haptic skin.
We were also interested in comparing the completion time
performance of our haptic skin with the most commonly used
modality in literature of vibration feedback for this virtual
wall positioning task. Despite having statistically significant
differences in the mean completion times between vibration
and skin stretch feedback, there was no statistically significant
difference in the time it took for participants to react to the
skin stretch feedback when comparing with the mean reaction
time to tactile stimuli from Forster et al. of 274.3 ms [52].
The authors hypothesise that one probable cause for this
disparity is due to the much slower “relaxation” phase of the
actuators when the feedback is turned off. Although the initial
overpowering of the TCP actuators allowed the most distal
pin to displace 2 mm at a velocity of > 15 mm/s during con-
traction leading to the quick reaction times, the rate at which
it returns back to its original displacement takes substantially
longer with a velocity of < 0.6 mm/s (estimated from Fig. 9).
Webers law states that the proportion of the Just-Noticeable
Difference (JND) to that of a reference stimulus is constant.
Following from this law, it would seem that a greater difference
in displacement is required going from skin stretch (feedback
on) to no skin stretch (feedback off) in order for it to be notice-
able. It is known that the initial ramp of the skin stretch pro-
vided activates both Fast Adapting (FA) and Slow Adapting
(SA) mechanoreceptors [51] which helps in the perception of a
skin stretch event, but it is possible that during the constant
stretch phase that amplitude of the stretch may not have been
large enough for substantial firing of SAII mechanoreceptors.
Another cause, which was also mentioned by a few partici-
pants, was the temporal limit we placed on overpowering the
actuators to prevent it from overheating excessively. Some
participants noted after the experiment that they were exclu-
sively utilising the overpowering event (which happened at
most once every 5 s) for converging on the tolerance region.
This could explain the large disparity between the reaction
and completion times. From Fig. 13, it is interesting to note
that the variance when comparing completion times were dra-
matically different despite the mean reaction time of using the
haptic skin being not statistically significantly different from
the mean value found in literature. This suggests that most
participants found the skin stretch event intuitive despite the
lack of prior exposure, however, it seems some participants
were able to quickly develop an effective strategy whereas
some did not. It is possible that with additional training and
exposure, participants could possibly perform similarly using
both feedback modalities, however additional studies would
need to be performed to confirm this.
In this experiment we only presented binary on/off haptic
cues, however, because skin stretch is directional, further
improvements could allow different directions to be elicited.
Indeed, Chen et al. [53] found that a skin stretch displacement
limited to 2 mm, at a velocity of 4 mm/s, was already sufficient
to yield directional perception accuracies of close to 100% on
the lower limb. Likewise, Gleeson et al. [54] found that displace-
ments of 0.5 mm at velocities of 4 mm/s were sufficient for close
to 100% accuracies at the fingertip. The authors hypothesise that
the use of antagonistic muscles in the futuremay address the lim-
itations found in the user study by allowing for fast transitions in
opposing directions as well as devices which are capable of vari-
ous strains and directions. Even though the device is safe, some
users still reported a sensation of heat. Future work will aim at
decoupling all haptic modalities and compare the device with
known sensory substitution techniques.
VI. CONCLUSION
The purpose of this paper was to propose and study a softer
alternative to current wearable haptic devices. We presented a
soft wearable skin-stretch device worn on the index finger.
The support structure and the actuators were manufactured
using off-the-shelf materials. The overall design is lightweight
comfortable, and restrains very little the user’s motion. The
paper details the manufacturing of the silicone skin and TCP
actuators as well as the assembly of the prototype. Insights
Fig. 13. Boxplot showing mean completion times for each condition.
530 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
regarding actuator force, silicone skin softness, and device
haptic capabilities were also provided. This device represents
the first use of TCP actuators to generate haptic feedback as
well as the first design of a haptic device using UHMWPE
fibers as precursors for the manufacture of the TCP actuators.
We finally conducted a simple haptic experiment in which our
device was used to provide cues to the user to detect a virtual
wall. Although the mean completion time was longer when
participants’ performed the haptic task using the skin com-
pared with vibrotactile feedback, the mean reaction time for
reacting to a haptic cue was not statistically significant as
compared with data from literature.
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Jean-Baptiste Chossat received the M.E. degree inembedded systems from the �Ecole Centraled’�Electronique, Paris, France, in 2012, and the Ph.D.degree in mechatronics from the �Ecole de Technolo-gie Sup�erieure, Montreal, QC, Canada, in 2018. He iscurrently a Postdoctoral Research Fellow with theWearable Systems Laboratory, Mechanical Engineer-ing Department, Shanghai Jiao Tong University,Shanghai, China. His research interests include actu-ation and sensing for soft robotics and haptics.
Daniel K. Y. Chen received the B.E. (Hons.) degreein mechatronics engineering and the Ph.D. degree inbioengineering, in 2013 and 2017, respectively, bothfrom the University of Auckland, Auckland, NewZealand. He is currently a Postdoctoral Research Fel-low with the Wearable Systems Laboratory, Mechan-ical Engineering Department, Shanghai Jiao TongUniversity. His research interests include human-computer interaction with a focus on haptics, softsensing, and robotics.
Yong-Lae Park received the M.S. and Ph.D. degreesin mechanical engineering from Stanford University,Stanford, CA, USA, in 2005 and 2010, respectively.He is currently an Associate Professor with theDepartment of Mechanical Engineering, SeoulNational University (SNU), Seoul, South Korea.Prior to joining SNU, he was an Assistant Professorwith the Robotics Institute, Carnegie Mellon Univer-sity, Pittsburgh, PA, USA, from 2013 to 2017. Hiscurrent research interests include soft robots, artificialskin sensors and muscle actuators, and soft wearablerobots and devices.
Peter B. Shull received the B.S. degree in mechani-cal engineering and computer engineering fromLeTourneau University, Longview, TX, USA, in2005, and the M.S. and Ph.D. degrees in mechanicalengineering from Stanford University, Stanford, CA,USA, in 2008 and 2012, respectively. From 2012 to2013, he was a Postdoctoral Fellow with the Bioengi-neering Department, Stanford University. He is cur-rently an Associate Professor in mechanicalengineering with Shanghai Jiao Tong University. Hisresearch interests include wearable systems, real-time movement sensing and feedback, gait retraining,and biomechanics.
532 IEEE TRANSACTIONS ON HAPTICS, VOL. 12, NO. 4, OCTOBER-DECEMBER 2019
