SmartTouch: Electric Skin to Touch the Untouchable
Hiroyuki Kajimoto(1)Masahiko Inami(2)Naoki Kawakami(1)Susumu Tachi(1)(1)Graduate School of Information Science and Technology, The University of Tokyo{kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp(2)Department of Mechanical Engineering and Intelligent Systems, The University of Electro-Communications
inami@computer.org

Abstract
A new type of Haptic AR system, SmartTouch, isintroduced. SmartTouch is composed of a thinelectro-tactile display and a sensor mounted on the skin.
The sensed information is converted to tactile sensation
through electrical stimulation. Thus, the wearer not only
makes physical contact with an object, but also can“touch” surface information of any modality, even thosethat are ordinarily non-touchable. The prototype of
SmartTouch has optical sensors. We endeavored torealize the tactile perception of luminance information,
which was achieved by imitating the sensory nerve
activity with electrical stimulation.
Keywords: Tactile Display, Electrocutaneous, Tactile
Primary Colors, Augmented Reality, Augmented Haptics,
SmartTouch
1.

Introduction

It is indisputable that a major part of our life is theinteraction with our surroundings. In this relationship,
information from the world is received through five
major sensory modalities.
It is also a fundamental limitation that tiny sensory
organs in charge of the respective sensations are the only
gates which connect us to the world. Natural ambition
arises; the ambition to acquire sensing ability beyond theusual physical limits, and to build new relationship withthe world. It is the ability to see the invisible, or to hear
the inaudible, often referred to as a sixth-sense.
Augmented reality (AR)[2] is an engineer's approachto this dream. In AR, artificial information is captured
from the world by some sort of a sensor and displayed
through existing sensing channels. Hence, we virtuallyacquire the physical ability of the sensor as our own(Figure 1).

Figure 1 Typical example of Augmented Reality(AR).
Sensed information (x-ray CT) is converted to visual
information so that the ability to see through is
obtained.

1.1.

Augmented haptics

This study focuses on the AR of haptics (augmented
haptics), which would allow a person to touch theuntouchable. The system is essentially composed of atactile display and a sensor. When contacting an object,
information acquired by the sensor is translated into atactile sensation, such as a vibration or force by a tactile
display. Thus, a person not only makes physical contact
with an object, but also “touches” surface information ofany modality.
SmartTool[1] is one realization of augmented haptics
with a hand tool, such as a scalpel or pen (Figure 2, left).
SmartTool captures information with a sensor attached at
the tip of the tool and conveys it to the operator througha haptic force display. One of the proposed applications
is for surgery. When a “smart” scalpel contacts a vital
region such as an artery, the sensor detects surface
information and the display produces a repulsive force toprotect the region.

display particle work together as a new receptor of the
skin.
2.

Figure 2 Examples of Augmented haptics.
(Left) SmartTool[1], (Right) SmartFinger[3].

Prototype system

Figure 4 and 5 depict the prototype of SmartTouch andits cross section. It is composed of three layers. The firsthas electrodes on the front side of a thin plate, the second
has optical sensors on the reverse side of the plate, andthe third is a thin film force sensor between the other twolayers.

Another development in augmented haptics is
SmartFinger[3](Figure 2, right), in which a vibratingtactile display and a sensor are both mounted on afingernail. The vibrator drives the finger vertically,
which induces force between the finger and the object
with which contact is made. Thus, skin sensations are
generated indirectly, while natural tactile sensations areunhindered by the display.
In this paper, we pursue the AR of skin sensations.

Figure 4 Prototype of SmartTouch: Visual image(black and white stripes) is captured by opticalsensors and displayed through electrical stimulation.

Figure 5 Cross section of prototype SmartTouch[9].
Figure 3 SmartTouch: A new functional layer of skincomposed of a sensor and tactile display[9].
The concept is illustrated in Figure 3. An object istouched with a finger. The skin of the finger is covered
with miniscule particles. Looking closer, each particle
is composed of a micro-sensor and a micro-tactile
display. When something is touched, information is
acquired through a sensor and converted into tactile
information. Hence, we could “touch” the surface
information of any modality, even those that are
ordinarily non-touchable. In other words, sensor andVisual images captured by the sensor are translated
into tactile information and displayed through electricalstimulation. As the system facilitates the recognition of
printed materials through the tactile sense, it could beapplied as a Braille display for the visually impaired.

2.1.

Related works

Extensive researches have been conducted on
visual-to-tactile conversion systems. Bliss[13] developed
the first converter system, and Collins[14] employedelectrical and mechanical stimulation on the skin on theback.
A representative commercial product,
Optacon[15] was developed in the 1960s; it uses a video
camera and a matrix of vibrating pins. However, theobjective was for a visually impaired person to readprinted material rather than to “augment” the real world.
With the system, a participant must have a video camera
in one hand and tactile information is displayed onto theother. In our system, the optical sensor and the tactile
display are located in practically the same place andwork in combination as a new skin “receptor”.

2.2.

(0.2[ms], 100-300[V], 1.0-3.0[mA] current controlled) inorder to generate the tactile sensation.

Electrical stimulation

By mounting a display directly on the skin, tactile
sensations can be presented with high spatial resolution.
At the same time, the display itself is separate from thecontact point with an object. But what kind ofinconvenience arises consequently?
Consider a horizontal motion of the finger. When thefinger moves horizontally, the contact generates africtional force. The force is perceived at the finger as a
torsional moment. As the display becomes thicker, theincreased distance between the finger and the object
surface generates greater torsional moment, which
results in an unnatural haptic sensation (Figure 6).

Figure 6 Horizontal motion of the finger andtorsional moment. F1: Finger force; F2: Friction; r:Distance between the center of the finger and skin; R:
Display thickness; M: Finger torsional moment[9].
This fact highlights the merits of electrical stimulation
as a means to display tactile information. Under thisparadigm, all that is needed to contact the skin is amatrix of electrodes, which can be readily fabricated intoa thin wafer.
The tactile display was composed of a 4x4 matrix ofstainless steel electrodes, each 1.0mm in diameter. The
longitudinal and transversal pitch of the electrodes was
2.5mm and 2.0mm respectively (Figure 7, left). The
electrodes applied electrical current pulses to the skin

Figure 7 (Left) Electrodes, (Right) Optical sensors[9].
Both electrodes and sensors were arranged at a
2.5mmx2.0mm pitch and a 4x4 matrix. The positionof each electrode was strictly aligned with an opticalsensor.

2.3.

Optical sensor

For an optical sensor, we used a phototransistor(SHARP PT600T, 1.6mm×1.6mm×0.8mm). We placed
the sensors just beneath the electrodes so that thehorizontal displacement between the stimulation point
and the sensing point was less than 1.0mm.
We used printed paper as a contact object. As we did
not embed a light source into the system, the paper was
lit with an LED lamp from below.
In the first preliminary experiment, each sensing
element was placed in direct contact with the surface ofan object; so that there was some gap region between thesensors where no sensor could see. Hence, when wemove the system on black and white stripes using aninterval identical to that of the sensors, the sensors, in
one case, could not locate the stripes; however, suddenly,
they all sensed the stripes simultaneously, resulting ininstability of the displayed tactile sensation. Therefore,
the field of view of each sensor must be widened so thateach sensor has an appropriate spatial property.
The sampling theorem states that, to reconstruct an
original signal from sampled data, the original signal
should not have a frequency component higher than 1/2d
(d: sampling interval)[16]. From this viewpoint, theabove-mentioned phenomenon is seen as an aliasing
effect.
Hence, we tried to design a spatial filter by broadening
the field of vision of the sensing. It was achieved by
mounting spacer on the sensor substrate and keeping thegap between the sensor and the object surface to 0.5mm.
We measured the spatial frequency characteristics ofthe sensor by measuring the response of a single
phototransistor when being sweeping on black and white
stripes with different spatial intervals (Figure 8). The
stripes are considered as approximated sin waves. Theinterval of the stripes was from 1.0mm(0.5mm white and0.5mm black) to 8.0mm.
Figure 8(top) depicts the response of the sensor. To
measure spatial frequency response, DC component wasremoved and amplitude was measured (Figure 8, bottom).
From the figure, we see that the cutoff (-3dB) frequencyof the sensor is 0.3[mm-1], which is equivalent to thestripes with a 3.3mm interval. This value agrees quite
well with the Nyquist interval (two times the samplinginterval) and, hence, demonstrates that the anti-aliasing
filter was well designed.
Similar analysis was done by Fearing[17][18] todesign a tactile information transmission system using atactile sensor and tactile display.

2.4.

To produce a natural tactile sensation, the stimulationmust correspond to finger pressure. We used a thin film
force sensor (NITTA FlexiForce, thickness: 0.3mm). The
sensor was placed between the electrode substrate andsensor substrate to measure finger pressure.
The history of the development of electrocutaneous
displays is long and includes many failures associatedwith the unpleasantness referred to as “electric shock.”
One reason that a “shock” is perceived as a result ofelectrical stimulation but not as a result of mechanicalstimulation are explained in the following. A mechanicalinteraction may provoke the same amount of sensation ormore than electrical stimulation, but it is also proactively
regulated by the contact force. It is this controllability ofmechanical stimuli that eliminates the perception ofshock.
Conversely, electrical stimulation itself does not havesuch a relationship with a contact force. Furthermore, thesensation peaks when the finger first makes contact with
an electrode because an electrical current is focused on asmall contact area.
This is why an electrical stimulus must be controlled
by a contact force. Pulse energy (height or width) was set
as a monotonically increasing function (temporary
logarithmic) of this pressure. This allowed the populationof the excited nerves to be actively controlled by force,
while the nerve firing rate remained constant. The user
can, therefore, actively modulate the intensity of thesensation [8].

2.5.

Figure 8 Response of the phototransistor when being
swept on black and white stripes with different
spatial frequencies.

Force sensor

System latency

By combining the above components, electricalstimulation can be processed based on visual information
obtained by optical sensors. As, from our preliminary
experiment, the shortest distance between the twoelectrodes is 2.0mm and the sweep velocity of the finger
is less than 100[mm/s], the shortest travel time between
the two adjacent electrodes is 20[ms]. To express thismovement, the cycle time should be much less than thetravel time. In our system, the waveform of the electricalstimulation pulse was dynamically generated and storedinadouble-buffered
memorymounted on
digital-to-analog
boards(National
Instruments
DAQ6713). This allowed parallel processing of othertasks, such as image capturing during stimulation, whichreduced the stimulation iteration period to 4.0[ms].
The latency between sensing and stimulation wasmeasured. Figure 9 shows the normalized phototransistor
output and stimulus current pulses as the system was
swept over a boundary separating the black and whiteareas. As mentioned in Section 3.2, an electricalcurrent pulse was applied when the time derivative of thephototransistor output reached a certain threshold. From
the figure, we observed that the latency was less than4[ms].

3.1.

Tactile primary colors

We have been developing a tactile display to present
realistic skin sensation for Virtual Reality. The idea is toselectively stimulate each type of skin receptor[20],especially the four types of mechanoreceptors: Meissnercorpuscles, Merkel cells, Ruffini endings, and Pacinian
corpuscles. By combining these stimuli, we canreconstruct complex tactile sensations. We call them“tactile primary colors,” analogous to the three primarycolors for vision.

Figure 10 Tactile primary colors. We can reconstruct
any tactile sensation by combining the activities ofeach receptor.
Figure 9 Phototransistor output and stimulus current(normalized). The latency between sensing andstimulation was less than 4[ms].
3.

Coding of electrical stimulation

After visual information of an object surface isobtained by the optical sensor, it is translated into tactile
information to be displayed through electricalstimulation. The translation technique is highly
dependent on the application. For this research, an effort
was made to generate a tactile sensation that was highly
realistic, because of the following reason.
We discussed in Sec.2.4 about the negative image of“electric shock” in electrocutaneous display, andexplained that one possible cause is the uncontrollabilityof the amount of sensation. The other reason is, of course,
the sensation quality itself. Hence, we tried to produce
realistic tactile sensation by electrical stimulation andconfront this problem.
We endeavored to realize the perception of luminanceinformation as the unevenness of the object surface. For
example, the black and white stripes in Figure 4 wereperceived as bumps with an identical interval.

Our approach uses electrical stimulation through theskin, or an electro-tactile display. An electrical current
from surface electrodes generates an electric field inside
the skin, which induces nerve activity.
Our findings about electrical stimulation of the finger
are summarized as follows. When using two coaxial
electrodes (a central electrode with 1.0mm diameter andouter electrode with 4.0mm inner diameter), theelectrical current pulse (0.2[ms], 1.0-3.0[mA],
10-50[pps]) generated vague pressure sensations when
the central electrode is a cathode (i.e., when the current
flows from the outer electrode to the central electrode).
On the contrary, if the current flows from a central
electrode (i.e., the central electrode works as an anode),an acute vibratory sensation is elicited[6][10].
Physiological studies revealed that there are two typesof mechanoreceptors in the shallow part of the skin,referred to as Merkel cells and Meissner corpuscles.
Merkel cells respond to static deformation, while
Meissner corpuscles are activated when the deformation
changes over time (Figure 11)[4]. Experiments insingle-nerve stimulation showed that Merkel cells
generate a pressure sensation, while Meissner corpuscles
produce a vibratory sensation[5].

Extending these observations to fingertip electricalstimulation, cathodic pulse seems to selectively stimulate
nerve fibers connected to Merkel cells, while anodic
pulse activates nerve fibers connected to Meissnercorpuscles.
More evidence of the achievement of this selective
stimulation is presented from electro-physiologicalstudies[8][21]. Mathematical analysis of a nerve-fiber
electrical model revealed that cathodic pulse selectively
stimulates nerve axons that run parallel to skin surface,
while anodic pulse efficiently stimulates verticallyoriented nerves (Figure 12). This fact and ourexperimental results agree quite well with the anatomical
observation by Cauna[22], who found that nerves ofMeissner corpuscles mainly run perpendicular to the skinsurface, while Merkel cell nerves generally run parallelto the skin.

displayed.

3.2. Translation from image to nerve firing
pattern
Our main goal is to generate a “natural” tactile
sensation. If this principle is reduced to the level ofindividual receptor activity, it becomes nothing morethan artificially producing a nerve-firing pattern thatmight arise in a real contact situation.
Our transformation formula is as follows: if theluminance (regarded as bump height) reaches a certain
threshold, a cathodic pulse is provided to produce apressure sensation. The pulse rate (10-50[pps]) is set
proportional to the height (Figure 13).
At the same time, an anodic pulse is produced when
the time derivative of the luminance reaches a certain
threshold, which generates a vibratory sensation.
It is worth noting that, in this stimulation method, eachelectrode requires only the information from an opticalsensor immediately beneath it.

Figure 11 Firing pattern of a mechanoreceptor whena finger touches and releases an object. SAI: Merkel
ending (pressure sensation), RA: Meissner corpuscles(vibratory sensation) (reconstructed from [3])Figure 13 Phototransistor output and its differentialand electrical stimulation pattern of a single
electrode.
Figure 12 Electrode polarity and nerve-orientation
selective stimulation.
In summary, a cathodic pulse stimulates horizontallyoriented nerves, which are mainly connected to Merkel
cells in the human finger, while an anodic pulse excites
vertically oriented nerves, which are mainly connected to
Meissner corpuscle. As a result, the pressure andvibratory sensation can be displayed separately.
These new findings gave us a possibility that byelectrical stimulation, natural tactile sensation could be

3.3.

Stimulation timing

When we stimulate one point, surrounding electrodes
are used as a return current electrode (ground) and, hence,
only one point is stimulated at a time, which requires
time-division scanning[8][11](Figure 14). Compared toconventional coaxial electrodes, this configuration
enables a much denser electrode density.
As mentioned in Section 2.5, the cycle time of thesensor and display system is 4.0[ms]. Since eachelectrical pulse requires at least 0.2[ms], 20 pulses could

be packed into one cycle. This may seem quite sufficientfor our 16-electrode system, but not if we consider thenecessity to provide both anodic and cathodic pulses.
Our current solution was to constrain the number ofstimulation points by regulating the threshold levelmentioned in Section 3.2.

Figure 15 Two-point discrimination with anodic pulse
stimulation. The static resolution is 2 to 4 mm.
Figure 14 Scanning in the anodic stimulation mode.
In this manner, the stripes with a large interval (about
3.0mm) mainly generated a pressure sensation and wereperceived as a rough bump, while stripes with a smallinterval (about 1.0mm) mainly generated a vibratory
sensation and were perceived as a fine texture. All
participants could clearly distinguish between the two
types of stripes by moving their fingers.
4.

4.1.

Psychophysical evaluation

Two-point discrimination

First, we measured the static resolution of the display.
We stimulated two electrodes simultaneously withanodic current pulses. The distance between eachelectrode is 0 to 6mm. After the stimulation, we asked
whether the sensation was “one point,” “short line,” or“two distinct points.”
The results from six participants, each with 40 trials,
are shown in Figure 15. The horizontal axis is theelectrode distance, and vertical axis is the answer rate.
From the graph, we see that at 2mm, two points are mostfrequently perceived as a short line, while at 4mm, twopoints are perceived as two distinct points. Hence, thestatic resolution is 2 to 4 mm.
The same experiment was conducted by applying acathodic current pulse. Contrary to an anodic pulse, theelicited sensation is typically “blurred” around theelectrode; therefore, we could not stably measure thespatial resolution. This crucial difference was firstobserved by Kaczmarek[10], and we gave the following
electro-physiological explanation [8].

As we have seen in Section 3.1, a cathodic current
activates nerve axons parallel to the skin surface (Figure
12, left). However, the brain mistakes the receptor that isconnected at the tip of the axon as being activated.
Therefore, there is always some “gap” between thestimulation point and the sensation point. The
accumulation of this gap results in an unfocusedsensation. This phenomenon is inherent to cathodic
stimulation and cannot be avoided by a simple
application with a coaxial electrode.
On the contrary, an anodic pulse selectively stimulates“vertical” axons (Figure 12, right). In this case, although
the stimulation point and the connected mechanoreceptor
still may have a gap, the gap is vertical, so its influenceon the sensation is negligible. As a result, an acute tactile
image can be obtained.
In a practical application of SmartTouch, thepresentation of a spatial pattern is quite important. Hence,from now on, we will only use anodic current pulses.
Though it does not dutifully follow our “tactile primarycolor” approach (mentioned in Section.3.1), we appliedan anodic pulse when the luminance and time derivativeof the luminance reached specific thresholds.

4.2.

Line-width discrimination

Next, we measured the dynamic resolution of the total
system because SmartTouch assumes an active motion of
the finger.
Two lines are written on normal paper. One is a 4
mm-width standard line, and the other is a 2 to 6
mm-width comparison line. The participants swept overthe lines and indicated which line was the widest
Figure 16 is the result of six participants, eight trials

for each line width. The horizontal axis is thecomparison line width, and the vertical axis is the answer
rate for which the participants indicated that thecomparison line was wider than the standard line. From
the figure, the 70% correct threshold are 3.5mm and
4.5mm. Hence, the 0.5mm width is discriminated with a
70% certainty.

Figure 16 Line-width discrimination. The standardline width is 4mm. The 70% correct thresholds are
3.5mm and 4.5mm.
5.

Future work

This paper proposes an AR system of skin sensation
called SmartTouch. A mounted optical sensor converts
visual information from a contact object into tactile
information, and electrical stimulation is employed as ameans to present tactile information.
Until now, there were only two types of applications
for tactile display. One is the Braille system for thevisually impaired, and the other is a haptic device that
adds realism to the virtual world by obtaining tactile
textures. What we try to emphasize with SmartTouch isthat when sensor and tactile displays are combined,tactile display will come out to the real world.
Although this paper focuses on visual-to-tactile
translation, the use of SmartTouch is not limited toBraille for the visually impaired. By changing the sensor,
other modalities of sensation can be translated to touch
as well. We are now considering the combination of atactile sensor matrix with an electro-tactile display to
perform tactile-to-tactile conversion. If the tactile sensor
is more sensitive than human perception (detection
threshold of skin surface deformation is about
0.01mm[4]), we can enhance the natural tactile
experience.
Although it is not commonly known, human tactile

sensitivity dramatically decreases with age[19]. Hence,as people age, many will need “tactile aids,” just as theymight need hearing aids..
Our goal for SmartTouch is to achieve a very thindisplay and sensor directly mounted on the skin to serve
as a new functional layer so that the system can be wornas an unconscious daily interface. Hence, it will beinteresting to learn just how thin a system could bemanufactured with the use of existing technology.
As the display component only needs electrodes,
fabrication of electrodes with less than 0.3mm inthickness is possible by using a film substrate[11]. If we
could “print” electrodes on the skin directly by using
conductive ink or a disposable tattoo, we could reduce
the thickness of the display to virtually zero.
The second component of the system is a sensor. By
placing the sensor not on the skin but around the finger,
the sensor thickness could be ignored. In thisconfiguration, finger motion is computed by the timecorrelation of the sensor's output. The information under
the skin is obtained by using past information of thesensor.
The final component of the system is a contact force
sensor. It is already known that an optical sensor
mounted on the fingernail can measure contact pressure
by observing the color of the blood vessel under the nail[3][12].
Ultimately, we could fabricate an ideal SmartTouch, a
new layer of a skin, which does not hinder natural tactile
sensation while detecting and presenting other surface
information. Our next step is to develop such a system.

Figure 17 SmartTouch of the near future[9].

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