Electrotactile Stimulation Based on
Strain Energy Density of the Fingertip
Katsunari Sato, Hideaki Nii, Naoki Kawakami, and Susumu Tachi
Graduate School of Information Science and Technology,
The University of Tokyo, Japan{Katsunari_Sato,Hideaki_Nii}@ipc.i.u-tokyo.ac.jp,{kawakami,tachi}@star.t.u-tokyo.ac.jp

Abstract. The shape recognition of an object is important for dexterous manipulation by humans. Therefore, we have developed a haptic display that
integrates both electrotactile and kinesthetic sensations to present shapeinformation. However, the electrotactile display only presents the contact fieldbetween the object and the fingertip. Therefore, we propose a method ofelectrotactile stimulation using the strain energy density model at the fingertip
to generate the tactile sensation of the fingertip deformation. The result of the
shape recognition experiment verifies the efficiency of the proposed method.
Keywords: electrotactile display, strain energy density, shape recognition.

1 Introduction
When humans manipulate an object, they recognize the object properties such as theshape, posture, weight, and stiffness. Therefore, it is important for a haptic display topresent such information by presenting both tactile and kinesthetic information.
Recently, haptic displays that present both types of information have been realized [1][2]. We have also constructed a haptic display that can present the shape of an object.
We integrated a small electrotactile display with a kinesthetic display [3]. The
electrotactile information allows us to perceive the exact shapes and reflective forceof an object such as an edge or a concavo-convex surface. Therefore, humans canefficiently recognize the shape of an object by electrotactile-kinesthetic integration.
Conventional electrotactile displays present a two-dimensional contact field
between the object and the fingertip by on-off signals [2] [3] [4]. However, humans
perceive the exact shape of an object not from two-dimensional contacts fields but
from a three-dimensional touch condition. When the fingertip touches an object, itgets deformed. Humans perceive this deformation via the firing of mechanoreceptors
and recognize the shape of the object. Therefore, an electrotactile display that canreproduce the deformation of the fingertip is desired.
In this paper, we propose a method for generating an electrotactile stimulus basedon the strain energy density (SED) to reproduce fingertip deformation at a peripherallevel (Fig. 1). First, we use the finite element method (FEM) model to simulate theM. Ferre (Ed.): EuroHaptics 2008, LNCS 5024, pp. 724–729, 2008.
© Springer-Verlag Berlin Heidelberg 2008

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SED around receptor
proportion

proportion
objectobjectfiring rate of receptor

equivalentfrequency ofelectrotactile

Fig. 1. Diagram of the proposed SED model that reproduces the fingertip deformation using anelectrotactile stimulus

SED of a human fingertip when it touches objects. Then, we test the effectiveness ofthe electrotactile stimulus based on the simulated SED by means of a shaperecognition test.

2 Electrotactile Stimulus Based on SED
In order to reproduce the fingertip deformation caused on touching an object, we use
focus on the SED at the fingertip. Srinivasan and Dandekar [5] investigated therelationship between the SED around a mechanoreceptor and the receptor’s firing rate
at the fingertip. They showed that the firing rate of a Merkel cell is proportional to theSED around the cell. Furthermore, Kobayashi and Maeno [6] showed that the firing
rate of the mechanoreceptors is proportional to and the SED around a Meissner’scorpuscle.
From these relationships, we consider reproducing the fingertip deformation
virtually by stimulating the mechanoreceptors at a firing rate proportional to the SED.
We used an electrotactile stimulus to stimulate the mechanoreceptors [3]. The
electrotactile display comprises a pin-electrode matrix. The stimulus directly activates
the nerve fibers on the skin surface by means of an electric current from the surface
electrodes. Since the electrotactile stimulus can stimulate the nerve fibers with anarbitrary frequency, it can reproduce the firing rate of the mechanoreceptors inproportion to the SED in the fingertip.

3 Simulation of SED
In order to stimulate the nerve fibers using the SED, we must calculate the SED
around the mechanoreceptors when the fingertip touches an object. In this chapter, wesimulate the SED using FEM software (FEMLAB, COMSOL AB.).
In the simulation, we constructed a simple two-dimensional fingertip model (Fig. 2).
Merkel cells and Meissner’s corpuscle were considered to assume a key role in therecognition of the exact shape of the object [7]. Therefore, the SED was measured on
the boundary between the epidermis and dermis where the Merkel cells and Meissner’s
corpuscles exist.
The SED was measured by pushing an object to the surface of the fingertip model.
As object shapes for the simulation, we focused on three extreme examples of726

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two-dimensionalcross section

measurement boundary

epidermis (136 kPa, 0.48)
dermis (80 kPa, 0.48)

bone (17 TPa, 0.3)

hypodermal tissue(34 kPa, 0.48)
nail (17 TPa, 0.3): fixed end

Fig. 2. Constructed fingertip model. We set the nail as the fixed end. Young’s modulus andPoisson’s ratio are indicated in the parentheses.

curvature radius: plane, curved face, and edge. The curvature radii of the plane, curved
face, and edge surfaces were 0 mm–¹, 0.1 mm–¹, and ∞ mm–¹, respectively. We set thepushing distance of the objects to the fingertip at 0.2 mm, 0.4 mm, and 0.6 mm.
Figure 3 shows the simulation results. The SED of the edge surface is very highand that of the plane is low at all points. The SEDs at the contact boundaries of thecurved face are higher than that of the intermediate. The SED increases with thepushing distance for all shapes. A high SED implies that the mechanoreceptors fire
frequently when a human being touches an object. Therefore, this result is considered
to be natural because humans perceive an edge surface and a contact boundary morestrongly than a plane and an intermediate of the contact field, respectively.
(a) plane

20 mm

20 mm20 mm

planecurved face

edge

400

SED (J/m³)

10 mm50010 mm

10 mm

objects-50.6-2.502.5
position (mm)

5500

300

0.2

200

0.4

1000.6400

SED (J/m³)

400

SED (J/m³)

0.4100(c) edge

500

0-2.5
02.5
position (mm)0.2

2000(b) curved face

-5300

300

0.2

200

0.4

100

0.6

05-5-2.50
2.5
position (mm)5Fig. 3. Simulation results. The SED is calculated from –5.0 mm to 5.0 mm at 1.25 mm intervals.
(a), (b), and (c) represent the SED when the fingertip touches the plane, curved face, and edge
objects, respectively. The solid, broken, and dotted lines represent SED pushing distances of 0.2mm, 0.4 mm, and 0.6 mm, respectively.

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4 Shape Recognition Experiment
Using the simulated SED, we considered that the electrotactile stimulus can generate
the sensation of the deformation of the fingertip when touching an object. In thischapter, we evaluate its efficiency by means of the shape recognition test.
4.1 Materials and Methods
The experiment was carried out in a virtual environment. The subjects were made topush a virtual object shape and identify the shape of the object they touched. Theobject shapes—plane (0 mm–¹), large curved face (0.05 mm–¹), medium curved face(0.1 mm–¹), small curved face (0.2 mm–¹) and edge (∞ mm–¹)—were selected on thebasis of the curvature radius.
Figure 4 shows the experimental setup used for the evaluation. An electrotactile
display was mounted on the PHANToM Omni (SensAble Tech.) to function as theinterface between the real and virtual worlds. The subjects placed the tip of theirindex finger on the electrotactile display and held the end effector of the PHANToM
with the other fingers. They operated a virtual finger in the virtual world as theywould operate their index finger by moving the end effector. The movement of thefinger was restricted to a two-dimensional area (xy plane).
frequency
based on SED

17 mm

electrotactile
display
virtualfinger

1.25 mmpush

SED model
constant
frequency

2.5 mm12 mm

end effectorof PHANToM

virtualobjectyx

contact field

Fig. 4. Experimental setup. The left-hand side shows the electrotactile display on the PHANToM.
The right-hand side shows the virtual world and the electrotactile stimulus corresponding to thetouching condition.

When a subject pushed a virtual object along the y-direction, he/she felt a reflectiveforce of the object along the same direction. The reflective force was proportional tothe involved distance of the fingertip and the object. In addition, the subjects felt anelectrotactile stimulus corresponding to the finger position and the reflective force.
In this experiment, we compared SED-based and contact-field-based electrotactile
stimuli. The frequency of the SED-based electrotactile stimulus varied in proportion
to the simulated SED same way as described in chapter 3. This experiment used fivefrequencies—20 Hz, 30 Hz, 60 Hz, 90 Hz, and 120 Hz—to reproduce the firing rateof the mechanoreceptors. Meanwhile, the frequency of the contact-field-based
electrotactile stimulus was set constant at 60 Hz.

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It should be noted that we used an anodic stimulus as the electrotactile stimulus inthis experiment. Cathodic and anodic stimuli are considered to capable of stimulating
Merkel cells and Meissner’s corpuscles, respectively [4]. However, this selective
stimulation has not been proven quantitatively, and the cathodic stimulus causes
undesirable pain. Therefore, we used an anodic stimulus to fire both receptors.
The subjects were nine adults in their 20s and 30s. They were made to practice theprocedure before the real experiment. Each shape was presented 10 times under bothelectrotactile conditions. The subjects were not provided with a visual key of theshapes for their reference. After the experiment, we asked the subjects for theirfeedback about the experiment.
4.2 Results and Discussion
The graphs in Fig. 5 show the results of the experiment. There is a tendency that theaccuracy of shape recognition is better for the SED-based electrotactile stimulus whensubjects touched the plane, curve L, curve S, and edge. Moreover, the feedback fromthe subjects was positive: they mentioned that they felt a realistic sensation in the caseof the SED-based electrotactile stimulus, especially in the case of the edge object.
Therefore, we consider that an SED-based electrotactile stimulus is effective for exact
shape recognition.
However, the significant difference does not appear other than the result of the plane(t(8) = 2.41, p < .05). Moreover, the correct answer ratios of the curved faces are notgood (< 50 %). Therefore, we consider that we could not clearly reproduce the threedimensional touch condition in this experiment. There could be two reasons for this.
First, the spatial resolution of the electrotactile display was not appropriate, that is, thediameter of the electrode (= 1.25 mm) and the distance between the centers of twoelectrodes (= 2.5 mm) did not favor the reproduction of the SED of the fingertip. Second,
selective stimulation of the mechanoreceptors is required. Since the role of different
mechanoreceptors is different [7], we have to measure the SED around the Merkel cells
and Meissner’s corpuscles, and then selectively stimulate each mechanoreceptor.
curve L(t = 2.41)(t = 1.42)

curve M (t =-0.69)

curve Sedge (t = 0.63)(t = 0.82)

Ave.

100

100

100

10080808080806060606060C4040404040D2020202020000contact
f ieldBE0SED

A0SED

contact
field

SEDcontact
f ieldSED

contact
fieldhFGSED

contact
f ield

20 mm

35 mm

curveM

10 mm

curveL

10 mm

10 mm

plane

20 mm

curveS
10 mm

10 mm

condition of electrotactile stimuli

edge

HI

10 mm

correct answer ratio (%)

plane
10020 mmFig. 5. Results of the shape recognition experiment. Each dot represents the result of each subject
and the bars represent their average. The error bar represents the standard deviation. The resultsof t-test are indicated in the parentheses.

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5 Conclusion
We proposed a method for generating electrotactile stimuli based on SED. First, wesimulated the SED of a human fingertip when touching some objects by using theFEM model. Then, we confirmed the effectiveness of the electrotactile stimulus basedon SED by a shape recognition test. We concluded that this method of generating
electrotactile stimuli can potentially improve the efficiency of shape recognition.
In future, we plan to examine the relationship between SED and the firing rate ofmechanoreceptors more closely. Then, we will construct a more rigorous simulation
model and electrotactile stimulation to generate a realistic sensation of touching theshape of an object.

Acknowledgment
We would like to express our sincere gratitude to Associate Professor Hiroyuki
Kajimoto (The University of Electro-Communications) who originally developed ourelectrotactile display.

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