Int J CARS (2010) 5:359–367DOI 10.1007/s11548-010-0426-7

ORIGINAL ARTICLE

Development of a real-time tactile sensing system for brain tumordiagnosis

Yoshihiro Tanaka · Qingyun Yu · Kazuki Doumoto · Akihito Sano ·Yuichiro Hayashi · Masazumi Fujii · Yasukazu Kajita ·Masaaki Mizuno · Toshihiko Wakabayashi · Hideo Fujimoto

Received: 11 January 2010 / Accepted: 6 April 2010 / Published online: 24 April 2010© CARS 2010

AbstractPurpose Tactile sensing techniques may distinguish tumorfrom healthy tissue and have potential for intraoperative braintumor diagnosis. The aim of this study is to develop a bio-compatible real-time sensing system to measure tactile infor-mation such as softness and smoothness, and its applicationto brain tumor diagnosis.Methods An active tactile sensor is developed using balloonexpansion. This compact system provides instantaneous tac-tile information and has potential for brain tumor diagnosis.Measurements are obtained on soft samples with differentstiffness and surface condition with testing of boundary con-dition influence on thickness and area of the object. Then,measurements on white matter and gray matter of porcineex vivo brain are done as the first step for brain tumor diag-nosis.Results The sensor can discriminate samples with differentstiffness and surface condition subject to influence by bound-ary conditions. The sensor can evaluate an object relativelyunder the same boundary conditions but requires enoughthickness and area to evaluate absolutely. Measurements on

Y. Tanaka (B) · Q. Yu · A. SanoDepartment of Engineering Physics, Electronics and Mechanics,Graduate School of Engineering, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466-8555, Japane-mail: tanaka.yoshihiro@nitech.ac.jp

K. Doumoto · H. FujimotoDepartment of Computer Science and Engineering,Graduate School of Engineering, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan

Y. Hayashi · M. Fujii · Y. Kajita · M. Mizuno · T. WakabayashiDepartment of Neurosurgery, Graduate School of Medicine,Nagoya University, 65 Tsurumai-cho, Showa-ku,Nagoya 466-8550, Japan

brain show that the sensor can discriminate between whitematter and gray matter.Conclusions Although the sensor has problems on absoluteevaluation, results show that the sensor can evaluate tactileinformation, and it has potential for brain tumor diagnosis.

Keywords Tactile sensor · Brain tumor diagnosis ·Balloon expansion · Surface condition · Stiffness

Introduction

Tactile sensing techniques have been focused in medicalfield. Surgeons perceive various information of body tis-sue such as stiffness and smoothness through active touchduring usual surgery. Tactile sensing has potential to beeffective for tumor diagnosis, area cognition, and so on. Fur-thermore, minimally invasive surgery such as endoscopic sur-gery has been performed without tactile sensitivity. As thefield to apply tactile sensing technique, we focus on braintumor diagnosis. It is empirically known that tumor is differ-ent from healthy tissue in tactile sensations. Image-guidedsurgery with pre-/intra-operative MRI or CT has been effec-tively used in this field but such systems are expensive andtime-consuming. Therefore, we aim at real-time sensing forbrain tumor diagnosis with a tactile sensor.

Tactile sensors applied to human body tissue have spe-cific problems. Tactile sensors for helping robot manipulation[1,2] and evaluating quality of industrial products [3] havebeen proposed. However, these sensors cannot be availableas medical sensors in the following issues. Sensors need to besterilized and electric power should not be used in the body.Sensors must keep the prevention of infection and leakagecurrent to the body.

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Fig. 1 Conceptual schematicillustration of proposed sensorsystem

Endoscope

Object

Inner body

Outer body

Expansionusing fluid

PushStickor slip

Computer

Tube

Balloon

Force sensor

Linear actuatorwith encoder

Syringe

Volume output

Pressure output

Sensor probe

Constant loading mechanismBalloon

Recently, tactile sensors for medical application have beendeveloped [4–10] and some of them have considered enoughsafety for body tissue. The force sensor using a balloon [7],the stiffness sensor using vibration [8], the optical tactilesensor using an air cushion [9], and the non-contact imped-ance sensor using air bursts and a laser displacement sen-sor [10] have been proposed. However, they focus on themeasurement of stiffness. Stiffness is an important factor fortumor diagnosis but surface condition such as sliminess andsmoothness are also important. Concerning human tactileperception, it has been described that feeling of surface con-dition is an independent factor from the feeling of stiffness[11]. It seems that surgeons evaluate the object comprehen-sively when they use tactile information. However, any tactilesensor for measuring surface condition to apply to inner bodyhas not been developed as far as we know.

The aim of this study is to develop a biocompatiblereal-time sensing system to measure softness, sliminess,smoothness, and so on, and its application to brain tumordiagnosis. We have proposed active tactile sensing methodusing balloon expansion [12,13]. A balloon is kept in contactwith an object and expanded by using fluid. Pressure changesand volume changes are measured in the balloon expansion,and stiffness and sliminess are instantly evaluated. The sen-sor system is compact. Concerning the safety for human, itcan ensure the sterilization and electric power is not usedin the body. We have confirmed potential to measure stiff-ness and surface condition through fundamental analysis andexperiments.

In this paper, the capability of the sensor and potentialfor brain tumor diagnosis are presented. First, the developedtactile sensing system is described. Then, measurementson soft samples with different stiffness and surface condi-tion are carried out. Influences of boundary condition on

thickness and area of the object are discussed. Next, mea-surements on white matter and gray matter of porcine ex vivobrain are conducted as the first step for brain tumor diagnosis.The results show that the sensor can evaluate tactile informa-tion in stiffness and surface condition and it has potential forbrain tumor diagnosis.

Biocompatible real-time tactile sensing system

Basic idea

In our proposed sensing, a balloon is contacted with an objectand expanded by using fluid. In the process of the expansion,the balloon pushes the object and the contact surface of theballoon slips or sticks. These phenomena depend on stiffnessand surface condition such as friction of the object. There-fore, by evaluating the expansion of the balloon, it is expectedthat various tactile information can be measured.

We have developed the sensor system with a syringe [13].Figure 1 shows the conceptual schematic illustration of thesensor system. The sensor probe has a balloon at the tip anda constant loading mechanism. It is filled up in biocompati-ble water and is connected to a syringe through a tube. Theballoon is expanded by pushing the syringe with a syringedriver. The pressure of the balloon can be measured by aforce sensor set on the syringe, and the volume of the bal-loon can be measured by pushing distance of the syringe.Tactile information is instantly evaluated by using pressurechanges and volume changes obtained in the expansion.

The proposed sensor provides safety for human body tis-sue. The balloon is soft and flexible, and load given to theobject is mechanically kept to be constant by the constantloading mechanism. The sensor does not use electric power

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Fig. 2 Sensor probe unit.a Photograph and geometry ofprobe. b Constant loading bybuckling

Silicone rubber sheet

Acrylic tube

Balloon

Slider

Tube coupling

Polyurethane tube

Fixer

(a)

(b)

in the body. Furthermore, the sensor probe, the tube, andthe syringe, which we call “sensor probe unit” in set, canensure sterilization including the fluid in the balloon byfabricating them by the set under sterile condition. Addi-tionally, it is realistic to make the sensor probe unit dis-posable because the sensor probe, tube, and syringe are notexpensive. Furthermore, the sensor probe has potential tohave compatibility with the endoscope since it has a linearshape.

The proposed sensing has another advantage for humanbody. It is generally to slide a sensor over an object for mea-suring surface condition of the object [3]. However, it is verydifficult to steadily slide a sensor over tissues since they aresoft and their surface geometries are not flat but usually var-ious. Our proposed sensing does not need to be slid over anobject. The sensor touches an object locally through the bal-loon expansion. Partial slippage occurs between the contactsurface of the balloon and the object and surface conditioncan be collected. It is known that partial slippage betweenfingers and an object is important for grasping [14,15]. Inthe proposed sensing, the physical phenomenon is similar tosuch human case.

Sensor system

Sensor probe unit

Photograph of the sensor probe unit and geometry of the sen-sor probe are shown in Fig. 2a. The sensor probe unit is filledup in water. The flexible polyurethane tube is 4 mm in outerdiameter, 2 mm in inner diameter, and 1.1 m in length, andthe syringe is 2.5 ml in volume. The sensor probe is presentedin detail as follows.

A latex balloon is fixed at the tip of an acrylic tube,which is 4 mm in outer diameter and 2 mm in inner diam-eter. The balloon is 4 mm in diameter and 0.15 mm in thick-ness before expansion. The other side of the acrylic tube isconnected to the polyurethane tube by a coupling. The con-stant loading mechanism is composed of two larger tubes,

which are a fixer made of a light curing resin tube and aslider made of an acrylic tube with inner diameter of 4 mm,and four silicone rubber sheets, which are 1 mm in thick-ness, 20 mm in length, and 45◦ in arc of cross-section. Asshown in Fig. 2a, the acrylic tube with the balloon is insertedin the fixer and the slider. The fixer is attached on the tipof the acrylic tube, and the balloon is a little out from thefixer. The slider is set on the acrylic tube. The silicone rub-ber sheets are set between the fixer and the slider by bondingtheir edges.

The constant loading mechanism performs the constantloading by buckling of the silicone rubber sheets. In order tocontact the sensor probe with an object, the slider is held andmoved to the object. As shown in Fig. 2b, the silicone rubbersheets are buckled after the contact with the object. Here, theyare deformed largely but the load is almost constant. There-fore, the load is kept constant within some moving distanceof the slider. This range is long enough to confirm by sight.The developed sensor probe gives a constant load of 0.51 Nand the range of the constant loading is about 5 mm. Here,these characteristics can be adjusted by changing physicalproperties of the silicone sheets such as Young’s modulus,thickness, etc.

Syringe driver and measurement setup

Figure 3a presents the syringe driver. The syringe driver iscomposed of a linear actuator, which is driven by a servomotor with an encoder, a load cell, and a base to mount thesyringe.

Figure 3b presents the measurement setup. The system iscompact and it is easy to move. The measurement setup iscomposed of a strain gauge amplifier, a controller box, anda PC for display of sensing result. The controller box hasa SH4A microcomputer. It receives output signals from theload cell and the encoder and sends a control voltage to themotor. The sampling frequency is 1 kHz. The sensing resultssuch as output signals and status of the sensor system are sentfrom the controller box to the PC for display through TCP/IP

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Fig. 3 Syringe driver andmeasurement setup. a Syringedriver. b Measurement setup

Syringe baseCouplingLoad cellServo motor with encoder

Syringe

Sensor probe

Syringe driver

Sensor Probe

Strain gauge ampliferController box(microcomputer)

Foot switch for on/offof system

PC for displayof sensing result

Electric power supply

Foot switch for sensingFoot switch for initialization

(a)

(b)

Fig. 4 Sensor output under freecondition (in case of no object).a Pressure and volume.b Relation between pressure andvolume

0 0.5 1 1.5

0

50

100

150

0

50

100

Time [sec]

Pres

sure

[kP

a]

Vol

ume

[mm

3 ]

Pressure

Volume

0 50 100

0

50

100

150

Volume [mm3]Pr

essu

re [

kPa]

(a) (b)

connection. The start/stop of the system, the start of sensingand the initialization of sensing are conducted by using footswitches.

Sensing process

Sensing method

First after the setup of the sensor system, the balloonis expanded under free condition, in case of no object.Figure 4 shows typically obtained output. The volume mono-tonically increases since the syringe is pushed continuously,while pressure has a peak and then decreases. This resultis well known as the characteristic of balloon [16]. Then,the volume at the peak of the pressure is recorded and thesyringe is pulled back from the recorded point at the set vol-ume (75 mm3) to set the volume zero as the start position forthe sensing. Through this initialization, the characteristic ofthe balloon is made the same condition.

In the sensing, the motor is driven at a constant torqueand is stopped when the volume reaches to 100 mm3 fromthe start position. The sensing time is about 2 s. Then, the

40−50

0

50

100

Time [sec]

Vol

ume

[mm

3 ]

Sensing start

Sensing end Position recovery control

Objetive position (start position)

35

Fig. 5 Position recovery control after sensing

syringe goes back to the start position for next sensing. Inthis position recovery, the proxy-based sliding mode control[17] is applied for the precise position control. Figure 5 showsan example of volume changes in the position recovery con-trol. It is confirmed that the volume precisely returns at thestart position after sensing.

In this paper, the sensor probe was held by a hand andmoved to be contacted with an object for the sensing. Here,the probe was contacted at right angle to the object by sightand the balloon was shortly cleaned with water before the

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Embrocation(water, yam water, and milky lotion)

Urethan gel(A and B)

Fig. 6 Sample with different stiffness and surface condition

measurement on the sample with different surface conditionto avoid mixing in the surface condition. The tip of the sensorprobe is contacted with the object. Through experiments, itwas confirmed that cleaning the tip with water is enough forthe reproducibility of the sensor output.

Signal processing

We calculate outer pressure of the balloon by using obtainedoutput signals. Outer pressure means the force given to theballoon from the object and the constant loading mechanismof the probe. The pressure of the balloon is equal to be sum offorce based on the membrane stress of the balloon and outerpressure as follows:

p = 2h

rσ + pout,

where p, r, and h are pressure, radius, and thickness of theballoon, respectively, and σ and pout are membrane stress ofthe balloon and outer pressure given to the balloon, respec-tively. Here, when the expansion in case of no object, pout =0. Therefore, pout can be obtained by subtracting pressureobtained in case of no object from pressure obtained in thesensing on the basis of the volume. This processing is done inreal-time, and results are online-displayed with the sensing.

Additionally, this processing has the other effect. Pressurechanges and volume changes are influenced by changes ofphysical characteristics of the balloon and tube depending ontemperature etc. Pressure output signals also include frictiondue to pushing the syringe. However, through the subtractionprocessing, such influences can be canceled.

Fundamental experiments

Discrimination of samples with different stiffness andsurface condition

As shown in Fig. 6, soft gels with liquid on the surface wereprepared as samples. Two kinds of urethane gels, A and B,and three kinds of embrocations, water, water laced with yam,which we call “yam water”, and milky lotion, were prepared.Young’s modulus of the gel A is 0.14 MPa and the gel B0.44 MPa. The milky lotion is smooth. The yam water isslimy. The water is not smooth compared with the others.

0 50 100

0

20

40

60

Volume [mm3]

Out

er p

ress

ure

[kPa

]

Gel B with water

Gel A with water

Gel B with yam water

Gel B with milky lotion

Gel A with yam water

Gel A with milky lotion

Fig. 7 Typical outer pressure obtained on samples with differentstiffness and surface condition

The size of gels is 65 × 65 mm and their thickness is 25 mm.They have plane surface.

Figure 7 presents typical outer pressure obtained on eachsample. It is found that output signals are different betweensamples. Concerning stiffness, pressure in small volumeon samples with large stiffness (gel B) is larger than sam-ples with small stiffness (gel A). Concerning embrocations,increasing gradient of the pressure in large volume is clearlydifferent between embrocations. Gradient on samples withmilky lotion is smaller than samples with other embrocations.Gradient on samples with water is large. The expansion ofthe balloon is influenced to stiffness in the beginning. Theballoon pushes the object in the beginning of the expansion,but it pushes little in the late expansion. In the beginning, theballoon and the part except the balloon in the sensor probecontact the object due to small pressure of the balloon andform of the sensor probe. However, the balloon becomes largewith the expansion, and only the part of the balloon contactsthe object. Under this condition, normal load given to theballoon by the object becomes almost constant on any objectdue to the constant loading mechanism. Surface conditioninfluences the balloon throughout the expansion, especiallylargely in the late expansion, since the contact area is largethere.

We extracted the average of outer pressure values from45–55 mm3 in volume as Pa and the outer pressure gradientfrom 75–85 mm3] in volume as Gb by least-square method.Distributions of calculated Pa and Gb are shown in Fig. 8.Each plot indicates the output in a single measurement. It isfound that outputs on each sample are located in differentarea. Pa is small on soft samples and Gb is small on slipperysamples.

Influence of boundary condition

The sensor measures tactile information of the objectincluding boundary condition like human tactile perception.

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10 200

0.5

1

1.5

Pressure around 50 mm3 Pa [kPa]

Pres

sure

gra

dien

t aro

und

80 m

m3

Gb

[kPa

/mm

3 ]Gel A with water

Gel B with water

Gel B with yam water

Gel B with milky lotion

Gel A with yam water

Gel A with milky lotion

Fig. 8 Sensor output distributions

Therefore, it seems that the sensor output is influenced byboundary condition of the object even if the object has thesame mechanical properties. Human might be able to eval-uate tactile information of the object considering bound-ary condition but the influence of boundary condition isan important problem for identifying and/or evaluating theobject from sensor output. We investigated influences of thethickness and the area of the object. Here, characteristics onstiffness are largely influenced to such boundary condition.Then, we carried out the following experiments under thesame surface condition (water).

Thickness

Urethane gels (Young’s modulus E = 0.199 MPa) in thicknessof 1.2, 2, 3, 4.5, 6, and 8 mm, were used as the object. The sizeof all objects is 20 × 20 mm. As samples, under the objects,a soft urethane gel in thickness of 5 mm (E = 0.047 MPa)and an acrylic plate in thickness of 8 mm were set as the base.Figure 9 shows photograph and geometry of the samples.

Measurement was done 3 times on each sample. Figure 10shows typical outer pressures on some samples. Figure 11aand b show extracted Pa and Gb. Gray circle plots indicatethe outputs on the case of the acrylic base. Gray square plotsindicate the outputs on the case of the soft gel base. Blacklines show averages of the sensor outputs on each sample. Itcan be seen that outer pressure is different from each samplealthough the object’s Young’s modulus is equal. When thethickness of the object is small (less than 5 mm), the sensoroutput Pa changes according to the base. It is found that Pa

has similar values under the condition of the larger thickness.The sensor output Gb changes very little.

As mentioned earlier, Pa has a strong relation with thestiffness and Gb has a strong relation with the surface

Acrylic base or soft urethane gel base

Object urethane gel(t = 8, 6, 4.5, 3, 2, 1.2 mm)

t

Acrylic base

Soft urethane gel base

Fig. 9 Samples used in experiments on thickness

0 50 100

0

20

40

Volume [mm3]

Out

er p

ress

ure

[kPa

]t = 1.2mm

t = 3mm

t = 8mm

t = 8mm

t = 3mm

t = 1.2mm

Accylic baseGel base

Fig. 10 Typical outer pressure obtained on samples under conditionof different thickness and base

condition. Sensor output on stiffness can be obtained sincethe balloon pushes the object with the expansion and sen-sor output on surface condition can be obtained since thecontact surface slips with the expansion. Therefore, it seemsthat stiffness is influenced according to the thickness of theobject when the thickness is small but surface condition isinfluenced very little. Experimental results are correspondingto these discussions.

Area

Urethane gels (E = 0.038 MPa) with different area wereused as the object. The areas are 2, 4, 5, 7, and 9 mm square.As samples, a hard urethane gel (E = 0.484 MPa) was set

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Fig. 11 Relationship betweensensor output and thickness. aPressure around 50 mm3 Pa . bPressure gradient around80 mm3 Gb

2 4 6 85

10

15

20

Pres

sure

aro

und

50 m

m3

P a [

kPa]

Thickness of object [mm]

Acrylic baseGel base

2 4 6 8

0.1

0.6

1.1

Pres

sure

gra

dien

t aro

und

80 m

m3

Gb

[kP

a/m

m ]

Thickness of object [mm]

Acrylic baseGel base

3

(a) (b)

Object urethane gel

Hard urethane gelL

L=0mm L=2mm L=4mm

L=5mm L=7mm L=9mm L=20mm

Fig. 12 Samples used in experiments on area

around each object gel. The size of the samples is 20 × 20 mmand its thickness is 15 mm. In addition, the hard gel with thesame size as the above-mentioned samples was used as a sam-ple. It means the case without the object gel (0 mm square).Moreover, the object gel with the same size as the above sam-ples was used as a sample. It means the case of 20 mm square.Figure 12 shows photograph and geometry of the samples.

Figure 13 shows typical outer pressures on some samples.Figure 14a and b show Pa and Gb extracted from each outerpressure. Gray circle plots indicate each output, and blackline shows average of the sensor outputs on each sample. Itcan be seen that outer pressure, Pa , and Gb change largelyunder the condition of small area (less than 4 mm square).From Fig. 14a and b, Pa and Gb change very little under thecondition of the larger area.

Changes of the sensor outputs are explicitly differentaround 4 mm. It seems that these results are attributed tothe contact area of the balloon since the balloon touches the

0 50 100

0

20

40

Volume [mm3]

Out

er p

ress

ure

[kPa

]

L = 0mm

L = 2mm

L = 20mm

L = 4mm

L = 9mm

Fig. 13 Typical outer pressure obtained on samples under conditionof different area

object in the sensing locally as mentioned earlier. Here, thediameter of the contact area might be around 4 mm inthe expansion since the diameter of the balloon is 4 mm andthe expansion is not large in the sensing.

Discussions

From the above-mentioned results and discussions, it can bedescribed that the sensor can evaluate the object relativelyunder the same boundary condition, but it requires enoughthickness and area to evaluate absolutely. Especially, in theinfluence of the thickness, it is necessary to carry out theoret-ical analysis to know the range of the influence. The influenceof the area might be determined according to the propertiesof the sensor (contact area of the balloon), but also theoret-ical analysis is necessary to know in more detail. In future

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Fig. 14 Relationship betweensensor output and area.a Pressure around 50 mm3 Pa .b Pressure gradient around80 mm3Gb

0 10 205

10

15

20

Pres

sure

aro

und

50 m

m3

P a [

kPa]

Length of object [mm]

0 10 20

0.1

0.6

1.1

Pres

sure

gra

dien

t aro

und

80 m

m3

Gb

[kP

a/m

m ]

Length of object [mm]

3

(a) (b)

Fig. 15 Photographs in experiment on porcine ex vivo brain

works, we will theoretically analyze the balloon expansionand the capability of the sensor using FEM analysis.

Experiments on porcine ex vivo brain

Mechanical differences between healthy tissue and tumortissue have not been quantitatively revealed yet. It is empir-ically known that tumor is different from healthy tissue intactile sensations. Concerning stiffness, mechanical proper-ties of healthy tissue of brain have been studied [18,19] andon some organs, stiffness of tumor have been compared withhealthy tissue [20,21]. In this paper, as the first step for braintumor diagnosis, we tried discrimination between white mat-ter and gray matter of healthy brain tissue. It is empiricallyknown that these have different tactile sensations.

We carried out measurements on a porcine ex vivo brain.Photographs of the brain and the measurement are presentedin Fig. 15. The weight of the brain was about 100 g. The por-cine brain was collected from a slaughter house. It was notfrozen at any time. It stored in the refrigerator after slaugh-tering and transportation of the brain took one night. Then, itreturned to room temperature in the laboratory. As soon as thebrain became at room temperature, it was cut by a scalpel asshown in Fig. 15, and measurements were conducted. Totaltime duration of the measurements was about 30 minutes.

0 50 100

0

5

10

Volume [mm3]

Out

er p

ress

ure

[kPa

]

White matterGray matter

Fig. 16 All outer pressure obtained on white matter and gray matter

During the measurements, the surface of the brain was keptto be wet with water.

Here, the load condition of 0.51 N is so large that the sen-sor probe might give damages to the brain. The brain is softerthan the used gels. Consequently, we used the constant load-ing mechanism with the small load of 0.10 N. Latex rubbersheets were used instead of the silicone rubber sheets. Themeasurements were done at three points in white matter andthree points in gray matter. These measurement points werein the cross-section cut with the scalpel and they had planesurface. The indentation made by the balloon on brain tissuewas about 1 mm.

Figure 16 shows all obtained outer pressures. We can seeouter pressure is different between white matter and graymatter. Outer pressure on white matter is large in the begin-ning of the expansion and outer pressure on gray matterincreases largely in the late expansion. It was confirmed thatthe sensor can discriminate between white matter and graymatter. In future work, we will investigate mechanical proper-ties and tactile sensations on healthy tissue and tumor tissue,

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and we will analyze relationship between these characteris-tics and sensor output for precise measurement.

Conclusions

The aim of this study is to develop a biocompatible real-timesensing system to measure softness, sliminess, smoothness,and so on, and its application to brain tumor diagnosis. Wehave developed the sensor system based on active tactile sens-ing method using balloon expansion. The sensor system iscompact and the sensing time is about 2 s. The capabilityof the sensor and potential for brain tumor diagnosis werepresented. The measurements on soft samples with differentstiffness and surface condition were carried out, and influ-ences of the boundary condition on thickness and area ofthe object were discussed. It was found that the sensor candiscriminate soft samples with different stiffness and sur-face condition subject to influence by boundary conditions.It can be described that the sensor can evaluate the objectrelatively under the same boundary condition, but it requiresenough thickness and area to evaluate absolutely. Then, themeasurements on white matter and gray matter of porcineex vivo brain were conducted as the first step for brain tumordiagnosis. It was confirmed that the sensor can discriminatebetween white matter and gray matter. Although the sensorhas problems on the absolute evaluation of the object, theresults showed that the sensor can evaluate tactile informa-tion in stiffness and surface condition and it has potential forbrain tumor diagnosis.

In future works, analysis using FEM will be conductedto determine theoretically the capability of the sensor. Fur-thermore, mechanical properties of tumor tissue and healthytissue will be investigated, and the analysis of tumor con-ditions intraoperatively and the adequate application of thedeveloped sensor will be conducted. On the basis of results,we will improve and minimize the sensor for the clinicalapplication.

Acknowledgments This study was funded by NEDO P08006 “Intel-ligent Surgical Instruments Project”, Japan.

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