Measurement of Mechatronic Property of Biological Gel
with Micro-Vibrating Electrode at Ultrasonic Frequency
Daisuke Inoue, Shigehiro Hashimoto, Shuichi Mochizuki, Tomohiro Sahara,Hideo Kondo, Kenichi Yamasaki, Masahide Okada, Takayuki Kita, Teruo Miyazaki
Dept. Biomedical Engineering, Osaka Institute of Technology, Osaka, 535-8585, Japanhashimoto@bme.oit.ac.jp http://www.oit.ac.jp/bme/
andHajime Otani, Hiroji Imamura, Toshiji Iwasaka
Cardiovascular Center, Kansai Medical University, Moriguchi, JapanABSTRACT
A measurement system has been designed witha micro-vibrating electrode at ultrasonicfrequency to measure local impedance of
biological gel in vitro. The designed system
consists of two electrodes, where one of theelectrodes vibrates with a piezoelectric actuator.
The component of variation at impedance
between two electrodes with vibration of one
electrode is analyzed at the corresponding
spectrum.
The manufactured system wasapplied to measure impedance of aphysiological saline solution, a potassium
chloride solution, a dextran aqueous solution,
and an egg. The experimental results show thatthe designed system is effective to measure
local mechatronic property of biological gel.
Keywords: Biomeasurement, Mechatronic
Property, Micro-Vibrating Electrode, BiologicalGel, Ultrasonic Frequency, Viscosity

1. INTRODUCTION
Ultrasonic frequency, which is transmitted alonga straight line and has concentrated energy, hasbeen applied to several clinical treatments. The
transmission property has been used to measure
the inside structure in a biological body. Theenergy also has been used to destroy a hardmass in organs.
A biological body includes various electrolyte
solutions. Many studies have tried to estimate
the structure of the human body with electricimpedance [1]. Macroscopic studies about theimpedance of a biological body have been
applied to estimate a body fat index. Somestudies tried to analyze the structure of organs
with measurement of local electric impedance [2,3]. A methodology for electric impedance
measurement has also been studied on blood [4,5]. Also the local structure of biological tissue
might be estimated with local mechanical andelectrical properties.
To minimize the change of structure bymovement of ions with a direct current, analternating current is applied to measure electric
impedance of electrolyte solutions. To measure
local impedance, electrodes should beminiaturized, but their high impedance on theelectric circuit makes it difficult to detect theimpedance of a target object. Four-terminalmethod, which is usually applied to measureelectric resistance with separated terminals forvoltage and current, is not appropriate for local
measurement of a micro object, because of thelarger number of electrodes [6].
In the present study, a novel measurement
system has been designed with a microvibrating electrode to measure local impedanceof biological gel, and has been applied tomeasure mechatronic property of biological gel
in vitro.

2. METHODS

spectrum and the amplitude at the corresponding
frequency was calculated [7].
Measurement system
The electrode was made of a glass microcapillary filled with a physiological saline
solution. A platinum wire of 0.2 mm diameter
was inserted in the center of the capillary with a
0.94 mm inner diameter, and 1.2 mm outer
diameter, respectively (Fig. 2). One of theelectrodes was vibrated with a piezoelectricactuator (Fig. 3). The measurement system
consists of an oscillator, an amplifier, aspectrum analyzer, a phase-contrast optical
microscope (Fig. 4-6), a charge-coupled device
camera, a recorder, and a monitor.

Measurement principle
The impedance between two electrodes soaked
in an electrolyte solution varies with thedistance between them (Fig. 1). When thedistance oscillates sinusoidally with amechanically vibrating electrode, the impedance
oscillates, too.
The electric oscillation
corresponds to the variation of impedance of thelocal space, where the electrode is vibrating(between A and B in Fig. 1).
When the frequency of input electric alternating
voltage is different from the frequency of themechanical vibration of the electrode, the output
electric voltage signal fluctuates.
The
fluctuating voltage corresponds to theimpedance of dx.
The magnitude of thevariation of the voltage is the product of local
impedance and electric current, and does notdepend on the impedance of the other part of theelectric circuit. The variation was analyzed withAdx

Platinum wireCB

PhysiologicalsalineXFig. 1: Distance between two electrodes.
One electrode is fixed at point C and anotherelectrode is vibrating between A and B.

Fig. 2: Electrodes.

yVzx・・・External electrode・・・Internal electrode（GND）

Fig. 4: Electric circuit.

SpringMicromovement stage
Ruby ball
Piezoelectric element

Fig. 3: Piezoelectric actuator.

Input signal

Fig. 5: Measurement
microscope.

systemElectrode controller
Digital oscilloscope

Oscillator

Electrode vibration signal
Output signal

Input signal
Amplification circuit
Oscillator

Fig. 6: Measurement system.

withbetween the two electrodes (0.1 mm tipdiameter) was 0.3 mm. The impedance wasmeasured with an alternating electric source of
200 mV at 75 kHz.

Fig. 7: Measurement on egg.
Experiments
The manufactured system was applied tomeasure the impedance of a physiological saline
solution, a potassium chloride solution, adextran aqueous solution, and an egg.
In the experiment with a physiological saline
solution and 10 wt% potassium chloride
solution, one of the electrodes was mechanically
micro-vibrated in the solution of 15 mL in a 60mm diameter dish at 150 kHz-400 kHz. The
average distance between the two electrodes
was 1.3 mm. The impedance was measured
with an alternating electric source of 200 mV at700 kHz.
In the experiment with a dextran aqueoussolution, one of the electrodes was mechanicallyvibrated in the solution of 5 mL in a 30 mm
diameter dish at 30 kHz. The average distanceIn the experiment with an egg, one of theelectrodes was mechanically micro-vibrated in
the yolk or in the albumen of an egg containedin a 60 mm diameter dish at 200 kHz-350 kHz(Fig. 7). The average distance between the twoelectrodes was 15 mm. The impedance wasmeasured with the alternating electric source of
200 mV at 700 kHz.
The spectrum of the voltage was analyzed ineach test.
Viscosity of the solution wasmeasured with a cone and plate type viscometer.
Every test has been performed at 25 degrees
centigrade.

3. RESULTS AND DISCUSSION
Figure 8 exemplifies input and output signals,
and power spectra of the output signal, where
both of spectra at the input signal frequency andat the mechanically vibrating frequency areindicated with arrows. Figure 9 shows theimpedance of saline and potassium chloride
solutions as a function of vibration frequency.
40 dBV
Input

Vvibration-40 dBVFig. 8: Input (upper) and output signals (left); power spectrum with 50 Hz/div forhorizontal axis (right).

The output signal of the experimental system isindependent from the impedance of electrodes
and the electrical circuit. The output signal of
the system depends on several factors: theconcentration of sodium chloride, and thevariation of electrolytes [7-10]. It does notdepend on viscosity at lower frequency (<200Hz), and on frequency at lower viscosity (<0.01Pa s) [8].

Impedance (kΩ)

110wt% Potassium chloridesolution
Physiological saline

0.8
0.6
0.4

Impedance (kΩ)

2.5
21.5
10.5
005

1015202530time (min)Fig. 10: Impedance of dextran aqueoussolution as a function of time.

4Impedance (kΩ)

The result shows that impedance of 10 wt%
potassium chloride solution is lower than that ofsaline solution, and tends to decrease withfrequency. Figure 10 shows the impedance of
dextran aqueous solution as a function of time.
The experimental data show that impedance of
dextran aqueous solution tends to increase withtime, which indicates that the property of
dextran solution changes with time, whereimpedance of physiological saline solution did
not change with time. Figure 11 shows theimpedance of albumen and yolk as a function ofvibration frequency. The figure shows that theimpedance of yolk is higher than that ofalbumen and that the impedance of yolk
decreases with frequency. The experimental
results show that coefficients of viscosity ofphysiological saline solution, 10 wt% potassium
chloride solution, dextran aqueous solution,albumen and yolk are 0.89 mPa s, 0.90 mPa s,
36 mPa s, 0.44 Pa s and 0.92 Pa s, respectively.

AlbumenYolk

3210200

250300350

Electrode vibration frequency (kHz)Fig. 11: Impedance of albumen and yolk
as a function of vibration frequency.
Four terminals method, in which the voltage andcurrent terminals are separated, is useful tominimize the fraction of impedance of anelectronic circuit to measure the voltage. The
numbers of electrodes decreases to one in thepresent system, which has the advantage ofbeeing easily accepted to a local area.

0.24. CONCLUSION

0150200250

300350400450

Electrode vibration frequency (kHz)Fig. 9: Impedance of saline andpotassium chloride solutions as afunction of vibration frequency.

The experimental data show that the designed
system is effective to measure the local
mechatronic property of biological gel at
ultrasonic frequency.

5. ACKNOWLEDGMENT
This work was supported in part by a Grant-inAid for Scientific Research and Academic
Frontier from the Japanese Ministry ofEducation, Culture, Sports, Science andTechnology.

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