Sensors and Materials, Vol. 20, No. 7 (2008) 341–349
MYU Tokyo

S & M 0731

Thermal-Type Blood Flow Sensor
on Titanium Microneedle
Takashi Mineta, Yuki Tanahashi, Eiji Makino, Satoshi Toh1,
Takahiro Kawashima2 and Takayuki Shibata2
Graduate School of Science and Technology, Hirosaki University,
3 Bunkyo-cho, Hirosaki, Aomori 036-8561 Japan
1Graduate School of Medicine, Hirosaki University,
5 Zaifu-cho, Hirosaki, Aomori 036-8562, Japan
2Department of Production Systems Engineering, Toyohashi University of Technology,
1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441-8580, Japan(Received August 1, 2008; accepted November 18, 2008)
Key words:

thermal flow sensor, Ti needle, blood flow, electrochemical etching

In this paper, we describe a thermal-type blood flow microsensor that is inserted in a
blood vessel and used to detect the flow rate change caused by a thrombus.  The sensor
is based on a Ti microneedle, on which a Pt microheater and a Pt-Au microthermocouple
are formed.   The microneedle was fabricated from a Ti sheet of 50 μm thickness byelectrochemical etching and chemical etching in succession. The characteristics of thefabricated sensor were evaluated in flows of water and a viscous fluid in an artificial flow
channel 1.2 mm in diameter.  At a safe heating level (≤ 42°C), the sensor could measure
a flow velocity of up to several cm/s.   The sensor has high sensitivity in the flow
velocity range of 0–1 cm/s, indicating that it can detect the blood flow reduction due to athrombus in a vein.

1.

Introduction

Advances in microsurgery have improved the results of free tissue transfer operations
and organ graft operations such as liver transplantation. However, thrombi tend to form
near anastomotic sites in blood vessels. Tissue failure occurs in 5–10% of free tissuetransfer operations, mainly owing to thrombosis within the first 24 h after surgery.  Thus,
there is a need for adequate and reliable monitoring techniques.(1)To monitor the microvascular blood flow, an implantable ultrasonic Doppler probe(2)and electromagnetic flow measurement method(3) have been developed. However, theimplantable devices require additional surgery for removal from the body.
We have focused on the detection of blood flow in a small blood vessel with adiameter of about 1 mm, typical of blood vessels in fingers.   In our previous work,
we developed a mechanical pulsation sensor with a shape memory alloy film actuatorthat clips a blood vessel and detects the pulsation decrease caused by a thrombus.(4,5)*Corresponding author: e-mail: mineta@cc.hirosaki-u.ac.jp

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Sensors and Materials, Vol. 20, No. 7 (2008)

Although the sensor was sufficiently sensitive to detect blood flow obstruction in anartery, it was not easy to use for blood flow detection in a vein owing to the weakpulsation.  The typical blood flow rate in a 1 mm vein is estimated to be several ml/min.(6)A highly sensitive flow sensor is necessary to detect a decrease in blood flow in a vein.
The anemometric method is suitable for the sensitive measurement of fluid flow.(7)A wire-shaped anemometer has been used to measure the blood flow in the ascending
aorta.(8)In this paper, we describe the fabrication of a thermal-type blood flow microsensor
for the purpose of thrombus detection in a small-diameter vein. The sensor, based on aTi microneedle with a microheater and a microthermocouple, is inserted in a blood vessel
to detect the flow rate change caused by a thrombus.  The detection characteristics of thefabricated sensor are also described, focusing particularly on its sensitivity in the slowflow-rate range.

2.

Structure and Principle

The schematic of the blood flow sensor is shown in Fig. 1.  The sensor is inserted intoa fine blood vessel with an inner diameter of about 1 mm.  Figure 2 shows the structureand design of the sensor. The inserting needle is made of Ti, which has features of highbiocompatibility and corrosion resistance.  The needle is 1.2 mm long, 50 μm thick, and300 μm wide.  A microheater pattern of Pt film (100 nm thick, 30 μm wide) is formed
on a polyimide layer on the Ti needle. A Pt (100 nm)-Au (200 nm) micro-thermocouple
is laminated on the heater layer on top of a polyimide interlayer. The Pt-Au junctionof the microthermocouple is located at the center of the meandering pattern of the Ptmicroheater.
The needle-shaped sensor is heated using the microheater with a constant electric
current, and the temperature of the sensor decreases with increasing blood flow rate in
the vessel. This change in sensor temperature is measured with the laminated Pt-Au
thermocouple.

eFig. 1.   Schematic of the blood flow microsensor.

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Sensors and Materials, Vol. 20, No. 7 (2008)

13.0 mm
1.2 mmPt

4.2 mmAu

Pt

Ti (Thickness: 50 m)

300 µmMicro-heater
220 µm 530 µm

Second layer
240 µmFirst layer

Thermocouple

400 µm 160 µm

Micro-heater

Thickness 100 nm
Width 30 µm

Thermo-couple

Thickness
Pt: 100 nm
Au: 200 nm

Fig. 2.   Structure and design of the blood flow microsensor.

3.

Electrochemical Etching of Titanium Sheet

The etching processes available for Ti sheet were examined prior to the fabricationof the sensor.   Conventional
chemical etching in a hydrofluoric (HF) solution was not
Width 30 µm
suitable for the precision patterning of Ti sheet owing to the wide side etching. Our
preliminary experiment showed that the etch factor (etched depth/side etching) of
chemical etching of Ti in a HF solution was less than 0.5. Electrochemical etching issuitable for the patterning of a Ti substrate.(9) We have also studied the electrochemical
etching of Ti-Ni shape memory alloy with a high etch factor in LiCl-ethanol and LiClmethanol electrolyte solutions.(10) It has also been found that Ti was dissolved similarly
to Ti-Ni alloy in the electrochemical etching process.
To apply this electrochemical etching process to the patterning of Ti, basic etching
characteristics were studied in detail. A Ti (99.5%) sheet (20×20 mm2) of 200 μm
thickness was used. A negative photoresist (Tokyo Ohka, Ltd., OMR-83) pattern wasformed on the Ti sheet as an etching mask (line and space: 30 and 30 μm).  Constant dc
voltage was applied between the Ti substrate anode and the counter cathode of stainlesssteel plate (50×50 mm2) in an electrolyte solution of LiCl-methanol (1 mol/L, 25°C).
With an applied voltage of 10 V, the Ti sheet was etched successfully with an etchrate of about 15 μm/min and an etch factor of about 1.5.  As shown in Fig. 3(a), uniform
etching with a smooth etched surface occurred when the Ti sheet was not etched clearlythrough. However, electrochemical etching tended to proceed non-uniformly, as shown

(b)
(b)

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Sensors and Materials, Vol. 20, No. 7 (2008)

(a)
(a)
(c)

100 µm
(a)

(c)
(c)

(b)

(c)

Fig. 3. Etching of Ti sheet. (a) Electrochemical etching (half-etching), (b) electrochemicaletching (sheet-through etching), (c) electrochemical etching (40 μm, half-etching) and additional
chemical etching (10 μm, sheet-through).

in Fig. 3(b), because the electrolytic current distribution became non-uniform.(10) To
overcome this problem, the electrochemical etching was stopped before the Ti sheet was100 µmµm in HF solution was used to pierce the sheet (Fig. 3(b)).
pierced, then chemical100etching
The two-step etching produced uniform through-the-sheet etching with only small side
etching, as shown in Fig. 3(c).

4.

Fabrication of Blood Flow Sensor

The sequence of the fabrication of the blood flow sensor is shown in Fig. 4, as follows.  (a)A cold-rolled Ti sheet of 50 μm thickness was used.  An insulator layer of photosensitive
polyimide (Toray, UR-3100E) was formed by spin coating and baking (2 μm).  (b) A Pt
circuit pattern (100 nm) for the microheater was formed by sputter deposition and liftoff technique. (c) A second polyimide layer was formed as interlayer insulation. (d) A
microthermocouple of Pt (100 nm) and Au (200 nm) was formed by sputter depositionand lift-off technique.  (e) A third polyimide layer was formed for final passivation.  (f)After a negative photoresist (OMR-83) pattern was formed, the Ti substrate was halfetched to about 40 μm depth by electrochemical etching in a LiCl-methanol solution.  (g)The Ti sheet was etched through in an HF solution. (h) The needle-shaped sensor wasseparated from the Ti sheet after the photoresist was removed.
Figure 5 shows the completed sensor. After the fabrication process, Pt, Au, and Cu
lead wires (φ0.1 mm) were attached to the bonding pad of the thermocouple (Pt), thethermocouple (Au), and the heater, respectively, with a conductive resin.

5.

Sensing Characteristics

The microthermocouple was calibrated in a temperature-controlled oven. Figure
6 shows the relationship between the measured thermal-electromotive force and the345

Sensors and Materials, Vol. 20, No. 7 (2008)(a) Polyimide patterning
Polyimide (2 µm)(e) Polyimide insulation

Ti (50 µm)(b) Formation of Pt microheater
Pt (100 nm)(c) Polyimide insulation
2nd Polyimide (4 mm)(d) Formation ofPt-Au thermocouple
Pt (100 nm) Au (200 nm)(f) Electrochemical etching (40 µm)
Resist
3rd Polyimide(g) Sheet-through etching in
HF solution (10 µm)(h) Sensor separation(photoresist removal)

Fig. 4.   Fabrication flow of the blood flow sensor.

1 mm
240 mm 530 mm

450 mmMicroheater

Thermocouple
300 µmFig. 5.   Completed blood flow sensor.

atmospheric temperature in the oven. The fabricated Pt-Au microthermocouple had asensitivity of about 7.1 μV/°C in the temperature range of 25 to 50°C, which showed
good agreement with the reported value of 7.2 μV/°C.(11)346

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7.1 µV/( )Fig. 6. Calibration curve of the Pt-Au microthermocouple.

The sensing properties of the blood flow sensor were evaluated in a flow channelof acrylic resin (inner diameter: 1.2 mm). The experimental setup is shown in Fig. 7.
The sensor was inserted in the flow channel.  The flow rate of the fluid was varied bychanging the height of the supply vessel.
Figure 8 shows the change in the sensor temperature, obtained from the calibration
curve in Fig. 6, during heating in standing water (36°C).   The sensor temperature
increased rapidly after the heating current was turned on, but it was saturated withinseveral seconds. When the heating current of the microheater was 11 mA or lower, themaximum temperature of the sensor was controlled below 42°C, which is the permitted
upper limit temperature in vivo. Under this heating condition, the output voltage of thesensor was measured as the flow rate of the water (36°C) in the channel was varied.
Figure 9 shows the sensor temperature as a function of water flow rate.  The average
flow velocity (flow rate/cross-sectional area of the flow channel) is also indicated in Fig. 9.  
The sensor temperature changed markedly in the flow rate range of 0 to about 1 ml/min(flow velocity of about 1.5 cm/s).  The sensor temperature finally dropped to 36°C withincreasing flow rate.  However, the sensor is sensitive to flow rates in the range of 0 to 8ml/min.  There was no hysteresis when the flow rate increased and decreased.
In addition to the characterization in water flow, the sensor was also characterized
in a flow of a viscous fluid (50% aqueous solution of glycerol, 36°C) whose viscosity issimilar to that of blood (3.5 cP at 36°C).  The temperature of the sensor was controlled
below 42°C when the heating current was 10 mA or less.  The maximum temperature of
the sensor in the viscous solution was slightly higher than that in water.
Figure 10 shows the output voltage of the sensor as a function of flow rate of theviscous solution (36°C) for various constant heating currents.  The sensor temperaturesin standing fluid were 41.2 and 39.1°C, when the heating currents were 10 and 8 mA,

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Sensors and Materials, Vol. 20, No. 7 (2008)(Flow channel, diameter: 1.2 mm)
Constant
current supply
SensorFluidsupply
vesselDigitalvoltmeter

Flow channel(acrylic resin)(controlled at 36°C)

sensor(Flow ratemeasurement)

Fig. 7.   Setup for flow sensing evaluation.

Temperature (°C)

In vivo limit temperature

13 mA
12 mA
11 mA
10 mA
9 mA
8 mA

Time (s)Fig. 8. Sensor temperature change during heating with various heating currents (in standing 36°Cwater).

respectively.  Similar to the case of water flow, the sensor temperature fell markedly inthe small-flow-rate region, then saturated to the temperature of the fluid (36°C).  Witha heating current of 10 mA, the sensor is sensitive in the flow rate range of 0 to 7 ml/min.  This result indicates that the sensor can detect a slight decrease in blood flow fromthe normal value (several ml/min in vein), when the blood flow is slightly obstructedin early-stage thrombosis. On the other hand, when the heating current was 8 mA, thesensor temperature fell markedly and was saturated at the smaller flow rate of about

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43
42
41
40
39
38
37
36
35

0Average flow velocity (cm/s)
810624270

In vivo limit temperature

260250Flow velocity: Increase: Decrease

240
230
220

Fluid temperature

01Sensor output ( V)

Temperature (°C)

Sensors and Materials, Vol. 20, No. 7 (2008)

2 3 4 5 6Flow rate (ml/min)

7843424140
3938
3736
35

0

Average flow velocity (cm/s)
810624270

In vivo limit temperature

260
250

Heating current: 10 mA: 8 mA

240
230
220

Body temperature

012 3 4 5 6Flow rate (ml/min)

Sensor output ( V)

Temperature (°C)

Fig. 9.   Output of the sensor as a function of water flow rate (heating current: 11 mA).

78Fig. 10.   Sensor output as a function of viscous fluid flow rate with various heating currents.

1 ml/min.   The sensor cannot detect the thrombus formation until the blood flow issignificantly obstructed.  However, with a heating current of 8 mA, the maximum sensor
temperature did not rise to 42°C even if it was heated in air.  It can safely be used without
risk of overheating.

Sensors and Materials, Vol. 20, No. 7 (2008)

6.

349

Conclusion

In this study, a thermal-type blood flow sensor consisting of a Pt microheater anda Pt-Au microthermocouple on a Ti needle was designed and fabricated. A novel
electrochemical etching in LiCl-methanol was successfully used to fabricate the needle
from a Ti sheet.
Useful sensing properties of the fabricated sensor were demonstrated in flows ofwater and a viscous fluid of aqueous 50% glycerol, whose viscosity is similar to that ofblood, in an artificial flow channel with an inner diameter of 1.2 mm.  When heated totemperatures below 42°C, which is the upper limit temperature in vivo, the sensor wassufficiently sensitive to measure a flow velocity of less than 1 cm/s.  This indicates that
our sensor has the ability to detect the blood flow decrease due to the obstruction by athrombus in a vein.

References
1 H. Machens, P. Mailaender, B. Rieck and A. Berger: Microsurgery 15 (1994) 778.
2 G. M. Kind, R. F. Buntic, G. M. Buncke, T. M. Cooper, P. O. Siko and H. J. J. Buncke: Plast.
Reconstr. Surg. 1 (1998) 1268.
3 H. Nornes: Scand. J. Thorac. Cardiovasc. Surg. 10 (1976) 144.
4 T. Sugawara, K. Hirota, M. Watanabe, T. Mineta, E. Makino, S. Toh and T. Shibata: Sens.
Actuators, A 130/131 (2006) 461.
5 T. Mineta, N. Kida, S. Satoshi, E. Makino, T. Sugawara, S. Toh and T. Shibata: Sens.
Actuators, A 143 (2008) 14.
6 K. Hayashi et al. Eds.: Biomechanical Engineering: A First Course (Maruzen Co., Ltd.,Tokyo, 1997) p. 72 (in Japanese).
7 F. Mailly, A. Giani, R. Bonnet, P. Temple-Boyer, F. Pascal-Delannoy, A. Foucaran and A.
Boyer: Sens. Actuators, A 94 (2001) 32.
8 T. Yamaguchi, S. Kikkawa, T. Yoshikawa, K. Tanishita and M. Sugawara: J. Biomech. Eng.
105 (1983) 177.
9 Y. Ferri, O. Piotrowski, P.-F. Chauvy, C. Madore and D. Landolt: J. Micromech. Microeng.
11 (2001) 522.
10 T. Mineta: J. Micromech. Microeng. 14 (2004) 76.
11 A. W. Van Herwaarden and P. M. Sarro: Sens. Actuators 10 (1986) 321.