A SEAMLESSLY INTEGRATED MICROFLUIDIC PRESSURE SENSOR

BASED ON AN IONIC LIQUID ELECTROFLUIDIC CIRCUIT Chueh-Yu Wu, Wei-Hao Liao, and Yi-Chung Tung

Research Center for Applied Sciences, Academia Sinica, Taipei, TAIWAN

ABSTRACT A novel pressure sensor with electrical readout based on

ionic liquid (IL) electrofluidic circuit is reported in this paper.

The pressure measurement is achieved by measuring the

electrical property variation of the circuit induced by the

pressure inside the microfluidic devices. The sensor

integrated microfluidic device is made of poly-

dimethylsiloxane (PDMS), and can be fabricated using the

well-developed multilayer soft lithography (MSL) without

additional microfabrication processes. Therefore, the

pressure sensor is fully disposable, and can be seamlessly

integrated into PDMS microfluidic devices. Moreover, an IL

electrofluidic Wheatstone bridge is designed to provide the

device linear output, great long-term and thermal stability. In

this paper, the sensor performance characterization using

pressurized gas and liquid flowing in the microfluidic

channel have been conducted. The experimental results

demonstrate the advantages of the device. The developed

pressure sensor has great potentials for the development of

next generation sensor-integrated microfluidic systems.

INTRODUCTION

Pressure sensors are key components in various

microfluidic systems, such as BioMEMS and micro-total

analysis systems (µTAS) [1]. An elastomeric material,

polydimethylsiloxane (PDMS), has been broadly exploited

for microfluidic device fabrication due to its optical

transparency and great manufacturability. Therefore,

pressure sensors capable of being seamlessly integrated into

the PDMS microfluidic devices are highly desired. Several

integrated pressure-sensing schemes have been developed in

the previous research; for instance: a pressure sensor based

on image-based analysis of PDMS membranes. The

measurement is achieved using modern optical technologies

with high sensitivity [2]. A conductive PDMS-based

pressure sensor, which provides an electrical interface, has

been developed for better integration with PDMS device [3].

However, the existing devices still suffer from drawbacks

such as, massive instrument requirements, complex signal

analysis, and tedious assembly, which retard their practical

exploitation in microfluidic devices.

This paper presents a novel fully disposable and

cost-effective pressure sensor with electrical readout. The

developed sensor is constructed using an ionic liquid

(IL)-based PDMS electrofluidic circuit, and it can be directly

fabricated on top of various microfluidic devices. By using

IL, which is electrical conductive and thermally stable, the

device possesses excellent long-term stability. Moreover, the

integrated device can be simply fabricated by well-developed

multilayer soft lithography (MSL) technique without further

sophisticated cleanroom processes [4].

DEVICE DESIGN

The pressure sensor integrated microfluidic device is

composed of a glass substrate and two PDMS microfluidic

layers: a pressure sensing electrofluidic circuit layer and a

fluidic channel layer, which are separated by a deformable

PDMS membrane with thickness of 100 µm as shown in

Figure 1. On the electrofluidic circuit layer, four identical

sets of electrofluidic resistors are arranged as a Wheatstone

bridge circuit. On the fluidic channel layer, a microfluidic

channel with a meander section is designed to provide fluidic

resistance for demonstration. In addition, a short branch

channel connected to a pressure transduction hole is designed

on the fluidic channel layer for pressure measurement. A 4

mm-diameter hole for pressure transduction is punched and

aligned to one of the electrofluidic resistor for pressure

sensing.

The operation principle of the pressure sensor is

measuring the electrical property variation of the

electrofluidic circuit induced by the pressure inside the

microfluidic devices. While the pressure inside the

microfluidic channel in the fluidic channel layer is increased,

it deforms the membrane sandwiched between two PDMS

layers according to the applied pressure. The membrane

deformation further alters the cross-sectional area of the

pressure sensing electrofluidic resistor, and causes the

resulted electrical resistance change. As a result, the pressure

inside the microfluidic channel can be estimated by the

resistance variation.

In order to precisely measure the electrical resistance

change, an electrofluidic Wheatstone bridge is designed to

transfer the resistance variation to a voltage signal.

Wheatstone bridge has been widely used in many detection

schemes, such as mechanical strain measurement [5], due to

its thermal stability. Fig. 1 illustrates the photo a Wheatstone

bridge constructed by IL electrofluidic circuit and its

equivalent electrical configuration. By Kirchhoff’s circuit

laws, the gate voltage, VG, across the bridge can be calculated

by:

1

4 1

1

2G S

RV V

R R

= − ⋅

+

(1)

where R1~R4 is the electrical resistance of each branch. For

the design with initially identical resistors, the gate voltage

approaches zero. Specifically, the resistor R4 is exploited for

pressure sensing, and its resistance value depends on the

material property and the channel geometry. According to

Ohm’s law, the resistance value is inversely

978-1-4244-9634-1/11/$26.00 ©2011 IEEE 1087 MEMS 2011, Cancun, MEXICO, January 23-27, 2011

proportional to the cross-sectional area:

c

c

A

LR ρ=4

(2)

where ρ is the resistivity, Lc is the length of the channel, and

Ac is the cross-sectional area of the channel. While the

pressure in the fluidic channel increases, the membrane will

deform upward and decrease the cross-section area of the

resistor R4. As a result, the resistance value of R4 will

increase correspondingly:

A

A

LR

c

c ∆⋅

=∆

24

ρ (3)

where ∆A is the cross-sectional area reduction. Assuming the

deformation ∆A is small, the resistance variation ∆R4 will

also be small. The gate voltage can be approximated by the

first order of the Taylor expansion:

4

14

SG

VV R

R

∆ = − ⋅∆

(4)

Thus, as ∆R4 is substituted by the result of Ohm’s law, the

gate voltage change can be estimated by linear relationship:

42

14

S cG

c

V LV A

R A

ρ ∆ = − ⋅ ⋅∆

(5)

As a result, applied pressure affects the resistance value, and

further causes the linear gate voltage variation in the

Wheatstone bridge circuit.

Besides the linear output, the integrated IL electrofluidic

Wheatstone bridge circuit provides the device two essential

advantages: long-term and thermal stability. The long-term

stability is resulted from the extremely low vapor pressure

and hygroscopic nature of IL. Therefore, the IL

electrofluidic circuit does not suffer the variation due to the

evaporation of IL, even the volume of IL (on the order of tens

of µl) is small. The thermal stability is resulted from the

utilization of the integrated Wheatstone bridge circuit. The

bridge circuit is constructed by the four identical resistors,

and their positions are close to each other. As a result, the

temperature fluctuation in PDMS will increase or decrease

the resistance values by the same order. Since, the gate

voltage in the Wheatstone bridge is determined by the ratios

between resistances, these identically changed values will be

cancelled out. Consequently, the gate voltage is insensitive

to the time and temperature fluctuation, which provides the

developed pressure sensor stable performances.

DEVICE FABRICATION Figure 2 shows the fabrication steps of the entire

microfluidic device with an integrated pressure sensor using

the well-developed MSL technique. The PDMS (Sylgard

184, Dow Corning, Midland, MI) used in the entire

fabrication process was prepared by PDMS precursor with

10:1 v/v of base to curing agent ratio. The patterns on the two

layers, electrofluidic circuit layer and fluidic channel layer,

were replicated from molds fabricated using a negative tone

photoresist, SU-8, patterned by conventional

photolithography. The electrofluidic circuit layer was made

by PDMS cast against a silicon wafer mold silanized by

1H,1H,2H,2H-Perfluorooctyl Trichlorosilane (97%)

(L16606, Alfa Aesar, Ward Hill, MA), and the PDMS was

cured in a 65oC oven for more than 4 hours. The cured

PDMS layer was peeled from the wafer, and punched with

four holes for IL injection and electrical connections. The

fabricated electrofluidic circuit layer was then bonded onto a

PDMS membrane using oxygen plasma surface treatments

(90 W, 40 sec). The membrane had thickness of 100 µm and

was made by spinning PDMS precursor onto a silanized

silicon wafer. The fluidic channel layer was also made by the

same method as the electrofluidic circuit layer. The fluidic

channel layer was punched by one hole for pressure

transduction before the bonding. The assembled

electrofluidic layer with the PDMS membrane was bonded to

the fluidic channel layer with the pressure transduction hole

aligned to pressure sensing resistor, R4. After assembling the

two PDMS layers, the inlet and outlet to access the

microfluidic channel in the fluidic channel layer were

punched through the entire device. Finally, the entire PDMS

structure was bounded to a glass substrate.

After the device fabrication, ionic liquid

(1-ethyl-3-methylimidazolium dicyanamide) was injected

into the electrofluidic circuit channels. The inlet of the

fluidic channel is then connected to a syringe pump for

fluidic actuation. The electrical interconnections to the

electrofluidic circuit were achieved by inserting stainless

steel blunt needles with electrical wires. Because of the

hygroscopic nature of the IL, it can be retained in

Figure 1. (A) Photo of the fabricated PDMS microfluidic

device with a seamlessly integrated pressure sensor based

on an ionic liquid electrofluidic circuit. Blue dye presents

the electrofluidic circuit layer (top), and red dye represents

the fluidic channel layer (bottom) with a circular pressure

transduction hole. (B) Close view of the Wheatstone bridge

circuit constructed using the ionic liquid electrofluidic

circuit. R1~R4 are the electrofluidic resistors, and R4 is the

pressure sensing one. A voltage signal VS is applied through

interconnections A and C, while the gate voltage VG is

monitored at interconnections B and D. (C) The equivalent

electrical circuit of the ionic liquid-filled microfluidic

channels constructed for pressure sensing.

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the electrofluidic channel without significant evaporation [6].

DEVICE CHARACTERIZATION

Figure 3 illustrates the experimental setup for the device

characterization. We utilized a personal computer with a

data acquisition (DAQ) system (PCIe-6363, National

Instruments, Austin, TX) and LabVIEW programs (Version

2009, National Instruments) for the syringe pump control

(Fusion 200, Chemyx, Stafford, TX), and the electrical signal

application and detection. The device characterization can be

categorized into three major parts: 1. Electrofluidic circuit

current-voltage (I-V) curve measurement, 2. Pressure sensor

calibration using pressurized gas, and 3. Liquid pressure

measurement using the developed pressure sensor.

Electrofluidic circuit I-V curve measurement. To

investigate the electrical characteristics of the constructed

electrofluidic Wheatstone bridge circuit, an I-V curve was

measured across the entire circuit (terminals A and C) in the

Figure 1C. In order to eliminate the influence of the pressure

for the circuit characterization, an electrofluidic layer

directly bonded on a glass substrate was utilized to measure

the I-V curve. The device was connected to a shunt resistor

(1 MΩ) in series for current measurement. The LabVIEW

program controlled DAQ system was exploited to apply

voltages through the entire circuit and measure the voltage

across the bridge circuit and the passing current. The current

was calculated by measuring the voltage across the shunt

resistor and dividing it by the resistance value.

Figure 4 shows the measured I-V curve of the electrofluidic

circuit, and the result of a linear regression analysis. The

calculated electrical resistance of the entire circuit (or each

resistor) is approximately 1.787 MΩ. The excellent linearity

(correlation coefficient R2 > 0.998) of the I-V curve suggests

the pure resistor behavior within the applied voltage range

with minimal parasitic capacitance or inductance effects.

Consequently, the aforementioned theoretical derivation for

the Wheatstone bridge using pure resistors can be expected to

correctly predict the device performance.

Pressure sensor calibration using pressurized gas. In

order to calibrate the sensor performance, a dead-end

microfluidic channel in the fluidic channel layer was

pressurized using nitrogen gas. The gas was supplied by a

nitrogen gas cylinder with a pressure regulator. During the

calibration, the DAQ system continuously recorded the gate

voltage while the gas pressure was regulated from 0 psi to 30

psi with the increment of 5 psi. Every pressure level was

hold for 2 minutes, and the gate voltage in the second minute

was averaged to present the gate voltage at each pressure.

Figure 5 shows the calibration results. The linear regression

analysis was also applied to examine the relationship

between the gate voltage change and pressure, and the

resulted correlation coefficient R2 was 0.994. The high

correlation coefficient suggests the highly linear

performance of the pressure sensor in the applied pressure

range as derived in equation (5). It also shows that 1 psi

increase results in 8.45 mV gate voltage decrease.

Liquid pressure measurement using the developed

pressure sensor. The developed device was further

exploited for liquid pressure measurement. A LabVIEW

program was coded to record the gate voltage using the DAQ

system, and control the syringe pump through a RS-232

serial interface simultaneously. The flow rate was set from 0

to 100 µL/min with the increment of 10 µL/min, and each

flow rate was kept for 900 seconds for reaching the steady

states. Figure 6 shows the measurement results while

injecting a fluid (water) into the device with controlled flow

rates. The minimum flow rate tested was 10 µl/min, and this

flow yielded a pressure of approximate 0.63 psi at the

upstream of the channel according to the aforementioned

calibration results. The result verifies that the pressure

sensor has the resolution of at least 0.63 psi or 10 µl/min in

the conventional microfluidic channel. The linear regression

analysis was also applied to examine the relationship

between flow rate and gate voltage change, which was

calculated by averaging the last 5 minutes data for each flow

rate. The result also showed high correlation coefficient, R2

=0.993, meaning the output gate voltage variation

Figure 3. Schematic of the experimental setup for the device

characterization.

Figure 2. The fabrication process for the multi-layer PDMS

microfluidic device with a seamlessly integrated pressure

sensor.

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was linearly proportional to the flow rates of the fluid. The

gate voltage is also proportional to the pressure; thus, the

pressure is proportional to the flow rate, which agrees with

the theoretical prediction of laminar flow inside the fluid

channel. In summary, the experimental results confirmed

that the developed IL electrofluidic circuit-based presser

sensor works as a linear measurement system due to the

integrated Wheatstone bridge circuit. Further, the developed

device can be seamlessly integrated into broadly adapted

PDMS microfluidic systems without sophisticated

fabrication and assembly processes.

CONCLUSION

This paper presents a novel pressure sensor based on IL

electrofluidic circuit. The simple configuration makes the

device capable of being seamlessly integrated to wide

varieties of PDMS microfluidic devices. The experimental

results demonstrate that IL-filled microfluidic channels can

be utilized as electrical resistors to construct functional

circuits, and an electrofluidic Wheatstone bridge circuit has

been designed to construct the pressure sensor. In the

pressure sensor performance characterization, the calibration

results show that the gate voltage is linear proportional to the

applied pressure with sensitivity of 8.45 mV/psi and the

pressure as small as 2.5 psi can be easily detected.

Furthermore, in the liquid pressure measurement, the gate

voltage is also linear to the flow rate of water, and the

pressure sensor has been verified to have the resolution of

0.63 psi pressure drop in the liquid phase. The experimental

results successfully demonstrate that the developed pressure

sensor with the IL Wheatstone bridge circuit provides a linear

measurement system with stable and sensitive electrical

readout. In conclusion, the high performance and great

characteristics make the developed pressure sensor being

promising for building next-generation integrated

microfluidic systems.

ACKNOWLEDGMENT

This work was supported by National Science Council in

Taiwan (99-2218-E-001-003), and the Academia Sinica

Research Program on Nanoscience and Nanotechnology.

REFERENCES

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[2] K. Chung et al., “Multiplex Pressure Mmeasurement in

Microsystems Using Volume Displacement of Particle

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[3] L. Wang et al., “Polydimethylsiloxane-integratable

Micropressure Sensor for Microfluidic Chips”,

Biomicrofluidics, vol. 3, pp. 034105-+, 2009.

[4] M. A. Unger et al., “Monolithic Microfabricated Valves

and Pumps by Multilayer Soft Lithography”, Science, vol.

228, pp. 113-116, 2000. [5] E. O. Doebelin, Measurement Systems - Application and

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Edn., Ch. 10, pp837-843,

2003. [6] W. Gu et al., “Multiplexed Hydraulic Valve Actuation

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Figure 4. I-V curve measurement results of the ionic liquid

electrofluidic Wheatstone bridge. The calculated electrical

resistance of the entire circuit (or each resistor) is

approximately 1.787 MΩ.

Figure 5. Calibration results of the device using pressurized

nitrogen gas with pressure range of 0 ~ 30 psi.

Figure 6. Measurement results of the device while injecting

water into the fluidic channel with flow rate range of 0 ~ 100

µl/min.

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