JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.13, NO.4, AUGUST, 2013 http://dx.doi.org/10.5573/JSTS.2013.13.4.355

Manuscript received Dec. 25, 2012; accepted May. 7, 2013 * School of Electronic Engineering, College of Information Technology,

Soongsil University, 511 Sangdo-Dong, Dongjak-Gu, 156-743, Republic

of Korea ** Nano-Bio Research Center, Korea Institute of Science and Technology,

39-1 Hawolgok-Dong, Seongbuk-Gu, Seoul, 136-791, Republic of Korea *** School of Electrical Engineering, College of Engineering, Soongsil

University, 511 Sangdo-Dong, Dongjak-Gu, 156-743, Republic of Korea

E-mail : pck77@ssu.ac.kr

A Wireless Intraocular Pressure Sensor with Variable

Inductance Using a Ferrite Material

Byungjoo Kang*, Hoyong Hwang

*, Soo Hyun Lee

**, Ji Yoon Kang

**, Joung-Hu Park

***, Chulhun Seo

*,

and Changkun Park*

Abstract—A wireless intraocular (IOP) pressure

sensor based on micro electro mechanical system

(MEMS) technology is proposed. The proposed IOP

sensor uses variable inductance according to the

external pressure. The proposed sensor is composed

of two flexible membranes: a ferrite bottom part, an

inductor, and a capacitor. The inductance of the

sensor varies according to the external pressure. The

resonance frequency of the sensor is also varied, and

this frequency is detected using an external coil. The

external coil is designed with an FR-4 printed circuit

board. The feasibility of the proposed sensor structure

using variable inductance to detect the external

pressure is successfully demonstrated.

Index Terms—Ferrite, IOP sensor, resonance frequency,

variable inductance

I. INTRODUCTION

Techniques in health-care are becoming more

important as living standards are increasing. Wireless

engineering has been adapted to many health-care

techniques because wireless engineering can provide

convenience for the detection of bionic signals [1-5]. For

example, radar engineering can provide a method of non-

contact heartbeat detection.

Fig. 1 shows a simplified version of a wireless IOP

monitoring system. An IOP sensor located in the eyeball

detects the intraocular pressure and the pressure is

converted to the impedance variance of the IOP sensor.

The varied impedance of the sensor is detected by the

external coil and the electrical signal from the coil is

interpreted by the impedance analyzer or by an external

reader. Finally, a mechanical valve located in the eyeball

controls the IOP according to the information from the

external reader. The design of the IOP sensor is therefore

the key technology for this type of wireless IOP

monitoring system. The IOP sensor needs to have the

ability to use the variation of the IOP pressure to vary the

electrical parameters.

In this study, a sensor for a wireless intraocular

pressure (IOP) detection system is proposed and

designed. Recently, IOP sensors that use the capacitance

as it varies with the pressure have been intensely studied

[6-8]. However, IOP sensors using variable capacitance

require complicated micro electro mechanical system

Fig. 1. Simplified wireless IOP sensor monitoring system.

356 BYUNGJOO KANG et al : A WIRELESS INTRAOCULAR PRESSURE SENSOR WITH VARIABLE INDUCTANCE USING …

(MEMS) processes. Although IOP sensors using variable

inductance were proposed in previous works [9], the

complexity of the MEMS processes in those works was

not moderated.

We propose an IOP sensor that uses variable

inductance with a simple MEMS process. We achieve

variable inductance using a variable distance between the

ferrite material used in the design and the pattern of an

inductor according to the external pressure. Additionally,

we propose a method to improve the pressure sensitivity

at high resolution. This method utilizes two flexible

membranes in the ferrite material and in the inductor

pattern.

Finally, the proposed IOP sensor is designed using

simple MEMS technology. It is composed of flexible

membranes, a ferrite bottom part, an inductor, and a

capacitor. Variation of the external pressure of the sensor

changes the values of the inductance of the inductor

pattern. If the external pressure increases, the distance

between the inductor pattern and the ferrite material

decreases and the resonance frequency of the sensor

decreases. Similarly, if the external pressure decreases,

the resonance frequency of the sensor increases. From

the detected resonance frequency of the sensor using an

external coil, the IOP pressure can be detected.

II. PROPOSED IOP SENSOR AND OPERATION

PRINCIPLE

We propose an IOP sensor in which the inductance is

designed to vary according to the external pressure. The

proposed IOP sensor is composed of a spiral inductor, a

silicon substrate, two flexible membranes, the afore-

mentioned ferrite material, and a metal-insulator-metal

(MIM) capacitor. It is implemented using MEMS

technology, as shown in Fig. 2. The inductor pattern and

the ferrite material are attached to the flexible

membranes. The inside of the IOP sensor is designed to

be isolated perfectly from the outside of the sensor. The

MEMS process steps for the proposed IOP sensor, which

uses variable inductance according to the external

pressure, is simpler than that of an IOP sensor that uses

variable capacitance.

Fig. 3(a) shows a cross-section of the proposed IOP

sensor. The resonance frequency of the IOP sensor is

determined using the inductance and the parasitic

capacitance. If the external pressure of the IOP sensor

increases, the distance between the spiral inductor and

the ferrite bottom part, located on the flexible membranes,

decreases, as shown in Fig. 3(b). The magnetic flux

generated by the AC current of the spiral inductor is then

focused on the ferrite, causing the self-inductance of the

IOP sensor to increase. With ferrite as the bottom

material, the increase in the external pressure of the IOP

sensor shortens the distance between the spiral inductor

and the bottom ferrite, finally decreasing the resonance

frequency. In the proposed IOP sensor, we use two

flexible membranes to increase the resolution.

Fig. 4 provides a simplified process flowchart of the

investigated IOP sensor. The flexible membrane and the

inductor pattern are implemented using polyimide and

copper, respectively. A high frequency NiZn ferrite is

attached on the bottom flexible membrane as shown in

Fig. 4.

Fig. 5 shows the simplified equivalent circuit of the

proposed IOP sensor. The R of the circuit denotes the

parasitic resistance of the spiral inductor. In addition, CS

is the parasitic capacitance between the adjacent metals

of the inductor, CM denotes the MIM capacitor, and L is

the inductance of the spiral inductor. Thus, the resonance

Fig. 2. Layers of the IOP sensor.

(a) (b)

Fig. 3. Cross-section of the proposed IOP sensor (a) without

external pressure, (b) with external pressure.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.13, NO.4, AUGUST, 2013 357

frequency Fr is

( )MS

rCCL

F+

=π2

1. (1)

III. IMPLEMENTATION OF THE PROPOSED

SENSOR

1. Implemented IOP Sensor

Fig. 6 shows the implemented IOP sensors. The size of

the bottom ferrite is 3 × 3 mm2. The metal width is 50µm,

the space between adjacent metal lines is 30µm, and the

number of turns is 23. The thickness of the metal line is

approximately 10µm. The determined design parameters

of the metal width and the thickness are intended to

maximize the phase-quality factor. The number of turns

and the space are determined to obtain the optimized

resonance frequency. The target resonance frequency and

the inductance are 10 MHz and 3µH, respectively.

The measured inductance and the resistance are 4.0µH

and 5.4 Ω, respectively. The capacitance is designed to

be 100 pF. The measured resonance frequency is 10.8

MHz. The inductance, resistance, and capacitance are

measured using a vector network analyzer.

2. Measurement Setup

Fig. 7 shows the wireless IOP sensor measurement

setup. The measurement setup is composed of an

impedance analyzer, air compressor, pressure controller,

and jig. We use an Agilent impedance analyzer to

measure the phase and magnitude variation of the

impedance of the sensor. An external coil is implemented

using a two-layer FR-4 printed circuit board. The

distance between the external coil and the sensor is fixed

at approximately 5 mm.

IV. MEASUREMENT RESULTS

We measured the impedance variation according to the

external pressure of the IOP sensor. In this work, the

Fig. 4. Simplified process flowchart of the IOP sensor.

Fig. 5. Simplified equivalent model of the IOP sensor.

Fig. 6. Photograph of the implemented IOP sensor.

Fig. 7. Measurement system to detect the impedance variation

according to the external pressure.

358 BYUNGJOO KANG et al : A WIRELESS INTRAOCULAR PRESSURE SENSOR WITH VARIABLE INDUCTANCE USING …

distance between the external coil and the sensor is fixed

at approximately 5 mm. The external pressure applied to

the IOP sensor ranges from 0 mmHg to 90 mmHg. For

this measurement, we used an air pressure controller.

Fig. 8(a) shows the measured results according to the

external pressure of the IOP sensor without a cornea. The

resolution frequencies according to the external pressure

are summarized in Fig. 8(b). The resonance frequency of

the IOP sensor under 0 mmHg pressure was found to be

10.8 MHz. If the external pressure increases, the

resonance frequency decreases because the inductance of

the sensor increases, as predicted in the previous section.

The resonance frequency of the sensor does not vary if

the external pressure is higher than 70 mmHg. At a

pressure of 70 mmHg, the ferrite bottom material and the

upper flexible membrane of the sensor adhere to each

other. The dynamic range of the designed sensor is

therefore 0 mmHg to 70 mmHg. The dynamic range of

the sensor is determined from the flexibility of the

membranes. The flexibility can be controlled using the

thickness of the flexible membrane.

A measurement similar to that shown in Fig. 8 is

performed with a cornea located between an external coil

and the designed IOP sensor, as shown in Fig. 9(a). The

resolution frequencies according to the external pressure

are summarized in Fig. 9(b).

V. DISCUSSION

The quality factor of the measured results shown in

Fig. 9(a) is lower than that shown in Fig. 8(a) because the

electrolyte of the cornea degrades the coupling

characteristics between the external coil and the designed

IOP sensor. In general, the conductivity of tissues is

higher than that of air. The eddy current generated on the

surface of the cornea therefore degrades the magnetic

coupling between the external coil and the IOP sensor

and hence the quality factor. The conductivity of the

cornea can vary according to the water concentration on

the surface of the cornea. However, the resonance

(a)

(b)

Fig. 8. Measured results according to the external pressure

without a cornea (a) phase, (b) resonance frequency of IOP

sensor.

(a)

(b)

Fig. 9. Measured results according to the external pressure with

a cornea (a) phase, (b) resonance frequency of IOP sensor.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.13, NO.4, AUGUST, 2013 359

frequency and dynamic range characteristics are nearly

identical to the results shown in Fig. 8(a).

VI. CONCLUSIONS

We have proposed an intraocular pressure (IOP)

sensor based on MEMS technology. We have also

proposed that the inductance varies under external

pressure, which is detected by the detector IOP. Ferrite

bottom material on a flexible membrane is used for the

variable inductance. We use two flexible membranes to

enhance the resolution of the sensor. We obtained the

measured results at a resonance frequency of 10.8 MHz

and a dynamic range of 70 mmHg. Finally, we

successfully demonstrated the feasibility of the proposed

sensor.

ACKNOWLEDGMENTS

This research was supported by the Bio &Medical

Technology Development Program of the National

Research Foundation (NRF) funded by the Korean

government (MEST) (No. 2009-0091931) and the

Soongsil University Research Fund of 2010.

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Byungjoo Kang received the B.S.

degree in electronic engineering from

Soongsil University, Seoul, in 2011.

His current research interests include

radar systems and RF power amplifier.

Hoyong Hwang received the B.S.

degree in electronic engineering from

Soongsil University, Seoul, in 2012,

and is currently working toward the

M.S. degree at Soongsil University.

His current research interests include

CMOS RF power amplifiers.

360 BYUNGJOO KANG et al : A WIRELESS INTRAOCULAR PRESSURE SENSOR WITH VARIABLE INDUCTANCE USING …

Soo Hyun Lee received the B.S.

degree from the Department of

Electrical Engineering, Korea

University, Seoul, Korea, in 2002,

the M.S. degree and Ph.D. degree

from the Department of Electrical

and Computer Engineering and

Computer Science, University of Cincinnati, in 2004 and

2007. In 2006, he joined the Korea Institute of Science

and Technology (KIST), Seoul, Korea, where he is

currently a Senior Researcher. His research interests

include implantable sensors, neural electrodes, biosensors,

and cell chips.

Ji Yoon Kang received the B.S.,

M.S., and Ph.D. degrees in Mechanical

Engineering from Seoul National

University, Korea, in 1990, 1992,

and 1997, respectively. From 1997 to

2001, he was a Research scientist at

Samsung Advanced Institute of

Science and Technology, Korea. From 2003 to 2004, he

was a visiting scholar at the University of Cincinnati. In

2001, he joined the Korea Institute of Science and

Technology, Seoul, Korea, where he is currently an

Principal Researcher. His research interests include

microfluidics, biosensor, and implantable micro-devices.

Joung-Hu Park received his B.S.,

M.S., and Ph.D. from the Department

of Electrical Engineering and

Computer Science of Seoul National

University, Seoul, Korea, in 1999,

2001 and 2006, respectively. He is

currently an Assistant Professor at

Soongsil University, Seoul, Korea. His current research

interests include the analysis of high-frequency switching

converters and renewable energy applications.

Chulhun Seo received the B.S., M.S.,

and Ph.D. degrees from Seoul

National University, Seoul, Korea, in

1983, 1985, and 1993, respectively,

all in electronics engineering. From

1993 to 1995, he was a Postdoctoral

Research Scientist with the Massachusetts Institute of

Technology (MIT), Cambridge. He has been a Professor

with Soongsil University, Seoul, since 1993. He has also

been a Member of the Research Staff with the Research

Laboratory of Electronics, MIT, since March 1995. His

current researchinterests include wireless and microwave

components, metamaterial, wireless power transfer

system, microwave integrated circuits and radar, as well

as microwave circuits and RFICs.

Changkun Park received the B.S.,

M.S., and Ph.D. degrees in electrical

engineering from Korea Advanced

Institute of Science and Technology

(KAIST), Daejeon, Korea, in 2001,

2003, and 2007, respectively. From

2007 to 2009, he was with the

Advanced Design Team of DRAM Development

Division, Hynix Semiconductor Inc., Icheon, Korea,

where he was involved in development of high-speed I/O

interfaces of DRAM. In September 2009, he joined the

faculty of the School of Electronic Engineering, Soongsil

University, Seoul, Korea. His research interests include

RF and millimeter-wave circuits, RF CMOS power

amplifier, and wireless chip-to-chip communications.