An RFID-based On-lens Sensor System for Long-Term IOP Monitoring Shun-Hsi Hsu1, Jin-Chern Chiou1,2, Yu-Te Liao1, Tzu-Sen Yang1 ,Cheng-Kai Kuei2, Tsung-Wei Wu2, Yu-Chieh Huang2 1 Department of Electrical and Computer Engineering, National Chiao-Tung University, Taiwan 2 Institute of Electrical and Control Engineering, National Chiao-Tung University, Taiwan Email-Address: [email protected] 

Abstract—In this paper, an RFID-based on-lens sensor system is proposed for noninvasive long-term intraocular pressure monitoring. The proposed sensor IC, fabricated in a 0.18um CMOS process, consists of capacitive sensor readout circuitry, RFID communication circuits, and digital processing units. The sensor IC is integrated with electroplating capacitive sensors and a receiving antenna on the contact lens. The sensor IC can be wirelessly powered, communicate with RFID compatible equipment, and perform IOP measurement using on-lens capacitive sensor continuously from a 2cm distance while the incident power from an RFID reader is 20 dBm. The proposed system is compatible to Gen2 RFID protocol, extending the flexibility and reducing the self-developed firmware efforts.

Fig 1. Concept of the proposed long-term IOP monitoring system

I. INTRODUCTION Glaucoma is one of the major causes of blindness. Most glaucoma cases in North America and Europe are associated with the raising of the intraocular pressure. From clinic studies, 1mmHg increase in intraocular pressure (IOP) increases the risk of glaucoma by 10%. Therefore, long-term monitoring of intraocular pressure attracts many research focuses in the past few years [1]. Generally, the Goldman method is the standard procedure for measuring IOP; however, it is highly invasive and uncomfortable for the patients [2]. IOP measurement using an embedded strain gauge on a contact lens provides a noninvasive way for long-term eye pressure data collection [3]. The strain gauge is able to measure the angular changes at the junction due to variations in IOP. In order to measure correct IOP, the lens must be attached closely to the eye surface. The readout circuitry for strain gauge is placed in a Wheatstone bridge configuration with two sensing resistive gauges to achieve double sensitivity and two compensation resistive gauges for thermal compensation [4][5]. However, complicated calibration and temperature compensation schemes consume large power and area, making the strain gauge impractical for long-term precise IOP measurement. An alternative way to measure IOP is using LC resonant devices for impedance measurement of the eye surface [6]. The capacitive sensor detects the capacitance changes from the eye curvature at various eye pressures. The main problem of this method is the disturbance from magnetic coupling by external coils for wireless data communication, which reduces the precision of the IOP measurement. To minimize the size of sensor device, a customized, fully-integrated CMOS sensory chip is implemented for implantable intraocular pressure sensor [7]. The CMOS-only integration relaxes the

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Fig 2. System architecture of the proposed IOP sensor tag

fabrication and assembly processes of sensors, providing a low-cost solution for IOP measurement. However, these designs require self-developed software and are not compatible with standard commercial communication protocols so far. Our previous work demonstrated a MEMS capacitive pressure sensor on a contact lens for long-term IOP monitoring [8]. In this paper, an RFID-based on-lens IOP monitoring system is proposed and implemented using a 0.18µm CMOS process. This paper presents an integrated IOP monitoring system, shown in Fig 1, consisting of on-lens integrated sensor, readout circuitry, and RFID-compatible communication systems that can communicate with on-glass reader. The proposed sensor tag measures on-lens capacitance changes corresponding to the IOP-induced curvature deviations by the on-lens MEMS capacitive sensor. To avoid batteries on the lens, the sensor tag is wirelessly powered by external electromagnetic energy sources. The detail design of the sensor tag is described in the following sections. Section II describes analog front end (AFE) including sensor readout and power management circuits of the sensor tag and digital cores for RFID communication. Section III shows the experimental results of the proposed sensor tag. Finally, the paper is concluded in Section IV. II. SYSTEM ARCHITECTURE AND CIRCUIT DESIGN Fig. 2 shows the architecture of the sensor tag. The sensor tag includes a front end circuitry for power scavenging and data demodulation, a digital processor for RFID-protocol format generation and sensing information processing, and

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(a)

(a)

(b)

(b) Fig 4. Schematic of (a) demodulator (b) modulator

(c) Fig 3. Schematic of (a) constant current generation (b) bandgap reference (c) regulator

capacitive sensor readout circuitry. The measured capacitance value is converted to a bit-stream format for further digital processing. The available power on the lens is limited by low antenna efficiency and low energy storage capability due to size constraints on the lens. Therefore, the efficient power management and low power electronic design are crucial in the proposed system. We will discuss the design of each block in AFE in the next sections. A. RF-DC Rectifier and Limiter To maximize power transfer and voltage boosting, 10-stage voltage-doubling charge pump is employed to perform RF-DC conversion. High sensitivity and efficiency are achieved by using low-Vt diode-connected NMOS transistors [9]. In addition, to protect the chip from breakdown, a voltage limiter is inserted at the output of rectifier, giving a maximum voltage of 4V to the cores of the whole system. B. Bias Circuitry Fig. 3 shows the schematic of bias circuits. To reduce the power supply fluctuations from the incident RF power, regulator with high power supply rejection (PSRR) is required in this design. The cascode architecture, shown in Fig. 3(a), is adopted in the bias circuitry to generate constant current and provide good PSRR [10]. The bias circuit offers a stable current of 115nA to bias analog circuits such as amplifiers and oscillator. Fig 3(b) shows the schematic of bandgap reference. It provides a temperature-stable reference voltage of 1.25V for the chip. Fig. 3(c) shows the schematic of regulators. To avoid noise coupling between digital and analog circuits, separated regulators are used in this design: unit-gain feedback configuration for digital circuits with low-voltage supply (VDD_LV) and resistor-feedback configuration for sensitive sensor readout circuitry that requires boosting supply voltage (VDD_MV). In addition, several MΩ on-chip resistors with well-matched layout are employed in the resistive feedback loop for lowering the quiescent current. The simulated supply noise rejection of the regulator is 40 dB at low frequency.

Fig 5. Schematic of readout circuitry for capacitive sensor.

C. Demodulator and Modulator Fig 4 shows the schematic of data demodulator and modulator. The demodulator consists of a charge pump, a moving average circuit and a comparator. The amplitude shift keying (ASK) modulated signal is first detected by envelop detector for signal extraction and then compared with its averaging value to generate digital codes. The moving average unit adopts a variable MOS resistor instead of lossy and nonlinear diode-connected transistor. However, unbalanced rising time and falling time of the demodulated signals potentially cause bit width distortion and increase the bit-error rate of signal detection. Operational Transconductance Amplifier (OTA) is adopted for the voltage comparator. For maintaining symmetric rising and falling time, the push-pull output stage is used to reduce bit width distortion [11]. An RFID system uses backscattering communication schemes for transmitting and receiving data. In our design, signal reflection amplitude is modulated by controlling the parallel capacitor CMOD, which varies the input impedance. The reflected signal is detected by the interrogator. D. Sensor Readout Circuitry Fig. 5 shows the schematic of capacitive sensor readout interface circuitry. The sensor readout circuit adopts a capacitance to digital converter (CDC) with a 1-bit ∆-Σ modulator. The measured capacitance is converted to a bit-stream format and the bit-stream density represents the IOP value. Digital circuits such as digital counter and decimation filter following CDC are used to filter out the undesired noise and signal fluctuations. The capacitance conversion resolution and power consumption can be

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Fig 6. Diagram of digital signal processor Fig 8. Die photo of the proposed sensor IC

Fig 7. RFID command format used in this design

adjusted by the integration period of filter and CDC clock frequency. E. Oscillator For Gen2 RFID system, the required clocking frequency range is from 1.15MHz to 1.4MHz. To keep the ring oscillator frequency within the frequency range, compensation over temperature and processes is needed. Thus, a compensated bias current is injected into the ring oscillator to adjust the oscillation frequency according to PVT variations [9]. By this way, the oscillator frequency can meet the requirement of RFID Gen2 specification [12].

Fig 9. Picture of measurement setup for IOP sensor tag with an on-lens capacitive sensor.

F. Digital Core Design Fig 6 shows the schematic of digital cores of the sensor tag. Digital processor processes the sensor data and generates EPC Global Class1 Gen2 protocol formats. Incoming communication data are decoded and recognized by edge detector, command parser, and decoder. Then the receiving demodulated data are secured with 16 bit CRC checking codes. The command format referred to the setting of SL900A [13] is shown in Fig 7. To save power, CDC is only activated when the enable command is received. The sensing information is packeted and filtered before data transmission to the external reader.

Fig 10. Measured pressure results vs. injected fluid volume

III. EXPERIMENT RESULTS Fig 8 shows the chip micrograph of the proposed sensor IC. The chip is implemented using TSMC 0.18μm CMOS technology. The chip size with I/O pads is 1mm x 1.58mm. To verify the functionality of proposed IOP sensor tag, the chip is connected to an on-lens capacitive sensor. The sensor lens and the commercial RFID reader are placed in a distance of 2cm. A capacitive sensor fabricated using contact lens materials is placed on an artificial eye that can mimic the pressure changes by controlled fluid injection using a micro-pump. The antenna on a PCB is used with the similar size of a contact lens and eyeglass for receiving and transmitting antennas, respectively. A 300 micro litter fluid is

Fig 11. Measured results of periodical fluid injection at a period of 10 seconds

injected and drained to the artificial eye in a time interval of 5 seconds periodically, which varies the curvature of the artificial eye. The sensor data is wirelessly monitored by an external RFID reader. The photograph of measurement setup is shown in Fig 9. The sensor tag is wirelessly powered by a 20dBm incident RF power from the RFID reader (CISC RFID Xplorer [14]) at a 2cm distance. Real-time periodical IOP measuring data are monitored on a PC directly and recorded for future clinic study.

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Fig 12. Proposed long term IOP monitoring procedure TABLE I PERFORMANCE COMPARISONS WITH OTHER WORKS Authors

Sensor Type

Sensing Technique

Sensor Readout Compatibility

[4]

Noninvasive

Strain-gauged

NO

[6]

Invasive

LC Resonance

NO

[7]

Invasive

Capacitive Sensor

NO

This Work

Noninvasive

Capacitive Sensor

UHF RFID

Fig 10 shows the relation between pressure and injected fluid volume. The pressure variation induced by 300μL fluid is equivalent to 109mmHg according to the measured characteristic of the artificial eye after calibration. Fig 11 shows the measured results of periodical fluid injection of 300μL . The sensor response is recorded using 11bit digital codes to present the capacitance changes due to curvature deviations from injection fluid to the artificial eye. In addition, according to safety requirement in IEEE Std C95.1-2005 [15], eye temperature should be kept below temperature threshold (41oC) under RF exposure for prevention of cataract. Therefore, the IOP measurement is performed in 5 seconds within a time period of 100 seconds. Fig 12 shows the operation time of proposed IOP sensor system. The equivalent specific absorption rate (SAR) with proposed IOP measuring sequence under 20dBm incident RFID reader power is 0.667W/kg, and the eye temperature increase only 0.113oC by our calculation for a typical human eye of 7.5g weight [16]. Table I shows the key features of this design and comparisons to other publications. The proposed RFID-based IOP sensor tag with embedded RFID-compatible data format can communicate with a commercial RFID reader directly to save efforts and costs on developing reader systems.

Number: MOST 104-2220-E-039-001, MOST 103-2221E-009-192-MY3, MOST-102-2221-E-009-193-MY3, and MOST 103-2218-E-009-007-), and "Aim for the Top University Plan" of the National Chiao Tung University and Ministry of Education, Taiwan, R.O.C. This work was also supported by the Biomedical Electronics Translational Research Center, National Chiao-Tung University. This work was also particularly supported in part by UST-UCSD International Center of Excellence in Advanced Bioengineering sponsored by the Ministry of Science and Technology I-RiCE Program under Grant Number: MOST 103-2911-I-009-101-. The authors would like to thank Advanced Semiconductor Engineering Group for chip fabrication. The authors would like to thank National Chip Implementation Center for chip fabrication. The authors express their gratitude to CISC Semiconductor for work-specific technical assistance for the CISC RFID Xplorer. REFERENCES [1] H. A. Quigley and A. T. Broman, “The number of people with [2] [3]

[4]

[5] [6]

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[8]

IV. CONCLUSION AND DISCUSSION An RFID-based on-lens sensor IC is proposed for noninvasive, long-term intraocular pressure monitoring in this paper. The architecture includes a sensor readout circuitry, digital data processing unit, RFID Gen2 protocol generation, and backscattering communication modulator. The proposed system is compatible with Gen2 RFID system, thus reducing extra firmware effort and reader design. The proposed sensor IC can be wirelessly powered, communicate with RFID compatible equipment, and perform IOP measurement with on-lens capacitive sensor. In the future, the sensor tag can be integrated on a soft contact lens for collecting long-term IOP data directly from the patients.

[9] [10] [11] [12] [13] [14] [15]

ACKNOWLEDGMENT This work was supported in part by the Ministry of Science and Technology, Taiwan, R.O.C. (under Contract

[16]

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