Vol. 35, No. 10
Journal of Semiconductors
October 2014
An ultralow power wireless intraocular pressure monitoring system Liu Demeng(刘德盟)1; 2 , Mei Niansong(梅年松)1; 2 , and Zhang Zhaofeng(张钊锋)1; 2; 1 Micro-Nano
Device Research Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China 2 University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: This paper describes an ultralow power wireless intraocular pressure (IOP) monitoring system that is dedicated to sensing and transferring intraocular pressure of glaucoma patients. Our system is comprised of a capacitive pressure sensor, an application-specific integrated circuit, which is designed on the SMIC 180 nm process, and a dipole antenna. The system is wirelessly powered and demonstrates a power consumption of 7.56 W at 1.24 V during continuous monitoring, a significant reduction in active power dissipation compared to existing work. The input RF sensitivity is 13 dBm. A significant reduction in input RF sensitivity results from the reduction of mismatch time of the ASK modulation caused by FM0 encoding. The system exhibits an average error of ˙ 1.5 mmHg in measured pressure. Finally, a complete IOP system is demonstrated in the real biological environment, showing a successful reading of the pressure of an eye. Key words: IOP; implanted medical; pressure measurement; RF powering; ultralow power ASIC DOI: 10.1088/1674-4926/35/10/105014 EEACC: 2570D
1. Introduction Glaucoma is a group of eye diseases that impact about 8.4 million people worldwideŒ1 , and which are characterized by elevated and large fluctuation intraocular pressure (IOP)Œ2 . Normally, pressure levels in the eye range from 10 to 21 mmHg, but they can be as high as 50 mmHg in a diseased eye. Continuous measurement of the IOP in glaucoma patients can help in better disease diagnosis, monitoring, and management, which can be achieved with an implanted monitor. Hence, there has been an increasing amount of interest in a wireless implanted intraocular pressure monitoring systemŒ3 7 . IOP monitoring systems can have two kinds of structure: an LC resonant-based structure and a system on chip (SOC) structureŒ8 . The LC resonant-based structure uses an inductance and a capacitive pressure sensor, and the resonance frequency changes with the capacitance of the sensor. The advantages of this structure are that it is simple, robust, and somewhat small. The main drawbacks are that it has a very limited functionality and requires a nearby external data acquisition unit. The SOC structure consists of an IC, a pressure sensor, and an antenna. The great advantage of this structure presented is that it provides more reliability and is able to work over a longer distance. Although great progress has been made in SOC structure of the implanted system, there are some challenges for achieving high-resolution measurement, reliable wireless communication, and ultralow power. The power dissipations of Refs. [4, 5, 11] are 47 mW, 202.43 W and 210 W, respectively, which are too high for long-term implant monitoring. In this paper, we describe in detail the design of a wireless implantable IOP monitoring system, including an ASIC chip, a microelectrome-
chanical systems (MEMS) capacitive pressure sensor (E1.3N, microFAB Bremen), and an antenna. The system is batteryfree, allowing small size and long lifespan. Power is transmitted by electromagnetic wave. We use intermediate-frequency (IF) modulated backscatter for uplink communication. The signal is encoded by FM0 to improve the reliability of the communication. A low power capacitance-to-frequency (C-to-F) converter is designed to read the pressure sensor. An antenna and rectifier are used to finish the radio-frequency (RF) electromagnetic energy harvesting. In this work, the operating frequency is in the unlicensed 915 MHz industrial, scientific and medical (ISM) band.
2. System design The ASIC is designed on the SMIC 180 nm technology and consists of a sensor interface, voltage regulators and references, an RF rectifier for remote powering, an amplitude shift keying (ASK) modulator, oscillator, and digital conversion, as shown in Fig. 1. The oscillator (OSC) provides the clock signal for
Fig. 1. System design.
* Project supported by the Science and Technology Commission of Shanghai Municipality (No. 12DZ1500900). † Corresponding author. Email:
[email protected] Received 5 March 2014, revised manuscript received 23 April 2014 © 2014 Chinese Institute of Electronics
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Fig. 3. The schematic of rectifier.
Fig. 2. Structure of (a) the antenna, (b) the simulation result of gain and (c) S11 parameter.
the digital module, the low dropout regulator (LDO) provides a stable supply voltage for other modules, and the power-on reset (POR) takes the reset signal to the OSC and digital module. The MEMS pressure sensor used in our system, the E1.3N, has a sensitivity of 1.6 fF/mmHg in the pressure range of 10– 50 mmHgŒ9 , and the capacitance of sensor changes within the scope of 6.0–6.1 pF. The C-to-F converter converts the capacitance to the frequency of pulse signal and then a frequency divider is used to amplify the period of the pulse signal. The digital logic counts the amplified pulse width, encodes the count results, and then modulates backscatter to the external reader. The FM0 code can reduce the mismatch time of ASK modulator and then improve the energy obtained from RF. The antenna and rectifier convert RF energy to DC energy to power the total system. Compared to the previous systemsŒ3 5 , the proposed system has a reduced number of building blocks and power, and it improves the power conversion efficiency and the reliability of communication by FM0 encoding.
3. Antenna Several previous designs on intraocular devices used inductive coupling in the kHz to MHz range, requiring a large, multi-turn coil inductor in the implantŒ10; 11 , and the larger device size requires a larger incision. In contrast to inductive coupling, full-wave electromagnetic analysis considering tissue loss shows that the optimal frequency for power transfer to a size-constrained antenna is in the GHz rangeŒ12; 13 . Other research shows that microwave (above 2.4 GHz) radiation induces irreversible damage to the lens epithelial cells. According to the analysis above, the 915 MHz band is considered to be the most suitable to be chosen as the wireless transmission frequency. The antenna is a dipole-like structure, as Figure 2(a) shows. Bio-compatible polymethyl methacrylate (PMMA) plastic is used as the substrate material. The central hole of the loop antenna is designed to avoid a decrease in the quantity of
light into the pupil. Normally, as the diameter of pupil changes from 2.5 to 4 mm, the diameter of the central hole is 5 mm. Given that the diameter of anterior chamber is about 12.11 mm, the diameter of the antenna is designed to be 11.4 mm. The thickness of the antenna is about 0.3 mm. This antenna is modeled using Ansoft’s high frequency structural simulator (HFSS) in eye tissue, Figure 2(b) shows that the simulated gain of the antenna is about –17 dBi, and the S11 parameter is about 29 dB, as Figure 2(c) shows.
4. Application-specific integrated circuit 4.1. Power transfer and power management The IOP monitoring system is wirelessly powered by RF electromagnetic energy using an external reader operating at 915 MHz. The on-chip CMOS rectifier picks up the RF electromagnetic energy on the antenna to provide power to the chip. The 2-stage rectifier employs a cross-coupled differential CMOS configuration with a bridge structure, as Figure 3 shows. The power conversion efficiency of the rectifier depends on the turn-on resistance and the leakage power of the MOSFET. The length of MOSFET employs the minimum length of technology, which is 180 nm, and the width of MOSFET is expected to be a compromise between the turn-on resistance and the leakage power. As the temperature of body almost keeps constant, a bandgap reference circuit is unnecessary for implantable medical devices. A bias circuit working in the subthreshold region is used to provide three stable reference voltages of 1.2, 0.9 and 0.3 V. The total current of the bias circuit is about 200 nA. A low dropout regulator is used to isolate the supply noise and the regulator employs a common PMOS with operational amplifier feedback topology, which has good stability and PSRR without external capacitors. The reference voltage of the regulator is 1.2 V and the output voltage is equal to the reference voltage. The static current of the regulator is about 100 nA. 4.2. Capacitance-to-frequency converter The C-to-F circuit consists of 555 timer and 100 nA current source, as Figure 4 shows. The 555 timer controls the charge and discharge of the capacitance under test. There is a linear
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Fig. 4. The capacitance-to-frequency converter.
relationship between the sensing capacitor and the period of Tsignal . The relationship is given by T D
CX .VH VL / CX .VH VL / C : Icharge Idischarge
(1)
This proportionality holds even when channel length modulation effect is taken into consideration, where VH D 0.9 V, VL D 0.3 V, and Icharge D Idischarge D 100 nA, when CX D 6 pF, the period of Tsignal is 72 s. When CX increases by 1 fF, the period of Tsignal would increase by 12 ns. It is hard to count by the digital unit, so 12 ns is amplified to 3.072 s by an 8-bit binary frequency divider. 4.3. Digital conversion The main modules of the digital conversion are the 14-bit counter and FM0 coding. The counter is controlled by a clock, which is generated using a current-starved ringoscillator topology, whose frequency is determined based on the pulsewidths, specifications of the MEMS sensor, desired pressure range of 50 mmHg, and required sensitivity of at least 0.5 mmHg. Since the output pulsewidth of the frequency divider has a sensitivity of 1.536 s/fF, and the sensitivity of the sensor is 1.6 fF/mmHg, the total sensitivity is 2.4576 s/mmHg. Therefore, the frequency of the clock is chosen to be 1.28 MHz. The timing diagram of the digital conversion is shown in Fig. 5. The pulse controls the clocking of the binary counter, and the falling edge of the pulse is used to stop and reset the counter and prepare it for the next conversion. After the count sequence, the value of the counter is fed into a parallel-to-serial converter, which is designed using a standard 14-bit shift register topology, to serially stream out the digital data to FM0 encoder. Then, the FM0 data is sent to an ASK modulator. The code of 1010 in Fig. 5 is the result of the counter. In the FM0 encoder, a logical one is encoded in a particular bit-cell by a zero-crossing transition only at the end of the cell, while a logical zero is encoded by adding an additional transition at mid-cell, the encoding result of 1010 is shown in Fig. 5. In order to reduce the power dissipation of proposed system, most of the modules are worked in the sub-threshold condition. The simulation results of power dissipation of main modules in the ASIC are shown in Table 1. The total power dissipation is about 6 A.
Fig. 5. Timing diagram of the digital conversion.
Table 1. Power dissipation of main modules. Module Power dissipation CFC 1.76 A OSC 3.23 A Bias 0.2 A LDO 0.2 A POR 0.25 A Digital 0.4 A
5. Experimental results The ASIC of the IOP monitoring system was fabricated in SMIC 0.18 m 2P4M CMOS technology, and it occupies 0.4 0.35 mm2 silicon area, including the pads, as shown in Fig. 6. The measurement results are summarized in the following section. 5.1. Wireless powering test The input impedance of the IOP monitoring system was matched to 50 by the PNA-X network analyzer N5242A, the S11 is about –25 dB. Then, the output DC voltage and power conversion efficiency is measured at different RF powers, as shown in Fig. 7. When the supply voltage is above 1.24 V, the total current is 6.1 A. The input RF sensitivity is –13 dBm (minimum power needed to allow chip operation). When the supply voltage is 1.246 V, the power conversion efficiency is
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Fig. 8. Pressure measurement.
Fig. 6. ASIC microphotograph.
Fig. 9. The tested IOP signal.
Fig. 7. RF powering IC power conversion efficiency (PCE).
15% with a –13 dBm RF input. The total power conversion efficiency depends on the matched-degree of the rectifier’s input impedance matching to 50 and on the rectifier’s efficiency. The input impedance of the rectifier changes with the input RF power, the maximal matched-degress happened on – 13 dBm RF input power. In addition, the output voltage keeps constant if the input RF power is higher than a certain level because the leakage power of the rectifier increases with the input power, which also causes the reduction of power conversion efficiency.
pressure is about ˙1.5 mmHg. A summary of the IOP monitoring system and its comparison with other state-of-the-art implementations is shown in Table 2. The RF sensitivity of the proposed system is the lowest, but the pressure resolution is higher than other mentioned system, which is caused by the phase noise of the clock of the counter. The measured power consumption of Ref. [3] is 2.3 W, which is lower than the proposed system, the reason is that on-chip digitalization circuitry is removed in Ref. [3] and the result of the capacitance-to-frequency converter is sent to ASK modulator directly, hence the count is achieved in the external reader. Wireless transmission deteriorates the measured resolution, so the pressure resolution of 0.9 mmHg is hard to achieve in a practical application. The high pressure resolution of other mentioned system is achieved at the expense of power.
5.2. Pressure measurement 5.3. Biological test results The ASIC is integrated with the MEMS capacitor and tested with the wireless enabled by the RFID test platform NI100. Our empirical tests involved enclosing the fully functional wireless prototypes inside a custom-built pressure chamber. We then kept the temperature constant at 37 ıC. Process variations would result in the discrepancy between different tested sample, a two point calibration can be taken on the external reader to get an accurately measured result. Figure 8 shows the measured pressure sensing linearity from 0 to 60 mmHg, the slope is 3.25 s/mmHg, and the average error of the measured
In order to verify the operation of the IOP monitoring system, the system which contains antenna, ASIC and pressure sensor was tested in pig eyes. The RFID test platform with an outside antenna powers the implanted system and receives the test result. Figure 9 shows the received results on the external reader, which is a demodulated signal. This is the original FM0 code, and the corresponding binary code is 011000000000010, the decimal value is 12290, the corresponding pressure is about 70 mmHg.
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Parameter Process Supply voltage (V, min) Power (W, min) Pressure resolution (mmHg) RF sensitivity (dBm)
This work 0.18 m CMOS 1.24 7.56 ˙1.5 13
JSSC’11Œ3 0.13 m CMOS 1.5 2.3 0.9 10:5
6. Conclusion An ultralow power wireless IOP monitoring system is presented in this work, which contains an antenna, an ASIC, and a pressure sensor. The measurements results of the RF powering system show a low RF sensitivity of 13 dBm, which is the lowest of the systems that have so far been reported. The average error of pressure measurements is ˙1.5 mmHg. Finally, a complete system is demonstrated, showing successful transmission of captured pressure data.
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ISSCC’11Œ4 0.18 m CMOS 3.6 47000 (with battery) 0.5 N.A.
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