236 • 2010 IEEE International Solid-State Circuits Conference

ISSCC 2010 / SESSION 12 / EMERGING MEDICAL APPLICATIONS / 12.2

12.2 Mixed-Signal Integrated Circuits for Self-Contained Sub-Cubic Millimeter Biomedical Implants

Eric Y Chow1, Sudipto Chakraborty2, William J. Chappell1, Pedro P. Irazoqui1

1Purdue University, West Lafayette, IN2Texas Instruments, Dallas, TX

Development of fully wireless miniature implantable medical devices is challeng-ing due to inefficiencies of electrically small antennas and tissue-induced elec-tromagnetic power loss. Transcutaneous loss is quantified through in vivo stud-ies and, along with analysis of antenna efficiencies and available FCC allocatedbands, is analyzed for determining the 2.4GHz operating frequency.Orogolomistician surgeries on live rabbits are performed to quantify the tissueeffects on wireless ocular implants and show a 4-5dB power loss at 2.4GHz [1].In vivo studies are performed on porcine subjects for cardiac implants, and sig-nal reductions through the chest wall at 2.4GHz are measured to be 33-35dB [2].

The system, shown in Fig. 12.2.1, is designed and fabricated on the TI 130nmCMOS process. The wireless transmitter uses a complementary cross-coupledVCO topology for feasibility of on-chip implementation, low phase noise, and lowpower consumption. The LC tank includes a fully-symmetric differential inductorwith a patterned ground shield, and has a measured Q of 10.36 at 2.4GHz. A self-biased class AB power amplifier, a topology with sufficient efficiency and lowdistortion, is used to feed the antenna. The fabricated transmitter consumes812µA from a 1.5V regulated supply and outputs -45dBm. OOK is used as a sim-ple low-power modulation scheme and the transmitter requires 84.4kHz of band-width within the FCC allocated 2.4GHz ISM band. The system utilizes a 42.2kb/sdata-rate and, with a signal strength of -90dBm at the receiver input, the meas-ured wireless-link BER is 10-5.

The wireless powering technique is based on RF electromagnetic wave propaga-tion, which has greater ranges and better orientation immunity than low-fre-quency magnetic coupling. A Cockcroft-Walton multiplier technique is used inthe rectifier to produce sufficiently high voltages with relatively low input-pow-ers [3]. The RF powering module operates at 2.4GHz allowing the use of oneantenna for both powering and telemetry. Schottky diodes, implemented on-chip, are used for their superior high-frequency performance and low-forwardbias voltage. Measurements of the RF powering IC, plotted in Fig. 12.2.2, showthat with a 20mW (13dBm) input, the rectifier has a power conversion efficien-cy (PCE) of 11.3% and produces 1.5V across a 1kΩ load which is sufficient forbiasing standard CMOS process nodes and supplying adequate current for mostlow-power designs [4]. Friis path loss formula estimates a 10dB reduction inpower across 10 cm, and coupled with the 4-5dB power loss though ocular tis-sue in an implanted setting necessitates a 0.5 – 0.6W transmit power from theexternal source. Increase in transmit power, improvement in rectifier efficiency,and decreased power consumption of the system allows for an increase in oper-ating distances and/or greater implantation depths.

The ASIC processes and digitizes measurements from a MEMS pressure sensor[5]. A low-power measurement technique is used in the sensor interface whichutilizes a 1nA current to charge the MEMS capacitor converting capacitancechanges to time varying pulse-widths. These pulses are digitized using an 8-bitcounter feeding a parallel-to-serial converter which streams the data into on-chipmemory.

Control circuitry operates the system on a sleep-wake cycle to minimize powerwhile storing data points every 5 minutes into on-chip FeRAM, a low-power non-volatile memory supported on the TI 130 nm process [6]. The system, exclud-ing the wireless transmitter, is measured to consume 202.43µW during its 1mswake period, and the power consumptions of each block are tabulated in Fig.12.2.3 alongside Spectre simulated results. In a wake period, the system firstreads an address out of FeRAM, then digitizes and writes the MEMS capacitancevalue into memory, and finally writes an incremented address into memory.From simulations, the FeRAM requires 180pC and 39.75µs for the 1 read and 2write operations, which results in a 270nW average power consumption over the1ms measurement cycle. A 24µF capacitive power storage array enables contin-uous independent operation and when fully charged, stores 48µC of usablecharge. Using a picowatt timer, similar to that presented in [7], to track the 5minute intervals, the sleep-mode charge consumption over a 24hr period is cal-culated to be 86.4nC. After subtracting the sleep-mode charge consumption, theremaining capacitor charge is able to supply the 202.7µW wake period powerdraw for 360 cycles, which allows for recordings every 5 minutes throughoutone day. The average power consumption of the device over this 24hr period is

simulated to be 675.67pW. At the end of a 24hr period, an external device isbrought nearby to wirelessly download the data and recharge the device. A com-parison with published state-of-the-art low-power active IOP monitors, tabulat-ed in Fig. 12.2.4, shows comparable power consumptions during wake-mode;however, since our system architecture along with the FeRAM allows for sleep-wake cycles, the average power consumption is over 200,000 times less thanother IOP systems.

Our 0.49mm2 ASIC, shown in Fig. 12.2.7, has several similarities with “theworld’s smallest RFID IC, µ-chip,” developed by Hitachi, with the key differencesof integration with a MEMS pressure sensor and independent operation [8].Empirical tests involved enclosing the fully functional wireless prototypes insidea custom-built pressure chamber while the wireless power source and datareceiver are positioned externally. Pressure measurements, plotted in Fig.12.2.5, are taken by sweeping pressure within the chamber while wirelessly sup-plying power to the device and receiving the RF telemetry. Measured pressuresshow the same trend as simulated and actual pressures and after multiplicationwith a calibration constant of 0.593, there is an average error of +/-1.268mmHg.Measurements demonstrate the ability to achieve a sensitivity of 0.5mmHg whenthe digitization module is clocked at 42.2kHz.

The electronics are packaged using liquid crystal polymer (LCP), a flexible, bio-compatible, hermetic, and radio-transparent material [9]. The glaucoma intraoc-ular pressure (IOP) monitor uses a tadpole-like structure with a 3x6x0.3mm3

head and 27mm tail, shown in Fig. 12.2.6, and enables implantation in the ante-rior chamber of the eye via a trochanter-based insertion, where the tail is usedas the antenna. For cardiac pressure monitoring, the electronics are integratedwith a medical stent used as both an antenna and structural support. The cardiacdevice is implanted via a minimally invasive catheter-based procedure into thepulmonary artery for heart failure diagnosis, although this implant has the flexi-bility for placement in any vessel. For studying the root causes of glaucoma, asmaller version of the tadpole, currently under-development, has a 300 x 300 x300µm3 head and 2.7mm tail, is an on-demand system without power-storage oron-chip memory, and is aimed at implantation in the eyes of mice. These minia-ture wireless implantable systems allow for the study, early diagnosis, and moreaccurate treatment of glaucoma and cardiac diseases.

References:[1] E. Y. Chow, C.-L. Yang, A. Chlebowski, S. Moon, W. J. Chappell, and P. P.Irazoqui, “Implantable Wireless Telemetry Boards for In Vivo TransocularTransmission,” Microwave Theory and Techniques, IEEE Transactions on, vol.56, pp. 3200-3208, 2008.[2] E. Y. Chow, B. Beier, W. J. Chappell, and P. P. Irazoqui, “Towards anImplantable Wireless Cardiac Monitoring Platform Integrated With an FDAApproved Cardiovascular Stent,” Journal of Interventional Cardiology, vol. 22,2009.[3] E. Edgar and L. Paul, “The Cockcroft-Walton Voltage Multiplying Circuit.” vol.24: AIP, 1953, pp. 221-226.[4] “International Technology Roadmap for Semiconductors,” 2008.[5] “Capacitive Pressure Sensor E1.3N,” MicroFab Bremen, Bremen, Germany,2008.[6] E. M. Philofsky, “FRAM-the ultimate memory,” in Nonvolatile MemoryTechnology Conference, 1996., Sixth Biennial IEEE International, 1996, pp. 99-104.[7] L. Yu-Shiang, D. Sylvester, and D. Blaauw, “A sub-pW timer using gate leak-age for ultra low-power sub-Hz monitoring systems,” in Custom IntegratedCircuits Conference, 2007. CICC ‘07. IEEE, 2007, pp. 397-400.[8] R. Imura, “The World’s Smallest RFID µ-Chip, bringing about new businessand lifestyles,” in VLSI Circuits, 2004. Digest of Technical Papers. 2004Symposium on, 2004, pp. 120-123.[9] M. J. Chen, A. V. H. Pham, N. A. Evers, C. Kapusta, J. Iannotti, W. Kornrumpf,J. J. Maciel, and N. Karabudak, “Design and Development of a Package UsingLCP for RF/Microwave MEMS Switches,” Microwave Theory and Techniques,IEEE Transactions on, vol. 54, pp. 4009-4015, 2006.[10] T. Eggers, J. Draeger, K. Hille, C. Marschner, P. Stegmaier, J. Binder, and R.Laur, “Wireless intra-ocular pressure monitoring system integrated into an arti-ficial lens,” in Microtechnologies in Medicine and Biology, 1st AnnualInternational, Conference On. 2000, 2000, pp. 466-469.[11] K. Stangel, S. Kolnsberg, D. Hammerschmidt, B. J. Hosticka, H. K. Trieu,and W. Mokwa, “A programmable intraocular CMOS pressure sensor systemimplant,” Solid-State Circuits, IEEE Journal of, vol. 36, pp. 1094-1100, 2001.[12] W. Mokwa and U. Schnakenberg, “Micro-transponder systems for medicalapplications,” Instrumentation and Measurement, IEEE Transactions on, vol. 50,pp. 1551-1555, 2001.

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237DIGEST OF TECHNICAL PAPERS •

ISSCC 2010 / February 9, 2010 / 2:00 PM

Figure 12.2.1: Schematic of monitoring system. Figure 12.2.2: RF powering IC power conversion efficiency (PCE).

Figure 12.2.3: Measured and simulated power consumption of ASIC components.

Figure 12.2.5: Measured performance with and without calibration alongsidesimulated results as a function of actual pressure.

Figure 12.2.6: Cardiac pressure monitor, mouse on-demand IOP monitor, andhuman IOP monitor for glaucoma applications.

Figure 12.2.4: Ultra low power active systems for IOP monitoring applications.

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• 2010 IEEE International Solid-State Circuits Conference 978-1-4244-6034-2/10/$26.00 ©2010 IEEE

ISSCC 2010 PAPER CONTINUATIONS

Figure 12.2.7: Chip micrograph.