University of Michigan
Electrical & Computer Engineering
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Ann Arbor, MI 48109
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The Intraocular Pressure Sensor (IOPM) collects sensor measurements and transmits them to an external device. The system consists of a wireless transceiver, a processor, a power management unit, sensor readout circuitry, solar cells, a commercial thin film batter and a MEMS pressure sensor.
Glaucoma is an eye disease affecting 67 million people worldwide and is the leading cause of blindness. Glaucoma patients have high eye pressure, which damages the optic nerve and leads to vision loss. Eye pressure is commonly monitored at a doctor’s office using tonometry. Due to circadian rhythms and physical activity, a single pressure reading may be misleading, resulting in patents obtaining delayed treatment since eye pressure may fluctuate outside of safe limits throughout the day. Continuous measurement of intraocular pressure (IOP) is desired to provide targeted treatment and prevent unnecessary vision loss and can be achieved with an implanted monitor. The most suitable implantation location is the anterior chamber of the eye. Given a maximum width of 2.8mm for a self-healing incision, the IOP monitor (IOPM) width is limited to 1mm while the height of 0.5mm and length of 2mm are set by the curvature of the cornea and size of anterior chamber, resulting in a 0.5×1×2mm IOPM. A 5.4mm3 (6×3×0.3mm) sensor was demonstrated that uses a 27 mm antenna. Implantation of this device may require a large incision and sutures to immobilize the IOPM, both resulting in undesirable eye trauma. The large antenna is used to inductively recharge the system, but uses power densities 10× the recommended value for human tissue. The aggressive size constraint of an ideal IOPM creates major challenges for wireless communication and achieving multi-year device lifetime. Small antennas or inductors are required, resulting in lower receiver power and higher transmission frequency, both increasing power requirements. Little energy can be stored in the available form factor, calling for ultra-low power operation and energy harvesting.
We present a 1mm3 IOPM with energy-autonomous operation and wireless communication that can be implanted with minimally-invasive surgery. The device is implanted through a tiny incision, routinely used for outpatient minor cataract surgery. The device is pushed into the iris, using the natural elasticity of the tissue to hold it in place. Glass haptics formed as part of the micromachined package hold the device in place without damaging the tissue permanently and allowing for simple removal. Since it sits under the transparent cornea, the IOPM harvests solar energy that enters the eye to achieve energy-autonomous operation. The microsystem contains a solar cell, battery, capacitive sensor, wireless link, microcontroller, and memory in a biocompatible housing.
The IOPM takes measurements every 15 minutes with a 7µW capacitance to digital converter (CDC), which represents continuous pressure monitoring. The measurement interval does not require high precision for medical diagnosis and is controlled by a slow timer. The CDC generates an IOP-dependent current source by dropping VDD/2-VTH (VREF) across an impedance generated by switching the MEMS pressure sensor (CMEMS) at 50kHz. Simultaneously, a larger fixed current is generated in the same manner and with the same clock using fixed capacitors (C1, C2). Two fixed capacitors with out of phase clocks are used to generate a stable current source. This fixed current source is mirrored and compared to the IOP-dependent current using ΣΔ modulation to digitize IOP. The IOP-dependent current is integrated by discharging capacitor CINT. The voltage on CINT (VINT) is compared to VREF with a clocked comparator. When VINT drops below VREF, the fixed current is also integrated onto CINT, increasing VINT. The CDC runs for 10k clock cycles (200ms) and achieves a pressure resolution of 0.5 mmHg, which exceeds the resolution of typical tonometric measurements used in glaucoma diagnosis. Since the CDC measures the ratio of two currents, it has low sensitivity to supply voltage, clock jitter, and temperature variations. After the CDC measurement completes, IOP data is logged into the 4kb SRAM using the 90nW 8-bit microprocessor (µP). Storing 24 hours of 10-bit results taken every 15 minutes requires 1kb of SRAM. The µP can also be wirelessly programmed to perform DSP or compression on the IOP data to extend storage capacity to over 1 week of data.
Every 3-7 days the user downloads IOP data by placing an external device (ED) near the eye. The system can be woken up by coupling RF energy from the ED onto an LC tank, rectifying the AC signal, and using a comparator to generate a digital wakeup signal. The transmitter consists of an oscillator that acts both as a carrier generator and power amplifier. It uses a dual-resonator tank to generate FSK-modulated signal with the two FSK tones chosen far apart at 570MHz (f0) and 690MHz (f1). This enables us to achieve higher transmission distance by relaxing the constraint on the phase noise of our integrated transmitter. To transmit a zero from the IOPM to the ED, LC1 is shorted and the oscillator runs at f0for 0.1µs using LC0. A one is sent by oscillating at f1with LC1. The signal transmits to the ED through the 3.1mm anterior chamber, 0.5mm cornea, and air. The transmitter has a measured BER of less than 10-6 at 10cm through 5mm of 5g/liter saline; this is ample distance to reach the ED and pessimistically models attenuation from aqueous humor in the eye. The 4.7nJ/bit transmitter achieves 4× improvement in energy efficiency over comparable work in miniaturized highly-integrated biomedical implants . Peak current output of the battery is 35-40uW, which cannot directly support wireless transmission. To prevent catastrophic supply voltage droop, 1.6 nF of integrated MIM and MOS decap is included. The isolated VDD drops by 0.5V when the radio transmits one bit every 131 µs and recharges the decap between transmissions. Wireless transmissions need only occur once every 3-7 days.
Desired IOPM lifetime is on the order of years, allowing suitable treatment to be found for each patient. However, the anterior chamber volume limits lifetime by constraining the size and capacity of on-sensor power sources. The IOPM uses a custom 1µAh thin-film Li battery capacity from Cymbet. To extend lifetime, the IOPM harvests light energy entering the eye with an integrated 0.07mm2 photovoltaic diode (PV) to recharge the battery. The solar cell is disconnected from the system in the zero-light condition, which is detected from the open circuit voltage of a smaller replica PV cell. Given the ultra-small solar cell size, energy-autonomous operation requires average power consumption of <10nW. Processor power is reduced using subthreshold operation and delivered using a switched capacitor network (SCN) with 75% efficiency. While IOP measurements and wireless transmissions require µWs and mWs of power, these events occur infrequently and are short in duration. When CDC and radio circuits are idle, their power consumption drops to 172.8 pW and 3.3 nW, respectively. The majority of its lifetime, the system in a 3.65nW standby mode where mixed-signal circuits are disabled, digital logic is power gated, and 2.4 fW/bitcell SRAM retains IOP instructions and data . The combination of energy harvesting and low power operation enables the IOPM to achieve zero-net energy operation in bright indoor lighting. In sunny conditions, it recharges the battery at 80.6nW, far exceeding average system power. The IOPM requires 10 hours of indoor lighting or 1.5 hours of sunlight per day to achieve energy-autonomous operation.
Gregory Chen, Hassan Ghaed, Razi-Ul Haque, Michael Wieckowski, Yejoong Kim, Gyouho Kim, David Fick, Daeyeon Kim, Mingoo Seok, Kensall Wise, David Blaauw, Dennis Sylvester, “A 1 Cubic Millimeter Energy-Autonomous Wireless Intraocular Pressure Monitor,” IEEE International Solid-State Circuits Conference (ISSCC), February 2011 ©IEEE