An Implantable Microsystem for Autonomous Intraocular Pressure Monitoring .

Abstract

Glaucoma, a leading cause of blindness worldwide, is a disease in which the pressure within the eye is too high for the eye to tolerate and must be reduced in order to slow or prevent damage to the optic nerve. Conventional methods for monitoring eye pressure are normally only used in the physician’s office and rely on indirect measurement methods, leading to inaccuracies. Furthermore, intraocular pressure can vary throughout the day and also depends on activity. An autonomous implantable microsystem capable of monitoring intraocular pressure with minimal patient intervention would provide useful information to the clinician in the management of glaucoma. This dissertation studies the feasibility of an integrated microsystem for autonomously measuring intraocular pressure. Small size ensures minimal impact on the patient, preventing the device from entering the field of view and simplifying implantation. Integrated haptics aid surgical implantation and minimize trauma while allowing the implant to be removed if needed. A touch-mode capacitive pressure sensor, fabricated using the dissolved wafer process, transduces intraocular pressure into capacitance with a linear response and a sensitivity of 26 fF/mmHg. A new fabrication technique has been developed to embed vertical interconnects within a glass package containing the pressure sensor, a microbattery, readout circuitry, and an antenna. This enables the vertical stacking of these components and very efficient use of limited volume. The 1.5 mm x 2 mm x 0.5 mm transparent parylene-coated glass package enables solar cells to be placed on the circuit chip for power generation, trickle charging an on-board microbattery formed using standard cleanroom materials and a non-toxic electrolyte. Flooded-cell tests verified the electrochemistry and achieved a current capacity of 8 µAh/mm2. A simple integrated readout circuit consuming 35 pW in the idle mode implemented a finite-state machine and used an optical wakeup trigger to further reduce power. The microsystem has also been demonstrated with a microprocessor to autonomously gather and store data, reading it out on demand. Finally, a pulse-based ultrawideband wireless transmission technique is proposed using non-resonant antennas. The all-digital transmitter is expected to consume much less power than conventional encoded wireless transmitters and eliminates complex circuitry.