Flexible Packaging for a Wireless Intraocular Pressure Sensor

Thesis (Ph.D.)--University of Washington, 2013This dissertation focuses on the prototype of an intraocular pressure (IOP) sensor as a major step towards building a device that can be permanently implanted during cataract surgery. The implantation will proceed through an incision of 2-3 mm using an injector, during which the complete device must be folded into a cross-section of 2 mm × 1 mm. The device uses radio frequency (RF) for wireless power and data transfer. First, a novel device design is introduced. Materials are chosen systematically for the device fabrication. Two fabrication methods, metal-on-elastomer and solder-filled microchannel, are tested for the antenna fabrication and device integration. Results indicate that an antenna resistance below 5 Ω can be achieved using the solder-filled microchannel method. A device fabrication and integration process based on that method is introduced. A device prototype including an antenna, an RF chip and a pressure sensor is presented. It is assembled on a printed circuit board (PCB) with several circuit components used for testing and calibration. The antenna is fabricated and integrated with the circuit using a fabrication method employing solder-filled microchannels embedded in an elastomer (polydimethylsiloxane, PDMS). The presented method can be used for biocompatible packaging of microsystems and sensors. The prototype is tested for antenna functionality through power and data transfer. The monitoring device is powered at 2.716 GHz from a distance of 1-2 cm. Transferred power is greater than the threshold power required for chip operation. Exposure to RF power is kept below the maximum permissible exposure limit. The backscattered signal is observed in each chip mode to confirm the wireless sensing capability. Structural flexibility of the PDMS-metal-PDMS stack is studied using a mockup device. The effect of applied stress on antenna resistance is monitored in order to evaluate its durability during the implantation process. The flexible antenna can withstand a stress of 33.4 kPa without any electrical disconnection. It did not show a significant increase in electrical resistance after 50 bending cycles. The prototype has undergone electrical tests for the effect of PDMS coating on pressure sensor and wireless sensing performance. An acceptable increase in sensor capacitance and pressure sensitivity is observed after PDMS coating. Wireless pressure measurement tests showed device operation with a pressure sensitivity of 16.66 Hz/mm-Hg (0.125 Hz/Pa). Three topics for future work are proposed for converting the prototype into a complete device. First, additional tests for evaluating device performance in an eye-like environment are proposed. They provide a complete characterization of the RF power and data transfer inside the eye. Second, requirements for the next generation of RF chip are stated. Device size will be shrunk by the elimination of surface mount technology (SMT) components and PCB. Third, building a multi-sensor platform based on the prototype is proposed. Salinity and pH sensors can be added to the prototype