A wireless, low-drift, implantable intraocular pressure sensor with parylene-on-oil encapsulation
This paper presents a wireless, implantable continuous intraocular pressure (IOP) monitoring system that features a parylene-on-oil sensor encapsulation method for achieving long-term low-drift in vivo. The system is implanted in the superotemporal quadrant of the eye between the sclera and conjunctiva. It consists of a commercial pressure sensor (STMicroelectronics LPS25H) with digital readout, a 65nm CMOS chip that supports wireless power/data telemetry and the I2C serial communication interface with the pressure sensor. The chip and pressure sensor are assembled on a flexible polyimide PCB, and then the sensor is submerged in biocompatible silicone oil and coated with parylene in situ. The implant uses an on-chip integrated RF coil to receive power from near-field RF coupling at 915 MHz and transmit measurement bits via RF-backscattering to an external reader. A 2 mm × 1.2 mm chip is fabricated in TSMC 65nm CMOS process. The IOP implant achieves a pressure sensitivity of 0.17 mm Hg with a total power consumption of 9.7μW. We demonstrate pressure offset drift of less than 0.5 mmHg for more than 4 months over a temperature range of 27–38 °C. The implant successfully tracks induced IOP variations in a porcine eye ex vivo, validating the system functionality and surgical implantation.
© 2018 IEEE. The authors thank Samson Chen (Caltech Nanofabrication Lab) for help with the wireless reader, Trevor Roper (Caltech MEMS Lab) for maintaining equipment, Cameron Sylber for help with longevity tests, and MOSIS for chip fabrication. This work was supported by the Rosen Bioengineering Center Pilot Grant, the Heritage Medical Research Institute, the L. K. Whittier Foundation, and an unrestricted departmental grant from Research to Prevent Blindness.