Miniature Continuous Intraocular Pressure Sensor

Abstract

Abstract: : Purpose: The need for constant monitoring of intraocular pressure (IOP) in glaucoma patients, especially in diagnosing the disease and managing its subsequent treatment has prompted us to develop an IOP measuring sensor. Methods: : We fabricated several prototypes with various modifications of machine and hand-assembled sensors. The sensors consist of a capacitative-inductive circuit formed from a spiral inductor-diaphragm based capacitor. When the IOP level is altered, the pressure induced displacement of the diaphragm changes the value of the circuit capacitance, which in turn changes the resonant frequency of the LC circuit. The IOP monitoring and measurement is performed telemetrically, without coming into direct contact with the eye, using an external electromagnetic excitation and receiving pickup coil, which can be placed in a device, such as spectacles, that can be worn safely, comfortably and conveniently without disturbance of vision or ocular physiology. An external energy source is used to excite the LC circuit and the resulting signal emitted by the circuit is received remotely via the external detector pickup coil. The signal is electronically processed to determine its resonant frequency and in turn correlated to the IOP level. The implanted device contains no internal energy source without concerns about implantable power sources such as batteries. Results: Pressure chamber and in vitro tests demonstrated that the IOP sensor could measure pressures with accuracy and consistency. We produced several prototypes varying from 1.3 to 6.0 mm. in diameter. Our prototypes had resolutions of 1.2 to 1.4 mmHg with routine test equipment. Pressure measurements from 0 to 120 mmHg correlated to frequencies between 202.577 to 201.744 MHz. The Q’s varied from 37 to 58. The device could potentially measure IOPs several times per second. Conclusions: An implantable microsensor will allow continuous monitoring of IOPs and could be used for glaucoma research in animals, but more importantly it could help ascertain IOPs more accurately in humans and determine when to treat glaucoma.

Keywords: intraocular pressure • circadian rhythms • anterior chamber