Development of contact lens sensor and wireless sensing system for intraocular pressure monitoring

Intraocular pressure (IOP) is a dynamic parameter with circadian rhythms and varies over 24 hrs. As a result, glaucoma screening and treatment based on a single measurement during regular clinic hours is often insufficient. IOP can be monitored by tracking the corneal curvature changes using contact lens tonometry (CLT). Existing contact lens sensors (CLS) with embedded silicon chip are thick and rigid due to the need for proper encapsulation of the chip. A thick lens is less likely to conform to the eye curvature and may result in large IOP reading errors from eyelid-included contact lens movements. Moreover, it would be easier for a thick contact lens to encounter problems such as poor oxygen permeability, dry eyes, discomfort and lens eccentricity. In this thesis, a capacitive and an inductive CLS with chipless design are designed and fabricated to continuously monitor IOP. A passive inductor-capacitor (LC) resonator is embedded into a silicone contact lens to track the corneal curvature change. IOP variations will deform the cornea, which can be tracked by the CLS and change the resonance frequency of the resonator. The change in the resonance frequency, which is directly correlated with the change in corneal curvature and IOP, is measured wirelessly using a reading coil coupled with the sensor. The sensors were tested on a silicone rubber model eye, enucleated porcine eyes and living rabbit eyes to characterize their electrical and IOP sensing performance with clinical relevant IOP range between 5 and 40 mmHg. During the tests, the change of the resonance frequency was calibrated with the IOP measured by a needle pressure sensor inserted into the eye. The results show that both the capacitive and inductive CLS can accurately track fluctuating IOP, and have excellent linearity with a frequency response of ~25 and ~8 kHz/mmHg, respectively. A wireless reader integrated into the frame of a pair of spectacles is also developed in this thesis for measuring the resonance frequency of the CLS. The reading accuracy, distance and electromagnetic emission power of the reader are calculated and optimized in this thesis. The wireless reader was tested in living rabbits. Results show that < 1 mmHg IOP reading resolution, >10 Hz sampling rate and >30 mm reading distance between the readout coil and the CLS were achieved in combination with the CLS in the in vivo rabbit tests. The safety and tolerability of the CLS were tested in healthy human eyes. No damage to the corneal surface was observed in the tests. IOP fluctuation profiles during specific postural changes and exercise are presented in this thesis. Highly individual and repeatable profiles were obtained. Results show that the CLS and the wireless reader can detect IOP fluctuations related to postural changes and exercise. With the chipless design, the thin CLS and the wireless reader being integrated in glasses frame, this new design can be potentially used for IOP monitoring in clinics.