A technique to measure eyelid pressure using piezoresistive sensors

Abstract

Novel procedures were developed to use a thin (0.17 mm) tactile piezoresistive pressure sensor mounted on a rigid contact lens to measure upper eyelid pressure. A hydrostatic calibration system was constructed and the influence of conditioning (prestressing), drift (continued increasing response with a static load) and temperature variations on the response of the sensor were examined. To optimally position the sensor-contact lens combination under the upper eyelid margin, an in vivo measurement apparatus was constructed. Calibration gave a linear relationship between raw sensor output and actual pressure units, for loads between 1 and 10 mmHg (R²=0.96). Conditioning the sensor prior to use regulated the measurement response and sensor output stabilised about 10 seconds after loading. While sensor output drifts slightly over several hours, it was not significant over the measurement time of 1 minute used for eyelid pressure. The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. Eyelid pressure readings were observed when the upper eyelid was placed on the sensor and removed during a recording. When the eyelid pressure was increased by pulling the lids tighter against the eye, the readings from the sensor significantly increased.