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A. J. Shaw, M. J. Collins, B. A. Davis, L. G. Carney; The Measurement of Eyelid Margin Pressure on the Ocular Surface. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5623.
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To estimate the pressure of the eyelid against the ocular surface we developed a novel method using a thin (0.18 mm) tactile piezoresistive pressure sensor (Tekscan Inc., Boston, MA, USA) mounted on a rigid contact lens.
Curvature of the sensor causes an offset and altered sensitivity, so a contact lens was designed with a flat region where the sensor could be adhered. We examined the response of the sensor in different conditions including the influence of conditioning (prestressing), drift (continued increasing response with a static load) and temperature variations. Techniques were developed to calibrate the sensor output into real pressure units (mmHg) and to hold and optimally place the sensor-contact lens combination under the eyelid margin.
Calibration gave a linear relationship between raw sensor output and actual pressure units, for loads between 1 and 10 mmHg. Conditioning the sensor prior to use regulated the measurement response. Sensor output stabilised about 10 seconds after loading. While the output drifted slightly over several hours, the drift was not significant over the measurement time required for eyelid pressure (<30 seconds). 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 could be seen when the upper eyelid was placed on the sensor and then removed three times during a recording. Also when the eyelid was pulled from the outer canthus (similar to the lid-pull technique for removing a contact lens) the readings from the sensor significantly increased.
A number of novel procedures were developed to use a tactile sensor to measure eyelid pressure. These included designing a custom contact lens with a flat area to which the sensor could be attached, a hydrostatic calibration system and an in vivo measurement apparatus to hold and position the sensor-contact lens combination on the eye.
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