A New Technique to Detect Ocular Pathologies Based on Electrical Measurement Implemented on Programmable Logic

We developed a technique that detects superficial ocular pathologies based on the measurement of electrical impedance spectra. The sensor used is a small microelectrode made of platinum insulated from a cylindrical counterelectrode built of surgical stainless steel. The sensor has the shape of a truncated cone made of acrylic with dimensions identical to that of a standard Goldman prism. The sensor is applied to normal and pathological subject eyes with a constant force provided by a commercial tonometer. The circuit is closed through the lacrimal layer and the epithelial and endothelial cells. We measure the electrical impedance with a programmable logic device in which we implemented all the significant functions. These are the synthesis of the seventeen sines for the excitation, one lock-in, and delta-sigma modulators for the digital-to-analog converter and analog-to-digital converter requirements. A simple analog circuit filters the output, implements a voltage divider, and acts as current limiter in order not to damage the cells. We convert the measurements to resistance and capacitance as a function of frequencies. Consistent results are obtained for left and right eyes of the normal subjects. Significant differences are detected between the results for normal eyes and pathological eyes.