Sensor for measuring surface fluid conductivity in vivo

Thin-film microelectronic technology was used to fabricate a miniature, flexible sensor that can be placed directly onto the surface of the airway to measure the electrical conductance of the fluids present. Gold was deposited on glass, oxidized silicon, and polyimide substrates. The parallel gold electrodes were etched into an interdigitated configuration that increased the effective sensing area without increasing the sensor size. A temperature-sensitive resistor was added to make calculations possible to account for the effect of temperature on fluid conductivity. Fluid conductivity was measured and from this information osmolarity was calculated. Physiologic evaluation of the device and corroboration of the calculations was obtained by placing the sensor in the airway of mongrel dogs and simultaneously collecting surface fluids for atomic absorption spectroscopy and freezing-point-depression analysis. Osmolarity calculations made from the sensor data were consistently about 25% lower than clinical laboratory osmolarity analysis. In vitro tests of the sensor included calibration in multiple dilute salt solutions, temperature sensitivity assessment, and evaluation of the effects of solution concentration on electrical conductance.