Fabrication and preliminary assessment of a fiber optic-based relative humidity sensor for application in mechanical ventilation

During mechanical ventilation, specific devices are used to heat and to humidify gases delivered by mechanical ventilators. Basically, they must replace the function of the upper airways that are bypassed by tracheostomy or endotracheal tubes. Among others heated wire humidifiers (HWHs) are the most popular to accomplish this task. Their control is not based on the measure of relative humidity (RH) but on the gas temperature in a single point of the breathing circuit. Owing to the mentioned control, HWHs' performance shows some weaknesses in particular when they work under demanding ventilatory settings. Moreover, also environmental conditions (e.g., temperature) can affect HWHs' performance. The direct control of the air humidity may be beneficial to overcome these hurdles. The aim of this work is to develop a fiber optic sensors, based on fiber Bragg grating (FBG) technology for RH measurements in mechanical ventilation. It consists of a coated FBG, whose output changes with RH due to the volume expansion of the coating material. The proposed sensor has been fabricated with a simple coating procedure and it is coated with a cost effective polymer (i.e., agarose), which can be used in a wide range of RH values. An experimental set-up allowing the delivering of air with different amounts of vapour content (relative humidity from about 0% to saturation) has been prepared to perform a preliminary assessment of the sensor in a wide range of RH values. The proposed sensor shows promising results in terms of both capability to follow RH changes, as well as sensitivity and range of measurement. Future testing will assess in depth the metrological properties of the proposed sensor and will evaluate the potential of the sensor for RH monitoring during mechanical ventilation. The implementation of a feedback based on RH may be useful both to improve the performance of HWHs, to minimize the influence of external factors on the humidification process, as well as to pave the way to new strategy of control in this field.