ZEOLITE-BASED FAST-RESPONDING SENSORS FOR RESPIRATORY RATE MONITORING

The respiratory rate can be conveniently monitored by measuring the concentration of water vapor in the exhaled air. Since human breathing is a quite fast process, fast-responding sensors are required for this application. In addition, high-resolution breathing patterns could provide reliable information on the performance of the respiratory system, useful in medicine (spirometry, general anaesthesia, etc.) and sport fields. Although a variety of humidity sensors are described in the literature, those with a fast response form only a small subset. The use of zeolites as mechanically/thermally stable and inexpensive water sensors is well known, however these sensors have also fast response and high sensitivity, and therefore could result as adequate for respiratory rate monitoring. Natural clinoptilolite has been selected for this study because it has a high Si/Al ratio (5.3) which allows the material to experience significant conductivity changes by exposure to humidity. The basic mechanism of humidity detection in zeolites is based on the change of ionic conduction with the humidity level. In particular, owing to the presence of extra-framework cations, electrical transport at the surface of clinoptilolite depends on the environmental humidity. In fact, at room temperature, the cation mobility is very low in a dry atmosphere, because of the strong electrostatic interactions with framework negative charge, while it becomes much higher after that a water molecule has joined the cationic site. Thus, water adsorption modifies the charge carrier density, which reflects in a change of the intensity of current moving at the sample surface. Here, the temporal-evolution of sinusoidal current signal (5kHz), moving in a sample of natural clinoptilolite during the water adsorption-desorption process, has been recorded and analyzed to investigate the involved kinetic mechanism.