Electrical Method for In Vivo Testing of Exhalation Sensors Based on Natural Clinoptilolite
Authors: Carotenuto Gianfranco , Nicolais Luigi 
Published: March 2022

Natural substances with a complex chemical structure can be advantageously used for functional applications. Such functional materials can be found both in the mineral and biological worlds. Owing to the presence of ionic charge carriers (i.e., extra-framework cations) in their crystal lattice, whose mobility is strictly depending on parameters of the external environment (e.g., temperature, humidity, presence of small gaseous polar molecules, etc.), zeolites can be industrially exploited as a novel functional material class with great potentialities in sensors and electric/electronic field. For fast-responding chemical-sensing applications, ionic transport at the zeolite surface is much more useful than bulk-transport, since molecular transport in the channel network takes place by a very slow diffusion mechanism. The environmental dependence of electrical conductivity of common natural zeolites characterized by an aluminous nature (e.g., chabasite, clinoptilolite, etc.) can be conveniently exploited to fabricate impedimetric water-vapor sensors for apnea syndrome monitoring. The high mechanical, thermal, and chemical stability of geomorphic clinoptilolite (the most widely spread natural zeolite type) makes this type of zeolite the most adequate mineral substance to fabricate self-supporting impedimetric water-vapor sensors. In the development of devices for medical monitoring (e.g., apnea-syndrome monitors), it is very important to combine these inexpensive nature-made sensors with a low-weight simplified electronic circuitry that can be easily integrated in wearable items (e.g., garments, wristwatch, etc.). Very low power square-wave voltage sources (micro-Watt voltage sources) show significant voltage drops under only a minimal electric load, and this property of the ac generator can be advantageously exploited for detecting the small impedimetric change observed in clinoptilolite sensors during their exposition to water vapor coming from the human respiratory exhalation. Owing to the ionic conduction mechanism (single-charge carrier) characterizing the zeolite slab surface, the sensor biasing by an ac signal is strictly required. Cheap handheld multimeters frequently include a very low power square-wave (or sinusoidal) voltage source of different frequency (typically 50 Hz or 1 kHz) that is used as a signal injector (signal tracer) to test audio amplifiers (low-frequency amplifies), tone control (equalizer), radios, etc. Such multimeter outputs can be connected in parallel with a true-RMS (Root-Mean-Square) ac voltmeter to detect the response of the clinoptilolite-based impedimetric sensors as voltage drop. The frequency of exhalation during breathing can be measured, and the exhalation behavior can be visualized, too, by using the voltmeter readings. Many handheld multimeters also include a data-logging possibility, which is extremely useful to record the voltage reading over time, thus giving a time-resolved voltage measurement that contains all information concerning the breathing test. Based on the same principle (i.e., voltage drop under minimal resistive load) a devoted electronic circuitry can also be made.