Model for suspended gate field effect transistors used in laboratory animal cage monitoring

Over the past century, great advances in medicine have been achieved through the use of laboratory animals, specifically rodents. The quality of the animal environment is important to the rodent's health and welfare, and their well-being directly affects the quality of research involving their use. There can be significant variability in air quality between cages depending on a number of factors such as population size and air flow. A way to accommodate for the variability between cages is to monitor environmental quality indicators within the cage, such as ammonia, carbon dioxide, temperature, and relative humidity. Since rodent cages are approximately the size of a shoebox, commercially available sensors would be too large for this application. Therefore, micro-sensors, or field effect transistors, were investigated for application in a rodent cage. Since these sensors were on the forefront of technology, a theoretical model was developed for the ammonia sensor to further understand the chemical reaction taking place on its surface.;The sensors were tested in a controlled environment, where the air quality was known. The magnitude and time of the response to different levels of contaminants (e.g., ammonia and carbon dioxide) were determined. The study showed that the sensors can detect changes in air quality in a sufficiently short amount of time (5 minutes) so that corrective action could be taken to prevent the rodents from overexposure to harmful levels of air contaminants. At the present development stage, the sensors used for this investigation will require further improvements before implementation in a laboratory animal cage. These improvements include but are not limited to eliminating drift of baseline signal, increasing sensitivity of sensor, amplifying signal output, and coupling each gas sensor with a humidity sensor.;The reaction mechanism selected for the model which was best supported by the literature and the experiments was molecular adsorption of ammonia on a titanium nitride surface. The experimental results were fitted to the model to obtain the adsorption and desorption rate constants, the equilibrium concentration constant, equilibrium constant, and Gibbs free energy, which respectively were 6.28 L/mol*s, 6.43 x 10-3 s -1,976.7 L/mol, 39.04, and -9.25 kJ/mol. Based on these values, it was determined that the forward reaction, or adsorption, occurs spontaneously. There was good correlation between the theoretical model and the experimental results, indicating that the theoretical model was sufficient for this application.