Surface Acoustic Wave Propagation in Anisotropic Layered Mediums for Biosensor Application: Characterisation and Numerical Method Development

Abstract

The main motivation for conducting this research is the continued development of multilayered mechanical biomedical sensors. Biosensors which make use of resonant surface acoustic waves are a contender for Point-of-Care diagnostic technology. An area of interest for this research group, is in microsystem devices that make use of the cyclic degeneracy of cylindrical geometry. Ideally, these modes would remain invariant in the angular axis of the polar form geometry, in reality this isotropy can not be guaranteed. In multilayered biosensor design, various properties of the material and geometry, will introduce anisotropic behaviour. The main focus of this research is the effect of these properties on this variation of the wave modes with, the propagation angle in a material cut. A minor focus is placed on the effect of the various attenuation mechanisms, due to these physical properties, on the quality of the wave resonance. A uniform numerical framework is developed, by making use of the multidimensional complex structure of the functions, capable of solving all the problems presented. The numerical method, which is well-suited to fixed velocity searches, is combined with optimisation methods to improve efficiency with other search variables. For biosensor application, the common attenuated mechanisms are modelled, these include bulk leaky, viscoelastic, fluid and thermoelastic losses. The effect of these on the annular variation, in addition to the stiffness anisotropic and waveguide dispersions, is investigated. The fixed velocity solutions are computed for several propagation angles, for both novel and currently feasible biosensor combinations. The non-fixed velocity versions of these surfaces are computed for use in shaped annular transducer designs. For the material combinations of interest for sensor application, the different sources of attenuation are compared using a Q-factor approach. The research conducted in this thesis, provides insight into the material properties that alter the variation of wave modes with angle. Design and research recommendations for mechanical biosensors, have been made based on the cyclic variation and attenuation investigations.