Graduation Year

2022

Document Type

Thesis

Degree

M.S.B.E.

Degree Name

MS in Biomedical Engineering (M.S.B.E.)

Degree Granting Department

Biomedical Engineering

Major Professor

Venkat R. Bhethanabotla, Ph.D.

Committee Member

Robert Frisina, Ph.D.

Committee Member

John Kuhn, Ph.D.

Keywords

biofouling, piezoelectric materials, Rayleigh wave, shear-horizontal wave, Surface acoustic wave (SAW)

Abstract

Detection of trace amounts of biomarkers is a critical requirement in sensing the presence of conditions such as early-stage cancer. The ST cut of the Quartz crystal (ST-Quartz) supports two fundamental surface acoustic waves (SAWs) in mutually perpendicular, or orthogonal, directions: the Rayleigh wave and the shear-horizontal wave (RSAW and SH-SAW, respectively). The orthogonality enables the use of ST-Quartz in biofouling removal by the RSAW and mass sensing via the SH-SAW simultaneously. The purpose of this study is to find an ideal guiding layer and wavelength configuration that can enhance the sensing capability without sacrificing the biofouling removal ability for a biosensor.

Finite-element modeling (FEM) has been used to predict the behavior of complex materials with fair reliability. First, the resonant frequency of the Love mode (which is a layer-guided SH-SAW) will be determined for a certain thickness of ZnO guiding layer via the transient analysis of its impulse response. Then, the thickness and frequency data will be used to create a perturbation-theory-based steady-state analysis model that calculates sensitivity of the configuration. Once the optimal guiding layer thickness has been found from the first two points above, different configurations of the RSAW transducers can be explored to maximize kinetic energy on the surface, or in this case, voltage and displacement power, using FEM.

From preliminary results, the Love wave and RSAW models were confirmed to be reliable by verifying wave velocity and nodal displacement profiles with previously reported data. The sensitivity results were plotted against the ZnO guiding layer thickness. Love wave models with different wavelengths were found to have distinct sensitivity peaks, indicating the presence of an optimal thickness. For the ZnO/ST-Quartz configuration, the optimal thickness for the Love wave was found to be a ratio of approximately 0.035 of layer thickness/wavelength. The RSAW model was tested with two IDT configurations: one with the IDTs below the guiding layer and another with IDTs above the guiding layer. Similar to the Love wave simulations, a peak power was observed in the RSAW models for a ZnO layer thickness/wavelength ratio of approximately 0.4.

Some drawbacks of these models include the crystal rotation mathematics, which assume a cubic nature of crystals; variability of material properties in practical applications; and the simplified nature of the FEM in comparison with actual devices.

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