Dissertations and Theses

Date of Award

2020

Document Type

Dissertation

Department

Mechanical Engineering

First Advisor

Ioana Voiculescu

Second Advisor

Glen Kowach

Third Advisor

Fang Li

Keywords

Surface acoustic wave, sensor, MEMS, QCM, quartz, biosensor

Abstract

Although many air-based sensing mechanisms exist, not all of them can be applied in the water-based environment. In order to obtain a cell-based biosensor, the sensing method needs to be reliable and repeatable in a liquid environment. Therefore, this study is focused on transforming existing air-based sensors to water-immersed applications. This study includes two types of sensors.

First, commercial quartz resonators are used to investigate live-cell activity in water-based toxic solutions. I perform toxicity tests using quartz crystal microbalance (QCM). The QCM used in the study has a resonant frequency of 10 MHz and consists of an AT-cut quartz crystal with gold electrodes on both sides. Rainbow trout gill epithelial cells (RTgill-W1) are cultured on the resonators as a sensorial layer. The fluctuation of the resonant/fundamental frequency, due to the change of cell morphology and adhesion, is an indicator of water toxicity. The shift of resonant/fundamental frequency provides information about the viability of the cell monolayer after exposure to toxicants. Experiment setup, fabrication process, and sensor sensitivity testing are also addressed.

Second, I use surface horizontal modes (SH modes) on ST-cut quartz. Both Rayleigh modes and surface horizontal modes are designed and tested. Surface acoustic wave (SAW) can be used for weight/mass sensing in the air environment, and surface horizontal modes (SH modes) can be used for the water-immersed application. Interdigitated transducers (IDT) induce the deformation of an ST-cut quartz crystal substrate under AC source, and generated waves can propagate along the surface. With a thin layer of polymer, like parylene and polyimide, the SH waves are confined between the interface of the substrate and polymer layer without suffering the energy loss due to the liquid damping from above. The fundamental frequency of the SAW device is defined by the spacing between the fingers of IDT. The frequency of interest for this research is below 100 MHz in a water-based environment. Electrode thickness, IDT designs, and waveguide effects are the key factors to perform qualified signals for SH modes. Experiment setup, fabrication process, and sensor characterization are also addressed. Numerical simulation is used for device validation.

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