Pathogens in drinking water are the cause of over 1.5 million deaths around the world every year, mostly in developing countries. Practical, cheap, and effective tools for detection of these pathogens are critical to advance public health in many areas around the globe. Micro electro-mechanical systems (MEMS) are miniaturized structures that can be used for a variety of purposes, including, but not limited to, small scale sensors. Therefore, MEMS can be used in place of expensive laboratory equipment and offer a cheap and practical tool for pathogen detection. The presented work's research objective is to introduce a new technique called embedded passivated-electrode insulator-based dielectrophoresis (EπDEP) for preconcentration, separation, or enrichment of bioparticles, including living cells. This new method combines traditional electrode-based DEP and insulator-based DEP with the objective of enhancing the electric field strength and capture efficiency within the microfluidic channel while alleviating direct contact between the electrode and the fluid. The EπDEP chip contains embedded electrodes within the microfluidic channel covered by a thin passivation layer of only 4 μm. The channel was designed with two nonaligned vertical columns of insulated microposts (200 μm diameter, 50 μm spacing) located between the electrodes (600 μm wide, 600 μm horizontal spacing) to generate the nonuniform electric field lines to concentrate cells while maintaining steady flow in the channel. The performance of the chip was demonstrated using Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial pathogens in aqueous media. Trapping efficiencies of 100% were obtained for both pathogens at an applied AC voltage of 50 V peak-to-peak and flow rates as high as 10 uL/min. / Master of Science

The focus of this research is the isolation of waterborne pathogens which are one of the grand challenges to human health, costing the lives of about 2.5 million people worldwide each year. The aim was to develop new microfluidic techniques for selectively concentrating and detecting waterborne pathogens. Detection of microbes in water can greatly help reduce deaths; however, analytical instruments cannot readily detect them due to the extreme dilution of these microbes, and hence, require significant sample concentration. Current methods are expensive and either require days to process or are not sufficiently robust for water monitoring. Microfluidic chips based on insulator-based dielectrophoresis (iDEP) provide a promising solution to these problems and have been previously used to selectively concentrate biological particle such as bacteria. The microfluidic devices in this work were created with a 3D mircofabrication technique, which we also developed as part of this project. The core process of the technique is the etching of 3D structures in silicon with a single plasma etch utilizing an effect known as reactive ion etch lag (RIE lag). Using this unique process, 3D devices are fabricated in both silicon and the polymer polydimenthylsiloxane (PDMS). Using both numerical modeling and experimental results, we show how these 3D structures enhance the performance of the dielectrophoretic devices. The main findings indicate that 3D structures can help reduce Joule heating in the devices and lower the applied voltage necessary to operate the devices. Additionally, within this work, we develop a new dielectrophoresis technique called off-chip passivated-electrode, insulator-based dielectrophoresis microchip (O"DEP). This technique combines the sensitivity of electrode-based dielectrophoresis (eDEP) with the high-throughput and inexpensive device characteristics of insulator-based dielectrophoresis. The result is a cartridge based system which is accessible, economical, high-performance, and high-throughput technologies allowing timely detection of pathogenic bacteria. / Ph. D.

This work was focused on the characterization of microparticles with particular emphasis on waterborne pathogens which pose a great health risk to human lives. The goal of this study was to develop microfluidic systems for enhanced characterization and isolation of bioparticles. Insulator-based dielectrophoresis (iDEP) is a promising technique for analyzing, characterizing and isolation of microparticles based on their electrical properties. By employing insulator-based constrictions within the microchannel in combination with microelectrodes within the vicinity of the electrodes, dielectrophoretic performance is enhanced. In this study, three dimensional insulator-based dielectrophoresis devices are fabricated using our in-house developed 3D micromachining technique. This technology combines the benefits of electrode-based DEP, insulator-based DEP, and three dimensional insulating features with the goal of improving trapping efficiency of biological species at low applied signals and fostering wide frequency range operation of the microfluidic device. The dielectric properties of bacteria as well as submicron polystyrene beads are discussed and the impact of these results on the future development of iDEP microfluidic systems is explored. / Master of Science

Nanobelts are a new class of semiconducting metal oxide nanowires with great potential for nanoscale devices. The present research focuses on the manipulation of SnO₂ nanobelts suspended in ethanol using microfluidics and electric fields. Dielectrophoresis (DEP) was demonstrated for the first time on semiconducting metal oxide nanobelts, which also resulted in the fabrication of a multiple nanobelt device. Detailed and direct real-time observations of the wide variety of nanobelt motions induced by DEP forces were conducted using an innovative setup and an inverted optical microscope. High AC electric fields were generated on a gold microelectrode (~ 20 µm gap) array, patterned on glass substrate, and covered by a ~ 10 µm tall PDMS (polydimethylsiloxane) channel, into which the nanobelt suspension was introduced for performing the DEP experiments. Negative DEP (repulsion) of the nanobelts was observed in the low frequency range (< 100 kHz) of the applied voltage, which caused rigid body motion as well as deformation of the nanobelts. In the high frequency range (~ 1 MHz - 10 MHz), positive DEP (attraction) of the nanobelts was observed. Using a parallel plate electrode arrangement, evidence of electrophoresis was also found for DC and low frequency (Hz) voltages. The existence of negative DEP effect is unusual considering the fact that if bulk SnO₂ conductivity and permittivity values are used in combination with ethanol properties to calculate the Clausius Mossotti factor using the simple dipole approximation theory; it predicts positive DEP for most of the frequency range experimentally studied. A fluidic nanobelt alignment technique was studied and used in the fabrication of single nanobelt devices with small electrode gaps. These devices were primarily used for conducting impedance spectroscopy measurements to obtain an estimate of the nanobelt electrical conductivity. Parametric numerical studies were conducted using COMSOL Multiphysics software package to understand the different aspects of the DEP phenomenon in nanobelts. The DEP induced forces and torques were computed using the Maxwell Stress Tensor (MST) approach. The DEP force on the nanobelt was calculated for a range of nanobelt conductivity values. The simulation results indicate that the experimentally observed behavior can be explained if the nanobelt is modeled as having two components: an electrically conductive interior and a nonconductive outer layer surrounding it. This forms the basis for an explanation of the negative DEP observed in SnO₂ nanobelts suspended in ethanol. It is thought that the nonconductive layer is due to depletion of the charge carriers from the nanobelt surface regions. This is consistent with the fact that surface depletion is a commonly observed phenomenon in SnO₂ and other semiconducting metal oxide materials. The major research contribution of this work is that, since nanostructures have large surface areas, surface dominant properties are important. Considering only bulk electrical properties can predict misleading DEP characteristics.