Unwanted gas bubbles are a challenge for microfluidic-based systems, as adherence to channel networks can disrupt fluid delivery. This is especially true for devices with biological applications, as the presence of a single bubble creates thin fluid films with extremely high shear stresses, which can damage biological samples. Current strategies to remove bubbles require complicated fabrication or off-chip components. This thesis describes an on-chip microfluidic strategy utilizing permeation for in-plane trapping and removal of occasional gas bubbles. The trap was demonstrated with nitrogen bubbles, which were consistently removed at a rate of 0.14 µL/min for a single trap, and shown to have long-term operation capability by removing approximately 4,000 bubbles during one day without failure. The trap was integrated with a microfluidic system for the study of small blood vessels. Experiments were complemented with analytical and numerical models to characterize the bubble removal process.

This thesis was on the development of a high-throughput electrokinetically-controlled heterogeneous immunoassay (EK-IA) microfluidic chip for clinical application. Through a series of experimental studies, a high-throughput EK-IA was developed. This EK-IA was capable of automatically screening multiple analytes from up to 10 samples in parallel, in only 26 min. Flow control in an integrated microfluidic network was realized by numerical simulation of the transport processes. This EK-IA was successfully applied to detect E. coli O157:H7 antibody and H. pylori antibody from human sera with satisfactory accuracy. Simultaneous screening of both antibodies from human sera was also achieved, demonstrating the potential of this EK-IA for efficiently detecting multiple pathogenic infections in clinical settings. Preliminary work on the application of EK-IA to detect biomarkers of embryo development in embryo culture media also yielded good results. In addition to the experimental studies, the reaction kinetics of this microfluidic EK-IA has also been investigated, using both numerical simulation and a modified Damköhler number. Targeted towards a more sensitive assay, the influences of several important parameters on the reaction kinetics were studied. This EK-IA holds great promise for automated and high-throughput immunoassay in clinical environments.

Microfluidics refers to devices and methods for controlling and manipulating fluid flows at length scales less than a millimeter. Miniaturization of a laboratory to a small device, usually termed as lab-on-a-chip, is an advanced technology that integrates a microfluidic system including channels, mixers, reservoirs, pumps and valves on a micro scale chip and can manipulate very small sample volumes of fluids. While several flow control concepts for microfluidic devices have been developed to date, here flow control concepts based on thermally responsive polymer solutions are presented. In particular, flow control concepts base on the thermally triggered reversible phase change of aqueous solutions of the polymer Pluronic will be discussed. Selective heating of small regions of microfluidic channels, which leads to localized gel formation in these channels and reversible channel blockage, will be used to control a membrane valve that controls flow in a separate channel. This new technology will allow generating inexpensive portable bioanalysis tools where microvalve actuation occurs simply through heaters at a constant pressure source without a need for large external pressure control systems as is currently the case. Furthermore, a concept for controlled cross-channel transport of particles and potentially cells is presented that relies on the continuous regeneration of a gel wall at the diffusive interface of two co-streaming fluids in a microfluidic channel.

Biofilms are highly organized bacterial structures that are attached to a surface. They are ubiquitous in nature and may be detrimental, causing numerous types of illnesses in living organisms. Biofilms in the human oral cavity are the main cause of dental caries and periodontal diseases and can act as a source for pathogenic organisms to spread within the body and cause various types of systemic diseases. Streptococcus mutans is the primary etiological agent of dental caries, the single most chronic childhood disease. In many cases, quorum sensing (QS) is required for initial formation and subsequent development of biofilms and the signaling molecule autoinducer 2 (AI- 2) has been well studied as an inter-species QS signaling molecule. However, recent reports also suggest that AI-2-mediated signaling is important for intra-species biofilm formation in both Gram-negative and positive bacteria. Therefore, there is significant interest in understanding the role of different QS signals such as AI-2 in oral biofilm formation. Microfluidic devices provide biomimetic environments and offer a simple method for executing multiple stimuli experiments simultaneously, thus, can be an extremely powerful tool in the study of QS in biofilms. In this study, we report conditions that support the development of S. mutans biofilms in microchannel microfluidic devices, and the effects of extracellular addition of chemically synthesized (S)-4,5-dihydroxy-2,3-pentanedione (DPD; precursor of AI-2) on mono-species S. mutans luxS (AI-2 deficient strain) biofilm formation using a gradient generating microfluidic device. S. mutans wild type (WT) and luxS biofilms were developed in nutrient rich medium (25% brain heart infusion medium, BHI + 1% sucrose) for up to 48 h. Maximum biofilm formation with both strains was observed after 24 h, with distinct structure and organization. No changes in S. mutans luxS biofilm growth or structure were observed upon exposure to different concentrations of AI-2 in a gradient generating device (0 to 5 M). These results were also validated by using a standard 96-well plate assay and by verifying the uptake of AI-2 by S. mutans luxS. Our data suggest that extracellular addition of AI-2 does not complement the luxS deletion in S. mutans with respect to biofilm formation.

The research presented within this dissertation explores the photocatalytic deposition of metal carried out within sealed microfluidic channels. Micro scale patterning of metals inside sealed microchannels is investigated as well as nanoscale control over the surface morphology of the nanoparticles making up the patterns. This is achieved by controlling solution conditions during deposition. Finally, the nanoparticle patterns are used in fabricating a sensor device, which demonstrates the ability to address multiple patches within a sealed channel with different surface chemistries. Also presented here is the construction of the first epifluorescence/total internal reflection macroscope. Its ability to carry out high numerical aperture imaging of large arrays of solid supported phospholipid bilayers is explored. For this, three experiments are carried out. First, imaging of a 63 element array where every other box contains a different bilayer is preformed, demonstrating the ability to address large scale arrays by hand. Next, a protein binding experiment is preformed using two different arrays of increasing ligand density on the same chip. Finally, a two-dimensional array of mixed fluorescent dyes contained within solid supported lipid bilayers is imaged illustrating the ability of the instrument to acquire fluorescent resonance energy transfer data. Additionally, the design and fabrication of an improved array chip and addressing method is presented. Using this new array chip and addressing method in conjunction with the epifluorescence/total internal reflection macroscope should provide an efficient platform for high throughput screening of important biological processes which occur at the surfaces of cell membranes.

Abstract A new technique for bonding of polymer micro-fluidic devices has been developed. This method can easily bond biochips with complex flow patterns and metal layer. Above all, using a patterned glass, the micro-channel structures on Poly-Methyl Meth-Acrylate (PMMA) substrates were generated by one-step hot embossing procedure. In contrast with the traditional thermal bonding, this paper presents low-temperature and low-pressure packaging for polymer micro-fluidic platforms. Furthermore, the disposable plastic biochip has successfully been tested by the measurement of tensile strength and surface roughness. This paper also reports details of the passive and active micro-mixers. According to experimental and numerical investigations, the mixing performance of passive micro-mixers is expectably to be found. In addition, to quantify the mixing concentration distribution in the micro-channel, it has been demonstrated by launching the image analysis programs. The bonding efficiency of the solvent is twenty four times as strong as thermal bonding efficiency.

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Colloidal suspensions of disk-like particles have been of interest in both colloidal and liquid crystal studies because they exhibit unique liquid crystalline phases different from those of rod-like molecules. Disk-like particles, such as asphaltenes in heavy oil industry, clay particles in agriculture, and red blood cells in biology, are of great interest in a variety of industries and scientific areas. However, to fabricate monodisperse microdisks, uniform in structure or composition with precise control of particle size and shape has not yet succeeded. In this thesis, we show an experimental strategy of using microfluidic technique to fabricate homogeneous α-eicosene microemulsions with chloroform in an aqueous solution of sodium dedecyl sulfate (SDS). The monodisperse chloroform emulsions, generated by the glass-based microfluidic devices, ensure the precise control on microdisk particle size and shape. A systematic investigation was performed to study the relation between the resulted microdisk size and the initial concentration of α-eicosene in chloroform before evaporation. The smectic liquid crystalline phase inside the wax particles controls the coin-like disk shape below the melting temperature of wax’s rotator phase. The kinetics of the disk formation is observed using a polarized light microscope. Dynamic light scattering is used to characterize the Brownian motion of the microdisks, and the rotational diffusion is estimated from the image sequences taken by the charge-coupled device (CCD) camera. Effort has been put into collecting a large quantity of microdisks to investigate the discotic liquid crystalline phases, which can be readily probed by light scattering and microscope. In comparison, X-ray and neutron have to be used for the atomic liquid crystalline phase investigation.

This study investigates the flow behaviors of the microflow in a sudden expansion microfluidic channel with a rectangular block structure. 2D and 3D numerical simulations are used to predict the vortex formation behavior and experimental approaches are adopted to confirm the simulated results. A novel microfluidic rectifier is proposed by operating the designed microfluidic device under opposite flow conditions. The performance of the flow rectifier is also evaluated under difference flow velocities. There are three parts finished in this thesis. Firstly, the vortex formation behavior is investigated for the microchannel with the block at different distances downstream the sudden expansion channel. The size of the fully developed vortices is measured and analyzed. Results show that the size of the vortex reaches stable while the distance between the block and sudden expansion channel is longer than 1000 £gm. Secondly, this study also investigates the sequence of the vortex formation under different flow velocity (Reynolds number). Results indicate that there are four stages for the vortex formation in the microfluidic channel. Vortices are formed firstly at the sudden expansion channel and then behind the block. Two small vortices are then formed once beside the block and then merge with the two big vortices behind the block under increasing velocity conditions. The flow becomes instable once the Reynolds number higher than 555, two symmetrical shedding flows are observed behind the block structure. This flow behavior is rarely observed in a microfluidic channel due to the big viscous force of the flow in the microchannel. Thirdly, this study measures the pressure drops for the forward and backward flows under different flow speeds. Results show that the vortex formation behavior in backward flow is different from it is in forward conditions. Two symmetric vortexes are formed beside the channel while the Reynolds number higher than 416. The squeezed vortices form a virtual valve structure and increase the flow resistance of the microflow, resulting in a high performance valve structure. The calculated results indicate that the diodicity (Di) of the designed microchannel is as high as 1.76 and 1.5 for the numerical result and experimental result, respectively. The rectifying performance of the developed microchip device is higher than the reported devices fabricated using delicate processes and designed. The results of this research will give valuable knowledge for the flow behavior in a microchannel and the design of microfluidic chips.

A series of sulfonate anionic lipids esters derived from 4-sulfobenzoic acid (single chain) or 5-sulfoisophthalic acid (double chain) with chain length from C14 to C18 were synthesized and characterized. The sodium salts were uniformly insoluble in ethanol; the tetramethylammonium salts of the single chain derivative from oleyl alcohol and the double chain derivative from 2-octyldodecan-1-ol were sufficiently soluble for subsequent experiments. Lipids in ethanol and aqueous buffers were mixed in a microfluidic system (NanoAssmblr ® microfluidic mixer) to prepare a lipid dispersion containing vesicles and/or nanoparticles. Initial studies on prediction and controlling vesicle size based on lipid geometric parameters showed that particle size could be successfully affected and controlled by altering lipid compositions consistent with the formation of vesicles. A survey using high resolution cryo-Scanning Transmission Electron microscopy of the sample made by the microfluidic mixer demonstrated that vesicles were formed but a majority of the sample reformed to other aggregates, which complicated the interpretation of the initial product distribution. Further investigation on the efficiency of incorporation of phospholipids into vesicles indicated that 55% of the initial phospholipid appeared in the vesicle fractions. Sulfonate anionic lipids are incorporated into vesicles with lower efficiency and reach a threshold beyond which the sulfonate lipid is not incorporated. Entrapment efficiency was studied with three dyes. Different concentrations of the hydrophobic neutral dye Nile red, the hydrophilic cationic dye neutral red and the hydrophilic anionic dye hydroxypyrene trisulfonate (HPTS) were prepared. The entrapment efficiency was quantitatively analyzed by HPLC, and electrospray mass spectrometry; up to 15% of the initial dye present could be entrapped. Vesicles permeability assays using the ion channel gramicidin and the ion carrier valinomycin with HPTS-loaded vesicle samples showed that vesicle samples made by the microfluidic mixer and made by a conventional extrusion method appeared to behave in the same manner. Addition of a sulfonate anionic lipid to the lipid mixture resulted in vesicle leakage. The unilamellar proportion of HPTS loaded vesicle samples was assessed using a mellitin assay. A vesicle sample made by the microfluidic mixer was 80% unilamellar; a vesicle sample made by the extrusion method on the same lipid mixture was 60% unilamellar. / Graduate / mengxiuzheng@gmail.com