The use of digital microfluidics (DMF) for sample handling in a microarray immunoassay was investigated. A two plate DMF device was used, with the top plate being used for the immobilization of antibodies for a sandwich immunoassay. A patterning procedure was developed for the top plate to expose patches of glass that were chemically modified, using silane chemistry, to allow for the covalent immobilization of antibodies. For creating microarrays, a set of parallel microchannels were used for the high density patterning of proteins onto the functionalized surface of the top plate. This patterning procedure was optimized to ensure the reproducibility of the immobilization and the physical integrity of the top plate. Preliminary work for a multiplexed immunoassay such as verification of cross-reactivity and detection schemes was also conducted. This work represents the initial efforts towards a microarray immunoassay on DMF, which has the potential to improve high throughput analysis.

Microfluidics was introduced in the early 1990’s and was posited to usher in a new age of integrated analysis systems in the form of labs-on-a-chip. To date, numerous embodiments of microfluidic technologies including fully integrated analysis systems have been described for various applications. Microfluidics can be sub-divided into two paradigms based on fluid manipulation in streams or droplets. In the former, streams of fluids flow through micron-dimension channels, and typical volumes manipulated are in the pico-liter to nano-litre range. These devices are mainly employed for rapid, high efficiency chemical separations, among other applications. In the latter, droplets are manipulated on a dielectric-coated array of microelectrodes in a process called digital microfluidics (DMF). In DMF each droplet is individually addressable, giving superior spatial control over fluid droplets with volumes in the pico-liter to micro-litre range. Independently addressable droplets make DMF amenable to carrying out sequential reactions. This thesis presents methods towards the integration of these two microfluidic paradigms into “hybrid microfluidic” platforms. Hybrid devices contain a DMF array for sample preparation and a microfluidic channel network for on-line analysis by chemical separation. Sample transfer between the platforms is made by way of a digital-channel interface, which has been fabricated in two geometries: side-on and vertical. Chemical separations on hybrid devices are performed in various open-channel and chromatographic modes. In open-channel methods analytes are separated by microchannel zone electrophoresis (MZE) or micellar electrokinetic chromatography (MEKC). In chromatographic separations porous polymer monolithic (PPM) columns were created in situ by UV-initiated polymerization of acrylate monomers. Prior to integration into hybrid microfluidic devices PPMs were optimized for use in gradient elution microchannel electrochromatography (MEC) of peptides. It is anticipated that hybrid microfluidic devices will bridge a large bottleneck for myriad analyses by combining sample preparation with on-line analysis by chemical separation.

This thesis describes a study of various aspects of chemical reactions conducted in microfluidic reactors. (i) In the first project, we proposed the application of the 'internal trigger' approach to multi-step microfluidic polymerization reactions conducted in droplets, namely, polyaddition and polycondensation. We hypothesized and experimentally established that heat generated in the exothermic free radical polymerization of an acrylate monomer triggers the polycondensation of the urethane oligomer. As a result, we synthesized monodispersed poly(acrylate/urethane) microparticles with an interpenetrating polymer network structure. (ii) In the second project, we developed a multiple modular microfluidic reactor with the purpose of increasing productivity in microfluidic synthesis. Compared to the productivity of the single microfluidic reactor < 1g/hr, we synthesized poly(N-isopropylacrylamide) particles at a productivity of approximately 50g/hr with a CV < 5%. We analyzed and addressed several challenges of this process, such as the fidelity in the fabrication of microfluidic reactors, crosstalk between individual reactors sharing a common liquid supply, and coalescence of droplets. (iii) We developed an integrated microfluidic reactor comprising four parallel individual reactors to study the effect of geometry and surface energy of the microchannels on the emulsification process. We spontaneously generated droplets with different volumes by integrating individual droplet generators in parallel with varying geometry. This approach is important in studies of the effect of droplet surface and volume on chemical reactions, and in the studies of diffusion-controlled processes. (iv) We conducted a microfluidic study of the reversible binding of CO2 to secondary amines in the process that mediates solvent polarity switch. We studied reaction rates and CO2 uptake by generating plugs of gaseous a CO2 and monitoring the change in their dimensions. We also demonstrated fast screening of reaction conditions, as well as the ability to reverse the reaction in situ.

The physical mechanism responsible for droplet manipulation in electrowetting on dielectric (EWOD) devices is not yet fully understood. This investigation will examine the role of capillary forces on droplet manipulation to further the physical understanding of these devices. An analytical model for the capillary force acting on a confined droplet at equilibrium is developed here. Model predictions were validated using optical measurements of the droplet interface in the vertical plane. It was found that the capillary force and interface shape predicted by the equilibrium model were over an order of magnitude more accurate than predictions from the model commonly used in EWOD investigations. The equilibrium model was adapted to droplets with arbitrary shapes to predict droplet dynamics in EWOD devices. It was found that droplet motion could be described using the driving capillary force and frictional forces from wall shear, the contact line, and contact angle hysteresis. Comparison with experimental data shows that this model accurately predicts the effects of applied voltage and droplet aspect ratio on the transient position and velocity of droplets. This model can be used to design EWOD devices and predict the simultaneous manipulation of droplets required to meet the high throughput demands of practical applications. A robust system for droplet monitoring must be automated before EWOD devices can be used reliably in practical applications. Although capacitance measurements have been used to automate droplet detection in EWOD devices, manual optical measurements are generally used to monitor droplet mixing. This may not be possible in high throughput applications with multiple droplets and limited optical access. Here, capacitance measurements are shown to be an accurate and repeatable means of monitoring droplet composition and real time mixing. Experiments were performed with this technique to show that mixing efficiency is better characterized by the number of translations required for full mixing, not mixing time.

A simple and efficient technique for the concentration enrichment of charged species within a microfluidic device was developed. The functional component of the system is a hydrogel microplug photopolymerized inside the microfluidic channel. The fundamental properties of the nanoporous hydrogel microplug in modulating the electrokinetic transport during the concentration enrichment were investigated. The physicochemical properties of the hydrogel plug play a key role in determining the mode of concentration enrichment. A neutral hydrogel plug acts as a physical barrier to the electrophoretic transport of charged analytes resulting in size-based concentration enrichment. In contrast, an anionic hydrogel plug introduces concentration polarization effects, facilitating a size and charge-based concentration enrichment. The concentration polarization effects result in redistribution of the local electric field and subsequent lowering of the extent of concentration enrichment. In addition, an electroosmotic flow originating inside the pores of the anionic hydrogel manipulates the location of concentration enrichment. A theoretical model qualitatively consistent with the experimental observations is provided.

The next step in miniaturization of analytical devices involves the use of MEMS and Lab-on-a-Chip applications, where many biological or chemical reactions are carried out on the device in real time. Since detection mechanisms occur almost immediately after the reactions, inefficient mixing of reagents could cause a decrease in sensing capability, especially on micro- and nano-scaled devices. Thus a microfluidic mixer has become a crucial component in these applications. Here we propose a new design of a passive microfluidic mixer that utilizes the theories of chaotic advection to enhance mixing. The micro-channels for the mixer have dimensions with width ranging from 10µm to 40µm, depth 40µm, and a total length of 280µm. First the designs are simulated using CFD-ACE+ for computational analysis. After the device geometry has been decided, the actual devices are fabricated using traditional UV photolithography on silicon and bonded with pyrex glass by anodic bonding. To test the actual device mixing efficiency, we used a fluorescent dye rhodamine B solution to mix with DI water and put the devices under fluorescent microscope observations for real-time analysis. Images of fluorescent light intensities are taken at different flow rates during the analysis and are later used to study the experimental results calculated using a published mixing efficiency formula for comparison.

In this study we are presenting a proof-of-concept microfluidic device that simultaneously applies the conditions required for microelectroporation and micro-sonoporation in a flow-through fashion that allows for high throughput, high efficiency transfection of mammalian cells. During the design stage, we developed a low-cost, high-resolution polymer microfabrication technique termed laser stenciling. While few other electro-sonoporation protocols have been reported, to the best of our knowledge, we are the first to incorporate microelectroporation, which has been well established in literature to be advantageous to conventional electroporation, with flow-through micro-sonoporation. When comparing transfection efficiency for our electro-sonoporation method to that of sonoporation or microelectroporation alone, we observed single batch improvements up to 20 percent and 17 percent, respectively. The average improvement in efficiency was approximately 15 percent greater than achieved with sonoporation and 10 percent greater than that of electroporation. Importantly, there was little difference in short term cell viability between the three methods (maintained at > 90 percent). The average transfection efficiency for electro-sonoporation was 81.25 percent and cell viability was 91.56 percent. Overall, we have presented a device and electro-sonoporation method that meets or outperforms the transfection efficiency and cell viability standards for HeLa cells set by other reported electroporation and sonoporation methods.

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Microfluidics." Includes bibliographical references (p. 115-122).

Thesis (Ph. D.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains xiii, 110 p. : ill. (some col.) + 4 video files. Includes four QuickTime video files. Vita. Includes abstract. Includes bibliographical references (p. 109-110) and index.

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on December 17, 2009. Major professor: David Clague, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in biomedical Engineering." "June 2009." Also available on microfiche.