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Doctor of Philosophy / Department of Chemistry / Christopher T. Culbertson / Chemical separations are a necessary component in many scientific analyses. Microfluidics, the use of micron-sized fluidic channels defined in glass or polymer blends, is a powerful branch of separation science that is developing rapidly. Miniaturized analytical devices offer important advantages compared to traditional bench-top techniques, most notably capillary electrophoresis (CE). This dissertation was focused on developing several novel methods to improve microfluidic based separations and techniques. The electrophoretic separation of small similarly charged analytes can be very difficult. Chapter 2 discusses a new buffer that has been developed for fast, high efficiency separations of amino acids by micellar electrokinetic chromatography (MEKC). This buffer is more environmentally friendly than the most commonly used surfactant containing buffers for MEKC separations. It uses a commercially available dish washing soap by Seventh Generation™ Inc. that contains three micelle forming agents; sodium lauryl ether sulfate (anionic), cocamidopropyl betaine (zwitterionic), and cocamide monoethanolamine (MEA) (non-ionic), and is completely void of organic solvents. Many biological samples contain analytes below the limit of detection of traditional detection systems; therefore, chapter 3 reports the fabrication of nanoporous membranes on microfluidic devices that are capable of analyte concentration enrichment. Donnan exclusion is responsible for the preconcentration of fluorescent dyes near a charged, porous titania membrane. The level of analyte enrichment was monitored, and enrichment factors greater than 4000 in 400 s were obtained for 2,7-Dichlorofluorescein. Chapter 4 describes the fabrication and characterization of paper based microfluidic devices. Mixtures of acrylate modified photocurable polymers were used to photolithographically define channels on multiple paper substrates. Flow characteristics are described and their use for monitoring complications associated with type 1 diabetes is demonstrated. Finally in Chapter 5, Sol-gel modified gold surfaces for preventing protein adsorption during surface plasmon resonance (SPR) detection are also presented.

Doctor of Philosophy / Department of Chemistry / Christopher T. Culbertson / During the last five years, great advancements in microfluidics have been achieved with the development of “sample-in-answer-out” systems. Such systems have begun to realize the true potential of analytical miniaturization since the concept of the “micro-Total Analysis System” was first envisioned. These systems are characterized by the elegant integration of multiple fluid-handling channel architectures that enable serial execution of sample preparation, separation and detection techniques on a single device. While miniaturization and portability are often identified as key advantages for microfluidics, these highly integrated systems are heavily reliant upon large off-chip equipment, i.e. the microchip is often tethered to the laboratory via multiple syringe pumps, vacuum pumps, solenoid valves, gas cylinders and high voltage power supplies. In this dissertation, a procedure for the facile integration of dielectric elastomer (DE) actuators (called IDEAs) onto microfluidic devices is described. Poly(dimethylsiloxane) (PDMS) is commonly used as a microchip substrate because it is cheap and easy to fabricate, mechanically robust and optically transparent. The operation of an IDEA exploits the ability of PDMS to behave as a smart material and deform in the presence of an electric field. In Chapter 2, the fabrication of IDEA units on a standard microchip electrophoresis device is described. IDEA-derived injections were used to evaluate the physical performance of this novel actuator configuration. In Chapter 3, the analytical merits of IDEA-derived injections were evaluated. Sampling bias caused by electokinetic injection techniques has been problematic for conventional microchip electrophoresis systems due to the lack of fluid access. The hydrodynamic injections created by IDEA operation were found to be highly reproducible, efficient, and possess a negligible degree of sampling bias. In Chapter 4, the spatial characteristics of microchannel deformation due to IDEA actuation have been investigated using fluorescence microscopy. It was determined that the DE compresses more along the edge of the channel than in the middle of the channel. This information can be used to design a new generation of more efficient IDEAs.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This thesis deals with the mathematical optimization of the detecting chamber of a thermal convective microfluidic gyroscope and the comparison of several different optimization strategies. An analytical model is developed for the gyroscope and some design considerations are discussed. Sequential approximate optimization strategies are explained and compared to each other by implementing test problems fromthe literature. The optimization problem is formulated from the analytical model and implemented using the different optimization strategies. Results are presented and compared to find the most effective optimization strategy. A sequential approximate optimization algorithm is implemented in MATLAB and tested using the gyroscope design problem and common test problems from the literature. Results and iteration history are compared with an existing FORTRAN implementation. / AFRIKAANSE OPSOMMING: Hierdie tesis handel oor die wiskundige optimering van die deteksiekamer van n termies-konvektiewe mikrovloeier giroskoop en die vergelyking van verskeie optimeringsstrategieë. ’n Analitiese model is opgestel vir die giroskoop en verskeie ontwerpsoorwegings word bespreek. Sekwensiëel benaderde optimeringsstrategieë word bespreek en met mekaar vergelyk, deur dit op toetsprobleme uit die literatuur toe te pas. Die optimeringsprobleem is geformuleer uit die analitiese model en geimplementeer deur gebruik te maak van verskeie optimeringsstrategieë. Resultate word getoon en vergelyk, omdie mees effektiewe optimeringsstrategie te vind. ’n Algoritme vir sekwensiëel benaderde optimeringsprobleme is inMATLAB geimplementeer. Die giroskoop probleem, asook probleme uit die literatuur, is gebruik om resultate en iterasie geskiedenis te vergelyk met ’n bestaande FORTRAN implementasie.

The development of a novel method of droplet levitation to be employed in lab-on-a-chip (LOC) applications relies upon the mechanism of thermocapillary convection (due to the temperature dependence of surface tension) to drive a layer of lubricating gas between droplet and substrate. The fact that most droplets of interest in LOC applications are aqueous in nature, coupled with the fact that success in effecting thermocapillary transport in aqueous solutions has been limited, has led to the development of a technique for the controlled encapsulation of water droplets within a shell of inert silicone oil. These droplets can then be transported, virtually frictionlessly, resulting in ease of transport due to the lack of friction as well as improvements in sample cross-contamination prevention for multiple-use chips. Previous reports suggest that levitation of spherical O(nL)-volume droplets requires squeezing to increase the apparent contact area over which the pressure in the lubricating layer can act allowing sufficient opposition to gravity. This research explores thermocapillary levitation and translation of O(nL)-volume single-phase oil droplets; generation, capture, levitation, and translation of O(nL)-volume oil-encapsulated water droplets to demonstrate the benefits and applicability to LOC operations.

The mechanical parameters of biological cells are relevant indicators of their function or of disease. For example, certain cancerous cells are more deformable than healthy cells. The challenge consists in developing methods that can measure these parameters while not affecting the cell. The Optical Stretcher is a microfluidic system that deforms single suspended cells without contact using lasers and determines the cells’ viscoelastic properties. The advantage compared to standard methods of molecular biology is that cells do not need to be treated with additional markers. Basic versions of the Optical Stretcher have existed for some years. These allow the measurement of homogeneous cell populations. Up until now, it was only possible to calculate average population values of compliance. To characterize inhomogeneous populations however, it is necessary to consider each single cell and measure additional mechanical or optical parameters such as the refractive index. This work highlights various extensions of the Optical Stretcher. A novel procedure, including an improved image processing algorithm, is presented to analyze mechanical data in real time. In combination with measurements of the optical refractive index, single cells can now be characterized in more detail. Moreover, it is now possible to extract interesting subpopulations that can be further examined with molecular biology techniques. Depending on the intended purpose, novel devices for cell measurements, based on microfluidic and optical considerations, are presented. The fundamental concept involves microstructured chips that can be integrated into a commercial microscope. These chips offer the possibility of separating measured cell populations according to their mechanical properties. This separation, including mathematical classification, is demonstrated. These methods are tested with cell types of differing mechanical properties to prove their applicability in practice. Single cells are sorted into their respective population of origin. These novel methods offer the possibility of a versatile device to be applied in biophysical research.

Three projects are described in this thesis that combine microfabrication techniques with optical micromanipulation. The aim of these projects is to use expertise in microlithography and optical tweezing to create new tools for Lab-on-Chip devices. The first project looks at the creation of microgears that can be moved using an optical force. The microgears include one dimensional photonic crystal that creates birefringence. This allows the transfer of angular momentum from a circularly polarised light beam to the microgear, making them spin. The microgears are simulated, fabricated and tested. Possible biological applications are suggested. The second project looks at creating microchannels to perform micromanipulation experiments in. Different methods of fabricating the microfluidic channels are compared, and the resulting chambers are used to find the maximum flow rate an optical sorting experiment can be performed at. The third project involves using a thin photoconductive layer to allow the optical control of an electrical force called dielectrophoresis. This light induced dielectrophoresis (LIDEP) allows similar control to optical tweezing but requires less irradiance than optical tweezing, allowing control over a larger area with the same input optical power. A LIDEP device is created and experiments to measure the electrical trap size that is created with a given optical spot size are performed. These three projects show different microfabrication techniques, and highlight how well suited they are for use in optical manipulation and microfluidic experiments. As the size of objects that can be optically manipulated matches well with the size of objects that can be created with microfabrication, it seems likely that many more interesting applications will develop.

The nexus of engineering and molecular biology has given birth to high-throughput technologies that allow biologists and medical scientists to produce previously unattainable amounts of data to better understand the molecular basis of many biological phenomena. Here, we describe the development of an enabling biotechnology, commonly known as microfluidics in the fabrication of high-throughput systems to study nerve degeneration and regeneration in the well-defined model nematode, Caenorhabditis elegans (C. elegans). Our lab previously demonstrated how femtosecond (fs) laser pulses could precisely cut nerve axons in C. elegans, and we observed axonal regeneration in vivo in single worms that were immobilized on anesthetic treated agar pads. We then developed a microfluidic device capable of immobilizing one worm at a time with a deformable membrane to perform these experiments without agar pads or anesthetics. Here, we describe the development of improved microfluidic devices that can trap and immobilize up to 24 individual worms in parallel chambers for high-throughput axotomy and subsequent imaging of nerve regeneration in a single platform. We tested different micro-channel designs and geometries to optimize specific parameters: (1) the initial trapping of a single worm in each immobilization chamber, simultaneously, (2) immobilization of single worms for imaging and fs-laser axotomy, and (3) long term storage of worms on-chip for imaging of regeneration at different time points after the initial axon cut. / text