ELECTROKINETIC PARTICLE MANIPULATIONS IN SPIRAL MICROCHANNELS

Recent developments in the field of microfluidics have created a multitude of new useful techniques for practical particle and cellular assays. Among them is the use of dielectrophoretic forces in \u27lab-on-a-chip\u27 devices. This sub-domain of electrokinetic flow is particularly popular due to its advantages in simplicity and versatility. This thesis makes use of dielectrophoretic particle manipulations in three distinct spiral microchannels. In the first of these experiments, we demonstrate the utility of a novel single-spiral curved microchannel with a single inlet reservoir and a single outlet reservoir for the continuous focusing and filtration of particles. The insulator-based negative-dielectrophoretic (repulsive) force is used in a parametric study of the effects of electric field strength, particle size, and solution concentration on particle focusing abilities. It was summarily determined that all three factors are positively correlated with increased particle focusing ability. From these results, a partial filtration of 10 μm particles from a binary solution of 3 and 10 μm particles was demonstrated. Also observed was a balance between dielectrophoretic and repulsive particle-wall interactions; thus yielding a novel approach for particle manipulation. Following the results of the first, we demonstrate in the second experiment a continuous-flow electrokinetic separation of both a binary mixture and a ternary mixture of colloidal particles based on size in a single-spiral microchannel with a single inlet reservoir and triple outlet reservoirs. This method also utilizes both curvature-induced dielectrophoresis to focus particles to a tight stream and the previously observed wall-induced electric lift to manipulate the aligned particles to size-dependent equilibrium positions. Due to the continuous nature of the flow through concentric spiral loops, both focusing forces influence particles simultaneously. This novel technique is useful for its compact geometry, robust structure, ease of manufacture, and ease of use in the manipulation of independent particle species. A theoretical model is also developed to understand this separation, and the obtained analytical formula predicts the experimentally measured particle center-wall distance in the spiral with a close agreement. We demonstrate in the third experiment a continuous-flow electrical sorting of spherical and peanut-shaped particles of similar volumes in an asymmetric double-spiral microchannel with a single inlet reservoir and triple outlet reservoirs. This experiment, unlike the first two, differentiates particle species based principally on shape. Shape is an intrinsic marker of cell cycle, an important factor for identifying a bio-particle, and also a useful indicator of cell state for disease diagnostics; therefore, shape can be a specific marker in label-free particle and cell separation for various chemical and biological applications. The double-spiral geometry exploits curvature-induced dielectrophoresis to initially focus particles to a tight stream in the first spiral without any sheath flow. Particles are subsequently displaced to shape-dependent flow paths in the second spiral without any external force. We also develop a numerical model to simulate and understand this shape-based particle sorting in spiral microchannels. The predicted particle trajectories agree qualitatively with the experimental observation

JOULE HEATING EFFECTS ON ELECTROKINETIC TRANSPORT IN CONSTRICTION MICROCHANNELS

Microfluidic technology involving multidisciplinary studies including MEMS, chemistry, physics, fluids and heat transfer has been developed into a promising research field in the recent decade. If offers many advantages over conventional laboratory techniques like reduced reagent consumption, faster analysis, easy fabrication and low chemical waste. Microfluidic lab-on-a-chip devices have been used to manipulate cells and particles like sorting, separating, trapping, mixing and lysing. Microfluidic manipulation can be achieved through many methods and insulator based dielectrophoresis (iDEP) is one of the highly used method in the recent years. In iDEP, both DC and AC voltages can be applied to the remote electrodes positioned in end-channel reservoirs for transporting and manipulating particles. The electric field gradients are caused by the blockage of electric current due to in-channel hurdles, posts, and ridges. However, iDEP devices suffer from the issue of Joule heating due to locally amplified electric field around the insulators. A parametric study of Joule heating effects on electroosmotic fluid flow in iDEP is studied under various electric fields. It was determined that depending upon the magnitude of DC voltage, a pair of counter rotating vortices fluid circulations can occur at either downstream end or each end of the channel constriction. Moreover, pair at the downstream end appears larger in size than the upstream end due to DC electroosmotic flow. A numerical model is developed to simulate the fluid circulations occurred due to the action of electric field on Joule heating induced fluid inhomogeneities in the constriction region. Focusing particles or cells into a single stream is usually a necessary step prior to counting and separating them in microfluidic devices such as flow cytometers and cell sorters. A systematic study of Joule heating effects on electrokinetic particle transport in constriction microchannels under DC and DC biased AC electric fields is presented in this work. A numerical model is developed to capture the particle trace observed in the experiments. It was determined that particle transport is greatly affected by electrothermal effects where Joule heating is high. At very low DC magnitude where the electrothermal effects dominate the electrokinetic flow, particles in the shallow depth channel are being trapped and particles in deep channels are transported to the downstream reservoir from the constriction in a single streamline. Electrothermal flow circulations should be taken into account in the design and operation of iDEP devices, especially when highly conductive solutions and large electric fields must be employed. They may potentially be harnessed to enhance microfluidic mixing and immunoassay for lab-on-a-chip applications. A numerical study of Joule heating effects on the sample mixing performance in constriction microchannels is presented in this work. It was determined that Joule heating induced electrothermal force enhanced the sample mixing by generating circulations at the ends of the constriction under DC biased AC electric fields. Furthermore, mixing performance was also studied for various parameters like applied electric field, channel structure, channel depth and number of constrictions

ELECTROKINETIC TRANSPORT AND MANIPULATION OF PARTICLES IN CURVED MICROCHANNELS

The investigation of electrokinetic particle transport in confined microchannels has practical significances in a variety of applications ranging from traditional gel electrophoresis to electrokinetic microfluidics-based lab-on-a-chip devices. To date, however, studies on particle electrokinetics have been limited to primarily theoretical or numerical analyses in straight microchannels of simple geometries. Very little work has been done on electrokinetic particle motions in real microchannels which usually consist of one or multiple turns. This thesis is dedicated to the fundamental and applied studies of electrokinetic transport and manipulation of particles in various curved microchannels using a combined experimental, theoretical, and numerical method. First, a fundamental study of particle electrokinetics in a microchannel U-turn, a typical unit in LOC devices, was investigated. A 2-D numerical model based on finite element method was developed to understand and predict the particle motion within the U-turn. It is demonstrated that particles are deflected to the outer wall of the turn by curvature-induced dielectrophoresis (termed cDEP) due to the locally intrinsic electric field gradients. Moreover, this lateral displacement increases with the rise of either the applied electric field or the particle size. Next, we utilize the cDEP in microchannel turns to implement a continuous electrokinetic focusing of particles in serpentine microchannels. Particles are demonstrated to gradually migrate to the centerline due to the periodically switched dielectrophoretic force they experience in a serpentine microchannel. This electrokinetic focusing favors large electric fields and large particles, and also increases when the number of serpentine periods increases. Such focusing also takes place in a spiral microchannel, where, however, particles are eventually focused to a stream flowing near the outer sidewall of the channel. Then, we explore the applications of cDEP to continuous electrokinetic separation of particles in curved microchannels. We develop two different approaches based on what we have acquired from the studies of particle electrokinetics in serpentine and spiral microchannels. The first approach employs a sheath flow to focus particles to one sidewall of a serpentine microchannel, where particles are then deflected to different flow paths by cDEP and thus sorted at the exit of serpentine section. We use this method to separate particles and cells by size at low DC electric fields. The second approach eliminates the sheath flow focusing of particles by the use of particle deflection and focusing in a double-spiral microchannel. Specifically, particles are focused by cDEP to one single stream near the outer wall of the first spiral, which is then displaced by cDEP and divided into two or more sub-streams in the second spiral, enabling the continuous sorting. We use this approach to implement the separation of particles by size and by charge, respectively. Moreover, we also demonstrate a continuous ternary separation of particle by size and charge simultaneously

MICROFLUIDIC PARTICLE AND CELL MANIPULATION USING RESERVOIR-BASED DIELECTROPHORESIS

Controlled manipulation of synthetic particles and biological cells from a complex mixture is important to a wide range of applications in biology, environmental monitoring, and pharmaceutical industry. In the past two decades microfluidics has evolved to be a very useful tool for particle and cell manipulations in miniaturized devices. A variety of force fields have been demonstrated to control particle and cell motions in microfluidic devices, among which electrokinetic techniques are most often used. However, to date, studies of electrokinetic transport phenomena have been primarily confined within the area of microchannels. Very few works have addressed the electrokinetic particle motion at the reservoir-microchannel junction which acts as the interface between the macro (i.e., reservoir) and the micro (i.e., microchannel) worlds in real microfluidic devices. This Dissertation is dedicated to the study of electrokinetic transport and manipulation of particles and cells at the reservoir-microchannel junction of a microfluidic device using a combined experimental, theoretical, and numerical analysis. First, we performed a fundamental study of particles undergoing electrokinetic motion at the reservoir-microchannel junction. The effects of AC electric field, DC electric field, and particle size on the electrokinetic motion of particles passing through the junction were studied. A two-dimensional numerical model using COMSOL 3.5a was developed to investigate and understand the particle motion through the junction. It was found that particles can be continuously focused and even trapped at the reservoir-microchannel junction due to the effect of reservoir-based dielectrophoresis (rDEP). The electrokinetic particle focusing increases with the increase in AC electric field and particle size but decreases with the increase in DC electric field. It was also found that larger particles can be trapped at lower electric fields compared to smaller counterparts. Next, we utilized rDEP to continuously separate particles with different sizes at the reservoir-microchannel junction. The separation process utilized the inherent electric field gradients formed at the junction due to the size difference between the reservoir and the microchannel. It was observed, that the separation efficiency was reduced by inter-particle interactions when particles with small size differences were separated. The effect of enhanced electrokinetic flow on the separation efficiency was investigated experimentally and was observed to have a favorable effect. We also utilized rDEP approach to separate particles based on surface charge. Same sized particles with difference in surface charge were separated inside the microfluidic reservoir. The streaming particles interacted with the trapped particles and reduced the separation efficiency. The influences from the undesired particle trapping have been found through experiments to decrease with a reduced AC field frequency. Then, we demonstrated a continuous microfluidic separation of live yeast cells from dead cells using rDEP. Because the membrane of a cell gets distorted when it loses its viability, a higher exchange of ions results from such viability loss. The increased membrane conductivity of dead cells leads to a different Claussius-Mossoti factor from that of live cells, which enables their selective trapping and continuous separation based on cell viability. A two-shell numerical model was developed to account for the varying conductivities of different cell layers, the results of which agree reasonably with the experimental observations. We also used rDEP to implement a continuous concentration and separation of particles/cells in a stacked microfluidics device. This device has multiple layers and multiple microchannels on each layer so that the throughput can be significantly increased as compared to a single channel/single layer device. Finally, we compared the two-dimensional and three-dimensional particle focusing and trapping at the reservoir-microchannel junction using rDEP. We observed that the inherent electric field gradients in both the horizontal and vertical planes of the junction can be utilized if the reservoir is created right at the reservoir-microchannel junction. Three-dimensional rDEP utilizes the additional electric field gradient in the depth wise direction and thus can produce three-dimensional focusing. The electric field required to trap particles is also considerably lower in three-dimensional rDEP as compared to the two-dimensional rDEP, which thus considerably reduces the non-desired effects of Joule heating. A three-dimensional numerical model which accounted for the entire microfluidic device was also developed to predict particle trajectories

Nano-orifice based Dielectrophoretic Manipulation and Characterization of Nanoparticles and Biological Cells

Dielectrophoresis (DEP) is the motion of a dielectric particle in an aqueous solution due to the polarization effects in a non-uniform electric field. Due to its property of label-free, scalable, and capable of generating both negative and positive forces to manipulate bio-particles, DEP shows crucial applications in various biological and clinical analysis such as trapping, sorting, separation and characterization of micro and nanoparticles, cells, viruses, bacteria, and DNA. However, to date, the traditional DEP techniques involve the issues of complex fabrication of arrays of the embedded microelectrodes, particles clogging, low sensitivity and resolution, as well as the Joule heating effect with high electric fields applied. This thesis investigates and develops a novel dielectrophoretic platform for the asymmetric-orifices based manipulation and separation of nanoparticles and micron droplets, as well as characterization and identification of droplets and biological cells by the pressure-driven flow in the polydimethylsiloxane (PDMS) microchannels. At the beginning of this thesis, a nano-orifice based dielectrophoretic microfluidic chip is developed. In such a chip, the non-uniform electric field is generated by applying the electric field via a pair of asymmetric orifices, a small orifice on one side of the channel walls and a large orifice on the opposite side of the channel walls. In order to obtain a strong gradient of the non-uniform electric fields, i.e., a large width ratio between the small orifice and the large orifice, a small microchannel or a nanochannel fabricated by the solvent-induced cracking method is used to form the small orifice. The electric field and the flow field inside the channel are simulated and studied. Then two fundamental research projects are conducted on the nano-orifice based direct current (DC) DEP microfluidic chips to investigate the separation of the nanoparticles and Janus particles in microchannels. In the first research project, the size-dependent separation of micro and nanoparticles and the separation of similar size nanoparticles by type are studied. The Clausius-Mossotti factors of the particles as a function of the electrical conductivity of the suspending medium are discussed, and the effects of the applied electric field, the flow rate as well as the width and length of the small orifice are investigated. The experimental results of the particle trajectory show good agreements with the numerical simulation results. Distinguishing of nanoparticles as small as 51 nm and 140 nm, as well as 140 nm polystyrene (PS) and 150 nm magnetic nanoparticles with high separation resolution, have been achieved. In the second research project, the dielectrophoretic manipulation and separation of the Janus particles are numerically investigated. Effects of the strength of the electric fields, as well as the coating coverage, thickness, and electrical conductivity of the Janus particles on their DEP behaviors and trajectories under DC electric field are systematically studied. The effect of the coating thickness of the Janus particles on their dielectrophoretic behaviors is negligible when using the DC-DEP method and the Janus particles with gold coating coverage over 50% will experience positive DEP effects. Afterward, the manipulation and separation of the oil and ionic liquid (IL)-in-water emulsion droplets are investigated under DC electric field in the asymmetric orifice based microfluidic chips. The effects of the type and content of the oil droplets and the ionic concentration and types of the electrolyte solutions on the trajectories of the emulsion droplets are analyzed. By using the pressure-driven flow and a stream of sheath flow, the mixed emulsion droplets move closely to the vicinity of the nano-orifice and experience the stronger DEP effects. As the magnitude of DEP forces exerting on the droplets is determined by the size of the droplet, the separation of smaller silicone oil droplets with a small size difference of only 3.5 µm is demonstrated. By selecting the surrounding solution with a specific electrical conductivity, the separation of the emulsion droplets of similar size but different contents is achieved by opposite DEP effects, i.e., p-DEP and n-DEP, respectively, providing a platform to manipulate different kinds of emulsion droplets carrying different biomolecules or bioparticles. Lastly, by using the alternating current (AC) DEP microfluidic chips, the tunable characterization and identification of droplets and biological cells are investigated. To generate DEP forces, two electrode-pads are embedded in a set of asymmetric orifices on the opposite sidewalls to produce the non-uniform electric fields. In the vicinity of a small orifice, the cells experience the strongest non-uniform gradient. The effects of the strength and frequency of the applied AC electric field, as well as the ionic concentrations, i.e., different electrical conductivities on their DEP behaviors, are investigated, respectively. By adjusting the frequency and strength of the AC electric field, the separation of live and dead yeast cells, as well as the cells with the targeted diameter and dielectric property, are achieved. To evaluate the critical frequency of the specific droplets and cells and manipulate the targeted cells, a microfluidic system is developed to measure the lateral distance between the cells center and the centerline of the main channel as a function of the AC frequency. The trends of measured lateral migrations of yeast cells are similar to the corresponding Clausius−Mossotti (CM) factors. This system provides a method to characterize the crossover frequency of the specific cells and manipulate the targeted cells. This thesis provides the microfluidic research platform with a comprehensive working procedure for the asymmetric orifice based DEP microfluidic applications. The fundamental studies in this thesis expand our understanding of the dielectrophoretic behaviors of the nanoparticles, micron droplets, Janus particles, and the biological cells and overcome the shortcomings of the conventional DEP methods, and the microfluidic systems developed on the asymmetric orifice based dielectrophoretic chips open a new avenue to nanoparticles separation as well as biological cells characterization

Dielectrophoretic characterization of particles and erythrocytes

Medical lab work, such as blood testing, will one day be near instantaneous and inexpensive via capabilities enabled by the fast growing world of microtechnology. In this research study, sorting and separation of different ABO blood types have been investigated by applying alternating and direct electric fields using class=SpellE\u3edielectrophoresis in microdevices. Poly(dimethylsiloxane) (PDMS) microdevices, fabricated by standard photolithography techniques have been used. Embedded perpendicular platinum (Pt) electrodes to generate forces in AC dielectrophoresis were used to successfully distinguish positive ABO blood types, with O+ distinguishable from other blood types at \u3e95% confidence. This is an important foundation for exploring DC dielectrophoretic sorting of blood types. The expansion of red blood cell sorting employing direct current insulative class=SpellE\u3edielectrophoresis (DC-iDEP) is novel. Here Pt electrodes were remotely situated in the inlet and outlet ports of the microdevice and an insulating obstacle generates the required dielectrophoretic force. The presence of ABO antigens on the red blood cell were found to affect the class=SpellE\u3edielectrophoretic deflection around the insulating obstacle thus sorting cells by type. To optimize the placement of insulating obstacle in the microchannel, COMSOL Multiphysics® simulations were performed. Microdevice dimensions were optimized by evaluating the behaviors of fluorescent polystyrene particles of three different sizes roughly corresponding to the three main components of blood: platelets (2-4 µm), erythrocytes (6-8 µm) and leukocytes (10-15 µm). This work provided the operating conditions for successfully performing size dependent blood cell insulator based DC dielectrophoresis in PDMS microdevices. In subsequent studies, the optimized microdevice geometry was then used for continuous separation of erythrocytes. The class=SpellE\u3emicrodevice design enabled erythrocyte collection into specific channels based on the cell’s deflection from the high field density region of the obstacle. The channel with the highest concentration of cells is indicative of the ABO blood type of the sample. DC resistance measurement system for quantification of erythrocytes was developed with single PDMS class=SpellE\u3emicrochannel system to be integrated with the DC- class=SpellE\u3eiDEP device developed in this research. This lab-on-a-chip technology application could be applied to emergency situations and naturalcalamities for accurate, fast, and portable blood typing with minimal error

Dielectrophoretic characterization of particles and erythrocytes

Medical lab work, such as blood testing, will one day be near instantaneous and inexpensive via capabilities enabled by the fast growing world of microtechnology. In this research study, sorting and separation of different ABO blood types have been investigated by applying alternating and direct electric fields using class=SpellE\u3edielectrophoresis in microdevices. Poly(dimethylsiloxane) (PDMS) microdevices, fabricated by standard photolithography techniques have been used. Embedded perpendicular platinum (Pt) electrodes to generate forces in AC dielectrophoresis were used to successfully distinguish positive ABO blood types, with O+ distinguishable from other blood types at \u3e95% confidence. This is an important foundation for exploring DC dielectrophoretic sorting of blood types. The expansion of red blood cell sorting employing direct current insulative class=SpellE\u3edielectrophoresis (DC-iDEP) is novel. Here Pt electrodes were remotely situated in the inlet and outlet ports of the microdevice and an insulating obstacle generates the required dielectrophoretic force. The presence of ABO antigens on the red blood cell were found to affect the class=SpellE\u3edielectrophoretic deflection around the insulating obstacle thus sorting cells by type. To optimize the placement of insulating obstacle in the microchannel, COMSOL Multiphysics® simulations were performed. Microdevice dimensions were optimized by evaluating the behaviors of fluorescent polystyrene particles of three different sizes roughly corresponding to the three main components of blood: platelets (2-4 µm), erythrocytes (6-8 µm) and leukocytes (10-15 µm). This work provided the operating conditions for successfully performing size dependent blood cell insulator based DC dielectrophoresis in PDMS microdevices. In subsequent studies, the optimized microdevice geometry was then used for continuous separation of erythrocytes. The class=SpellE\u3emicrodevice design enabled erythrocyte collection into specific channels based on the cell’s deflection from the high field density region of the obstacle. The channel with the highest concentration of cells is indicative of the ABO blood type of the sample. DC resistance measurement system for quantification of erythrocytes was developed with single PDMS class=SpellE\u3emicrochannel system to be integrated with the DC- class=SpellE\u3eiDEP device developed in this research. This lab-on-a-chip technology application could be applied to emergency situations and naturalcalamities for accurate, fast, and portable blood typing with minimal error

Higher Order Electrokinetic Effects for Applied Biological Analytics

abstract: Microfluidic systems have gained popularity in the last two decades for their potential applications in manipulating micro- and nano- particulates of interest. Several different microfluidics devices have been built capable of rapidly probing, sorting, and trapping analytes of interest. Microfluidics can be combined with separation science to address challenges of obtaining a concentrated and pure distinct analyte from mixtures of increasingly similar entities. Many of these techniques have been developed to assess biological analytes of interest; one of which is dielectrophoresis (DEP), a force which acts on polarizable analytes in the presence of a non-uniform electric fields. This method can achieve high resolution separations with the unique attribute of concentrating, rather than diluting, analytes upon separation. Studies utilizing DEP have manipulated a wide range of analytes including various cell types, proteins, DNA, and viruses. These analytes range from approximately 50 nm to 1 µm in size. Many of the currently-utilized techniques for assessing these analytes are time intensive, cost prohibitive, and require specialized equipment and technical skills. The work presented in this dissertation focuses on developing and utilizing insulator-based dielectrophoresis (iDEP) to probe a wide range of analytes; where the intrinsic properties of an analyte will determine its behavior in a microchannel. This is based on the analyte’s interactions with the electrokinetic and dielectrophoretic forces present. Novel applications of this technique to probe the biophysical difference(s) between serovars of the foodborne pathogen, Listeria monocytogenes, and surface modified Escherichia coli, are investigated. Both of these applications demonstrate the capabilities of iDEP to achieve high resolution separations and probe slight changes in the biophysical properties of an analyte of interest. To improve upon existing iDEP strategies a novel insulator design which streamlines analytes in an iDEP device while still achieving the desirable forces for separation is developed, fabricated, and tested. Finally, pioneering work to develop an iDEP device capable of manipulating larger analytes, which range in size 10-250 µm, is presented.Dissertation/ThesisDoctoral Dissertation Chemistry 201

Micropatterning in BioMEMS for Separation of Cells/Bioparticles

Biofluids remain a difficult issue in some drug delivery processes for separation of bioparticles through microchannels. This chapter reviews the techniques which have been substantiated and proven helpful for the separation of particles depending on mass and size with some constraints of high throughput. In this study, a key focus will be on separation criterion by patterning of a microchannel and utilize sieve type channels based on spherical bioparticles. The first part focuses on the designing of the pattern for issues of the network like clogging and theoretical experiments by both hydrodynamic and other passive methods for sorting/separation. The second part focuses on the simulations for separation for small and larger bio particles depending on mass and size, samples of blood and other Klebsiella infected fluidic samples for the experiment. The theme provided for mass and size-based separation is simple and can accomplish operations in microfluidics for several biological experiments, diagnosis approaches and zoological researches