Many applications for the analysis and processing of biological materials require the enrichment of cell subpopulations. Conventional cell sorting systems are large and expensive with complex equipment that necessitates specialized personnel for operation. Employing microfluidics technology for lab-on-a-chip adaptation of these devices provides several benefits: improved transport control, reduced sample volumes, simplicity of operation, portability, greater accessibility, and reduced cost. The designs of microfluidic cell sorting chips vary widely in literature; evaluation and optimization efforts are rarely reported. This study intends to investigate the primary components of the design to understand the effect of various parameters and to improve the performance of the microfluidic chip. Optimized individual elements are incorporated into a proposed cell sorter chip with the ability to dynamically sort target cells from a non-homogeneous solution using electrical driving forces. Numerical and experimental results are used to evaluate the sample focusing element for controlled cell dispensing, the sorting configuration for target cell collection, and the flow elements for reduced pressure effects and prevention of flow blockages. Compact models are adapted to solve the potential field and flow field in the chip and to predict the focused sample stream width. A commercial CFD package is used to perform 2-D simulations of the potential, velocity, and concentration fields. A fluorescence microscopy visualization system is implemented to conduct experiments on several generations of chip designs. The data from sample focusing experiments, performed with fluorescent dye samples, is analyzed using a Gaussian distribution model proposed in this work. A technique for real-time monitoring of fluorescent microspheres in the microfluidic chip enables the use of dynamic cell sorting to emulate fully autonomous operation. The performance values obtained from these experiments are used to characterize the various design configurations. Sample focusing is shown to depend largely on the relative size of the sheath fluid channel and the sample channel, but is virtually independent of the junction shape. Savings in the applied potential can be achieved by utilizing the size dependency. The focusing performance also provides information for optimizing the widths of the channels relative to the cell size. Successful sorting of desired cells is demonstrated for several designs. Key parameters that affect the sorting performance are discussed; a design employing the use of supplemental fluid streams to direct the particle during collection is chosen due to a high sorting evaluation and a low sensitivity to flow anomalies. The necessary reduction of pressure influences to achieve reliable flow conditions is accomplished by introducing channel constrictions to increase the hydrodynamic resistance. Also, prolonged operation is realized by including particle filters in the proposed design to prevent blockages caused by the accumulation of larger particles. A greater understanding of the behaviour of various components is demonstrated and a design is presented that incorporates the elements with the best performance. The capability of the microfluidic chip is summarized based on experimental results of the tested designs and theoretical cell sorting relationships. Adaptation of this chip to a stand-alone, autonomous device can be accomplished by integrating an optical detection system and further miniaturization of the critical components.

Conventional cell cytometers are often bulky and thus not convenient for bio-medical analysis where portable devices are desired. They also suffer from the drawback of high cost due to the complicated and expensive optical detection system involved. Therefore miniaturizing conventional cell cytometer is highly demanded as it offers an opportunity to transform the conventional bulky systems to more cost-efficient and portable microfluidic cell sorting devices. In addition to the advantages reduced cost and enhanced portability, microfluidic cell sorting devices require only a tiny amount of sample for analysis. In this thesis, one common microfluidic cell sorting device is developed using similar conventional functions and concepts but different sorting method. Unlike most of the conventional cell cytometers in which an electrical field or magnetic field is employed to deflect the charged target cells to the collecting container, microfluidic cell sorting devices use the fluid flow to control the movement of the targeted cells to the collecting reservoir. By using an electroosmitic pump, the response time of the flow switch is significantly lowered, leading to a much higher sorting efficiency. Despite the advantages of microfluidic cell sorting devices, there are some issues need to be addressed before realization of such devices. For example, more studies are required on the successful integration of the optical elements in the devices. In microfluidic, the transport phenomena is also different from that in macroscopic. Unlike that in macroscopic, surface forces are important in microfluidics. They result in pressure-induced flow which gives the parabolic profile of the velocity along the channel. Also, a plug-like velocity which is generated by the electoosmitic flow is required for the more controllable and accurate detection. To suppress the pressure-driven flow, hydro-resistance elements (Shallow channel network) are implemented on the microfluidic devices. Fabrication of optical elements by deposition of optical materials on glass or silicon wafer has been reported. However, this Micro Electro-Mechanical (MEM) technique requires special equipment and cleanroom facilities used in the semiconductor industry. A good alternative to the MEMS technique is soft lithography where optical elements can be created using polymers. In this work, ultraviolet-sensitive photo resists SU8 is used to fabricate the microfluidic cell sorting devices and the optical elements. By using the mask with the patterns of the microchannel network and optical elements, the optical elements can be fabricated with the microchannel, eliminating the problem of alignment. Experiments are also conducted to evaluate the integrated optical elements. To prevent cross-contamination, samples are usually prepared and are only mixed inside the microfluidic devices by the embedded mixers. Such embedded mixers, however, pose a great challenge as the small characteristic length of a microfluidic device tends to give a laminar flow and diffusion-dominated mixing. A simple passive micromixer is investigated to find the possibilities to integrate it to the microfluidic devices. To truly understand the diffusional mixing, a Y channel mixer is studied through the numerical and experimental investigations. Based on the results found, a possible design is also proposed and evaluated by experiments.

Droplet based microfluidics is a developing technology with great potential towards improving large scale combinatorial studies that require high throughput and accurate metering of reagents. Each droplet can be thought of as a miniature microreactor where complex reactions can be performed on the micro-scale by mixing, splitting and combining droplets. This thesis investigates the operation and control of droplet microfluidic devices operating using constant pressure sources to pump fluids where feedback from the droplets influences the overall performance of the device. For this purpose, a model system consisting of a single T-junction droplet generator and a single network node is used to understand how pressure source control effects droplet generation and transport through microfluidic networks. The first part of this thesis focuses on the generation of Newtonian liquid-liquid droplets from a microfluidic T-junction operating within the squeezing-to-transition regime with stable flow rates. An experimental study was performed to characterize the effects of geometry (height/width ratio, channel width ratio) and flow parameters (Capillary number, flow rate ratio, viscosity ratio) on the droplet size, spacing and rate of production. Three stages of droplet formation were identified (lag, filling and necking), including the newly defined lag stage that appears at the beginning of the formation cycle once the interface pulls back after a droplet detaches. Based on the experimental observations, a model was developed to describe the formation process which incorporates a detailed geometric description of the drop shape with a force balance in the filling stage and a control volume analysis of the necking stage. The model matches well with the experimental results as data falls within 10% of the predicted values. Subsequently, the effect of surfactants on the formation process was investigated. Surfactant transport occurs on a timescale comparable to the production rate of droplets resulting in dynamic interfacial tension effects. This causes strong coupling between the mass transport of surfactants and the drop production process. Using the previously defined force balance, the apparent interfacial tension at the end of the filling stage was measured. The results show that there is a significant deviation from the equilibrium interfacial tension at normal operating conditions for the T-junction generators due to the rapid expansion of the interface. A model was developed to calculate the dynamic interfacial tension for pre and post micellar solutions, which was then incorporated into the overall model for droplet formation in T-junction generators. Next, the behaviour of microfluidic droplet generators operating under pressure source control was investigated. Coupling between the changing interface and hydrodynamic resistance of the droplets and the flow rate of the two phases creates fluctuations in the output of the droplet generator. Oscillations were found to occur over the short term (one droplet formation cycle) and long term (many formation cycles). Two metrics were developed to quantify these oscillations. Short term oscillations were quantified by tracking droplet speed in the output channel and long term oscillation were quantified by measuring changes in droplet spacing. Analysis of experimental and numerical data shows that long term oscillations have a periodicity that matches the residence time of droplets in the system. A simple model is developed to determine the influence of Laplace pressure, droplet resistance, T-junction generator design and network architecture on the magnitude of these oscillations. From the model a set of design rules are developed to improve the overall operation of T-junction generators using pressure driven flow. The final part of this thesis studies the transport of droplets through a single microchannel junction under various geometric and flow conditions applied to the two outlet channels. Droplets alter the hydrodynamic resistance of the channel they travel within which creates a feedback effect where the decision of preceding droplets influences the trajectory of subsequent droplets. Multifaceted behaviour occurs where sometimes the trajectory of droplets follows a repeatable pattern and other times it is chaotic. As part of this work, a discrete analytical model was developed that predicts droplet transport through the junction including transitions between filtering and sorting, bifurcation in the patterns, composition of the patterns, and an estimation of when patterns will disintegrate into chaos. The model was validated by comparing it to compact numerical simulations and microfluidic experiments with good agreement.The complex behaviour of a simple junction emphasizes the challenge that remains for more highly integrated droplet microfluidic networks operating with pressure driven flow.

The development of microneedles that penetrate the skin barrier, but are small enough not to stimulate nerves, has the potential to deliver drugs across skin in a painless way. Controlled injection by convective flow into skin using hollow microneedles, however, has remained a challenge. To address this challenge, the goals of this study were (i) to provide experimental measurements coupled with numerical simulations to quantitatively describe fluid mechanics of flow within microneedles over a range of experimental conditions and needle geometries, (ii) to demonstrate and study the effects of diffusion-based delivery of insulin to diabetic rats in vivo using solid and hollow microneedles and (iii) to determine the effect of experimental parameters on microinfusion through hollow microneedles into skin to optimize drug delivery protocols and identify rate-limiting barriers to flow. Experimentally, we quantified the relationship between pressure drop and flow rate through microneedles as a function of fluid viscosity and microneedle length, diameter, and cone half-angle. Microneedle tip diameter and taper angle were the primary controlling parameters for flow through conically tapered microneedles as shown by numerical simulations. Flow rates over a range of 1.4 56 l/s were achieved through microneedles (in the absence of skin) with pressure drops in the range of 4.6 196.5 kPa. This work also studied the use of solid and hollow microneedle arrays to insert into the skin of diabetic animals for transdermal delivery of insulin. Blood glucose levels dropped by as much as 80% in diabetic rats in vivo. Larger drops in blood glucose level and larger plasma insulin concentrations were shown due to higher donor solution insulin concentration, shorter microneedles insertion time and fewer repeated insertions. The final scope of this work was to determine the effect of microneedle geometry and infusion protocols on microinfusion flow rate into skin in vitro. Infusion flow rates ranged from 21 to 1130 l/h was demonstrated using glass microneedles. The presence of a bevel at the microneedle tip, larger retraction distance and insertion depth, larger infusion pressure and the presence of hyaluronidase led to larger infusion flow rates.

Recent advances in microfluidic technologies have enabled integration of the functional units for biological and chemical analysis onto miniaturized chips, called Labon- a-Chip (LOC). However, the effective manipulation and control of colloidal particles suspended in fluids are still challenging tasks due to the lack of clear characterization of particle control mechanisms. The aim of this dissertation is to develop microfluidic techniques and devices for manipulating colloids and biological particles with the utilization of alternating current (AC) electric fields and acoustic waves. The dissertation presents a simple theoretical tool for predicting the motion of colloidal particles in the presence of AC electric field. Dominant electrokinetic forces are explained as a function of the electric field conditions and material properties, and parametric experimental validations of the model are conducted with particles and biological species. Using the theoretical tool as an effective framework for designing electrokinetic systems, a dielectrophoresis (DEP) based microfluidic device for trapping bacterial spores from high conductivity media is developed. With a simple planar electrode having well defined electric field minima that can act as the targetattachment/ detection sites for integration of biosensors, negative DEP trapping of spores on patterned surfaces is successfully demonstrated. A further investigation of DEP colloidal manipulation under the effects of electrothermal flow induced by Joule heating of the applied electric field is conducted. A periodic structure of the electrothermal flow that enhances DEP trapping is numerically simulated and experimentally validated. An acoustic method is investigated for continuous sample concentration in a microscale device. Fast formation of particle streams focused at the pressure nodes is demonstrated by using the long-range forces of the ultrasonic standing waves (USW). High frequency actuation suitable for miniaturization of devices is successfully applied and the device performance and key parameters are explained. Further extension and integration of the technologies presented in this dissertation will enable a chip scale platform for various chemical and biological applications such as drug delivery, chemical analyses, point-of-care clinical diagnosis, biowarfare and biochemical agent detection/screening, and water quality control.

Thesis (M.S.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 73-79).

Fast mixing remains a major challenge in droplet-based microfluidics. The low Reynolds number operating regime typical of most microfluidic devices signify laminar and orderly flows that are devoid of any inertial characteristics. To increase mixing rates in droplet-based devices, a novel technique is presented that uses a high Reynolds number gaseous phase for droplet generation and transport and promotes mixing through binary droplet collisions at velocities near 1m/s. Control of multiple gas and liquid streams is provided by a newly constructed microfluidic test bed that affords the stringent flow stability required for generating liquid droplets in gaseous flows. The result is droplet production with size dispersion and generation frequencies not previously achievable. Limitations of existing mixing diagnostic methods have led to the development of a new measurement technique for measuring droplet collision mixing in confined microchannels. The technique employs single fluorophore laser-induced fluorescence, custom image processing, and meaningful statistical analysis for monitoring and quantifying mixing in high-speed droplet collisions. Mixing information is revealed through three governing statistics that that separate the roles of convective rearrangement and molecular diffusion during the mixing process. The end result is a viewing window into the rich dynamics of droplet collisions with spatial and temporal resolutions of 1μm and 25μs, respectively. Experimental results obtained across a decade vi of Reynolds and Peclet numbers reveal a direct link between droplet mixing time and the collision convective timescale. Increasing the collision velocity or reducing the collision length scale is the most direct method for increasing droplet mixing rates. These characteristics are complemented by detaching droplets under inertial conditions, where increasing the Reynolds number of the continuous gaseous phase generates and transports smaller droplets at faster rates. This work provides valuable insight into the emerging field of two-phase gas-liquid microfluidics and opens the door to fundamental research possibilities not offered by traditional oil-based architectures. / text

This work presents the first electrochemical observation of single polystyrene microbead collisions with an electrode within a microchannel. We have observed that detecting single microbead collisions is facile with this system. Additionally, we have shown that by increasing flow within the channel, one can increase both the frequency and magnitude of collision signals. This technique may provide a means of signal amplification in future sensing work. / text

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy