Abstract

This thesis presents the development of four different microfluidic technologies that can be used as stand-alone devices or integrated in point-of-care systems. The first technology is a rapid, low-cost, portable microfluidic system for assessing the somatic cell count and fat content of milk in 15 min using a “sample-in, answer-out” approach. The system consists of twelve independent microfluidic devices, essentially flattened funnel structures, fabricated on the footprint of a plastic compact disc (CD). The assay separates cells and fat globules based on their densities (by differential sedimentation), concentrating white cells in the closed-end channel near the outer rim of the CD for estimation of total “cell pellet” volume, while fat globules move toward the center of disc rotation, forming a fat “band” in the funnel. The closed-end channel provides accurate cell counts over the range 50,000 to over 3,000,000 cells per mL. A technique is also presented to recirculate liquids in a microfluidic channel by alternating the predominance of centrifugal and capillary forces. With this technique, liquid volumes of μL to mL can be sampled with many sizes of microfluidic channels that contain only a fraction of the sample at one time, provided the channel wall with greatest surface area is hydrophilic. We present a theoretical model describing the balance of centrifugal and capillary forces in the device and validate the model experimentally. Towards the development of an integrated pathogen identification system, two other technologies are demonstrated and implemented. The design, fabrication, and characterization of a polymer centrifugal microfluidic system for the specific detection of bacterial pathogens is presented. This single-cartridge platform integrates bacteria capture and concentration, supernatant solution removal, lysis, and nucleic-acid sequence-based amplification (NASBA) in a single unit. The unit is fabricated using multilayer lamination and consists of five different polymer layers. Bacteria capture and concentration are accomplished by sedimentation in five minutes. Centrifugation forces also drive the subsequent steps. A wax valve is integrated in the cartridge to enable high-speed centrifugation. Oil is used to prevent evaporation during reactions requiring thermal cycling. Device functionality was demonstrated by real-time detection of E. coli cells from a 200-μL sample. Finally, the laser-printer-based fabrication of pressure-resistant microfluidic single-use valves is reported, along with their implementation on pressure-driven and centrifugal microfluidic platforms. A laser printer is used to selectively deposit toner on a plastic substrate in the form of circular dots. After assembly into a microfluidic device, the valve is opened (melted) with a pulse of laser light. This is an easy approach to connect multiple fluidic levels. This simple technology is compatible with a range of polymer microfabrication technologies and should facilitate the development of fully integrated, (re)configurable, and automated lab-on-a-chip systems, particularly when reagents must be stored on chip for extended periods, e.g. for medical diagnostic devices, lab-on-a-chip synthetic systems, or hazardous bio/chemical analysis platforms.