Microfluidic polymer- and paper-based devices for in-vitro diagnostics

Existing microfluidic devices transport fluids by a variety of physical means. Among them, pump-based and capillary-based fluid delivery systems are commonly adapted to move fluids. The pump-based systems intrinsically require sub systems consisting of mechanical and/or electrical instruments, which poses tremendous difficulties to system-level miniaturization. Herein, the investigations of two capillary-based microfluidic platforms are described. The first platform encapsulates biological reagents by capillary force in carefully designed microchannels, and detects biomolecules by moving magnetic beads in it. The second platform provides functional microfluidic devices made of paper—a fabric and abundant material that autonomously wicks fluids. Chapter 1 describes the development of the polymer-based platform, whereas Chapter 2 and 3 present the paper-based platform. Specifically, Chapter 1 reports a the polymer-based microfluidic device for immuno-diagnostics. Rather than handling fluid reagents against a stationary solid phase, the platform manipulates analyte-coated magnetic beads through stationary plugs of fluid reagents to detect an antigenic analyte. These isolated but accessible plugs are pre-encapsulated in a microchannel by capillary force. We call this platform microfluidic inverse phase enzyme- linked immunosorbent assay (μIPELISA). μIPELISA has distinctive advantages in the family of microfluidic immunoassay. In particular, it avoids pumping and valving fluid reagents during assaying, thus leading to a lab-on-a-chip format that is free of instrumentation for fluid actuation and control. We use μIPELISA to detect digoxigenin-labeled DNA segments amplified from E. Coli O157:H7 by polymerase chain reaction (PCR), and compare its detection capability with that of microplate ELISA. For 0.259 ng per &mgr;L−1 of digoxigenin-labeled amplicon, μIPELISA is as responsive as the microplate ELISA. Also, we simultaneously conduct μIPELISA in two parallel microchannels. Chapter 2 reports a fluidic diode, valves and a circuit fabricated entirely on a single layer of paper to control wicking of fluids. Our fluidic diode is a two-terminal component that promotes or stops wicking along a paper channel. We further constructed a trigger valve and a delay valve based on the fluidic diode. Furthermore, we demonstrate a high-level functional circuit, consisted of a diode and a delay valve, to manipulate two fluids in a sequential manner. Our study provides new, transformative tools to manipulate fluid for microfluidic paper-based devices. Chapter 3 reports the 3D counterparts of the fluidic components described in the previous chapter. By using printing, stacking and taping, we create channels defined by wax contours in multilayer of paper to further reduce the footprints of microfluidic paper-based devices. This fabrication method is simple with a high yield: the channels are ready for assembling in minutes, and the typical turn-round time from a design to an end assembly is less than an hour. We believe these features are attractive for rapid prototyping of microfluidic paper-based devices.

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