UC Irvine

Compact disc (CD)-based centrifugal microfluidics is an increasingly popular choice for academic and commercial applications as it enables a portable platform for biological and chemical assays. By rationally designing microfluidic conduits and programming the disc’s rotational speeds and accelerations, one can reliably control propulsion, metering, and valving operations. This centrifugal force-based propulsion in flow patterns of recirculation and reciprocation can be supplemented by the capillary flow through nitrocellulose membrane toward more sensitive bioassays. By studying the effect of various process parameters affecting the mass transport in relation to reaction kinetics, we can enhance the sensitivity of microfluidic tests. We can enhance assays by processing larger sample volumes passing through the nitrocellulose membrane, by forcing the fluidic sample to pass close to the test line or test patch, and by re-concentration or post-concentration of the analyte. In combining lateral flow assay with the centrifugal microfluidics we are able to control the advancement of the liquid front along the nitrocellulose (NC) membrane where the rate of the advancement of liquid front in the radially positioned NC strip is retarded by spinning disc at higher angular velocity balancing the centrifugal force with the capillary force. Thus, the flow rate can be well controlled and slowed sufficiently to increase binding of the analytes to the probes at the test line of NC strips. Through study of the ratio of reaction rate to advective transport rate (expressed by Damkohler number Da) we observe how mass transport can be optimized for various reaction kinetics. We further introduce a new merit number Pa (Transport reaction constant) that modified Da by introduction of the film thickness that affect the final sensitivity of the assays. Additional way to enhance performance of complex microfluidic assays is the development and utilization of reusable valves. Valves that either stop the fluid flow or allow it to proceed are critical components of a CD platform. Among the valves on a CD, wax valves that liquify at elevated temperatures to open channels and that solidify at room temperature to close them have been previously implemented on CD platforms. However, typical wax valves on the CD fluidic platforms can be actuated only once (to open or to close fluidic passage) and they require complex fabrication steps. Here, we present two new multiple-use wax valve designs, driven by capillary or magnetic forces. One wax valve design utilizes a combination of capillary-driven flow of molten wax and centrifugal force to toggle between open and closed configurations. The phase change of the wax is enabled by heat application (e.g., a 500-mW laser). The second wax valve design employs a magnet to move a molten ferroparticle-laden wax in and out of a channel to enable reversible operation. In addition to valving operation, some of the major methods of the mass transport phenomenon on centrifugal platforms are reciprocation and recirculation. In reciprocation, the sample volume is moved over the biosensing array in a reciprocating fashion (back and forth) using a novel CD design mechanism described in this work. In reciprocation the test strip remains submerged in the sample, but the sample moves over the test strip. In contrast to reciprocation, in recirculation, the sample moves over the test strip and then is removed completely, it goes through a fluidic network to wash over the test strip again (the cycle can be performed multiple times). The boundary layer close to the test line is retained during reciprocation, but it is completely removed and then reforms during the recirculation process, enhancing sensitivity of the bioassays. Besides scientific advances, engineering implementation is required to integrate multiple fluidic processes on a single CD since running multiple sequential processes on separate discs. The alternative to integration of multi-step assays on a single disc is slow and expensive process of performing some assay steps on one disc and pipetting the product from one disc to transfer it to another disc to continue subsequent process steps. Integration of blood plasma separation step and reciprocation for enhanced bioassay was demonstrated in the present work (present state-of-the-art procedure requires a separate CD for blood plasma separation).