Droplet Feedback Mechanisms on a Digital Microfluidic Platform and Development of Hyperbilirubinemia Panel
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Digital microfluidics employing an array of electrodes to manipulate pico-through-micro liters of fluid is one of the most reliable methods in delivering fluids in Lab-on-Chip platform. The objective of this dissertation is to "Design and demonstrate droplet feedback methodologies using electrical and optical methods on a digital microfluidic cartridge and to employ the methods of feedback to demonstrate the feasibility on Hyperbilirubinemia Assay". Three types of droplet feedback mechanisms are demonstrated: Measurement of droplet impedance over an electrode pair to identify the presence or absence of a droplet, measurement of absorbance of a droplet over an electrode using spectrometer to determine the concentration of a particular species in a droplet and fluorescence imagine of a droplet to obtain the footprint of a droplet using a charge coupled device (CCD) to obtain the droplet volume. Further, the measurement of impedance has also been employed to characterize the mass produced digital microfluidic cartridges used in this platform to detect cartridges with defects. The impedance detection has been found to be robust enough to identify droplet presence, droplet absence, transport and merging of droplets. However, this method suffers from the disadvantage of necessitating the droplets to be within a certain volume, leading to inability in identifying failure modes involving merging of two or more droplets. Thus, the absorbance and fluorescence method has been adopted to demonstrate presence, absence and merging failures of a droplet with the presence of an internal dye. As the droplets are unitized over merging operations, the absorbance would be proportional to the concentration of the dye present after dilution enabling prediction of the final concentration of a species to determine proper merging. Fluorescence imaging in digital microfluidics would provide an accurate footprint of the droplet resulting in the measurement of the droplet volume and its variation. After successfully demonstrating the feedback mechanism employing impedance, fluorescence and absorbance, we borrowed the absorbance measurement technique to demonstrate feasibility of assay performance on lab-on-chip assays. The impedance measurements were demonstrated on Lysosomal Storage Disease (LSD Assay) and the fluorescence and absorbance were employed on the Hyperbilirubinemia assay. The Lysosomal Storage Disorder (LSD) assay was chosen to demonstrate the impedance measurements due to the presence of very high number of reaction droplets (220). The protocol employed would identify 220 droplet dispense, transport and merge. The Hyperbilirubinemia assay consists of three parts, measurement of Total Bilirubin (T.Bili) in plasma/serum employing absorbance, measurement of Thyroid Stimulating Hormone (TSH) in blood using an immunoassay and fluorescence and measurement of Glucose-6-Phospate Dehydrogenase (G6PD) from blood employing fluorescence.
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