Control and Modeling of Imbibition in Paper-Based Microfluidic Devices
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Control and Modeling of Imbibition in Paper-Based Microfluidic Devices


Access to medical care is a significant challenge facing developing countries. The World Health Organization developed the ASSURED criteria, which specifies requirements for the ideal diagnostic. ASSURED stands for Affordable, Sensitive, Specific, Rapid/Robust, Equipment-free and Deliverable. Traditional microfluidic diagnostics (comprised of glass/PDMS) require expensive fabrication procedures and user-intervention to manipulate fluids on the device. For those reasons, paper shows great promise as a substrate for an ASSURED microfluidic diagnostic. It is affordable and does not require external pumping to move fluids. Initially, microfluidic paper-based analytical devices (μPADs) performed simple, colorimetric assays. However, the field has evolved rapidly. Modern μPADs have been developed for a plethora of applications such as nucleic acid amplification. Wider adaptation of μPADs relies on the incorporation of more complicated assays (i.e. involving multiple fluids), which would allow μPADs to replace more expensive benchtop equipment. This requires more robust fluid control. Typically, fluid control on μPADs has been achieved by slowing fluid down to create delays between different channels. However, adding delays increases overall assay time and creates other complications such as fluid loss due to evaporation. Instead, creating ‘delays’ by accelerating wicking (relative to native paper) is being investigated. Previous approaches include sandwiching the paper between polymer films or creating ‘macro capillaries’ within the paper for the liquid to flow through. Of particular interest is the etching of grooves onto paper channels using either a plotter or a CO2 laser. These grooves create hollow regions in the paper which lead to faster wicking speeds. This study aims to characterize the behavior of grooved channels in paper and assess their performance as ‘delay’ mechanisms for a multi-fluid paper-based sensor. Typically, μPAD designs evolve in a trial-and-error basis, where devices are fabricated, tested and updated. Having accurate models that characterize the imbibition process could streamline development by allowing direct translation of in-silico designs to fully-functioning paper-based tools. Current imbibition models do not adequately describe the complex transport phenomena occurring within the paper matrix. This study also aims to develop an in-silico simulation that can reliably predict imbibition in both native paper and grooved channels.

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