Manipulating fluid flow behavior in microporous paper to achieve better mixing in lateral flow devices

Paper microfluidics-based tools have emerged as low-cost, portable diagnostics platform; yet they face challenges when used for whole blood sample analysis. Whole blood samples cause pore clogging, phase separation, and reduced wicking on the porous media during lateral transport; this leads to poor quality of mixing, which is an inherently diffusion driven phenomenon. This study investigates the manipulation of fluid flow behavior in a microporous matrix (filter paper) to enhance mixing efficiency in lateral flow devices. By exploring various design parameters, specifically the curvature of a microporous paper strip, and the wettability characteristics of backing layers, we aimed to manipulate the fluid flow behavior within these systems. Experiments demonstrated that curved paper strips significantly increase fluid velocity compared to traditional straight paper strips, with an average velocity increase of 65%. The backing layer's wettability—hydrophilic, hydrophobic, or ambient air proved to play a critical role, affecting fluid flow behavior and velocity. The hydrophilic surface facilitated improved fluid movement due to the lower contact angles and reduced drag resistance, while the hydrophobic surfaces posed greater resistance. Additionally, the degree of curvature and radius of the strips were crucial factors influencing fluid velocity, with lower degrees and radii enhancing the fluid flow rate. The curvature assisted asymmetry on the paper strip allowed larger interface propagation on the porous matrix, which led to better mixing by transverse dispersion of analytes. Further investigation into mixing efficiency was further analyzed using food dye and methylene blue in skimmed milk solutions, revealing the significance of curved paper strip and different configuration of fluid introduction points. These findings provide valuable insights into improved mixing processes in porous media, optimizing paper-based microfluidic device designs, potentially overcoming current limitations in scaling and accuracy, and advancing their practical application in diagnostics.