A Geometry-Based Optimization for Enhanced Mixing in Paper-Based Microfluidics

Abstract

Introduction: Paper microfluidic devices despite proven to be effective, encounter challenges when complex biofluids such as blood, mucus or cerebrospinal fluid are used as samples1. These fluids exhibit high flow resistance due to their non-Newtonian nature, arising from cellular & biomolecular components. As a result, complex fluids show slow wetting uneven spreading and unintended separation effects2. This study examines different design parameters to manipulate fluid flow more effectively within a microporous filter‑paper matrix. We have investigated channel curvature and backing‑layer composition as design parameters to optimize flow behaviour in lateral‑flow devices to enhance mixing efficiency. Methods: Whatman grade 1 paper strips of equal area were kept on top of surfaces with varying nature (absent backing layer and hydrophilic, hydrophobic). A 30μL skim milk solution (analogous to complex biofluid) of different concentrations was dispensed at the centre of the paper strip. Then the fluid wicking length was captured using a video camera to later analyse the average velocity of the fluid front with respect to time. Later using the optimized geometrical designs of the paper strip, mixing of two different colour dyes were performed, and the mixing efficiency was quantified. The mixing quality then quantified and compared across different configuration of porous strips. Results & Discussion: Experiments demonstrated that presence of backing layer and its nature—hydrophilic, hydrophobic, proved to affect fluid velocity (Figure 1). The presence of backing layer created a parallel flow along the porous strip which has aided the flow in comparison to its absent backing layer counterpart. Hence, hydrophilic surfaces facilitated faster fluid flow due to lower contact angles and reduced drag resistance (Figure 1d). However, hydrophobic surfaces posed more resistance and possibly have created localized fluid reservoir. Moreover, curved design has shown significant increase in fluid velocity compared to traditional straight strips, with an average increase of 65% in every concentration of skim milk sample tested (Figure 3). Additionally, the degree of curvature and the angle subtended by the curvature of the strips were crucial factors influencing fluid velocity. It was found that higher degree of curvature enhances the average velocity in fixed time. The similar pattern was observed when measured across different angles subtended by the curvature. Based on the results the mixing efficiency was further analysed using food dye and methylene blue in SM solutions. The mixing indexes also revealed that smaller radii and specific angle subtended by the curvature promotes improvement through higher fluid velocities and enhanced diffusion (Figure 2). These findings provide valuable insights into optimizing paper-based microfluidic device designs, potentially overcoming current limitations in scaling and application in diagnostics. Conclusion: This study highlights the role of design parameters in the overall performance of an LFA device. By systematically varying the curvature and angle subtended by the curvature, we have observed significant improvement in average wetting speed and mixing on porous matrix. This study offers an effective strategy for the improvement of traditional LFA devices which are robust, easily scalable and can potentially solve the challenges posed by the complex biofluid samples.