Multilayered Microfluidic Paper-Based Devices: Characterization, Modeling, and Perspectives

Abstract

Microfluidic paper-based analytical devices (μPADs) are simple but powerful analytical tools that are gaining significant recent attention due to their many advantages over more traditional monitoring tools. These include being inexpensive, portable, pump-free, and having the ability to store reagents. One major limitation of these devices is slow flow rates, which are controlled by capillary action in the hydrophilic pores of cellulosic paper. Recent investigations have advanced the flow rates in μPADs through the generation of a gap or channel between two closely spaced paper sheets. This multilayered format has opened up μPADs to new applications and detection schemes, where large gap sizes (>300 μm) provide at least 169× faster flow rates than single-layer μPADs, but do not conform to established mathematical models for fluid transport in porous materials, such as the classic Lucas-Washburn equation. In the present study, experimental investigations and analytical modeling are applied to elucidate the driving forces behind the rapid flow rates in these devices. We investigate a range of hypotheses for the systems fluid dynamics and establish a theoretical model to predict the flow rate in multilayered μPADs that takes into account viscous dissipation within the paper. Device orientation, sample addition method, and the gap height are found to be critical concerns when modeling the imbibition in multilayered devices.

Figure 1.

Schematic and orientation of multilayerd μPADs.

Figure 2.

The 1/distance fit of experimental data with 3 and 5 layers of double-sided tape (height = 234 and 390 μm, respectively).

Figure 3.

a) Side view picture and b) illustrative schematic of fluid flow in multilayered μPADs (390 μm gap height).

Figure 4.

Decay of velocity with distance with different sampling methods namely addition through pipetting (grey triangles), connection to aliquots on a super omniphobic surface (orange squares), and connection to aliquots in tape wells (blue circles), flow of a dilute solution of dye down a multilayer μPAD, 390 μm gap height, Whatman 3MM paper, n = 5.

Figure 5.

Comparison of experimental data (actual), the Lucas Washburn equation (LW) and our viscous dissipation (ViDi) model, velocity of fluid flow is taken at 5.5 cm.

Figure 6.

The liquid front velocity as a function of time in horizontal operation. The circle symbols () correspond to the experimental values while the black line represents the model predictionl. The gap heights are (a) 234 μm, (b) 312 μm, and (c) 390 μm.