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

Microfluidic paper-based analytical devices (mu PADs) 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 mu PADs through the generation of a gap or channel between two closely spaced paper sheets. This multilayered format has opened up mu PADs to new applications and detection schemes, where large gap sizes (>300 mu m) provide at least 169X faster flow rates than single-layer mu PADs, 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 mu PADs 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.