National Cheng Kung University,
Keywords: porosity, paper-based microfluidics
Summary:A method is proposed for estimating the porosity of paper by comparing the electric current passing through a wax-printed channel patterned on the paper surface with that passing through a hollow PDMS channel. To ensure the accuracy of the estimation results, the paper channel is sandwiched between two flat polydimethylsiloxane (PDMS) plates in order to minimize evaporation losses and to emulate the surface condition of the hollow PDMS channel used for reference purposes. The feasibility of the proposed method is demonstrated using three paper samples with different porosities (Whatman filter paper, Grade 1, Grade 2 and Grade 5). For each sample, the current is measured using a KCl electrolyte solution under applied voltages of 10, 20, 30, 40 and 50 V. It is shown that the measured electric current in all three samples is lower than that in the hollow PDMS reference channel due to the obstruction of the ion flow by the paper fibers. In addition, a good agreement is observed between the estimated values of the paper porosity and the porosity values determined from the basis weight and thickness data provided by the paper manufacturer. Finally, it is shown that the porosity estimates are insensitive to the magnitude of the applied voltage. Consequently, the proposed method provides a low-cost and effective means of evaluating candidate materials for microfluidic paper-based analytical devices (μPADs).