Paper-based microfluidic devices have recently attracted attention for their ability to utilize the inherent capillary force, or absorptivity, of paper to generate flows instead of requiring an external force. This allows for a simple sensor system that can be readily manufactured at low costs; however, sophisticated flow control is still necessary to implement and analyze diverse functions. In this paper, we propose a facile flow rate control method by varying the permeability of chromatography paper using a wax printing method frequently used to make channel walls in paper-based microfluidic devices. We found that the flow rates in paper channels can be precisely controlled by varying either the brightness of wax patterns, which is inversely proportional to the amount of printed wax droplets, or the length of wax patterns printed on the paper channel. Based on these results, we developed a paper-based micromixer that minutely controls the mixing ratio of two dye flows as prescribed in printed wax patterns. We found experimental mixing ratio values that were in excellent agreement with predicted values.
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