A comparative study of paper-based microfluidic devices with respect to channel geometry

Since 2007, the world of microfluidics saw an emergence of a new era of low cost, simple, quick fabrication, abundantly available, and environmentally friendly microfluidic paper-based analytical devices (μPADs) capable of clinical diagnostics, food quality control as well as environmental monitoring. Although many fabrication techniques have evolved as a result of its broad application spectrum and ease of use, the technology has still barely scratched the surface of its potential in terms of its underlying fundamental principle, i.e., fluid flow analysis. In this study, we report the comparison of flow profile attained by using two of the most promising techniques of photolithography and wax printing from a hydrodynamic point of view. A modified protocol for synthesizing an epoxy based negative photoresist (SU-8) channel and wax-based channel was created by optimizing a few process parameters of our equipment. Water and oil (oleic acid) are selected as the hydrophilic and hydrophobic fluids, respectively, and their flow was analyzed in straight channels using a paper device. A new approach to vary the flow velocity is described in detail involving cylindrical dots as the resistance inside the paper channel. Observing the length-time curve for the two fluids, it becomes evident that both follow the Lucas-Washburn equation if the width of the channel is large enough. Various configurations of dots indicate that different longitudinal flow velocity implying its application in simultaneous addition of chemicals without the need to change the channel width or length.