Search

Spatially resolved fluid-flow modulation and its corresponding integration becomes a crucial aspect to consider when designing high-performing paper-based analytical devices. As such, high demand exists for cost-effective techniques able to selectively control flow in patternable regions of the device. In this regard, a solution composed of potassium alum and animal glue; sizing materials used in the fabrication of calligraphy Xuan paper, is successfully adapted for the first time in a revolutionary approach to efficiently modulate the capillary flow in paper microfluidic channels. Flow delays are achieved by coating the substrate systematically through the variation of parameters such as the ratio of alum to glue, the concentration of alum-glue in the solution, and the number of subsequent coating layers of alum-glue. Moreover, digitized patterning with alum-glue is developed to achieve programmable wicking profiles of accelerating, decelerating, and quasi-linear flow displacement by alternating coated and uncoated zones in the channel. Additionally, the influence of alum-glue when performing biometric assays is investigated by performing a horseradish peroxidase activity colorimetric assay and found to have negligible effects on reading, where a negligible difference of 5.2% was observed in an assay performed in a coated paper substrate, compared to an uncoated one.

Abstract One of the main objectives of microfluidic paper-based analytical devices is to present solutions particularly, for applications in low-resource settings. Therefore, screen-printing appears to be an attractive fabrication technique in the field, due to its overall simplicity, affordability, and high-scalability potential. Conversely, the minimum feature size attained using screen-printing is still rather low, especially compared to other fabrication methods, mainly attributed to the over-penetration of hydrophobic agents, underneath defined patterns on masks, into the fiber matrix of paper substrates. In this work, we propose the use of the over-penetration to our advantage, whereby an appropriate combination of hydrophobic agent temperature and substrate thickness, allows for the proper control of channel patterning, rendering considerably higher resolutions than prior arts. The implementation of Xuan paper and nail oil as novel substrate and hydrophobic agent, respectively, is proposed in this work. Under optimum conditions of temperature and substrate thickness, the resolution of the screen-printing method was pushed up to 97.83 ± 16.34 μm of channel width with acceptable repeatability. It was also found that a trade-off exists between achieving considerably high channel resolutions and maintaining high levels of repeatability of the process. Lastly, miniaturized microfluidic channels were successfully patterned on pH strips for colorimetric pH measurement, demonstrating its advantage on negligible sample-volume consumption in nano-liter range during chemical measurement and minimal interference on manipulation of precious samples, which for the first time, is realized on screen-printed microfluidic paper-based analytical devices.