In addition, Kalish et Combining etching with geometric patterning offers a greater range of wicking control through a single fabrication step. This technique has potential applications in various fields such as medical diagnostics and environmental monitoring. Another research paper presents a novel paper-based capillary-driven flow system that uses narrow gap geometry between two parallel surfaces to enhance flow rates. The bottom surface of the system is a hydrophobic paper substrate with a microfluidic channel defined by a hydrophilic pathway, while the top surface is hydrophobic. The liquid flows on the hydrophilic path in the gap without spreading onto the hydrophobic regions. The system is closed-channel, meaning that there are no side walls, which results in higher spreading distances and accelerated liquid flow. The closed-channel microfluidic system was found to increase the flow rate by 200% and 100% for nanoparticle-coated paperboard and blotting papers, respectively. The system offers several advantages, including fast liquid delivery to detection zones or reaction and reduced liquid drying and evaporation. The closedchannel flow system can be produced utilizing existing paper converting processes, and laser etching, and lamination techniques can be used to create and close the channel. The improvements in flow rates are attributed to the increased capillary force created by the non-wetting top cover, which modifies the liquid-air interface curvature in the narrow gap. An optimal gap distance exists and is determined by the balance of the flow driving wetting force and the viscous drag of the flow in the channel, both of which increase as the gap distance is reduced. The closed-channel flow system is a significant improvement over traditional filter-paper-based microfluidic paper analytical devices (μPADs) and can find applications in various fields where rapid and efficient detection or reaction is needed.