Paper-based microfluidics is gaining traction as a cost-effective and user-friendly platform for point-of-care diagnostics. Expanding the capabilities of these simple devices with efficient fluid control mechanisms is crucial for wider applications. While capillary action offers inherent fluid transport, achieving consistent and uniform mixing in paper-based microfluidic systems remains a challenge.
Passive mixing in paper microfluidics, driven by capillary forces, often suffers from limitations. The irregular fiber structure of paper leads to inconsistent mixing, heavily influenced by variations in paper texture and fiber alignment. This method provides limited control, is prone to backflow, and the scale of paper fibers relative to microfluidic channel dimensions hinders precise manipulation. These inconsistencies impede reliable and uniform mixing essential for quantitative assays.
To overcome these drawbacks, surface acoustic waves (SAW) present a promising alternative for active mixing in paper-based devices. By integrating a Y-channel structure onto paper, researchers have demonstrated rapid and consistent mixing driven by 30 MHz acoustic waves. This surface acoustic wave mixing method offers significant advantages over passive approaches. It provides enhanced control over the mixing process, is less susceptible to paper variations, and effectively eliminates backflow issues.
The efficiency of uniform mixing using SAW has been rigorously compared to passive mixing methods using a novel hue-based colorimetric technique. This technique allows for accurate quantification of mixing speed and efficiency, even with samples exhibiting low color contrast, offering improvements over traditional grayscale analysis. Studies have explored the impact of input power, channel design (tortuosity), and the alignment of paper fibers relative to the flow direction on the performance of acoustically-driven mixing. The hue-based method is particularly valuable for quantifying mixing in applications like on-chip immunochromatographic assays, showcasing the practical utility of uniform mixing in paper-based microfluidic systems enabled by surface acoustic waves.
In conclusion, surface acoustic waves offer a robust and efficient solution for achieving uniform mixing in paper-based microfluidic systems. This active mixing approach overcomes the inherent limitations of passive capillary mixing, paving the way for more reliable and sophisticated paper-based diagnostic devices. The combination of SAW-driven mixing and hue-based analysis represents a significant advancement for point-of-care testing and other microfluidic applications.
