Enhanced passive mixing for paper microfluidics.

Area of Science:

• Microfluidics
• Analytical Chemistry
• Materials Science

Background:

• Paper-based devices offer low-cost diagnostics but suffer from imprecise fluid control, impacting assay performance.
• Low reaction yields and reproducibility hinder the sensitivity and detection limits of paper-based assays.
• Optimizing fluid dynamics is key to unlocking the full potential of paper-based analytical devices.

Purpose of the Study:

• To investigate the impact of flow constriction geometry on molecular mixing in paper-based devices.
• To enhance passive mixing and improve reaction yield and reproducibility in paper-based assays.
• To establish design principles for optimizing reagent mixing ratios and reaction stoichiometry.

Main Methods:

• Colorimetric analysis using the iron(III) thiocyanate reaction (Fe³⁺ + SCN⁻ → Fe(SCN)²⁺) to quantify mixing.
• Fabrication of paper-based devices with patterned hourglass structures to create controlled flow constrictions.
• Systematic variation of constriction gap width (0.32–4.69 mm) and input channel ratios for reagent delivery.

Main Results:

• Decreasing constriction gap width significantly enhanced passive mixing and reaction yield.
• A transition window for mixing enhancement was identified, with complete mixing observed below 0.5 mm gap width.
• The hourglass structure enabled precise control over reagent mixing ratios (e.g., 2:1, 1:1, 1:2 HCl-NaOH).

Conclusions:

• Implementing flow constrictions smaller than 0.5 mm is an effective strategy for achieving complete mixing and reproducible results in paper-based devices.
• The patterned hourglass structure provides a versatile platform for optimizing reagent stoichiometry and improving assay performance.
• These findings pave the way for more sensitive, reliable, and implementable paper-based analytical systems.