Automatic flow delay through passive wax valves for paper-based analytical devices.

Area of Science:

• Microfluidics
• Analytical Chemistry
• Materials Science

Background:

• Microfluidic paper-based analytical devices (μPADs) are promising for point-of-care testing.
• Precise flow control, particularly flow delay, is crucial for multi-step reactions in μPADs.
• Existing methods often lack reliable and automatic flow-delay mechanisms.

Purpose of the Study:

• To investigate and develop a novel flow-delay system for μPADs using wax valves and surfactant diffusion.
• To establish numerical, mathematical, and experimental understanding of the flow-delay mechanism.
• To demonstrate the application of this system in sequential reactions and analyte detection.

Main Methods:

• Numerical simulations to analyze wax valve performance based on contact angle, porosity, and pore size.
• Fabrication of PDMS walls to prevent leakage.
• Experimental control of surfactant quantity, diffusion distance, and type to regulate delay time (1.6–20 minutes).
• Development of a mathematical model based on Fick's second law to predict delay times.
• Application in sequential mixing and detection of glucose and alcohol.

Main Results:

• Wax valves effectively controlled flow, allowing surfactant solutions while preventing water.
• PDMS walls successfully prevented leakage.
• Achieved tunable flow delay from 1.6 to 20 minutes by manipulating surfactant properties.
• Developed a predictive mathematical model for delay time.
• Demonstrated successful sequential detection of glucose and alcohol with improved LOD (1 mg dL⁻¹).

Conclusions:

• The developed wax valve system with surfactant diffusion provides an effective and automatic flow-delay mechanism for μPADs.
• This approach significantly enhances the performance of μPADs, particularly in improving detection limits.
• The findings offer valuable guidelines for designing and implementing advanced μPADs for point-of-care diagnostics.