Triggerable Fluidic Gates in Paper-Based Microfluidic Devices Using Printed Wax Barriers

Paper-based microfluidic devices offer a low-cost, portable platform for diagnostics, yet user-controlled fluidic switching for multi-step assays remains challenging. We present a scalable method to fabricate triggerable fluidic gates in paper substrates using wax printing. Hydrophobic wax barriers (0.5-1 mm thick) block flow ("OFF" state) until locally heated above their melting point (~ 100°C, "ON" state), enabling on-demand fluid release at ~$0.1/device. Using a Xerox ColorQube 8580 and biocompatible wax, this approach requires no solvents or complex equipment. We demonstrate its efficacy via global (hot plate, 5 s trigger) and localized (wax pen, 3 s; c.w. laser, 2 s) heating, validated by microscopy and assays for nitrite detection (LOD: 0.1 mM) and C-reactive protein (CRP) sandwich ELISA (LOD: 0.01 µg/mL). This contamination-free strategy enhances paper-based sensors for point-of-care diagnostics.