Sliding rail-enabled capillary flow control on μPADs for programmable fluid delivery

Precise control of fluid transport is critical for paper-based microfluidic analytical devices (μPADs), especially in point-of-care testing (POCT) applications involving multistep reactions and timed reagent delivery. Here, we propose a simple yet versatile strategy to regulate capillary flow on μPADs by overlaying a film above a paper strip as a sliding rail for liquid samples. By systematically investigating the surface wettability of the sliding rail and its distance (0.1-2.4 mm) from the NC paper strip, we demonstrate that flow velocity on μPADs can be efficiently modulated through geometric control and surface modification. The influence of the width of sliding rails is also investigated. For hydrophilic sliding rails, increasing the width of film will accelerate flow velocity, whereas for hydrophobic sliding rails, narrower film widths yield higher flow velocity. Furthermore, we use a three-branch μPAD to show that both sequential and synchronized fluid arrival can be achieved by fine-tuning gap distance and paper strip width. In addition, a proof-of-concept delay valve implemented by introducing a continuous hydrophilic sliding rail above the segmented NC paper strips is successfully validated. A 1 mm interval located at various positions of μPADs will lead to different time delays. Beyond flow control, the sliding rail also enables localized droplet delivery and transport using a Janus hydrophobic-hydrophilic film with defined apertures. This platform supports sequential and parallel reagent delivery, allowing chemical reactions on-chip. The feasibility of this approach is validated through precipitation reactions, colorimetric reactions, and an enzymatic colorimetric glucose assay, which exhibits a strong correlation (R² = 0.9980) between gray intensity and glucose concentration under controlled laboratory conditions, suggesting its potential for quantitative applications in high-concentration samples. Overall, this work introduces a highly adaptable and fabrication-friendly μPAD platform with rapid flow tuning and multifunctional reaction capability. The mechanical support and alignment structure (3D-printed frame and spacers) are reusable across experiments, while the paper strips and surface-modified films are single-use consumables to ensure reproducible flow performance. This offers a powerful solution for efficient POCT applications and paper-based microfluidic systems.