Surface Directed Flows and Fluidic diodes: Transforming lateral flow assays into multistep diagnostics devices

Abstract Lateral flow assays (LFAs) are among the most widely deployed point-of-care diagnostic platforms, yet their inherent single-flow architecture limits their operation to single-step binding assays and precludes the sequential delivery of multiple reagents within a single device. Herein, we report the design, fabrication, and characterization of hybrid paper-plastic fluidic diode modules that may be seamlessly integrated into conventional LFA architectures to enable controlled, unidirectional, multi-step reagent delivery. Single diode characterization confirmed reliable forward flow initiation and robust reverse flow prevention across a range of buffer compositions with results in good agreement with the theoretical predictions. Cascaded diode assemblies demonstrated well-defined sequential reagent delivery with minimal inter-zone mixing. Integration of a two-stage cascaded diode assembly into a SARS-CoV-2 lateral flow assay demonstrated a 2.5-fold enhancement in test line signal intensity relative to a conventional single-flow LFA employing identical assay chemistries. Owing to the simple geometry of the fluidic diode, these components may be assembled as a planar laminated stack along with other LFA components and singulated into individual test strips through the same cutting operations employed in conventional LFA manufacturing, without any requirement for precision microfabrication or specialized equipment. This positions the fluidic diode as a manufacturable and scalable solution for introducing multi-step reagent sequencing into lateral flow diagnostics.