Autonomous railing and trapping of microbeads for continuous flow multi-stage microfluidic processes

"Multi-stage" fluidic mixing processes are critical to a wide range of chemical and biological assays (e.g., immunoassays). Unfortunately, the majority of biochemical assays suffer from laborious and time-intensive fluidic mixing procedures in which distinct reagents and/or washes are loaded sequentially and separately (i.e., one-at-a-time). Previously, we introduced the first microfluidic railing system capable of hydrodynamically guiding microbeads into discrete, adjacent flow streams in order to autonomously accomplish multi-stage fluidic mixing reactions on-chip. However, microbead immobilization and signal detection were only possible after reaction completion, which poses a significant problem for bead-based assays that require microbead visualization during intermediate phases of multi-stage processes. To overcome this limitation, here we present a single-layer, "continuous flow" microfluidic system that utilizes microposts arrayed in sections at angles of 1°, 15°, and 1° (with respect to the flow direction) to: (i) rail suspended microbeads into a trapping area, (ii) trap select numbers of microbeads, and then (iii) transport subsequent microbeads into adjacent flow streams, respectively. By enabling both autonomous bead-based mixing reactions as well as microbead immobilization during each fluidic mixing stage, the presented continuous flow microfluidic "rail-and-trap" system offers a simple, yet powerful methodology for applications in chemical and biological fields, such as molecular diagnostics.