Stacking of beads into monolayers by flowthrough flat microfluidic chambers

This work presents the investigation of hydrodynamic filling of beads into flat microfluidic devices. A periodical hexagonal monolayer as aggregation pattern is favorable for parallel optical detection. Several microfluidic devices for bead-based analyses were designed. Each microfluidic device consists of one inlet channel, one flat aggregation chamber for the beads and several outlet channels. Suspensions of beads with 180 μm in diameter are loaded into a flat chamber measuring 190 μm in depth by a pressure driven flow. With the depth smaller than a bead diameter, the outlets act as barriers to the beads and force them to accumulate in the chamber. Therefore, the decisive impact parameters are the geometry, the particle concentration of suspension, and the inlet pressure. Reproducible filling ratios of more than 94 % have been achieved. We found an optimum filling behavior for a rhombus-like aggregation chamber connected to a single outlet channel at the same width as the chamber. Here, the aperture angle of 60° fosters hexagonal aggregation patterns which leads to the highest package density. The rhombus-like chamber also shows the least rise of the hydrodynamic resistance during filling and the best rinsing behavior which allows to minimize the volume of washing detergents used for an immunoassay. Areas of accumulated beads redistribute the hydrodynamic resistance of the microfluidic device. CFD-simulations, embedded in an iterative master-routine, are carried out to describe the complete process of filling and to assist the process of design optimization.