High resolution particle separation
For sepsis application, the spiral device needs to focus blood cells at the outer wall while allowing the bacteria to migrate away from the wall. To this end, we evaluated the spiral device using particles to find the particle size cut-off above which particles remain focused. Experimentally, 1 µm particles were mixed with particles of different sizes and the migration and focusing was observed (Fig.4A). Using the current spiral design, it is possible to separate 1 µm from 3 µm particles with resolution. While the 2 µm particles are not separated from the 1 µm particles at the outlet, it is noteworthy to mention that while the 1 µm particles have made a turn along the inner wall, the 2 µm particles are yet to make the complete turn. Hence, while outside the scope of this work, by cleverly designing the device it should be possible to differentiate those particles as well. Furthermore, the 3 and 5 µm particles clearly migrate toward the inner wall while the 7 µm particles are fully focused. For a low-aspect-ratio channel geometry (width >> height), focusing is strongly dependent on the particle size to channel height ratio (a/h). Using low-aspect-ratio channel geometry, we previously suggested a minimum a/h ratio > 0.1 for focusing in inertial microfluidics53. Interestingly, our findings also indicate stable focusing at a/h ratio > 0.1. Stable focusing in our case means particles remaining fully focused at the outer wall. This would translate to a particle size above 5 µm in the current spiral design (h=50 µm). As a proof of principle for high resolution particle separation, 1 µm and 3 µm particles were mixed and pushed through the device and we analyzed the collected fractions at the outlets (Fig.4B). A hemocytometer was used to count the two fractions. The yield of the 1 μm particles, calculated as fraction of 1 μm particles recovered through the inner outlet (O1) to the total count, was 96%, and the yield was 100% for 3 μm particles in the outer outlet (O2). To reiterate, the total volumetric flow rate used was 1 mL/min, a throughput previously only reported for inertial microfluidics.