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.