Lastly, the distribution of monocytes was evaluated to determine whether they could be separated from lymphocytes and granulocytes, based on deformability. Monocytes make up 2–8% of the total leukocyte population. They average 7.5 µm in diameter22, making them slightly larger than either lymphocytes (6.2 µm) or granulocytes (7.0–7.3 µm). We sorted monocytes from whole blood after pre-staining with anti-CD14 marker and observed that they were retained primarily in outlets O8–O9, by the 6.5 µm constrictions (Fig. 7). This result suggests that these cells could be efficiently enriched from lymphocytes but not from granulocytes using the current version of the microfluidic ratchets device. The purity of monocytes in O8 range 4.5–20%, relative to granulocytes, which represents only a modest increase from the standard frequency of these cells in peripheral blood leukocyte population. However, the nearly exclusive accumulation of monocytes within these outlets suggests that future iterations of this device could resolve monocytes from granulocytes by optimization of the pore size (in the range between 6.5 and 8 µm) and funnel thickness.Figure 7Distribution of monocytes sorted from whole blood from the same donor in three trials. Inset graphs show the composition of leukocytes subsets in O8 and 9.