Effects of microfluidic channel geometry on leukocyte rolling assays

Microfluidic cell adhesion assays have emerged as a means to increase throughput as well as reduce the amount of costly reagents. However as dimensions of the flow chamber are reduced and approach the diameter of a cell (D(c)), theoretical models have predicted that mechanical stress, force, and torque on a cell will be amplified. We fabricated a series of microfluidic devices that have a constant width:height ratio (10:1) but with varying heights. The smallest microfluidic device (200 μm ×20 μm) requires perfusion rates as low as 40 nL/min to generate wall shear stresses of 0.5 dynes/cm(2). When neutrophils were perfused through P-selectin coated chambers at equivalent wall shear stress, rolling velocities decreased by approximately 70 % as the ratio of cell diameter to chamber height (D(c)/H) increased from 0.08 (H = 100 μm) to 0.40 (H = 20 μm). Three-dimensional numerical simulations of neutrophil rolling in channels of different heights showed a similar trend. Complementary studies with PSGL-1 coated microspheres and paraformaldehyde-fixed neutrophils suggested that changes in rolling velocity were related to cell deformability. Using interference reflection microscopy, we observed increases in neutrophil contact area with increasing chamber height (9-33 %) and increasing wall shear stress (28-56 %). Our results suggest that rolling velocity is dependent not only on wall shear stress but also on the shear stress gradient experienced by the rolling cell. These results point to the D(c)/H ratio as an important design parameter of leukocyte microfluidic assays, and should be applicable to rolling assays that involve other cell types such as platelets or cancer cells.