SUMMARY

In this proposal, we demonstrate a new strategy to continuously and non-destructively separate cells into subpopulations by exploiting the variation in mechanical stiffness between individual cells. We use a microfluidic channel decorated by periodic diagonal ridges that compress the flowing cells in rapid succession. The compression in combination with secondary flows in the ridged microfluidic channel translates each cell perpendicular to the channel axis in proportion to its stiffness. The difference in mechanical resistance to compression of cells with different stiffness gives rise to a stiffness-dependent force associated with cell passage through constrictions formed by the consecutive channel ridges. This elastic force is directed normal to the compressive diagonal ridges and, therefore, has a component that deflects cells propelled by the flow in the transverse direction with a rate proportional to their stiffness. In addition to the elastic force, cells experience a transverse hydrodynamic force due to circulatory flow created by diagonal ridges. The elastic and hydrodynamic forces act in the opposing transverse directions and the balance between these two forces sets cell trajectories that rapidly diverge for cells with different stiffness. We demonstrate the physical principle of the cell sorting mechanism and show that our microfluidic approach can be effectively used to separate a variety of cell types which are similar in size but of different stiffnesses, spanning a range from 210 Pa to 23 kPa. Atomic force microscopy is used to directly measure the stiffness of the separated cells and we found that the trajectories in the microfluidic channel correlated to stiffness. microfluidic separation technique opens new ways for conducting rapid and low-cost cell analysis and disease diagnostics through biophysical markers.