Circulating tumor cells (CTCs) from the blood has potential to provide understanding of the biology of metastasis,noninvasive measurement of disease progression,and many clinical applications such as cancer screening and diagnosis [1].Due to the rarity of the CTCs,high throughput process is necessary and recent advances in inertial microfluidics show promising results with continuous size-based separation [2].However,the overlap in size distribution of white blood cells (WBCs) and CTCs limits the separation efficiency of size-based separation methods by reducing the recovery of the injected CTCs and the purity of the collected sample.We present an approach to improve the efficiency of CTC separation from blood cells in a contraction-expansion array (CEA) microfluidic device by selectively increasing the size of cancer cells.Particles in the CEA microchannel experience Dean drag force at the entrance of contraction region and as they pass through the contraction region they are also exposed to inertial lift force.Balance between the two forces,which depends on the particle size,determines the particle trajectory and allows size-based separation.To reduce the overlap in size distribution of blood cells and cancer cells,we used 3 μm-diameter antibody-coated microbeads that bind specifically to epithelial cell adhesion molecules (EpCAM) expressed on the surface of MCF-7 cells (breast cancer cell line).This increased the overall size of the cancer cell,and thereby reducing the overlap in size distribution.Using this method we observed that average lateral position of bead-labeled MCF-7 cells differs from that of unlabeled MCF-7 cells.