Circulating tumor cells (CTCs) shed from either primary or metastatic cancers have been identified in the peripheral blood of patients,and are often associated with cancer metastasis and tumor recurrence.A commonly used biological parameter to separate CTCs from blood samples in microfluidic systems is the cell size.The size of epithelial tumor cells ranges from 15 to 25 μm in diameter,larger than the surrounding blood cells.The clever manipulation of the well-controlled laminar flow inside curved microfluidic channels can alter the cell movement,and achieve size-dependent label-free tumor cell separation.[1] In this work,we design a double spiral microchannel with 6-spiral loops for each direction with a very low aspect ratio to enrich the tumor cells from blood in a label-free manner (Fig.1).When the magnitude of Dean drag is comparable to that of inertial lift forces,cells are focused to a single equilibrium position or array due to the secondary flow,resulting in different equilibrium positions for small blood cells and large tumor cells.[2] We characterized the position and distribution of blood (stained red) and MCF-7 cells (stained green) at different positions inside the double spiral at the flow rate of 20 mL/hr (Fig.2).The first spiral ordered the cells to narrow streams in a size-dependent manner.As tumor and blood cells passed the S-shape junction and traversed along the clockwise direction,a separation between tumor and blood cells was finally achieved in the outlet channel.After cell separation,we employed an integrated polycarbonate (PC) membrane located on top of the outlet of microchannels to trap and enrich MCF-7 cells (Fig.3).Nucleic acids from MCF-7 cells were extracted and detected by loop-mediated isothermal amplification (LAMP).