Circulating tumor cells (CTCs) are viable cells found in blood of cancer patients; isolation of these cells from blood sample is significant in understanding the cancer metastasis through a non-invasive monitoring.Conventional immune-mediated techniques like FACS or MACS are extensively applied for CTC detection.The disadvantages are high cost,larger sample volume and loss of cells during the sorting process.Recent advances in the micro-fabrication technology led to the discovery of efficient microfluidic devices that are "label-free" and dependent on the bio-physical characteristics of cells.We developed a microfluidic device that utilizes the principle of size-dependent flow based separation combined with micropillars for isolation of viable tumor cells.The basis of separation in our device is that tumor cells are larger in size than the most of blood cells.Samples mixtures were injected from the sample inlet and inclined micropillars are stationed downstream for diverging cells of certain size away from others instead of trapping them,in our case the tumor cells.Sample mixtures were prepared at hematocrit (Hct) condition of 0.05-0.1% and in a (0.01-0.05):1 ratio between tumor and red blood cells.Our experiments were carried out using breast cancer MCF-7 cells.Sample mixtures were injected from the inlet and cells collected at two outlets were counted and isolation efficiency was calculated.Viability of the isolated tumor cells was compared to that of the normal cells.The variable parameters in our devices are the angle of inclination of the micropillar array (5,10 and 15 degrees) and width of the outlets (50,100 and 600 microns).Devices with outlet width of about 50 microns were tested at flow rates of 2.4 ml/hr and 3.6 ml/hr.At flow rate of 2.4 ml/hr,the isolation efficiency was maximum 25%.Conversely,at flow rate of 3.6 ml/hr,the efficiency was up to 40%.Hence the angle of inclination had moderate influence on the efficiency for these devices.A similar trend was seen in the case of devices with 100 microns outlet width with a maximum efficiency reaching about 58% at 2.4 ml/hr and about 75% at 3.6 ml/hr.For devices with an outlet width of 600 microns,the efficiency at both flow rates was about 70%,although a high amount of blood cell lysis was observed.As such,we could infer that the angle of inclination had a certain influence on the isolation efficiency but the width of the outlet channel had a more dominant role,reaching a maximum efficiency of about 75%.Tumor cells isolated from the devices were found to be viable and grew in a similar way to those in the normal culture.In addition,our design offers the advantage of containing cell traps that can fix the tumor cells and enable their staining with various fluorescent markers.Overall,we have successfully developed a microfluidic device for "label free" isolation of tumor cells with additional cell traps for further cell characterization within the device.