This study integrated the microfluidic system and ODEP technology for the isolation of CTC clusters from the background leukocytes. The working principle is based on the size difference between the CTC clusters and leukocytes, and thus different magnitude of ODEP force acting on them. ODEP mainly use a controllable light pattern, acting as a virtual electrode, to generate a non-uniform electric field that is in turn utilized to manipulate the electrically-polarized cells. The utilization of ODEP-based mechanism for CTCs isolation has been successfully demonstrated in our previous study .
Since 1970, a series of clinical studies have shown that single CTCs may not be the main cause of cancer metastasis, but two or more aggregated CTCs [2, 3]. In order to isolate CTC clusters for back-end analysis. For the biological-based methods (e.g. HBCTC-Chip , or CTC-iChip ), although CTC clusters can be specifically separated by antibody-based schemes, but the surface-area-to-volume ratio of CTC clusters is relatively low which might affect the binding efficiency of CTC clusters and antibody. Alternatively, some studies proposed physical-based methods to separate CTC clusters (e.g. ISET , FMSA , Cluster-chip ). Although these methods have been demonstrated to effectively isolate the CTC clusters from the background cells mainly based on their size difference, the influence of shear stress on physical size of CTC cluster, or the viability of the cells isolated is still a problem.
To address this issue, The key advantages of ODEP mechanism for cell isolation including: (1) no need of complex microfabrication process for constructing microfilter structures, and (2) the reduction of shear stress acting on the cells manipulated.
However, the feasibility of using ODEP-based mechanism for the isolation of CTC cluster (i.e. CTC cell aggregates) has not yet been explored. To test its feasibility, a T-shaped microfluidic chip was designed (Fig.1). A virtual microfilter consisting of multiple light patterns was designed at the CTC clusters isolation zone (Fig.1). By continuously moving and rotating the light patterns in the microfilter, the larger CTC clusters can be separated from the background leukocytes, and also transported to the side microchannel (Fig.2). In this work, the optimum ODEP operating conditions (e.g. moving velocity of light pattern) was explored. Results revealed that moving velocity of light pattern that can manipulate the CTC clusters (containing 2-13 cells) was significantly higher than the background of leukocytes (Fig.3). Based on this, the moving velocity of light pattern was set at 100 μm/sec (Fig.3). At a given sample flow rate of 0.5 μl/min, moreover, we found that the rotation speed of light patterns at 14 RPM could significantly increase the purity of CTC clusters isolation (Fig.4). Based on the set operating conditions, the recovery and purity of the isolated CTC clusters were experimentally evaluated to be 70.1 ± 7.1% and 60.8 ± 2.7%, respectively (Fig.4). As a whole, we have established a high purity CTC clusters isolation method that is easy to operate, and is possible to avoid the problem caused by the shear stress acting on the cells or particles.