Microfluidics for label-free sorting of rare circulating tumor cells.

Circulating tumor cells (CTCs) have been widely considered as promising novel biomarkers for molecular research and clinical diagnosis of cancer. However, the sorting of CTCs is very challenging due to the rarity of CTCs in blood and the morphological similarity to blood cells. Although affinity-based CTC sorting methods could capture CTCs using specific biochemical markers, there are limitations such as the loss of cell viability after labeling and a requirement for expensive biochemical marker reagents. Emerging label-free CTC sorting methods rely on the physical properties of cells and can potentially overcome the aforementioned limitations. In this review, we highlight recent advances in label-free CTC sorting methods, with emphasis on device structures and performances. Specifically, we present a detailed discussion on label-free CTC sorting methods, including passive ones that depend on the channel structure or specific fluidic effects and active ones that use external force fields, as well as provide an overview of the principles, advantages, limitations, and applications of state-of-art label-free CTC sorting devices. Finally, we provide a future perspective of microfluidics for label-free CTC sorting and hope to inspire readers to develop new devices for applications in clinical cancer diagnoses and research.