Over the past two decades, circulating tumor cells (CTCs) have been widely recognized for their importance in clinical trials. While most enrichment methods for these cells have been conducted through the batch process due to their rarity in blood and the need for large sample volumes, the batch process leads to unavoidable cell loss. Given the heterogenetic features of CTCs, this cell loss may limit the validity of research that relies on the isolation of CTCs; such research includes cancer prognosis, diagnosis of minimal residual diseases, assessment of tumor sensitivity to anticancer drugs, and the personalization of anticancer therapies. Recent advances in microfluidic approaches have made it possible to enrich CTCs with a small degree of cell loss. In this review, we highlight several microfluidic-based positive and negative enrichment methods that are the subject of considerable research interest (e.g. EpCAM-dependent assay and EpCAM-independent assay) and suggest a microfluidic-based single cell analysis platform for the down-stream analysis of CTCs. We also discuss critical concerns and future directions for research.

Circulating tumor cells (CTCs) have attracted a great deal of attention, as they can be exploited to investigate metastasis. The molecular and cellular characteristics of these cells are little understood because they are rare and difficult to isolate. Many methods of isolation have centered on affinity-based positive enrichment (i.e., capturing target cells and eluting nontarget cells) using epithelial cell adhesion molecule (EpCAM) antibodies. It is known, however, that not all CTCs express the EpCAM antigen because they are heterogeneous by nature. In addition, negative enrichment (i.e., capturing nontarget cells and eluting target cells) has advantages over positive enrichment in isolating CTCs since the former can collect the target cells in an intact form. In this paper, we introduce a geometrically activated surface interaction (GASI) chip with an asymmetric herringbone structure designed to generate enhanced mixing flows, increasing the surface interaction between the nontarget cells and the channel surface. CD45 antibodies were immobilized inside the channel to capture leukocytes and release CTCs to the outlet. Blood samples from breast, lung, and gastric cancer patients were analyzed. The number of isolated CTCs varied from 1 to 51 in 1 mL of blood. Because our device does not require any labeling processes (e.g., EpCAM antibodies), intact and heterogeneous CTCs can be isolated regardless of EpCAM expression.

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Much research has been performed over the past several decades in an attempt to conquer cancer. Tissue biopsy is the conventional method for gathering biological materials to analyze cancer and has contributed greatly to the understanding of cancer. However, this method is limited because it is time-consuming (requires tissue sectioning, staining, and pathological analysis), costly, provides scarce starting materials for multiple tests, and is painful. A liquid biopsy, which analyzes cancer-derived materials from various body fluids using a minimally invasive procedure, is more practical for real-time monitoring of disease progression than tissue biopsy. Biomarkers analyzable through liquid biopsy include circulating tumor cells (CTCs), exosomes, circulating cell-free DNA (cfDNA), miRNA, and proteins. Research on CTCs has been actively conducted because CTCs provide information on the whole cell, unlike the other biomarkers mentioned above. However, owing to the rarity and heterogeneity of CTCs, CTC research faces many critical concerns. Although exosomes and cfDNA have some technical challenges, they are being highlighted as new target materials. That is because they also have genetic information on cancers. Even though the number of exosomes and cfDNA from early stage cancer patients are similar to healthy individuals, they are present in high concentrations after metastasis. In this article, we review several technologies for material analyses of cancer, discuss the critical concerns based on hands-on experience, and describe future directions for cancer screening, detection, and diagnostics.