Fig. 1. Source of liquid biopsy tumor material. Solid tumor masses may shed circulating tumor cell (CTC), circulating cell-free nucleic acids (ctDNA), and exosome into the bloodstream.
Several instruments have been developed to achieve optimal LB assessment (Fig. 2, Table 1). CellSearch (Veridex, USA), the first instrument approved by the United States Food and Drug Administration as an instrument for separating and detecting CTCs in LB, has been widely used for breast, colorectal, and prostate cancer diagnosis. Using this instrument, CTCs are separated from blood cells by magnetic nanoparticles coated with anti-epithelial cellular adhesion molecule (EpCAM) antibodies [16]. Immunofluorescence analysis is then performed on the separated cells for CTC characterization. This approach provides a CTC detection specificity of > 99% and sensitivity of > 97%. As the detection of circulating cell-free nucleic acids depends on molecular biology techniques that are complex to operate and require additional analytical platforms (such as quantitative polymerase chain reaction [qPCR], BEAMing, Safe-SeqS, and CAPP-Seq) [9,17], LB-based instruments are commonly used to extract cell-free nucleic acids. For example, the KingFisher Flex instrument has been applied for cfDNA extraction from blood samples based on the magnetic enrichment technique [18], wherein cfDNA becomes positively charged by magnetic beads that bind to the negatively charged phosphate DNA backbone. For exosome analysis, serial centrifugation or ultracentrifugation steps are performed to eliminate nanoscale contaminants [19]. However, these methods require time-consuming sample preparation protocols and cannot achieve high purity or high-throughput analysis. To address these limitations, an instrument named EXODUS was developed based on a novel ultrafiltration strategy [20]. In this instrument, exosomes are filtered through nanoporous membranes, and double-coupled harmonic oscillations are introduced to these membranes to generate transverse waves to inhibit fouling effects, resulting in enhanced processing speed, yield, and purity.
Nowadays, although many LB instruments have been proposed for cancer diagnosis, most of them still exist a wide gap between the laboratory devices and the commercialization of instrument. According to the differences of working mechanism and application object, the developed instruments also face various limitations. In this article, the challenges of the existed LB instrument, as well as the opportunities for next-generation LB instruments are discussed, aiming at improving the sensitivity, efficiency, and accuracy of cancer diagnosis.