Salivary exosome and cell-free DNA for cancer detection

Liquid biopsies are easier to acquire patient derived samples than conventional tissue biopsies, and their use enables real-time monitoring of the disease through continuous sampling after initial diagnosis, resulting in a paradigm shift to customized treatment according to the patient's prognosis. Among the various liquid biopsy samples, saliva is easily obtained by spitting or swab sucking without needing an expert for sample collection. In addition, it is known that disease related biomarkers that exist in the blood and have undergone extensive research exist in saliva even at a lower concentration than the blood. Thus, interest in the use of saliva as a liquid biopsy has increased. In this review, we focused on the salivary exosome and cell-free DNA (cfDNA) among the various biomarkers in saliva. Since the exosome and cfDNA in saliva are present at lower concentrations than the biomarkers in blood, it is important to separate and concentrate them before conducting down-stream analyses such as exosome cargo analysis, quantitative polymerase chain reaction (qPCR), and sequencing. However, saliva is difficult to apply directly to microfluidics-based systems for separation because of its high viscosity and the presence of various foreign substances. Therefore, we reviewed the microfluidics-based saliva pretreatment method and then compared the commercially available kit and the microfluidic chip for isolation and enrichment of the exosome and cfDNA in saliva.