Microfluidic Separation, Detection, and Engineering of Extracellular Vesicles for Cancer Diagnostics and Drug Delivery

Extracellular vesicles (EVs) are cell-derived submicron biopartides composed of lipid bilayer membrane and molecular cargos, acting as important mediators of physiopathological cellular processes. The analysis and engineering of EVs hold significant therapeutic potential in noninvasive cancer diagnostics and innovative drug delivery systems. Despite significant improvements in technologies for EV investigation, the clinical use of EVs has been hampered by several challenges including the requirement of expensive equipment such as ultracentrifuge for EV isolation from clinical samples, laborious and time-consuming procedures for EV analysis, and large batch-to-batch variation for EV engineering. In this respect, microfluidic technologies have attracted increasing attention as promising avenues to accelerate the study of EVs by offering advantages of small-volume capacity, cost effectiveness, precise manipulation of bioparticles, streamlined workflows, high levels of sensitivity and specificity, and good reproducibility and stability. In this Account, we review the state-of-the-art advances in the development of microfluidic platforms for EV separation, detection, and engineering with key applications in cancer diagnostics and drug delivery. We first elaborate a variety of passive and active microfluidic approaches for label-free, high-resolution separation of EVs from biological matrix based on their physical properties. As an example of passive method, viscoelastic microfluidics exploits the size-dependent elastic lift force imposed on EVs in a viscoelastic medium, allowing for the high-resolution isolation of EVs from biofluids. The active methods leverage the use of externally applied physical fields (e.g., electric and acoustic fields) to achieve rapid separation of submicron-sized EVs. We then summarize different signal amplification and detection strategies implemented with microfluidic platforms for sensitive and specific characterization of EVs in clinical samples. The combination of microfluidics with thermophoresis-assisted fluorescence detection, surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS), and magnetic detection have been employed to profile EV surface proteins, miRNAs, mRNAs, etc. These EV-associated biomarkers reveal great potential for the diagnosis, monitoring, and prognosis of cancer. We also survey the progress in microfluidic engineering of EVs that utilizes the intensive physical (acoustic and electric fields) or mechanical force fields to load active cargo into EVs in a reproducible, continuous manner. The engineered EVs have been developed as advanced delivery systems with improved immune evasion, targeting capability, and therapeutic effectiveness. Finally, we condude this Account by outlining the challenges, opportunities, and future directions in the microfluidic investigation of EVs in the clinic and in vivo.