Volume 24 Issue 6, December 2019
Dave Pechter discusses with Chao Li the article, "Automated System for Small-Population Single-Particle Processing Enabled by Exclusive Liquid Repellency."
Lossless processing and culture of rare cells (e.g., circulating tumor cells, drug-persistent microorganisms) at single-cell level is of great significance in understanding the heterogeneity of carcinogenesis or human pathogenesis caused by microbial infection. Current single-cell isolation techniques like fluorescence-activated cell sorting (FACS) require relatively large sample volume and cell number to work with, and inflict sample loss and reduced cell viability from the processing. While microfluidic and single-cell printing techniques allow the handling of minute amounts of cellular samples, they either come with limited physical access to the sample of interest due to the closed-channel design (e.g., droplet microfluidics) or sample loss during aspiration, transfer, and sample retrieval from culture.
Recently, we reported an extreme wettability phenomenon, named exclusive liquid repellency or ELR. ELR is observed in solid-liquid-liquid three phase systems, where a solid surface shows complete repellency to a liquid (with Young’s contact angle, CA = 180o) when exposed to a second immiscible liquid. This phenomenon is observed when a particular thermodynamic boundary condition is satisfied (i.e., γS/Lcp + γLdp/Lcp ≤ γS/Ldp, where γ - interfacial tension, S - solid, Lcp - liquid of continuous phase, and Ldp - liquid of dispersed phase). Neither surfactant nor flow condition is required, e.g., compared with droplet microfluidics. ELR enables additional fluidic control, robust on-chip cell culture, and improved processing of rare cell samples in open aqueous fluid under oil. ELR is distinct from other liquid repellent systems with CA < 180o (i.e., non-ELR), showing no compromise of liquid adhesion on solid surfaces and enabling unique applications.
In this work, we developed an automated platform using ELR microdrops for lossless single-particle (or single-cell) isolation, identification, and retrieval. It features the combined use of a robotic liquid handler, an automated microscopic imaging system, and real-time image-processing software for single-particle identification. The automated ELR technique enables rapid, hands-free, and robust isolation of microdrop-encapsulated rare cellular samples, and further on-chip cell culture or down-stream analysis (e.g., RNA extraction and RT-qPCR).