Microfluidics, as its name suggests, is the science of manipulating fluids on a microscopic scale, and it has positively impacted biology from the aspects of research and industry. With significant advantages over traditional technologies, including precise controllability, high sensitivity, and high throughput with reduced costs and sample volumes, microfluidics is increasingly benefiting immunology research.
In the development of immunology and immune engineering, conventional methods require bulk measurements to obtain average phenotypes of a population of cells, which appears to be inefficient when handling rare subsets of cells and returning characteristics of a specific single cell. On the contrary, microfluidic devices can efficiently filter and sort subpopulations of cells and are ideal for studying single cells.
Microfluidic devices have been widely used to study immune cells at a single-cell resolution, including isolation, manipulation, and analysis.
Single-cell Isolation
The first necessary step to startingsingle-cell measurements is to isolate single immune cells from a tissue. Different from approaches like flow cytometry displaying some limits in the selection and transfer of single cells into wells, microfluidic methods can be designed for high throughput meanwhile with minimized risk of dilution and contamination. Therefore, microfluidic enrichment devices are used upstream of cell isolation methods. Single-cell molecular cloning and screening using high-throughput microfluidic-integrated devices could be performed in perfusion flow mode, droplet mode, and microarray mode.
Single-cell Manipulation
The majority of immune cells are only micrometer-sized, which means using traditional tools like Petri-dishes and well-plates to manipulate those cells at the single-cell level could be extremely difficult. Therefore, microfluidic strategies, which can manipulate fluids in the range of micro to pico-liters, have been developed into various platforms and techniques based on hydrodynamic, electrical, optical, acoustic, magnetic, and micro-robotic methods. Microfluidic chips with more flexibility could meet the demands of diverse single-cell manipulation tasks, including single-cell encapsulation, sorting, trapping, rotation, pairing, patterning, stretching, transportation, lysis, and stimulation.
Single-cell Analysis
Moreover, microfluidics can also be applied in single immune cell analyses with a combination of microscopy, microelectrodes array, mass spectrometry, and chromatography.
Most microfluidic chips use optically transparent materials, which is convenient for the real-time monitoring of immune cellular behavior through imaging. Therefore, microfluidic platforms have been developed to analyze the morphology, migration, proliferation, differentiation, and apoptosis behaviors of immune cells.
Besides, measurements that can analyze the transcriptome of individual cells are highly desirable for immune research, inwhich unravelment of immune cells' heterogeneity is important. Single-cell sequencing on microfluidic chips has allowed possible breakthroughs in understanding single-cell genetic information and cellular phenotypes. Commonsingle-cellsequencingstrategiesbased on microfluidic techniques include droplet-based single-cell sequencing, diffusion-based single-cell RNA sequencing, and single-cell whole-transcriptome sequencing. And microfluidic droplet-based single-cell sequencing has been used in single-cell immunology research to tackle various challenges.
To get information about a single immune cell and study the heterogeneity of a cell population, single cells can be confined to picoliter-sized water droplets contained in an oil phase. Droplet microfluidics enables the analysis of thousands of cells in parallel for mRNA transcripts while keeping a record of the transcript's cell of origin via genomic barcodes. Droplet microfluidic technologies for single-cell sequencing, both genome-wide and RNA-sequencing, are expected to help identify rare immune cell populations and develop personalized immune cell-based therapies for complex diseases like cancer.
Conclusion
The immune system is complicated as several cell types with unique genetic information and intercellular interactions are involved. Thus, challenges in separating and studying cell behaviors in controlled environments should be addressed before researchers can further understand the immune system and develop efficienttherapeutics for autoimmune diseases and cancer. Compared to traditional methods based on cell populations, microfluidics enables scientists to study immune cells with single-cell resolution under a controlled and dynamic microenvironment. Microfluidics has advanced single-cell immunology in the field of genomics, proteomics, secretion dynamics and signaling, and cell interactions.
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