The sorting and isolation of specific cells from heterogeneous mixtures is a crucial task in life science research, clinical diagnostics and therapeutics. Initially developed in the late 1960s, fluorescence activated cell sorting (FACS) instruments are specialized flow cytometers that sort and collect cells based on specific fluorescent emissions from antibodies, probes and tags. As cell sorting technology evolved, magnetic activated cell sorting (MACS) was introduced, which operates on the same principles as FACS, but uses magnetic probes and tags to sort and collect cells. Modern cell sorters are capable of high-throughput separation of tens-of-thousands of cells per second.
Conventional FACS- and MACS-based cell sorters pose several challenges for users. As cell suspensions flow through the instrument, they are pressure forced through a nozzle to aerosolize and separate cells into individual, charged droplets, which are laser-scanned, and droplets containing the cells of interest are diverted and separated based on charge. The aerosolizing process creates shear-stress, which may damage and decrease the viability of sorted cells. Aerosolized particles also create biohazardous exposure risks for users. Because the cell suspensions must travel through the instruments, there is the potential for contamination, even when proper cleaning protocols are followed. Finally, as a result of the complexity, large footprints and high costs of most cell sorter instruments, they are typically incorporated into core facilities rather than individual labs, creating accessibility and availability barriers.
Recent advances in cell sorting technology have addressed these issues with the introduction of on-chip sorting. The heterogeneous cell mixture to be sorted is loaded into a sterile, single-use cartridge, within which all cell sorting functions are performed. Since the cells do not flow through the instrument itself, the risk of contamination is greatly reduced. Instead of aerosolizing the cell suspension, fluorescent-tagged cells flow through microfluidic channels. This mechanism removes aerosol-exposure risk and is much gentler on the sorted cells. Each supplier utilizes a different mechanism for diverting the target cells of interest into collection channels. On-chip cell sorters are much smaller than their conventional counterparts. They are small enough to place into a cell culture hood or on a benchtop, making them more accessible to individual labs.
However, microfluidics-based cell sorters do have some limitations compared to conventional cell sorters. Since they are smaller, they generally use fewer lasers than conventional sorters. This narrows the absorbance wavelengths they are able to detect, limiting the number of parameters that the instruments can analyze. They are also not as fast as conventional cell sorters, some of which can reach throughputs of up to 200,000 events/second. The volume of cells that can be processed in one run in limited by the capacity of the cartridges. Because the cartridges are single use, there is an additional consumables expense that is not associated with conventional cell sorters.