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Publication

Micro Bubble-jet Cell Sorter

Book - Dissertation

FACS enables the analysis and sorting of cells from a heterogeneous cell population at single cell level, hence representing an indispensable tool for biological research and clinical diagnostics and therapeutics. A traditional FACS consists of a flow cell in which biological cells pass an optical detection point one by one. There they are discriminated based on optical scatter signals and fluorescent labels. The cells are then encapsulated in droplets and electrostatically deflected into collection vials depending on set optical signal gates. Selection of cells of interest is typically done based on cell surface antigens, viability, DNA and RNA content or fluorescent proteins. Novel applications, such as rare cell isolation and cell therapy are demanding extreme high throughput and sterile sorting with multi-marker selection. The formation of aerosols in a FACS during sorting raises issues regarding biosafety and cross-contamination, which is especially problematic for these applications.Several microfluidic sorters on chip have been developed to overcome the limitations of FACS. The actuation method in these devices either relies on physical cell properties or is independent of cell properties with selection based on the optical scatter and fluorescence signature of the cell i.e. µFACS. For many applications, the physical properties of the target cells do not defer enough to be sorted out from the heterogeneous cell mixture and only a µFACS is capable of isolating the cells of interest. Many actuation methods for µFACS devices have been developed such as a MEMS valve, piezoelectric induced jet flow or vapor bubble induced jet flow. The latter method can be divided in bubble generation from laser induced optical heating and Joule heating of a thin-film resistor.In this dissertation, a strategy has been developed to create a high throughput, biosafe and disposable µFACS, the micro bubble-jet sorter. It leverages lithographical fabrication techniques to fabricate hundreds of small thermal hotspots in a thin-film microheater. Pulsed resistive heating at the hotspots produces numerous micro vapor bubbles with ultra-short lifetimes compared to a single large vapor bubble in other jet flow sorters. On top of that, all external actuators are integrated which removes costly components such as with the laser induced bubble generation. The fabrication process and design of two versions of the micro bubble-jet sorter chip are introduced. Two new photo-patternable adhesive polymers were characterized for microfluidic cytometry applications. Powerful cavitation forces from collapsing vapor bubbles at the end of the bubble cycle deteriorate the passivation layer on top of the microheater. A second-generation sorter chip was developed with an improved material stack to increase the heater lifetime. The vapor bubble cycle was studied with thermal simulations of the heater stack and stroboscopic imaging of the vapor bubbles during heating pulse application. The jet flow for cell sorting was characterized with finite element analysis, and dye trace imaging. High rate sorting experiments with beads showed that the sorting performance was limited by coincidence events in which cells arrive simultaneously and the jet flow period. Moreover, a significant enrichment of spiked cells demonstrated that this sorting chip is particularly suitable for CTC isolation from PBMCs and prenatal cell isolation from cervical samples. To check the influence of the sorting process on cell well-being, cell viability tests were performed. In a preliminary test, the expression levels of genes related to stress of sorted cells were measured to further investigate the influence of the sorting process. Affordable solutions for massive high throughput sorting can only be achieved with parallel sorting on chip. Therefore, thermal simulations and measurements were performed to investigate the limitations of parallel sorting. The parallel sorting capabilities were demonstrated with bead and cell isolations.Cell sorting based on an array of micro sized vapor bubbles provides an eminent tool to increase the throughput of µFACS devices required for next generation cell therapy applications. To this end, a microfluidic sorting chip has been demonstrated that will enable contamination-free, enclosed, and affordable cell sorting that is compatible with mass manufacturing.