Development of all-aqueous based high throughput microfluidic workflow for biological applications

Abstract

Microfluidic technology has gained remarkable research attention in the past decades. With the ability to generate or manipulate a large quantity of microdroplets without increasing the device's size or complexity, microfluidic technology has found applications in many fields, such as single-cell analysis, material synthesis, and pharmaceutical engineering. However, till now, most droplet microfluidic platforms for biological applications hinge on water/oil single-emulsion systems, where some limitations are involved. The confined environment of droplets prevents necessary cell-cell interactions, completely obstructing this vital communication pathway. Meanwhile, restricted nutrition and oxygen within a single droplet may reduce cell viability and shorten the available time for culture. This thesis explores and develops an oil-mediated all-aqueous droplet microfluidic workflow that has the potential to overcome these drawbacks of conventional water/oil single emulsion systems. As the beginning of the whole work, the widely used droplet generation and manipulation methods are firstly overviewed in Chapter 2. Following this, to ensure stable and uniform droplet generation using PDMS based microfluidic devices, a multifunctional superhydrophobic coating is synthesized in Chapter 3. The durability and stability of the coating are proved by crack-free photonic crystal synthesis in an oil bath. The coating can resist 70℃ heating for over 60 hours, indicating it is suitable for most droplet microfluidic applications. The PDMS surface treated by the coating shows superhydrophobicity under silicone oil. Finally, droplet generation shows that the superhydrophobic coating can ensure stable generate of water-in-oil (W/O) single emulsion. After that, a flow-focusing microfluidic device is designed and fabricated to generate Fe3O4 MNP-laden alginate droplets in Chapter 4. The droplets are then crosslinked and released into the culture medium through evaporation of the continuous phase to achieve an oil-mediated all-aqueous culturing environment. The manipulation of Fe3O4 MNP-laden alginate droplets within the bulk phase is carried out to prove the feasibility of magnetic manipulation. Finally, E. coli cultivation using the fabricated Fe3O4 MNP-laden alginate droplets demonstrates the biocompatibility of the proposed method. To further improve the biocompatibility of the workflow, a new microfluidic device containing two flow-focusing sections is designed in Chapter 5 to generate a core-shell structure droplet, which can separate the Fe3O4 MNPs from the biological targets. PEG/DEX ATPS systems are encapsulated into a core-shell droplet using the fabricated device, proving the feasibility of the core-shell droplet. Finally, the oil-mediated all-aqueous-based microfluidic workflow, including the generation, release, manipulation, and degradation of the Fe3O4 MNP-laden core-shell droplet, is demonstrated in Chapter 6. The flow rate condition governing the droplet generation is studied in detail. E. coli is utilized as a model microorganism to encapsulate within the droplet to perform single bacterium cultivation. The biocompatible degradation of the alginate droplets is also demonstrated to complete the whole workflow for cell or bacteria culturing applications.