Fabrication and characterization of polyelectrolyte microcarriers for microorganism cultivation through a microfluidic droplet system

Abstract

This study experimentally investigates the generation of polyelectrolyte droplets, and subsequently, cured microcarriers for application in microorganism cultivation. Microcarriers containing an mature microorganism culture can replace in situ biofilms in microfluidic bioprocesses to reduce the startup duration and facilitate the replenishment of functional microorganisms. This study examines the effects of particulate solutes (polystyrene (PS) microparticles, Chlorella vulgaris, and Escherichia coli) on polyelectrolyte solution properties (zeta potential, contact angle, and interfacial tension) and droplet/microcarrier formation. The results indicated that, except for interfacial tension and shear stress, the colloidal stability and morphology of particulate solutes should also be considered when generating droplets. A particulate solute resulting in a dispersed phase with colloidal stability was beneficial for the predictable droplet generation. A particulate solute increasing the affinity between the disperse phase and the channel wall hastened the droplet generation to shift from the dripping region to the jetting region. Adding particulate solutes with consistent size and morphology into the dispersed phase was less likely to affect the droplet formation and the droplet size. In this study, adding PS microparticles and C. vulgaris into a diallyldimethylammonium chloride (DDA) solution resulted in a more hydrophilic solution and higher interfacial tension compared with adding DDA solution alone. These particulate solutes also tended to cause incipient colloidal instability. The incipient instability of the C. vulgaris solution led to various droplet sizes. However, the droplet sizes of the PS microparticle solution, which had solution properties similar to those of the C. vulgaris solution, were more uniform because of the consistent size and morphology of PS microparticles. Adding moderate amounts of E. coli resulted in a more hydrophobic solution, lower interfacial tension, and satisfactory colloidal stability. The droplets containing microorganisms were cured to generate poly-DDA (PDDA) microcarriers, and E. coli cultivated in PDDA microcarriers had the same viability as those cultivated in suspension. However, C. vulgaris cultivated in PDDA microcarriers failed to proliferate possibly due to the blockage of the nutrient intake by the quaternary ammonium cation of DDA.