Atomisation Techniques for Immobilisation of Cells in Micro Gel Beads

Abstract

Entrapment of cells in natural polymer gel beads is one of the most extensively studied methods of cell immobilisation. A mixture of pre-cultured cells and polymer solution is extruded through a nozzle into a stirred gelling agent solution, resulting in formation of gel beads entrapping the cells. The diameters of the resulting gel beads depend on the diameter of the nozzle and also on the physical properties (mainly the viscosity and surface tension) of the polymer. Gel beads entrapping microbial cells are used in either fluidised or packed bed bioreactors for various bioprocesses such as production of useful metabolites, waste water treatment and in biofiltration systems. However, most of the gel beads currently used for the various processes are more than 2.0 mm in diameter. Mass transfer limitation is often a major problem encountered in bioprocesses using cells entrapped in large diameter gel beads [1]. Oxygen and nutrients have to diffuse from the broth through the gel matrix before reaching the immobilised cells. Furthermore, as oxygen and nutrients diffuse from the broth into the gel beads, they are consumed by the cells at and near the surface of the gel beads, leading to oxygen and nutrient concentration gradients inside the gel beads. These gradients depend on the specific rates of nutrient and oxygen consumption and on the concentration of the immobilised cells. When the concentration of the immobilised cells is very high, oxygen and nutrients are completely consumed within the periphery of the gel beads. Thus, it is difficult to maintain active cells at the centre of large diameter gel beads due to oxygen and nutrient limitations. Furthermore, the produced metabolites and gasses must also diffuse out from the beads into the broth. For products with feed back inhibition, diffusion of the produced metabolites from the large diameter gel beads into the broth may be the rate-limiting step in the overall process. When the product is acidic, the pH inside the large diameter gel beads may be much lower than that in the bulk medium, making it difficult to maintain the optimum pH for the cells inside the gel beads [2,3]. Also, when the rate of gas (carbon dioxide) evolution by the immobilised cells is higher than the rate of its diffusion out of the gel beads, the gel beads crack due to increase in the internal pressure. Depending on the fluid dynamics in the bioreactor, cracked beads float with resultant decrease in the productivity of the process.