INVESTIGATIONS ON OXYGEN LIMITATIONS OF ADHERENT CELLS GROWING ON MACROPOROUS MICROCARRIERS

Macroporous microcarriers are commonly applied to fixed and fluidized bed bioreactors for the cultivation of stringent adherent cells. Sever al investigations showed that these carriers are advantageous in respe ct to a large surface area(Griffiths, 1990; Looby, 1990a). When growin g a rC-127 cell line on Cytoline 2 (Pharmacia Biotech), no satisfactor y product yield could be achieved. A possible limitation in the supply of nutrient components was investigated to explain these poor results . No significant concentration gradients could be detected. Neverthele ss, fluorescence staining revealed a decreasing viability, particularl y inside the macroporous structure. Therefore, oxygen transfer to and into the carriers was examined by means of an oxygen microprobe during the entire process. Additional mathematical modeling supported these results. The maximum penetration depth of oxygen was determined to be 300 mu m. A critical value influencing the oxygen uptake rate of the r C-127 cells occured at a dissolved oxygen concentration of 8% of air s aturation. A significant mass transfer resistance within a laminar bou ndary film at the surface of the carrier could be detected. This bound ary layer had a depth of 170 mu m. The results showed that even a 40% air saturation in the bulk liquid could not provide an efficient oxyge nation of the surface biofilm during the exponential growth phase. Flu orescent staining reveals a poor viability of cells growing inside the carrier volume. Thus, oxygen supply limits the growth of rC-127 cells on macroporous microcarriers. Poor process performance and low produc t yield could be explained this way.