Augmentation of mass transfer through electrical means and nutrient enrichment for suspension and entrapment cell cultures

Nutrient limitations (oxygen, glucose and amino acids) and cellular waste product accumulation (lactate and ammonia) are two factors limiting cell growth and protein production in suspension and entrapped cell cultures. In this research, the possibility of using enriched medium and electrokinetic transport phenomena (electroosmosis and electrophoresis) to overcome the above limitations is explored.

E. coli was entrapped in $\kappa$-carrageenan and agarose hydrogel slabs. The total cell densities increased by 140% and 80%, respectively, in $\kappa$-carrageenan and agarose under an electric current density of 180 A/m$\sp2$, as compared to the control cultivations in the absence of the electric field. A mathematical model considering the intra-hydrogel transport and cell growth kinetics was developed, and this model successfully simulated the time-dependent cell growth in the hydrogels under different electric current densities. The model showed that in $\kappa$-carrageenan, 80% of the increase in cell density was attributed to the removal of the toxic waste products and 20% of the increase was due to the augmented glucose transport.

In suspension hybridoma cultures, an electric current density of 50 A/m$\sp2$ was applied to the cultures using commercial medium, DMEM. Cell-produced ammonia and lactate were effectively removed and hence both cell growth and antibody production were enhanced. However, the enhancement was limited (less than 50%) due to the nutrient depletion. Therefore, enriched medium in conjunction with a higher electric current density of 75 A/m$\sp2$ were used in the subsequent cultures. The final cell density and antibody titer increased from $3.9\times 10\sp{6}$ to $9.1\times 10\sp{6}$ cells/ml and from 170 to 510 mg/L, respectively, compared to the control culture using the same enriched medium in the absence of the electric field. The enhanced cell growth and antibody production were mainly attributed to the complete removal of ammonia (maintained below 2 mM) and the higher nutrient concentrations. Cellular metabolism was analyzed, and the results suggested that the enriched medium caused the overflow metabolism of nutrients, especially glucose, and thus resulted in an excess production of lactate and a significant increase in osmolarity.

In order to decelerate cellular metabolism and increase specific antibody productivity in the suspension hybridoma cultures using the enriched medium in the presence of 75 A/m$\sp2$, 0.7 mM sodium butyrate was added during the mid-exponential growth phase of the culture. It was found that 0.7 mM sodium butyrate was able to decrease the glucose metabolism by 25% and increased the specific antibody productivity by 60% compared to the culture in the absence of sodium butyrate. Sodium butyrate, however, slightly retarded cell growth. As a result, a final antibody titer of 650 mg/L was achieved. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.)