ENGINEERING CHALLENGES IN HIGH-DENSITY CELL-CULTURE SYSTEMS

High density cell culture systems offer the advantage of production of bio-pharmaceuticals in compact bioreactors with high volumetric produ ction rates; however, these systems are difficult to design and operat e. First of all, the cells have to be retained in the bioreactor by ph ysical means during perfusion. The design of the cell retention is the key to performance of high density cell culture systems. Oxygenation and media design are also important for maximizing the cell number. In high density perfusion reactors, variable cell density, and hence the metabolic demand, require constant adjustment of perfusion rates. The use of cell specific perfusion rate (CSPR) control provides a constan t environment to the cells resulting in consistent production. On-line measurement of cell density and metabolic activities can be used for the estimation of cell densities and the control of CSPR. Issues relat ed to mass transfer and mixing become more important at high cell dens ities. Due to the difference in mass transfer coefficients for oxygen and CO2, a significant accumulation of dissolved COP is experienced wi th silicone tubing aeration. Also, mixing is observed to decrease at h igh densities. Base addition, if not properly done, could result in lo calized cell lysis and poor culture performance. Non-uniform mixing in reactors promotes the heterogeneity of the culture. Cell aggregation results in segregation of the cells within different mixing zones. Thi s paper discusses these issues and makes recommendations for further d evelopment of high density cell culture bioreactors.