CHO immobilization in alginate/poly-L-lysine microcapsules: an understanding of potential and limitations

Microencapsulation offers a unique potential for high cell density, high productivity mammalian cell cultures. However, for successful exploitation there is the need for microcapsules of defined size, properties and mechanical stability. Four types of alginate/poly-L-Lysine microcapsules, containing recombinant CHO cells, have been investigated: ( a) 800 mu m liquid core microcapsules, (b) 500 mu m liquid core microcapsules, ( c) 880 mu m liquid core microcapsules with a double PLL membrane and ( d) 740 mu m semiliquid core microcapsules. With encapsulated cells a reduced growth rate was observed, however this was accompanied by a 2 - 3 fold higher specific production rate of the recombinant protein. Interestingly, the maximal intracapsular cell concentration was only 8.7 x 10(7) cell mL(-1), corresponding to a colonization of 20% of the microcapsule volume. The low level of colonization is unlikely to be due to diffusional limitations since reduction of microcapsule size had no effect. Measurement of cell leaching and mechanical properties showed that liquid core microcapsules are not suitable for continuous long-term cultures (> 1 month). By contrast semi-liquid core microcapsules were stable over long periods with a constant level of cell colonization (phi = 3%). This indicates that the alginate in the core plays a predominant role in determining the level of microcapsule colonization. This was confirmed by experiments showing reduced growth rates of batch suspension cultures of CHO cells in medium containing dissolved alginate. Removal of this alginate would therefore be expected to increase microcapsule colonization.