Suspension of Microcarriers for Cell Culture with Axial Flow Impellers

The suspension characteristics, the agitator speed, N_js, and the mean specific energy dissipation rate, (${\overline{\epsilon }}_{\text{T}}$)_js, required to just fully suspend ∼20% wet weight Cytodex 3^® microcarrier beads have been studied using a range of different diameter Chemineer HE-3 hydrofoils, a pair of Ekato InterMIG impellers and a six-blade mixed-flow impeller in a baffled vessel of 19.2 l operating volume containing phosphate buffer saline solution. Flat and modified tank bases were used. N_js values were observed to be in the range of 50 to 90 rev min⁻¹. The use of Zwietering's correlation using existing literature geometric suspension parameters, S, to predict N_js would give values up to 50% higher. The low N_js values obtained in the experiments are attributed to the very small particle–liquid density difference (40 kg m⁻³), which eased the lifting of the particles from the tank bottom, compared to those studied previously. Also, it was found that at N_js, it took up to about 40 min to suspend all particles as the upper surface of the initially settled bed was scoured away. Previous work has not reported such a long-term phenomenon. With the liquid/particle properties used in this study, the three-blade hydrofoil HE-3 impeller of D/T = 0.39 in a cone-and-fillet based tank was marginally the most efficient; that is, it had the lowest (${\overline{\epsilon }}_{\text{T}}$)_js of ∼0.5 x 10⁻³ W kg⁻¹. However, (${\overline{\epsilon }}_{\text{T}}$)_js was less than 1 x 10⁻³ Wkg⁻¹ in most cases with the different size HE-3 hydrofoils, which implies that these geometries would probably be suitable for application in shear-sensitive animal cell culture systems using such microcarriers.