Scalable, High-Density Expansion of Human Mesenchymal Stem Cells on Microcarriers Using the Bach Impeller in Stirred-Tank Reactors.

This paper describes the results of process developmental experiments to achieve higher cell densities in the manufacturing of hMSCs using the novel Bach impeller in a stirred-tank bioreactor. Engineering experiments have previously shown that the Bach impeller represents an efficient mixing device that suspends particles in fluids at low power inputs. To assess the impeller during biological experiments, the growth performance of Wharton Jelly (WJ)-hMSCs in a 1 L STR equipped with the Bach impeller was evaluated at a variety of culture conditions. The cells attached to Cytodex 1 microcarriers at a concentration of 5.6 g/L and were cultured for 5-7 days. The growth phase was carried out at varying impeller speeds N $N$  = 75, 115, and 150 rpm. Cell growth was additionally evaluated at a microcarrier concentration of 11.2 g/L Cytodex 1. Here, a maximum cell density of up to 1.7 × 106 cells/mL and cell viability > 90% was achieved within 5 culture days, which is amongst the highest cell densities ever attained for a hMSC batch culture. Critical cell quality attributes of the WJ-hMSCs were assessed upon completion of the growth phase, that is, FACS to identify stem cell surface markers, tri-lineage differentiation, and capacity of the cells to form colonies. In addition, informed by the previously described engineering characterization, the 1 L process at N $N$  = 75 rpm was scaled up to the 5 L scale, where WJ-hMSCs were again confirmed to have retained the relevant cell quality attributes. The reported findings are important to determine the design space to which scale-ups to even larger tank sizes can adhere.