“…For example, Cormier et al attributed the decline in viability of mESC aggregates after 6 days in batch culture to the associated build-up of ammonia (to 2.5 mM) and decrease in pH (from 7.6 to 6.6) [15]. Perfusion, frequent feeding, and/or the use of gas-permeable culture surfaces increased the expansion of MSCs [4,5,6•], ESCs [32,33] and mammary epithelial stem cells [12], while retaining stem cell potential [5,9,32,33]. Under conditions for which oxygen was not limiting, Zhao et al reported greater human MSC expansion at a lower perfusion rate [7].…”