Expansion of mammalian neural stem cells in bioreactors: effect of power input and medium viscosity.

Multipotent neural precursors can be cultured in suspension bioreactors as aggregates of stem cells and progenitor cells. However, it is important to limit the size of the aggregates, as necrotic centers may develop at very large diameters. Previously, we have shown that the hydrodynamics within a suspension bioreactor can be used to control the diameter of NSC aggregates (D(MAVG)<150 microm) below sizes where necrosis would be expected to occur. In the present study, power law correlations were developed for our bioreactors showing the dependence of the maximum mean aggregate diameter on both the kinematic viscosity of the medium and the power input per unit mass of medium. The power input was manipulated by changing the agitation rate (60-100 rpm), and the viscosity was manipulated through the addition of non-toxic levels of carboxymethylcellulose. The study also confirmed that the maximum liquid shear generated at the surface of the aggregates was sufficient to dislodge single cells, thus limiting the maximum diameter of the aggregates, without causing cell damage (tau(max)=9.76 dyn/cm(2)). This is a first step in the development of a reproducible, scaled-up process for the production of neural stem cells for therapeutic applications including the treatment of neurodegenerative disorders and acute central nervous system injuries.