RIM-PIV Measurements of Solid–Liquid Flow in a Stirred Tank Used for Mesenchymal Stem Cell Culture

Mesenchymal stem cells are widely cultivated in stirred tank bioreactors. Due to their adhesion properties, they are attached to small spherical spheres called microcarriers. To understand the hydromechanical stresses encountered by the cells, it is essential to characterize the flow using the PIV technique. However, the usual solid–liquid system used in cell cultures has poor optical properties. Thus, shifting to one with better optical properties, while respecting the physical characteristics, is mandatory to achieve a relevant representation. PMMA microparticles suspended with 61 wt% ammonium thiocyanate solution NH4SCN were found to be a robust candidate. The refractive index (RI) of both sides is of the order of 1.491 with a density ratio of ρf/ρp≈ 0.96, and particle size averaged around 168 μm. Using the RIM-PIV (refractive index matched particle image velocimetry) technique for a 0.7 L volume stirred tank equipped with an HTPG down-pumping axial impeller and operating at full homogeneous speed N=150 rpm, mean and turbulence quantities of the liquid phase were measured as a function of PMMA particle volume fractions αp, which ranged from 0.5 to 3 v%. This corresponds to a particle number density of n=12 particles/mm3, which is considered original and challenging for the PIV technique. At 3 v%, the addition of particles dampened the turbulent kinetic energy (TKE) of the liquid phase locally by 20% near the impeller. This impact became trivial (<10%) at the local-average level. The structure and direction of the recirculation loop also shifted.