Structure and assembly of PEO-PPO-PEO co-polymers in mammalian cell-culture media

We investigate the structure of a poly(ethylene oxide)-poly(propylene oxide)-poly (ethylene oxide) co-polymer, Pluronic F127, in mammalian cell-culture media. It is well known that aqueous solutions of F127 gel at physiological temperatures with this transition, which corresponds to the formation of a close-packed cubic assembly of spherical micelles. Previous work has shown that in both mammalian cell minimum essential medium (MEM) and MEM with added fetal bovine serum, the gel phase boundary shifts to lower temperatures and concentrations as compared to pure water. Using DLS, we have found that at 25 degrees C the critical micelle concentration (CMC) decreases in the presence of MEM. Our SANS studies at 25 degrees C indicate that F127 in MEM-D2O also forms a close-packed cubic micellar gel, suggesting that the mechanism of gelation is the same in both pure water and MEM. Fits to the neutron spectra on 2 wt% F127 in D2O and MEM-D2O show a large difference in the micelle aggregation number (Nagg) and a small difference in the micelle size, with both Nagg and the micelle size larger in the presence of MEM. In addition, our SANS spectra in MEM-D2O indicate repulsive interactions between micelles at 2 wt% polymer, whereas no correlation peak is observed for this concentration in water. Finally, moderately concentrated samples (5-18 wt% polymer) in MEM exhibit slightly stronger ordering and sharper peaks, perhaps indicating stronger intermicellar interactions in the presence of MEM. This stronger repulsive interaction may be the cause of the shift in the liquid-gel phase boundary that is observed.