Characterization of the properties of PEI as an attachment factor. Graph A: Increasing doses of PEI were tested to determine the concentration range for optimal coating. PEI showed full attachment enhancement at concentrations of 2.5 μg/ml and higher. Graph B: PEI promoted attachment of both PC-12 and HEK-293 cells over a wide range of cell numbers (numbers tested are indicated in X axis). Graph C: PEI coating remained stable onto the culture dish surface over long incubation periods and medium changes (10d-PBS: PEI-treated dish incubated for 10 days in PBS at 4°C; 96 h-3MC: PEI-treated dish incubated for 96 h with 3 medium changes at 37°C). For all the graphs, the bars show the average ± standard deviation of triplicate assays, which are representative of at least 3 independent experiments (* significantly higher than untreated control, p < 0.01).