## 4 Discussion  
This study evaluated the mechanical properties of GO spray-coated onto the surface of a large-area polystyrene film. The possibility of developing a large-area cell culture  
Figure 11: Flow cytometric analysis of MSC markers. The expression of MSC positive markers was all over 95% and of MSC negative markers was less than 1%. Figure 12: Secretion of a cytokine. The concentration of secreted VEGF from PS or GO substrates-cultured hBM-MSCs did not appear to have a significant difference. vessel was analyzed by evaluating cell compatibility and cell proliferation on the GO-coated surface. Step difference measurements across the GO-coated polystyrene film revealed a curve of 100 nm or higher on the film surface. The change in the flexural difference according to the number of coatings became more pronounced as the coating speed decreased. In general, the roughness values (Ra, Ra) of the entire surface reduced with increases in coating speed and increased with increases in the number of coatings. In particular, the difference in the average size of the surface curvature was large in the group coated at a low speed. In the case of medium speed (M group), the average size of surface curves similar to that of the low speed group (L group) appeared, but the number of surface curves was higher. Therefore, the magnitudes of the surface roughness and the number of curves can be controlled by appropriately adjusting the speed of the coating application. In the case of cells, it is known that cells adhere well to a curved surface rather than a flat surface during the cell adhesion process, and growth is good on a curved surface in the cell growth field [25]. In this study, an appropriate shape of the GO coating on the polystyrene surface appeared to facilitate cell growth. The G peak in the Raman spectra of all groups confirmed the GO coating, and the ID/IG ratios (0.88-0.94) confirmed the same type of GO with similar structural defects in each group. The transmittances of GO-coated surfaces (relative to that of uncoated polystyrene film) decreased as the number of coatings increased and the coating speed decreased. The contact angle measurements of the surface of the GO-coated polystyrene film confirmed a hydrophilic surface with higher surface energy than the hydrophobic polystyrene surface. Moreover, the cells better adhered to and proliferated on the hydrophilic surface provided by the GO coating [21]. The cells grew poorly on the uncoated polystyrene film but thrived on the GO-coated polystyrene film. After adhering to the surface, the cells increased in number and eventually formed colonies that flattened and extended across the surface. At this time, the surface energy and roughness ratio were appropriate for attachment and growth; however, when the number of coatings increased, the surface roughness became unfavorable which possibly explains the comparatively low cell adhesion rates on these surfaces [20; 25; 26]. As the cell proliferation rate was highest on the H3 sample, the cell culture efficiency was determined to be sufficiently increased by coating with small amounts of GO. As confirmed in the growth-rate analysis, the cell growth was enhanced by the surface roughness and hydrophilicity introduced by the GO coating, requiring no complex chemical treatment. Moreover, the polystyrene film inhibited cell growth, but the GO-coated polystyrene film was entirely nontoxic, thus confirming its improved cell compatibility. Among the coating conditions set in this study, the smallest amount of GO (three coatings) deposited at the highest speed (93.25 mm/s) achieved the highest cell proliferation, indicating that a small amount of GO sufficiently increases the cell culturing efficiency. The cell viability on the H3 coating was 1.43 times higher than in the conventional cell culture vessel. That is, by optimizing the GO coating, we can realize an economical and practical large-area cell culture vessel that can maximize the cell culture efficiency.