Oxygen-dependent generation of a graded polydopamine coating on nanofibrous materials for controlling stem cell functions

Substrates modified with gradient surface chemistry are of fundamental importance for designing a new bio-interface in biomaterial research and tissue engineering. However, current gradient fabrication strategies are not easily accessible to most laboratories due to complex, expensive, and expertise-requiring procedures. In this study, we generated a gradient of polydopamine (PD) coating on a PLLA nanofiber surface using a spatially restricted supply of oxygen in the reaction solution. Analysis of the oxygen distribution revealed that oxygen availability varied along different reaction solution depths during dopamine polymerization. We then extensively investigated the effects of different parameters, such as tilting angle, reaction time, pH of the reaction solution, and concentration of dopamine, on PD gradient formation, which should be appropriately modulated for PD gradient on nanofibers. Further, culturing of human mesenchymal stem cells (hMSCs) on the PD gradient nanofiber resulted in a gradient of adhesion and spreading from high to low PD coating. However, the proliferation rate was not affected by the PD gradient, with an approximately 3-fold change after 5 days of culture. Maintenance of the stem cell density gradient on the PD gradient nanofiber resulted in controlled osteogenic differentiation, which was greater in the higher PD-coated area. Interestingly, stemness analysis showed a reverse trend relative to osteogenic differentiation of hMSCs. In summary, the spatially controlled polymerization of dopamine can be a versatile tool to generate substrates with gradient surface chemistry, which holds promise to direct stem cell behavior.