In situ self-assembly of graphene oxide/polydopamine/Sr2+ nanosheets on titanium surfaces for enhanced osteogenic differentiation of mesenchymal stem cells

Abstract

Bioinert surface is the greatest obstacle to the biomedical applications of titanium (Ti)-based materials. Herein, a bioactive coating as a carrier of strontium ions (Sr2+) on Ti surfaces was prepared by in situ self-assembly of graphene oxide/polydopamine/Sr2+ (GO/PDA/Sr2+) nanosheets. Surface physicochemical characteristics of the coating were evaluated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle (CA) and surface free energy (SFE) measurements. The results showed an ordered and Sr-incorporated nanosheets coating that may be beneficial to cell responses of mesenchymal stem cells (MSCs). Furthermore, 4.51 at.% Sr was introduced into the coating. During 30 days of immersion, GO/PDA/Sr2+ nanosheets coated Ti surfaces cumulatively released 20.36 ± 2.09 ppm Sr2+ in low glucose Dulbecco's Modified Eagle's Medium (L-DMEM), but only 16.62 ± 1.00 ppm in simulated body fluid (SBF). Moreover, the coating possessed outstanding capacity of protein adsorption. In vitro biological evaluation displayed that the synergies of PDA, GO, and released Sr2+ promoted the adhesion, spreading, and proliferation of MSCs grown on GO/PDA/Sr2+-coated Ti surfaces, subsequently enhancing the expressions of bone-specific genes and proteins, in turn accelerating extracellular matrix (ECM) mineralization. This work provided us an alternative for bio-functionalizing the surfaces of Ti-based implants.