Strontium confinement in polyacrylic acid brushes: a soft nanoarchitectonics approach for the design of titania coatings with enhanced osseointegration

Polyacrylic acid (PAA) brushes synthesized by reversible addition fragment chain transfer (RAFT) polymerization on titania (TiO2) surfaces were applied for the surface confinement and slow delivery of strontium (Sr2+) ions during the osseointegration process on TiO2 surfaces. Sr2+ is an essential element in the bone remodeling cycle and has large potential for application in therapies for bone-related diseases and bone replacement. Sr2+ complexation to the carboxylate groups of PAA was demonstrated by XPS while Sr2+ release in media was studied by ICP-MS. The Sr2+ concentration was varied by preparing PAA brushes at two grafting densities. At the higher grafting density, the brushes offer more carboxyl groups for Sr2+ binding. The architecture of the brushes and the amount of Sr2+ is controlled by the density of polymer chains. The morphology and brush thickness were characterized by AFM and ellipsometry. The significant effects of PAA brush coated TiO2 complexing Sr2+ on osteoblast cells is shown. The use of PAA brushes for Sr2+ confinement restricts Sr2+ activity to the cells in direct contact with the TiO2 surface. Initial adhesion of osteoblast cells is superior on coatings with brushes compared to bare TiO2. Moreover, a rather rapid cell tissue formation is observed for the cells cultured on the brushes with higher Sr2+ content. Enhanced biocompatibility and osseoactivity is confirmed by an increased alkaline phosphatase activity.