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

Abstract

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.