Strontium-substituted hydroxyapatite stimulates osteogenesis on poly(propylene fumarate) nanocomposite scaffolds

Incorporation of hydroxyapatite (HA) into polymer networks is a promising strategy to enhance the mechanical properties and osteoinductivity of the composite scaffolds for bone tissue engineering. In this study, we designed a group of nanocomposite scaffolds based on cross-linkable poly(propylene fumarate) (PPF) and 30 wt % strontium-hydroxyapatite (Sr-HA) nanoparticles. Four different Sr contents [Sr:(Sr + Ca), molar ratio] in the Sr-HA particles were studied: 0% (HA), 5% (Sr5-HA), 10% (Sr10-HA), and 20% (Sr20-HA). Two-dimensional (2D) disks were prepared using a thermal crosslinking method. The structure and surface morphology of different Sr-HA and PPF/Sr-HA composites were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). To detect cellular responses in vitro, MC3T3-E1 cells were seeded and cultured on the different PPF/Sr-HA composite disks. Cell morphology after 24 h and 5 days were imaged using Live/Dead live cell staining and SEM, respectively. Cell proliferation was quantified using an MTS assay at 1, 4, and 7 days. Osteogenic differentiation of the cells was examined by alkaline phosphatase (ALP) staining at 10 days and quantified using ALP activity and osteocalcin assays at 7, 14, and 21 days. The sizes of the HA, Sr5-HA, Sr10-HA, and Sr20-HA particles were mainly between 10 × 20 nm and 10 × 250 nm, and these nanoparticles were dispersed or clustered in the composite scaffolds. in vitro cell studies showed that the PPF/Sr10-HA scaffold was significantly better than the other three groups (PPF/HA, PPF/Sr5-HA, and PPF/Sr20-HA) in supporting MC3T3-E1 cell adhesion, proliferation, and differentiation. PPF/Sr10-HA may, therefore, serve as a promising scaffold material for bone tissue engineering.