The role of hydroxyapatite in citric acid-based nanocomposites: surface characteristics, degradation, and osteogenicity in vitro.

The incorporation of nanoscale hydroxyapatite (HA) into biodegradable polymers can potentially mimic the native structure of bone and influence the mechanical properties and the extent of bioactivity. In this study nanocomposites of poly(1,8-octanediol-co-citrate) (POC) containing 40, 50, and 60 wt.% HA (POC-HA) were fabricated and characterized. Nanocomposite hydrophilicity and the degradation properties in vitro were evaluated via contact angle measurements, scanning electron microscopy (SEM), and mass loss measurements. Human mesenchymal stem cells (hMSC) were cultured on POC-HA nanocomposites in both growth and osteogenic media. Cell proliferation, alkaline phosphatase activity, and osteocalcin were measured. The equilibrium water in air contact angles confirmed all of the nanocomposites to be hydrophilic (23.4 ± 8.1°, 27 ± 9.1°, and 27.7 ± 3.5° for 40, 50, and 60 wt.% HA, respectively). Over a period of 26 weeks the degradation rate increased with decreasing HA content and pore formation was evident for POC-HA containing 40 wt.% HA, whereas POC with 50 and 60 wt.% HA lacked pores (mass loss at 26 weeks for 40, 50, and 60 wt.% HA, 27.4 ± 1.6%, 17.7 ± 1.6%, and 6.3 ± 2.6%, respectively). hMSC adhered and proliferated well on all composites, confirming biocompatibility for at least 21 days. An increase in adhesion and proliferation was found with increasing HA nanoparticle content (ng DNA at day 21 for 40, 50, and 60 wt.% HA, 130.4 ± 49.4, 184.4 ± 86.4, 314.1 ± 92.3). Alkaline phosphatase activity and osteocalcin concentration correlated with HA content (alkaline phosphate activity in expansion medium and osteogenic medium for 40, 50, and 60 wt.% HA, 256.1 ± 71.8%, 304.0 ± 128.7%, and 500.2 ± 89.9%, and 358.4 ± 124.1%, 653.7 ± 216.5%, and 814.4 ± 68.8%, respectively; osteocalcin concentration in expansion medium and osteogenic medium for 40, 50, and 60 wt.% HA, 236.9 ± 7.8%, 253.0 ± 7.5%, and 285.2 ± 11.4%, and 265.8 ± 15.0%, 288.3 ± 17.9%, and 717.3 ± 38.7%, respectively). This study provides insight into how the HA nanoparticle content can modulate the cell compatibility and physical properties of POC-HA nanocomposites.