Electro-thermally polarized bulk and coated hydroxyapatite (HAp) ceramics---Understanding the role of surface charge, wettability and dopants on physical, mechanical and biological properties

The objective of this study was to investigate the role of surface charge, wettability, and dopants on physical, mechanical and in vitro biological properties of bulk and coated electrothermally polarized hydroxyapatite (HAp) ceramics. For this purpose, bulk pure HAp compacts were prepared by sintering at 1200°C for 2h and HAp coating was fabricated on Ti substrate. Thermally stimulated depolarization current (TSDC) analysis revealed that a maximum surface charge of 4.05+/-0.33 microC/cm2 can be stored in bulk sintered HAp compacts while HAp coating samples exhibited a stored surface charge of 1.41+/-0.30 microC/cm2. It has been observed that increasing the surface potential led to decreasing of hydrophobicity of HAp surface without introducing any other volumetric effects in the material. In vitro bioactivity and bone cell-materials interaction results proved that increasing surface energy and wettability on negatively charged HAp surface significantly accelerated the apatite mineralization process by selectively adsorbing Ca2+ ions which promoted better hFOB cell attachment, proliferation and faster cells differentiation, especially at early time points, over unpoled HAp surface. In contrast, positively poled HAp surface exhibited inhibited hFOB cell proliferation and growth. Furthermore, dopants were incorporated into pure HAp in different single, binary and ternary composition to achieve combined benefits of matching bone chemistry and polarization effect. Small addition of Mg2+ and combined addition of Mg 2+-Sr2+ were found most optimal in enhancing density, mechanical properties as well as polarizability and charge storage ability of pure HAp. In vitro cell materials interaction study showed that presence of Mg2+ and Sr2+ can also significantly improved osteoblast activities on negatively charged surface. Finally, the interaction cell adhesive fibronectin protein on polarized doped and undoped HAp surfaces was investigated to get comprehensive understanding on cell-materials interactions. Protein study results confirmed that higher adsorption and better structural stability of fibronectin protein led to superior cell-materials interactions on negatively charged binary doped HAp surfaces over undoped HAp. Overall, the present research findings demonstrated the potency of developing electro-thermally polarized HAp ceramics which can provide a new basis for the development of a new generation of HAp based implants that can promote faster healing.