Bioactive materials: In vitro investigation of different mechanisms of hydroxyapatite precipitation

Bioactive materials, able to induce hydroxyapatite precipitation in contact with body fluids, are of great interest for their bone bonding capacity and act through different mechanisms: ions exchange with the solution and/or chemical reactivity of surface OH groups. The aim of this paper is to compare bioactive materials with different surface features to verify mechanism of action and relationship with kinetic and type of precipitated hydroxyapatite over time. Four different surface treatments for Ti/Ti6Al4V alloy and a bioactive glass were selected because a different mechanism of bioactivity is supposed for each of them. Apart from conventional techniques, such as FESEM, XPS and EDX, less common characterizations such as zeta potential measurements on solid surfaces and FTIR chemical imaging through bulk zeta potential and FTIR image analysis were applied. Results suggest that OH groups on the surface have several effects: the total amount of OH groups mainly affects hydrophilicity of surfaces, while isoelectric point, surface charge and ions attraction mainly depend on OH acidic/basic strength. Kinetic of hydroxyapatite precipitation is faster when it is due to a mechanism of ion exchange while it is slower when it is due to electrostatic effects such as the presence of deprotonated and negatively charged OH groups on the surface. The electrostatic effect can cooperate with ion exchange and it speeds up the kinetic of hydroxyapatite precipitation. The occurrence of an acidic local environment on a highly porous surface, able to get a positive charge because of strong basic OH groups, can significantly speed up bioactivity based on an electrostatic mechanism. Different bioactive surfaces are able to differently induce precipitation of type A and B of hydroxyapatite, as well as different degree of crystallinity and carbonation.