Strengthening Adhesion of the Hydroxyapatite and Titanium Interface by Substituting Silver and Zinc: A First Principles Investigation

A basic understanding of the influences of substituted silver and zinc on the adhesion between hydroxyapatite (HA) and α-Ti surfaces is obtained through first-principles electronic structure calculations. Since the (0001) plane presents the most thermodynamically stable surface of HA, the HA/Ti interfaces with Zn or Ag dopants were constructed between the HA(0001) with dopants and Ti(0001) surfaces. Applying the universal binding energy relation (UBER), the optimal interfacial distance for HA/Ti interface models are estimated and then the optimized interfacial models are determined after full geometrical relaxation, and the affections of Zn dopants on the interfacial distances for the HA/Ti interface are studied. The work of adhesion of interfaces with various stoichiometries was evaluated via the optimized HA/Ti interface models with dopants. It is strongly affected by substituted silver and zinc dopants and reaches the largest value of -3.52 J/m² for Zn doping and of -3.48 J/m² for Ag doping in the PO₄-terminated interfaces, which is significantly larger than the one of the metal-atom-terminated stacked interfaces (1.31-2.55 J/m²) and of the stoichiometric HA(0001)/Ti(0001) interfaces of without Ca termination (-2.33 J/m²). Therefore, the bond strengths of HA/Ti interfaces are increased by silver and zinc dopants. Analysis of electronic structure reveals that charge transfer between HA and Ti slabs is increased by doping Ag or Zn in the HA. More charge transfers lead to stronger the Ti-O bonds and drive the HA/Ti interface system to be more metallic.