Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

First-principle study of the influence of hydroxyapatite on magnesium surfaces

Anthony Veit Berg, Ablai Forster, Tim Hansson, Alexandra J. Jernstedt, Emmy Salminen, Elsebeth Schröder

Abstract

Hydroxyapatite (HA) on a magnesium (Mg) surface is studied using density functional theory, to help understand the effect of HA coating and alloying in the surfaces of Mg-based biodegradable implants. We determine the adsorption energies and structural changes of a single layer of HA on pure Mg(0001) and on sparsely calcium (Ca) or zinc (Zn) doped Mg(0001) and find that both Zn and Ca doping improves the adsorption, except in a few positions of HA relative to the dopant position. All adsorption configurations, whether with pure or doped Mg surfaces, show deformation of the surface and HA layer. For Ca doping, we found that for a certain adsorption configuration, the dopant Ca atom moves out of the Mg surface and into the HA layer, leaving behind a Mg vacancy in the top layer of the Mg surface. Plots of electron density changes show that electrons accumulate around the Ca dopant and the neighboring Mg atoms, while in Zn doping this is less pronounced. Overall, our results demonstrate that the dopant choice and relative position of HA influence the interaction between HA and Mg-surfaces, and affect both adsorption energies and atomic and electronic structures.

First-principle study of the influence of hydroxyapatite on magnesium surfaces

Abstract

Hydroxyapatite (HA) on a magnesium (Mg) surface is studied using density functional theory, to help understand the effect of HA coating and alloying in the surfaces of Mg-based biodegradable implants. We determine the adsorption energies and structural changes of a single layer of HA on pure Mg(0001) and on sparsely calcium (Ca) or zinc (Zn) doped Mg(0001) and find that both Zn and Ca doping improves the adsorption, except in a few positions of HA relative to the dopant position. All adsorption configurations, whether with pure or doped Mg surfaces, show deformation of the surface and HA layer. For Ca doping, we found that for a certain adsorption configuration, the dopant Ca atom moves out of the Mg surface and into the HA layer, leaving behind a Mg vacancy in the top layer of the Mg surface. Plots of electron density changes show that electrons accumulate around the Ca dopant and the neighboring Mg atoms, while in Zn doping this is less pronounced. Overall, our results demonstrate that the dopant choice and relative position of HA influence the interaction between HA and Mg-surfaces, and affect both adsorption energies and atomic and electronic structures.
Paper Structure (16 sections, 1 equation, 10 figures, 2 tables)

This paper contains 16 sections, 1 equation, 10 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Materials
  3. Mg(0001) with alloys
  4. Hydroxyapatite
  5. Methods
  6. Density functional theory modelling of Mg(0001)
  7. Convergence tests and computational parameters
  8. Adsorption energy
  9. Hydroxyapatite
  10. Zinc or calcium alloying of Mg(0001)
  11. Results and discussion
  12. Hydroxyapatite on pristine Mg(0001)
  13. Alloying of Mg(0001)
  14. Adsorption of hydroxyapatite on alloyed Mg(0001)
  15. Electron density
  16. ...and 1 more sections

Figures (10)

  • Figure 1: Hydroxyapatite (HA) on the Mg(0001) surface, shown from three perspectives to illustrate the structural arrangement and relative positioning. (a) Side view ($x$-$z$ plane). (b) Top view ($x$-$y$ plane). (c) Angled view. Color coding of atoms: magnesium - grey, oxygen - red, phosphorus - orange, calcium - blue, hydrogen - white. All structures and electron densities visualized with XCrySDen.
  • Figure 2: HA structures with OH group orientations (a) as energetically favoured for a single layer of HA (b) as in bulk HA. The two OH groups in each panel are marked with yellow circles. Atom colors as in Figure \ref{['fig:HA_mg_olika_vinklar']}.
  • Figure 3: Atomic configurations showing the positioning of the substituted atoms. (a) All of HA above the top layer of Mg. (b) Substituted atom positions in green and including only the atoms in HA that are directly above them. Notation Ca-X means HA-bound Ca atom over substituted atom X (Zn/Ca). Only one Mg atom at a time is substituted. Other atom colors as in Figure \ref{['fig:HA_mg_olika_vinklar']}.
  • Figure 4: Potential energy contour plot for relative position of HA on Mg(0001), for estimating the optimal positioning of HA over the Mg surface. Energies relative to the energy of the optimal position.
  • Figure 5: Structural changes of HA in the optimal translational position, starting from (a) the structure of an isolated layer of HA (slightly stretched in the lateral directions) and clean Mg(0001) brought together at 2 Å separation, and (b) after optimization of the atomic positions. Atom colors as in Figure \ref{['fig:HA_mg_olika_vinklar']}.
  • ...and 5 more figures