Role of Zn and Mg substitutions on the mechanical behaviour of biomimetic hydroxyapatite and insight of the emergence of hydroxyapatite-ZnO nanocomposite

Abstract

A simple and inexpensive biomimetic method is attempted to produce hydroxyapatite (HAp), Mg-/Zn-doped HAp and HAp-ZnO nanocomposite. An aqueous calcium solution with and without precursors of Mg2+ or Zn2+ and a carbonate containing phosphate solution are prepared at a temperature of 37 °C with pH of 7.4 and mixed in equal volumes to obtain the calcifying solution, which has similar ionic constituents as that of human blood plasma. At a temperature of 37 °C, reactions drive the calcifying solution to supersaturation for the crystallization of carbonated HAp, Mg-/Zn-doped HAp, and HAp-ZnO nanocomposite. The microstructural and morphological analyses reveal that the carbonated HAp can sustain low doping of Mg and Zn substituting at Ca sites, wherein Zn is likely to replace screw axis Ca preferentially. Significant lattice distortion leads to the transformation of nanophase HAp to amorphous calcium phosphate when Mg content in the calcium solution is increased to 40% of (Ca + Mg) solute concentration. The role of Zn is more intriguing – Ca-vacancy mediated substitution is favourable when Zn content is below 15% of (Ca + Zn) solute concentration and subsequent increase leads to the emergence of spherical ZnO nanocrystals coexisting with needle-like HAp nanocrystals. The present work describes a new way of synthesizing HAp-ZnO nanocomposite – a potential candidate for the orthopaedic and the prosthodontic applications. Simply increasing Zn content in the calcium solution and without using additional precursors, reaction pathways lead to the precipitation of HAp-ZnO nanocomposite. Field emission transmission electron microscopy reveals that when Zn/(Ca + Zn) molar ratio in the calcifying solution is set at 40%, the resulting nanocomposite is characterized by 5 nm average diameter needle shaped HAp with 15–30 nm spherical ZnO particles as an intimate mixture of two nanophase systems. The mechanical properties of HAp evaluated using the X-ray elastic constants and the modified Williamson-Hall analyses suggest a clear decreasing trend of the apparent domain size with increasing Mg and Zn incorporations while microstrain, isotropic stress, and uniform deformation energy density of nanophase HAp rise only with increasing Mg substitutions.