Enhancing early osteogenic differentiation on Ti6Al4V implants via MAO coatings doped with strontium and calcium

Abstract

Titanium alloys such as Ti6Al4V are widely used in orthopaedic implants due to their excellent mechanical and corrosion-resistant properties. However, insufficient early-stage osseointegration remains a critical challenge, often leading to implant loosening and failure. To overcome this issue, bone implant design and surface modification increasingly focus on incorporating bioactive elements. This study explores the enhancement of osseointegration through micro-arc oxidation (MAO) surface modification of Ti6Al4V, incorporating bioactive strontium (Sr) and calcium (Ca) ions. Two surface pretreatments - polishing (P) or shot-blasting (B) - were applied prior to MAO to generate coatings with distinct topographies and compositions: TAV(P), TAV(P)-Sr, TAV (P)-Sr-Ca, TAV(B), TAV(B)-Sr, and TAV(B)-Sr-Ca. Surface characterization of the resulting oxide layers confirmed the incorporation of Sr, Ca, and phosphate ions, with Sr releasing continuously over 60 days. Wear resistance analysis revealed the highest coefficient of friction for the TAV(P)-Sr coatings and the lowest values for TAV(B)-Sr. Surface modification by blasting led to reduced corrosion resistance in the TAV(B), TAV(B)-Sr, and TAV(B)-Sr-Ca samples. Nevertheless, enhanced corrosion resistance was observed in all samples modified with Sr and Sr+Ca. In vitro analyses using human bone marrow-derived mesenchymal stromal cells (hBMSCs) demonstrated excellent biocompatibility of all coatings. Notably, Sr-doped and especially Sr-Ca-doped surfaces significantly enhanced early osteogenic differentiation, as evidenced by increased alkaline phosphatase activity and type I collagen deposition. These findings highlight the synergistic effects of surface topography and Sr-Ca ion doping in promoting early-stage osseointegration, offering a promising strategy for improving the performance of titanium-based bone implants, particularly in patients with compromised bone healing.