Biocompatible Ti3Au-Ag/Cu thin film coatings with enhanced mechanical and antimicrobial functionality

Abstract

Background: Biofilm formation on medical device surfaces is a persistent problem that shelters bacteria and encourages infections and implant rejection. One promising approach to tackle this problem is to coat the medical device with an antimicrobial material. In this work, for the first time, we impart antimicrobial functionality to Ti₃Au intermetallic alloy thin film coatings, while maintaining their superior mechanical hardness and biocompatibility.

Methods: A mosaic Ti sputtering target is developed to dope controlled amounts of antimicrobial elements of Ag and Cu into a Ti₃Au coating matrix by precise control of individual target power levels. The resulting Ti₃Au-Ag/Cu thin film coatings are then systematically characterised for their structural, chemical, morphological, mechanical, corrosion, biocompatibility-cytotoxicity and antimicrobial properties.

Results: X-ray diffraction patterns reveal the formation of a super hard β-Ti₃Au phase, but the thin films undergo a transition in crystal orientation from (200) to (211) with increasing Ag concentration, whereas introduction of Cu brings no observable changes in crystal orientation. Scanning and transmission electron microscopy analysis show the polyhedral shape of the Ti₃Au crystal but agglomeration of Ag particles between crystal grains begins at 1.2 at% Ag and develops into large granules with increasing Ag concentration up to 4.1 at%. The smallest doping concentration of 0.2 at% Ag raises the hardness of the thin film to 14.7 GPa, a 360% improvement compared to the ∼4 GPa hardness of the standard Ti₆Al₄V base alloy. On the other hand, addition of Cu brings a 315-330% improvement in mechanical hardness of films throughout the entire concentration range of 0.5-7.1 at%. The thin films also show good electrochemical corrosion resistance and a > tenfold reduction in wear rate compared to Ti₆Al₄V alloy. All thin film samples exhibit very safe cytotoxic profiles towards L929 mouse fibroblast cells when analysed with Alamar blue assay, with ion leaching concentrations lower than 0.2 ppm for Ag and 0.08 ppm for Cu and conductivity tests reveal the positive effect of increased conductivity on myogenic differentiation. Antimicrobial tests show a drastic reduction in microbial survival over a short test period of < 20 min for Ti₃Au films doped with Ag or Cu concentrations as low as 0.2-0.5 at%.

Conclusion: Therefore, according to these results, this work presents a new antimicrobial Ti₃Au-Ag/Cu coating material with excellent mechanical performance with the potential to develop wear resistant medical implant devices with resistance to biofilm formation and bacterial infection.

Keywords: Antimicrobial; Biocompatible; Controlled doping; Super hard coating; Ti3Au.