Study of the stability under in vitro physiological conditions of surface silanized equimolar HfNbTaTiZr high-entropy alloy: A first step toward bio-implant applications

Abstract

A methodology of surface chemical functionalization of equimolar HfNbTaTiZr high-entropy alloy (HEA) is presented herein. The aim is to improve the biocompatibility of this material for its application as bio-implants. X-ray Photoelectron Spectroscopy (XPS) showed that air or aqueous oxidation of HfNbTaTiZr induced the formation at its surface of unmixed monometallic oxides: TiO2, Nb2O3, ZrO2, Hf2O3, and Ta2O5. Besides, it was observed that short (2 h) mild aqueous oxidation (without H2O2) retains the surface stoichiometry of these elements compared to a more aggressive one (with H2O2). The mild oxidation was retained to form a sol-gel chemistry platform for HfNbTaTiZr surface silanization. In practice, the oxidized HfNbTaTiZr surface was reacted with 2-methoxy [(polyethyleneoxy)6–9 propyl] trimethoxysilane (MPTMS), dissolved in distilled ethanol, under an inert atmosphere. Both, the precursor concentration and the reaction time, were varied to define the optimized operating conditions. Infrared spectroscopy performed on all prepared samples, within an attenuated total reflectance (ATR) sampling configuration, evidenced the well-known vibrational signatures of SiO and CH bonds at 1090 cm−1 and 2980 cm−1, respectively, confirming silane grafting, and showed that the intensity of these bands increases when the reaction time and/or the siloxane precursor concentration increase. Moreover, XPS showed that a reaction time of 48 h is required to completely cover the alloy surface with an organic layer (exhibiting a thickness larger than the XPS analysis depth). Finally, as a preliminary study, mildly oxidized alloy and the grafted one for a reaction time of 48 h were contacted to a Phosphate-Buffered Saline (PBS) solution and a standard Nutrient Mixture for cell cultures, the well-known Dulbecco's Modified Eagle Medium (DMEM), up to one month to check their chemical stability. None metal release was evidenced for the treated sample highlighting the importance of surface processing before any bio-implant use.