Beta-titanium alloys are promising materials for long term/permanent bone implants due to their mechanical compatibility with bone. For testing, the beta titanium alloy Ti39Nb (Ti-39wt%Nb) produced using powder metallurgy was chosen. A set of cylindrical implants (length 8 mm, diameter 4 mm) was prepared for in vivo tests in minipigs. To enhance the osseointegration with this material, we deposited a ferroelectric (i.e. also piezoelectric) BaTiO3 coating on the sandblasted implants. The interaction between the electrical activity of the ferroelectric coating and the piezoelectricity of bone should accelerate the bone-implant interaction. The BaTiO3 coating was prepared by hydrothermal reaction of TiO2 with Ba(OH)2.which was formed by reaction of BaCl2 and NaOH in the autoclave. A Parr hydrothermal reactor with custom made titanium insert (300°C / 48h) was used. The diffusion of Nb to the BaTiO3 film was found to deteriorate its ferroelectricity. To limit the diffusion of Nb, the Ti39Nb alloy was coated with an interlayer of Ti (cca 5µm) deposited by magnetron sputtering. The surface of Ti interlayer was subsequently oxidized in situ to TiO2 in an O2 atmosphere (500°C / 6h). X-ray diffraction confirmed the presence of tetragonal BaTiO3 in the coating, with crystallite size 80 – 100 nm (some crystallites were larger than ~200 nm). The ferroelectric properties were confirmed by micro-Raman spectroscopy by the presence of a peak at 300 cm-1. The morphology of the coating was determined by scanning electron microscopy (SEM). The composition was confirmed by SEM/Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS), as well. Piezo force microscopy (PFM) shows that the BaTiO3 film has non-zero piezoelectric response and the spontaneous polarizations of individual grains are randomly oriented and do not have any preferred direction within the studied area. Mechanical properties were determined by nanoindentation and roughness measurements.
In vitro tests (presented at the Ceramics 2022 conference) were followed by in vivo tests in minipigs. For comparison, Ti6Al4V and Ti39Nb implants without BaTiO3 coating were used in addition to Ti39Nb/BaTiO3 implants. The implants were operated into trochanter of the porcine femur. Healing of all pigs was without any infection. After 6 and 12 weeks following implantation, histological sections of bone with implants were stained with toluidine blue and the percentage of implant surface in direct contact with bone tissue (i.e. bone- implant contact, BIC) was analyzed. Fibrotic tissue and cartilage were considered negative. Due to a rather large variability among individual pigs, no statistically significant changes in osseointegration were observed among three sample groups after 6 and 12 weeks. However, there was a trend of decreasing the BIC parameter in time. However, the Ti39Nb/BaTiO3 samples evinced the highest BIC value after 6 and 12 weeks. For biomechanical evaluation, pull-out tests were performed with at 12 weeks. The parameters of maximum force, displacement at failure, and shear strength did not show any statistical difference between the groups of samples.
This research was funded by the Czech Science Foundation, grant No. 20-01570S.