The recently developed additively manufacturing techniques have enabled the fabrication of porous biomaterials that mimic the characteristics of the native bone, thereby avoiding stress shielding and facilitating bony ingrowth. However, aseptic loosening and bacterial infection, as the leading causes of implant failure, need to be further addressed through surface biofunctionalization. Here, we used a combination of (1) plasma electrolytic oxidation (PEO) using Ca-, P-, and silver nanoparticle-rich electrolytes and (2) post-PEO hydrothermal treatments (HT) to furnish additively manufactured Ti-6Al-4V porous implants with a multi-functional surface. The applied HT led to the formation of hydroxyapatite (HA) nanocrystals throughout the oxide layer. This process was controlled by the supersaturation of Ca²⁺ and PO₄³⁻ during the hydrothermal process. Initially, the high local supersaturation resulted in homogenous nucleation of spindle-like nanocrystals throughout the surface. As the process continued, the depletion of reactant ions in the outermost surface layer led to a remarkable decrease in the supersaturation degrees. High aspect-ratio nanorods and hexagonal nanopillars were, therefore, created. The unique hierarchical structure of the microporous PEO layer (pore size < 3 μm) and spindle-like HA nanocrystals (<150 nm) on the surface of macro-porous additively manufactured Ti-6Al-4V implants provided a favorable substrate for the anchorage of cytoplasmic extensions assisting cell attachment and migration on the surface. The results of our in vitro assays clearly showed the important benefits of the HT and the spindle-like HA nanocrystals including a significantly stronger and much more sustained antibacterial activity, significantly higher levels of pre-osteoblasts metabolic activity, and significantly higher levels of alkaline phosphatase activity as compared to similar PEO-treated implants lacking the HT.

The performance of biomaterials in general and orthopaedic biomaterials in particular is dependent on both the chemistry and topography of their surfaces. It is therefore important to tailor both of those aspects through an appropriate surface modification technique. Here, we examined the influence of hydrothermal treatment on the surface characteristics and electrochemical behavior of Ti-6Al-4V specimens whose surfaces were modified using plasma electrolytic oxidation (PEO). Even though no calcium-phosphorous related crystalline compound was identified in the XRD spectra of PEO layers, hydroxyapatite crystals were clearly detectable after the applied hydrothermal treatment. The partial water absorption of the HA crystals and their needle-like morphology resulted in a significant increase in the wettability of the surfaces. However, the application of post-PEO hydrothermal treatment also decreased the corrosion resistance of the PEO layers. The numerical results of electrochemical impedance spectroscopy demonstrated that the optimized surface properties and corrosion resistance were achieved in one of the groups, namely PEO-HT3, where the HA nanocrystals homogenously covered the entire surface of the specimens.