Functionalization of Semicrystalline Hydroxyapatite Coatings with Citrate as New Approach in Orthopaedic Applications

Introduction: Hydroxyapatite (HA)-based nanocoatings are used in orthopedics to promote osseointegration due to their chemical similarity to the mineral component of human bone. Functionalization of these coatings with citrate can enhance tissue mineralization, as citrate plays a key role in the activity of osteogenic cells and in stabilizing the bone matrix.

Methods: In this work, HA coatings were fabricated using Ionized Jet Deposition (IJD) and biomimetic deposition by simulated body fluid (SBF). Subsequently, they were loaded via citrate adsorption to identify which coating was more adequate for combination with citrate for future perspectives. Differences in morphology, roughness, and crystallinity between the two deposition techniques were evaluated. Citrate loading efficacy at three different concentrations (1, 10 and 50mM) was assessed by measuring citrate release in phosphate-buffered saline (PBS). Fibroblast cells (L929) were cultured for 24h with material extracts to test cytotoxicity.

Results: All coatings were composed of semicrystalline HA, as confirmed by FT-IR and XRD analyses. Moreover, post-deposition thermal treatment at 400°C increased the films' crystallinity. IJD-coatings are formed by globular aggregates, as observed by Atomic Force (AFM) and Scanning Electron Microscopy (SEM), with a thickness around 400nm. In contrast, SBF coatings exhibited similar morphology to IJD films and a needle-like shape, typical of HA crystals. SEM images, after citrate absorption, showed that IJD-made films maintained their original morphology, whereas SBF-coatings dissolved completely, making their functionalization impossible. FT-IR analysis of PBS collected after 5 days of immersion of IJD coatings confirmed the presence of a peak around 1550cm^-1, corresponding to the antisymmetric stretching of COO^- groups in citrate, more evident for loading concentrations of 10 and 50mM. L929 cells showed no cytotoxic effect against materials released from the coatings.

Conclusion: Based on these results, IJD-made coatings represent a suitable solution for citrate functionalization and may be used for further in-depth analysis and future orthopedic applications.