Cobalt substitutes beta-tricalcium phosphate in nanocoatings to increase the pro-osseointegration and pro-angiogenic properties of metallic implants

Tricalcium phosphate (TCP)-based nanocoatings are used in orthopedics as a bone substitute to favor osseointegration and vascularization processes. Cobalt ions are able to induce hypoxia and stimulate mesenchymal stem cells (MSCs) to produce the pro-angiogenic factor VEGF. Thus, cobalt-substituted TCP (Co-TCP) coatings, with different amounts of cobalt ions (0, 2, 5, 10 and 15 wt%), were realized through Ionized Jet Deposition.

Morphological and physico-chemical properties of the coatings were analyzed. The influence of deposition parameters, such as deposition temperature and amount of Co ions, on the properties were investigated. Fibroblast cells (L929) were cultured for 24-48 hours with material extracts to test the cytotoxic effects. Then, adhesion and proliferation of MSC directly seeded on the coatings were analyzed with also the evaluation of VEGF secretion, as representative pro-angiogenic factor.

All Co-TCPs could be deposited with a good chemical fidelity with respect to the targets, as observed by FT-IR and EDS analyses. Coatings were formed by globular aggregates. The use of 400°C as deposition temperature permitted us to obtain films with higher roughness and crystallinity degrees. Thus, this condition was used for the subsequent analyses. The films were hydrophilic and stable during medium immersion for up to 7 days. No significant cytotoxic effect was noticed on L929, which proliferated and were metabolically active. MSCs adhered and were able to proliferate from 24h up to 7d in all coatings. However, a lower area covered by MSCs was recorded for 15% Co-TCP coatings, a sign of cytotoxic effects due to the higher amount of Co ions. VEGF secretion by BM-MSCs increased significantly when cells were seeded on a 10% Co-TCP coating.

Based on these results, 5% and 10% Co-TCP coatings can be a possible solution for improving VEGF production, and they appear to be a promising tool for future applications.