Antibacterial and cell-friendly copper-substituted tricalcium phosphate ceramics for biomedical implant applications

The development of new materials with antibacterial properties and the scope to decrease or eliminate the excessive antibiotic use is an urgent priority due to the growing antibiotic resistance-related mortalities. New bone substitute materials with intrinsic antibacterial characteristics are highly requested for various clinical applications. In this study, the choice of copper ions as substitutes for calcium in tricalcium phosphate (TCP) has been justified by their pronounced broad-spectrum antibacterial properties. Copper-substituted TCP (Cu-TCP) ceramics with the copper content of 1.4 and 0.1 wt% were synthesized by mechano-chemical activation. X-ray diffraction (XRD) analyses established that both pure and copper-containing compounds adopted the structure of whitlockite (β-TCP). XRD and electron paramagnetic resonance (EPR) spectroscopy revealed the partial isovalent substitution of calcium ions with copper ions in the β-TCP lattice. With the use of infrared and EPR spectroscopies, it was detected that carbonate ions got incorporated into the β-TCP structure during the synthesis procedure. By releasing the tension in the M(5)O6 octahedron consequential to the lower Ca[sbnd]O bond length than the corresponding sum of ionic radii, the substitution of calcium with smaller copper ions stabilizes the structure of β-TCP. As concluded form the thermal analyses, the introduction of Cu prevented the polymorphic transformation of β- to α-TCP. At the same time, the introduction of Cu to the β-TCP structure enhanced the crystal growth and porosity of the ceramics, which had a positive effect on the cytocompatibility of the material. The MTT colorimetric assay showed that the metabolic activity of the mouse fibroblast NCTC L929 cell line during 24 h of incubation with 3-day extracts from Cu-TCP (1.4 wt%) and β-TCP pellets in the cell culture medium was similar to the negative control, indicating the absence of any inhibitory effects on cells. The seeding and the growth of human dental pulp stem cells on the surface of Cu-TCP (1.4 wt%) and β-TCP ceramics also showed the absence of any signs of cytotoxicity. Finally, microbiological assays demonstrated the antibacterial activity of Cu-TCP ceramics against Escherichia coli and Salmonella enteritidis, whereas β-TCP did not exhibit such an activity. Overall, the addition of Cu ions to β-TCP improves its antibacterial properties without diminishing the biocompatibility of the material, thus making it more attractive than pure β-TCP for clinical applications such as synthetic bone grafts and orthopaedic implant coatings.