Osteosarcoma cell viability, proliferation, and differentiation into osteoblasts on a silicon nitride bioceramic were examined as a function of chemical modifications of its as-fired surface. Biological and spectroscopic analyses showed that (i) postsintering annealing in N2 gas significantly improved apatite formation from human osteosarcoma (SaOS-2) cells; (ii) in situ Raman spectroscopic monitoring revealed new metabolic details of the SaOS-2 cells, including fine differences in intracellular RNA and membrane phospholipids; and (iii) the enhanced apatite formation originated from a high density of positively charged surface groups, including both nitrogen vacancies (VN3+) and nitrogen N-N bonds (N4+) formed during annealing in N2 gas. At homeostatic pH, these positive surface charges promoted binding of proteins onto an otherwise negatively charged surface of deprotonated silanols (SiO-). A dipole-like electric-charge, which includes VN3+/N4+ and SiO- defective sites, is proposed as a mechanism to explain the attractive forces between transmembrane proteins and the COO- and NH2+ termini, respectively. This is analogous to the mechanism occurring in mineral hydroxyapatite where protein groups are specifically displaced by the presence of positively charged calcium loci (Ca+) and off-stoichiometry phosphorus sites (PO42-).
Keywords: SaOS-2 cells; hydroxyapatite formation; nitrogen vacancies; silicon nitride bioceramic; surface treatment.