Silicon nitride surface chemistry: A potent regulator of mesenchymal progenitor cell activity in bone formation

Abstract Polycrystalline silicon nitride (Si3N4), sintered with the addition of minor fractions of yttrium and aluminum oxides (i.e., Y2O3 and Al2O3), possesses uniquely adjustable surface chemistry that results in improved cell metabolism and enhanced bone formation. Building upon previous in vitro mineralization studies using osteosarcoma cells, this study examined interactions between various chemically modulated Si3N4 surfaces and murine mesenchymal progenitor cells (KUSA-A1). It was discovered that various pressurized thermal-treatments coupled with adiabatic and non-adiabatic cooling of sintered Si3N4 samples resulted in partial or full coverage of their surfaces with different Si–Y–O–N compounds. Full coverage by mostly yttrium silicate (β-Y2Si2O7) was obtained by non-adiabatic cooling, whereas partial coverage with N-apatite (Y10(SiO4)6N2) occurred under adiabatic conditions. These peculiar phases were found to be particularly efficient in stimulating the in vitro differentiation of KUSA-A1 cells into osteoblasts, although according to different microscopic mechanisms. The final amount of bone formation was nearly identical for both phases. Cell differentiation was monitored by assessing the concentration of the osteogenic marker γ-carboxyglutamate (i.e., Gla-osteocalcin). It was found to be ∼45% higher for Si3N4 samples possessing the N-apatite phase than for biomedical titanium alloy controls tested under exactly the same conditions. Concurrent measurements of the bone resorption marker Glu-osteocalcin (i.e., an undercarboxylated form of γ-carboxyglutamate) showed significant inhibition of osteoclastogenesis on these surface-treated Si3N4 samples as compared to the controls. Bone formation was assessed using in situ Raman microprobe spectroscopy and ex situ laser microscopy. These two independent analytical techniques consistently found an increase of ∼80% in expressed hydroxyapatite when compared to a biomedical titanium alloy. This study suggests that surface-treated Si3N4 may have a powerful anabolic, differentiating, and antiapoptotic effect on osteoblasts in vitro, and a concurrent inhibitive action on osteoclastogenesis. Given additional research, Si3N4 may represent a new therapeutic solution for bone disorders and for engineered implants that physiologically regulate bone growth processes.