Bioactivation of a cobalt alloy by coating with wollastonite during investment casting.

Cobalt alloy samples were bioactivated during investment casting. The cavities of the investment mold were previously coated with wollastonite. Additionally, before coating with wollastonite, some mold cavities were filled out with graphite rods to avoid a chemical reaction between the wollastonite powder and the investment material. Half of the cast samples were heat treated at 1220 degrees C for 1 h. To perform the in vitro bioactivity assessment, the cast and heat-treated samples were immersed in a simulated body fluid solution (SBF) for a period of 21 days. The surface of the samples before and after immersion in SBF was characterized by SEM, EDX, and XRD analyses. During the casting, particles of pseudowollastonite were embedded on the metallic surface. After immersion of the samples in SBF, a ceramic layer was formed on both the alloy obtained by using the investment mold and the alloy obtained by using the graphite-filled cavity. The ceramic layer was thicker on the alloy cast in the investment mold. The layer was identified as hydroxyapatite by XRD analysis, in all the cases. The heat-treated samples after immersion in SBF showed the formation of a thin homogeneous layer consisting of fine grains of apatite.

Effect of wollastonite ceramics and bioactive glass on the formation of a bonelike apatite layer on a cobalt base alloy.

A biomimetic method was used to promote a bioactive surface on a cobalt base alloy (ASTM F-75). The metallic substrates were alkali treated and some of the samples were subsequently heat treated. The treated samples were immersed in simulated body fluid (SBF) on granular particles of either bioactive glass or wollastonite. For comparative purposes, no bioactive system was used in some tests. Three different methods were used for the immersion of the samples in SBF: 1) 21 days in SBF, 2) 21 days in 1.5 SBF, and 3) 7 days in SBF followed by 14 days in 1.5 SBF (re-immersion method). A bonelike apatite layer was formed on all the samples placed on wollastonite and bioactive glass particles. The morphology of the apatite layer formed by using the re-immersion method and wollastonite closely resembled the existing bioactive systems. No apatite layer was observed on the samples treated without bioactive material and soaked for 21 days in SBF or 1.5 SBF, apart from the substrates treated by using the re-immersion method. The heat treatment delayed the apatite formation in all the cases studied.