Carbonate-containing hydroxyapatite (CO3-HAp) shows higher osteoconductivity than pure hydroxyapatite but the underlying mechanism is unclear. As specific adsorption of proteins on CO3-HAp may affect cellular responses and osteoconductivity, we examined the contribution of protein adsorption to the high osteoconductivity observed with CO3-HAp. We prepared CO3-HAp powders with different carbonate contents (0.3, 4.8, or 8.3 wt%) and investigated the adsorption behaviour of fibronectin (Fn) on the powders. We found that the Fn adsorption capacity of CO3-HAp decreased with increasing carbonate content. Although the Hill or Langmuir isotherm was the best-fit for the adsorption of Fn onto CO3-HAp with a carbonate content of 0.3 or 4.8 wt%, the Brunauer–Emmett–Teller isotherm was the best-fit for Fn adsorption on CO3-HAp with an 8.3 wt% carbonate content. These results show that the carbonate content of CO3-HAp influences the adsorption behaviour of Fn and suggest that its specific adsorption contributes to the high osteoconductivity of CO3-HAp. The findings do not affect the clinical application of CO3-HAp directly, but they might lead to the development of bone substitutes with high osteoconductivity.
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