Surface Physiochemistry Affects Protein Adsorption to Stoichiometric and Silicate-Substituted Microporous Hydroxyapatites

An important factor in the bioactivity and success of a bone‐graft substitute is the nature of the adsorbed protein layer, which plays a vital role in orchestrating cell attachment and development through the presence of adhesion proteins such as fibronectin (Fn) and vitronectin (Vn). In this study, microporous hydroxyapatite (HA) and silicate‐substituted hydroxyapatite (SA) discs with matched porosity and surface morphology are developed to mimic the topography found in commercial bone‐graft substitutes in order to identify whether the introduction of microporosity and associated surface roughness eliminates the beneficial effect that silicate substitution has on protein adsorption. The introduction of microporosity does not abolish the relative enrichment of the protein layer that is adsorbed to the microporous SA discs, as opposed to HA, but appears to accelerate it. Fibronectin and Vn adsorption in a range of competitive environments at physiological temperatures confirm that the microporous SA discs have a greater affinity for Fn and Vn compared with HA, suggesting differences in the mechanisms behind the surface affinity to SA. Thus, development of a surface protein layer on SA and HA is likely to be dependent on the nature of the local protein environment and a combination of factors that are associated with the addition of silicate: the surface charge, the nature of the ionic species at the interface and the resultant hydrophilicity of the surface. Total protein adsorption is not found to be a good indicator of potential implant performance, particularly at early time points. Discs of microporous hydroxyapatite (HA) and silicate‐substituted hydroxyapatite (SA) with matched porosity and surface morphology were developed to match commercial bone‐graft substitutes. The pattern of protein adsorption to the HA and SA was examined to identify whether the microporosity and the associated surface roughness eliminated the affinity that adhesion proteins (fibronectin (Fn) and vitronectin (Vn)) have for dense SA, compared with HA. The variation in the specific (Fn and Vn) and non‐specific (total) protein adsorption to the HA and SA was investigated and the protein adsorption was measured both non‐specifically and specifically.