Biological Response of and Blood Plasma Protein Adsorption on Silver-Doped Hydroxyapatite

Hydroxyapatite (HAP) is an extensively used orthopedic biomaterial because of its high biocompatibility and osteoconductivity. Implant-related infection is a major cause of orthopedic device failure. Previous research showed that silver-doped hydroxyapatite nanoparticles (Ag-HAP NPs) have prominent antimicrobial activity, but their biocompatibility and plasma protein response remained unexplored. Here we investigated the effects of synthesis conditions on Ag-HAP NP antimicrobial (E. coli and S. epidermidis) activity, biocompatibility, and the adsorption of two blood plasma proteins, human serum albumin (HSA) and fibrinogen (Fib). It was found that synthesis pH affected the Ag content of Ag-HAP NPs and subsequent Ag⁺ release from the NPs in solution. This, in turn, affected antimicrobial efficiency and cytotoxicity to murine preosteoblast cells (MC3T3-E1). More HSA than Fib was adsorbed on a molar basis. The conformation of HSA changed drastically from predominantly α-helix and minor β-sheet content in solution to greater β-sheet than α-helix content when adsorbed. Correspondingly, the melting temperature T_m of HSA changed significantly from 76 °C in solution to ∼65-66 °C when adsorbed. Fib exhibited a modest decrease in α-helix content while the β-sheet content increased modestly upon adsorption and its T_m remained unchanged at ∼60 °C. These differences in behavior of HSA and Fib are ascribed to the much smaller size of HSA, which allows a greater molecular packing density on the surface, which induces greater conformational changes. The protein adsorption behavior on Ag-HAP was similar to that on pure HAP. Thus, we show that Ag-HAP NPs have antimicrobial activity without deleterious effects on biocompatibility and blood plasma protein adsorption.