In vitro and in vivo reactivity of porous, electrosprayed calcium phosphate coatings

The dissolution and/or precipitation behaviour of porous calcium phosphate (CaP) coatings, deposited using electrostatic spray deposition (ESD), was investigated (a) in vitro after soaking in simulated body fluid (SBF) for several time periods (2, 4, 8, and 12 weeks), and (b) in vivo after subcutaneous implantation of CaP-coated implants in the back of goats for identical time periods. Physical and chemical properties of coatings were characterized before and after in vitro/vivo testing by means of scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and energy dispersive spectroscopy. Moreover, part of the explants was prepared for light microscopical evaluation of the tissue response. In vitro, all apatitic ESD-coatings induced the formation of homogeneous and adherent CaP precipitation layers. Amorphous CaP, however, displayed a delayed precipitation of poorly adherent CaP layers, whereas heterogeneous calcification was observed on top of β-TCP-coated substrates, indicating that β-TCP and amorphous CaP coatings exhibit a poor ability of inducing calcification in SBF as compared to crystalline apatitic coatings. In vivo, no adverse tissue reactions (toxic effects/inflammatory cells) were observed using light microscopy, and all coatings became surrounded by a dense, fibrous tissue capsule after implantation. All ESD-coatings degraded gradually at a dissolution rate depending on the chemical phase (order of relative solubility: amorphous CaP≈carbonate apatite>β-TCP>carbonated hydroxyapatite), thereby enabling synthesis of CaP coatings with a tailored degradation rate.