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Biodegradable polymers that can be processed into injectable hydrogel matrices are promising candidates for bone-substituting purposes. Furthermore, by incorporating degradable calcium phosphate (CaP) particles and growth factors into these hydrogel matrices, a bone construct can be designed which stimulates the formation of new bone by the surrounding tissue, thereby compensating for the loss of structural integrity of the degrading synthetic bone-substitute. Generally, a major challenge in synthesis of nanoceramic-reinforced polymers is the achievement of a fine dispersion of nanoparticles throughout the polymer, since the unique properties of nanocomposites are lost when nanoparticles aggregate. In the current study, composite hydrogels consisting of oligo(poly(ethylene glycol)fumarate) (OPF) matrices and CaP dispersions of varying crystallinity were successfully developed using physical or chemical preparation strategies. Physical mixing of dried, micrometer-sized CaP powders resulted into formation of irreproducible composites with a highly heterogeneous dispersion of large and agglomerated CaP microparticles throughout the OPF matrix. On the contrary, reproducible and homogeneous hydrogels were fabricated using a chemical mixing strategy, whereby CaP crystals were formed in the presence of dissolved OPF macromers. This co-precipitation technique resulted into a much higher degree of dispersion of the CaP crystals, which can enable higher CaP contents in organic matrices such as OPF. By using these CaP suspensions instead of dried powders, the nanosized structure of separated CaP crystals was preserved, resulting into a higher reactivity of the CaP phase, as indicated by a reduced swelling behavior of these hydrogels. This effect was most likely caused by a physicochemical interaction between Ca(2+) and unreacted COOH end-groups, thereby leading to increased physical cross-linking of the composite hydrogels.

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 beta-TCP-coated substrates, indicating that beta-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 approximately carbonate apatite>beta-TCP>carbonated hydroxyapatite), thereby enabling synthesis of CaP coatings with a tailored degradation rate.