Calcium phosphates (CaP) are known to be bioactive, i.e. able to bond to bone. This makes CaPs very suitable to be aplied as thin coatings on bone-implants. In this work we studied the physicochemical behaviour of CaP coatings applied with radio frequency (RF) magnetron sputtering, a deposition technique that can produce thin (~100 nm), homogeneous, and well-adhereing coatings. As-deposited CaP coatings are amorphous and can be crystallized by a heat-treatment of 30 minutes at 650C, resulting in a mainly apatitic structure.
Firstly, we have studied the behaviour of these CaP coatings in so-called simulated body fluid (SBF or SBF1), an anorganic equivalent of human blood plasma. Amorphous CaP coatings dissolve in SBF and even in SBF with twice the Ca and PO4 concentrations (SBF2). After a heat-treatment CaP coatings remain inert in SBF for days, i.e. coatings do not dissolve and no crystals are formed from the solution on the coating surface. However, formation of crystals is possible in SBF2. At room temperature, the formation of crystals is preceeded by an induction time, in which rod-shaped sediments can be found on the coating surface, but no significant growth is observed. Only after completion of the induction period growth of CaP crystals is allowed. Growth can proceed in solutions with lower concentrations like SBF1. Only within a limited range of Ca over PO4 ratio of the coatings, formation of CaP crystals from SBF2 is possible.
In a rat bone marrow (RBM) cell-culture CaP was compared to RF-sputtered coatings of other bioceramics like alumina (Al2O3) and titania (TiO2). Alumina is a known bioinert material. However, there is still discussion on the biocompatibility of titania. It was found that cell behavior on CaP coatings significantly differed from alumina. CaP coatings showed decreased early proliferation, increased differentiation, and increased mature osteoblast activity compared to alumina. Results for titania were intermediate compared to CaP and alumina. That is, early proliferation followed the alumina results, whereas the mature osteoblast activity and the matrix production confirmed the CaP findings.
Coinciding with the differentiation of cells towards mature, extracellular matrix forming osteoblasts, both the CaP and titania coatings showed the formation of a directly bonded CaP layer (~1 micron after 16 days), quite similar to the precipitate grown in simulated body fluids. The directly bonded CaP layer is thought to be indicative of bone-bonding bioactivity.
In simulated body fluids we showed that the presence of CaP nuclei, obtained by 40-60 minutes pre-immersion in SBF2, is required to allow growth in SBF1. Therefore, we also studied the effect of pre-immersion in a RBM cell-model. The coatings that were not pre-immersed showed the formation of a directly bonded \cap layer, again after the start of osteoblast-differentiation. However, the pre-immersed samples allowed the growth of a this layer without an offset. Thus by applying CaP nuclei, the formation of the directly bonded CaP layer has been decoupled from the ECM calcifications.