Bioactivity and Surface Reactivity of RF-sputtered Calcium Phosphate Thin Films

Field: Materials Science (Physical Sciences)

Calcium phosphates (CaP) are known to be bioactive, i.e. able to bond\nto bone. This makes CaPs very suitable to be aplied as thin coatings\non bone-implants. In this work we studied the physicochemical\nbehaviour of CaP coatings applied with radio frequency (RF) magnetron\nsputtering, a deposition technique that can produce thin (~100 nm),\nhomogeneous, and well-adhereing coatings. As-deposited CaP coatings\nare amorphous and can be crystallized by a heat-treatment of 30\nminutes at 650C, resulting in a mainly apatitic structure.\nFirstly, we have studied the behaviour of these CaP coatings in\nso-called simulated body fluid (SBF or SBF1), an anorganic equivalent\nof human blood plasma. Amorphous CaP coatings dissolve in SBF and even\nin SBF with twice the Ca and PO4 concentrations (SBF2). After a\nheat-treatment CaP coatings remain inert in SBF for days, i.e.\ncoatings do not dissolve and no crystals are formed from the solution\non the coating surface. However, formation of crystals is possible in\nSBF2. At room temperature, the formation of crystals is preceeded by\nan induction time, in which rod-shaped sediments can be found on the\ncoating surface, but no significant growth is observed. Only after\ncompletion of the induction period growth of CaP crystals is allowed.\nGrowth can proceed in solutions with lower concentrations like SBF1.\nOnly within a limited range of Ca over PO4 ratio of the coatings,\nformation of CaP crystals from SBF2 is possible.\nIn a rat bone marrow (RBM) cell-culture CaP was compared to\nRF-sputtered coatings of other bioceramics like alumina (Al2O3) and\ntitania (TiO2). Alumina is a known bioinert material. However, there\nis still discussion on the biocompatibility of titania. It was found\nthat cell behavior on CaP coatings significantly differed from\nalumina. CaP coatings showed decreased early proliferation, increased\ndifferentiation, and increased mature osteoblast activity compared to\nalumina. Results for titania were intermediate compared to CaP and\nalumina. That is, early proliferation followed the alumina results,\nwhereas the mature osteoblast activity and the matrix production\nconfirmed the CaP findings.\nCoinciding with the differentiation of cells towards mature,\nextracellular matrix forming osteoblasts, both the CaP and titania\ncoatings showed the formation of a directly bonded CaP layer (~1\nmicron after 16 days), quite similar to the precipitate grown in\nsimulated body fluids. The directly bonded CaP layer is thought to be\nindicative of bone-bonding bioactivity.\nIn simulated body fluids we showed that the presence of CaP nuclei,\nobtained by 40-60 minutes pre-immersion in SBF2, is required to allow\ngrowth in SBF1. Therefore, we also studied the effect of pre-immersion\nin a RBM cell-model. The coatings that were not pre-immersed showed\nthe formation of a directly bonded \\cap layer, again after the start\nof osteoblast-differentiation. However, the pre-immersed samples\nallowed the growth of a this layer without an offset. Thus by applying\nCaP nuclei, the formation of the directly bonded CaP layer has been\ndecoupled from the ECM calcifications.