Abstract
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,
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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.
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