Biomedical Engineering Reference
In-Depth Information
BAG Coatings
Due to the nature of their mechanical properties, BAGs need a strong backing.
Several techniques have been proposed for the application of BAG coatings (hot
dipping, electroforming, plasma spraying, you name it!). None has found its way to
commercial products yet. The composition is more subtle than HA and the compo-
sition is subject to change during heating. It already starts at the production stage
of the glass. For melting, the basic compounds inert crucibles (platina) are a must
to avoid contamination during heating (1,300-1,450
ı
C). Next, if plasma spraying
is the coating technique of choice, a smooth feeding of the plasma torch needs a
free-flowing powder. Preparing such a powder is another time consuming, costly
and occasionally contaminating step with considerable loss of material (sequestra-
tion of the required particle size). Helsen and collaborators met these disadvantages
by developing
Reactive Plasma Spraying
, the essence of which is the synthesis of
the glass in the plasma itself. The production of the free-flowing powder happens,
except the last step, at room temperature. It starts from base chemicals as calcium
carbonate, tricalcium phosphate, sodium silicate. An aqueous slurry of appropri-
ate composition (already corrected for losses during the residence in the plasma) is
spray dried, cold isostatically pressed, broken, milled, sieved to separate the right
fraction (40-70
m) and finished with a thermal treatment at 950
ı
C; the fraction
outside the right size range is easily reprocessed. This production flow sheet is
flexible and cost efficient. As proven by X-ray diffraction analysis, the coating con-
sist of a true nonporous glass with homogeneous composition of thicknesses up
to 50
84 MPa were routinely obtained; adhesion strength
was tested by a newly developed technique for cylindrical specimens [
141
,
142
]. The
excellent osteoconductivity was demonstrated in vivo by implantation in the jaw of
beagle dogs and the result was already shown in Fig.
4.8
. But even that cost-efficient
process with excellent in vivo results could not seduce industry.
m. Adhesion strengths
>
9.6
General Conclusion
This chapter opened by addressing the superior skills of ancient ceramists. The link
to contemporary ceramics is surprisingly straightforward. For any practical pur-
poses, despite our present day insight in the basic physical and chemical principles,
technical proficiency is still the-stay-behind for a good ceramist...as catastroph-
ically demonstrated by the Prozyr case. Where indicated, the micro- or nanos-
tructural particularities leading to the actual performance of the selected ceramic
products were analyzed.
One particular aspect of ceramics like barium titanate has not been addressed.
The favorable promotion of osteogenesis by the combination of hydroxyapatite-
barium titanate (piezoelectric) after implantation in jawbones of dogs was announ-
ced but it is not translated into a clinical application in humans in as far as we are
aware of [
311
].
