Biomedical Engineering Reference
In-Depth Information
of al, Cr, nb, Ta, Ti and V are stable owing to a more negative heat of
formation than that of water, the oxides of Co and ni are unstable because
of a less negative heat of formation.
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The interaction between the oxides
forming the passive layer and the body fluid increases with increasing heat
of formation of the oxide, resulting in a higher solubility. Consistently,
Co-, Fe- and ni-oxides have a considerable solubility, whereas Ti-oxide is
practically insoluble. experimental determination of the rate of nickel release
from the ConiCrMo alloy and 316L stainless steel in the ringer's solution
reveals that although the cobalt alloy has a greater initial release of nickel
ions, the rate of release was about the same (3 ¥ 10
-10
g/cm
2
/day) for both
alloys despite the nickel content for the former alloy being about three times
that of austenitic steel.
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recently, it has been reported that austenitic stainless steels show a higher
sensitivity to corrosion in cross-sectional specimens (transversal cuts with
respect to the rolling direction) and compared to longitudinal sections they
release nickel ions at rates 10-100 times higher.
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The effect is less intense
for medical grade steels (asTM-138) for which a remelting process is added
to the standard metallurgical process in order to reduce the inclusion content
and notably the sulphur concentration. Its nickel release rate is very low and
increases from 0.2 to 0.25 mg cm
-2
week
-1
when going from an
S
trans
/
S
long
ratio of 0.2 to 0.25, where
S
refers to section. The release of nickel increases
with cold working (23%), except for the medical grade steels.
6.4.3 Physical surface properties
Because the interactions between cells and tissues with biomaterials at
the tissue implant interface are almost exclusively surface phenomena, the
surface properties of implant materials are of great importance.
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surface
parameters are frequently grouped in three categories: geometrical, chemical
and physical. next we will describe the physical properties (surface charge
and surface energy) that have been demonstrated to play an important role
in
in vitro
biocompatibility.
Surface charges
The first event that takes place soon after implantation of a biomaterial is the
adsorption of a monolayer of soluble proteins from physiological fluids that
cover the implant. The protein adsorption is selective and depends on the
biophysical and biochemical characteristics of the surface, particularly the
electrical charges. This protein layer transforms the implant in a biologically
recognizable surface through specific receptors located in the cell membrane.
The composition of the adsorbed protein layer influences the later cellular
behaviour. Thus, if the proteins required for cellular adhesion adsorb on the
