Biomedical Engineering Reference
In-Depth Information
thought to affect directly this activation alongside shear forces and blood
contact with the biomaterial. This release is the result of the slow passive
dissolution of ions from the protective metal oxide layer; this also gives rise
to discoloration of the implant (Gotmann, 1997).
Body fl uids such as blood, urine and tear are liquids of a very complex
composition. Among the solutes found in them are various amino acids and
proteins which have been proven to infl uence metallic corrosion (Siebert,
1985). In addition, the reduced concentration of dissolved oxygen in venous
blood (1/4 of the air concentration) may prevent the complete passivation
of the device further accelerating its corrosion (Hanawa, 2002). Metallic
materials as such do not show toxicity, but some dissolved metal ions and
corrosion products may induce toxicity when in combination with bio-
molecules and cells (Hanawa, 2002).
In general,
in vivo
degradation of biomaterials is caused by chemical
hydrolysis, oxidation or physical/mechanical degradation, and other biologi-
cal factors which are diffi cult to replicate in an
in vitro
model. However,
many researchers have investigated the effects of different types of cells on
stent metals (Harmand, 1994; Wataha,
et al.
, 2000; Hanawa, 2002; Kocijan &
Milosev, 2003).
It is believed that the corrosion rate of a cobalt-chrome-molybdenum
alloy (orthopaedic alloy) signifi cantly increased in the presence of cells, in
comparison with medium alone (Harmand, 1994). Studies have shown that
both cobalt and chromium accumulated inside the cells over a 9 day period.
It has been proposed that the intracellular uptake of chromium could be
responsible for either a cytotoxic or genotoxic effect on cells (Harmand,
1994).
The longer-term effects of metal ion release on monocytes, key infl am-
matory cells associated with ISR have also been studied. Nickel was found
to have a signifi cant antiproliferative effect
in vitro
at concentrations of 5,
10 and 40
mol/L which is 1-10% of the concentration known to cause
cytotoxicity (Wataha,
et al.
, 2000). One of the most important groups of
molecular species in physiological solutions are proteins. The data pub-
lished in this area show that the addition of proteins to solutions
in vitro
increases the dissolution of pure metals and stainless steels while minimis-
ing the formation of protective oxide layers (Harmand, 1994). However, a
complex picture emerged where different effects were observed depending
on the protein and metal types under investigation (Kocijan & Milosev,
2003). Hanawa (2002) commented that even trace elements in an alloy are
not negligible from the viewpoint of metal ion release and toxicity.
It is as yet unknown whether metal ions released from implants are
soluble in tissue fl uids and if they cause any toxicity or long-term complica-
tions. It is known that stainless steel does interact with the body undergoing
corrosion reactions, whereas conversely titanium does not (Steinemann,
μ
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