Biomedical Engineering Reference
In-Depth Information
healing response of the body to the presence of mechanical pressure from
an implanted stent. It is part of the immune response by which blood clots
can form or tissue cells may proliferate to reduce the diameter of the artery,
again restricting blood l ow. Drug eluting stents reduced the incidence of
ISR to between 4% and 15% [325]. However, so long as the stent is present,
the body may experience neointimal tissue proliferation from the aggrava-
tion caused by the stent.
h e risk of ISR can be eliminated by introducing non-permanent bio-
absorbable stents. h is prospect drives the ongoing research and product
development to create magnesium stents that dissolve. h e amount of
magnesium per stent is between 3 mg and 6 mg, which is released over
several months as the stent corrodes. h is amount of magnesium is small
compared to the normal blood concentration of magnesium in the blood
between 0.7 mmol/l and 1.05 mmol/l [325]. Preclinical and clinical trials
since 2004 have shown no toxic reactions, contributing to the growing
body of clinical data to demonstrate the safety of magnesium stenting.
Multiple magnesium alloy systems have been proposed which incorporate
strontium [326], selenium [327], and neodymium [328] to alter the
in situ
degradation rates. However, the biocompatibility of these and other alloy-
ing elements still needs to be established. Bioactive coatings of Ca and P
and other surface modii cations on pure magnesium are also being evalu-
ated for their ef ects on corrosion rates [329, 330]. Other coatings such as
these are being proposed and evaluated.
Nanostructuring of magnesium alloys of ers several advantages and
alternatives to alloying for stent applications. First, reducing the grain size
alters the corrosion rates. Hao
et al.
[331] subjected an AZ31 alloy to ECAP
and found the corrosion rate in Hank's solution to be reduced, but not to
an extent to make it suitable for stent applications. Hadzima
et al.
[332]
subjected AZ80 alloy to ECAP and extrusion to obtain an ultrai ne grain
structure that enhanced the electrochemical properties to produce polar-
ization layers that remained stable, completely resisting degradation up to
96 hours. Most recently Minárik
et al.
[333] evaluated the electrochemi-
cal characteristics of AE21 and AE42 alloys subjected to 8 ECAP passes.
h ey found that the smaller grain size resulting from ECAP enhanced
the corrosion rate in AE21 due to increased chemical activity at the grain
boundaries. In contrast, the corrosion rate in AE42 subject to the same
ECAP treatment was reduced. In this case, the larger ef ect of increased
uniformity of the spatial distribution of alloying elements of set the ef ect
of having a smaller grain size. Clearly the ef ects of nanostructuring are
sui ciently complex and alloy dependent that they must be carefully evalu-
ated for any prospective magnesium alloy.
