Biomedical Engineering Reference
In-Depth Information
and function of implant grade 316L stainless steel and ASTM F-86 suggests
a process in which all medical implants should be passivated using dilute
HNO 3 at room temperature. The resulting stable oxide layer on the passiv-
ated metal surface is produced to improve the materials corrosion resis-
tance and biocompatibility in physiological conditions (Shih et al. , 2006).
Other surface treatments including electropolishing are also adopted by
many manufactures to improve the degree of passivity by modifying the
thickness, morphology or chemical composition of the surface oxide layer
(Shih et al. , 2006).
In general, biocompatibility is determined by the interfacial process
between the surface of a given biomaterial, the bodily fl uids and living
cells it comes into contact with (Lemons, 1996). Material surface proper-
ties have been shown to infl uence cell adhesion, proliferation and phe-
notype (Langenhove et al. , 1996; Bren et al. , 2004; Pecheva, et al. , 2004;
Stewart et al. , 2009). The cell-surface interaction is specifi cally controlled
by the surface morphology (e.g. the surface roughness), and by the
chemical state of the surface via hydrophobic interactions, free chemical
bonds and functional groups (Lemons, 1996; Su, 1998; Bren et al. , 2004).
The physicochemical properties of the surface are controlled mainly by
the material selection and by the process technology (Su, 1998). The
processing of 316 stainless steel through traditional hot and cold defor-
mation can generate several permanent defects on the fi nished product.
Cracks and pits on the steel rolls of the hot rolling process can create
foreign-particle inclusions, and rolled-in defects in the materials (Su,
1998). High friction encountered during the drawing process can
form severe surface defects such as deep scratches, check marks and
mechanical dents. The presence of these surface defects can radically
change the corrosion resistance and considerably modify the mechanical
properties of a device (Trethewey & Chamberlain, 1998). Surface imper-
fections can create areas of concentrated high stress and are often the
initial point of fatigue failure (Su, 1998). Fatigue failure has also been
shown to originate from inclusions where crevice corrosion starts from
the seam, between the inclusions and the metallic matrix. A combina-
tion of corrosion and cyclic loading could destroy the prosthesis (Su,
1998).
To this end, it has been postulated that the infl ammatory response and
the development of ISR may be increased by a contact allergic reaction to
metal compounds (nickel, chromium or molybdenum ions) released from
stainless steel stents (Koster et al. , 2000). A higher frequency of ISR was
found in patients with delayed-type hypersensitivity to metals, particularly
to nickel. The same study also found symptoms and the need for repeat
intervention in patients with no hypersensitivity to the metals (Berger-
Garbet et al. , 1996).
￿ ￿ ￿ ￿ ￿
Search WWH ::




Custom Search