Biomedical Engineering Reference
In-Depth Information
state at room temperature for the high Cr content (>18 wt%) used. Co base
alloys, used for heavily loaded joints (wrought products), usually contain
nickel contents below 0.5%, although much higher values of up to about 37
wt% are sometimes present in hip or knee prosthesis components (F562).
It has been known for a number of years that nickel ions leach into the
human body and can lead to localized irritation and infection, even in the
absence of any evident corrosion mechanism. Contact dermatitis and toxicity
caused by nickel and its salts have been well documented.
76, 77
as a matter
of fact, intracutaneous tests have shown that in europe 10-15% of adult
females and 1-3% of adult males can be susceptible to allergic reactions
to nickel.
78
Consequently, the trend in legislation in a number of countries
is currently towards limiting or even eliminating nickel from alloys which
come into contact with skin or which are used to manufacture temporary or
permanent prosthetic material.
Fortunately, nickel is not alone in being an austenite former. ni-free
austenitic stainless steels with a large amount of nitrogen (up to 4.2 at%)
and manganese (up to 23 at%) have been recently developed to enhance the
strength, corrosion resistance and biocompatibility of conventional stainless
steels.
79-82
on the other hand, ni-free austenitic stainless steels without
manganese but with a higher n content (up to 5.1 at%) have been also
developed.
83
one of these new types of steel has been subjected to biological
studies and shown promising results.
84
asTM F2229 is a ni-free (< 0.05%
ni) austenitic stainless steel with a nominal composition of 23Mn-21Cr-
1Mo and about 1.0% n. The higher n content contributes to high levels of
corrosion resistance and strength when compared to type 316 L.
1
ni-free Fe-base oDs (oxide dispersion strengthened) alloys have been
developed as potential biomaterials for surgical implants. evidence of good
biocompatibility was found
in vitro
when challenging different types of cells
to the alloy either as fine particles
85
or solid samples.
86
The soft ferromagnetic
behaviour makes the alloy a useful candidate for the preparation of medical
devices where biocompatible and soft magnetic materials are sought.
21
a further achievement concerns the development of new FealCr intermetallic
alloys. Their major advantages are their low density (< 6 g cm
-3
), low raw
materials cost and low content of strategic elements. Biocompatibility
was evaluated
in vitro
by testing human osteoblast-like saos2 cells with
mechanically alloyed particles
85
or solid samples.
87
overall, none of the
investigated new alloys was found to be cytotoxic. Instead, a good level of
biocompatibility was assessed.
6.5.3 Nanostructured alloys
another recent approach to improving orthopaedic implants has centred
on the development of alloys with an extremely fine grain size. Grain
