Biomedical Engineering Reference
In-Depth Information
Steel
Nitinol
d
b
c
e
a
Strain
(a)
(b)
(c)
Figure 6.2
Metals and alloys. (a) Stainless steel stent with a polymer coating (TAXUS Express² Atom
Paclitaxel-eluting coronary stent system, Boston Scientifi c). (b) MEDPOR TITAN cranial temporal im-
plant. (Used with permission from Porex Surgical, Inc.) (c) Stress-strain behavior of SMAs.
corrosion resistance. Cobalt alloys are used in both cast condition and wrought
condition; the wrought condition provides superior mechanical and chemical
properties due to finer grain sizes and a more homogenous microstructure. The
ability to form an inert thin oxide layer on the surface of aluminum and titanium is
also advantageous in many applications.
While the widely used implant metals are generally biocompatible, they may
release ions. For example, stainless steels could potential release Ni
2+
, Cr
3+
, and
Cr
6+
ions on a long-term basis into the body, due to which their use is restricted to
temporary devices. Further, the linear elastic deformation (following Hooke's law,
discussed in Chapter 5) of many metals including stainless steel cobalt-based alloys
is limited to nearly 1-2% strain. This limits usage in many medical devices where
broader elastic range is necessary. For example, stainless steel arch-wires have been
employed as a corrective measure for misaligned teeth. Owing to the limited ex-
tensibility and tensile properties of these wires, considerable forces are applied to
teeth, which can cause a great deal of discomfort. When the teeth succumb to the
corrective forces applied, the stainless steel wire has to be retensioned. Repeated
visitation to the orthodontist may be needed for retensioning every 3 to 4 weeks in
the initial stages of treatment.
There is a special class of alloys, called
shape memory alloys
(SMAs),
which
have the ability to return to a predetermined shape with thermal changes, and de-
formation of more than 10% strain can be elastically recovered. The most common
SMA is that of nickel and titanium, called Nitinol. SMAs have also been formed by
blending nickel and titanium with other components such as copper, hafnium, pal-
ladium, and zirconium. Further, niobium-aluminum, nickel-iron-zinc-aluminum,
titanium-niobium, gold-cadmium, iron-zinc-copper-aluminum, uranium-niobium,
iron-manganese-silicon, copper-aluminum-iron, and zirconium-copper-zinc also
behave as SMAs. When an SMA is cold or below its transformation temperature, it
has a very low yield strength and can be deformed easily into any new shape. When
the material is heated above its transformation temperature it undergoes a change
