Biomedical Engineering Reference
In-Depth Information
Fig. 10.12
Body heat activated NiTi superelastic spring, exerting nearly constant force (Sentalloy,
Dentsply Int.)
NiTi contains nickel and its presence always elicits suspicion. In a paper by
Lijima et al., galvanic corrosion is described of the following couples: 304-NiTi,
304-Elgiloy, 304-
-Ti [334, 335]. A few con-
clusions of this work are worth to be reproduced here. The combinations of alloys
were coupled in NaCl 0.9%. In all couples, the galvanic current decreased with time
and becomes nearly constant after 24 h. The coupling SS304 with NiTi is unfavor-
able because the galvanic current density is 10 times higher than for other couples.
An odd result is obtained for the couple Ti-NiTi. Initially, Ti was the anode and
corroded but that reversed after 1 h or so and NiTi got corroded. At the start, the
electrode potential of Ti was less than that of NiTi but that reversed after about
1 h: NiTi became the anode. An XPS study (X-ray Photo-electron Spectrometry)
revealed that the subsurface layer of NiTi was rich in Ni and depleted in Ti. The
growing oxide film needed Ti which diffused to the surface enriching the subsur-
face in nickel. These results may be somewhat premature and not the end of the
discussion but clinicians should remain alert, when they treat patients sensitive to
nickel. By the way, we never fully trusted NiTi in a biomedical environment.
We are not aware of biomedical devices in which damping, the third characteris-
tic of SMAs, is the leading actor. However, it is something to keep in mind because
the Specific Damping Capacity (SDC) of typical SMAs is in excess of 40% (per-
centage of energy dissipated as heat), a value close to hard rubber (application in
scaffolds?).
An increasing number of medical devices are developed utilizing one- or two-
way SME or pseudoelasticity in orthopedics, dentistry, vascular and prostatic stents,
micro-actuators, etc. and instruments.
Nitinols are offered on the market in a range of mechanical properties, some are
summarized in Table
10.5
.
ˇ
-Ti, Ti-NiTi, Ti-CoCr, Ti-304 and Ti-
ˇ
Other Alloys
Two classes of titanium alloys are used for archwires: NiTi discussed above and a
ˇ
-titanium.
Beta-titanium.
TMA (Ormco,Sybron) is
ˇ
-titanium orthodontic alloy; some basic
properties were shortly reviewed in Sect. 2.2.3. The major elements are molybdenum
