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Author:sunriseti Date:8/22/2012 12:21:18 AM
Ni-Ti alloys have extremely good corrosion resistance based on formation of a tenacious
titanium oxide surface. Recent technical articles show conclusively that the pitting resistance
of Nitinol is significantly higher than stainless steel (Epit >800mV compared with <250mV)
with a corrosion current density on the order of 10 nA/cm² (24,25). Furthermore, Trépanier, et
al. (26) have shown that optimal corrosion resistance is obtained through passivation
treatments, such as electropolishing the surface. Cell culture and implant studies were
conducted to assess the cytotoxicity response of Nitinol compared with stainless steel and
titanium alloys. Trépanier, et al. (27) and Ryhänen, et al. (28) have shown that although the
initial Ni ion release was slightly higher in Nitinol, the overall Ni release rate is similar to
stainless steel. Furthermore, it was shown that NiTi induced no toxic effects, decrease in cell
proliferation or inhibition in the growth of cells in contact with Nitinol.
Figure 11 shows fibro-cellular tissue surrounding a Nitinol stent after twelve weeks
implantation in rabbit para-vertebral muscle (29). Note that there is excellent acceptance of
the Nitinol device in the muscle tissue. Based on early indications of exceptional
biocompatibility of Nitinol, products such as the Mitek suture anchor (Figure 12) and the
Simon vena cava filter (Figure 4) have been routinely implanted in the human body for over
ten years with no reports of toxic behavior.

Figure 11: Image of an ex-planted Nitinol stent in rabbit muscle tissue after 12 weeks. note the uniformity of cell
growth near the stent (29).

Figure 12: Mitek suture anchors have been implanted into bones for over 15 years with no reports of rejection.