No abstract

Nitinol (NiTi) has become one of the most important implant materials in biomedical applications such as endovascular stents, embolic protection filters, and endodontic files. However, NiTi stents sometimes do not provide satisfactory visibility under fluoroscopy during angioplasty. A new ternary alloy, radiopaque nitinol (NiTiPt), exhibits much better radiopacity than NiTi and has great potential for future biomedical applications. This paper presents the first characterization of anisotropic stress-strain behavior in micron-sized specimens cut directly from hypotubes of this new alloy NiTiPt. Experimental results show that NiTiPt hypotube has very different anisotropic characteristics and high stress-strain nonlinearity when compared to its NiTi counterpart. Its ultimate tensile strength and strain are approximately 15-40% higher than NiTi with statistically significant difference for all three cutting angles measured. Its stress-strain behavior has several unique features including lower initial austenite elastic modulus, smaller hysteresis loop, and sharper tails during the loading-unloading cycle. The 458 behavior is particularly different from other angles, with stronger hardening effect and an approximately 25-30% reduction in ultimate strain when compared to 08.

Nitinol fatigue strain limit versus both strain amplitude (range 0.25-1.25%) and mean strain (1.0, 2.0, and 4.0%) was investigated using a tension-tension method. In order to apply the fatigue testing results to a nitinol stent and evaluate stent fatigue performance, the dog-bone style specimens were processed from the same raw material common to implantable stent manufacturing, i.e., similar nitinol tubing, surface quality, and electropolished surface. To simulate a physiological environment, the tension-tension fatigue tests were conducted in water at 37°C. This strain-controlled fatigue test was conducted with a run-out set at 10 6 cycles. The fatigue strain limit at 10 6 cycles as well as the mean strain effect and the effects of inclusions are discussed. Fatigue results appeared in a bi-modal pattern when the strain amplitude was at a level between too high, which made all specimens to fail, and too low, which allowed all specimens to survive.

A new wire test specimen was developed as part of the Safe Technology consortium project to produce a nitinol algorithm for the fe-safe Ô fatigue postprocessor. The requirements for the design were permit tensile and compressive loading, accommodate data collection of strains from ±0.1 to 6% at high-frequency cycling. The requirements for the wire were inexpensive, stable, consistent, and commercially available. The requirements for the processing were incorporate deformation and multiple anneal cycles to simulate manufacturing methods associated with vascular stent production. The final design was 0.508 mm (0.020 in.) diameter wire formed in a two-stage process to produce a Z-shaped specimen. The final part met all requirements for testing. Subsequently, a second specimen, Z3, and a straight wire specimen were produced to supply additional data to the consortium.

An experimental study using a new fiberoptic sensor for the continuous intraarterial measurement of oxygen tension is described. This "optode" sensor uses the phenomenon of fluorescence quenching to determine the oxygen tension of the surrounding medium. To assess the accuracy of this device, we anesthetized 4 dogs and monitored them continuously with arterial catheters and an intraarterial optode probe, and intermittently with arterial blood gas analysis. The inspired oxygen fraction was varied from 1.0 to 0.1, and arterial blood gases were measured for comparison with the optode reading. Two hundred ninety data sets yielded a correlation coefficient of 0.96, with a linear regression slope of 0.98 and intercept of 5.1 mm Hg. In the 72 data sets from the last dog, the bias and precision of the optode arterial oxygen tension values were -10.3 mm Hg and 20.0 mm Hg, respectively. The optode probe was easily inserted through a 20-gauge catheter and did not interfere with continuous arterial pressure measurement or blood sampling. This study suggests that the optode has great potential as a continuous, real-time monitor of arterial oxygen tension.