Nitinol Shape Memory Research Articles

Effects of design geometry of intravascular endoprostheses on stenosis rate in normal rabbits

To investigate the effects of gaps between the individual wire coils of a shape memory Nitinol alloy intravascular endoprosthesis, 20 stents with and without gaps were implanted transluminally into the infrarenal abdominal aortas of 10 normal rabbits after balloon angioplasty. Digital subtraction angiography (DSA) was done at 4, 12, 16, 20, and 24 weeks after stent implantation to examine the stenosis rate and major side branch patency. Stenosis rate within stents with gaps were significantly lower than those without gaps: 8.1 ± 5.0% versus 15.0 ± 6.8% at 12 weeks, 13.6 ± 6.0% versus 26.0 ± 9.4% at 24 weeks by DSA, p > 0.005 and p < 0.01 respectively. The maximum and mean neointimal thickness measured histologically at the time of animal death correlated significantly to the narrowest diameter obtained from the DSA studies, (r = 0.84 and r = 0.79, respectively, p < 0.01). Greater hyperplasia of the neointima was evident in the stented arterial segments with stents without gaps compared with those with gaps (83 ± 22 versus 187 ± 46 μm mean thickness, p < 0.001). The patency rate of the side branches in the stented arterial segment was significantly (p < 0.05) higher in stents having gaps. These results suggest that the placement of gaps between wire pitches reduced the neointimal thickness within stents and prevented the obstruction of arterial side branches.

Study of the martensitic transformation in shape-memory nitinol alloy by time-of-flight neutron diffraction techniques

The present paper reports a study of thermally induced martensite (M) and its parent (P) structure in the shape-memory NiTi alloy (Nitinol) by neutron time-of-flight diffraction. The specimen was a 0.6-mm-dia wire in which the martensitic reaction occurred at 33 ± 7°C with zero applied stress. We made seven measurements in a closed cycle of temperature between − 18 to 90°C, using neutrons with wavelengths between 0.72 and 4.15 Å, sorted into 120 channels of 1.5% resolution. Neutron scattered at angles between 30 and 60° were detected by an area detector. The provided scattering data were used in these programs for obtaining the spectrum of the Bragg peaks for given angle θ and T, the lattice constants of the unit cells of both P and M phases, and the specimen texture at a given T. We found, as did previous authors, that the P phase has a CsCl-type cell with the lattice constant 3.032 ± 0.5%, and the M phase is monoclinic which can be formed with both P112 1/ m and P12/ c 1 unit cells. The lattice constants of these cells were found to deviate ±0.4 to ±1.4% from their average values. Although the martensite is predominantly long-range ordered with either Ni or Ti atoms at the cell corners, our data lead us to suggest that in our specimen there are some small fractions of this structure in which the cell corners are occupied by both Ni and Ti atoms in four different ordered arrangements. These domains, together with the atomic positions deduced from our results, eliminate the controversies among the models proposed by previous X-ray and TEM workers, and unifies their findings with our experimental results. The Bragg peak line spectra obtained with a given angle θ, reveal that during the P → M reaction, each peak of the P phase is replaced by two peaks, each of which is composed of a number of peaks of the M phase. We have phenomenologically formulated this so-called “Bp transition”, and identified some families of habit planes. The “transformation twins” are discussed, and it is found that the habit planes {100} are the sources of the two twin systems (1 10) [111] and (1 12) [111] which are formed on {111} planes. The branching angles of all twins are estimated to be within the range 57–62°. The cycle of M → P → M transformations revealed a hysteresis of about 15°C, but no anomaly or premartensitic effects in the Bragg peak spectra.

SHS synthesized Ni-Ti based shape memory alloys for both low- and high-temperature applications

The thermal explosion mode of self-propagating hightemperature synthesis (SHS), or combustion synthesis, was used to successfully produce NiTi (Nitinol) shape memory alloys (SMA) for the first time [1]. The explosion mode of ignition of the powder pellet mix is preferred to the combustion mode because it can provide combustion temperatures higher than the melting point of the synthesized intermetallic, thereby providing a cast product [2]. Compared with the conventional process of melting the two metals in an induction furnace, the SHS method provides advantages with respect to both time and energy-savings and easier processing. In this letter, the results of producing shape memory alloys with a range from low- to high-transition temperatures using the SHS technique are reported. Patent

Intravascular Endoprostheses Effect of Surface Geometry on Restenosis and Side Branch Patency

To investigate the effects of gaps between the wire pitches of a shape memory Nitinol alloy intravascular endoprosthesis (IVEP), 42 stents with and without gaps were implanted transluminally into the infrarenal abdominal aortae of 11 normal and 10 atherosclerotic rabbits. Digital subtraction angiography was done every 4 weeks to examine the restenosis rate and major side branch patency. Restenosis rates within IVEPs with gaps were significantly lower than in stents without gaps in both normal and atherosclerotic rabbits. The side branch patency rate was significantly higher with IVEPs having gaps. These results suggest that the placement of gaps between wire pitches reduced the restenosis rate within IVEPs, and prevented the obstruction of major side branches.

Intravascular Endoprostheses Effect of Surface Geometry on Restenosis and Side Branch Patency

To investigate the effects of gaps between the wire pitches of a shape memory Nitinol alloy intravascular endoprosthesis (IVEP), 42 stents with and without gaps were implanted transluminally into the infrarenal abdominal aortae of 11 normal and 10 atherosclerotic rabbits. Digital subtraction angiography was done every 4 weeks to examine the restenosis rate and major side branch patency. Restenosis rates within IVEPs with gaps were significantly lower than in stents without gaps in both normal and atherosclerotic rabbits. The side branch patency rate was significantly higher with IVEPs having gaps. These results suggest that the placement of gaps between wire pitches reduced the restenosis rate within IVEPs, and prevented the obstruction of major side branches.