Effect of modification of oxide layer on NiTi stent corrosion resistance

Effect of chemical etching and aging in boiling water on the corrosion resistance of Nitinol wires with black oxide resulting from manufacturing process

The effect of chemical etching in a HF/HNO(3) acid solution and aging in boiling water on the corrosion resistance of Nitinol wires with black oxide has been evaluated with the use of potentiodynamic, modified potentiostatic ASTM F746, and scratch tests. Scanning-electron microscopy, elemental XPS, and Auger analysis were employed to characterize surface alterations induced by surface treatment and corrosion testing. The effect of aging in boiling water on the temperatures of martensitic transformations and shape recovery was evaluated by means of measuring the wire electroresistance. After corrosion tests, as-received wires revealed uniformly cracked surfaces reminiscent of the stress-corrosion-cracking phenomenon. These wires exhibited negative breakdown potentials in potentiostatic tests and variable breakdown potentials in potentiodynamic tests (- 100 mV to + 400 mV versus SCE). Wires with treated surfaces did not reveal cracking or other traces of corrosion attacks in potentiodynamic tests up to + 900-1400-mV potentials and no pitting after stimulation at + 800 mV in potentiostatic tests. They exhibited corrosion behavior satisfactory for medical applications. Significant improvement of corrosion parameters was observed on the reverse scans in potentiodynamic tests after exposure of treated wires to potentials > 1000 mV. In scratch tests, the prepared surfaces repassivated only at low potentials, comparable to that of stainless steel. Tremendous improvement of the corrosion behavior of treated Nitinol wires is associated with the removal of defect surface material and the growth of stable TiO(2) oxide. The role of precipitates in the corrosion resistance of Nitinol-scratch repassivation capacity in particular-is emphasized in the discussion.

The endovascular repair of abdominal aortic aneurysm: an update analysis of structural failure modes of endovascular stent grafts

Over the last decade, more than 25,000 endovascular stent grafts have been used to treat aortic aneurysms. Although results have been promising thus far, problems with endoleaks, material failure, device migration, and aneurysm rupture continue to be reported. Improvements in device material and design have contributed to the rapid growth and utility of these devices; however, material failure still remains a concerning mode for potential procedure failure. A review of the authors'experience with material failure and a review of the literature was conducted to help understand and comprehend the magnitude of this problem and try to determine its clinical significance.

Mechanical failure of prosthetic human implants: a 10-year experience with aortic stent graft devices

OBJECTIVE

The first endovascular stent graft was implanted to treat an abdominal aortic aneurysm more than a decade ago. This technique has evolved dramatically with the growing understanding of metallurgic and fabric sciences and improved device designs. However the potential for stent graft material failure remains. This investigation describes the incidence of material failure, potential modes of device fatigue, and the clinical significance of these failures.

METHODS

Six hundred eighty-six endovascular stent grafts were used to treat patients with aortic aneurysms. Device fatigue in the form of stent, suture fracture, or graft wear was identified with an analysis of follow-up radiographs and explanted stent grafts. A review of patient clinical histories, spiral computed tomographic scan studies, scanning electron microscopy, and energy dispersion spectroscopy of explanted devices was conducted to evaluate the modes and consequences of failure.

RESULTS

Sixty patients were identified with device fatigue, 49 of whom had abdominal endovascular repairs and 11 of whom had thoracic repairs. Of the 60 patients with stent graft fatigue, 43 patients had metallic stent fractures, 14 had suture disruptions, and three had graft holes. These material failures occurred within seven distinct stent graft designs. The average time to the recognition of failure was 19 months, with a mean follow-up period of 8 months since the event was identified. Eleven patients died, and one was lost to follow-up 2 years after identification of a stent fracture. The remaining patients are presently being followed eoyj physical examination, plain film radiograph, and computed tomographic scans for clinical sequelae of device fatigue.

CONCLUSION

Endovascular stent graft fatigue has been recognized in numerous devices after aortic implantation. Fatigue may take the form of stent, graft, or suture failure, with certain modes unique to specific stent graft devices. The clinical significance of stent graft material failure remains uncertain.

Corrosion resistance of polyurethane-coated Nitinol cardiovascular stents

Corrosion of metal stents implanted inside an artery can have two adverse effects: (1) tissue reaction and possible toxic effects from the metal ions leaching out of the stent, and (2) loss of mechanical strength of the stent caused by corrosion. The corrosion resistance of Nitinol (Nickel-Titanium) stents and its modulation with different film thickness of polymer coating was studied against an artificial physiological solution using a Potentiostat/Galvanostat and an electrochemical corrosion cell. The corrosion rate decreased rapidly from 275 microm/year for an uncoated surface down to less than 13 microm/year for a 30 microm thick polyurethane coating. Stainless steel (316L) and Nitinol both contain potentially toxic elements, and both are subject to stress corrosion. Minimization of corrosion can significantly reduce both tissue reaction and structural degradation.

Easy assessment of the biocompatibility of Ni-Ti alloys by in vitro cell culture experiments on a functionally graded Ni-NiTi-Ti material

The biocompatibility of nickel-titanium alloys was investigated by single-culture experiments on functionally graded samples with a stepwise change in composition from pure nickel to pure titanium, including an Ni-Ti shape memory alloy for a 50:50 mixture. This approach permitted a considerable decrease of experimental resources by simultaneously studying a full variation of composition. The results indicate a good biocompatibility for a nickel content up to about 50%. The cells used in the biocompatibility studies comprised osteoblast-like osteosarcoma cells (SAOS-2, MG-63), primary human osteoblasts (HOB), and murine fibroblasts (3T3).

Effect of surface treatment of NiTi alloy on its corrosion behavior in Hanks' solution

The effect of surface modification of NiTi shape memory alloy on its corrosion behavior in Hanks' solution was determined. The near-equi-atomic super elastic NiTi (Ni 55.8 wt %) alloy used for this study was provided by Memry USA. The surfaces of heat-treated samples were modified by mechanical polishing (MP), electropolishing (EP), and electropolishing followed by chemical passivation (CP). As-heat-treated samples with straw-colored oxide finishes (SCO) and blue-colored oxide finishes (BO) also were included in the study. Surface analysis was performed using auger electron spectroscopy (AES), atomic force microscopy (AFM), and contact angle measurements (CAM). It was shown that surface roughness increased in the order CP < EP < SCO < BO < MP. The nickel release within the five groups of NiTi samples, as determined by atomic absorption spectrophotometry, reduced in time over the measured period. The level of Ni ions released over a 25-day immersion period was highest in the SCO sample (0.002 microg/day). This Ni level is negligible compared with the daily intake of Ni in an ordinary diet. The auger electron spectroscopy (AES) analyses indicated that before immersion in Hanks' physiologic solution, the main surface composition of all the samples was titanium and nickel, with a small amount of oxygen, carbon, and sulphur as contaminants. And the surface oxide thickness of the different samples increased in the order CP < EP < MP < BO < SCO. On the other hand, for the electrodes treated under the same conditions, the mean breakdown potential value decreased in the order BO > MP > CP > EP > SCO while the corrosion current density and rate increased in the order CP < SCO < EP < BO < MP.

Nitinol versus stainless steel stents: acute thrombogenicity study in an ex vivo porcine model

Acute and subacute stents thrombosis along with thrombus mediating neointimal proliferation within the stent struts remain major concerns in coronary stenting. Up to date, there is an obvious lack of data on the thrombogenicity of stent materials in physiological conditions. This study was performed to compare the relative thrombogenicity of nitinol versus stainless steel stents. Nitinol stents were laser cut to reproduce the exact geometry of the stainless steel Palmaz stents and tested in an ex vivo AV shunt porcine model under controlled conditions. Nitinol stents presented only small amounts of white and/or red thrombus principally located at the strut intersections while Palmaz stents clearly exhibited more thrombus. As a result, 125I-fibrin(ogen) adsorption and (111)I-platelets adhesion were significantly lower on nitinol than on stainless steel devices (36%, p = 0.03 for fibrin(ogen) and 63%, p = 0.01 for platelet). These results were confirmed by scanning electron observations showing different thrombus morphologies for nitinol and stainless steel. Along with the unique mechanical properties of nitinol, its promising haemocompatibility demonstrated in our study may promote their increasing use for both peripheral and coronary revascularization procedures.

On the properties of two binary NiTi shape memory alloys. Effects of surface finish on the corrosion behaviour and in vitro biocompatibility

The present paper compares the transformation behaviour and mechanical properties of two orthodontic wires of close chemical compositions. The effects of surface topography and surface finish residues on the potentiodynamic corrosion behaviour and biocompatibility are also reported. The cytotoxicity tests were performed on both alloys in fibroblast cell cultures from human gingiva using the MTT test. It is shown that the surface finish and the amounts of surface finish residues affect dramatically the corrosion resistance. Bad surface finish results in lower corrosion resistance. The in vitro biocompatibility, though not affected to the extent of corrosion resistance, is also reduced as the surface roughness and the amounts of residues increase. This is thought to be due to surface effects on corrosion and metallic ions release.

Passivation of nitinol wire for vascular implants--a demonstration of the benefits

This study investigated a passivation process for polished nitinol wires and vascular stent components, after being given a typical shape setting heat treatment. Heat treated samples were passivated in a nitric acid solution and a series of corrosion tests, surface analysis and chemical analysis was performed. Potentiodynamic polarization tests demonstrated a significant increase in breakdown potential for passivated samples, compared to heat treated surfaces. Surface analysis indicated that the passivation reduces Ni and NiO content in the oxide and increases TiO2 content. Chemical analysis of passivation solutions suggests that the improvement in corrosion resistance is proportional to the quantity of nickel removed. Long term immersion tests demonstrate that nickel release from the surface of the material decreases with time and the quantity of nickel released is lower for passivated samples. The improved corrosion resistance is maintained after extended periods of immersion in saline solution.

Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials

The corrosion susceptibility of Ti, Ti-6A1-4V and Ti-45Ni was studied in a buffered saline solution using anodic polarisation and electrochemical impedance measurements. Pitting potentials as low as + 250 mV(SCE) were recorded for Ti-45Ni and once initiated pits continued to propagate at potentials as low as -150 mV(SCE). It was possible to increase the pitting potential of Ti-45Ni to values greater than +800 mV(SCE) using a H2O2 surface treatment procedure; however, this surface modification process had no beneficial effect on the rate of pit repassivation. Impedance spectra, recorded under open-circuit conditions, were modelled using a dual oxide film model; a porous outer layer and an inner barrier oxide layer. The nature of this porous outer layer was found to depend on the nature of the electrode material and the presence of phosphate anions in the saline-buffered solution. The porous layers formed on Ti-45Ni and Ti-6Al-4V in the presence of phosphate anions had low resistances typically between 10 and 70 ohm cm2. Much higher porous layer resistances were recorded for Ti and also for Ti-45Ni and Ti-6Al-4V in the absence of the phosphate anions.

Effect of surface treatment and sterilization processes on the corrosion behavior of NiTi shape memory alloy

Nickel-titanium (NiTi) alloy derives its biocompatibility and good corrosion resistance from a homogeneous oxide layer mainly composed of TiO(2), with a very low concentration of nickel. In this article, we described the corrosion behavior of NiTi alloys after mechanical polishing, electropolishing, and sterilization processes using cyclic polarization and atomic absorption. As a preparative surface treatment, electropolishing decreased the amount of nickel on the surface and remarkably improved the corrosion behavior of the alloy by increasing the mean breakdown potential value and the reproducibility of the results (0.99 +/- 0.05 V/SCE vs. 0.53 +/- 0. 42). Ethylene oxide and Sterrad(R) sterilization techniques did not modify the corrosion resistance of electropolished NiTi, whereas a steam autoclave and, to a lesser extent, peracetic acid sterilization produced scattered breakdown potential. In comparing the corrosion resistance of common biomaterials, NiTi ranked between 316L stainless steel and Ti6A14V even after sterilization. Electropolished NiTi and 316L stainless-steel alloys released similar amounts of nickel after a few days of immersion in Hank's solution. Measurements by atomic absorption have shown that the amount of released nickel from passive dissolution was below the expected toxic level in the human body. Auger electron spectroscopy analyses indicated surface contamination by Ca and P on NiTi during immersion, but no significant modification in oxide thickness was observed.

First-generation aortic endografts: analysis of explanted Stentor devices from the EUROSTAR Registry

PURPOSE

To examine the structure and healing characteristics of chronically implanted Stentor endografts that were explanted due to migration, endoleak, thrombosis, or aneurysm expansion.

METHODS

The devices were harvested following reoperation (n = 5) or autopsy (n = 1) with implantation times ranging from 13 to 53 months. Structural modifications to the metal components were examined using radiography, endoscopy, and magnetic resonance imaging (MRI). Specimens taken from components of the modular stent-grafts were examined histologically and with scanning electron microscopy (SEM) to assess healing behavior. Physical and chemical stability of the nitinol wires and woven polyester graft material was evaluated using SEM and electron spectroscopy for chemical analysis.

RESULTS

Although the endografts were retrieved for a variety of reasons, they exhibited similar healing and structural modifications. The woven polyester sleeve showed evidence of yarn shifting and distortion, yarn damage, and filament breakage leading to the formation of openings in the fabric. The luminal surface endografts showed incomplete healing characterized by a poorly organized, nonadherent thrombotic matrix of variable thickness. Radiographic and endoscopic observations indicated that structural failure of the grafts, particularly in the main aortic component, was related to severe compaction and dislocation of the metallic frame due to suture breaks. Corrosion marks were observed on some nitinol wires in all devices. Chemical analysis and ion bombardment of the nitinol wires revealed that the surface concentrations of titanium and nickel were not homogenous. The first layer was composed of carbon or organic elements, followed by a stratum of highly oxidized titanium with a low nickel concentration; the titanium-nickel alloy lay beneath these layers.

CONCLUSIONS

Although the materials selected for construction of endovascular grafts appears judicious, the assembly of these biomaterials into various interrelated structures within the device requires further improvement.

First-generation aortic endografts: analysis of explanted Stentor devices from the EUROSTAR Registry

PURPOSE

To examine the structure and healing characteristics of chronically implanted Stentor endografts that were explanted due to migration, endoleak, thrombosis, or aneurysm expansion.

METHODS

The devices were harvested following reoperation (n = 5) or autopsy (n = 1) with implantation times ranging from 13 to 53 months. Structural modifications to the metal components were examined using radiography, endoscopy, and magnetic resonance imaging (MRI). Specimens taken from components of the modular stent-grafts were examined histologically and with scanning electron microscopy (SEM) to assess healing behavior. Physical and chemical stability of the nitinol wires and woven polyester graft material was evaluated using SEM and electron spectroscopy for chemical analysis.

RESULTS

Although the endografts were retrieved for a variety of reasons, they exhibited similar healing and structural modifications. The woven polyester sleeve showed evidence of yarn shifting and distortion, yarn damage, and filament breakage leading to the formation of openings in the fabric. The luminal surface endografts showed incomplete healing characterized by a poorly organized, nonadherent thrombotic matrix of variable thickness. Radiographic and endoscopic observations indicated that structural failure of the grafts, particularly in the main aortic component, was related to severe compaction and dislocation of the metallic frame due to suture breaks. Corrosion marks were observed on some nitinol wires in all devices. Chemical analysis and ion bombardment of the nitinol wires revealed that the surface concentrations of titanium and nickel were not homogenous. The first layer was composed of carbon or organic elements, followed by a stratum of highly oxidized titanium with a low nickel concentration; the titanium-nickel alloy lay beneath these layers.

CONCLUSIONS

Although the materials selected for construction of endovascular grafts appears judicious, the assembly of these biomaterials into various interrelated structures within the device requires further improvement.

Effects of sterilization processes on NiTi alloy: surface characterization

Sterilization is required for using any device in contact with the human body. Numerous authors have studied device properties after sterilization and reported on bulk and surface modifications of many materials after processing. These surface modifications may in turn influence device biocompatibility. Still, data are missing on the effect of sterilization procedures on new biomaterials such as nickel-titanium (NiTi). Herein we report on the effect of dry heat, steam autoclaving, ethylene oxide, peracetic acid, and plasma-based sterilization techniques on the surface properties of NiTi. After processing electropolished NiTi disks with these techniques, surface analyses were performed by Auger electron spectroscopy (AES), atomic force microscopy (AFM), and contact angle measurements. AES analyses revealed a higher Ni concentration (6-7 vs. 1%) and a slightly thicker oxide layer on the surface for heat and ethylene oxide processed materials. Studies of surface topography by AFM showed up to a threefold increase of the surface roughness when disks were dry heat sterilized. An increase of the surface energy of up to 100% was calculated for plasma treated surfaces. Our results point out that some surface modifications are induced by sterilization procedures. Further work is required to assess the effect of these modifications on biocompatibility, and to determine the most appropriate methods to sterilize NiTi.