Intravascular functional element and method of manufacture

Abstract

The invention relates to a method of manufacture of an intravascular functional element that can be introduced into a hollow organ and that comprises at least one wire (10) of an alloy having nickel and titanium as alloying elements, with the following steps: preparation of a metal body of the wire (10) with a metallic surface, then formation of a first oxide layer on the metallic surface of the metal body, then performance of a heat treatment of the wire (10) in a nitrogen-containing salt bath for thermal formation of a second mixed oxide layer on the first oxide layer, wherein the total layer thickness is 15 nm to 100 nm and the mixed oxide layer contains TiO.sub.2 and at least one nitride, especially titanium oxynitride and/or titanium nitride.

Claims

1. A method of manufacturing an intravascular functional element, the intravascular functional element for introduction into a hollow organ; the intravascular functional element comprising a wire comprising an alloy of nickel and titanium as alloying elements; and a mixed oxide layer formed on the surface of the wire, the mixed oxide layer comprising a layer thickness from 15 nm to 100 nm, TiO.sub.2, and a nitride, the nitride being a titanium nitride or an titanium oxynitride, wherein the wire forms a wire structure comprising the wire; wherein the wire structure comprises a coil for aneurysm treatment or a braid; wherein the wire structure comprises a contact zone, the contact zone having a plurality of cross-overs of the wire where the wire crosses-over itself or a plurality of self-contact points of the wire where the wire contacts itself; and wherein the mixed oxide layer is homogeneous and has a substantially constant thickness on the wire even at cross-overs or self-contact points; the method comprising the steps of: (a) preparing a metal body of the wire; (b) subsequent to step (a), forming a first oxide layer on the surface of the wire; and (c) subsequent to step (b), heat treating the wire in a nitrogen-containing salt bath for thermal formation of the mixed oxide layer on the first oxide layer.

2. The method according to claim 1, wherein the layer thickness is at least 55 nm.

3. The method according to claim 1, wherein the layer thickness is at most 95 nm.

4. The method according to claim 1, wherein a peak of a oxygen concentration in the mixed oxide layer is formed as a plateau.

5. The method according to claim 4, wherein a ratio of intensities between nitrogen and oxygen comprises a range of 1:2.5 to 1:10 in the plateau, and decreases toward an interior of the wire, wherein the intensities are determined respectively by Auger electron spectroscopy (AES).

6. The method according to claim 1, wherein the nitride extends from an exterior surface of the mixed oxide layer to a depth in the wire of of the total thickness of the mixed oxide layer.

7. The method according to claim 1, wherein the mixed oxide layer comprises nickel, the nickel in a region extending from an exterior surface of the mixed oxide layer to a depth of 30% of the total thickness of the mixed oxide layer is at most 6% by weight.

8. The method according to claim 1, further comprising a step of forming an enrichment of nickel oxide in an inner portion of the mixed oxide layer.

9. The method according to claim 1, further comprising a step of forming the contact zone of the wire prior to step (c).

10. The method according to claim 9, wherein the step of forming the contact zone of the wire is performed by braiding the wire.

11. The method according to claim 9, wherein the step of forming the contact zone of the wire is performed by winding the wire into a coil.

12. The method according to claim 1, wherein a contact angle measured with distilled water is smaller than 90 and at least 30.

13. The method according to claim 1, wherein the salt bath comprises an alkali metal-nitrogen salt, a potassium nitrate, a sodium nitrite, or a mixture of potassium nitrate and sodium nitrite.

14. The method according to claim 13, wherein the content of potassium nitrate is greater than the content of sodium nitrite.

15. A method according to claim 13, wherein the salt bath comprises 30-40 wt % KNO3 25-35 wt % NaNO2 rest usual carbon compounds and impurities, wherein a content of potassium nitrate is greater than a content of sodium nitrite.

16. The method according to claim 1, wherein step (c) is performed after performing a step of introducing a functional item, the functional item being a radiologically visible markings, or after performing a step of joining process.

17. The method according to claim 1, wherein step (a) is performed by electro-polishing the wire.

18. An intravascular functional element for introduction into a hollow organ, the intravascular functional element comprises: a wire comprising an alloy of nickel and titanium as alloying elements; and a mixed oxide layer formed on the surface of the wire, the mixed oxide layer comprising a layer thickness from 15 nm to 100 nm, TiO.sub.2, and a nitride, the nitride being a titanium nitride or an titanium oxynitride; wherein the wire forms a wire structure comprising the wire; wherein the wire structure comprises a coil for aneurysm treatment or a braid; wherein the wire structure comprises a contact zone, the contact zone having a plurality of cross-overs of the wire where the wire crosses-over itself or a plurality of self-contact points of the wire where the wire contacts itself; and wherein the mixed oxide layer is homogeneous and has a substantially constant thickness on the wire even at cross-overs or self-contact points.

19. A functional element according to claim 18, wherein the mixed oxide layer is disposed in the contact zone.

20. A functional element according to claim 18, wherein the layer thickness is from 15 nm to 60 nm or is from 30 nm to 100 nm.

21. A functional element according to claim 18, wherein the wire structure is non-unitary.

22. A functional element according to claim 18, wherein a peak of a concentration of TiO.sub.2 in the mixed oxide layer forms a plateau.

23. A functional element according to claim 22, wherein a ratio of intensities between nitrogen and oxygen comprises a range of 1:2.5 to 1:10 in the plateau, and decreases toward an interior of the wire, wherein the intensities are determined respectively by Auger electron spectroscopy (AES).

24. A functional element according to claim 18, wherein the nitride extends from an exterior surface of the mixed oxide layer to a depth in the wire of of the total thickness of the mixed oxide layer.

25. A functional element according to claim 18, wherein the mixed oxide layer comprises nickel, the nickel in a region extending from an exterior surface of the mixed oxide layer to a depth of 30% of the total thickness of the mixed oxide layer is at most 6% by weight.

26. A functional element according to claim 18, wherein an enrichment of nickel oxide is formed in an inner portion of the mixed oxide layer.

27. An intravascular functional element for introduction into a hollow organ, the intravascular functional element comprises: a plurality of wires comprising a first wire and a second wire, each wire comprising an alloy of nickel and titanium as alloying elements; and a mixed oxide layer formed on the surface of each wire, the mixed oxide layer comprising a layer thickness from 15 nm to 100 nm, TiO.sub.2, and a nitride, the nitride being a titanium nitride or an titanium oxynitride; wherein the plurality of wires form a non-unitary wire structure; wherein the wire structure comprises a coil for aneurysm treatment or a braid; wherein the wire structure comprising comprises a contact zone, the contact zone having a plurality of wire cross-overs where the first wire crosses-over the second wire or a plurality of wire self-contact point where the first wire contacts the second wire; and wherein the mixed oxide layer is homogeneous and has a substantially constant thickness of each wire even at wire cross-overs or wire self-contact points.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail hereinafter on the basis of exemplary embodiments with reference to the attached schematic drawings, wherein:

(2) FIG. 1 shows several wires in an electropolishing bath;

(3) FIG. 2 shows a schematic braid of several wires;

(4) FIG. 3 shows the braid according to FIG. 2 in a salt bath;

(5) FIG. 4 shows a depth profile of a mixed oxide layer of a functional element according to an inventive exemplary embodiment with a layer thickness of approximately 60 nm (Probe1.6pro);

(6) FIG. 5 shows a depth profile of a mixed oxide layer of a functional element according to an comparison example with a layer thickness of approximately 220 nm (Auger2.5+3.pro);

(7) FIG. 6 shows a corrosion curve (0611-170-01) of a functional element as a comparison example, in which a non-electropolished wire is used;

(8) FIG. 7 shows a corrosion curve (1586-170-04) of a functional element according to an inventive exemplary embodiment;

(9) FIG. 8 shows an SEM photograph of an untreated comparison sample, and

(10) FIG. 9 shows an SEM photograph of a wire surface generated according to an embodiment of the inventive method.

(11) FIG. 10 is a schematic of a portion of an intravascular functional element comprising a wire comprising an alloy of nickel and titanium as alloying elements and a mixed oxide layer formed on the surface of the wire.

DETAILED DESCRIPTION OF THE INVENTION

(12) FIG. 1 shows a first step for the production of a stent. Several wires 10 (four wires in the schematic drawing) are immersed in an electropolishing bath 11 of an electrolyte. This step may be carried out as in US 2004/0117001 A1, except for the timing sequence.

(13) FIG. 2 shows (schematically) a braid 12 of wires 10. This braid 12 is illustrated in the expanded condition, so that the entire circumferential surface of the braid 12 is shown in the drawing plane. After the braiding step, braid 12 is immersed and heat-treated in a salt bath 13 (see FIG. 3). In salt bath 13, braid 12 or the stent receives its final structure, including the passivated surface. This does not rule out the possibility that still further processing steps will be carried out.

Example

(14) The invention will be explained by means of an example of a functional element produced from a binary NiTi alloy, such as Nitinol. Other NiTi-containing alloys are possible. In this case the modification of the surface is represented by the thermal treatment in the salt bath, which treatment is responsible for adjusting the nitrogen concentration in the TiO.sub.2 mixed oxide layer. The basic component of the functional element, namely the wire, is electropolished in the first step. The electropolishing may be carried out as is usual in the prior art, for example at a temperature T<20 C., using a methanolic sulfuric acid solution. A homogeneous natural oxide layer with a layer thickness of approximately 5 nm is spontaneously formed on the electropolished wire upon contact with the ambient air.

(15) In the second step, a functional element, a stent, is braided using the electropolished wire.

(16) In the third step, the functional element is heat-treated in the salt bath in order to increase the layer thickness. For this purpose there is used a salt-bath composition consisting of the following components: approximately 35-36 wt % KNO.sub.3 approximately 27-29 wt % NaNO.sub.2 rest usual carbon compounds and impurities.

(17) It has been found that good results may be achieved when the content of potassium nitrate is greater than the content of sodium nitrite in the salt bath.

(18) The process temperatures are approximately 490 C. to 510 C. In the first treatment step, the functional element is immersed for approximately 2 to 3 minutes in the salt bath. Formation of the oxide layer takes place during this time. The treatment time in the second step is approximately 30 sec or shorter.

Measuring Technique

(19) The measurements to determine the AES depth profile according to FIGS. 4 and 5 were made with the following parameters: Primary electron energy (excitation): 5 keV Beam current: 20 nA Electron beam raster (analyzed zone): 20 m2 m Ion beam energy: 3 keV (FIG. 4, Probe1.6.pro, treated in each case) Beam current: 2 A Ablation rate: 59.3 nm/min Ion beam raster: 0.8 mm0.8 mm Ion beam energy: 1 keV (FIG. 5, Auger2.5+3.pro, untreated) Beam current: 0.5 A Ablation rate: 8.24 nm/min Ion beam raster: 0.8 mm0.8 mm

(20) Sample angle (between electron beam and normal to the sample): 30

(21) The following element peaks were used for determination of the intensities: Ti1: Ti LMM at 390 eV Ti2: Ti LMM at 421 eV N1: N KLL at 389 eV Ni1: Ni LMM at 849 eV O1: O KLL at 510 eV

(22) The measurements for the corrosion curves according to FIGS. 6 and 7 were carried out according to ASTM F2129 Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices.

Results and Discussion

(23) The depth profile according to FIG. 4, where the sputtering depth is normalized to 500 nm, shows the concentration profile obtained for the layer elements after the process explained in the foregoing has been carried out. In the process, the layer thickness is generally determined from the sputtering parameters. Alternatively to the determination of the layer thickness, 50% of the peak value of TiO.sub.2 is calculated. Accordingly, the layer has a thickness of approximately 60 nm, which is obtained from the intersection of the oxygen peak and the peak for metallic Ti and Ni. The following peaks are marked in FIG. 4: Oxygen Nitrogen Ti in the titanium oxide Titanium (metallic titanium) Ni in the Ni oxide Ni

(24) It is particularly obvious that the oxygen peak has the form of a plateau. The plateau extends over a layer depth between approximately 10 nm and 40 nm. One possible explanation is that the oxygen also combines with nitrogen toward the outer part of the layer and then combines with titanium toward the inner part of the layer. The nitrogen is incorporated as a chemical compound in the layer, and specifically as titanium oxynitride. This follows from the shape of the oxygen signal, which forms a plateau. It may also well be that the nitrogen is additionally present even as titanium nitride. In general, the plateau shape means that the oxygen intensity is zonally constant, especially over a layer depth of at least 10 nm.

(25) In the region of the oxygen plateau, the most obvious ratios N/O of the intensities of nitrogen and oxygen in the layer are approximately 1:3; 1:6; 1:10, where the ratio 1:3 is located in the immediate boundary layer of the outer surface of the layer (approximately 5 nm to 10 nm), the ratio 1:6 at a layer depth of approximately 20 to 25 nm, for example, and the ratio 1:10 at a layer depth of approximately 35 to 40 nm, for example. The ratio at the layer surface is approximately 1:2.5.

(26) From FIG. 4 it is further apparent that a distinct enrichment of Ni oxide is present in the inner part of the layer, in other words close to the metallic wire body. The rest of the layer contains hardly any nickel. In particular, the outer boundary layer is low in nickel. This concentration profile could be imposed by the nitrogen, which may well combine preferentially with oxygen rather than with nickel and thus suppress nickel enrichment in the outer part of the layer.

(27) FIG. 5 shows the depth profile of an untreated sample with an oxide layer thickness of approximately 220 nm (see Ni/O intersection). The investigated oxide layer was formed by the heat treatment during wire manufacture. No treatment was carried out for the sample according to FIG. 5, i.e. the manufacturing-related oxide layer was left on the surface of the wire. FIG. 5 shows that the oxygen profile does not form a plateau. The oxygen intensity increases to approximately 50 nm then decreases. In contrast to the layer according to FIG. 4, moreover, a slight enrichment of nickel oxide is apparent in the near-surface zone of the layer. Enrichment of nickel oxide in the zone of the layer close to the metal body is absent. On the whole, the nitrogen intensity in the layer is significantly lower than in the layer according to FIG. 4.

(28) FIG. 10 is a schematic of a portion of an intravascular functional element 5 comprising a wire 10 comprising an alloy 10a of nickel and titanium as alloying elements and a mixed oxide layer 10b formed on the surface 10c of the wire 10.

(29) The protective behavior of the layer is assessed on the basis of the corrosion curves according to FIGS. 6 and 7, from which the electrochemical behavior of the layers and therefore the layer properties of interest, such as the release of nickel ions, for example, can be deduced. In FIGS. 6 and 7, the current density J in A/cm.sup.2 is plotted against the voltage E in V (SCE).

(30) FIG. 7 shows the corrosion curve (1586-170-04) of an inventively produced layer, which exhibits a very low corrosion current density (<110.sup.8 A/cm.sup.2). This means that the layer has low permeability for metal ions and therefore exhibits a good protective effect. It is particularly important, as follows from the almost linear increase, that no perforation, i.e. no pitting corrosion occurs. Accordingly the layer properties are excellent.

(31) In contrast to this, as shown in FIG. 6, the corrosion current in the conventionally produced layer is greater than 110.sup.7 A/cm.sup.2. Perforations suggestive of the onset of pitting corrosion, i.e., the formation of small holes, can be observed at approximately 400 mV.

(32) The good surface properties are obvious from the comparison between the surface of an untreated wire having a manufacturing-related oxide layer, illustrated in FIG. 8, and the surface of a wire heat-treated according to the invention, shown in FIG. 9. The oxide layer of the wire according to FIG. 9 is uniformly dense and pore-free.

(33) By means of the inventive method it is possible to produce very corrosion-stable and hard mixed oxide layers, which develop a good protective effect and protect safely against abrasion.

LIST OF REFERENCE SYMBOLS

(34) Braid angle 5 Intravascular functional element 10 Wire 10a Alloy of nickel and titanium as alloying elements 10b mixed oxide layer formed on the surface 10c of the wire 10c surface of the wire 11 Electropolishing bath 12 Braid 13 Salt bath