WO2007098884A1

WO2007098884A1 - Method of making stainless steel hypotubes

Info

Publication number: WO2007098884A1
Application number: PCT/EP2007/001533
Authority: WO
Inventors: Vladimir Gergely; Conan Cavanagh
Original assignee: Creganna Medical Devices Ltd.
Priority date: 2006-03-01
Filing date: 2007-02-22
Publication date: 2007-09-07

Abstract

A method (50) of the manufacture of steel hypotubes is provided, wherein steel strips are formed into tubes (52), the tubes are drawn (54), and the drawn hypotubes are subjected to hardening by heat treatment (56). A synergistic effect of the material composition, tube drawing and heat-treatment process results in production of hypotubes with significantly enhanced combination of mechanical performance attributes.

Description

METHOD OF MAKING STAINLESS STEEL HYPOTUBES

Technical Field The present invention relates generally to a catheter tube, and more particularly to a catheter tube, also termed a hypotube, for intravascular use in the body, for example to deliver cardiac treatment devices, brain treatment devices and the like.

Background Art

A hypotube is a tube portion of a type of catheter used to deliver stents, angioplasty devices and other instruments into the body and in particular intravascularly into the cardiac arteries or into the brain, for example. In a typical use of a hypotube device, the elongated hypotube is provided with a treatment device at its end and is introduced into the femoral artery at the leg and is fed along the arterial system to the heart, where it is guided into selected arteries so that the hypotube device may deliver the treatment device to the desired location.

The performance characteristics of the hypotube shaft have established the hypotube as the device shaft of choice for PTA (Percutaneous Transluminal Angioplasty) applications. Recognizing its superior performance benefits, leading companies are now adopting hypotube-based device shafts in new application areas such as neurology, peripheral vascular interventions and catheter-based imaging.

A hypotube is a long shaft that often has micro-engineered features along its length. It is one of the components of minimally-invasive catheter systems. A hypotube is used as a shaft for delivering balloons, stents and other devices into a human or animal anatomy. The hypotube enters the body and pushes the attached device along what may be a torturous path. This journey requires hypotubes to resist kinking as well as to possess other attributes known as push, track and torque.

The commonest materials currently used for manufacture of metallic hypotubes are AISI 300-series austenitic stainless steels. While hypotubes manufactured from these steels can tolerate a reasonably high displacement before they kink when loaded either in bending or axial compression, the actual force required to kink these hypotubes is relatively low. The column strength of 300-series steel hypotubes is also relatively low. On the other hand, hypotubes manufactured from a 17-7PH stainless steel can exhibit higher column strength than 304-series steels and require a higher force to kink them when loaded in an axial compression but this is at the expense of displacements they can be subjected to before they kink.

It is well known issue encountered by hypotube designers that improvement of one of the hypotube performance attributes comes at the expense of another one. The present invention offers a solution for this problem.

In U.S. Patent No. 5,411,613 is disclosed a heat treatment of surgical needles manufactured from 1RK91 steel.

Disclosure Of The Invention

The present invention provides a catheter or hypotube having superior performance characteristics. In particular, the hypotube is formed according to a method wherein a steel alloy strip is shaped, welded, drawn and heat treated to an elongated thin tube having a desired flexibility, column strength, kink resistance and shape set resilience. In a preferred embodiment, the invention provides a hypotube and method for its manufacture from martensitic steel where a significant improvement of one of the mechanical performance features does not notably degrade another one.

A preferred martensitic precipitation hardening alloy for use in the present hypotube is a Sandvik Bioline 1RK91 steel (also know as Nanoflex^®), developed by Sandvik A.B., Sandviken, Sweden. A drawing process of hypotubes from the 1RK91 steel strip is similar to the drawing of hypotubes from AISI 300-series austenitic stainless steels. It is within the scope of the present invention that other alloys may be used in place of the 1RK91 steel alloy. Such additional alloys may share components and/or proportions of components of the 1RK91 alloy, although this is not necessary in every instance.

In order to tailor the 1RK91 material for the subsequent heat treatment, it is important to design the tube drawing process such that the tensile properties of 1RK91 hypotubes are within the range of the tensile properties exhibited by half- to full-hard hypotubes drawn from the AISI 300-series austenitic stainless steels.

The heat treatment of as-drawn 1RK91 hypotubes in the present invention comprises heating the hypotubes under vacuum and/or inert atmosphere at a temperature in the temperature range from about 555 °C to about 635 °C from about 5 minutes to about 10 hours. This hardening heat treatment range not only significantly increases the column strength of the hypotubes but can also increase force required to kink the hypotube and/or the displacement a hypotube can tolerate before kinking in comparison with the 304L and 17- 7PH (RH950) steel hypotubes.

As noted in the foregoing section, U.S. Patent No. 5,411,613 describes a heat treatment of surgical needles manufactured from 1RK91 steel. However, as shown below, the hardening heat treatment procedure of as-drawn 1RK91 steel hypotubes within the temperature range described in the patent has a detrimental effect on the amount of displacement the hypotubes can tolerate before they kink.

Brief Description Of The Drawings

The foregoing and other advantages of the invention will be appreciated more fully from the following description thereof, with reference to the accompanying drawing wherein:

Figure Ia is a schematic illustration of a kink test being performed on a hypotube; Figure Ib is a graph of force over displacement as recorded during the kink test of a hypotube;

Figure 2a is a schematic illustration showing a beginning of a column strength test on a hypotube;

Figure 2b is a schematic illustration of a further stage in the column strength test of Figure 2a;

Figure 2c is a schematic illustration of yet a further stage in the column strength test of Figure 2a;

Figure 2d is a graph of a typical force-displacement curve recorded during a column strength test on a hypotube;

Figure 3 is a graph providing a diagrammatic summary of the displacement at kink and the force at kink values of evaluated hypotubes tested according to the test depicted in Figure 1;

Figure 4 is a graph providing a diagrammatic summary of the displacement at kink and force at kink values of evaluated hypotubes tested according to the column strength test shown in Figure 2;

Figure 5 is a bar chart providing a diagrammatic summary of the critical buckling force (see Figure 2d) values of selected types of hypotubes; and

Figure 6 is a graph providing a diagrammatic summary of the shape set resilience values, h, of selected types of hypotubes.

Modes For Carrying Out The Invention

The present invention provides a method and product produced by the method, or process, for producing a hypotube having a very attractive combination of kink resistance and columnar strength characteristics. According to a preferred embodiment, the hypotubes are manufactured from a Sandvik 1RK91 steel strip. An example of the steel composition range of this steel strip is provided in Table 1. The composition of the steel is given there in weight percent.

Forming, welding, inter-stage annealing and drawing of hypotubes from the 1RK91 steel strip is carried out. These steps are similar to those used by those skilled in the art of drawing hypotubes from AISI 300-series austenitic stainless steels. In order to condition the 1RK91 steel for the subsequent heat treatment, the amount of final cold-work during the drawing process should be selected in such a way that the tensile property range of as-drawn hypotubes manufactured from 1RK91 material falls into the tensile property range typically exhibited by half- to full-hard hypotubes drawn from AISI 300-series austenitic stainless steels.

The heat treatment of as-drawn 1RK91 hypotubes typically comprises heating the hypotubes under vacuum and/or under an inert atmosphere at a temperature in the temperature range from about 555 ⁰C to about 635 °C from about 5 minutes to about 10 hours. The temperature and duration of heat-treatment are varied depending on the exact composition of the 1RK91 strip material and the amount of cold- work the material is subjected to during the tube drawing process.

It is demonstrated by the following examples that the present combination of material composition, tube drawing process, and heat-treatment process provides significant mechanical performance advantages over AISI 304L and 17-7PH steel-based hypotubes. In the present examples the as-drawn 1RK91 hypotubes are preferably subjected to heat treatment at temperatures from about 555 °C to about 635 °C for about up to 3 hours.

Table 2 lists some of the types of evaluated hypotubes, their steel grade, hypotube dimensions (OD: outer diameter and ID: inner diameter), conditions (as-drawn or heat treated) and typical recorded tensile properties (UTS: ultimate tensile test and YS: yield strength at 0.002 offset strain). In particular, ten types of hypotube samples were tested, the first being an AISI 304L steel, the second being a 17-7PH steel, and types three through ten being of 1RK91 steel as-drawn and with various heat treatments. Mechanical performance characteristics of the hypotubes listed in Table 2 were evaluated using the following test methods:

Test 1 : Standard kink test. This test involves a compression loading of an arched hypotube 10 between two parallel plates 12 and 14 as shown in Figure Ia. The hypotube 10 is positioned in an arch between the plates 12 and 14 so that the opposite ends of the hypotube 10 are lying against the plates and are generally parallel to one another. The mid portion of the hypotube is curved about an approximately 180 degree bend and is unsupported in the middle. The start distance between the plates is 76.2 mm for one embodiment of the test. A load is applied as indicated by the arrow F to move the plate 12 towards the plate 14. The load is applied until a kink occurs in the hypotube. A kink in the tube occurs as a sudden collapse of the wall of the tube when the wall on one side of the tube collapses toward the other side wall at a distinct point along the tube length. Prior to the occurrence of the kink, the cross section of the tube is almost constant along its length, but following the kink a point along the length of the tube has an abrupt cross sectional shape discontinuity.

The kink test records the force applied to the plate 12 as a function of the plate 12 displacement. A typical load/displacement curve 16 recorded during the kink test is depicted in Figure Ib. A kink of the hypotube occurs at the moment of sudden force drop on the load (or force)/displacement curve 16, as indicated at 18. After a kink has occurred in the hypotube 10, the ends of the hypotube may be moved toward one another much more easily, but more importantly, the tube has been permanently damaged.

Test 2: Column strength test. This test is conducted by applying an axial compression load to a hypotube 10 as shown in Figure 2a. In the test, the hypotube 10 is gripped at two locations along its length by grippers 20 and 22. The grippers 20 and 22 grasp the hypotube 10 tightly enough to avoid slipping on the tube. In one embodiment, the start distance between the grippers 20 and 22 is 90 mm. As shown in Figure 2a, a force as indicated by arrow F is applied to move the gripper 20 towards the gripper 22. As the force F is applied, the gripper 20 moves only negligibly. This is as a result of the columnar strength of the tube 10. As greater force is applied, the hypotube 10 begins to bend outward at the unsupported middle section 24. To bend outwardly as indicated in Figure 2b, the tube actually bends in three different bend directions with bends in a first direction nearer the grippers 20 and 22, respectively, and a reverse bend at the mid portion 24. In Figure 2c, the hypotube 10 has been bent to a relative extreme deformation and a kink (wherein one side of the tube wall collapses) has occurred in the tube, typically at 26. The bending of the hypotube 10 causes the tube to offer less resistance to further bending than when the tube 10 remained straight and unbent.

A graph of the typical load/displacement curve for a column strength test is shown in Figure 2d. The curve 30 plots the positions of the critical buckling force at 32 and the kink point 34 along its plot. These are not only locations not only of changes in the shape of the tube but changes in the curve of the force plot.

Test 3: Shape set resilience test. This test involves storing a hypotube in a coiled shape for a period of time and then measuring the curvature retained by the tube after the hypotube is permitted to return to its relaxed shape. In one example, a hypotube 10 is stored in a 152 mm diameter coil for two hours. A graph in Figure 6 shows the shape resilience test results for the selected hypotubes listed in Table 2, as will be discussed in further detail hereinafter. After removal of the 1 m long hypotube from the storage case coil, the shape set resilience value, h, is defined as a perpendicular distance between a ruler (that is touching both ends of the hypotube) and a highest point of the hypotube arc.

The foregoing tests were applied to batches of hypotubes prepared according to the characteristics listed in Table 2. The results for each batch were summarized or averaged over the plurality of hypotubes in each batch and are indicated as a single entry for each table entry. Also shown in Table 2 are the tensile properties (UTS: ultimate tensile test and YS: yield strength at 0.002 offset strain). The sample summary for the tube types for the application of the three tests to the hypotubes as listed in Table 2 is set forth below. The test results from the standard kink test (Test 1) are presented in Figure 3. Ten different hypotube samples (sample summaries) listed in the Table 2 are identified on the graph by different shaped markings, as listed to the right of the graph. Of the ten hypotubes tested, hypotube samples 9 and 10 had the highest force and displacement at kink. It can been seen by comparing Figure 3 and Table 2 that a heat treatment of as-drawn 1RK91 hypotubes within the 555-635 °C temperature range increases the force required to kink the 1RK91 hypotubes in comparison with the 304L (sample 1) and 17-7PH (sample 2) steel based hypotubes. Further, the displacement at kink of the 1RK91 hypotubes heat treated within the 575-635 °C temperature range (samples 7-10) is comparable or better to that of 17- 7PH hypotubes. On the other hand, the 1RK91 hypotubes heat treated at a temperature of 475 °C or 520 °C (samples 4 and 5) exhibit markedly smaller displacement at kink. They also kink at a lower force.

The column strength test (Test 2) results for the samples listed in Table 2 are shown in Figure 4. It can be seen in Figure 4 that the 1RK91 hypotubes heat-treated at 595 °C for 3 hours (sample 8) and heat treated at 635 °C for 1 hour (sample 10) require significantly higher kink force than AISI 304L hypotubes (sample 1). The 1RK91 hypotubes (sample 8 and 10) can also tolerate a comparable or higher displacement before they kink than the AISI 304L hypotube (sample 1). Similarly, the 1RK91 hypotubes heat treated at the temperature range between 555 °C-620 °C from 1 to 3 hours (sample 6 - 9) exhibit higher kink force and comparable or higher displacement at kink than the 17-7PH hypotube (sample 2).

The 1RK91 hypotubes heat treated at a temperature of 475 °C or 520 °C (samples 4 and 5) can only tolerate a small displacement before they kink (see Figure 3 and Figure 4). These small displacement values for use of these 1RK91 steel hypotube samples are unlikely to be acceptable in catheter-based applications.

Turning to Figure 5, a graph of the ten samples (sample summaries) of Table 2 is set forth showing the critical buckling force at point 32 of the graph of Figure 2d according to Test 2. It can be seen that the critical buckling force 32 of the heat treated 1RK91 hypotubes (samples 4 - 10) is higher than that of as-drawn 1RK91 hypotubes (sample 3). The critical buckling force of the heat treated 1RK91 hypotubes (samples 4 - 10) is also higher than that of AISI 304L and 17-7PH steel hypotubes (samples 1 and 2).

Shape set resilience values, h, as tested in Test 3, of the nine selected types of hypotubes listed in Table 2 are shown in Figure 6. The values are illustrated as bars 36 in the graph. Hypotubes with smaller h values have better shape set resilience. It can be seen that the shape set resilience of the heat treated (up to 620 ⁰C) 1RK91 hypotubes (samples 5 - 9) is markedly better than that of the as-drawn 1RK91 hypotubes (sample 3). It is also better than the shape set resilience of the AISI 304L and 17-7PH steel-based hypotubes (samples 1 and 2). This means that the heat treated 1RK91 hypotubes are much less likely to acquire a permanent shape set after being stored in a coiled state, during handling, etc.

In Figure 7, a method 50 according to a preferred embodiment of the present invention provides for the manufacture of steel hypotubes exhibiting enhanced mechanical performance characteristics, particularly a favorable combination of columnar strength and kink resistance. The method of a preferred embodiment includes the followings steps. A tube is formed in step 52 from a strip of steel where a strip is shaped and welded, such as a strip of steel having a composition corresponding to 1RK91 steel. The tube is drawn to form a drawn hypotube, according to step 54. The drawn hypotube is interstage annealed, for example in an atmosphere of an inert gas or in a reducing atmosphere, as shown at step 56. This can provide a hypotube according to some embodiments, but more commonly at least one further drawing and annealing, step is performed. Thus, a further drawing step 58 is performed on the hypotube, followed by a hardening heat treatment 60.

In one example, the method includes the steps of (a) drawing hypotubes from precipitation hardenable steel; and (b) heat treating to harden the as-drawn hypotubes under vacuum and/or an inert atmosphere in the temperature range from about 555 °C to about 635 °C from about 5 minutes to about 10 hours. In one embodiment, the present method provides that the composition of the steel is within the composition range of 1RK91 steel (Nanoflex^®), developed by Sandvik A.B., Sandviken, Sweden.

In a further development, the method provides that the hypotubes are manufactured from a strip of the foregoing steel.

According to preferred embodiments, the strip is shaped, fusion welded, drawn and inter-stage annealed to provide as-drawn hypotubes from the 1RK91 steel with the ultimate tensile strength of that is within the ultimate tensile strength range typically exhibited by half- to full-hard hypotubes manufactures from AISI 300-series austenitic stainless steels.

Preferably, the method provides that the hardening heat treatment time of as- drawn hypotubes from the 1RK91 steel ranges from about 1 hour to about 3 hours.

Industrial Applicability

The present invention has industrial applicability in that it is useful for intravascular delivery of medical devices and treatments in the body. Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

Claims

Patent Claims

1. A method for manufacture of hypotubes, comprising the steps of: drawing precipitation hardenable steel to form a drawn hypotube; and heat treating to harden the drawn hypotube under one of a vacuum and an inert atmosphere, said heat treating being carried out in a temperature range from about 555 °C to about

635 °C from about 5 minutes to about 10 hours.

2. A method as claimed in claim 1, wherein said precipitation hardenable steel is a steel having a composition range of 1RK91 steel.

3. A method as claimed in claim 1, further comprising the step of: forming a tube from a strip of steel prior to said step of drawing.

4. A method as claimed in claim 1, wherein said step of heat treating is performed for up to 3 hours.

5. A method as claimed in claim 1, wherein said step of heat treating is performed in a temperature range of 575 °C to 635 °C.

6. A method as claimed in claim 1, wherein said step of heat treating is performed at a temperature of approximately 595 °C for approximately 3 hours.

7. A method as claimed in claim 1, wherein said step of heat treating is performed at a temperature of approximately 635 °C for approximately 1 hours.

8. A method as claimed in claim 1, wherein said step of heat treating is performed in a temperature range of 555 °C to 620 °C for 1 to 3 hours.

9. A method for manufacture of a hypotube for use in intravascular delivery of medical devices or treatments, comprising the steps of: forming a shaped element from a strip of precipitation hardenable steel, said steel having proportions of components corresponding to 1RK91 steel; fusion welding said shaped element to form a tube; drawing said tube to form a drawn hypotube; annealing said drawn hypotube wherein said step of annealing includes interstage annealing between drawing steps, followed by a further drawing step or steps.

10. A method as claimed in claim 9, wherein the drawn hypotubes are subjected to hardening heat treatment to provide a steel hypotube having a combination of columnar strength and kink resistance, said step of hardening heat treatment being performed in a range of 555 °C to 635 °C.

11. A method as claimed in claim 10, wherein said step of hardening heat treatment time of the drawn hypotube is from about 1 hour to about 3 hours.