The term shape memory alloy (SMA) is applied to a group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory. Some materials also undergo a change in shape upon re-cooling. These materials have a two-way shape memory. A relatively wide variety of alloys are known to exhibit the shape memory effect. They include:
______________________________________ Alloy Composition ______________________________________ AgCd 44-49 at. % Cd AuCd 46.5-50 at. % Cd CuAlNi 14-14.5 wt. % Al 3-4.5 wt. % Ni CuSu .about.15 at. % Sn CuZn 38.5-41.5 wt. % Zn CuZnX a few wt. % X X = Si, Sn, Al InTi 18-23 at. % Ti NiAl 36-38 at. % Al NiTi 49-51 at. % Ni FePt .about.25 at. % Pt MnCu 5-35 at. % Cu FeMnSi 32 wt. % Mn 6 wt. % Si ______________________________________
To date only the nickel-titanium alloys (NiTi or Nitinol) and copper-base alloys such, as CuZnAl and CuAlNi, can recover enough strain or generate enough force upon changing shape to be of commercial interest.
Shape memory alloys may be characterized by several general methods including chemical, thermochemical, crystallographic, and stress/strain. Chemical analysis of a shape memory alloy may be further defined as an alloy that yields a thermoelastic martensite. In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent phase.
Crystallographic analysis of a shape memory alloy shows a herringbone structure of athermal martensites essentially consisting of twin-related self-accommodating variants. The shape change among the variants tends to cause them to eliminate each other and, as a result, little macroscopic strain is generated. In the case of stress-induced martensites, or when stressing a self-accommodating structure, the variant that can transform and yield the greatest shape change in the direction of the applied stress is stabilized and becomes dominant. This process creates a macroscopic strain which is recoverable as the crystal structure reverts to austenite during reverse transformation.
In addition to their ability to return to some previously defined shape or size when subjected to an appropriate thermal procedure, shape memory alloys also have the useful mechanical characteristic of being highly elastic or super-elastic. Super-elastic metals can appear to be stressed beyond their elastic yield point but still return to their original shape after the stress is removed. As can be seen from the stress-strain diagram of FIG. 1, a super-elastic metal that is stressed has a first portion Q where the stress and the strain are proportional. The diagram further shows the classic flagged shaped curve of a superelastic alloy with the transition point X marking the beginning of plateau P where the metal continues to elongate while the stress is unchanged. Finally, if the stress is removed, the alloy will return to its original shape without any plastic deformation. Super-elastic alloys are then able to take more of a load without permanent deformation than conventional metals.
Elastic metals or super-elastic precursors may also be shape-memory alloys but elastic metals do not have the stress-strain plateau of a super-elastic alloy. FIG. 2 is a stress-strain diagram of an elastic metal which again shows a proportional region Q. Similar to conventional metals, an elastic metal would break if stressed much beyond its yield point Y. However, unlike a conventional metal, an elastic metal will take much more strain than conventional metals before yielding. Elastic metals then are able to take a large load with only a small amount of permanent deformation and are generally stiffer than super-elastic metals.
To date NiTi shape memory alloys have been the most commercially successful. Processing of NiTi shape memory alloys include selective work hardening, which can exceed 50% reduction in some cases. Proper heat treatment can also greatly improve the ease with which the martensite is deformed, give an austenite with much greater strength, and create material that spontaneously moves itself both on heating and on cooling (two-way shape memory). One of the biggest challenges in using this family of alloys is in developing the proper processing procedures to yield the properties desired.
Because of the reactivity of the titanium in these alloys, all melting of them must be done in a vacuum or an inert atmosphere. Methods such as plasma-arc melting, electron-beam melting, and vacuum-induction melting are all used commercially. After ingots are melted, standard hot-forming processes such as forging, bar rolling, and extrusion can be used for initial breakdown. The alloys react slowly with air, so hot working in air is quite successful. Most cold-working processes can also be applied to these alloys, but they work harden extremely rapidly, and frequent annealing is required. Wire drawing is probably the most widely used of the techniques, and excellent surface properties and sizes as small as 0.05 mm (0.002 in.) are made routinely. Super-elastic wires have a relatively high kink resistance but lack both axial and torsional stiffness. Linear elastic wires have slightly lower kink resistance than super-elastic wires but higher torsional rigidity. Unfortunately, elastic wires also are very difficult to keep straight during processing.
Fabrication of articles from the NiTi alloys can usually be done with care, but some of the normal processes are difficult. Machining by turning or milling is very difficult except with special tools and practices. Welding, brazing, or soldering the alloys is also generally difficult. Heat treating to impart the desired memory shape is often done at 500 to 800.degree. C. (950 to 1450.degree. F.). The SMA component may need to be restrained in the desired memory shape during the heat treatment; otherwise, it may not remain there.
The most common medical use of these materials to date is as core wires in guide wires. Guide wires are used in minimally invasive medical procedures. Typically, a guide wire is inserted into an access point and then advanced through a body lumen, such as a blood vessel, to a site to be treated. Another medical device that actually performs the treatment is then advanced over the guide wire.
A typical guide wire 20 is shown in FIG. 3. Guide wire 20 has a core 25 and a polymer sleeve 10. Best performance in guide wire cores is based on a combination of factors which include a small diameter, smooth finish, straightness, pushability, kink resistance, and torqueability. The diameter of the wire core ultimately determines the diameter of the lumen that can be treated. For example, in the neurovasculature where the vessels may be extremely small, having a small diameter wire core is very important.
The finish of a guide wire often affects the performance of therapeutic devices that are slid over the wire since a rough surface will increase the drag on any device. Surface friction may be reduced by polishing or through the use of lubricious coatings. Similarly, it is important that the wire core and ultimately the guide wire be as straight as possible to reduce the number of points where the guide wire contacts the therapeutic device. Wire cores may be mechanically straightened or ground to remove uneven surfaces.
Pushability, kink resistance, and torqueability are closely related and important features of a guidewire. It is important that force applied at the proximal end of the guide wire is completely transferred to the distal end of the guide wire. Very stiff wire cores often provide good pushability (axial rigidity) but poor kink resistance. Kink resistance is measured by the ability of the guide wire to be forced into a relatively tight bend radius without permanently deforming the wire core. Finally, torqueability is closely related to the torsional rigidity of the wire core. That is, how well rotation imparted to the proximal end of the guide wire is translated to the distal end of the guide wire.
Conventional guide wire cores are made of carbon steel or stainless steel. More recently, guide wire cores made of super-elastic alloys have been used. A super-elastic or pseudoelastic metal guide wire core was taught in U.S. Pat. No. 4,925,445 to Sakamoto. In U.S. Pat. Nos. 5,238,004 to Sahatian and U.S. Pat. No. 5,230,348 to Ishibe the use of an elastic metal alloy was taught. Sahatian '004 further teaches that elastic metals may be heat treated to form bends in the wire core and that centerless grinding may be used to create certain wire core profiles.
It is well known in the art to centerless grind guide wire cores to provide desired core profiles. Generally, centerless grinders are used to grind the outer surface of the wire core. The object of the grinding operation is to produce a wire core that is round, straight and has a diameter and surface finish in accordance with given specifications at any given cross-section along its length.
Typically, a wire core is fed into a centerless grinder at one end and guided between two grinding wheels that rotate in the same direction at different speeds, known as the work wheel and the regulating wheel. The wire core rotates as a result of its contact with the regulating wheel and is ground to a specified diameter dictated by the distance between the faces of the two grinding wheels. One of the grinding wheels, typically the regulating wheel, can be moved so that the distance between the faces of the grinding wheels may be varied during the grinding process. The wire core advances through the grinding machine as a result of its contact with the grinding wheels. Specifically, one of the grinding wheels, typically the regulating wheel, rotates along an axis that is almost parallel to the axis of rotation of the wire core being ground, but slightly skewed in a vertical plane, so that its contact with the wire causes the wire to move forward through the machine.
A number of factors can affect the rate at which the wire moves through the grinding machine and the rate at which wheels must be changed. For example, temperature, regulating wheel RPM, regulating wheel tilt angle, slippage, type of coolant used, and grinding wheel material may affect feed rate, wire core diameter, wire core material, and wire core uniformity. As may be appreciated from the description of the centerless grinding process, having a straight and preferably uniform wire is essential to effective centerless grinding.
As previously described, a typical linear elastic wire is not straight and is, in fact, roughly sinusoidal following the typical processing regime. As can be seen in FIG. 3, it is desirable to grind a taper 15 into the distal end of the guide wire core to make the wire more flexible near its distal tip. Attempts to grind linear elastic wire cores have proven to be destructive to the grinding equipment since the wire core is not straight and is relatively stiff. It is therefore desirable to provide a linear elastic wire for use as a guide wire core which is straight enough to be easily ground to a desired shape.
In addition to guide wires, many other devices may benefit from the characteristics of a linear elastic elongate member. General applications of elastic alloys may include medical wires and hypotubes. Specific applications of wires may include but are not limited to guide wires, pull wires in catheters and endoscopes, wire stents and drive shafts for ultrasound or atherectomy/thrombectomy catheters. Specific applications of hypotubes may include but are not limited to guide wires, stents, needles, needle stylets, drive shafts and catheter components. It may therefore be desirable to provide a linear elastic wire or hypotube which is easily ground to a desired shape and to use that wire or hypotube in any of the applications described above.
In U.S. Pat. No. 4,445,509 to Auth a rotary atherectomy device is taught. This device essential consists of a catheter with a bur located on the distal end. Within the catheter is a drive shaft which rotates the bur at high speed, greater than 20,000 RPM. The proximal end of the drive shaft is connected to a motor which powers the entire assembly.
Another common drive shaft application is in ultrasound catheters. In U.S. Pat. No. 4,794,931 to Yock a flexible drive shaft connects a proximal power source outside the body, through a catheter, to a distal ultrasonic transducer. The transducer is rotated and provides an ultrasonographic image of the interior of a body lumen. In this and other well known drive shaft applications the shaft must be flexible, have high kink resistance, and excellent torsional rigidity.
Small tubes or hypotubes are also commonly used in the medical device industry. In some applications like drive shafts and guide wires, hypotubes perform a similar function to wires but also have the advantage of a hollow space to perform some other action. As an example U.S. Pat. No. 4,953,553 to Tremulis teaches a hypotube used as a guide wire which may further be used to measure pressure insitu or to infuse liquids. Needle stylets are similar to guide wires in that they are advanced through other medical devices. Stylets are commonly used to add support to the medical device that is slid over it.
Another application for medical hypotubes is in catheter shafts. Examples of such catheters include but are not limited to catheters for angiography or catheters for dilating blood vessels. Angiography catheters typically have a main body formed of a somewhat soft thermoplastic resin and a rigidity imparting member consisting of a metallic braided wire (generally a stainless-steel wire). The rigidity imparting member is disposed around the main body such that kinking of the catheter is inhibited while its high flexibility is maintained. The rigidity imparting member further improves the torque transmission efficiency.
Design of balloon catheters is similar with the addition of a distal inflatable member for dilating a stenosis portion in a blood vessel. These catheters often comprise an inner tube made of a flexible polymer, an outer tube made of a flexible polymer and disposed coaxially with the inner tube, and a balloon attached to the outer tube at the balloon's proximal end and attached to the inner tube at the balloon's distal end. The inner or outer tube may be provided with a rigidity imparting member consisting of a metallic wire braid (e.g., a stainless-steel wire).
The rigidity imparting member used in the above catheters can inhibit kinking and improve torque transmission efficiency to some extent. Rigidity, pushability, and torque transmission were further improved by Peters et al. in U.S. Pat. No. 5,549,552 which teaches the use of a super-elastic metal hypotube as the rigidity imparting member described above.
Yet another application of super-elastic metal hypotubes is in stents. It is well known in the art to make stents of Nitinol. These stents are often made by laser cutting a Nitinol hypotube and then further processing the cut stent depending on specific applications or desired geometries.
In each of the applications previously described and in many other related applications it is therefore desirable to provide a metal alloy which is flexible, axially and torsionally rigid, kink resistant and straight.