Linear elastic member

Details Linear elastic member Linear elastic member

1999-08-04

2001-01-02

O'Connor, Cary (Department: 3736)

Surgery

Diagnostic testing

Flexible catheter guide

C604S528000

Utility Patent

active

06168571

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to improved processing methods for linear elastic alloys and applications of these alloys. General applications of these 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, shafts for balloon catheters and cytology brushes, stents, braid within catheters 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. Those skilled in the art will recognize the benefits of applying the present invention to similar fields not discussed herein.
BACKGROUND OF THE INVENTION
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
CuSn
~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
~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 super-elastic 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° C. (950 to 1450° 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 t

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