AU2402397A - Manufacture of two-way shape memory devices - Google Patents

Abstract

A process is provided for the manufacture of a two-way shape memory alloy an d device. The process of the invention allows a reversible adjustment of the characteristic transformation temperatures, as well as the direction of the two-way shape memory effect, at the final stage of manufacture.

Description

WO 98/49363 PCT/IL97/00134 MANUFACTURE OF TWO-WAY SHAPE MEMORY DEVICES FIELD OF THE INVENTION The present invention generally relates to shape memory alloys (SMA) i.e. alloys which can switch from one shape to another, "memorized" state upon a change in temperature. More specifically, the present invention 5 relates to an SMA which is nickel-titanium based, also known as Nitinol. BACKGROUND OF THE INVENTION Various metal alloys possess the ability to change their shape as a result of a change in temperature. Such SMA can undergo a reversible 10 transformation from a martensitic state, in which the material is relatively soft and deformable, to an austenitic state in which the material possesses super elastic properties and is relatively firm. The transformation from the martensitic state to the austenitic state will be referred to herein as the "austenitic transformation ", and the other transformation, from the austenitic 15 state to the martensitic state, will be referred to herein as the "martensitic transformation". The austenitic transformation occurs over a range of temperature which is higher than the range of temperatures in which the reverse transformation occurs. This means, that once transformed to an austenitic state, an SMA will remain in that state even when cooled to a 20 temperature below that in which the austenitic transformation began, as long as the temperature is above that in which the martensitic transformation begins.

WO 98/49363 PCT/IL97/00134 A particular class of SMAs are alloys of nickel and titanium NiTi alloys. NiTi alloys have found a variety of uses in medical as well as other fields. Medical uses of SMAs, particularly an NiTi-based alloy has been described in U.S. Patents, 4,665,906, 5,067,957, European Patent 5 Application 143,580, U.S. Patent 4,820,298 and many others. For medical uses it is usually desired that the alloy will undergo an austenitic transformation over a narrow, well defined range. For example, a vascular stent of the two-way SMA type, such as that described in European Patent Application, Publication No. 625153, is typically 10 deployed in the body while being in the martensitic state at body tempera ture, and then after heating, it transforms into the austenitic state, and then remains in the austenitic state when cooled to the body temperature. It can be appreciated that if excessive heating to transform the SMA from the martensitic to the austenitic state is required, this can be damaging to the 15 surrounding tissue and is thus undesirable. Thus, it would ideally be desired that the austenitic transformation will begin at a temperature several degrees above body temperature and will be over a temperature range which will not cause tissue damage owing to the excessive heating. 20 GENERAL DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a process for the treatment of a NiTi-based alloy to obtain an alloy with a two-way shape memory effect (SME). It is more specifically an object of the invention to provide such 25 a process to obtain a two-way SME which does not require a multi-cycle "training"to yield a two-way SME. It is still further an object of the invention to provide a process to obtain a two-way SME, with a narrow range of temperatures over which the austenitic transformation occurs. 30 The process of the invention has two aspects. By one aspect, to be referred to herein at times as the "saidfirst aspect", the process yields an alloy with a direction of the austenitic and the martensitic transformations WO 98/49363 PCT/IL97/00134 -3 dictated by the direction of a conditioning transformation in the martensitic state. In accordance with another aspect of the invention, to be referred to herein at times as the "said second aspect", the process yields an alloy with a direction of a martensitic or austenitic transformations which is indepen 5 dent on the deformation introduced in the martensitic state. In the following description and claims the term "NiTi alloy" will be used to denote an alloy comprising primarily nickel and titanium atoms but may contain also trace amounts of other metals). A NiTi alloy has typically the following empiric formula: 10 Nil Tim A, wherein A represents Na, Cu, Fe, Cr or V 1, m, and n representing the proportions of the metal atoms within the alloy, the value of 1, m and n being about as 15 follows: 1= 0.5 m = 0.5-n n = 0.003 to 0.02. 20 In accordance with the invention, there is provided a process for treating a raw NiTi alloy having an initial form to obtain an alloy with a final form in which it exhibits a two-way shape memory effect (SME) whereby it has an austenitic and a martensitic memory state with associated austenitic and martensitic shapes, respectively, the process comprising the 25 steps of: (a) testing the raw NiTi alloy so as to obtain an estimate of the alloy's internal structure, by measuring the difference between As and Af; (b) subjecting the raw NiTi alloy to a first heat treatment based on 30 results obtained in (a) so as to yield alloys with an initial internal structure condition having essentially equal random dislocation density; WO 98/49363 PCT/IL97/00134 - 4 (c) subjecting the alloy to thermo-mechanical treatment (TMT), comprising plastic deformation of the alloy with simultaneous heating (e.g. by warm rolling or warm drawing) to yield, during dynamic ageing process (ageing while depressing), a polygonal 5 sub-grain dislocation structure decorated by precipitation; (d) if the deformation in step (c) did not yield the final form, subjecting the alloy to an intermediate heat treatment to conclude one cycle of sub-grain dislocation structure formation; (e) repeating steps (c) and (d) until yielding said final form; and 10 (f) subjecting the alloy to a final heat treatment and to a memorizing treatment. The TMT, while it may in some instance be performed in a single step, occasionally it is necessary to do it in a few steps if the total strain value could exceed a critical value which may give rise to formation of pre 15 cracks (crack nucleation) in the alloy. The TMT is performed while warming the alloy typically to a temperature of about 0.3 - 0.6 Tm (Tm = melting temperature in 'K). In accordance with one embodiment of the invention, the process comprising the steps of: 20 (a) heating a sample of the raw NiTi alloy, to a temperature of about 450-550'C for about 0.5-2.5 hours, and then testing the sample for temperature difference between As and Af; (b) subjecting the raw NiTi alloy to a first heat treatment based on the Af-A, difference obtained in step (a), as follows: 25 - where the difference is less than about 7oC, heat treating the alloy to a temperature of about 450-500 0 C for about 0.5-1.0 hours; - where the difference is more than about 7 0 C, heat treating the alloy to a temperature of about 510-550 0 C for about 30 1.0-2.5 hours; (c) subjecting the alloy to thermo-mechanical treatment, comprising plastic deforming the alloy at a strain rate, of less than 5 sec

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WO 98/49363 PCT/IL97/00134 with simultaneous internal heating of a portion of the alloy where the deformation occurs to a temperature of about 250-550oC, the deformation of this step being less than 55%, preferably less than 40%; 5 (d) if the deformation in step (c) did not yield the final form, subjecting the alloy to an intermediate heat treatment at a temperature of about 500-550oC, for about 0.5-2 hours, and then repeating step (c); and (e) subjecting the alloy to a final heat treatment and to a memorizing 10 treatment. The particulars of the final heat treatment and the memorizing treatment, are different in said first aspect and in said second aspect. In accordance with said first aspect, this treatment comprises: (i) forming the alloy into the form to be assumed by it in the 15 austenitic state, (ii) subject the alloy to a polygonization heat treatment to yield arrangement of random dislocation, then to solution treatment to release non arranged dislocation from precipitation and provide for their rearrangement and then to an ageing treatment; 20 (iii) deforming the alloy to assume a conditioning form and treating it to memorize said austenitic state, which is the state into which it was formed under (i) above, and a martensitic state, in which the alloy has a martensitic form with an intermediate degree of deformation between the austenitic form and the conditioning 25 form. Preferably, steps (ii) and (iii) of said first aspect, comprise: (ii) subjecting the alloy to a polygonization heat treatment at about 450-550oC for about 0.5-1.5 hours, then to solution treatment at about 600-800'C for about 2-50 mins., and then to an ageing 30 treatment at about 350-5000 for about 0.15-2.5 hours, and (iii) deforming the alloy to assume a conditioning form, the deforma tion being less than about 15%, and preferably less than 7%, and WO 98/49363 PCT/IL97/00134 -6 being performed at a temperature T, which meets the following formula T < Ms + 30 0 C wherein Ms is a temperature where the martensitic transfor 5 mation begins, and then heating the alloy to a temperature of or above that in which the austenitic transformation of the alloy ends. It should be pointed out that although a single cycle of deforma tion in step (iii) above is usually sufficient, it may at times be desired to 10 repeat this cycle one or more times. In accordance with said second aspect, the final heat and memorizing treatment comprises: (i) forming the alloy into a form other than the form to be assumed by it in the austenitic state, 15 (ii) subjecting the alloy first to heat treatment, then to polygonization and solution treatment and then optionally to ageing treatment; (iii) forming the alloy into a form to be assumed by it in the austenit ic state, (iv) subjecting the alloy to a memorizing heat treatment and to an 20 ageing treatment; whereby the alloy is conditioned to memorize an austenitic state in which it has an austenitic form assumed by it in (iii) above, and a martensitic state, wherein it has a martensitic form being a form with an intermediate degree of deformation between the 25 form in which the alloy was formed in (i) above and the austen itic form. By a preferred embodiment of said aspect, steps (ii) and (iv), comprise: (ii) subjecting the alloy to a heat treatment at about 450-500 0 C for 30 about 0.5-2 hours, then subjecting the alloy to polygonization and solution treatment at about 600-800oC for about 2-50 mins., WO 98/49363 PCT/IL97/00134 -7 and then subjecting the alloy to aging treatment at about 350 500oC for about 0-2 hours, (iv) subjecting the alloy to a memorizing heat treatment at about 500 600oC for more than about 10 mins., and then subjecting the 5 alloy to ageing treatment at about 350-500oC for about 0.15-2.5 hours. Following the treatment in accordance with both said first and said second embodiments, Af will be between about 10 to about 60 0 C. In order to increase Af and As, the alloy may then be subjected to ageing heat 10 treatment at a temperature of about 350-500'C. In order to decrease Af and As, the alloy can then be subjected to a solution treatment at a temperature of about 510 to about 800'C. By differential ageing or solution treatment in different portions the alloy will have different temperatures of austenitic transformation. This 15 is at times desired, for example, in the case of a medical stent, to have portions thereof with different transition temperatures of austenitic transformation and/or martensitic transformation. By the above process, SMAs for a variety of applications may be prepared. Examples are medical devices, e.g. various orthopaedic devices, 20 tooth rooth implants, medical stents, intrauterine implants, as well as non medical devices, e.g. tube joints. A process for the preparation of such medical devices, as well as devices prepared by such process also form an aspect of the invention. 25 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 in the drawings shows the relation between Af and the ageing time, in different ageing temperatures. DETAILED DESCRIPTION OF THE INVENTION 30 The temperature range over which the austenitic transformation takes place, is critical in a variety of medical applications. A specific case in point are medical stents such as those made of a two-way shape memory WO 98/49363 PCT/IL97/00134 -8 alloy (SMA) described in European Patent Application No. 626153. Such a shape memory (SM) device, is deployed in a tubular organ at body temperature, and then heated so as to allow the occurrence of the austenitic transformation. Once heated, it remains in the austenitic state in body 5 temperature and supports the wall of the tubular organ. Such SM devices are designed so that the beginning of the austenitic transformation will occur at a temperature at or above 40 0 C. It will however be appreciated, that the range of temperature over which the austenitic transformation occurs should desirably be narrow since excessive heating where the temperature range is 10 large, can cause tissue damage. Furthermore, a narrow temperature range will generally ensure also a more rapid transition from the martensitic to the austenitic states. In the following description the invention will be described at times with particular reference to its application for the preparation of 15 medical SM devices with a narrow range of austenitic transformation. It will however, be appreciated, that the invention is not limited thereto and the application of the invention to the preparation of medical stents is exemplary only. In accordance with the present invention, a raw Niti alloy, which is typically provided by manufacturers in the form of a wire or a rod 20 is first tested for the difference between A s and Af. For this purpose a small sample of the material is taken. Based on the As-Af difference, the alloy, e.g. the wire or the rod, is then subjected to a first heat treatment. Following the first heat treatment, the alloy is subjected to a thermo-mechanical treatment (TMT) where the alloy is simultaneously 25 heated and subjected to a mechanical deformation. In the case of a process intended for the manufacture of a medical SM device, the mechanical deformation typically involves changing the form of the alloy, from an initial form of a wire or rod, to that of a ribbon, band, etc; or alternatively, changing the wire or rod into a wire or rod of a smaller diameter. In order 30 to retain the shape memory effect (SME) of the alloy, the total degree of deformation during the TMT, should be less than 55%, preferably less than WO 98/49363 PCT/IL97/00134 9 40%. Where the total required eventual deformation is more than 55%, the TMT is performed in two steps with an intermediate heat treatment. The thermo-mechanical treatment may, for example, be: warm roling where the alloy is processed to be used as a medical stent; warm 5 drawing where the alloy is processed to be used as an orthopaedic tooth root implant; etc. In warm rolling or drawing the alloy is typically heated to a temperature of about 0.3-0.6 Tm (Tm being the melting temperature in 'K). The heating of the deforming portion must be by electro-stimulation, e.g. at a current density of about 500-2000 A/cm 2 . A big advantage of such a 10 treatment is that in addition to causing a mechanical deformation, it leads also to heating of the pre-cracks with a high dislocation density owing to the relatively high electrical resistance at such pre-cracks which gives rise to a selective overheating at such points and heating of the pre-cracks. Moreover, electro-stimulated warm TMT at the above current density 15 accelerates dislocation reaction, which results in a perfect dislocation subgrain structure formation. Furthermore, the heating electrical current gives rise to a dynamic ageing process with a second phase precipitation on the walls of the subgrain dislocation cells. This structure provides for a very narrow thermal interval of austenitic transformation Af-As for the shape 20 memory alloy, and for a variety of other advantageous properties to be explained below. In an electrically stimulated warm rolling, where the current density decreases below 500 A/cm 2 , or the strain rate of the deformation is above about 5 sec - 1 , there is an increase of the random dislocation density 25 which will decrease the degree of perfection of the subgrain structure. For a narrow Af-As interval, a subgrain structure as perfect as possible is required. Accordingly, with an increase in the random dislocation density there is an increase in the A,-As interval. For example, where the current density is about 400 A/cm 2 , or where strain rate is about 8 sec-1, the Af-As 30 interval after final heat treatment will be about 10-12oC. Furthermore, increasing of the current density to above about 2000 A/cm 2 , leads to a WO 98/49363 PCT/IL97/00134 - 10 recrystallization process, that prevents formation of the necessary subgrain cells with precipitation on the cell walls. The memorizing treatment involves a conditioning step in which microscopic changes within the alloy condition it to "memorize" the two 5 forms which the alloy assumes during its use, that in the martensitic state ("martensitic form" ) and that in the austenitic state ("austenitic form"'). In accordance with said first aspect, the alloy is formed into a shape to be assumed by it in the austenitic state, e.g. in the case of a stent this involves winding on a mandrel having a diameter of a stent in the 10 austenitic state. The alloy is then typically placed in a vacuum or inert atmosphere furnace, in which it is first subjected to a memorizing and internal structure polygonization treatment, at a temperature of about 450 550 0 C for about 0.5-1.5 hours, and then heated to about 600-800oC for about 2-50 mins. During this latter heating, the alloy undergoes a solution 15 treatment with re-arrangement of dislocations which are freed after solution treatment. Subsequently, the alloy is subjected to a final aging treatment at a temperature of about 350-500oC for about 0.15-2.5 hours. The result of the above treatment is a subgrain structure which imparts the alloy with several features. For one, the temperature of the 20 austenitic transformation, Af, can be adjusted within a range of 10-60 0 C with a very narrow interval of Af-AS of about 1-5oC. In case it is desired to decrease Af, the alloy may be subjected to a solution treatment at a temperature of about 510-800'C. In order to achieve a desired Af, both the temperature as well as the ageing time can be 25 controlled. For example, where the Nitinol alloy has after the final heat treatment an As of about 45oC and an Af of about 48 0 C, after a solution treatment at 640 0 C for about 5 mins., the As and Af decrease to about 23oC and 27 0 C, respectively; following solution treatment at 640 0 C for 10 mins.,

A

s and Af decrease to about 11 0 C and 15 0 C, respectively. 30 In order to increase A,, the alloy is subjected to an ageing heat treatment at a temperature of about 350-500oC. Here again, in order to achieve a desired Af, both the temperature as well as the ageing time can be WO 98/49363 PCT/IL97/00134 - 11 controlled. This is demonstrated, for example, in Fig. 1 which gives the relation between the ageing time at two different temperatures (380oC and 480 0 C) and the resulting Af, following a solution treatment at 640 0 C for 20 mins. As can be seen, for example, ageing treatment at 380'C for about 100 5 mins. yields an Af of about 40 0 C, with the same Af being reached with an ageing treatment at 480 0 C of about 40 mins. Ageing at a temperature of about 450'C for about 80 mins. will yield an A s of about 46oC and an Af of about 49oC (not shown in Fig. 1). A unique feature of the process of the invention is the fact that 10 the two-way SME is induced by only one cycle of deformation. In the case of said first aspect of the invention, this may be achieved by deforming the alloy into a conditioning form at a temperature of T < M s + 30 0 C followed by heating to a temperature at or above the Af of the alloy. The deforma tion should be less than 15% and preferably less than 7%. A deformation 15 above 15% will effect the internal structure of the material and yield a total or partial loss of the memory form of the austenitic state. A deformation between 7% and 15% will have only such a partial harming effect. The martensitic memory form which the alloy assumes after the above condi tioning step, is an intermediate form between the austenitic memory form 20 and the conditioning form. The direction of the two-way SME following such memorizing treatment, coincides with the direction in the martensitic state deformation. For example, where a deformation in the martensitic state involves a decrease in diameter, the diameter of the alloy in the martensitic state will be less than that of the austenitic state, and vice versa. 25 Generally, the process in accordance with said first aspect allows a reversible adjustment of the characteristic transformation temperatures as well as the direction of the two-way SME at the final stage of manufacture. A final memorizing treatment in accordance with the second aspect of the invention, gives rise to a two-way SME without the need for 30 a final deformation to induce the two-way SME. This effect is not determined when indirect SME occurs. The second aspect is useful, for example, for the manufacture of a stent with a two-way SME, and the WO 98/49363 PCT/IL97/00134 - 12 description below will refer to this specific embodiment. The Niti ribbon or wire is wound on a mandrel having a diameter equal to 2R 1 constrained and placed into a vacuum furnace, at a temperature of about 450-550'C for about 0.5-1.5 hours, so that internal structure normalization and textural 5 formation takes place. Similarly as above, the alloy is then subjected to solution treatment and structure improvement at a temperature of 600-800oC for 2-50 mins. and then to an ageing treatment at 350-500oC for 0.15-2.5 hours. The ribbon or wire is then rewound on a mandrel with a diameter 2R 2 , which is the diameter to be assumed by the stent in the austenitic state 10 and then subjected to a memorizing and ageing treatment at temperature of 450-550oC for 0.15-2 hours. If the strain of this treatment treat. = 2w(1/R 2 - 1/R 1 ) < 0 (w being the thickness in case of a ribbon and the diameter in case of a wire) the corresponding strain of the two-way SME during cooling C, = /2w(1/R, - 1/R 2 ) > 0 (R, being the diameter of the stent when 15 assuming its martensitic state) and vice versa. As a result of this treatment, there is a very narrow temperature range in which the austenitic transforma tion takes place, Af - As = 1-5 0 C, with a possibility to change Af between 10 and 60'C, similarly as above. The two-way SME in cooling may either coincide or oppose the 20 direction of the deformation in the martensitic state. In case R 2 is larger than R 1 , and R, will be smaller than R 2 , the device shrinks when it is cooled. In case R 2 is less than 0, i.e. a reverse bending, and R 2 is larger than Rw, the device will expand when cooled. Finally, another result of the process of the present invention is 25 a high resistance of the formed alloy, to pitting corrosion and hydrogen embrittlement which may occur in the biological media with their relatively high chlorine ion content. The invention will now be illustrated further by several specific examples. 30 WO 98/49363 PCT/IL97/00134 - 13 Example 1 - Preparation of a Biliary Stent The starting material was a super-elastic Niti wire, with a diameter of 1.5 mm. The Ti and Ni content of the alloy was 50 to 50.8 at % (at % = % atoms out of the total number of atoms in the alloy) and 5 49.1 at %, respectively. A sample of the wire was treated at a temperature of 500'C for 1.5 hours and the temperature interval Af-A was determined and was found to be 15'C. The wire was then subjected to a first heat treatment at 550 0 C for two hours, and then to a thermo-mechanical electro-simulated treatment, 10 with the current density being 900 A/cm 2 and the strain rate being 0.3 sec'. The TMT was repeated three times with two intermediate heat treatments at 500 0 C for one hour each. The ribbon thickness was eventually reduced to 0.25 mm. The ribbon was then wound and constrained on a mandrel having 15 a diameter of 8 mm, and placed into a vacuum furnace and heated to 500'C for 0.6 hours, and then subjected to a solution treatment at 650 0 C for 30 mins. This was followed by an aging treatment at 400'C for 1 hour. The spiral coiled stent which was obtained had an As of 40 0 C and an Af of 43°C. 20 The stent was then wound on a 3 mm. diameter mandrel at a temperature of 25oC and heated to above 43oC for shape recovery. Thus, a stent with a two-way SME was obtained, having an austenitic memory form in which its diameter was 8 mm, a martensitic memory form to which it shrank when cooled below 25 0 C in which it had a diameter of 7.3 mm. 25 In order to install the stent, in situ within the body, it is wound on a catheter, and then inserted into the desired place within the bile duct. The stent is then activated by raising its temperature to more than 43oC. To remove the stent it has to be cooled below 25oC and after shrinking it can be pulled away. 30 WO 98/49363 PCT/IL97/00134 - 14 Example 2 - Esophageal Stent A stent was prepared from the same TiNi wire as used in Example 1. The wire was subjected to a first heat treatment and then to a TMT, similarly as described in Example 1, the difference being that the final 5 thickness of the wire which was obtained was 0.28 mm. The ribbon was then wound on a mandrel having a diameter of 70 mm, was constrained and then heated to 500 0 C for 1 hour and then to a solution treatment at 650 0 C for 20 mins. The ribbon was then wound on a mandrel having a diameter of 16 mm., was constrained and subjected to a 10 memorizing treatment at 520 0 C for 30 mins., and then to aging treatment at 400 0 C for 2 hours. The stent which was obtained after this procedure had the following parameters: As = 42 0 C; Af = 45 0 C; temperature of martensitic transformation being 27 0 C, with the stent expanding when cooling from a diameter of 16 mm. which it had in the austenitic state to a 15 diameter of 18 mm. in the martensitic state. For deployment, the stent is wound on a catheter with a diameter of 5 mm, inserted into the desired place within the esophageal tract and is activated by heating above 45 0 C. When the stent is cooled, it expands which prevents the stent from falling into the stomach. 20 Example 3 - Esophageal Stent A stent was prepared in a similar manner as that of Example 2, with the difference being that the ribbon was wound on a mandrel having a diameter of 5 mm. and after heat treatment was rewound on the mandrel 25 with the opposite direction. After heat treatment, similarly as in Example 2, the stent expands when cooled from a diameter of 16 mm. to a diameter of 25 mm.

WO 98/49363 PCT/IL97/00134 - 15 Example 4 - Shape Memory Force Element for Orthopedic Compres sion Screw Starting material was 1.5 mm diameter NiTi wire (the composi 5 tion of the alloy was 50.5 at % Ni and 49.5 at % Ti). The wire was subjected to a first heat treatment and then to a TMT, similarly as described in Example 1 (however using warm drawing instead of warm rolling). The wire was then heat treated in straight constrained condition under a temperature of 500 0 C for 0.5 hours and then subjected to a solution 10 treatment at 650'C for 20 mins. The wire was then released and subjected to a memorizing treatment at 520'C for 30 mins., which was followed by an ageing treatment at 450'C for 1 hour. After wire elongation from 20 to 21 mm, shape memory effect was obtained with As = 39 0 C and Af = 41 0 C and after cooling up to 25 0 C two way shape memory effect occurs just after 15 heat treatment (without training) and increases after the training process (stretching-heating). Example 5 - Shape Memory Medical Staples Starting material and treatment was similar to that of Example 4. 20 The final diameter which was achieved (by warm drawing) was 0.25 mm. The wire was constrained in the necessary shape and heat treated after TMT at 520 0 C for 0.5 hours, solution treatment at 680'C for 10 mins. and ageing at 450 0 C for 1.5 hours. After staple bending, shape memory effect was obtained with 25 As = 420C and Af = 45oC. Example 6 - Tooth Root Implants The starting material was a super-elastic Nitinol (50.8 at % Ni) rod with diameter of 10 mm. The rod was subjected to a first het treatment 30 at 550 0 C for 2 hours and then to a TMT - drawing at 500'C, with a strain rate of 0.5 sec

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'. The TMT was repeated 2 times with intermediate heat treatment at 500 0 C for 1 hour. The rod had a final diameter of 6.0 mm.

WO 98/49363 PCT/IL97/00134 - 16 The rod was machined into a shape of a tooth root implant with 6 force segments (legs) for anchoring into the jaw bone. The leg's length wasd 3, 4 and 5mm for different implants. The implant was then subjected to poligonization heat treatment at 500'C, for 1 hour, then the implant's legs 5 were distorted on the mandrel, following which the implant was subjected to heat treatment at 650 0 C, for 30 min and to ageing at 480 0 C for 1,5 hour. Then the implant's legs were forced together with a conic cup, (from distorted diameter 5.0mm to closed conditions with diameter 3,0mm). While heating the implant, it causes the legs to open at temperatures: 10 As=38 C and A,= 4 2 C, that yields very gentle pressure on the jaw bone and very safe implant activation. A single cycle of straightening of the implant's legs and subsequent heating induces two way SME in the direction of joining of the legs with cooling, which feature is useful for removing the implant. 15 Example 7 - Two way SM tube coupling with narrow As-Af interval A 10 mm NiTi rod identical to that which served as the starting material in Example 6 was treated in the same manner as described in Example 6 to yield a rod with a diameter of 6 mm. This rod was then 20 machined into the shape of the hollow cylinder with an internal diameter (ID) of 4.4 mm. Then the cylinder was subjected to poligonisation and solution heat treatment: 500'C for 1 hour and then of 680 0 C for 20 min. It was then cooled, expended on a mandrel with diameter of 4.5mm and subjected to memorising and ageing heat treatment: 530 0 C for 30 min and 25 430 0 C for 40 min. The tube coupling was then cooled and expanded on the mandrel to yield an internal diameter of 4.75mm. The coupling joins tubes after heating (As=15C, Af=18 0 C) and it has two way SME in direction of reduction of ID. Thus, even with cooling it keeps pressure on the joined tubes. In comparison, conventional 30 couplings where the two way SME is induced during installation (expansion and heating) in direction of expansion, cooling results in loosening of the coupling.

Claims (3)

1. A process for treating a raw NiTi alloy having an initial form to obtain an alloy with a final form in which it exhibits a two-way shape 5 memory effect (SME) whereby it has an austenitic and a martensitic memory state with associated austenitic and martensitic shapes, respectively, the process comprising the steps of: (a) testing the raw NiTi-based alloy so as to obtain an estimate of the alloy's internal structure, by measuring the difference between 10 As and Af; (b) subjecting the raw NiTi alloy to a first heat treatment based on results obtained in (a) so as to yield alloys with an initial internal structure condition having essentially equal random dislocation density; 15 (c) subjecting the alloy to thermo-mechanical treatment (TMT), comprising plastic deformation of the alloy with simultaneous heating during a dynamic ageing process, to yield a polygonal sub-grain dislocation structure decorated by precipitation; (d) if the deformation in step (c) did not yield the final form, 20 subjecting the alloy to an intermediate heat treatment to conclude one cycle of sub-grain dislocation structure formation; (e) repeating steps (c) and (d) until yielding said final form; and (f) subjecting the alloy to a final heat treatment and to a memorizing treatment. 25 2. A process according to Claim 1, comprising the steps of: (a) heating a sample of the raw Niti alloy, to a temperature of about

450-550'C for about 0.5-2.5 hours, and then testing the sample for temperature difference between As, and Af; (b) subjecting the raw Niti alloy to a first heat treatment based on the 30 Af-As difference obtained in step (a), as follows: WO 98/49363 PCT/IL97/00134 - 18 - where the difference is less than about 7oC, heat treating the alloy to a temperature of about 450-500 0 C for about 0.5-1.0 hours; - where the difference is more than about 7oC, heat treating 5 the alloy to a temperature of about 510-550 0 C for about 1.0-2.5 hours; (c) subjecting the alloy to TMT, comprising plastic deforming the alloy at a strain rate, of less than 5 sec - 1 , with simultaneous internal heating of a portion of the alloy where the deformation 10 occurs to a temperature of about 250-550oC, the deformation of this step being less than 55%, preferably less than 40%; (d) if the deformation in step (c) did not yield the final form, subjecting the alloy to an intermediate heat treatment at a temperature of about 500-550oC, for about 0.5-2 hours, and then 15 repeating step (c); and (e) subjecting the alloy to a final heat treatment and to a memorizing treatment. 3. A process according to Claim 1 or 2, wherein the final heat and memorizing treatment comprises: 20 (i) forming the alloy into the form to be assumed by it in the austenitic state, (ii) subject the alloy to a polygonization heat treatment to yield arrangement of random dislocation, then to solution treatment to release non arranged dislocation from precipitation and provide 25 for their rearrangement and then to an ageing treatment; (iii) deforming the alloy to assume a conditioning form and treating it to memorize said austenitic state, which is the state into which it was formed under (i) above, and a martensitic state, in which the alloy has a martensitic form with an intermediate degree of 30 deformation between the austenitic form and the conditioning form. WO 98/49363 PCT/IL97/00134 - 19 4. A process according to Claim 3, wherein the final heat heat and memorizing treatment comprises: (i) forming the alloy into the form to be assumed by it in the austenitic state; 5 (ii) subjecting the alloy to a polygonization heat treatment at about 450-550 0 C for about 0.5-1.5 hours, then to solution treatment at about 600-800 0 C for about 2-50 mins., and then to an aging treatment at about 350-5000 for about 0.15-2.5 hours, and (iii) deforming the alloy to assume a conditioning form, the deforma 10 tion being less than about 15%, and preferably less than 7%, and being performed at a temperature T, which meets the following formula T < Ms + 30 0 C wherein Ms is a temperature where the martensitic transformation 15 begins, and then heating the alloy to a temperature of or above that in which the austenitic transformation of the alloy ends. 5. A process according to Claim 4, wherein the deformation of the alloy to asume the conditioning form in step (e) (iii), is less than about 7%. 6. A process according to Claim 1 or 2, wherein the final heat and 20 memorizing treatment comprises: (i) forming the alloy into a form other than the form to be assumed by it in the austenitic state, (ii) subjecting the alloy first to heat treatment, then to polygonization and solution treatment and then optionally to ageing treatment; 25 (iii) forming the alloy into a form to be assumed by it in the austenit ic state, (iv) subjecting the alloy to a memorizing heat treatment and to an ageing treatment; whereby the alloy is conditioned to memorize an austenitic state 30 in which it has an austenitic form assumed by it in (iii) above, and a martensitic state, wherein it has a martensitic form being a form with an intermediate degree of deformation between the WO 98/49363 PCT/IL97/00134 - 20 form in which the alloy was formed in (i) above and the austen itic form. 7. A process according to Claim 6, wherein the final heat and memorizing treatment comprising: 5 (i) forming the alloy into a form other than the form to be assumed by it in the austenitic state, (ii) subjecting the alloy to a heat treatment at about 450-500oC for about 0.5-2 hours, then subjecting the alloy to polygonization and solution treatment at about 600-800 0 C for about 2-50 mins., 10 and then subjecting the alloy to aging treatment at about 350 500 0 C for about 0-2 hours, (iii) forming the alloy into a form to be assumed by it in the austenit ic state, and (iv) subjecting the alloy to a memorizing heat treatment at about 500 15 600 0 C for more than about 10 mins., and then subjecting the alloy to aging treatment at about 350-500oC for about 0.15-2.5 hours. 8. A process according to Claim 4 or 7, comprising: (a) adjusting the temperature in which the austenitic transformation 20 occurs, by either - an aging treatment at a temperature of about 350-500 0 C, to increase the temperature in which the austenitic transforma tion occurs, or - a solution treatment at a temperature of about 510-800 0 C, 25 to decrease the temperature in which the austenitic transfor mation occurs. 9. A process according to Claim 2 or 6, wherein the deformation in step (c) is less than 40%. 10. A process according to Claim 4 or 7, wherein the internal heating 30 in step (c) comprises electro-stimulation with a current density of about

500-2000 A/cm 2 WO 98/49363 PCT/IL97/00134 -21 11. A process for preparing a medical device comprising a shape memory alloy (SMA) embodying a two-way shape memory effect, comprising treating the SMA in accordance with the process defined in Claim 1 or 4. 5 12. A process according to Claim 11, wherein said medical device is a stent. 13. A process for the manufacture of a medical stent from a Niti alloy, being a wire having a first diameter, the stent having either the form of a wire with a second diameter or a form of a band, the stent exhibiting 10 a two-way shape memory effect (SME) having an austenitic and a martensitic memory state with associated austenitic and martensitic shapes, respectively, the process comprising the steps of: (a) heating a sample of the Niti wire to a temperature of about 450 550'C for about 0.5-2.5 hours, and then testing the sample for 15 temperature difference between As and A,, wherein As is a temperature wherein austenitic transformation, namely transfor mation between the martensitic to the austenitic state, begins, and Af is a temperature where the austenitic transformation ends; (b) subjecting the wire to a first heat treatment based on the Af-A, 20 difference obtained in step (a), as follows: - where the difference is less than about 7 0 C, heat treating the wire to a temperature of about 450-500'C for about 0.5-1.0 hours; - where the difference is more than about 7oC, heat treating 25 the wire to a temperature of about 510-550 0 C for about 1.0-2.5 hours; (c) subjecting the wire to a thermo-mechanical treatment, comprising warm rolling of the wire at a strain rate of less than 5 sec - 1 , with simultaneous internal heating of a portion of the wire where the 30 deformation occurs, the heating by electro-stimulation at a current density of about 500-2000 A/cm 2 , the deformation in this step being less than 55%; WO 98/49363 PCT/IL97/00134 -22 (d) where the deformation in step (c) did not yield a cross-sectional shape of the final form, subjecting wire to an intermediate heat treatment at a temperature of about 500-550'C, for about 0.5-2 hours and then repeating step (c); and 5 (e) subjecting the wire to a final heat treatment and to a memorizing treatment, which comprises: (i) winding the wire or band obtained in step (c) on a mandrel having a diameter to be assumed by the stent in the austen itic state, 10 (ii) subjecting the wire to a polygonization heat treatment at about 450-550oC for about 0.5-1.5 hours, then to solution treatment at about 600-800 0 C for about 2-50 mins., and then to an aging treatment at about 350-500 0 C for about 0.15-2.5 hours, 15 (iii) deforming the wire by winding it on a mandrel having a conditioning diameter, the deformation being less than about 7%, and being performed at a temperature T, which meets the following formula T < Ms + 30 0 C 20 wherein Ms is a temperature where the martensitic transfor mation begins, and then heating the wire or band to a temperature at or above that in which the austenitic trans formation ends; whereby a stent is obtained with an austenitic state in which it 25 has a diameter assumed in (i) above and a martensitic state in which it has a diameter which is an intermediate diameter between the conditioning diameter and the austenitic diameter. 14. A process for the manufacture of a medical stent from a Niti wire having a first diameter, the stent having either the form of a wire with a 30 second diameter or a form of a band, the stent exhibiting a two-way shape memory effect (SME) having an austenitic and a martensitic memory state WO 98/49363 PCT/IL97/00134 - 23 with associated austenitic and martensitic shapes, respectively, the process comprising the steps of: (a) heating a sample of the Nitinol wire to a temperature of about 450-550'C for about 0.5-2.5 hours, and then testing the sample 5 for temperature difference between A, and Af, wherein As is a temperature wherein austenitic transformation, namely transfor mation between the martensitic to the austenitic state, begins, and Af is a temperature where the austenitic transformation ends; (b) subjecting the wire to a first heat treatment based on the Af-As 10 difference obtained in step (a), as follows: - where the difference is less than about 7 0 C, heat treating the wire to a temperature of about 450-500 0 C for about 0.5-1.0 hours; - where the difference is more than about 7oC, heat treating 15 the wire to a temperature of about 510-550 0 C for about 1.0-2.5 hours; (c) subjecting the wire to a thermo-mechanical treatment, comprising warm rolling of the wire at a strain rate of less than 5 sec - 1 , with simultaneous internal heating of a portion of the wire where the 20 deformation occurs, by electro-stimulation at a current density of about 500-2000 A/cm 2 , the deformation in this step being less than 55%; (d) where the deformation in step (c) did not yield a cross-sectional shape of the final form, subjecting wire to an intermediate heat 25 treatment at a temperature of about 500-550 0 C, for about 0.-2 hours and then repeating step (c); and (e) subjecting the wire to a final heat treatment and to a memorizing treatment, which comprises: (i) winding the wire or band obtained in step (c) on a mandrel 30 having a conditioning diameter being different than the diameter to be assumed by the stent in the austenitic state, WO 98/49363 PCT/IL97/00134 - 24 (ii) subjecting the wire to a heat treatment at about 450-500 0 C for about 0.5-2 hours, then to polygonization and solution treatment at about 600-800 0 C for about 2-50 mins., and then to aging treatment at about 350-500'C for about 5 0-2 hours, (iii) winding the wire or band on a mandrel having a diameter to be assumed by the stent in the austenitic state, (iv) subjecting the alloy to a memorizing heat treatment at about 500-600oC for more than about 10 mins., and then to aging 10 treatment at about 350-500'C for about 0.15-2.15 hours; whereby a stent is obtained having an austenitic state with a diameter into which the wire was formed in step (iii), and a martensitic state in which the stent has a diameter which is an intermediate diameter between the conditioning diameter and the 15 diameter of the stent in the austenitic state. 15. A process for the manufacture of tooth root implant from a NiTi alloy, exhibiting a two way SME having an austenitic and a martensitic memory states with associated austenitic and martensitic shapes, respectively, the process comprising the steps of: 20 (a) heating a sample of a NiTi rod to a temperature of about 450 550oC for about 0.5-2.5 hours, and then testing the sample for temperature difference between A, and A,, wherein As is a temperature wherein austenitic transformation, namely transform ation between the martensitic to the austenitic state, begins, and 25 Af is a temperture where the austenitic transformation ends; (b) subjecting the rod to a first heat treatment based on the Af-A, difference obtained in step (a), as follows: - where the idfference is less than about 7oC, heat treating the wire to a temperature of about 450-500 0 C for about 0.5-1.0 30 hours; WO 98/49363 PCT/IL97/00134 - 25 - where the difference is more than about 7oC, heat treating the wire to a temperature of about 510-550'C for about 1.0-2.5 hours. (c) subjecting the rod to a TMT comprising warm drawing with a 5 strain rate less than 5sec - 1 with simultaneous heating, the total strain in this step being less than 55%; (d) where the deformation in step (c) did not yield a cross-sectional shape of the final form, subjecting the rod to an intermediate heat treatment at a temperature of about 500-550'C, for about 0.5-2 10 hours and then repeating step (c); (e) maching the rod to yield the shape of the implant; (f) subjecting the implant to the final heat treatment and to a memorising treatment, which comprises: (i) expanding implant's force segments to a diameter to be 15 assumed by the implant in the austenitic state, (ii) subjecting the implant to a poligonization heat treatment at about 450-550oC for about 0.5-1.5 hours, then to solution treatment at about 600-800 0 C for about 2-50 min, and then to an ageing treatment at about 350-500 0 C for about 0.15 20 2.5 hours, (iii) deforming the implant force segments to a conditioning diameter with a stain less than about 7% and being per formed at temperature T < M, + 30 0 C, wherein Ms is a temperature where the martensitic transformation begins and 25 then heating the implant to a temperature at or above that in which the austeritic transformation ends; whereby an implant is obtained with an austenitic state in which it has a diameter assumed in (i) above and a martensitic state in which it has a diameter, which is an intermediate diameter between the conditioning 30 diameter and the austenitic diameter. 16. A process for the manufacture of tube coupling from a NiTi alloy, exhibiting a two sway SME having an austenitic and a martensitic memory WO 98/49363 PCT/IL97/00134 - 26 stakes with associated austenitic and martensitic shapes, respectively, the process comprising the steps of: (a) heating a sample of a NiTi rod to a temperature of about 450 550'C for about 0.5-2.5 hours, and then testing the sample for 5 temperature difference between As and Af, wherein As is a temperature wherein austenitic transformation, namely transfor mation between the martensitic to the austenitic state, begins, and Af is a temperature where the austenitic transformation ends; (b) subjecting the rod to a first heat treatment based on the Af-A s 10 difference obtained in step (*a), as follows: - where the difference is less than about 7C, heat treating the wire to a temperature of about 450-500 0 C for about 0.5-1.0 hours; - where the diference is more than about 7oC, heat treating 15 the wire to a temperature of about 510-550 0 C for about 1.0-2.5 hours; (c) subjecting the rod to a TMT comprising warm drawing with a strain rate less than 5sec - 1 with simultaneous heating, the total strain in this step being less than 55%; 20 (d) where the deformation in step (c) did not yield a cross-sectional shape of the final form, subjecting the rod to an intermediate heat treatment at a temperature of about 500-550oC, for about 0.5-2 hours and then repeating step (c); and (e) machining the rod to yield the shape of the implant; 25 (f) subjecting the implant to the final heat treatment and to a memorising treatment, which comprises: (i) subjecting the coupling to a poligonizatin heat treatment at bout 450-550'C for about 0.5-1.5 hours, then to solution treatment at about 600-800'C for about 2-50 min and then 30 to an ageing treatment at about 350-500 0 C for about 0-2.5 hours, WO 98/49363 PCT/IL97/00134 - 27 (ii) expanding the coupling to a diameter to be assumed by the coupling in the austenitic state, (iii) subjecting the coupling to a memorizing heat treatment at about 500-600 0 C for more than about 10 mins, and then to 5 an ageing treatment at about 350-500'C for about 0.15-2.5 hours, whereby a coupling is obtained having an austenitic with a diameter into which it was formed in step (ii) and a martensitic state in which the oupling has a diameter which is intermediate diameter between the conditioning ID 10 and the diameter of the coupling in the austenitic state.