United States Patent c19J
`Flomenblit et al.
`
`I
`
`11111
`Ill
`US005624508A
`[11] Patent Number:
`[45] Date of Patent:
`
`11111111
`
`1111
`
`5,624,508
`Apr. 29, 1997
`
`[54] MANUFACTURE OF A TWO-WAY SHAPE
`MEMORY ALLOY AND DEVICE
`
`[76]
`
`Inventors: Josef Flomenblit; Nathaly Budigina.
`both of 15/12 Akiva St.. Holon 58824.
`Israel
`
`[21] Appl. No.: 432,802
`May 2, 1995
`
`[22] Filed:
`Int CI.6
`........................... A61M 29/00; C22C 19/03
`[51]
`[52] U.S. Cl. ........................... 148/510; 148/563; 606/198
`[58] Field of Search ..................................... 148/510. 511,
`148/563; 606/195. 198. 200; 623/1, 12
`
`[56]
`
`References Cited
`
`U.S. PAT~ DOCUMENTS
`
`4,665,906
`4,753,689
`4,820,298
`
`5/1987 Jervis ...................................... 606/108
`6/1988 Rizzo et al. • ........................... 148/563
`4/1989 Leveen et al ............................... 623/1
`
`4/1990 Homma ................................... 148/563
`4,919,177
`6/1990 Duerig .................................... 148/563
`4,935,068
`5,067,957 11/1991 Jervis ...................................... 606/108
`5,254,130 10/1993 Poncet et al ............................ 606/206
`
`FOREIGN PATENT DOCUMENTS
`
`143580
`0625153
`
`6/1985 European Pat. Off ..
`8/1993 WIPO .
`.
`
`Primary Examiner-George Wyszomierski
`Attorney, Agent, or Finn-Nath &Associates; Gary M. Nath
`ABSTRACT
`
`[57]
`
`A process is provided for the manufacture of a two-way
`shape memory alloy and device. The process of the inven(cid:173)
`tion 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 manu(cid:173)
`facture.
`
`15 Claims, 1 Drawing Sheet
`
`Af°C
`60
`
`50
`
`40
`
`30
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`20
`
`4ao0c
`
`3ao0c
`
`10
`
`20
`
`40
`80
`60
`AGEING TIME, MIN
`
`100
`
`120
`
`GOLD STANDARD EXHIBIT 2043
`US ENDODONTICS v. GOLD STANDARD
`CASE PGR2015-00019
`
`

`

`U.S. Patent
`
`Apr. 29, 1997
`
`5,624,508
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`5,624,508
`
`1
`MANUFACTURE OF A TWO-WAY SHAPE
`MEMORY ALLOY AND DEVICE
`
`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 relates to an SMA
`which is nickel-titanium based, also known as Nitinol.
`
`BACKGROUND OF THE INVENTION
`
`2
`times as the ''first embodiment". the process yields an alloy
`with a direction of the austenitic and the martensitic trans(cid:173)
`formations dictated by the direction of a conditioning trans(cid:173)
`formation in the martensitic state. In accordance with
`5 another embodiment of the invention. to be referred to
`herein at times as the "said second embodiment". the process
`yields an alloy with a direction of a martensitic or austenitic
`transformations which is independent on the deformation
`introduced in the martensitic state.
`In the following description and claims the term "Nitinol"
`will be used to denote an alloy comprising primarily nickel
`and titanium atoms. A Nitinol alloy has typically the fol(cid:173)
`lowing empiric formula:
`
`10
`
`Various metal alloys possess the ability to change their
`shape as a result of a change in temperature. Such SMA can
`undergo a reversible transformation from a martensitic state, 15
`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 20
`transformation, from the austenitic state to the martensitic
`state. will be referred to herein as the "martensitic transfor(cid:173)
`mation". The austenitic transformation occurs over a range
`of temperature which is higher than the range of tempera(cid:173)
`tures in which the reverse transformation occurs. This 25
`means. that once transformed to an austenitic state, an SMA
`will remain in that state even when cooled to a temperature
`below that in which the austenitic transformation began, as
`long as the temperature is above that in which the marten(cid:173)
`sitic transformation begins.
`A particular class of SMAs are alloys of nickel and
`titanium-Nitinol. Nitinol has found a variety of uses in
`medical as well as other fields. Medical uses of SMAs,
`particularly Nitinol. has been described in U.S. Pat. Nos.
`4,665,906. 5,067.957. European Patent Application 143,580. 35
`U.S. Pat. No. 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 40
`Application, Publication No. 625153, is typically deployed
`in the body while being in the martensitic state at body
`temperature, 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 45
`excessive heating to transform the SMA from the martensitic
`to the austenitic state is required, this can be damaging to the
`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 tempera- 50
`ture and will be over a temperature range which will not
`cause tissue damage owing to the excessive heating.
`
`wherein A represents Ni. 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
`follows:
`1=0.5
`m=0.5-n
`n=0.003 to 0.02.
`In accordance with the invention, there is provided a
`process for treating a raw Nitinol 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
`30 comprising the steps of:
`(a) heating a sample of the raw Nitinol 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 Ar wherein As is a tempera(cid:173)
`ture wherein austenitic transformation. namely trans(cid:173)
`formation between the martensitic to the austenitic
`state, begins, and A_,is a temperature where the auste(cid:173)
`nitic transformation ends;
`(b) subjecting the raw Nitinol alloy to a first heat treat(cid:173)
`ment based on the Ar As difference obtained in step (a).
`as follows:
`where the difference is less than about 7° C., heat
`treating the alloy to a temperature of about
`450°-500° C. for about 0.5-1.0 hours;
`where the difference is more than about 7° C., heat
`treating the alloy to a temperature of about
`510°-550° C. for about 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-1
`, with simultaneous internal heating
`of a portion of the alloy where the deformation occurs
`to a temperature of about 250°-550° C., the deforma(cid:173)
`tion 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°-550° C., 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 treatment.
`The particulars of the final heat treatment and the memo(cid:173)
`rizing treatment, are different in said first embodiment and
`in second embodiment. In accordance with said first
`65 embodiment, this treatment comprises:
`(i) forming the alloy into the form to be assumed by it in
`the austenitic state.
`
`GENERAL DESCRIPTION OF THE INVENTION
`
`55
`
`It is an object of the present invention to provide a process
`for the treatment of a Nitinol 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 a process to obtain a two-way SME which does not 60
`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.
`In accordance with the invention two embodiments are
`provided. By one embodiment. to be referred to herein at
`
`

`

`5,624,508
`
`4
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 in the drawings shows the relation between ~and
`the aging time, in different aging temperatures.
`
`3
`(ii) subjecting the alloy to a polygonization heat treatment
`at about 450°-550° C. for about 0.5-1.5 hours, then to
`solution treatment at about 600°-800° C. for about
`2-50 rnins., and then to an aging treatment at about
`350°-500° for about 0.15-2.5 hours,
`(iii) deforming the alloy to assume a conditioning form,
`the deformation being less than about 15%, and pref(cid:173)
`erably less than 7%, and being performed at a tempera(cid:173)
`ture T, which meets the following formula
`
`T<M,+30° C.
`
`5
`
`10
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The temperature range over which the austenitic trans(cid:173)
`formation 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 SMA described in European
`Patent Application No. 626153. Such a medical stent, 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 temperature and supports the wall of the
`tubular organ. Such medical stents arc designed so that the
`beginning of the austenitic transformation will occur at a
`temperature at or above 40° 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 large,
`can cause tissue damage. Furthermore, a narrow temperature
`range will generally ensure also a more rapid transition from
`25 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 medical stents 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 Nitinol alloy, which is typically provided by manufac(cid:173)
`turers in the form of a wire, is first tested for the difference
`35 between A, and Ar For this purpose a small sample of the
`material is taken. Based on the A,-~ difference, the alloy,
`e.g. the wire, is then subjected to a first heat treatment.
`Following the first heat treatment, the alloy is subjected to
`a thermo-mechanical treatment where the alloy is simulta-
`40 neously heated and subjected to a mechanical deformation.
`In the case of a process intended for the manufacture of a
`medical stent, the mechanical deformation typically
`involves changing the form of the alloy, from an initial form
`of a wire, to that of a ribbon or band; or alternatively,
`45 changing the wire into a wire of a smaller diameter. In order
`to retain the shape memory effect (SME) of the alloy, the
`total degree of deformation during the mechanical treatment,
`should be less than 55%, preferably less than 40% Where the
`total required eventual deformation is more than 55%, the
`50 thermo-mechanical treatment is repeated following an inter(cid:173)
`mediate heat treatment
`The thermo-mechanical treatment particularly where the
`alloy is processed to be used as a medical stent, is typically
`warm rolling, with the heating of the deforming portions
`55 being a result of electro-stimulation at a preferred current
`density of about 500-2000 A/cm2
`• A big advantage of such
`a 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
`60 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 thermo(cid:173)
`mechanical treatment at the above current density acceler(cid:173)
`ates dislocation reaction, which results in a perfect disloca-
`65 tion subgrain structure formation. Furthermore, the heating
`electrical current gives rise to a dynamic aging process with
`a second phase precipitation on the walls of the subgrain
`
`20
`
`15
`
`wherein M, is a temperature where the martensitic
`transformation begins, and then heating the alloy to a
`temperature of or above that in which the austenitic
`transformation of the alloy ends;
`whereby the alloy is conditioned to memorize an austenitic
`state in which it has deformed into which it was formed
`under (i) above, and an austenitic state, in which it has a
`martensitic form with an intermediate degree of deformation
`between the austenitic form and the conditioning form.
`It should be pointed out that also a single cycle of
`deformation in step ( e) (iii) is usually sufficient, it may at
`times be desired to repeat this cycle one or more times.
`In accordance with said second embodiment, 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,
`(ii) subjecting the alloy to a heat treatment at about
`450°-500° C. for about 0.5-2 hours, then subjecting the 30
`alloy to polygonization and solution treatment at about
`600°-800° C. for about 2-50 rnins., and then subjecting
`the alloy to aging treatment at about 350°-500° C. for
`about 0-2 hours,
`(iii) forming the alloy into a form to be assumed by it in
`the austenitic state,
`(iv) subjecting the alloy to a memorizing heat treatment at
`about 500°-600° C. for more than about 10 mins., and
`then subjecting the alloy to aging treatment at about
`350°-500° C. for about 0.15-2.5 hours;
`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 deforma(cid:173)
`tion between the form in which the alloy was formed in (i)
`above and the austenitic form.
`Following the treatment in accordance with both said first
`and said second embodiments, ~ will be between about 10
`to about 60° C. In order to increase ~and A,. the alloy may
`then be subjected to aging heat treatment at a temperature of
`about 350°-500° C. In order to decrease ~and A,. the alloy
`can then be subjected to a solution treatment at a temperature
`of about 510° to about 800° C.
`By differential aging or solution treatment in different
`portions the alloy will have different temperatures of aus(cid:173)
`tenitic transformation. This is as time 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. Particularly preferred in accordance with
`the invention are SMAs useful as medical devices.
`Examples are various orthopaedic devices and medical
`stents. A stent made of an SMA prepared by the above
`process, is a particularly preferred example in accordance
`with the invention. A process for the preparation of such
`medical devices also form an aspect of the invention.
`
`

`

`5,624,508
`
`15
`
`20
`
`5
`dislocation cells. This structure provides for a very narrow
`thermal interval of austenitic transformation ArA, for the
`shape memory alloy, and for a variety of other advantageous
`properties to be explained below.
`In an electrically stimulated warm rolling, where the 5
`current density decreases below 500 A/cm2
`, or the strain rate
`of the deformation is above about 5 seC1
`, there is an
`increase of the random dislocation density which will
`decrease the degree of perfection of the subgrain structure.
`For a narrow ArA, interval, a subgrain structure as perfect 10
`as possible is required. Accordingly, with an increase in the
`random dislocation density there is an increase in the ArA,
`interval. For example, where the current density is about 400
`, the ArA,
`A/cm2
`, or where strain rate is about 8 sec-1
`interval after final heat treatment will be about 10°-12° C.
`Furthermore, increasing of the current density to above
`about 2000 A/cm2
`, leads to a 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 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 embodiment, the alloy is 25
`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 austenitic state.
`The alloy is then typically placed in a vacuum or inert
`atmosphere furnace, in which it is first subjected to a 30
`memorizing and internal structure polygonization treatment,
`at a temperature of about 450°-550° C. for about 0.5-1.5
`hours, and then heated to about 600°-800° C. for about 2-50
`mins. During this latter heating, the alloy undergoes a
`solution treatment with re-arrangement of dislocations 35
`which are freed after solution treatment. Subsequently, the
`alloy is subjected to a final aging treatment at a temperature
`of about 350°-500° C. 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 austenitic transformation, Ar can be
`adjusted within a range of 10°-60° C. with a very narrow
`interval of ArA, of about 1-5° C.
`In case it is desired to decrease Ar the alloy may be
`subjected to a solution treatment at a temperature of about
`510°-800° C. In order to achieve a desired Ar both the
`temperature as well as the aging time can be controlled. For
`example, where the Nitinol alloy has after the final heat
`treatment an A, of about 45° C. and an A_,of about 48° C.,
`after a solution treatment at 640° C. for about 5 rnins., the
`A, and.A_,decrease to about 23° C. and 27° C., respectively;
`following solution treatment at 640° C. for 10 mins., A, and
`A_, decrease to about 11 ° C. and 15° C., respectively.
`In order to increase Ar the alloy is subjected to an aging
`heat treatment at a temperature of about 350°-500° C. Here 55
`again, in order to achieve a desired Ar both the temperature
`as well as the aging time can be controlled. This is
`demonstrated, for example, in FlG. 1 which gives the
`relation between the aging time at two different temperatures
`(380° C. and 480° C.) and the resulting Ar following a
`solution treatment at 640° C. for 20 rnins. As can be seen, for
`example, aging treatment at 380° C. for about 100 mins.
`yields an A_, of about 40° C., with the same A_, being reached
`with an aging treatment at 480° C. of about 40 mins. Aging
`at a temperature of about 450° C. for about 80 mins. will
`yield an As of about 46° C. and an A_, of about 49° C. (not
`shown in FlG. 1).
`
`6
`A unique feature of the process of the invention is the fact
`that the two-way SME is induced by only one cycle of
`deformation. In the case of a first embodiment of the
`invention, this is achieved by deforming the alloy into a
`conditioning form at a temperature of T<M,+30° C. fol(cid:173)
`lowed by heating to a temperature at or above the A_, of the
`alloy. The deformation should be less than 15% and pref(cid:173)
`erably less than 7%. A deformation 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 conditioning step, is an
`intermediate form between the austenitic memory form 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.
`Generally, the process in accordance with said first
`embodiment allows a reversible adjustment of the charac(cid:173)
`teristic 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 embodiment of the invention, gives rise to a two-way
`SME without the need for a final deformation to induce the
`two-way SME. This effect is not determined when indirect
`SME occurs. The second embodiment is particularly useful
`for the manufacture of a stent with a two-way SME, and the
`description below will refer to this specific embodiment. The
`Nitinol ribbon or wire is wound on a mandrel having a
`diameter equal to 2R1 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 formation takes place. Similarly as above, the alloy
`is then subjected to solution treatment and structure
`improvement at a temperature of 600°-800° C. for 2-50
`40 mins. and then to an ageing treatment at 350°-500° C. for
`0.15-2.5 hours. The ribbon or wire is then rewound on a
`mandrel with a diameter 2R2, which is the diameter to be
`assumed by the stent in the austenitic state and then sub(cid:173)
`jected to a memorizing and ageing treatment at temperature
`45 of 450°-550° C. for 0.15-2 hours. If the strain of this
`treatment Eirea,.=~w( l/R2-l/R1)<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
`f.tw=Y2w(1JR.w-l/R2)>0 (R,w being the diameter of the stent
`50 when assuming its martensitic state) and vice versa. As a
`result of this treatment, there is a very narrow temperature
`range in which the austenitic transformation takes place,
`ArAs =1 °-5° C., with a possibility to change A_, between
`10° and 60° C., similarly as above.
`The two-way SME in cooling may either coincide or
`oppose the direction of the deformation in the martensitic
`state. In case R2 is larger than R 1, and Rtw will be smaller
`than R2 , the device shrinks when it is cooled. In case R2 is
`less than 0, i.e. a reverse bending, and R2 is larger than Rtw,
`6o the device will expand when cooled.
`Finally, another result of the process of the present
`invention is 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
`65 content.
`The invention will now be illustrated further by several
`specific examples.
`
`

`

`5,624,508
`
`7
`EXAMPLE 1
`
`Preparation of a Biliary Stent
`
`8
`EXAMPLE3
`
`Esophageal Stent
`
`The starting material was a super-elastic Nitinol wire. 5
`with a diameter of 1.5 mm. The Ti and Ni content of the
`alloy was 50.8% and 49.1 %. respectively. A sample of the
`wire was treated at a temperature of 500° C. for 1.5 hours
`and the temperature interval ArA, was determined and was
`found to be 15° C.
`The wire was then subjected to a first heat treatment at
`550° C. for two hours. and then to a thermo-mechanical
`electro-simulated treatment. with the current density being
`900 A/cm2 and the strain rate being 0.3 sec-1
`• The thermo(cid:173)
`mechanical treatment was repeated three times with two l5
`intermediate heat treatments at 500° 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 a diameter of 8 mm. and placed into a vacuum 20
`furnace and heated to 500° C. for 0.6 hours. and then
`subjected to a solution treatment at 650° C. for 30 ruins. This
`was followed by an aging treatment at 400° C. for 1 hour.
`The spiral coiled stent which was obtained had an A, of
`40° C. and an A:r of 43° C.
`The stent was then wound on a 3 mm. diameter mandrel
`at a temperature of 25° C. and heated to above 43° C. 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° C. in which it had a diameter
`of 7.3 min.
`In order to install the stent. in situ within the body. it is
`wound on a catheter. and then inserted into the desired place 35
`within the bile duct. The stent is then activated by raising its
`temperature to more than 43° C. To remove the stent it has
`to be cooled below 25° C. and after shrinking it can be pulled
`away.
`
`30
`
`25
`
`EXAMPLE2
`
`Esophageal Stent
`
`40
`
`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 with the oppo(cid:173)
`site direction. After heat treatment. similarly as in Example
`2. the stent expands when cooled from a diameter of 16 mm.
`10 to a diameter of 25 mm.
`We claim:
`1. A process for treating a raw nickel-titanium based 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 steps of:
`(a) heating a sample of the raw nickel-titanium based
`alloy. to a temperature of about 450°-550° C. for about
`0.5-2.5 hours. and then testing the sample for tempera(cid:173)
`ture difference between A, and Ar wherein A, is a
`temperature wherein austenitic transformation. namely
`transformation between the martensitic to the austenitic
`state. begins. and A:r is a temperature where the auste(cid:173)
`nitic transformation ends;
`(b) subjecting the raw nickel-titanium based alloy to a first
`heat treatment based on the ArA, difference obtained
`in step (a). as follows:
`where the difference is less than about 7° C .• heat
`treating the alloy at a temperature of about
`450°-500° C. for about 0.5-1.0 hours;
`where the difference is more than about 7° C .. heat
`treating the alloy at a temperature of about
`510°-550° C. for about 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-1
`• with simultaneous internal heating
`of a portion of the alloy where the deformation occurs
`to a temperature of about 250°-550° C .• the deforma(cid:173)
`tion of this step being less than 55%;
`(d) if the deformation in step (c) does not yield the final
`form. subjecting the alloy 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
`( e) subjecting the alloy to a final heat treatment and to a
`memorizing treatment. which comprises:
`(i) forming the alloy into the form to be assumed by it
`in the austenitic state.
`(ii) subjecting the alloy to a polygonization heat treat(cid:173)
`ment at about 450°-550° C. for about 0.5-1.5 hours.
`then to solution treatment at about 600°-800° C. for
`about 2-50 ruins .• and then to an aging treatment at
`about 350°-500° for about 0.15-2.5 hours.
`(iii) deforming the alloy to assume a conditioning form.
`the deformation being less than about 15%, and
`being performed at a temperature T. which meets the
`following formula
`
`A stent was prepared from the same TiNi wire as used in 45
`Example 1. The wire was subjected to a first heat treatment
`and then to a thermo-mechanical treatment. similarly as
`described in Example 1. the difference being that the final
`thickness of the wire which was obtained was 0.28 mm.
`The ribbon was then wound on a mandrel having a
`diameter of70 mm. was constrained and then heated to 500°
`C. for 1 hour and then to a solution treatment at 650° C. for
`20 ruins. The ribbon was then wound on a mandrel having
`a diameter of 16 mm., was constrained and subjected to a
`memorizing treatment at 520° C. for 30 ruins .• and then to
`aging treatment at 400° C. for 2 hours. The stent which was
`obtained after this procedure had the following parameters:
`A,=42° C.; A_,.=45° C.; temperature of martensitic transfor(cid:173)
`mation being 27° C .. with the stent expanding when cooling
`from a diameter of 16 mm. which it had in the austenitic
`state to a 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° C. 65
`When the stent is cooled, it expands which prevents the stent
`from falling into the stomach.
`
`50
`
`55
`
`60
`
`T<M,+30° C.
`
`wherein M, is a temperature where the martensitic
`transformation begins. and then heating the alloy to
`a temperature at or above that in which austenitic
`transformation of the alloy ends;
`whereby the alloy is conditioned to memorize an austenitic
`state into which it was formed under (i) above. and a state
`
`

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`5,624,508
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`15
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`20
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`25
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`9
`in which it has a martensitic form with an intermediate
`degree of deformation between the austenitic form and the
`conditioning form.
`2. A process according to claim 1. wherein the deforma(cid:173)
`tion of the alloy to assume the conditioning form in step ( e) 5
`(iii), is less than about 7%.
`3. A process according to claim 1. comprising:
`(a) adjusting the temperature in which the austenitic
`transformation occurs, by either
`an aging treatment at a temperature of about 350°-500° 10
`C .• to increase the temperature in which the austen(cid:173)
`itic transformation occurs, or
`a solution treatment at a temperature of about
`510°-800° C., to decrease the temperature in which
`the austenitic transformation occurs.
`4. A process according to claim 1. wherein the deforma(cid:173)
`tion in step (c) is less than 40%.
`5. A process according to claim 1. wherein the internal
`heating in step ( c) comprises electro-stimulation with a
`current density of about 500-2000 Ncm2
`•
`6. 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 such that said final form
`is in the shape of said medical device.
`7. A process according to claim 6. wherein said medical
`device is a stent.
`8. A process for treating a raw nickel-titanium based alloy
`having an initial form to obtain an alloy with a final form in
`which it exhibits a two-way shape memory effect (SME) 30
`whereby it has 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 raw nickel-titanium based
`alloy, to a temperature of about 450°-550° C. for about 35
`0.5-2.5 hours, and then testing the sample for tempera(cid:173)
`ture difference between As and Ar, wherein As is a
`temperature wherein austenitic transformation, namely
`transformation between the martensitic to the austenitic
`state. begins, and~ is a temperature where the auste- 40
`nitic transformation ends;
`(b) subjecting the raw nickel-titanium based alloy to a first
`heat treatment based on the ArAs difference obtained
`in step (a). as follows:
`where the difference is less than about 7° C., heat
`treating the alloy at a temperature of about
`450°-500° C. for about 0.5-1.0 hours;
`where the difference is more than about 7° C.. heat
`treating the alloy at a te