180
`
`Recent Patents on Biomedical Engineering 2008, 1, 180-196
`
`Enhanced Nitinol Properties for Biomedical Applications
`
`Andrea Biscarini1,*, Giovanni Mazzolai1 and Ausonio Tuissi2
`
`1Department of Physics, University of Perugia, Via Pascoli, 06123 Perugia, 2Istituto per l’Energetica e le Interfasi,
`CNR-IENI, Lecco, Italy
`
`Received: June 30, 2008; Accepted: July 16, 2008; Revised: July 17, 2008
`
`Abstract: In recent years, Nitinol producers and medical products have experienced an exponential growth, driven by
`advanced manufacturing techniques and the use of progressively less invasive medical procedures. Concurrently, new
`processing techniques have been developed to further enhance the valuable properties of Nitinol used in medical devices;
`recent patents on these techniques include changing the composition of nickel and titanium, alloying the nickel-titanium
`with other elements, improving melting practices, heat-treating the alloy, and mechanical processing of the alloy. For
`example, alloying the nickel-titanium with ternary elements may widen the superelastic temperature operating window,
`maximize/minimize the stress-strain hysteresis, and improve the radiopacity of a Nitinol intraluminal device comparable
`to that of a stainless steel device of the same strut pattern coated with a thin layer of gold. Limiting to less than 30% the
`final cold work step (after a full anneal, and before the shape-setting step) may improve the Nitinol fatigue lifetime of
`about 37%, the fatigue lifetime being a primary factor limiting the performances of Nitinol endoluminal prosthetic
`implants. Local selective and differential thermo-mechanical treatments have also been devised to achieve different
`physical properties in different portions of a Nitinol medical device in order to improve its performance under expected
`operating conditions.
`
`Keywords: Nitinol, NiTi, titanium, nickel, NiTi based alloys, superelastic, superelaticity, shape memory, processing, fatigue,
`hysteresis, radiopacity.
`
`1. INTRODUCTION
`
`In recent years, equiatomic or near-equiatomic inter-
`metallic compounds of nickel and titanium (commonly
`referred to as Nitinol or Nitinol alloys) have been used in a
`wide and growing variety of insertable and implantable
`medical devices, such as guide wires [1], cannulae [2],
`catheters [3], needles [4], introducer sheaths [5]; intraluminal
`filters [6], vascular (coronary and peripheral) and non
`vascular stents [7] and graft support systems [8]; dental
`instruments [9], endodontic files for use in the cleaning,
`shaping, and widening stages of root canal procedures [10];
`surgical instruments [11], devices for occluding anatomical
`defects [12], manipulation [13] and retrieval [14] devices;
`components for binding together a pair of biological tissues
`[15], anchor devices [16], sutures [17], staples for bone
`fixation [18]; devices for correcting spinal deformities [19],
`orthodontic brackets [20] and archwires [21]; heart anatomy
`reshaping devices [22], valves [23]; these represent the most
`relevant applications and related recent patents.
`
` These applications are made possible by the most
`celebrated superelastic and shape memory properties [24-
`26], as well as by a number of lesser-known and more
`specific properties properly reviewed in a series of papers
`and reports by Duerig and co-workers (see for example refs
`[27, 28] ). These properties include elastic deployment, ther-
`mal deployment, kink resistance, biocompatibility, constant
`unloading stress, biomechanical compatibility, dynamic
`interference, hysteresis, MR compatibility, fatigue resistance,
`and uniform plastic deformation. The reference international
`
`*Address correspondence to this author at the Department of Physics,
`University of Perugia, Via Pascoli, 06123 Perugia, Italy; Tel: +39-075-
`5852703; Fax: +39-075-44666; E-mail: biscarini@fisica.unipg.it
`
`organizations and conferences, i.e., International Conference
`on Martensitic Transformations
`(ICOMAT), European
`Symposium on Marensitic Transformation (ESOMAT), and
`particularly International Conference/Organization on Shape
`Memory and Superelastic Technologies (SMTS), have
`chronicled over the years the tremendous progress in
`understanding these Nitinol properties and the success that
`Nitinol technology has recently experienced in the medical
`industry. Nevertheless, researchers involved in the field are
`still constantly discovering new and fascinating attributes of
`Nitinol, and some aspects of Nitinol have not yet been fully
`clarified, such as the ductility of the B2-phase as it relates to
`pseudo-twinning, unique observations of the R-phase, the
`stiffness of Nitinol during unloading, an unusual situation in
`which austenite can be stress induced from martensite, and
`an unusual deformation memory associated with subsequent
`modes of deformation [29].
`
` New processing techniques have been developed since
`the early 1980's to further enhance the valuable properties of
`Nitinol. These techniques include changing the Ni/Ti compo-
`sitional ratio in binary NiTi, alloying the nickel-titanium
`with other elements, improving melting practices, heat
`treating the alloy, and mechanical processing of the alloy.
`Examples of such techniques have been extensively reported
`in patents: Fountain et al. (1982, [30]) disclose processes for
`producing a shape memory Nitinol alloy having a desired
`transition temperature; DiCarlo and Walak (2000, [31])
`disclose a process for improving ductility of Nitinol; Pelton
`and Duerig (1998, [32]) disclose cold working and annealing
`a Nitinol alloy to lower a transformation temperature; Thoma
`et al. (1989, [33]) describe a process for adjusting the
`physical and mechanical properties of a shape memory alloy
`member by increasing the internal stress level of the alloy
`through cold work and heat treatment; Simpson et al. (1988
`
` 1874-7647/08 $100.00+.00
`
`© 2008 Bentham Science Publishers Ltd.
`
`Medtronic Exhibit 2002
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`
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`IPR2019-00123
`Page 00001
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`Enhanced Nitinol Properties
`
`Recent Patents on Biomedical Engineering, 2008, Vol. 1, No. 3 181
`
`[34]), Mitose and Ueki (1999, [21]), Duerig et al. (1998,
`[35]), Boylan and Huter (2004, [36]) teach adding a ternary
`element to a nickel-titanium alloy to enhance engineering
`properties.
`
` Recently, the availability of micro-tubing and high
`precision laser cutting, as well as the use of progressively
`less invasive medical procedures, have focused attention on
`the quickly growing and technologically demanding intralu-
`minal applications, particularly on self-expanding stents.
`Accordingly, new and complex manufacturing and pro-
`cessing techniques have been developed to improve radio-
`pacity, thermo-mechanical properties, and fatigue resistance
`of Nitinol intraluminal medical devices. New selective and
`differential themo-mechanical treatments have also been
`devised to spatially modulate the local mechanical properties
`of such devices, in order to improve their performances
`under expected operating conditions. In this paper we review
`some of the most relevant and recent patents related to these
`new techniques. The large field of Nitinol surface treatments
`is outside the scope of this paper and will be neglected. We
`begin with a brief introductory description of superelasticity,
`shape memory effect, and common Nitinol processing
`techniques.
`
`2. NITINOL PROPERTIES AND PROCESSING
`TECHNIQUES
`
` NiTi alloys having shape memory effect and superelastic
`properties generally have at least two phases: a martensitic
`phase (M) and an austenitic phase (A). Martensite is stable at
`relatively low temperatures, has a relatively low tensile
`strength, is soft and malleable, its structure is composed of
`self-accommodating twins, and can be easily deformed by
`de-twinning the structure via an applied stress. Austenite is
`stable at higher temperatures, has a relatively high tensile
`strength, is a strong and hard phase of the alloy, exhibiting
`properties similar to those of titanium, and is characterized
`by a B2 structure. The properties of the austenitic and
`martensitic phases of NiTi alloys have been deeply discussed
`by Otsuka and Ren in a recent review paper entitled
`“Physical metallurgy of Ti-Ni based shape memory alloys”
`[26]. The direct A(cid:1)M (reverse M(cid:1)A) martensitic phase
`transformation can be induced by a decrease (increase) in
`temperature or by the application (removal) of an external
`load. Shape memory and superelaticity are closely related to
`these
`different
`temperture-driven
`and
`stress-driven
`transformation modes, respectively.
`
`2.1. Shape Memory
`
` Conventionally, MS and MF indicate the martensite start
`and finish temperatures, at which the transformation from
`austenite to martensite begins and is completed, respectively,
`as the temperature decreases (MS > MF). Likewise, AS and
`AF indicate the austenite start and finish temperatures, at
`which the transformation from martensite to austenite starts
`and is completed, respectively, as the temperature increases
`(AS < AF). Since AS > MF and MS < AF, a hysteresis loop is
`displayed when the percent austenite (or martensite) phase in
`the alloy is plotted as a function of the temperature for a
`thermal cycle involving the direct A(cid:1)M and reverse M(cid:1)A
`transformations.
`
`Shape memory means that a metal alloy, plastically
`
`deformed in the martensitic phase, spontaneously returns to
`its previous undeformed shape (“remembered” shape) when
`heated to the austenitic phase. Shape memory effect is
`imparted by heat treating the alloy at temperature above AF,
`at which the austenitic phase is stable (see paragraph 2.6 for
`the details of the imparting-memory treatment); the shape of
`the metal during this heat treatment is the shape “remem-
`bered”. Fig. (1) illustrates the shape memory effect in a strain
`versus temperature ((cid:1)-T) plot. The heat-treated metal is
`cooled to a temperature below MF at which the martensitic
`phase is stable, causing the austenitic phase to transform to
`the martensitic phase with no changes in the macroscopic
`shape of the alloy (1). The metal in the martensitic phase is
`then plastically deformed into a particular desired shape (2).
`With subsequent heating (3,4), the metal gradually reverts to
`its original shape as the martensite transforms to austenite
`(4). Ultimately, at a temperature above AF, the material com-
`pletes the return transformation to the austenitic phase and
`fully recovers the applied strain. The alloy may accom-
`modate several percentage points of recoverable strain (8%
`in the example of Fig. (1).
`
`Shape memory effect
`
`8
`
`Strain
`(%)
`
`,]
`
`2
`
`3
`
`4
`
`I - A->M cooling
`2 - M deformation
`3, 4 - M->A heating
`
`MF
`
`Ms
`
`As
`
`AF
`
`Temperature
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Fig. (1). Diagram of strain versus temperature illustrating the shape
`memory effect for an exemplary shape memory alloy.
`
`
`
`2.2. Superelasticity
`
`Superelasticity (or pseudoelasticity) refers to the ultra
`
`high elastic behavior of the alloy under stress: typical
`reversible strains of up to 10% elongation can be achieved in
`a superelastic Nitinol wire as compared to 0.5% reversible
`strain in a steel wire, for example. The superelastic behavior
`appears in the austenitic phase when stress is applied to the
`alloy and the alloy changes from the austenitic phase to the
`martensitic phase. This particular martensitic phase is more
`precisely described as stress-induced martensite (SIM),
`which is unstable at temperatures above AF. As such, if the
`applied stress is removed, the stress-induced martensite
`reverts back to the austenitic phase.
`
`Figure (2) illustrates an idealized stress-strain curve for a
`
`superelastic nickel-titanium alloy at a temperature above AF.
`With applying an increasing stress, at first, the superelastic
`specimen deforms elastically until it reaches a particular
`stress level where the alloy undergoes a stress-induced phase
`transformation from the austenitic phase to the martensitic
`
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`182 Recent Patents on Biomedical Engineering, 2008, Vol. 1, No. 3
`
`Biscarini et al.
`
`phase. The line from point O to point B represents the elastic
`deformation of the nickel-titanium alloy. After point B the
`strain is no longer proportional to the applied stress and it is
`in this region between point B and point C that the stress-
`induced transformation of the austenitic phase to the mar-
`tensitic phase begins to occur. As the phase transformation
`progresses, the alloy undergoes significant increases in strain
`with little or no corresponding increase in stress. The strain
`continues to increase while the stress remains essentially
`constant until the transformation of the austenitic phase to
`the martensitic phase is complete. This constant or plateau
`region C-D is known as the loading stress, since it represents
`the behavior of the material as it encounters continuous
`increasing strain. It is in this plateau region C-D that the
`transformation from austenite to martensite occurs. At point
`D the transformation to the martensitic phase due to the
`application of stress to the specimen is substantially com-
`plete. Thereafter, further increase in stress is necessary to
`cause further deformation. The martensitic metal first yields
`elastically upon the application of additional stress and then
`plastically with permanent residual deformation. In the
`figure, beyond point D, the martensitic phase begins to
`deform elastically, and beyond point E, the deformation is
`plastic or permanent.
`
`If the load on the specimen is removed before any
`
`permanent deformation has occurred, the stress-induced
`martensite elastically recovers and transforms back to the
`austenitic phase, and the specimen returns to its unstrained
`state. The reduction in stress first causes a decrease in elastic
`strain until the martensite recovers its original shape,
`provided that there was no permanent deformation to the
`martensitic structure. The line from point E to point F
`represents the elastic recovery of the martensite. As stress
`reduction reaches the level at which the martensitic phase
`begins transformation to the austenitic phase, the stress level
`in the specimen remains essentially constant until the
`transformation to the austenitic phase is complete; i.e., there
`is significant recovery
`in strain with only negligible
`corresponding stress reduction. In the figure, at point F in the
`recovery process, the metal begins to transform from the
`stress-induced, unstable, martensitic phase back to the more
`stable austenitic phase. In the region from point G to point H,
`which is also an essentially constant or plateau stress region,
`the phase transformation from martensite back to austenite
`takes place. This constant or plateau region G-H is known as
`the unloading stress. After the transformation back to
`austenite is complete, further stress reduction results in
`elastic strain reduction. The line from point I to the starting
`point O represents the elastic recovery of the austenitic metal
`to its original shape.
`
` This ability to incur significant strain at relatively
`constant stress upon the application of a load and to recover
`from the deformation upon the removal of the load is
`commonly referred to as “superelasticity” or “pseudo-
`elasticity”, and sometimes “rubber-like” effect. Regarding
`the terminology, the authors believe that the term “pseu-
`doelasticity” should be used when the closed loop of the
`stress-strain curve is due to “non elastic” physical pheno-
`mena such as SIM. This should help engineers to consider
`Nitinol properties as non-linear and strongly temperature
`dependent thermo-mechanical behaviours. From the practical
`
`point of view, “superelasticity” or “pseudoelasticity” are
`synonyms for describing the stress-strain curve reported in
`Fig. (2).
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Stress
`
`Superelasticity
`
`E
`
`A.._SIM
`
`H
`
`G
`
`Strain
`
`Fig. (2). Diagram of stress versus strain, at a temperature above the
`austenitic final temperature AF, illustrating the superelasticity of an
`exemplary shape memory alloy.
`
`
` As emphasized above, the pseudoelastic stress-strain
`curve is characterized by two regions of nearly constant
`stress, identified above as loading plateau stress C-D and
`unloading plateau stress G-H. The loading plateau stress
`represents the period during which martensite is being stress-
`induced in favor of the original austenitic crystalline
`structure. As the load is removed, the stress-induced mar-
`tensite transforms back into austenite along the unloading
`plateau stress part of the curve. Naturally, the loading plateau
`L(cid:1) always has a greater magnitude than the unloading
`U(cid:1) . The difference
`L (cid:2)(cid:1)(cid:2)
` defines the
`plateau stress
`stress hysteresis of the system.
`
`stress
`
`U
`
`2.3. Use of Superelasticity in Medicine
`
` The superelastic effect is frequently utilized for delivery
`and deployment of medical devices that pass through very
`small openings inside the body and then elastically spring
`back into the desired shapes (elastic deployment) at the
`target site. For example, a self-expandable stent is typically
`maintained at a compressed diameter by a tubular delivery
`sheath which overlies the stent to facilitate the insertion and
`delivery within a blood vessel. When the constraining
`member (e.g., the delivery sheath) is removed and the stress
`is released, the martensite transforms to austenite and the
`stent recovers its deployed configuration. The stent may
`expand from the compressed diameter to an expanded
`diameter and come into contact with the vessel wall to
`support it. According to this embodiment, the nickel-titanium
`alloy has an austenite finish temperature AF which is less
`than or equal to human body temperature (37°C) so that
`removal of the constraining member is sufficient to trigger
`the transformation to the austenitic phase.
`
` The constant unloading stress exhibited by a Nitinol
`material over a large strain range is widely utilized in
`orthodontic and orthopedic devices. In contrast to stainless
`steel and other conventional appliances, a Nitinol device may
`
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`Enhanced Nitinol Properties
`
`Recent Patents on Biomedical Engineering, 2008, Vol. 1, No. 3 183
`
`apply a constant optimized force over a wide range of
`shapes, i.e., over a very broad treatment time, without the
`need of continuous force adjustments. Even the hysteresis,
`commonly considered a drawback due to the reduced energy
`storage efficiency, may be a desirable feature in stent design,
`as will be discussed in paragraph 5.
`
` A great number of medical devices are based on stress
`induced martensite. The most commercially
`important
`patents now pending for medical devices incorporating SIM
`alloys include the Jervis’s series which is listed in Jervis
`patent [37].
`
`2.4. Use of Shape Memory in Medicine
`
` Besides superelasticy, the shape memory effect may be
`utilized to deliver and deploy the medical device comprising
`the nickel-titanium alloy. In other words, a change in
`temperature instead of an applied (removed) stress may
`control the transformation from martensite to austenite. For
`example, a stent may be deployed by heating instead of
`retraction of a delivery sheath. According
`to
`this
`embodiment, the nickel-titanium alloy has an austenite finish
`temperature (AF) which is less than or equal to body
`temperature (37°C). The medical device is maintained at a
`temperature of less than AS, prior to and during delivery of
`the device into the body, thereby maintaining a martensitic
`structure of the nickel-titanium alloy. The device transforms
`to the austenitic structure and thus deploys when warmed up
`to about body temperature. Cooling of the device during
`delivery is desirable to prevent the martensitic structure from
`prematurely transforming to austenite. As the device is being
`advanced in the body, the cooling may entail keeping the
`device at a temperature below AS by, for example, flushing a
`cold fluid through the device or through a delivery system of
`the device itself. Preferably, the nickel-titanium alloy has a
`value of AF of at least about 32°C, yet no higher than about
`37°C.
`
`2.5. Transformation Temperature Tuning
`
` The AF and the other transformation temperatures of
`Nitinol alloys can be fine-tuned within a desired temperature
`range by controlling the alloy composition and processing.
`
` The transformation temperatures are very sensitive to
`small changes in the ratio of nickel to titanium. Increasing
`the proportion of nickel to titanium in the alloy provides a
`means of reducing the AF temperature to a desired level, thus
`stabilizing austenite. For example, AF is generally above
`100°C for equiatomic NiTi, whereas it is generally around
`100°C for slightly off-stoichiometric alloys including an
`excess of nickel (e.g., from about 50.6 to about 50.8 at. %
`Ni). As discussed above, Nitinol alloys, which are austenitic
`at and slightly below body temperature, are known to be
`useful for medical devices. Specifically, alloys including
`50.6-50.8 at. % Ni and 49.2-49.4 at. % Ti are considered to
`be medical grade Nitinol alloys.
`
` The transformation temperatures are also very sensitive
`to the presence of minute quantities of alloying elements or
`trace amounts of
`impurities. Additions of chromium,
`palladium, cobalt and/or iron may be effective in reducing
`AF, whereas additions of vanadium and/or cobalt may be
`effective in reducing MS. Thus, Nitinol's properties thus can
`
`the
`tailored for specific applications by selecting
`be
`appropriate concentration, type, and combination of alloying
`elements. On the contrary, oxygen and carbon are conta-
`minants which should be excluded from the system as they
`can affect
`the
`transformation
`temperature and reduce
`mechanical properties. Oxygen has the ability to tie up
`titanium by forming Ti4Ni2Ox inclusions, thus depressing
`MS. The exact impact of oxygen on Nitinol properties is still
`unclear. Excess oxygen concentrations may not significantly
`affect shape memory effect or superelasticity, but may
`negatively impact ductility and fatigue resistance. Carbon
`forms TiC inclusions which also tie up Titanium and depress
`MS as a consequence. ASTM F 2063-05, the standard
`specification for wrought Nichel-Titanium Shape Memory
`Alloys for Medical Devices and Surgical Implants, accepts
`Oxygen and Carbon contents up to 500 ppm in NiTi
`compound.
`
`2.6. Nitinol Processing Techniques
`
` Nickel-titanium alloys have a variety of characteristics
`and behaviors depending on composition and processing
`conditions. Products made from Nitinol alloys nevertheless
`typically undergo a common series of processing steps.
`However, the following processing sequence may be gene-
`rally employed to produce Nitinol alloys and Nitinol medical
`devices.
`
`Melting
`
` A melt incorporating the desired amounts of alloying
`elements is formed and then cooled into a solid (e.g., an
`ingot). High purity raw materials (usually Ti sponge and
`electrolytic grade Ni) are preferably melted in an inert gas or
`vacuum atmosphere: the constituent components are placed
`in a crucible, then induction heated or electrical arc heated in
`a vacuum induction melting (VIM) process or vacuum
`consumable arc melting (VAR) process, respectively.
`Remelting is generally desirable to obtain satisfactory
`microstructural homogeneity in the ingot. For example,
`successive VAR processes or a VIM/VAR double melting
`process may be employed. The Nitinol ingot after VIM/VAR
`processing has the desired general composition of nickel to
`titanium as well as trace elements of carbon, oxygen, iron,
`and other impurities.
`
`Hot working
`
` After the melting process, the Nitinol ingot has little
`ductility. The ingot may then be hot worked into a first shape
`(e.g., bar, rod, hollow tube, or plate) by extruding, hot
`rolling, or forging. Hot working is generally employed to
`refine the as-cast structure of the ingot and more specifically
`to achieve a microstructure that exhibits better workability.
`The hot working is generally carried out at temperatures in
`the range of approximately 700°C to 950°C, and may require
`multiple hot working and reheating cycles. The reheating
`may be carried out over an eight hour period. Preferably, the
`ingot undergoes a minimum deformation of about 90%
`during hot working in order to homogenize the as-cast,
`dendritic microstructure. Prior to hot working, it may be
`beneficial to carry out a solution heat treatment that involves
`soaking the ingot at an elevated temperature for a given time,
`followed by quenching. The solution heat treatment may aid
`in homogenizing the microstructure of the alloy and,
`
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`Biscarini et al.
`
`preferably, is carried out at temperatures in the range of
`approximately 1000°C to 1150°C.
`
`Cold working
`
` The Nitinol first shape is moved closer to the target
`component with
`the desired mechanical and physical
`properties by means of a series of cold working steps.
`Typically, the Nitinol receives cold working in the range of
`30 to 50 % at each step, and is also annealed at about 600 to
`800°C for stress release after each cold work step. The
`interspersed anneal cycles minimize work hardening of the
`Nitinol caused by the repeated cold work, through recrys-
`tallization and growth of the austenite grains. The cold
`working is typically performed by cold drawing for wires
`through a series of dies, cold rolling for tapes, and tube
`drawing with an internal mandrel for tubes.
`
`Machining operations
`
` Machining operations, such as drilling, cylindrical
`centerless grinding, or laser cutting may also be employed to
`fabricate the desired component. Other operations, such as
`wire braiding or winding, may also be carried out.
`
`Shape Setting
`
` A heat treatment is employed after the last cold work step
`to
`impart a “memory” of
`the desired
`final shape
`(“remembered” shape) and to optimize the shape memory,
`superelastic and mechanical properties of the component.
`Usually, shape setting involves annealing the component
`while constrained in a final shape at a temperature in the
`range of approximately 350 to 550°C. The number, duration
`and the temperature of the heat treatments may alter the
`transformation temperatures. In alloys having an excess of
`nickel atoms (e.g., from about 50.6 to about 50.8 at. % Ni),
`for example, the heat treatment described above may cause
`nickel-rich precipitates to form, thereby reducing the nickel
`content of the matrix and causing the transformation
`temperatures to increase. The precipitates may also improve
`the tensile strength of the nickel-titanium alloy. At this stage,
`the Nitinol component has been
`transformed
`into a
`standardized, nearly finished condition for consumption in
`the industry.
`
`3. IMPROVED RADIOPACITY
`
` Nickel-titanium alloys are commonly used for the
`manufacture of intraluminal biomedical devices, such as
`self-expandable stents, stent grafts, embolic protection
`filters, and stone extraction baskets. A distinct disadvantage
`with these self-expanding Nitinol devices is the fact that they
`are not sufficiently radiopaque as compared to similar struc-
`tures made from gold or tantalum. As a result of this poor
`radiopacity, such devices may be difficult to visualize from
`outside the body using non-invasive imaging techniques,
`such as x-ray fluoroscopy. Visualization is particularly
`problematic when the intraluminal device is made of fine
`wires or thin-walled struts. Consequently, a clinician may
`not be able to accurately place and/or manipulate a Nitinol
`stent or basket within a human blood vessel.
`
` Conventional approaches to improving the radiopacity of
`nickel-titanium medical devices include the use of radio-
`paque markers or coatings. For example, gold markers
`
`attached to ends of a stent may guide the positioning of the
`device and delineate its length during an x-ray procedure.
`Alternatively, a medical device may be plated, clad or
`otherwise coated with gold or another heavy metal to create
`a radiopaque surface or outer layer. In another approach, a
`heavy metal cylinder may be included within the lumen of a
`stent to produce a radiopaque core. These techniques to imp-
`roving radiopacity, however, create a number of important
`complications such as material compatibility, galvanic corro-
`sion, high manufacturing cost, coating adhesion or dela-
`mination, biocompatibility, increased delivery profile of the
`device, loss of coating integrity following collapse and
`deployment of the device, interference with the mechanical
`behavior of the device, etc.
`
` Radiopacity can be appreciably improved by increasing
`the wall or strut thickness of the Nitinol self-expanding
`device. But increasing strut thickness detrimentally increases
`the stent chronic outward force, and decreases the flexibility
`of the device, which is a quality necessary for ease of
`delivery.
`
` Radiopacity can also be improved by alloy addition.
`Some relevant and recent patents reviewed in the following
`paragraphs discuss alloying nickel-titanium with a ternary
`element, which increases Nitinol’s radiopacity yet preserves
`its biocompatibility, its superelastic qualities and all the other
`associated properties that are advantageous for intraluminal
`biomedical applications.
`
`3.1. Addition of Tungsten
`
` A patent entitled “ Thermoelastic and superelastic Ni-Ti-
`W alloy”, published in January 2005 by Pelton [38],
`discloses a radiopaque NiTiW stent for implantation in any
`body lumen. The added tungsten in specified amounts
`improves the radiopacity of the Nitinol stent comparable to
`that of a stainless steel stent of the same strut pattern coated
`with a thin layer of gold. Furthermore, the stent retains its
`superelastic and shape memory behavior and further
`maintains a thin strut/wall thickness for high flexibility.
`
`Composition
`
`In order to achieve a sufficient degree of radiopacity
`
`while maintaining the superelastic engineering properties of
`a binary nickel-titanium,
`the radiopaque stent of
`the
`invention should include tungsten whose atomic percent is
`greater than or equal to 5 and less than or equal to 12. In
`various preferred embodiments, the atomic percent of the
`nickel is approximately 50.8, the atomic percent of the
`titanium is a maximum of approximately 40, and the atomic
`percent of the tungsten is approximately 10. A preferred
`method of fabricating the superelastic radiopaque stent is
`also indicated by the inventor, together with suggested
`temperature ranges for the AF temperature of NiTiW ingot (0
`(cid:1) AF (cid:1) 20°C), NiTiW tubing (-15 (cid:1) AF (cid:1) 0°C), and final
`laser cut NiTiW stent (0 (cid:1) AF (cid:1) 30°C).
`
`Properties
`
` This invention provides a Nitinol stent with improved
`radiopacity and without the downside of increasing the stent
`wall thickness or strut thickness. As explained above,
`increasing wall or strut thicknesses detracts from the flexi-
`bility of the stent, which in turns is detrimental to deliver-
`
`IPR2019-00123 Page 00005
`
`

`

`Enhanced Nitinol Properties
`
`Recent Patents on Biomedical Engineering, 2008, Vol. 1, No. 3 185
`
`ability. This superelastic NiTiW stent has a thin wall/strut
`thickness and/or strut cross-sectional area akin to a conven-
`tional stainless steel stent, and has comparable radiopacity to
`a stainless steel stent with a thin coating of gold. The
`wall/strut thickness is defined by the difference between the
`inside diameter and the outside diameter of the tube. In an
`exemplary embodiment of this invention, an approximately
`21 mm long stent with an expanded diameter of about 8 mm,
`has a wall thickness of approximately 0.0045 inch (0.114
`mm). If the exemplary embodiment stent strut has a square
`shape cross-sectional area, its dimensions would be 0.0045
`inch (0.114 mm) by 0.0045 inch (0.114 mm) with a cross-
`sectional area of approximately 0.000020 in2 (0.013 mm2).
`
` Within the specified compositions, the stress-strain
`hysteresis curve of the radiopaque NiTiW alloy closely
`approximates the idealized stress-strain hysteresis curve of
`binary nickel-titanium. Thus, the tungsten addition (in the
`specified amounts) generally preserves the engineering
`qualities of the Nitinol alloy, and the Nitinol stent retains its
`superelastic and shape memo