Nitinol and its application in Self Expanding Stents

NITINOL AND ITS
USE IN MAKING
SELF EXPANDING
STENTS
BY-
YASH CHANNE - PA
29
VIVEK VIJAYAN - PA
214
SHIVALI YADAV - PA
185
SHUBHANGI PRASAD - PA
181
Guided by – Prof. Swanand Pachpore

INTRODUCTION & EXECUTIVE
SUMMARY

WHAT ARE BIOMATERIALS?
 Biomaterials are those materials that are used in the
human body. Biomaterials should have two important
properties: bio functionality and biocompatibility.
 A Good bio functionality means that the biomaterial
can perform the required function when it is used as a
biomaterial. Biocompatibility means that the material
should not be toxic within the body.
 Because of these two rigorous properties required for
the material to be used as a biomaterial, not all
materials are suitable for biomedical applications.
 The use of biomaterials in the medical field is an area
of great interest as average life has increased due to
advances in the use of surgical instruments and the
use of biomaterials.

WHAT ARE SHAPE MEMORY ALLOYS?
 Shape memory alloys have the ability to recover their original shape.
Shape memory alloys remember their original shape.
 Fig. shows the mechanism of shape memory effect. Here, the parent
austenite phase is stable above austenite finish temperature and
transforms to diffusion less twinned oriented martensitic phase
upon cooling to a temperature below the martensite finish
temperature (𝑀𝑓).
 For shape memory effect, the material in general is in martensitic
state at test temperature. When we apply an external force,
martensite changes to detwinned martensite.
 When we heat this material above the austenite finish temperature (
𝐴f), reverse transformation occurs from detwinned/deformation-
induced martensite to parent phase and the original shape is
recovered.

NITINOL: HISTORY
 Nitinol (NiTi) was discovered in 1959 by William J. Buehler while
working in the U.S. Naval Ordnance Laboratory in White Oak,
Maryland. 60. The word Nitinol was derived from the combination of
words nickel (Ni), titanium (Ti), and Naval Ordnance Laboratory (nol).
 Nitinol is a metal alloy of Nickel and Titanium that contain nearly equal
amount of Nickel and Titanium and exhibit two closely related unique
properties of Superelasticity or Pseudoelasticity and Shape Memory Effect.
That means nitinol can remember its original shape and return to it when
heated. It also shows great elasticity under stress.
 Prior to the use of nitinol stents, a balloon and stainless steel stent were
placed across the plaque. The balloon was expanded leaving the stent to
prop open the artery. However, after a short time, restenosis would set in,
whereby scar tissue builds up around the stent, causing flow restriction.
Today, a nitinol stent can be inserted in a relatively thin constraining guide
tube, which is positioned by the doctor. When it is deployed, the nitinol
stent, with a composition keyed to the body temperature of 38°C, expands
to its original shape, transforming to the austenite phase.

TRANSFORMATION TEMPERATURES AND THEIR IMPORTANCE
 Applications of shape memory alloys depend upon their phase transformation
temperatures.
 These transformation temperatures are
 Martensite start temperature (Ms)
 Martensite finish temperature (Mf)
 Austenite start temperature (As)
 Austenite finish temperature (Af)
 Transformation temperatures of nitinol are well below or close to body temperature,
which is why nitinol has a large number of applications as a biomaterial compared to
copper based and iron based shape memory alloys where transformation
temperatures are well above the body temperature.
 Transformation temperatures dictate the use of shape memory alloy. If austenite
transformation temperatures are below the body (test) temperature, then the shape
memory alloy can be used as a biomaterial due to its superelasticity, and if the test
temperature is below the martensitic transformation temperatures, then shape
memory alloy can be used as a biomaterial due to its shape memory effect

FUNDAMENTALS OF SHAPE MEMORY SYSTEMS
 The unique properties of shape memory alloys (SMAs)
revolve around what is known as the martensite
transformation, whereby a solid-state change from one
phase to another is induced, through a change in
temperature or stress. Irrespective of the alloy system, the
higher temperature phase is identified as austenite, while
the lower temperature state is martensite.
 In most commercial SMAs the crystal structure of the
austenite is a cubic B2 or caesium chloride (CsCl) while the
martensite is a more complex twinned monoclinic structure.

 Fig. schematically illustrates the austenite to martensite
transformation, with changes in temperature. In addition,
this diagram demonstrates how the transformation is
exploited to bring about macroscopic shape changes. When
the structure is deformed in the martensite condition, the
twin boundaries readily shift such that the twins are
predominantly oriented in one preferential direction; this
process is known as de-twinning.
 Upon heating of the deformed martensite, the structure
reverts to austenite as it becomes more thermodynamically
stable; in doing so, the deformation induced in the
martensite fully recovers with the material returning to its
undeformed state – thereby giving the shape memory
effect.

 The other feature of note is that the “forward” and
“reverse” transformations do not occur at the same
temperature, ie, the austenite to martensite change
occurs at a lower temperature than the reverse
martensite to austenite transition.
 This hysteresis effect and the incremental nature of the
transformation are both schematically shown in Fig. 1
 Fig. 2 which shows the super-elastic strain being induced
up to Point A.
 These stress levels are respectively identified as the
loading and unloading plateau stresses. If the material is
stressed above the load plateau, the de-twinned
martensite elastically and plastically deforms and
ultimately fails, as indicated by Point B in Fig. 2

PRACTICAL SHAPE MEMORY ALLOY
 There are several alloy systems that exhibit shape memory effects, though very few have achieved successful
engineering application; fewer still have been used in medical device applications. There are a number of
copper-based shape memory systems, with Cu–Zn–Al and Cu–Al–Ni being the most commercially successful.
 There are also a number of iron-based shape memory materials, including Fe–Mn–Si and Fe–Mn–Si–Cr–Ni,
though these materials tend to have low recoverable strain and require complex thermomechanical
treatments.
 However, by far the most significant shape memory alloy to-date is that based on equiatomic and near-
equiatomic nickel and titanium compositions. These NiTi materials have accounted for the majority of
commercial applications, particularly in the medical device industry, and this trend is likely to continue.
 The material was originally developed at the US Naval Ordnance Laboratory, leading to it being now widely
known as NiTiNOL.
 NiTi is classified as an intermetallic material with the respective atoms bonded to each other in a long-range
ordered structure; this is unlike many common alloys where the solute atoms randomly substitute for atoms of
the solvent crystal structure or sit in the interstices of the crystal structure; the most widely used NiTi material
has 50.8 at% nickel and 49.2 at% titanium.

MANUFACTURING PROCESS OF NITINOL
 NiTi is most usually produced by either vacuum induction melting (VIM) or
vacuum arc remelting (VAR).
 The main disadvantage of the VAR process is that only part of the ingot is
molten at any one time, thereby resulting in less alloy homogenization than
is possible with VIM. This creates the risk of transformation temperature
variations throughout the material; multiple repeat VAR steps can be used
to address this aspect. A VIM/VAR combination process can also be used to
produce NiTi, with the initial homogenous induction melted material being
re-melted to reduce impurities. However, it should be noted that
contamination from oxygen is also highly detrimental to NiTi materials and
each additional melting or re-melting step increases the risk of such
contamination.
 ISM has features of both VIM and VAR in that the charge is induction
melted within a copper water-cooled crucible.
 However, it is important to note that neither ISM is used as of yet to
manufacture commercial material and that the VIM and VAR processes are
still being widely and successfully used.

 After melting, the cast ingot is processed, using primarily
conventional metalworking technologies, to obtain wrought
products such as wire, strip or tubing.
 The ingot is typically hot rolled or forged in the temperature
range 800–9501C, breaking down and refining the cast structure
and thereby improving the mechanical properties.
 However, nitinol has a high work hardening rate and several
passes with inter-stage anneals (600–8001C) are needed to get
the material down to the required dimensions.
 Though it should be noted that there are fewer facilities
producing the original ingot material and often material from
different suppliers can originate from a single melt source and
therefore have similar melt chemistries and inclusion contents.

HEAT TREATMENT TO CONTROL PERFORMANCE
 Cold worked nitinol does not exhibit full shape memory or
superelastic behavior and needs to be heat treated to
activate these effects.
 At a most basic level the heat treatment is used to set the
final desired shape of the product, ie, the shape to which it
would thermally recover after being deformed in a
martensitic state, or mechanically (superelastically) recover
after being released from a constrained condition.
 The temperature for the treatments can range from 300 to
6001C, though most are typically at 5001C. Durations can
be as short as 2 or 3 min or as long as 2 or 3 h, though are
typically in the 5 to 30 min range, depending on the
temperature and the desired properties.

BIOMEDICAL APPLICATIONS OF NITINOL
 Biomedical applications of nitinol are related to transformation temperatures of nitinol that are close to body
temperature (310 K). Due to thermoelastic martensitic phase transformation and reverse transformation to
parent austenite upon heating (shape memory effect) or upon unloading (superelasticity), nitinol has a large
number of biomedical applications. Another important property of nitinol is its low elastic modulus close to
natural bone material and compressive strength higher than natural bone material which makes it an ideal
material for biomedical implant applications. In the medical field, nitinol has many applications; for example, it
can be used as guided wire and heart valve tool, can be used in joining of fractured bones, and can be used as
stent, as a guided wire, and as an orthodontic wire or brace. Attachments to each tooth in front of the teeth
are called brackets. When NiTi arch wires are attached to brackets, teeth can move in a controlled manner .
Pitting corrosion of nitinol is better than SS304 stainless steel in saliva solution.
 Some of the biomedical applications of Nitinol:
 orthodontic arch wire
 guided wire
 bone fixation
 Stent
 Artificial Organs

SELF EXPANDING METALLIC STENTS: INTRODUCTION
 Expandable biliary stents are used primarily for the palliation of
malignant biliary obstruction. There are two main categories of biliary
stents: fixed-diameter plastic stents (FDPS) and self-expanding metallic
stents (SEMS). FDPS, introduced in 1980 were preceded their SEMS
counterparts.
 While FDPS are a safe and effective means to overcome biliary
stenoses, they eventually become occluded.
 Bile flow rate in FDPS is impacted on by the stent lumen diameter. The
internal diameter of an FDPS is limited by the accessory channel size
of the duodenoscope. Because the diameter of the accessory channel
of a “therapeutic” duodenoscope is 3.2 mm, FDPSs are available with
internal diameters up to 12 Fr. SEMS were developed to overcome this
limitation as they deliver a larger diameter stent (10 mm) via a small
diameter (7.5 Fr) delivery device. Because malignant biliary obstruction
is typically associated with a survival of less than one year, SEMS are
intended to yield “lifelong” palliation of obstructive symptoms.

TYPES OF SELF EXPANDING METALLIC STENTS
WALLSTENT
 The Wallstent is the original SEMS and is considered
the industry standard.
 It is a braided stainless steel mesh with soft barbed
ends. The Wallstent is available in 40, 60 and 80mm
lengths. The available diameters of the fully
expanded Wallstent are 8 and 10 mm. The delivery
device has an outside diameter of 7.5 Fr.
DIAMOND ULTRAFLEX STENT
 The Ultraflex Diamond stent is made of nitinol. The
outer sheath measures 3 mm (8.5 Fr) in diameter.
The stent is available in 4, 6, and 8 cm in length and
10 mm in diameter.
 It easily permits cannulation of the interstices and
dilation for placement of another stent to create a
“Y” configuration; this may be potentially helpful in
the palliation of hilar strictures.

Z STENT
 There have been multiple iterations of the Z stent. The original
Gianturco-Rosch “Z” stent was a stainless steel wire bent in a continuous
Z shaped pattern forming a cylinder. The Spiral Z stent is available in 5.7
cm and 7.5 cm lengths and 10 mm in diameter.
 One of the iteration of the design, the Za-stent, incorporates nitinol in
place of stainless steel making the stent more flexible. The available
lengths of the Za-stent are 4, 6 and 8 cm with a diameter of 10 mm.
There are gold radiopaque markers in the middle and at the end of the
Za-stent for fluoroscopic visualization.
 A multi-center trial comparing the Wallstent with Spiral Z stent was
performed by Shah et al. and included 145 patients. There were 64
patients in the Z stent group and 68 in the Wallstent group. There was a
100% success in the placement of the stents.

RECOMMENDATION AND
IMPLEMENTATION METHODOLOGY

DEPLOYMENT CONSIDERATIONS
 The extensive use of nitinol in cardiovascular stent applications can be directly attributed to its characteristic
shape memory and super-elastic behavior. The possibility of achieving small compressed device
configurations, inserting these with minimal trauma and then having them recover to their larger deployed
functional configuration has intrigued physicians and device designers for many years now.
 In addition, the unique super-elastic “durability” has made nitinol even more attractive in applications where
device flexibility, conformance and crush resistance are critical. However, to fully appreciate how nitinol is
ideally suited to stent applications, the strains and loads involved during all stages of stent deployment need
to be considered.
 In summary, the stent is first compressed down to a small diametrical profile and retained within the delivery
tube or sheath. Upon tracking of the catheter to the treatment site, the stent is deployed in the artery by either
pushing out the stent or retracting the sheath. The stent is usually over-sized, so that the unconstrained
diameter of the stent would be larger than that of the vessel, thereby developing a force between the stent
and the vessel wall which keeps the stent in position.

 All of these steps can be considered further with reference to the
figure, which illustrates a plot of stent hoop force versus stent
diameter.
 As the stent is initially compressed to fit into the catheter, it deforms
elastically and when the stress reaches the level of the load plateau,
the superelastic deformation commences. This accounts for the bulk
of the deformation strain taken by the stent during crimping.

MANUFACTURING METHODS
 Given the extensive use of nitinol in cardiovascular stents
and bearing in mind the high sensitivity of the material to
heat treatment, a brief review of stent manufacturing
processes is useful. The majority of nitinol stents are now
laser cut from tubes, typically using Nd-YAG lasers.
 Electopolishing of nitinol is typically performed in mixtures
of alcohols and acids, though most processes tend to be
proprietary and there is a scarcity of detailed published
information.
 Electropolished devices are most often passivated, to
improve corrosion resistance and enhance biocompatibility.
There are many ways to implement this surface treatment
but the process objective is to remove free surface nickel
and to preferentially oxidize the titanium, thereby creating a
predominantly titanium oxide surface.

 While nitinol heat treatment has already been described, the positioning of this step in the overall process flow
needs to be considered. Normally, heat treatment is carried out after laser cutting and before surface finishing.
This ensures that any chemical pickling or cleaning step, employed as a pre-treatment for electropolishing, will
remove all dross and oxides including those oxides that may develop during heat treatment. However, the
number of heat treatment steps required will depend on the tube diameter selected, relative to the desired
final stent diameter.
 One option is to laser cut from a tube size of the same diameter as the finished stent size. This configuration
requires only one heat treatment step, primarily to tune in the transformation temperatures and to impart
some stress relief and shape setting.
 The other alternative is to laser cut from a smaller tube diameter and to gradually expand out the stent to the
required diameter. This expansion may take several steps, with the stent being put on increasing sized
mandrels for each step and being given a shape setting and stress relief heat treatment each time, typically at
approximately 5001C. Once the desired size is achieved the device is given the final heat treatment, to tune in
transformation behavior. This has the obvious disadvantage of adding more steps to the overall manufacturing
process, as well as requiring development of an initial laser cutting tool path that is different to the ultimate
stent geometry required. This approach does however generate less scrap metal during laser cutting and the
smaller diameter tubes are also usually less expensive.

CARDIOVASCULAR STENTS - CLINICAL EXAMPLES
 Whilst there is a vast number of nitinol stents developed and approved, there are surprisingly few available for
cardiovascular use. The majority are approved for non-vascular indications, such as biliary stenting, and do not
meet the regulatory requirements for cardiovascular devices.
 The situation has therefore improved in recent years with an increasing number of stents being specifically
designed and developed for a whole range of peripheral vessel anatomies. Even still, there are many design
challenges to be addressed in these applications.
 While many new additional designs are being developed, the basic principal features remain the same. These
cardiovascular devices are mainly tube-based designs, ie, laser cut, as already described. (Wire-based designs
are more widely used in non-vascular applications).
 These tube-based designs typically consist of cylindrical segments or rings comprised of several struts
spanning around the device circumference.
 These rings are connected to each other via “connector” or “bridge” struts.

 These features are best described with the aid of Fig; which shows
a drawing of the Radius coronary stent, one of the first
commercial nitinol cardiovascular stents. This shows a number of
such rings consisting of a zigzag set of struts, with these rings
connected at a number of points around the circumference.
 Without doubt, however, the area of biggest application
development for nitinol over the last 20 years has been the field of
peripheral artery stenting. Balloon-expandable stents were being
used in many peripheral indications for several years, but the only
competing self-expanding product was the Wallstent.
 There has now been a number of other nitinol stents developed
and approved for various peripheral indications. Some of these
may have only been initially approved for non-vascular biliary
applications, but as off-label use of these stents has now been
restricted, manufacturers have been going back, gathering data
and obtaining the approvals for vascular indications.

 There are a number of other important vascular anatomies
in which nitinol stents are being implanted; most notable
being the placement of stent grafts in the abdominal and
thoracic aorta, for aneurysm treatment.
 By way of illustration Fig; shows a CT image of an infra-
renal aortic aneurysm, showing the severely dilated vessel,
as well as an angiogram and CT image after placement of
the stent graft

BIOCOMPATIBILITY ISSUES OF NITINOL
 Nickel is a toxic element and causes contact allergy. For good biocompatibility, nitinol should have good
corrosion resistance so the release of nickel should be minimum.
 Corrosion resistance of nitinol decreases significantly with medium acidification. During daily life, pH usually
ranges from 4 to 5.5 (acidic), and after a meal it even falls below this value. Toothpastes used for the cleaning
of teeth contain up to 1% sodium fluoride (NaF) and/or Na2FPO4. It is also reported that the corrosion
resistance of nitinol significantly reduces in fluoridated saliva solution.
 The biomaterial, when implanted into the human body or as braces, experiences specific mechanical and
electrochemical interactions with the environment. For this reason, biomaterials like nitinol, should have
properties to remain stable under such hostile environment. It is reported that titanium and nickel are released
from nitinol into the surrounding body environment due to interaction of the biomaterial with the surrounding
environment.
 Potential danger of nitinol is associated with the negative effects of the release of nickel ions into the human
body. Study has reported 13 μg/day on average nickel ions release from nitinol braces in saliva environment.
 Nitinol has poor corrosion properties in halide containing environment.

CONCLUSION
 Shape memory effects can be observed in several alloys but from a medical device perspective nitinol remains
to be the most significant shape memory material. The use of nitinol in a diverse array of medical devices has
been described. The majority of the applications to-date have been in the cardiovascular field and this trend is
likely to continue given the growing demand for peripheral interventional technologies. However, it is likely
that on-going developments in the production of porous structures will also enable an increasing number of
applications in orthopedics.
 There are many aspects of current nitinol technologies that require further investigation and development.
These range from continuous improvements needed in design and manufacturing methodologies through to
collection and understanding of more extensive fatigue data.

REFERENCES: JOURNALS
1. Comprehensive Biomaterials II : Page 51 – 65 by “B O’Brien,
FM Weafer, and MS Bruzzi, National University of Ireland,
Galway, Ireland” 2017 Elsevier Ltd.
2. ERCP: Page 165 – 169 by “Ann Marie Joyce and Gregory G.
Ginsberg”

REFERENCES: RESEARCH PAPERS
1. Abdul Wadood, “Brief Overview of Nitinol as Biomaterial”, Hindawi Publishing Corporation Advances in
Materials Science and Engineering Volume 2016, Article ID 4173138, Page 1-2.
2. A review of shape memory alloy research, applications and opportunities by Jaronie Mohd Jani, M. Leary,
Aleksandar Subic & Mark Gibson.
3. Self-expanding Nitinol stents: Material and design considerations by Alan R. Pelton.
4. Hornung, M.; Bertog, S. C.; Franke, J.; Id, D.; Grunwald, I.; Sievert, H. Evaluation of Proximal Protection
Devices During Carotid Artery Stenting as the First Choice for Embolic Protection. EuroIntervention 2015

Nitinol and its application in Self Expanding Stents

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