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A Review on Nickel Titanium and it’s Biomedical Applications

Agnesh Rao K1, Ayush Kothari2, Dhanush L Prakash3, Jeevan S Hallikeri4, Nesar V Shetty5
1,2,3,4,5Bachelor’s in Mechanical Engineering, Dept. of Mechanical Engineering, BNM Institute of Technology,
Karnataka, India
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Abstract - NiTinol is a super alloy that has influenced Fields like Aeronautics and Biomedicine. Modern Medicine
tremendous change in science and technology, since its has accepted and acknowledged the usage of Shape Memory
inception in 1959. It has provided novel solutions to our pre- Alloys. Industrial and commercial applications of SMA haven’t
existing challenges and has affected many fields in our been exploited still but rather given trivial importance. This
paper pertains only to Nitinol, Its properties and its
scientific community bringing funding for research and
applications in the field of Biomedical Engineering.
positively impacting many industries to help in their
development. One such impact is in the bio medical industry.
The bio medical industry is the most fast paced section of our 1.2 An Overview about the Material Science of
scientific community and sees a lot of development in a short Nickel-Titanium Alloy
span of time. This review provides an understanding of the
history, manufacturing methods, shape memory effect and the
use of NiTinol in biomedicine & orthodontics; how NiTinol has William J. Buehler along with Frederick Wang, discovered the
helped improve pre-existing solutions to medical problems that SME of Nitinol and its properties during research at the Naval
affect a large demographic and how NiTinol has helped in Ordnance Laboratory in 1959. Different types of Nitinol can
dental corrections and orthodontic repairs. be obtained by varying its composition. Majority of the
Nitinol that is being used has Nickel and Titanium in 50:50
ratio however most of the Biomedical applications of Nitinol
Key Words: Nitinol, Biomedicine, Shape Memory, have minor increase in nickel. The composition of Nitinol
Pseudoelasticity, Superelasticity depends on its internal energy(U). Nitinol alloys exhibit two
closely related and unique properties: shape memory effect
1. INTRODUCTION (SME) and superelasticity (SE); also called pseudoelasticity,
(PE). Shape memory is the ability of nitinol to undergo
Mechanical Engineering is a vast discipline with a range of deformation at one temperature, then recover its original,
activities and functions that derives its spread from the need undeformed shape upon heating above its "transformation
to design, manufacture and provide efficient solutions in temperature". Superelasticity occurs at a narrow
development of processes and products. The application of temperature range just above its transformation
principles of STEM in the field of Medicine has yielded temperature without any need for heating and the material
Bioengineering having a great potential to treat patients exhibits enormous elasticity. SME of Nitinol involves phase
better. transformations between the two unique temperature
dependent crystal structures (phases) named Martensite
phase (lower Temperature phase) and Austenite phase (the
1.1 Shape Memory Alloys higher temperature parent phase). Body centered cubic
(BCC) structure is found in the Austenitic Nitinol. Upon
Smart Materials are materials that are able to respond to cooling, the austenite transforms spontaneously to a
external force and then undergo some property changes. martensite phase. This Transformation is diffusionless and
SMAs (Shape Memory Alloys) are one among many varieties involves an orderly shift of large groups of atoms, very
of Smart materials. SMAs are relatively new kind of metals rapidly. The degree of transformation depends on
that will show unique ability of “retaining’’ it’s set shape and a temperature. The crystals in Martensite phase have Face
practical phenomenon of shape shifting. Deformation occurs Centred Cubic structure (FCC). There are two types of
at a relatively low temperature, whereas shape memory effect martensite transformations: the temperature-induced
(SME) occurs on heating. Nickel–titanium alloys (was named transformation which causes the SME and the stress-induced
Nitinol, Nickel-Titanium Naval Ordnance Laboratory), some transformation which results in SE [1]. Shear Mechanisms
copper-base alloys (Cu–Zn–Al and Cu–Al–Ni alloys) and some are responsible for these transformations. The
other materials have been found to be capable of showing transformation temperatures are denoted as follows:
shape memory effect up to good extent of deformations. The
accidental discovery of the unique behaviour of SMAs is a Ms : Martensite start temperature upon cooling;
story in itself. All though Early research into SMAs took place Mf : Martensite finish temperature upon cooling;
in the 1930s, In 1932 Swedish chemist Arne Ölander first As : Austenite start temperature upon heating;
observed the pseudoelastic property in gold-cadmium alloys Af : Austenite finish temperature upon heating.
and the formation and disappearance of martensitic phase
observed by decreasing and increasing the temperature for a When Nitinol is heated starting at room temperature, it starts
Cu-Zn alloy. Use of SMAs have been seen greatly in Advance to change into Austenite Phase at temperature As and this

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transformation is completed at Af temperature. In the similar (VAR) . Subtle adjustments in the ratio of the two elements
fashion when it is cooled from Austenite phase, it starts to can make a large difference in the properties of the NiTi alloy,
transform into Martensite at Ms temperature and completes particularly its transformation temperatures. If there is any
this transition at temperature Mf. Crystals in Martensite excess nickel over the 50:50 ratio, one sees a dramatic
phase can have different variants (orientations in which they decrease in the transformation temperatures and an increase
can shear). Variants of Nitionol assemble either by Twinned in the austenite yield strength. Increasing the nickel-to-
Martensitic structure and Detwinned martensitic structure. titanium ratio to 51:49 causes the active Af to drop by over
Figure 1 shows the highly twinned Martensitic Structure of 373.15K [3]. Other non-conventional methods used are
Nitinol. Growing and Shrinking of Twin Boundaries occurs electro beam melting (EBM) and plasma arc melting (PAM).
dur to the applied external stress. Furthermore, under the Undeniably, every machining process has its benefits and
removal of the applied external stress, the deformed shape is limitations and hence a suitable process must be selected
retained at this temperature. Lastly, upon subsequent according to the application.
heating to the initial temperature, the material reverts back
to its original size and shape (stage 4). This process is The process of alloying of nickel – titanium is a very tactful
accompanied by a phase transformation from the deformed process. VIM ensures control over the elements that the
martensite to the original high-temperature austenite phase. molten metals may come in contact with but the inclusion of
For these shape-memory alloys, the martensite-to-austenite oxides or carbides is very hard to control in VIM. It prevents
transformation occurs over a temperature range, between the interaction of melted titanium with reactants in air
temperatures As and Af. preventing the formation of oxides and carbides. VIM is the
most widely used process for the commercial production of
NiTi alloys [4]. Carbon contamination is a major issue since
graphite crucible is used , carbon is highly soluble in nickel
and titanium reacts with carbon to form carbides, but
research on VIM of nitinol from 2000-2005 has found better
practices to get less inclusions and maintain relative high
purity. VAR is a secondary process which improves the purity
of the pre-cast.

In VAR the feedstock materials are pressed into a large

compact, which is fed as a consumable electrode downward
onto a hearth, where an electrical arc is struck; sufficient
current is passed to continuously melt the electrode and, as
molten metal is formed, the hearth is slowly moved
downward as the metal freezes at the bottom of the pool. The
molten pool is contained by a water-cooled copper collar,
which forms a frozen skull on the outside of the pool,
Figure 1 Diagram illustrates the SME of Nitinol and
preventing any contamination of the melt by a crucible. This
Schematic representations of the crystal structure at the four
keeps the purity of the material at the highest level possible.
stages.
[5]
The original shape (the shape given initially) is created by
heating to well above the Af temperature and then
restraining the material to the desired memory shape for a
sufficient time period. For example, for Nitinol alloys, a one-
hour treatment at 773.15K is necessary [2].

2. Material Processing of Nitinol

Nitinol is very much dependant on its internal grain structure

and the purity of the materials used since it gets its shape
memory attributes by them. Any small inclusions in the raw
materials or during the manufacturing may result in the final
product being weaker or may affect the temperatures at
which the shape memory effect take place. Since nitinol
contains a high percentage of titanium it is known to react Figure 2 Photograph showing the charging of nickel plates and
with air and also air may cause for inclusion of foreign bodies Titanium sponge rods in graphite crucible [6]
during manufacturing therefore it is desirable for a process
that eliminates air and which takes place in a closed Electro beam melting is a process that guarantees a high
environment. Nitinol ingots are melted using combinations of purity end product, it requires an expensive setup and has
vacuum induction melting (VIM) and vacuum arc remelting limited scalability that prevents its use in mass production.
By using EBM, the carbon contamination is completely
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eliminated due to melting in a water-cooled copper crucible, and cons of hot or cold methods we can assume the best
and oxygen contamination is minimized due to operation in method which has a minimal impact on the life of the end
high vacuum (better than 10-20 Pa). Therefore, the carbon product and does not affect the shape memory effect in a
and oxygen contents in the final product depend mainly on negative way. By exposing nitinol to high temperatures, we
their levels in the initial raw material [7]. There is need for risk the inclusion of oxides in the product, which may induce
extensive research here as pure alloys will crucially benefit brittleness and shorten the life of the product. Finally, the
Biomedicine for better quality of Apparatus and implants. product is cold worked and annealed at 873.15 – 1073.15 K.
After the ingots are casted and have undergone a secondary A combination of cold working and ageing treatments is
melting to remove inclusions, they are subjected to hot/cold necessary in order to achieve optimal superelastic
forging to form a cylindrical shape and then they are performance. [8]
hot/cold drawn to a suitable diameter. By weighing the pros

Table 1 : Strengths and limitations of casting/melting manufacturing methods of nitinol.

Method Strengths Limitations

VIM Easy operation, flexibility due to small batch High processing costs, TiC and carbon particles present
sizes, graphite crucibles are inexpensive and easy increasing Ni concentration and thus depressing phase
to handle, good melt chemical homogeneity transformation temperatures, extreme melt reactivity and
segregation possible, rapid grain growth.

VAR No need for a crucible, superior purity compared Susceptible to presence of small inclusions, remelting can
to VIM result in carbon and oxygen pick-up in case of vacuum leak,
extreme melt reactivity and segregation possible, rapid grain
growth

PAM High energy concentration, high plasma flow Low melted metal degassing, insufficient homogeneity;
velocity, quick heat transfer hence quick melting. multiple torches required for homogeneity

EBM No further carbon contamination, carbon content [Small volume production, poor chemical composition control
is 4–10 times lower than in VIM.

Table 2 : Comparison based on suitability to machine nitinol for medical applications.

Method Determining factors and their allowable ranges. Values exceeding recommended range are Suitability rating
shown in bold

VIM Carbon content: (300–700 ppm against a ≤500 ppm recommended , Can reach 0.22 wt.-% Good
(recommended ≤0.05 wt.%) on first crucible use ) Oxygen content: ≈0.025 wt.%
(recommended ≤0.05 wt.%) , Homogeneity: good due to electrodynamic stirring , Inclusions:
5–40 μm

VAR Carbon content: (≤100 ppm against a ≤500 ppm recommended ,Oxygen content: 0.03 wt.% Excellent
(recommended ≤0.05 wt.%) Inclusions: ≈ 17 μm (≤39 μm recommended)

PAM Carbon content: 0.0094 wt.% (recommended ≤0.05 wt.%), Oxygen content: 0.031 Good
(recommended ≤0.05 wt.%) Inclusions lesser than 5 μm, poor homogeinity as compared to
VIM

EBM Carbon content: 0.012–0.016 wt.% (recommended ≤0.05 wt.%) , Oxygen content: 0.01 wt.% Good
(recommended ≤0.05 wt.%), poor chemical composition

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3. PROPERTIES OF NITINOL DESIRABLE IN against failure.” “In order to analyze the lifetime of a device
subjected to both a cyclic and static strain, a modified
BIOMEDICINE
Goodman diagram may be used (Fig) [10]. First a strain
approach is used in the constructing the axes. But then, as
3.1 Corrosion and Surface Properties one might expect based on the shape of stress-strain
curves, the line itself is non-linear, departing from the
All the studies of surface modifications of Nitinol have been endurance limit along a tangent to the first yield, then
aimed at improving its corrosion behavior and It has been finishing at a second yield.” [11]
shown that localized corrosion resistance of bare Nitinol
may vary significantly, depending on its surface state.
However, in scratch corrosion tests when surface damage
is caused mechanically, the ability of Nitinol happens to be
inferior to that of pure Ti, though comparable with the
scratch healing ability of stainless steel. The potentials of
NiTi determined in scratch tests are low (from 150 to 300
mV) compared with PD and PS polarization, and this is the
problem to be targeted in the development of Nitinol
surface modifications [9]. When Nitinol implants receive
appropriate surface treatment through electropolishing
and passivation, they develop a passive titanium oxide
layer, which forms a barrier that will prevent corrosion and
release of toxic Ni ions into the bloodstream.

3.2 Fatigue in Nitinol based Biomedical Devices

It was wrongly thought that nitinol is poor in fatigue

resistance because of the non fatal fractures reported
during the end of 20th century. There was a wrong notion
that fatigue was not an issue for superelasticity as the Figure 3: Modified Goodman diagram may be used to analyze
material could recover from large strains with negligible the effects of mean strains on fatigue lifetime NiTi.
residual stresses. These medical devices started being used
clinically without even enough knowledge about the 3.3 Biocompactibility
fatigue in nitinol and the effect of the environment of use
on the device. Any material used for biomedical use must be
biocompatible, so as to avoid the induction of immune
Nitinol being a superelastic material also has its own responses, resist corrosion, and be radio-opaque.
drawbacks and limitations. Like the property of Biologically inert stent material is of atmost importance to
superelasticity provides high fatigue performance even in avoid all problems and safety. Biocompatibility is defined
the condition where the amplitude of force is high and as the ability of a material to perform with an appropriate
strain is high. But the minimum threshold after which host response in a specific application [Williams, 1987]. An
fatigue crack growth starts is lower than most metal which important aspect of the biocompatibility of implant devices
are used today in the devices with medical application. is their ability to perform desirable functions in close
Since there is no prior material degradation like plastic contact with body tissues. The major criteria for traditional
deformation or strain hardening prior to the fracture, it is medical alloys were high strength and corrosion resistance.
therefore difficult to observe and detect fatigue in nitinol. The higher the elastic modulus of an implant material,
Longer fatigue cycles, higher confidence levels on fatigue however, the higher the stress level exerted onto a bone.
limits and more specific device fatigue tests and fatigue Thus, good mechanical compatibility required carefully
failure modes must be evaluated. “Although the matching an implant’s elastic modulus to that of the elastic
thermodynamics and phase transformations of the shape- modulus of a specific bone. The elastic modulus of cortical
memory and superelastic effects have been widely studied, bone is 15– 30 GPa, while that of cancellous bone is 0.1–1.5
currently, there is a lack of information and understanding GPa [Cooke, 1996]. Nitinol has a low elastic modulus that,
of the micromechanisms that contribute to the fracture in the presence of porous material, can be reduced to even
resistance of Nitinol. At present, there are only very limited lower values (0.1–1.5 GPa), thereby making it a suitable
data in the literature which describe crack propagation in match for either bone type [Assad, 2003a]. Another
Nitinol alloys under monotonic or cyclic loading [9-11]; approach to the design of biomaterials, which is based on a
furthermore, none of the data in these studies concept of biomechanical compatibility, is rooted in the
characterizes the fatigue behavior of superelastic Nitinol.” knowledge of the mechanical behaviour of body tissues
“Thus, it is critical that the crack-propagation rates in [Fung, 1991, Gunther, 2000]. A functional implant material
superelastic NiTi are known in order to determine should be similar in its mechanical behaviour to that of
expected device lifetimes and provide a rational design living tissue.

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Patterns of nickel release from Nitinol is a central issue in rather large embolized blood clots. One more example is
Nitinol biocompatibility. Our understanding of the patterns osteosynthesis plates and staples, these can be inserted
of Ni release from Nitinol evolved significantly. and placed in the body. Then after heating it can help
Observations on Ti–Ni alloys prepared in laboratories closure the facture of the bones. [12] [13] [14]
[Ryhänen, 1997, Wever, 1998, Michiardi, 2006a] showed
that Ni release from Nitinol might be higher than from SS 3.6 Elastic Deployment
during the first days of exposure to biological solutions,
even though it dropped to almost undetectable levels after One of the reasons to use nitinol in medical device is that
10–14 days. Studies of Ni release from commercial with one or two small incisions will be certainly more
material, however, pointed at different patterns, where an appealing than the patient undergoing surgery with a long
increase was noted at the beginning of exposure and its scars and other complications that result from traditional
stabilization was observed only after a few months [Cisse, surgery. With using nitinol devices, it can be inserted
2002, Kobayashi, 2005, Sui, 2006, Clarke, 2006]. The through small openings and then expand to the desired
amount of released Ni differed significantly [Clarke, 2006], size and function. The concept of making a curved device
pointing at the effects of processing, which is in agreement through a straight needle is probably the most common
with variable Ni surface concentrations reported for use of Nitinol in medical instrumentation. One of the first
Nitinol wires (0.4–15 at.%) [Shabalovskaya, 2003a]. High- products was Homer Mammalok (shown in the figure
temperature treatments, which promote the formation of a below), which radiologists use to 'mark' the location of a
thicker external TiO2 layer, result in Ni accumulation in the breast tumour. It has a Nitinol wire hook inside a Stainless
internal surface layers. These buried layers can be easily Steel annulated needle. [12], [13], [14]
activated due to surface damage.

3.4 Kink Resistance

Nitinol is known to have good kink resistance. It is evident

from its stress-strain curve due to the presence of the
plateau. When strains are locally increased beyond the
plateau, stresses increase noticeably. Resulting in strain
partition in the areas of lower strain, uniformly distributing
the peak strain itself. Thus a more uniform strain is
realised, preventing kinking.

Figure 5 : Homer Mammalok Nitinol wire needle localizer.

4. BIOMEDICAL APPLICATIONS OF NITINOL

4.1 Nitinol Stents for Coronary Treatments

Coronary Artery Disease (CAD) and Peripheral artery

disease (PAD) are the narrowing of the arteries in the heart
and near the limbs respectively caused by the build-up of
fat and Calcium deposits called plaque. Apart from the
bypass surgery, Baloon Angioplasty and Coronary artery
stenting are sought out treatments. Dotter [15] was the
pioneer in the interventional cardiovascular world. He
Figure 4 : Presence of plateau region indicates high kink opened the way for intravascular stenting at the end of the
resistance 1980s. Subsequently, the field of Intravascular stenting has
seen stupendous progress due to advances in Material
3.5 Thermal Deployment Science and stent design. The composition of the stent
influences many of its properties, including radial strength,
One of the characteristic feature of Nitinol is shape deliverability and potential for restenosis. Conventionally
memory effect which means that increase in the stents are made of metals and alloys. The use of Alloys
temperature will trigger the pre set shape of it. One helps in achieving desirable stent properties much easily.
example is the Simon vena cave filter, this is preloaded Originally Bare metal stents (MBS) were made up of
into the delivery system in the martensitic state flushed stainless steel alloy 316L SS which possessed excellent
with chilled saline then positioned and released. The mechanical properties like Youngs modulus, Yield strength,
flowing blood will trigger the shape memory motion to tensile strength and corrosion resistance. Unfortunately, It
return to its original shape or pre-set shape so that it can was found that it lacks radio opaqueness. Inclusion of other

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metals for mitigation has not proved beneficial. Cobalt- forces to be produced when the wire is engaged in the
chromium Alloy has higher density, Young’s modulus, yield bracket slots of teeth. High strength would prevent any
strength, tensile strength and overcomes the limitations of permanent deformation when the wire is engaged in teeth
stainless steel as it possesses better radio-opacity and which are severely crowded [19].
better overall strength. These properties have enabled
stent struts to become thinner and still have the same The use of NiTi alloy in this field also demands for higher
ability to resist deformation as a thicker strut with a lower grade and pure nickel-titanium as any inclusions that are
elastic modulus [16]. Thinner struts improve the flexibility, not suitable may result in undesired effects or even
increase the inner diameter of the stent and mainly medical complications. Nitinol is preferred overall for
decreases the potential injuries. orthodontic use as it has general corrosion resistance as
compared to stainless steel wires that have shown to
The mechanical properties of the superelastic Nitinol alloy corrode after some use. Nitinol is also found to have high
have played a major role in the explosion of CAD and PAD working range compared to stainless steel or twist flex
stenting, with modern stents demonstrating reasonable wires shown in a study by Andrearson. In his study he
resilience and durability. The majority of nitinol stents are mentions in conclusion ”the time-linked performance of
of the self-expanding type. Self-expandable vascular stents one 0.019 inch cobalt unannealed nitinol wire will
made up of Nitinol exploits its pseudoelasticity property. approximate that of having to change two to four single
The stenting procedure using a Nitinol stent consists of: (i) strands of round levelling wire with the slope of the curve
setting the stent in open condition i.e. austenitic phase (Af following Hooks’ law to the elastic limit of approximately a
is lower than body temperature), (ii) compressing and 0.014 inch round stainless steel wire” [20].
inserting the stent into the catheter (iii) removal of the
sheath and expansion of the stent. The inverse 4.3 Clinical Instruments
transformation from martensite to austenite occurs during
this, which is due to the martensitic instability at a There is a growing market for nitinol in clinical
temperature higher than Af. Hence avoiding the usage of instruments. Instruments that are steerable, hingeless, kink
balloon and minimizing inflation problems caused by the resistant, highly flexible and that provide constant force
forces exerted by the cardiovascular tissues. The have all been developed. These include: Biopsy forceps
Biocompactibility of Nitinol stents are exceptional tissue ablators hingeless graspers and retrieval baskets for
compared to other MBS. However the mechanical laparoscopy. [21]
behaviour of Nitinol under multiaxial conditions remains
poorly understood and hence the deformation and fracture 4.4 Magnetic Resonance Imagining
behaviour for long term safe use is still to be researched.
Magnetic Resonance Imagining (MRI) is a non-invasive
imaging technology which is used as a diagnostic
4.2 Nitinol in Orthodontics
procedure to get high quality results in the form of
pictures. MRI scanners use strong magnetic fields to
Nitinol alloy in orthodontics was a revolutionary feat of generate the images of the organs in the body. Stainless
engineering as it allowed for newer research, faster and steel is susceptible to magnetic fields and as a result
easier ways to make dental corrections. George F interferes with the image to the point where it is
Andrearson was a determined researcher in this field a unrecognizable. Nitinol is very much less sensitive to
published numerous articles in the 1970’s. He mainly magnetic resonance and therefore yields a much better
focused on nitinol’s use in dental archwire. Archwire is results. [21]
used along with metal brackets to align the teeth to make a
person look cosmetically appealing. They also help to align 5. CONCLUSIONS
disfigured or even impacted teeth so as to make them more
functional or to prevent future misalignment. in
orthodontics an archwire also helps in conforming to the Our review has ventured far into the understanding of
alveolar or dental arch that can be used with dental braces NiTinol and its biomedical uses, but it is evident that far
as a source of force in correcting irregularities in the more research can be done with regards to this super alloy
position of the teeth. An archwire can also be used to and we expect it to provide solutions to problems we might
maintain existing dental positions; in this case it has a not know yet. Research about NiTinol in the field of Bio-
retentive purpose [17]. Levelling is the process in which medicine has many uses as discussed above and further
the incisal edges of the anterior teeth and the buccal cusps more in surgical tools and self tightening stitches that are
of the posterior teeth are placed on the same still under development and require further research .Our
horizontal level; and alignment is the lining up of teeth of review has exhibited the ways in which NiTinol has
an arch in order to achieve normal contact point improved pre-existing solutions by showcasing its
relationships [18]. Wires used in this initial phase in an material properties which has yielded the name super alloy
orthodontic treatment requires them to have low stiffness, .NiTinol has proven itself to be a robust, versatile and
high strength and long working range. The ideal wires to progressive super alloy that will bring forth a monumental
use in this phase of treatment is a Nickel-Titanium change in science and technology in the future.
archwires. Low stiffness will allow small

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