Junior Undergraduate Thesis

7/31/2019 Junior Undergraduate Thesis

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NiTi based Shape Memory Alloys and its Medical

Applications

Seminar Report

By

Divyanshu Singh

Roll No. 08011002

Aniket Patni

Roll No. 08011018

Department of Metallurgical Engineering and Materials Science

Indian Institute of Technology Bombay

(April 2011)


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4.2.1 Crystallographic Aspects in MT4.2.2 Superelasticity

4.3Transformation Thermodynamics4.3.1 Thermodynamic Characteristics

4.4Transformation Kinetics4.4.1 Reaction Order

4.5Thermodynamic & Kinetics Conclusion4.6Memory in MT4.7Stability in Material Response4.8Macroscopic Characterization of Shape Memory Alloys

4.8.1 Reaction Order4.8.2 StressStrain Temperature Tests

4.8.2.1Isobaric Tests4.8.2.2Isothermal Tests

5. Medical Applications ofNitinol.... 325.1Inferior Vena-Cava Filters5.1.1 Medical Complications5.1.2 Treatment Techniques5.1.3 Requirements of an Ideal Filter5.1.4 Advantages & Properties of Nitinol making it suitable for use as filters5.1.5 Disadvantages associated with using SMAs other than Nitinol

5.2Intra Vascular Stents5.2.1 Medical Complications5.2.2 Treatment Involved5.2.3 Requirements of an Ideal Stent5.2.4 Suitability of NiTi and properties that render it useful

5.3Stent Grafts5.3.1 Medical Complications5.3.2 Treatment Involved & Application of Nitinol

5.4Nitinol Guidewires & Other Surgical Instruments5.4.1 Medical Complications5.4.2 Suitability of NiTi for this application

5.5Orthopaedic Applications5.5.1

Medical Complications

5.5.2 Properties of Nitinol making it suitable for use5.6Dental Applications

References54


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1. INTRODUCTIONShape memory alloys (SMAs) exhibit two fascinating characteristic behaviors, viz., shape

memory (SME) and superelasticity (SE) or pseudoelasticity (PE). Hence, they are also

classified as smart materials or intelligent materials or active materials. Two broad

classes of SMAs exist, viz., thermally activated and magnetically activated.

The focus of the present review is on the more prevalent thermally activated SMAs. The key

characteristic of SMAs is the martensitic phase transformation, brought about by temperature

change and/or by application of stress. In SMAs, Martensitic Transformation (MT) is

between a high-symmetric, usually cubic, austenitic/parent phase (A/P) and a low symmetric

martensitic phase (M), such as their monoclinic variants. This reversible phase change can be

brought about by much smaller temperature change compared to conventional phasetransformation like solidification. MT is usually accompanied by significant changes in

mechanical, electrical and thermal properties that render them as prime candidates for the

development of smart structures and devices. Typical examples of SMAs are NiTi

(commonly referred to as nitinol), NiTiCu, CuAlZn and CuAlNi.

Shape Memory Effect

Fig 1: Schematic of phase transformation in SMAs; (a)-(b)-(c) is a typical shape memory

cycle. (a)-(c) under stress cycling represents Superelastic cycle; (a)-(c) under thermal cycling

represents another shape memory path.[2]


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2. Shape Memory AlloysA shape memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart

alloy) is an alloy that "remembers" its original, cold-forged shape: returning the pre-

deformed shape by heating. This material is a lightweight, solid-state alternative to

conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape

memory alloys have applications in industries including medical and aerospace Definition

by Wikipedia

Mf is the temperature at which the transition to martensite completes upon cooling.

Accordingly, during heating As and Afare the temperatures at which the transformation from

martensite to austenite starts and finishes. Repeated use of the shape memory effect may lead

to a shift of the characteristic transformation temperatures (this effect is known as functional

fatigue, as it is closely related with a change of microstructural and functional properties of

the material).

The transition from the martensite phase to the austenite phase is only dependent on

temperature and stress, not time, as most phase changes are, as there is no diffusion involved.

Similarly, the austenite structure receives its name from steel alloys of a similar structure. It

is the reversible diffusionless transition between these two phases that results in special

properties. While martensite can be formed from austenite by rapidly cooling carbon-steel,

this process is not reversible, so steel does not have shape memory properties.


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Fig 2.1 (T) represents the martensite fraction. The difference between the heating transition

and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost

in the process. The shape of the curve depends on the material properties of the shape

memory alloy, such as the alloying and work hardening. [B1]

2.1 Classification Shape Memory Alloys

Shape memory alloys are classified in many ways (like Two broad classes of SMAs exist,

viz., thermally activated and magnetically activated) but most it as they have different shape

memory effects. Two common effects are one-way and two-way shape memory. A schematic

of the effects is shown below.

Fig 2.2 - The procedures are very similar: starting from martensite (a), adding a reversible

deformation for the one-way effect or severe deformation with an irreversible amount for the

two-way (b), heating the sample (c) and cooling it again (d). [B1]

2.1.1 One-way memory effect

When a shape memory alloy is in its cold state (below As), the metal can be bent or stretched

and will hold those shapes until heated above the transition temperature. Upon heating, the

shape changes to its original. When the metal cools again it will remain in the hot shape, until
http://en.wikipedia.org/wiki/File:SMAtwoway.jpghttp://en.wikipedia.org/wiki/File:SMAoneway.jpghttp://en.wikipedia.org/wiki/File:SMAtwoway.jpghttp://en.wikipedia.org/wiki/File:SMAoneway.jpg


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deformed again. With the one-way effect, cooling from high temperatures does not cause a

macroscopic shape change.

2.1.2 Two-way memory effect

The two-way shape memory effect is the effect that the material remembers two different

shapes: one at low temperatures, and one at the high-temperature shape. A material that

shows a shape memory effect during both heating and cooling is called two-way shape

memory. This can also be obtained without the application of an external force (intrinsic two-

way effect). The reason the material behaves so differently in these situations lies in training.

Training implies that a shape memory can "learn" to behave in a certain way. Under normal

circumstances, a shape memory alloy "remembers" its high-temperature shape, but upon

heating to recover the high-temperature shape, immediately "forgets" the low-temperature

shape. However, it can be "trained" to "remember" to leave some reminders of the deformed

low-temperature condition in the high-temperature phases. There are several ways of doing

this. A shaped, trained object heated beyond a certain point will lose the two-way memory

effect; this is known as "amnesia".

2.2 General Properties and Manufacture

2.2.1 Uniqueness of Shape Memory Alloys

Many metals have several different crystal structures at the same composition, but most

metals do not show this shape memory effect. The special property that allows shape

memory alloys to revert to their original shape after heating is that their crystal

transformation is fully reversible. In most crystal transformations, the atoms in the structure

will travel through the metal by diffusion, changing the composition locally, even though the

metal as a whole is made of the same atoms. A reversible transformation does not involve

this diffusion of atoms, instead all the atoms shift at the same time to form a new structure,

much in the way a parallelogram can be made out of a square by pushing on two opposing

sides. At different temperatures, different structures are preferred and when the structure is

cooled through the transition temperature, the martensitic structure forms from the austenitic

phase.


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2.2.2 Superelasticity

Pure mechanical loading can induce MT when the material is in the austenitic phase. Fig1

schematically illustrates this effect. Upon loading Stress Induced Martensite (SIM) is directly

produced from pure austenite phaseleading to large macroscopic strains which are recovered

by unloading when the temperature is above Af. This is referred to as Superelastic or

Pseudoelastic effect and is illustrated in Fig 2.3. Critical stresses at which the forward and

reverse transformation occurs again illustrate the associated hysteretic behavior. Most of the

bio-medical and damping applications of shape memory materials exploit this effect.

Fig 2.3: Superelasticity effect; (a) load path a-b-c-d-e-a on stress-temperature diagram; (b)

stress-strain hysteresis response. [2]

2.2.3 Mechanical Properties

The yield strength of shape memory alloys is lower than that of conventional steel, but some

compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti

can reach 500 MPa. The high cost of the metal itself and the processing requirements make it

difficult and expensive to implement SMAs into a design. As a result, these materials are

used in applications where the super elastic properties or the shape memory effect can be

exploited. The most common application is in actuation.

One of the advantages to using shape memory alloys is the high level of recoverable plastic

strain that can be induced. The maximum recoverable strain these materials can hold without

permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5%

for conventional steels.
http://en.wikipedia.org/wiki/Pascal_%28unit%29http://en.wikipedia.org/wiki/Pascal_%28unit%29


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2.2.4 Manufacturing

The copper-based and NiTi-based shape memory alloys are considered to be engineering

materials. These compositions can be manufactured to almost any shape and size.

Shape memory alloys are typically made by casting, using vacuum arc melting or induction

melting. These are specialist techniques used to keep impurities in the alloy to a minimum

and ensure the metals are well mixed. The ingot is then hot rolled into longer sections and

then drawn to turn it into wire.

The way in which the alloys are "trained" depends on the properties wanted. The "training"

dictates the shape that the alloy will remember when it is heated. This occurs by heating the

alloy so that the dislocations re-order into stable positions, but not so hot that the material

recrystallizes. They are heated to between 400 C and 500 C for 30 minutes. Typical

variables for some alloys are 500 C and for more than 5 minutes.

They are then shaped while hot and are cooled rapidly by quenching in water or by cooling

with air.


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3. NitinolOur case of study

Fig 3.1 Nitinol Strip [3]

Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two

elements are present in roughly equal atomic percentages.

Nitinol alloys exhibit two closely related and unique properties: shape memory and

superelasticity (also called pseudoelasticity). Shape memory refers to the ability of nitinol to

undergo deformation at one temperature, and then recover its original, undeformed shape

upon heating above its "transformation temperature". Superelasticity occurs at a narrow

temperature range just above its transformation temperature; in this case, no heating is

necessary to cause the undeformed shape to recover, and the material exhibits enormous

elasticity, some 10-30 times that of ordinary metal.

3.1 History

The term nitinol is derived from its composition and its place of discovery: (Nickel Titanium

Naval Ordnance Laboratory). William J. Buehleralong with Frederick Wang, discovered its

properties during research at the Naval Ordnance Laboratory in 1962.

While the potential applications for nitinol were realized immediately, practical efforts to

commercialize the alloy didn't take place until a decade later. This delay was largely because

of the extraordinary difficulty of melting, processing and machining the alloy. Even these


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efforts encountered financial challenges that weren't really overcome until the 1990s, when

these practical difficulties finally began to be resolved.

The discovery of the shape-memory effect in general dates back to 1932 when Swedish

researcher Arne Olander first observed the property in gold-cadmium alloys. The same effect

was observed in Cu-Zn in the early 1950s.

3.2 NitinolMechanism Introduction

Fig 3.2 Austenite and Martensite structures of the NiTi compound. [B2]

Nitinol's unusual properties are derived from a reversible, solid state phase transformation

known as a martensitic transformation.

At high temperatures, nitinol assumes an interpenetrating simple cubic crystal structure

referred to as austenite (also known as the parent phase). At low temperatures, nitinolspontaneously transforms to a more complicated monoclinic crystal structure known as

martensite. The temperature at which austenite transforms to martensite is generally referred

to as the transformation temperature (To).

Crucial to nitinols properties are two key aspects of this phase transformation. First is that

the transformation is reversible, meaning that heating above the transformation

temperature will revert the crystal structure to the simpler austenite phase. Upon heating,

however, there is a slight upward shift in the temperatures, now beginning at the A s

temperature, and finishing at the Af temperature. The second key point is that the

transformation in both directions is instantaneous.

Martensite's crystal structure (known as a monoclinic, or B19' structure) has the unique

ability to undergo limited deformation in some ways without breaking atomic bonds. This
http://en.wikipedia.org/wiki/File:Nitinol_Austenite_and_martensite.jpg


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type of deformation is known as twinning, which consists of the rearrangement of atomic

planes without causing slip, or permanent deformation. It is able to undergo about 6-8%

strain in this manner. When martensite is reverted to Austenite by heating, the original

austenitic structure is restored, regardless of whether the martensite phase was deformed.

Thus the name "shape memory" refers to the fact that the shape of the high temperature

austenite phase is "remembered," even though the alloy is severely deformed at a lower

temperature.

A great deal of force can be produced by preventing the reversion of deformed martensite to

austenite - in many cases, more than 100,000 psi. One of the reasons that nitinol works so

hard to return its original shape is that it is not just an ordinary metal alloy, but what is

known as an intermetallic compound. In an ordinary alloy, the constituents are randomlypositioned on the crystal lattice; in an ordered intermetallic compound, the atoms (in this

case, nickel and titanium) have very specific locations in the lattice. The fact that nitinol is an

intermetallic is largely responsible for the difficulty in fabricating devices made from the

alloy.

Fig 3.3 The effect of nitinol composition on the Ms Temperature [B2]

The scenario described above (cooling austenite to form martensite, deforming the

martensite, then heating to revert to austenite, thus returning the original, undeformed shape)
http://en.wikipedia.org/wiki/File:Nitinol_Ms_vs_Ni_content.jpg


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is known as the thermal shape memory effect. A second effect, called superelasticity or

pseudoelasticity is also observed in nitinol. This effect is the direct result of the fact that

martensite can be formed by applying a stress as well as by cooling. Thus in a certain

temperature range, one can apply a stress to austenite, causing martensite to form while at the

same time changing shape. In this case, as soon as the stress is removed, the nitinol will

spontaneously return to its original shape. In this mode of use, nitinol behaves like a super

spring, possessing an elastic range some 10 to 30 times greater than that of a normal spring

material. There are, however, constraints: the effect is only observed some 0-40 degrees C

above the Aftemperature.

Nitinol is typically composed of approximately 50 to 51% nickel by atomic percent (55 to

56% weight percent). Making small changes in the composition can change the transitiontemperature of the alloy significantly. One can control the Af temperature in nitinol to some

extent, but convenient super elastic temperature ranges are from about -20 degrees to +60

degrees C.

One often-encountered complication regarding nitinol is the so-called R-Phase. The R-Phase

is another martensitic phase that competes with the martensite phase mentioned above.

Because it does not offer the large memory effects of the martensite phase, it is, more often

than not, an annoyance. We will study the complete mechanism in just a short while

3.3 Making nitinol and nitinol devices

Nitinol is exceedingly difficult to make due to the exceptionally tight compositional control

required, and the tremendous reactivity of titanium. Every atom of titanium that combines

with oxygen or carbon is an atom that is robbed from the NiTi lattice, thus shifting the

composition and making the transformation temperature that much colder. There are two

primary melting methods used today:

Vacuum Arc Remelting: This is done by striking an electrical arc between the rawmaterial and a water-cooled copper strike plate. Melting is done in a high vacuum,

and the mold itself is water cooled copper, so no carbon is introduced during melting.


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Fig 3.4 Annealing Line [3]

Vacuum Induction Melting: This is done by using alternating magnetic fields to heatthe raw materials in a crucible (generally carbon). This is also done in a high vacuum,

but carbon is introduced during the process.

While both methods have advantages, there are no substantive data showing that material

from one process is better than the other. Other methods are also used on a boutique scale,

including plasma arc melting, induction skull melting, and e-beam melting. Physical vapor

deposition is also used on a laboratory scale.

Hot working of nitinol is relatively easy, but cold working is difficult because the enormous

elasticity of the alloy increases die or roll contact, leading to tremendous frictional resistance

and tool wear. For similar reasons, machining is extremely difficultto make things worse,

the thermal conductivity of nitinol is poor, so heat is difficult to remove. Grinding (abrasive

cutting), Electrical discharge machining (EDM) and laser cutting are all relatively easy.

Heat treating nitinol is delicate and critical. It is the essential tool in fine-tuning the

transformation temperature. Aging time and temperature controls the precipitation of various

Ni-rich phases, and thus controls how much nickel resides on the NiTi lattice; by depleting

the matrix of nickel, aging increases the transformation temperature. The combination of heat

treatment and cold working is essential in controlling the properties of nitinol.


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4. SHAPE MEMORY EFFECT: MECHANISM4.1 Martensitic Transformations

The face centered cubic austenite transformed into lens shaped or plate like regions with

body centered cubic lattices or body centered tetragonal lattices. The crystals created by such

transformations are called Martensite and lattice transformations without atomic diffusion

are called Martensitic Transformations or simply, MTs.

Diffusionless martensitic transformations have since been observed in many metals, alloys

and compounds other than steel. Today the term martensitic transformation is widely used

and signifies one type of phase transitions in solids.

Martensitic transformation can be defined simply and precisely: a lattice transformation

involving shearing deformation and resulting from cooperative atomic movement. The atoms

within the lens or plate shaped areas in the parent phase are not shifted independently but

undergo shearing deformation as a unit while maintaining a domino- like coordination until

the parent lattice transform into martensite. With this kind of cooperative movement of

atoms, a 1-to-1 correspondence, called a lattice correspondence, persists between the lattice

points in the parent phase and the points in the martensite phase. When the parent phase has a

superlattice structure, this ensures that the martensite phase obtained from transformations

maintaining such a lattice correspondence will have a specific superlattice.

The properties listed below follow from the martensitic transformations:

1. The martensite phase is a substitutional or interstitial solid solution.

2. The transformation is diffusionless. The concentration of solute atoms dissolved in the

martensitic phase is equal to that in the parent phase. There is no long distance diffusion such

as occurs in eutectoid transformations.

3. The transformation is accompanied by shape changes (or surface relief) of a definite value.

If the surface of a specimen which begins to transform below room temperature is, in the

parent(P) stage, polished and made planar and then cooled below room temperature to induce

the transformation, some regions where the martensite (M) phase appears on the surface will


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exhibit relief effects like those shown in Fig(a).. Further, if a scratch line is constructed on

the surface in the line at the boundaries between the P and M phases as shown in (b). The

inclination of the surface relief and the bends in the scratch line will have definite values

depending upon the crystal orientation of the P phase. This is because the shape changes

accompanying the transformation are of a definite value and is typically taken as proof that

the transformation mechanism involved is shearing deformation.

Fig 4.1 Formation of Surface Relief and Bending of Scratch Line Accompanying Martensitic

Transformation [1]

4. An M crystal has a specific habit plane. This is the interference between the P and M

phases in Fig (4.1a), that is the plane along which shear occurs during the transformation.

Habit planes are specified by indices of planes in the parent phase: there are many cases in

which they are irrational.

5. There is a definite orientation relationship between the P and M phase lattices.

6. Lattice defects necessarily exist in the M crystal. Even if a shearing deformation equal to

the measured shape change is induced along the habit plane in the P phase lattice, a true M

phase lattice cannot be obtained. This contradiction can be resolved from the following

considerations. As shown in Fig 4.2, when a shearing deformation (b) like that which

changes P phase lattice to the M phase lattice is induced, complimentary slip (c) or twinning

(d) deformations occur.


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Fig 4.2 Lattice Deformation and Complimentary Slip Shear and Twinning Shear (dotted lines

show the actual shape change) [1]

Such complimentary deformations are called lattice invariants strains and their traces, such

as dislocation and stacking or twinning faults have been observed by electron microscopes.

The twin faults especially perform a major role in the shape memory effect and two closely

related phenomena, twinning pseudoelasticity and bending pseudo-elasticity.

4.2 Characteristics of the Martensitic Transformation in Shape Memory AlloysMartensitic transformation (MT) which has several fascinating features has been investigated

extensively in the context of both single and polycrystalline SMAs. Even after nearly four

decades of effort, several unresolved and challenging aspects in MT make it still an open

problem for active research. Thus, various disciplines in materials research ranging from

molecular dynamics to continuum irreversible thermodynamics at macroscale are being

employed. A brief discussion on several relevant aspects of MT in SMAs is presented here to

facilitate better understanding of modeling approaches for SMA response.

4.2.1 Crystallographic Aspects in MT

As noted earlier, MT is a diffusionless reversible solid-solid phase transformation occurring

by activation and/or nucleation and growth of the martensitic phase from a parent austenitic

phase.


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Generally inelastic shear or distortion associated with the crystal lattice is the dominant

deformation mechanism with only cooperative and collective motion of atoms on distances

smaller than the lattice parameters. The characteristic size of martensitic plates is very small,

typically of the order of few microns. The deformation mode is twinning rather than slip

(occurring during conventional plastic deformation) of the crystallographic planes which

gives it the reversible nature. A lattice invariant strain-free plane, also called habit plane

exists at the interface between the product and parent phases. Depending on the alloy, the

lattice vectors of the two phases possess well defined mutual orientation relationships (the

Bain correspondences).

Fig 4.3: Different martensitic transformation pathways in NiTi based alloys [2]

These bring out the coherency aspects of the two different microstructures and play a critical

role in the kinetics and morphology of phase transformation process. The absence of

diffusion makes MT almost instantaneous and also athermal or thermoelastic. Theoretically,

the transformation occurs at the speed of sound in the material . However, in polycrystalline

materials, it is possible that under certain conditions, the transformation occurs at much

slower rates.

Generally monoclinic B19 yields maximum transformation strain, followed by orthorhombic

(B19) with least transformation strain seen in trigonal (Rhombohedral- R) phase. The number

of variants of the lower symmetry structure that can form from the parent cubic structure is

determined by the nature of symmetry of the parent structure and the product structure.

Hence, even in a single crystal parent, when there is transformation, the product is


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polycrystalline. For instance, in NiTi, 24 monoclinic variants can form from the parent cubic

phase. One of the important aspects in MT is the arrangement of these multiple variants and

the product and parent phases. In the absence of stress, these variants form twins wherein

the lattice strain is self-accommodated such that no macroscopic or long-range strain occurs.

For instance, pure thermal cycling of SMA under no stress would lead to cyclic

transformation between the parent austenite and twinned martensites. However, stress biases

the type of variant that forms (detwins and/or reorients the variant) giving rise to net

transformation strain. This is responsible for the SME and SE described above. In a single

crystal, the transformation strains can be really large (>>10%). However, in polycrystalline

SMAs, presence of grain boundaries, defects, precipitates etc., influence the morphology of

the evolving phase leading to higher complexity and heterogeneity. Thus, the reversible

macroscopic strains are much less (typically 5-6% for NiTi). One of the important aspects in

SMAs is the dependence of the phase morphology on the deformation history of the material;

this will be discussed later in this section. This is microscopically related to the memory or

path dependency in the response of material giving rise to hysteretic response. In this context,

in polycrystalline SMAs, the texture or the orientation of the grains in the material plays an

important role in the transformation characteristics and hence in SE and SME. Concomitant

to MT are significant changes in some of the material properties like moduli, specific heat,

thermal conductivity, electrical resistivity, which influence the thermomechanical response.

Issues of compatibility of microstructure, strain energy associated with lattice distortion,

transformation strain and the stress that arises due to incoherency and heterogeneity play a

vital role in determining the evolution of the MT. Since MT is treated as a first order

transformation, notion of order parameter associated with breaking of symmetry is used to

describe or characterize the transformation. Evolution of the phase can be related to

evolution of the order parameter and hence evolution of the inelastic strain associated with

the transformation. Another aspect that influences MT is the generation, growth and

stabilization of dislocations and/or defects in the material during transformation. These

introduce complexities in the microstructure evolution and influence the macroscopic

response. The crystallographic aspects outlined above give a brief sketch of the kinematics of

MT and factors influencing it.


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4.3 Transformation Thermodynamics

Driving forceIn order to induce the P M transformation, the chemical free energy of the

M phase must be lower than that of the P phase. However, since the transformation requires

an excess of non-chemical free energy ( such as transformation strain energy and interface

energy ), if the difference between the chemical free energies of both phases is not greater

than the necessary non-chemical free energy, the transformation will not begin Fig 4.4 In

other words, a driving force is necessary. If the specimen is not supercooled to a suitably low

temperature Ms below the equilibrium temperature T0 ( where the chemical free energy of the

M and P phases are equal), the transformation will not progress. A driving force is also

necessary for the reverse transformation, the specimen must be superheated to a suitably high

temperature As above T0.

Fig 4.4 Temperature Dependence of the Chemical Free Energies of the Parent and

Martensitic Phases and their relations to Martensitic Transformation [1]

Now,

Assume a lens shaped M crystal with radius r and average thickness 2t (r >> t) has nucleated.

Then,

Interface Energy = 2r2,

Here, 2r

2

is the approximate surface area

is the interface energy per unit area. The value ofthe expression varies widely according to the degree of coherency strain along the P-M

interface. Assuming the interface involves Frank dislocation loops, is estimated to be the

order of ( 1.2 ~ 2.4 ) X 10-5

cal/cm2.

The elastic strain energy is = r2t(At/r) = rt

2 A


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r2t is the M crystals approximate volume and A(t/r) is the elastic strain per unit volume.

This strain is estimated to be 500 cal/cm3

at 250c. This sort of elastic strain is generally not

large enough to promote a reverse transformation when the specimen is heated, but in certain

alloys it does promote the reverse transformation which then becomes thermoelastic.

Apart from the above, there are other energies due to plastic deformations and elastic

oscillations. The former requires twinning or slip deformations in the form of lattice invariant

strain in the M crystal, the energy required for both types of plastic deformations is thought

to be enormous. If we assume for the time being that plastic deformation occurs only within

the M crystal, then by analogy with elastic energy,

r2t(Bt/r) = rt

2B

Since the energy of elastic oscillations is thought to be small, the main forms of non-

chemical free energies produced during the transformation are given by above formulas.

Accordingly, the total energy change due to M crystal nucleation is

G = r2tgc+ 2r

2 + r

2t(A + B)

Where gc is the change in the chemical free energy per unit volume.

At temperature Ms, when the radius r of a lens shaped M crystal nucleus exceeds a critical

value, the change in the chemical free energy, the first term on the right side of the above

equation will exceed the sum of the non-chemical free energies given by the second and third

terms. The nucleus will therefore grow and the transformation will proceed. The difference

between T0 and Ms is called the degree of supercooling. The degree of supercooling in

martensitic transformations in ferrous alloys and steels can be as much as 2000C, but in shape

memory alloys it is only 5~30oC. We can observe different degrees of cooling in Table 1.

4.3.1 Thermodynamic Characteristics

It has been experimentally observed that, in thermoelastic martensitic transformations, the M

crystals first formed at the temperature Ms are the last to undergo the reverse transformation

at temperature Af. In these transformations, the change in the total free energy can be written

G(T)PM

= gcPM

(T) + (gncPM

) + gsPM

For the reverse transformation, the change is,


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G(T)MP

= gcMP

(T) + (gncMP

) + gsMP

gcis the change in the chemical free energy and gnc is the increase in the non-chemical

free energy (considering only the elastic energy accumulated in the thermoelastic

transformation). The energy term gs corresponds to the forces resisting either the growth

and shrinkage of existing M crystals or the creation and annihilation of new M crystals.

Table 1 Table of Data for Alloys which exhibit a Complete SME. [1]

4.4 Transformation Kinetics

Generally martensitic transformation advances only when the temperature drops below M s.

In other words, it is an athermal transformation. When this type of transformation occurs in

ferrous alloys or steels, the M crystals do not continue to grow after they are form. The

transformation process proceeds by nucleating new M crystals in the remaining P phase

material. Individual M crystals are then nucleated within the solid and grow quickly at about

1/3 the speed of elastic waves in solids. However, there are also cases when, if a temperature

above Ms is maintained, or if the specimen is super cooled below Ms and that temperature is

maintained after a partial athermal transformation has been induced, the transformation will

commence after an incubation period specific to the material. The extent of transformation

will increase with the passage of time. This type of behaviour is called an isothermal


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transformation. Here too, the transformation advances with the nucleation of new M crystal,

and the M crystals are created and grow at very high speeds.

Regardless of whether the transformation is isothermal or athermal, the individual M crystals

appear and grow quickly into their final size and do not grow further even if the temperature

drops or time passes. When considering the nucleation and growth of individual, we call this

sort of transformation non-thermo-elastic. Conversely, in thermo-elastic transformations,

once M crystals are nucleated, they grow at a velocity proportional to the cooling rate during

dropping temperatures; some grow at rates detectable with the naked eye. Similarly, the

crystals shrink when heat is applied. Such thermo-elastic martensitic transformations are also

crucial to the realization of the shape memory effect.

4.4.1 Reaction Order

Thermodynamically, MT in SMAs is classified as either a weak first order or a second order

transformation (Ehrenfests criterion). First order transition requires only continuity of free

energy of individual phases at the equilibrium temperature (T0) whereas the second order

transformation additionally requires continuity in gradient of free energy at T 0. In a regular

first order phase transformation like solidification, there is a sharp change in the energy

parameter with respect to a state variable like temperature. Also, there is long range

movement of atoms (diffusion). However, in SMAs, though there is no diffusion, significant

strains result in sharp change in the gradient of the energy parameter. Thus, the

transformation in SMAs is displacive in nature and is a weak first-order or second order

transformation. The transformation usually occurs over a temperature range (Mf < Af) and

the start and end temperatures are affected by the stress. Generally, the transformation

temperatures increase almost proportionately with stress. Further, the difference between the

start and the end temperatures of martensitic and austenitic phases are approximately same.

The interdependence of transformation temperature and stress is usually expressed using the

Clausius-Clapeyron relation, in this context given by,


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where, is the stress, H, the change in enthalpy due to transformation and 0, the total

inelastic strain, 0 is the molar volume, , the change in entropy due to transformation.

Usually the stress-transformation temperature relationship is obtained by conducting physical

experiments.

The athermal aspect of kinetics in MT implies that, in general, the process depends on the

instantaneous temperature (or stress) and not on the sweep rates of these driving parameters.

This implies a certain rate-independence in MT. Apparent rate effects seen in macroscopic

response like changes in shape of hysteresis due to rate of the loading parameter is more due

to the latent heat effects associated with phase transformation. Due to unstable (metastable)

nature of transformation, thermodynamic formulation based on equilibrium approach is

useful only in bringing out some essential aspects of transformation. Heat transfer, diffusion

and propagation effects that are not accounted for in the equilibrium approach are to be

considered in order to fully describe the thermoelastic behavior.

4.5 Thermodynamic & Kinetics Conclusion

From the above discussion it is clear that both thermal and elastic energies play a vital role in

phase transformation in SMAs. Further, there is a strong interaction or coupling between

these two energies manifesting as a highly nonlinear and coupled thermomechanical

behavior. For instance, even when MT is purely stress induced, it is accompanied by

exchange of latent heat of transformation. The forward (PM) transformation is exothermic

while the reverse transformation is endothermic. The amount of latent heat involved depends

on the alloy composition and processing history. In addition, MT is dissipative in nature

involving inelastic deformation.

Kinetics of phase evolution in terms of conditions for onset, direction of transformation

(forward PM or reverse MP), depending on the nature of loading and the amount ofphase evolution for a given load increment is central to understanding of MT. In this context,

several thermodynamic concepts like free-energy, dissipation potential, entropy production

and driving force are used extensively to describe MT. Usually, Helmholtz or Gibbs form of

free energy is used to explain MT in terms of the relative stability of the parent and product

phases. The temperature at which the chemical parts of free energies of the two phases are


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equal is termed as equilibrium temperature (T0). In a defect free single crystal, the

transformation occurs instantaneously at T0, the direction of which is determined by the sign

ofT.

This unstable transformation at the microscopic level manifests as jump in the stress-strain

response. The slope of the stress-strain curve during transformation is associated with the

interaction between the parent and product phases and the thermal equilibrium due to latent

heat.

Fig 4.6: Potential for A and M phases as a function of temperature. To is the equilibrium

temperature. Due to friction and other internal factors, additional energy is needed to initiate

transformation (undercooling to Ms or superheating to As) [2]

However, presence of defects, grain boundaries etc., alters the transformation temperature

requiring either undercooling or superheating (Fig 4.6). Once initiated, the transformation is

not self-sustaining (as in an initiation controlled process) and additional driving force is

necessary for further transformation (propagation controlled process). Additional driving

force is needed to overcome friction and other dissipative forces that exist due to interaction

energy. From a macroscopic perspective, MT is assumed to occur through a sequence of

metastable states associated with athermal nature of kinetics. Minimization of free energy is

used as the criterion to determine the state of SMA. For example, cubic austenite transforms

into several energetically equivalent, yet crystallographically different variants of martensite.


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Fig 4.7 shows the influence of stress on Landaus free energy parameter (based on Gibbs

free energy). It shows two symmetric low energy wells corresponding to the two variants of

martensite considered. Instead of the initial symmetry in the free energy structure

corresponding to different self-accommodating variants (under a stress free condition) the

direction of applied stress biases the free energy wells leading to the formation of martensites

with specific orientations thereby accumulating large macroscopic strain. The isothermal

stress-strain response shows two stable branches corresponding to the pure phases (P and M).

During transformation, equilibrium of the phase mixture line gives a line with negative slope

(corresponding to fully reversible and dissipation less transformation). Unstable branches at

different temperatures are identified based on the free energy structure as a function of

temperature and order parameter (in this case, the strain).

Fig 4.7 Schematic of free energy as a function of strain (order parameter) for a two variant

martensite and austenite transformation. [2]

4.6 Memory in MT

Path dependency in MT is attributed to the (rate of) entropy production in these materials. A

suitable notion of memory or information about the evolution for the load history within

that transformation zone is essential while defining the kinetics under arbitrary loading.

Ideally, due to history dependency, to predict the instantaneous state of the material during


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transformation, the information about the entire load-deformation history is essential. In

order to capture the history dependency, suitable state variables are introduced into the free

energy function. However, under arbitrary thermodynamic loading, definition of memory

poses a significant challenge. Another aspect adding to the complexity is that the nature of

memory changes during transformation history over many cycles. The response for arbitrary

thermomechanical loading manifests as:

Shakedown or training or stabilization of material (stability in the hysteretic response)

Incomplete transformation (partial and inner hysteresis loops).

4.7 Stability in Material Response

An important aspect that governs the hysteretic response is the stabilization of the material

behavior under cyclic thermomechanical loading. Under repeated loading, the material

behavior is said to be stable, if the hysteresis for each load cycle is stable (shows no drift).

Else, the behavior is said to be non-stabilized and this could typically manifest as remnant

strain and/or stress at the end of each cycle, which could accumulate during subsequent

cycles (ratcheting response). This leads to non-closure of hysteresis loops. Stability is

desirable from a device or an application perspective and hence special thermomechanical

treatments are imparted to the material to achieve the same. For instance, dislocations and

defects are deliberately introduced and grown through thermomechanical process to obtain

desired levels and repeatability of SME and SE. Factors influencing stabilization are the load

levels, history and number of cycles at the given load level. Due to history dependency of the

material response, stability is observed only under loads not greater than the previous

maximum load levels. For higher loads, a certain number of additional cycles are necessary

before stabilization occurs. Such transformation behavior can be interpreted as imperfect

memory which needs multiple cycles to become perfect.


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Fig 4.8: Different inner or partial loop behavior; (a) return point memory behavior in a

commercially available NiTi ; (b) Schematic sink point behavior. [2]

4.8 Macroscopic Characterization of SMAs

4.8.1 Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) is a thermo-analytical technique that is often used

to characterize thermally induced stress-free transformation in SMAs. ASTM F2004 provides

a standard procedure to conduct the tests and report the results. In DSC, essentially, the

difference in the amount of heat required to increase the temperature of a small quantity of

the sample (about 10 mg) and the reference are measured as a function of temperature. Both

the sample and reference are maintained at nearly the same temperature during thermal

cycling over the entire transformation temperature range at a typical rate of 10 oC/min. A

typical output from DSC test on SMA with and without R-Phase in terms of heat flow rate

versus temperature is shown in Fig 9. From these, the transition temperatures and the

enthalpy of transformation (H) associated with MP (heating) and PM (cooling)

transformations are determined. H is measured as the area under the transformation peaks

during the heating and cooling cycles. A more sophisticated test is the modulated DSC that

can differentiate both the reversible and irreversible components of heat flow. This is useful

to resolve multimode transformations or other second order effects.


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Fig 4.9: Heat flow vs. temperature trace in DSC for SMAs (a) without R-phase and (b) with

R-phase. Transformation temperatures are identified by drawing tangents as shown. [2]

4.8.2 Stress-strain-temperature tests

Although DSC is an excellent experimental technique to obtain the SMA transition

temperatures, it is suitable only under stress free condition. In order to characterize the

thermomechanical hysteresis of SMAs, an experimental technique to study the material

behavior under heating and cooling under a constant stress is required. Generally, two types

of tests are frequently employed for this purpose.

1. Constant stress thermal cycling (iso-baric test)

2. Constant temperature mechanical cycling (isothermal test)

Several important material parameters like moduli, total transformation strain (both one-way

and two-way), and Clausius-Clapeyron relation are estimated from these tests. This

information is used to obtain the phase diagram (discussed in detail in next section).

4.8.2.1 Isobaric tests

On either the tensile coupons or wires, constant stress is applied and the SMA is thermally

cycled between the martensite and austenite conditions. This experiment is usually conducted

with SMA under a dead weight or in a UTM under constant force mode. The rate of heating

and cooling is kept sufficiently low (order of 5 oC/min). In this case, SMA shows a hysteretic

strain-temperature (-T) response. A typical -T hysteresis response is plotted for a given


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stress level as shown in Fig10. It is not always easy to identify the transformation

temperatures from this plot. For engineering purposes, usually, tangents are drawn to the

heating and cooling segments (Fig 4.10) to obtain the transformation temperatures (Ms, Mf,

As and Af). More analytical methods such as numerical differentiation of the curve can be

used to identify change in slope or curvatures and associate them with the critical

temperature. Another parameter that is obtained from this test is the amount of

transformation strain.

Fig 4.10: Strain-temperature hysteresis at 100 MPa stress for a commercially available NiTi.

[2]

4.8.2.2 Isothermal tests

Tensile tests involving complete load-unload cycle covering the range of SIM and its reverse

transformation are often performed to characterize SMA response. The tests have been done

in a controlled thermal environment at different constant temperatures; the range depending

on the transformation temperatures. The strain rates are typically small to obtain quasistatic

response (order of 0.001 /s). Typical stress-strain curves at a few temperatures are given in

Fig 11. Critical transformation stresses for both forward and reverse transformation are

obtained from this test as shown in Fig 4.11. The transformation strain can also be

determined from this test.


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Fig 4.11: Stress-strain response and tangents drawn to determine critical transformation

stresses for a commercially available NiTi. [2]


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5 Medical Applications of Nitinol

5.1 Inferior Vena Cava Filters

Fig 5.1Nitinol Filter [5]

5.1.1Medical Complication -

Pulmonary embolism occurs when a fragment of blood clot detaches and reaches the lungs

via veins. As a result it causes shortness of breath, pain during inspiration and may ultimately

lead to heart failure and death. The blockage due to the emboli (fragment from the blood

clot) occurs more often from the lower extremities or pelvis.


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Fig 5.2 Diagrammatic representation of vessels of interest in a) Venous System b) Arterial

System [7]

5.1.2 Treatment Techniques:

(a)The complication is removed either by the use of anti-coagulant drugs (such as heparin or

warfarin which reduce the clotting ability of blood) or is surgically removed. Anti-coagulants

or drugs administered to inhibit thrombosis may be ineffective or unsuitable in certain cases

owing to the danger to patients health

(b)Another method involves surgery in which the inferior vena cava is closed by external

means such as clips, ties or sutures. As a result of this the venous blood reaches the right

hand side of heart through the other venous pathways which are small enough to disallow

emboli from entering.

The above method involves surgery causing further pains to the patient already ill from the

pulmonary embolism. In order to overcome this drawback a method employing minimum

invasive surgery was investigated and employed.


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(c)A method requiring minimum invasive surgery uses a filter which is deployed inside the

inferior vena cava. It is placed inside the veins where it traps the emboli in its frame where

they dissolve over a period of time.

Procedure: The filter consists primarily of two parts: the filter and the locking system. The

locking system is made of sharp tipped leading and trailing wires which penetrate the

endothelium to a depth of 1-2mm to hold the filter in place. The sharp tipped ends are

prevented from further penetration by means of small metal studs.

Fig 5.3 Diagram for Delivery System for Filter [7]

Outside the body the filter is straightened in ice cold water and placed inside a catheter which

travels over a guidewire and releases the filter at the desired location. The filter is placed

inside the catheter in a manner such that the rear stud of the locking system is attached to a

special notch which helps to retain the filter inside the catheter. Throughout this process a

constant flow of cool saline is maintained through the catheter in order to prevent the filter

from taking shape before reaching the intended location. Once at the deployment site the

flow of the chilled saline solution is stopped and the straightened wire is pushed out of the

catheter where due to the body temperature it reverts back to its original shape of a filter.


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Fig 5.4 Deployment of Filter [7]

5.1.3 Requirements of an Ideal Filter

Be deliverable into the IVC using a standard angiographic catheter. Be able to be delivered quickly, accurately and have no problem of wrongful

orientation. Problem can be dealt by filter shapes. However implanting complex

shapes can be difficult which is assisted by SMA effect.

Look itself into position and not migrate heart ward. Capture all dangerous emboli. Be non-thrombogenic and have good biocompatibility.

5.1.4 Advantages & Properties of Nitinol making it suitable for use as a Filter

Shape Memory Effect and Thermal Deployment- The one way shape memory effect of

Nitinol alloys is utilized is to collapse the filter into a straightened wire and revert it back to

its original shape that is of a filter once it has been deployed at the desired spot inside the


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Inferior vena cava. The shape memory effect allows for deployment of filters having

complex shapes into the vein. Properties such as Shape Memory Effect and Superelasticity

present in Nitinol further assist in making it more suitable for use as Inferior Vena Cava

Filter in the following ways

A correspondingly smaller entry hole is likely to reduce the risk of bleedingcomplications in a patient who is on anticoagulant drugs.

It allows deployment through an anticubital vein(in the elbow) where the deploymentand the puncture site is more easily controlled. {diagram}

One of the main characteristics exhibited by SMAs has already been utilized to greateffect. This is the ability of large complicated memorized structures to be drawn into

small diameters allowing percutaneous implantation and avoiding the risks of

surgery. On placement the alloy retains its original shape.

Biocompatibility- Nitinol is an alloy of Nickel and Titanium and though nickel is a

carcinogenic and biologically incompatible material it is more than compensated by the use

of Titanium which forms a layer of Titanium oxide on the surface of the alloy and thus

prevents the presence of pure nickel on the surface. Titanium being a biocompatible material

together with Titanium oxide renders biocompatibility to the alloy. Nitinol has been shown to

be non-thrombogenic.

Another concerns regarding the biocompatibility of nitinol is concerned with its cytotoxicity.

Cytotoxicity refers to the damage that the material may cause on cells and is usually

measured during in vitro testing. It has been shown that the cytotoxicity of nitinol is

comparable to that of other alloys used for implantation. Surface processing has been

observed to have a significant impact on cytotoxicity.

MR compatibility- Nitinol is non-ferromagnetic material with a lower magnetic susceptibility

than stainless steel. MRI compatibility is directly related to the susceptibility of the material

relative to human tissue. Nitinol thus provides a clear crisp image. MRI is a diagnostic

technique that yields high quality cross-sectional images of the body. It involves short bursts

of a powerful magnetic field and does not employ X-rays or other forms of radiation.

Stainless steel is susceptible to magnetic fields and as a result interferes with the image to the


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point where it becomes unrecognizable, nitinol however is very much less sensitive to

magnetic resonance and therefore yields a much cleaner image and can thus be easily tracked

inside the body. The nitinol filter when compared with permanent filters has the additional

advantages of easy surveillance and retrievability.

5.1.5 Disadvantages associated with using SMAs other than Nitinol such as stainless

steel-

It uses a cut down procedure of a major vein requiring dissection of either neck orgroin, of an already ill patient so that the relatively large delivery capsule can be

placed through a venotomy.

The orientation of the filter is difficult to control on delivery within the Inferior venacava and unless the apex of the filter is centralized the efficiency of the filter may be

reduced.

5.2 Intravascular Stents

Fig 5.5 Intravascular Stent [5]

5.2.1 Medical Complication- Angioplasty is a technique for treating occlusion (blockage) or

stenosis (narrowing) of a blood vessel or heart valve. It is a medical treatment to remove the

constriction of the blood vessels. It is used extensively to restore correct blood flow and for

the treatment of coronary heart disease.

5.2.2 Treatment Involved


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All vascular stents work on the same basic principlethey all hold up the walls of the artery

by mechanical means, ensuring a clear pathway. Stenting prevents recoil exhibited by arterial

walls and may also trap other debris against the vessel walls thus allowing a reduction in the

possible thrombus generation at the dilation site.

a) Balloon Expandable Stent- Essentially the treatment involves the passing of a thin

guidewire through the femoral artery until it is just past the blockage. A balloon tipped

catheter is then passed over the guidewire and pushed along it until the balloon reaches the

occlusion or stenosis. The cylindrical shaped balloon is then inflated to open and widen the

blocked vessel. Subsequently the catheter, the balloon and the guidewire are withdrawn.

However it may happen due to elasticity of the vessel wall, that it may recoil and fail to

remain dilated after the balloon is removed. This procedure thus suffers from a relatively

high rate of restenosis in the dilated region.

b) Self Expanding Stent- Permanently implantable metal cylinders known as stents are often

used to support the walls of the vessel and maintain arterial lumen. Stents are either balloon

expandable where the angioplasty balloon is employed to both open the blocked vessel and

expand the stent or as self-expanding where as the stent is pushed out of the catheter it

immediately opens out to support the already diluted solution.

The stent is shape set into the open condition and then compressed and inserted into the

delivery catheter. When the distal end of the catheter is in the correct position the stent can be

pushed out and will self-expand against the vessel wall. The catheter may then be withdrawn

leaving the stent permanently in place.


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Fig 5.6 Intravascular Stent portrayed in a cut away view of the internal carotid artery [5]

Two techniques used to deploy Ni-Ti stents

Cold Saline Technique- this involves a Ni-Ti wire whose TransformationTemperature Region is around body temperature. The stent is transformed at a

temperature well below its TTR and inserted into the delivery catheter where it is kept

cool by the flow of cold saline around it. On reaching the desired site it is pushed out

of the catheter and regains its original shape as it warms to body temperature.

Hot Saline Technique- This involves Ni-Ti with TTR just above body temperature. Itis inserted using a catheter into the placement site where warm saline is flushed over

the stent and warms the shape memory alloy above its transition temperature hence

returning it to its original shape.

5.2.3 Qualities required of an Ideal Stent

Stable support Good visibility Ease of deployment High expansion ratio


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Biologically inert Thrombus resistant Reliable Expandability

5.2.4 Suitability of Ni-Ti & Properties that render it useful.

a) Superelasticity or Elastic DeploymentOne of the most common reasons to use Nitinol is

to allow the efficient deployment of a medical device. Modern medicine is constantly striving

towards less and less invasive procedures. Entire operations of increasing complexity are

performed through small leak tight portals into the body called trocars. These procedures

require instruments and devices that can pass through very small openings and then

elastically spring back into the desired shapes. The concept of elastically deploying a curved

device through a straight needle or cannula is the most common use of Nitinol for medical

instrumentation. A correspondingly smaller entry hole is likely to reduce the risk of bleeding

complications in a patient. It allows deployment through an anticubital vein(in the elbow)

where the deployment and the puncture site is more easily controlled.

The compliance or elasticity of an engineered component depends upon design as well as the

inherent elasticity of the material used. For instance one can increase the compliance of a coilspring by adding coils, but this would increase weight and size. Material properties dictate

the total elastic energy stored in the device. The use of Nitinol allows one to design more

compact, stiffer and more elastic devices by increasing the elastic energy storage.

Superelasticity allows one to pass a rather complex instrument through a straight trocar and

the instrument to elastically return to the deployed configuration once through. Nitinol

provides the ability to the stent to deal with large strains.

b) Dynamic Interference- Dynamic interference refers to the long range nature of nitinolstresses. For example in case of a balloon expandable stainless steel stent, following balloon

expansion after the balloon is deflated there is a spring back causing the stent to revert to a

smaller undeformed shape. This phenomena is called acute recoil and is highly undesirable as

it leads to the expansion of the stent to large dimensions which can either damage the vessel

or cause restenosis. In case if the vessel diameter relaxes with time or undergoes a temporary


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spasm a stainless steel stent will not follow the vessel wall causing a reduction in the applied

stresses or the stent could even embolize. In contrast the forces in a nitinol stent are of a long

range nature. The stent is oversized for the vessel and continues to apply an outward force

until it fully reaches its preset diameter.

Fig 5.7 Stress-Strain Curve for Nitinol [4]

c) Hysteresis Loop: The stress strain behaviour of bone and tendon fits with the hysteresis of

nitinol. High elasticity, low deformation forces and constant force over wide ranges of strain

are characteristics of human tissue and bone as well as nitinol. Stainless steel stents will tend

to force the vessel straight; Nitinol is much more compliant to bends in the vessel con

contours in the lumen

The other aspect of the hysteresis loop that can be exploited in stenting applications is that of

the upper and lower plateau stresses. Ideally a stent should resist crushing during normalphysiological processes (radial resistive force) yet exert a small outward force on the vessel

wall during recovery (chronic outward force). In the hysteresis curve of Nitinol the upper

plateau represents the force required to deform the stent or the force that resists crushing

(radial resistive force) and the lower plateau represents the force exerted on the vessel during

self expansion. With appropriate heat treatments and alloy selection these plateaus may be


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fine tuned for a given stent application. The plateau of the chronic outward force also means

that the stent continues to exert constant force over a considerable strain recovery range.

Fig 5.8 Stress vs Strain response for Nitinol, Stainless Steel, Bone & Tendon [4]

d) Shape memory effect or Thermal DeploymentThe shape memory effect of the Nitinol is

utilized to convert the complex shape of the stent into a relatively simpler one which can be

transported via the catheter to the desired spot inside the body where it is made to revert back

to the original shape by employing the higher temperature of the body.

e) Bio-compatibilty- Bio-compatibility is defined as the ability to perform with anappropriate host response in a specific application. It is well known that titanium is not toxic

when used inside the human body, nickel on the other hand is extremely toxic. Nickel is a

carcinogenic material that is cancer inducing. In Nitinol however there is the formation of a

passive layer of titanium oxide on the surface that protects the surrounding body from

contacting nickel. The titanium oxide layer also acts as both a physical barrier to nickel

oxidation as well as protects the bulk metal from corrosion.

f) MR compatibility- Nitinol is non-ferromagnetic material with a lower magneticsusceptibility than stainless steel. MRI compatibility is directly related to the susceptibility of

the material relative to human tissue. Nitinol thus provides a clear crisp image. MRI is a

diagnostic technique that yields high quality cross-sectional images of the body. It involves

short bursts of a powerful magnetic field and does not employ X-rays or other forms of

radiation. Stainless steel is susceptible to magnetic fields and as a result interferes with the


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image to the point where it becomes unrecognizable, nitinol however is very much less

sensitive to magnetic resonance and therefore yields a much cleaner image and can thus be

easily tracked inside the body.

5.3 Stent Grafts

5.3.1 Medical Complication :A major problem involved with the installation of stents inside

the body is the possibility ofrestenosis. Restenosis involves the growth of tissue on the open

pattern of stent and thus creating a blockage. This can be prevented by covering the open

stent frame with a graft material.

Another problem involves the repair of abdominal aortic aneurysms(AAA). An abdominal

aortic aneurysm occurs when the pressure from the blood flow causes the wall of the

weakened artery to swell and expand. With time the aneurysm can become so swollen and

weakened that it may rupture thus leading to death. Abdominal Aortic Aneurysm results in

mortality rates approaching 90%.

5.3.2Treatment Involved & Application of Nitinol: The surgical repair of AAAs involves

by passing the constricted section of the artery and therefore the removal of the aneurismal

sac from circulation. Conventional treatment involves surgery and is thus traumatic for the

patient as well as carries a high risk. The surgical method involves accessing and opening the

aorta, cleaning of any debris and insertion of a graft to bypass the aneurysm and sewing of

the aorta back together. This operation has both high morbidity as well as high patient

recovery times (upto 6 months).

The treatment involving the use of Nitinol involves delivering the graft through a catheter via

the femoral artery and carries with it average recovery periods of just 11days. Nitinol has

found application in this procedure as the material for the supporting frame of the graft

material.


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Fig 5.9 AAA Stent Graft System [4]

5.4 Nitinol Guidewires & Other Surgical Instruments

5.4.1 Medical Application :

Angiography- Angiography or arteriography is a medical imaging technique involving the

passage of catheters into arteries and veins. It is used for the examination of blood or lymph

vessels by application of X-rays. In this process a radiopaque substance is introduced into the

body. Angiography involves introducing a radiopaque contrast medium through a catheter

into the cardiac chambers or coronary arteries. This assists in the X-ray imaging of blood

flow and vessels within the cardiovascular system and can also be used to locate blockages in

circulation.

Interventional radiologists employ X-ray imaging techniques such as X-ray fluoroscopy and

MRI to guide sophisticated instruments and carry out advanced medical procedures.

Fig 5.10 Nitinol Guidewire


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Deployment of Catheters: The catheters are deployed by passing them over guidewires which

have earlier been inserted through an artery and manoeuvred into the desired position. In this

respect the guidewire acts as a rail for the catheter to travel along until it reaches the distal

location.

Complications of the Procedure :

If a catheter is inserted through the femoral artery at the groin and the final target is for

instance the heart or the brain, then the route through which the guidewire must pass to

deploy the catheter can be very long and tortuous. The brachial artery(just above the elbow)

or carotid arteries (in the neck) are also possible insertion points. These insertion points are

however smaller in size.

The route through which the guidewire must pass often involves very small vessels with tight

radii and branching points. The strains imposed on the guidewires can thus result in

permanent deformation and kinking of the wire.

Hingeless Instruments: Minimally invasive endoscopic surgery is intended to reduce the pain

and trauma of access to the body without compromising exposure of the operating field for

the surgeon. By working through an endoscope with small precision instruments, large

painful access wounds are avoided, internal tissue trauma is reduced, hospital stays are

shortened and recovery is accelerated. Conventional instrument development efforts have

taken the form of miniaturization of mechanical linkages in hinged type designs, resulting in

highly complicated systems with many individual parts which are difficult to assemble.

Hingeless instruments use the elasticity of spring materials instead of pivoting joints to open

and close the jaws of grasping forceps or the blades of scissors. Because of their simple

design without moving parts and hidden crevices, they are easier to clean and sterilize. A

new generation of hingeless instruments uses superelastic Nitinol as the actuating componentof these instruments as it provides higher elasticity. This results in an increased opening span

and/or reduced displacement of the constraining tube for better handling.


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Fig 5.11 Hingeless vs Conventional Instrument Design [6]

The nonlinear stress/strain characteristic of Nitinol provides a constant force gripping of

large and small objects. This reduces the risk of tissue damage.

5.4.2 Suitability of Ni-Ti for this Application :

a) High Recoverable Strains : Recoverable strains of upto 8% are possible if appropriate

processing of nitinol is carried out. This high elastic limit and associated plateau stress make

nitinol behave in a very flexible manner.

b) Good Kink Resistance : The kink resistance of nitinol is very closely related to the plateau

in the stress vs. strain curve of the alloy. If the localized strain in a guidewire is such that it

exceeds beyond the end of the plateau region then the localized stress increases sharply. This

increased stress is accommodated by surrounding areas of lower strain. This means that the

localized peak strain is more uniformly distributed and kinking is prevented. These features

allow bending through tortuous paths while preventing strain localization and plastic

deformation.


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Fig 5.12 Stress Strain response of Nitinol & Stainless Steel [4]

c) Steerability & Torquability : The ability of the nitinol wire to pass through tortuous paths

during angiographic procedures without permanently deforming or kinking greatly improves

its torquability (The ability to translate or twist one end of the guidewire and into a turn of

nearly identical degree at the other end) and steerability. They are both dependent on the

ability of the guidewire to translate twist and motion from the proximal end to the distal end.

Any small kink or permanent deformation in the wire will cause the distal end to whip and

accurate steering and torque translation becomes almost impossible.

5.5 Orthopaedic Applications

5.5.1 Medical Complication:

a)Scoliosis- Scoliosis is by basic definition a deformity of spine. A normal spine is notabsolutely straight, instead it curves in the lower region however it should still remain

vertical. In scoliotic backs the spine is seen to have one or more lateral curves. These curves

may vary in severity and can cause disability or internal organ difficulties.


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Fig 5.13 Lateral Curvature Caused by Scoliosis [7]

Treatment Involved : Surgical internal correction which involves the use of a distraction rod

to correct spinal curvature. The distraction rod technique involves the attachment of hooks

under the laminae of vertebrae above and below the curve to be corrected. The spine is then

longitudinally straightened by an external device and a distraction(Harrington) rod attached

to the hooks. Bone material is then applied around the spine to provide a basis for tissue

growth and eventual bone fusion in the straightened position. In conventional treatments

which did not involve the use of Nitinol after the correcting force exerted by the distraction

rod decreases to approximately 30% of its original value, the re-establishment of this force

required a second operation to be performed. The use of Nitinol however eliminates a second

surgery as after a set period of time the alloy may be externally heated above its austenitic

forming temperature and hence transform into its preset shape and exert the same or nearly

the same force as originally applied.


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Fig 5.14 L shaped rod used for stepwise correction of Scoliosis [7]

b) Surface Arthroplasty- Knee replacement or knee arthroplasty is a surgical procedure to

replace the weight bearing surfaces of the knee joint to relieve pain. It involves the placement

of a corrosion resistant cup on the top of the femoral head. The use of shape memory alloy

simplifies this procedure by the utilization of inverted memorized hooks. At zero degree the

hooks are deformed so that the cups can be placed on the femoral head. Once set the

warming created by body temperature reverts the hooks back into their original position and

in doing so clamps the alloy cup to the bone.

Fig 5.15 Diagrammatic representation of a Ni-Ti cup used in Surface Arthroplasty [7]

c) Treatment of Fractures- In cases of osteotomy, bone fracture or bone fusion the hospitals

involved used wavy staples with sixty degree spikes at each end. The staples are elongated at

zero degree along their U-section and spikes placed within the pre-drilled holes in the bones.

Upon warming by a hot saline pack the wavy section returns to its original shape and thus

produces a compressive force between the two subject bones. This compressive force

encourages tissue growth and hence quick fracture recovery.


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Fig 5.16 Diagrammatic representation of Ni-Ti orthopaedic staples a)Before deformation

b)After deformation at the time of implant c)Return to the original shape causing a

compressive force [7]

Fig 5.17 Schematic of Compressive Staple used. [7]

5.5.2 Properties of Nitinol making it suitable for Use

a) Constant Stress: An important feature of Nitinol is that it exhibits constant unloading stress

over large strains. Thus the force applied by the Superelastic device is determined by

temperature, not strain as in conventional materials. Since body temperature is substantially

constant, one can design a device that applies a constant stress over a wide range of shapes.


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This property of Nitinol is utilized in correction of spine by use of distraction rods as well as

the treatment of fractures of bone.

Fig 5.18 Stress-Strain response of Nitinol highlighting the region of constant stress over

varying strain [5]

b) Biomechanical Compatibility: Stainless steel, titanium and other metals are very stiff

relative to biological materials yielding very little in response to pressure from the

surrounding tissue. The extraordinary compliance of nitinol makes it the most mechanically

similar to biological materials. This improved physiological similarity promotes bone in

growth and proper healing by sharing loads with surrounding tissue. A large number of

orthopaedic devices take advantage of this property, including hip implants, bone spacers,

bone staples, skull plates etc.


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Fig 5.19 Stress vs Strain response for Nitinol, Stainless Steel, Bone & Tendon [4]

5.6 Dental Applications

Dental Arch Wires or Braces- The adjustments that the teeth are subjected to during an

orthodontic treatment are due to remodelling of the bone by the force exerted by braces. As a

result the mechanical stimulation allows the remodelling of the periodontal level. The

strength of the force must in a narrow range to permit a proper correction of the dental

malformations. Too much force leads to the absorption of the bone while too little force

increases the possibility that the remodelling will not occur.

The use of Nitinol for orthodontic arch wires is favoured because of its superelasticity. The

unloading curve for Nitinol is flat over large strains. Stainless steel and other conventional

wires are tightened by the orthodontist often to the point of causing pain. As treatment

continues the teeth move and the forces applied by stainless steel quickly relax. This causes

treatment to slow retarding tooth movement. Re-tightening by the orthodontist recycles the

process, with only a narrow optimum treatment period. In contrast, Nitinol wires are able to

move with the teeth applying a constant force over a broad treatment time and tooth position.


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Fig 5.20 Stress-Strain Response of Nitinol highlighting the region of constant stress over

varying strain. [5]


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References

1. H. Funakubo, Shape Memory Alloys, translated from Japanese by J.B. Kennedy,Standford University , Gordon and Breach Science Publishers, (1987), pp 1-58

2. Ashish Khandelwal and Vidyashankar Buravalla, Models for Shape Memory AlloyBehavior: An overview of modeling approaches, GM R&D, India Science Lab,

Bangalore, India, International Journal of Structural Changes in Solids Mechanics

and Application, Volume 1, Number 1, December 2009, pp. 111-148

3. Diego Mantovani, Shape Memory Alloys: Properties and Biomedical Applications,JOM October 2000

4. N.B Morgan, Medical shape memory alloy applications- the market and itsproducts, Materials Science & Engineering A 378 (2004) 16-23.

5. T. Duerig, A. Pelton and D. Stockel, An overview of Nitinol Medical Applications,Materials Science and Engineering A273-275 (1999) 149-160.

6. Dieter Stockel & Andreas Melzer, The use of Ni-Ti alloys for Surgical Instruments,Materials in Clinical Applications 1995.

7. Lipscomb and L D M Nokes, The Application of Shape Memory Alloys inMedicine Antony Rowe Ltd. Wiltshire (1996) pp 27-110.

Bibliography

1. http://en.wikipedia.org/wiki/Shape_memory_alloy2. http://en.wikipedia.org/wiki/Nickel_titanium


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Declaration

In the preparation of this `B.Tech. Seminar Report the following members of the

team have contributed to different parts of the report, nevertheless all of us have gonethrough the whole report carefully.

Name & Roll No. Contributed mainly

to the following

chapters / sections

Contents

(titles / subtitles only)

(i) Aniket Patni

08011018

All Shape Memory Effect: Mechanism

(ii) Divyanshu Singh

08011002

All Medical Applications of Nitinol

Further, we are aware that if the report write-up does not conform to the standard

format (given on the MEMS website), it (report) is likely to be rejected.

The oral presentation of different parts of the seminar shall be made by the respective

student(s).

Signatures:

Date: 5th April 2010

__________________

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Junior Undergraduate Thesis