Source: http://www.onlinejet.net/article.asp?issn=0976-8580;year=2015;volume=5;issue=1;spage=1;epage=7;aulast=Sharma;type=3
Timestamp: 2019-04-18 21:06:27+00:00

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Nickel-titanium (Ni-Ti) is mostly used in medical industries for orthopedic implants, orthodontic devices, cardio stents etc., due to its good bio-compatibility, osseointegration and corrosion resistance  . Because of its shape memory effect (SME) and super elasticity, Ni-Ti (Nitinol) is also quite suitable for automatic actuators, spacecraft, electrical devices, and micro-electromechanical systems (MEMSs), etc. Therefore, Nitinol is generally categorized as a smart material.
Shape memory effect is the ability to regain the original shape by an alloy of some fixed shape upon subsequent heating above the transition temperature that had been subjected to deform at low temperatures  . Ni-Ti exhibit equiatomic inter-metallic compound of Ni-Ti, which can undergo a reversible solid state phase transformation from ordered cubic crystal structure (B2) called austenite to distorted monoclinic (B19') called martensite. Crystal structure of martensite can be obtained when binary Ni-Ti alloys are quenched from high temperature. The temperature at which austenite cooled and it begins to convert into martensite is called martensite start temperature (Ms) and the temperature correspond to martensite finish is called martensite finish temperature (Mf). Similarly temperature correspond to heating of martensite to convert it into austenite is called austenite start temperature (As) and austenite finish temperature (Af)  as shown in [Figure 1]. Shape memory behavior can be classified as one-way effect and two-way effects. In one way, only the shape of the parent phase is recovered, while in two-way, shape memory alloy (SMA) remember the shape of both parent and product phase, and it can be accomplished by heating and cooling the specimen  .
Super elasticity or pseudoelasticity (PE) is the other distinct property of Nitinol. Super elasticity is the ability of material to recover reversibly high strain values significantly higher than those of the classic metal or alloys. This SME and PE behavior can be manipulated by controlling the composition, grain size and performing suitable heat treatment  .
Due to the unique properties of Ni-Ti such as shape memory effect, super elasticity, bio-compatibility, osseointegration etc., it has growing applications in vast areas of engineering from aero-space to biomedical industries. These applications are discussed in detail below.
In medical field, NiTi alloy has significant applications like in stent technology; orthopedics; orthodontics and cardiovascular, neurosurgical field etc., since it possess good bio-compatibility and osseointegarion. Bio-compatibility is the ability of the material to perform with an appropriate host response in a particular application  . Osseointegration is a direct bone-to-biomaterial interface, in which there is no fibrous tissue  . It is not applicable to the description of biomaterial interactions and only for description of clinical performance. Ni-Ti in the form of wire, are used in endovascular prosthesis  while Ni-Ti prosthesis has been successfully used in clinical surgery like fixation of spinal dysfunctions, etc  . In orthodontic treatment, NiTi wires which are in austenitic phase at the temperature of the buccal cavity, have been successful used with multi brackets. Super elasticity is exploited for generating constant force, after positioning of the wire into the brackets, for wide dental movements.
Thrombosis is a tendency of a material in contact with the blood to coagulate the white or red thrombus on its surface  . The probability of corrosion at the implants due to hostile electrolytic environment of human body is very high. The anti-thrombosis property, SME and PE effect makes the NiTi most favorable for vascular treatment , . Ni-Ti alloy forms a protective layer of TiO 2 , that prevent the release of Ni ions and improves the corrosion and shows good bio-compatibility. NiTi alloys are also used in numerous applications of the self-expandable vascular stents. Self-expandable stents are used to treat atherosclerotic lesions in the coronary arteries, the carotid arteries, and in the peripheral arteries  .
The tendency of corrosion of Ni-Ti with human biological system is more at mechanical and thermally loaded conditions. Therefore, prolonged application of orthodontic wire that are under tensile loading, may release unwanted Ni + ions and bio-compatibility of such material becomes an important issue  . Nickel ion depletion can also be prevented after surface treatment of Ni-Ti by creating a nonreactive oxide layer. The coating increases the surface resistance and bio-compatibility. The peel strength can also be increased by 300% after surface modification and is an innovative approach in self-expanding coronary stents preparation. The release of nickel ion can be reduced if surface nickel concentration is in the same range as in the bulk of super-elastic material. NiTi alloys in the neurosurgical field are used for producing three types of devices: (1) coils, (2) stents and (3)microguidewires. Coils are devices used for the treatment of cerebral aneurysms, which are localized dilations of the intracranial arteries. Stents are also used for the treatment of intracranial atherosclerotic disease. Microguidewires made of NiTi are used for stent positioning with the advantage of obtaining a higher strain recovery and torsion resistance as well as a better stress distribution (due to the plateau in the stress-strain curve), which reduces the guidewire bending problems.
Corrosion and stress shielding are the major problem in orthopedic implants made of steel alloy (316 L stainless steel) and Ti alloy (Ti-6Al-4V). Mismatch of Young's moduli of the biomaterials and the surrounding bone has been identified as the foremost reason for implant loosening following stress shielding of bone. Nitinol has a lower value of stress shielding effect as compared to steel alloys  . Strain recovery in Nitinol is several times more than ordinary steel alloys [Figure 2]. [Table 1] compares the physical and mechanical properties of Nitinol, stainless steel and titanium alloys and Ni-Ti comes out as a better choice for biomedical implants.
Although elastic modulus of Ni-Ti is higher than the elastic modulus of a natural bone but it can be lower down to the value of bones by increasing the porosity of Ni-Ti structure. This reduces the problem of stress shielding ,, . Porous structure favors long-term stability by providing high in-growth of living tissues and firm fixation  . In porous metallic implants, pore size and its interconnectivity significantly affect the adhesion and new tissue in-growth ,,, . If the pore size is too large or too small, the cell will fail to spread and form networks throughout the scaffold  . Large value of porosity increases the surface area, which results in high cell seeding efficiency, migration and neovascularization  . Macro pores (≥50 μm) influence tissue function and micro pores (<50 μm) influence cell function (e.g., cell attachment). Optimum pore size of 20 μm have been repeated for fibroblast in-growth, 20-125 μm for hepatocytes and 100-250 μm for regeneration of bone  .
Porosity of Ni-Ti has also a noticeable influence on super-elasticity, that is, the ability to recover strain after deformation as shown in [Figure 3], representing the stress-strain curve of loading and unloading compressive test conducted by Xiong et al.  . The plateau stress for Ni-Ti observed a remarkable change with the change in porosity. With the increase in porosity, the value of stress and elastic moduli are observed to be decreased. The residual strains are 2.2%, 2.8%, and 4.5% for samples with porosities of 71%, 80% and 87%, respectively. Therefore, mechanical and bio-properties of nitinol can be altered pore-size and pore inter-connectivity which depends upon fabrication method.
Nickel-titanium has gained wide acceptance as actuators in mechanical and aero-space industries. A great deal of attention of Ni-Ti is mainly due to the high-strain rate response  more flexible and resistance to cyclic fatigue. The desired shape memory and/or pseudo-elasticity of Ni-Ti can be achieved through optimized cold worked and heat treatment  , while the transition temperature depends on the chemical composition of the alloy.
Shape memory alloys have a significant role in space systems where SMAs are utilized in antenna and solar array, etc ,, . In air-crafts, the morphing wings and nozzle used for noise reduction in gas turbine engine are also made of SMA  . These SMA are thermally stimulated through joule heating to regain its original position. Due to high electrical resistivity of Ni-Ti alloy, joule heating is provided by passing an electric current ,[ 34] . Various SMA actuators such as wire, compression and tension springs and cantilever had been used in both electrical and thermal actuation systems. Thermal actuators of SMA are used as both sensors and actuators. Another application of SMA is the actuation of various components for orientation of solar flaps of satellites to optimize the performance.
MEMS have grown from laboratory fabrication to large scale manufacturing. High-abrasion resistance, pseudo-elasticity and corrosion resistance are the main properties which cause the Ni-Ti to be used as MEMS. The work done per unit volume is excellent in Ni-Ti as contrast to other available micro-level actuators. The actuation speed of Ni-Ti based MEMS depends upon heating speed/cooling speed of Ni-Ti. A high-heating rate can be reached by applying a high electrical current, but the cooling speed depends on the heat dissipation capacity  . NiTi films are sputter-deposited on silicon substrates and then etched to form the actuator element. The film is then back etched to separte it from silicon substrate. The MEMS devices are used for the control of liquid and gas flow in manufacturing processes, as pneumatic controls in instruments, and potentially, for medical delivery systems.
The main challenging task in Ni-Ti based actuators is how to prestrain the Ni-Ti thin films, so that when some thermal practice is provided, actuation can be triggered simultaneously by the shape memory effect in prestrained SMA thin films. The difference in coefficient of thermal expansion is a driving force to prestrain the Nitinol. This is the main working principle for prestraining most likely in silicon wafer ,, , but the maximum value to prestraining is limited.
Number of applications comes into existence for various electrical devices, because of the invention in the field of Ni-Ti SMA , . Working principle of an electrical circuit protection system includes that the prestretched Ni-Ti SMA wire breaks the circuit because of over-heating. The reaction speed is at 1/100 s level. SMA thermal actuators are also used in domestic safety devices. One of the most frequent causes of injury in the household and in hospitality buildings such as hotels is excessively hot water in the sink, tub and shower. An antiscald valve is now being produced which employs a small cantilever NiTiCu element which, when heated to critical temperature, the temperature above which scalding will occur, closes the valve. The valve automatically reopens when the water temperature is safe  .
Another recent SMA safety device is a thermally activated current interrupt mechanism for protecting high energy density batteries such as lithium ion cell from uncontrollable temperature increase due to overcharge or short circuit.
Shape memory alloy based heat engines are used to extract mechanical energy from low-grade energies for instance, warm wastewater, geothermal energy, etc. Turbine engine, offset crank engine, field engine and kick engine comes under SMA heat engines.
In SMA heat engine, the temperature required for stretching is slightly lower than the temperature which is required for SMA coil shape recovery. The difference in the work required to enhance shape memory characteristics of coils in heat engines.
SMA spring/coil heat engines have low efficiency less than 5%, and this can be improved by difference between the lowest and highest temperature ,,, . High-temperature hot water is capable of high-speed rotation, whereas low-temperature hot water is capable of low-speed rotation. In short if difference between high and low temperature is small, SMA heat engine can be designed with high efficiency.
Before new era of the energy crisis, many researchers drop the idea to work on SMA heat engine because of low efficiency. Few old techniques come into existence because of dispersion for a new source of clean energy. From a number of year's ocean thermal energy conversion has been investigated, which consume the difference of temperature between sea surface and deep sea.
It has a number of benefits like high consistency and high stability, which can be used other than energy produced by wind, tide in the region where water temperature at sea surface level is constant. Ocean thermal energy on earth could be probable endless new clean energy for human forever if properly investigated. For extracting ocean thermal energy with high efficiency, large temperature difference as possible.
Maximum possible temperature difference between deep sea and surface is practically <24°C even below to 1000 m from the sea plane.
Shape memory alloy heat engine could be resourceful even temperature difference is minute, and this can be formed using SMAs for utilizing ocean thermal energy with small hysteresis and narrow transition temperature range.
To provide energy for long-term, solar energy used for supplementary power supply. Currently, solar cells are used mainly for these purposes, which convert solar energy into electrical energy. Solar cell can be used directly or in the form of battery for later usage. SMA solar engine for outer missions have been proposed because surface temperature of the part of satellite facing sun can be over 120°C whereas the part of the shadow as low as 150°C.
Shape memory alloy solar engine can directly convert solar energy into mechanical energy. To investigate its performance under different conditions  , numerical simulation has been carried out. Solar engine integrated as part of the satellite structure, and it can be used as driving the mechanism, deployable structures and dust removal.
As compare to conventional energies, solar energy is ecological and clean and beginning of almost all kinds of energies. A lens for concentration on earth heated SMA through sunlight.
0.25 W is the average power predicted. Chemical energy of high-energy-density fuels is to be converted into mechanical energy, instead of heating SMAs openly upon exposing to a heat source. Such approach is used in our daily life  .
Another latest topic now a day for high efficiency is the hydrogen produced is ideal, for particular fuel cell. TiO 2 under ultraviolet light radiation, split hydrogen and oxygen (photo-catalyst effect), which is 4% part of incoming solar energy. Efficiency can be enhanced if we use visible light, which is 43% part of incoming solar energy. In recent times, some progress has been carried out to enable TiO 2 to work under visible light. An internal stress is introduced in the patterned TiO 2 thin film of the polymer substrate  , apart from nitrogen-doped TiO 2 nano-tube array  . The difficulty of adhesion and durability between polymeric substrate and brittle TiO 2 layer and high expenditure in fabrication of nano-tube is of high concern.
This is new research area for Ni-Ti SMAs in conserving energy utilizing water and solar energy in the greenest way. It is very important to make certain right type of TiO 2 to be generated atop Ni-Ti films for the photo-catalyst effect.
Shape memory alloys not only used in the engineering community, but also in the artist community. SMAs have been used as actuation material to move sculptress. Thin SMA wire rises and falls upon heating and power cutoff respectively in artificial mimosa. In built-in sensor upon touch, the response becomes fully automatic. Similarly, very thin SMA wire can be used to design artificial wigs, which are capable suddenly to change the styles, e.g., from straight to curly, according to someone choice. To reverse the visual presentation of information  , computer interfacing and virtual reality applications have been proposed with SMA technology.
Shape memory alloy has been planned for a seismic isolation and passive control of buildings, because of high-energy absorption ,,,,,, .
In recent times, reinforcement and repair of the damaged structure using SMAs is to be introduced in civil engineering ,,,,,,,, . Files and Olson  might be founders in crack healing SMA, followed by many others (e.g., ,, ). In almost all previous survey, Ni-Ti SMAs were embedded inside a matrix and have excellent corrosion resistance. Ni-Ti used in civil structure, because of high density.
"Lab on the chip" attracts a lot of interest of many communities because it shrinks a full-sized laboratory down to the size of a coin  . Flow of liquid is managed via micro-valves and micro-pumps within micro-channels  . SMA has been proposed as actuation material for micro-valves/pumps as it is ideal for minimum actuators ,, . Hirose et al.  are founders of SMA based active endoscope. SMA's stent is recently application  . Once SMA stent is implant, it will stay there forever ,, . Retractable stent are highly in demand ,, , for recovering patients from complications of bladder and cancer operations. About 18% patients required repeated dilations because stenosis may develop at anatomizes site. Removable stent require only treatment once a year instead of reconstructive bladder neck repair procedure. Sometimes retractable stent is embedding SMA inside elastic polymer. To embed SMA ring into a polymer tube, is to be converted into a circular shape from original star shape. We can easily design SMA to deployed at proper position in terms of dimension of polymer/SMA, how SMA embedded within the polymer matrix, stiffness of the polymer. SMA will try to recover its original star shape, after heating the stent above the transition temperature of SMA.
Biocompatibility, shape memory effect, pseudoelasticity are very special properties of NiTi (Nitinol). However, these properties can be tailored by modifying the composition and with suitable thermal and mechanical treatments.
Porosity of Ni-Ti also has a noticeable influence on pseudo-elasticity, that is, the ability to recover strain after deformation. Hence, the effect of parameters on the porosity can be a future aspect of research. The effect of porosity on various medical implants and after that creating a database of the same is still pending so that engineers and doctors can work on the same platform.
During the use of this alloy in automotive actuators, pre-strain of SMA wire is observed as a challenging task.
Multi-smart multi-energy is very much valuable for momentary coupling. Platforms having multidegree-of-freedom are essential for position control in many engineering applications.
Renewable energy resources are another thrust area for future research using Ni-Ti SMA. Latest topic now a day for high efficiency is the hydrogen produced is ideal, for particular fuel cell. This is new research area for Ni-Ti SMAs in conserving energy utilizing water and solar energy in greenest way. It is very important to make certain right type of TiO2 to be generated atop Ni-Ti films for the photo-catalyst effect.
Because of pseudo-elasticity this material has been used in seismic isolation and passive control of buildings. The effect of additives can be considered on pseudo-elasticity of this SMA.
This SMA has been proposed as actuation material for micro-valves/pumps as it is ideal for minimum actuators.
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Neeraj Sharma received B.Tech. and M.Tech. in 2006 and 2012 respectively. Now he is Pursuing his research from YMCA University of Science and Technology (Faridabad). His area of interest is non-conventional Machining methods and bio-compatible materials. He is Associate member of Institution of Engineers (India) and Life member of Indian Society of Technical Education.
Tilak Raj is working as a Professor in the Mechanical Engineering Department at the YMCA University of Science and Technology, Faridabad, India. He passed his BSc in Mechanical Engineering from the NIT, Kurukshetra, in 1987 with honours, ME in Production Engineering from the Delhi College of Engineering, Delhi, in 2004 with distinction and PhD from the Jamia Millia Islamia, New Delhi in 2009. His area of expertise is manufacturing technology. His research papers have been published in the International Journal of Flexible Manufacturing Systems, International Journal of Production Research, International Journal of Manufacturing Technology and Management and International Journal of Manufacturing Research etc.
Dr. Kamal Kumar is presently working as an Assistant Professor in Department of Mechanical Engineering at PEC University of Technology, Chandigarh, India. He has obtained his Ph.D. in Mechanical Engineering in 2012. He has more than 6 years of teaching experience and 4 years of research experience. His area of research interest is non-conventional machining processes, bio-compatible metallic materials, smart materials etc. He has published ten research articles in SCI indexed journals and some publications in National and International conferences in India.

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