Applications of Shape Memory Alloy

Biswajeet BedantaEEE, MSRIT

Bangalore, Indiabbedanta@gmail.com

Abstract— After the Shape Memory Effect was first discovered in 1930s, the whole shape memory alloy technology has experienced a dramatic development. Due to the biocompatibility of these alloys it is now extensively used in orthopedic and cardiovascular applications as well as in the manufacture of new surgical tools. There are many applications in the industrial sector. Eyeglass frames were an early example and cellular phone antennas consume millions of feet of superelastic wires. SMA actuators continue to achieve steady growth in safety valves for both consumer and industrial applications.

Index Terms— Shape Memory Alloys (SMA), Minimally Invasive Surgery (MIS), Micropumps, Microactuators, Application, Pumps, TiNi, cardiovascular surgery, orthopedic surgery

I. INTRODUCTION

A shape-memory alloy (SMA, smart metal, memory metal, smart alloy) is an alloy that “remembers” its original, cold-forged shape; returning to the pre-defined shape when heated The behavior of memory metals is caused by a change in the internal structure of the material by a martensitic transformation, fig.1 gives the reversible temperature induced transformation. During the heating the memory effect take place in the trajectory As-Af and during cooling the reverse transformation to martensite occurs in the trajectory Ms-Mf.

Fig. 1. Influence of the temperature on the transformation of martensite to austensite in a NiTi- shape memory alloy.

Fig.2 shows a typical stress-strain curve of superelastic NiTi-alloy, which can be deformed until 8% strain without permanent deformation. Loading and unloading curves both show a plateau, where the stress level is almost independent of the strain. These values of loading and unloading plateau vary

with the testing temperature and are higher for increased temperature.

Fig. 2. Stress-Strain curve for steel and superelastic NiTi, where the dashed areas represent the stored energy for the same stress.

From As to Af the memory metal tries to return to the predefined shape, but if this free recovery is constrained it will build up recovery stress. In fig.3 this recovery stress is shown as the vertical line which runs from the lower, unloaded and deformed position to the unloading plateau of the curve, which belongs to the higher temperature level

Fig. 3. Stress-Strain curve for martensite and austensite. Building up of

recovery stress when free recovery is constrained upon heating.

II. WET SHAPE MEMORY ALLOY ACTUATED ROBOTIC PUMP

Just as the human heart provides energy to the muscles in the body, the robotic SMA pump distributes thermofluidic energy to arrays of SMA actuators that function as robotic muscles. The robotic pump draws from its own fluidic output to assist in the actuation of its own internal SMA actuators, just as a portion of the blood pumped by the human heart supplies en-ergy to its own muscles. The volumetric output-to-in-put ratios of 1.4 using fluid-only actuation and 2.1 with the assistance of Joule heating. This SMA pump is capable of pumping a net output of 66 mL/min, which is two orders of magnitude larger than the output of any other SMA pump. The maximum theoretical efficiency that this system can achieve is 3.4%.

Fig. 4. wet SMA actuator concept

III. CARDIOVASCULAR SURGERY

The first Nitinol stent was made by Dotter’s group in 1983; it was a simple coiled Nitinol wire and was delivered into a dog’s femoral artery by guide catheter (Fig. 5). The stainless steel stent was introduced into clinical use by Palmaz and Schatz in 1987. The concept of Nitinol stent covered with fab-ric graft was first introduced in 1993. Since then stent has evolved into an enabling technology as drug delivery device rather than a pure mechanical scaffold. Although stent is the most widely known SMA device in minimally invasive car-diovascular therapy to revascularize arteries, Nitinol stent have also been used in other parts of human body including stents for esophagus, gastrointestinal tract, ureter, tracheal air-way, and vascular anastomosis device, radiofrequency abla-tion catheter and prosthetic heart valves.

IV. ORTHOPEDIC SURGERY

The early applications of Nitinol in orthopaedic surgery were staples and clamps to treat adolescent scoliosis and bone frac-tures. The fusionless scoliosis correction using minimally in-vasive SMA staple was examined in animal models and the early results were promising in correcting moderate to severe scoliosis and halting the malignant progression without fusion. The Nitinol implants found no negative effect on new bone formation, and even the bone modeling could be controlled by a constant bending force applied to the bone through a func-tional SMA implant. Recent development of porous Nitinol has shown good biocompatibility and excellent bone in-growths, which could be used as ideal bone substitute.

V. THIN-FILM SMA ACTUATED MICROPUMPS

Micropumps capable of precise handling of low-fluid volumes have the potential to revolutionize applications in fields such as drug delivery, fuel injection, and micrototal chemical analy-sis systems (µTAS). Traditional microactuators used in mi-cropumps suffer from low strokes and, as a result, are unsuit-able for achieving large fluid displacement. They also suffer low-actuation work densities, which translate to low forces. The use shape-memory effect (SME) in thin-film shape-mem-ory alloy (SMA) titanium nickel (TiNi) as an actuator for mi-croelectromechanical systems (MEMS)-based microfluidic de-vices, as it is capable of both high force and high strains. The resistivity of the SMA thin film is suitable for Joule heating, which allows direct electrical control of the actuator. A maxi-mum water flow rate of 50 µl/min was achieved.

VI. EYEGLASS FRAME

Today, NiTi SMA has achieved a permanent place in high-end eyeglass frames. The use of superelastic SMA components for the nose piece (bridge) and ear pieces (temples) provide im-proved wearer comfort as well as great resistance to accidental damage. To achieve the highly kink-resistant superelasticity over a wide range of environmental temperatures, these com-ponents are usually highly cold-worked followed by a low temperature heat treatment to impart “work-hardened pseu-doelasticity”. Because of the difficulty in welding NiTi to dis-similar materials, the ends of a NiTi bridge are mechanically crimped into a silver-nickel casing before being soldered onto the rims. NiTi frames are now manufactured worldwide in the many designs with a wide variety in surface finish and coating dictated by fashion (Fig.6).

Fig. 1. Nitinol stents. a) First Nitinol coil wire stent, the compacted shape is for catheter placement and the expanded shape after saline heating at 60˚C. b)The latest Abbott Acculink stent with complex patterns and optimized tapered design to fit individual patient anatomies

Fig. 5. NiTi eyeglass frames.

VII. CELLULAR PHONE ANTENNA

The cellular phone antenna, formerly of stainless steel, is now universally manufactured from superelastic NiTi alloy due to great resistance to permanent set on bending and accidental damage. Utilizing the same principle for manufacturing su-perelastic NiTi eyeglass frame, significant cold work is often used to enhance the low temperature superelasticity. Ni-rich chemistry or ternary addition is also used to achieve this de-sired property. A photograph of typical cellular phone antenna is shown in Fig.7

Fig. 6. Cellular phone antenna

VIII. BRAILLE SMARTPHONE

Braille Phone is a smartphone for the visually impaired. The smartphone uses Shape Memory Alloy technology. A touch screen made of tiny, height-variable bumps allows users to 'feel' information — and brings printed and visual resources like maps and animations to life. This screen is capable of elevating and depressing the contents to form patterns in braille. The phone’s screen has a grid of pins, which will

move up and down as per requirement. The grid has a Braille display, where pins come up to present character. The phone is currently being prototyped. Fig.7 shows a prototype.

Fig. 7. A prototype display of Braille Smartphone

IX. CONCLUSION

Memory metals offer very interesting properties to designers of a wide range of products. The application of super elasticity has been successful in eyeglass frames and antennas for cellular phones. The application for medical stents and guidewires are becoming very important these days. Since TiNi wires are not only flexible, but their high corrosion resistance and good biocompatibility also make them ideal for these purposes. Medical applications are leading the way at present times. After successful completion of braille smartphone, it can empower the visually impaired population.

X. REFERENCES

[1] Shape Memory Alloy-Wikipedia

[2] P.A. Besselink, Recent Developments on Shape Memory Application, colloque C5, Supplement au Journal de Physique III de November 1997.

[3] Matthew D. Pierce and Stephen A. Mascaro, Biologically Inspired Wet SMA Actuated Robotic Pump, IEEE/ASME Transactions on Mechatronics, Vol.18, No.2, April 2013

[4] Chengli Song, SMA Devices for Minimally Invasive Surgery, The Open Medical Devices Journal, 2010, 2, 24-31

[5] William L. Benard, Harlod Kahn, Michael A. Huff, Thin-Film SMA Actuated Micropumps, Journal of Microelectromechanical Systems, Vol.7, No.2, June 1998

[6] Ming H. Wu and L. McD. Schetky, Industrial Applications for Shape Memory Alloy,Proceedings of the International Conference on Shape Memory Alloy and Superelastic Technologies, Pacific Grove, California, P.171-182(2000)

[7] Sunday Times of India, Bangalore, April 21, 2013