A shape-memory alloy (SMA) is a mixture of two or more metals that has the special property of being able to 'memorize' a certain shape, and return to that shape even after being deformed. Usually the return to a memorized shape is triggered by heating up the material. The change can happen very fast, often within seconds.
The Strong Man would only have to heat up the metal bar and it would unbend itself, ready almost immediately to be bent again. Of course, this example is a bit unrealistic, but in real life SMAs have a variety of uses in mechanical engineering, medical science, and even in everyday life. Even though SMAs are all around us, few people realize the role that shape-memory alloys play in our lives. However, the wide use of SMAs is a direct result of their remarkable properties, which in turn originate from their unique internal structures. Therefore, the tale of this widely-used but little-known class of materials begins at the microscopic level.
The key to an SMA's ability to shape-changing ability is that its structure differs, depending on the temperature. At high temperatures, the atoms in an SMA possess a very stiff, rigid structure, called the "austenitic" structure (named after British metallurgist Sir William Chandler Roberts-Austen). The shape of an SMA is thoroughly linked to its austenitic structure. Any change in the shape of the metal while it is in the austenitic phase causes the structure to change, and vice versa.
As the metal cools and reaches a critical temperature range, the atoms begin to realign themselves into a different structure, called the "martensitic" structure (named after German metallurgist Adolf Martens). This structure is also linked to the austenitic structure, but is flexible and allows the metal to be visibly bent, stretched, and manipulated without changing the underlying atomic structure. When the metal is heated up again to its critical temperature range, the metal transforms back into the rigid austenitic structure which, being linked to the shape of the metal, causes it to regain its original, "memorised" or "programmed" shape. If the metal is cooled down once again to the flexible martensitic phase, it will retain the memorized shape until otherwise changed by an outside influence.
A high recovery force from an SMA can be compared to the stretching or compression of a spring, such as one found in a mattress or a trampoline. When a person jumps onto a trampoline, the springs get squeezed and in trying to return to their normal state, they push back - sending that person flying into the air. Changing the shape of an SMA in its martensitic phase is like compressing the springs on a trampoline, and when the SMA returns to its memorized austenitic shape, the force is released. SMAs usually operate on a smaller size scale than do trampolines, but they can cause similar or greater forces.
SMAs found their first uses in the various fields of engineering, especially in the military. It's not much of a surprise, considering that the first SMAs were developed by the Navy. The very first SMA, a nickel-titanium alloy, was developed at the US Naval Ordnance Laboratory in 1961, where they discovered the shape-memory properties by accident. They dubbed it "Nitinol" - Ni for nickel, Ti for titanium, and NOL for Naval Ordnance Lab. Since then it has been the most prominently used SMA in engineering applications.
SMAs have found a niche in the biomedical world as well. They do well at tasks which require shape changes within the body during operation. Without SMAs, the surgeon would have to make larger incisions to make the change - for example, to tie a knot in a suture. But when implants and tools can make their own changes, incisions can be smaller and operations can be less traumatic. In addition, the impressive flexibility of SMAs means that they can be used for catheters and probes, which must twist and turn through small passages. SMAs are well-suited for tasks in the operating room. In most of these cases, the temperature change to activate the shape-memory characteristics of the SMAs is caused by hot water, simply body heat, or electric current, which causes the metal to heat up.
SMAs can also be used in longer term, corrective situations, as in dental and orthopedic practice. Dental arch wires and braces are perhaps the most commonly used corrective procedure using SMAs. Most people are familiar with the concept of braces. Wires are attached to crooked teeth and tightened over time, to pull the teeth into line. Although almost any metal would do the job, most metals would be stretched out of shape and eventually break. The superelasticity of SMAs allows the wires to be tightened and stretched without breaking (Swardz, 2001). Through the medium of braces, most teenagers nowadays learn to hate SMAs without even knowing it.
Scoliosis can be treated in a similar manner. Previously, scoliosis was treated by simply setting the spine in the desired new position and using "Harrington rods" made out of conventional metals as splints to keep the spine in place. This places huge, unhealthy stresses on the bones, sometimes causing bone fractures. It requires a body cast, and surely puts the patient out of action for weeks. The newer procedure uses SMA Harrington rods with the programmed contour of a straight spine, but a temporary form of a curved spine, fitted to the patient. These rods are wired to the spine and as the SMA is heated, it will attempt to straighten over time, pulling the spine along with it. Once the spine is straightened, the rods are removed. The stresses on the bone in this situation are much lower, spread out over time, and the treatment requires no body cast, allowing the patient to be mobile after the operation.
It has been shown that bone fractures heal better when subjected to compressive stress. To aid this, an SMA can be fashioned into a so-called "staple," which hooks onto the bone on either side of the fracture. Since the programmed shape of the metal is smaller than when the staple is put into the bone, the staple will tend to try to shrink when triggered, pulling the bones together. This process has been particularly successful in healing jaw and arm fractures. A similar process has been used for correcting bone deformities in the legs and feet.
Other common objects use SMAs for their remarkable flexibility, such as mobile phone antennas, eyeglass frames, and brassiere underwires.
The tips on some modern fountain pens contain SMAs, because the shape-memory and elasticity provides excellent resistance to damage. An aesthetic use for SMAs has even been found; shape-changing jewelry has been made from SMAs which have a critical temperature near body temperature. In fact, SMAs are so versatile that they have been found to improve one's golf game - through SMAs inserted into golf clubs. The superelasticity and damping of the SMA gives the ball an extra spin and more "bite on the green".
Titanium rods have gained use in scoliosis surgery due to their excellent biocompatibility, while allowing medical personnel to obtain undistorted magnetic resonance imaging scans following surgery. However, the impression of several clinicians has been that when screw pullout and/or loss of sagittal balance occurs, it may be due to the rods losing some of their curvature. Five 6-mm rods of differing compositions and lengths (titanium 300 and 100 mm, stainless steel 300 and 100 mm, prebent titanium 85 mm) were bent at room temperature with a 3-point rod bender, then placed in an incubator at 37 C. Digital photographs were taken every 2 weeks and analyzed to extract the radius of curvature of each rod.
The Ti rods had a significantly decreasing curvature with time. The prebent Ti and stainless steel rods did not exhibit significant change in curvature. Titanium rods bent at room temperature and then exposed to body temperature over time tend to exhibit "metal memory"; they gradually revert to their original shape. This may result in loss of sagittal balance and/or proximal screw pullout.