Shape Memory Alloys - Medical Applications

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Smart materials are an interesting field that is growing quickly, and shape memory alloys (SMAs) are one such materials that represent one of the most exciting areas.

An SMA can experience considerable plastic deformation, but it can be subsequently stimulated to return to its original shape by applying heat. These properties have resulted in an explosion of diverse applications in various industries (refer to Table 2).

From early applications like greenhouse window openers where an SMA actuator delivered temperature-reliant ventilation, the list of applications has grown tremendously in the 1990s. One of the advanced application is plastic-coated mobile phone antennas produced from a super-elastic SMA that can regain its shape even after extreme deformation, such as dropping the phone.

SMA’s shape recovery and superelastic properties are especially interesting and advantageous in the medical applications, which are increasing rapidly.

History

Although shape memory alloys have not been around a long time, they have a surprising number of applications. An important discovery took place in 1962 when a mechanically deformed binary alloy, made up of equiatomic titanium and nickel, was found to display a shape recovery effect after being heated.

Many other reversible phase change materials were known at that time, but the Ni-Ti alloys displayed a large recoverable strain value when compared to other binary, ternary, or quaternary shape memory alloy systems.

The Ni-Ti alloy’s physical performance made it a milestone discovery, and the extent of commercially feasible applications that have been created for the materials is proof of the significance of the Ni-Ti shape memory alloys. This discovery was perhaps a happy accident.

It has been said that William Buehler who was working with high nickel-bearing alloys for gas turbine parts, had left a tiny ingot of Ni-Ti alloy produced in a vacuum melt furnace, on a desk in direct sunlight. When Buehler and his coworkers returned from lunch, they observed a change in the ingot shape.

Currently, Ni-Ti alloy is referred to as Nitinol (made from Naval Ordinance Laboratories, part of the US Department of Defense), and the name has become one of the oft-used titles for the SMAs originating from the laboratory of Buehler.

Alloy Types

Following the discovery of Ni-Ti, at least 15 varied binary, ternary, and quaternary alloy types have been discovered that display shape variations and rare elastic properties after deformation. A few of these alloy types and alternatives are illustrated in Table 1.

Table 1. Shape memory alloy types

The original nickel-titanium alloy has some of the most beneficial properties in terms of its active temperature range, recoverable strain energy, cyclic performance, and comparatively simple thermal processing.

Alloys, including Ni-Ti, have two generic properties—that is, thermally induced shape recovery and super- or pseudo-elasticity. The latter means that an SMA in its elastic state can endure a deformation about ten times more than that of a spring-steel equivalent, and total elastic recovery to the original geometry may be anticipated.

This could be possible through several million cycles. The alloy’s energy density can be used to good effect to produce high-force actuators—for example, a contemporary DC brushless electric motor has a mass of 5 to10 times that of a thermally activated Ni-Ti alloy, to perform the same task.

The superelastic Ni-Ti alloys are “stressed” by merely working the alloy. These stresses can be eliminated, just as with several other alloys, by an annealing process. The stressed condition is called stress-induced martensite, which is the same as being cold/hot worked.

SMAs, mainly nickel-titanium, are commercially available from numerous sources. However, global production is small compared to other metal products (approximately 200 tons were manufactured in 1998) owing to problems in the melt/forging production process. Hence, the cost of the material in 1999 was high—US$0.30–US$1.50 (UK£0.20–UK£1.00) per gram of wire.

Favorably, a majority of present applications need only a small amount of the material. As global production increases (as it has done quite considerably in the 1990s), prices should drop. Wires, bar, rod, strip, and sheet are all easily available, and sintering powders, alloy foams, and sputtering targets of high purity are also manufactured.

Medical Applications

The range of forms and the properties of SMAs make them very useful for a variety of medical applications. For instance, a wire in its “deformed” shape has a small cross-section that can be added to a body cavity or an artery with minimal chance of causing trauma. After it is in position and released from a constraining catheter, the device is triggered by the body heat and returns to its former “memorized” shape.

Expanding the volume of a device by direct contact or remote heat input has resulted in the advancement of new methods for minimally invasive or keyhole surgery. This includes instruments with dynamic properties, such as tiny clamps, forceps, and manipulators. SMA-based devices that can dilate, pull together, constrict, push apart and so on have made challenging or problematic tasks in surgery quite feasible (refer Table 2 for medical and other applications).

Table 2. Current examples of applications of shape memory alloys.

Stents

The property of thermally induced elastic recovery allows a small volume to be changed to a larger one. The stent is an example of a device applying this concept. A stent, either along with a dilation balloon or just by self-expansion, can support or dilate a blocked conduit in the human body.

Coronary artery disease, which is a leading cause of death worldwide, is caused by the development of plaque in-growth on and within the inner wall of an artery. This minimizes the cross-section of the artery and subsequently decreases the blood flow to the heart muscle.

A stent can be added in a “deformed” shape, in other words, with a smaller diameter. This is achieved when the stent contained in a catheter travels through the arteries. When deployed from the catheter, the stent expands to a suitable diameter with adequate force to open the vessel lumen and restore blood flow.

The same method can be used in a number of the body’s conduits, including the esophagus, biliary system, trachea, and urinary system. The balloon-assisted expansion or self-expansion technology is applied to millions of these stents annually, and the numbers are gradually increasing.

It is not possible to directly introduce a catheter into the complex arterial channels through a small external incision. This is due to the relative inflexibility and lack of steerability of the catheter alone. To make sure that the catheter reaches the right site, a guide-wire must first be added. Superelastic Ni-Ti alloys can be used very effectively for this application. Their deformability, torquability, recovery, and low whipping effect permit the surgeon to place a highly flexible guide wire in the appropriate position.

The end of the guide wire is fed via a side or central hole in the catheter. The catheter can travel only up to where the guide wire is placed—it serves like a railway line. Mostly, the guide wire may be maintained in place while other catheters for diverse tasks use the same guide wire.

Vena-Cava Filters

Vena-cava filters have a comparatively long record of positive in-vivo applications. The filters are designed using Ni-Ti wires, and are used in one of the outer heart chambers to capture blood clots, which could be fatal if allowed to move freely around the blood circulation system. The exclusively designed filters capture these small clots, stopping them from entering the pulmonary system and leading to pulmonary embolism.

The vena-cava filter is added in a small cylindrical form measuring approximately 2.0–2.5 mm in diameter. When discharged, it forms an umbrella shape. The construction is engineered with a wire mesh spacing that is small enough to trap clots. This is an example of the application of superelastic properties, but there are also a few commercially available thermally actuated vena cava filters.

Dental and Orthodontic Applications

Another commercially significant application is the use of superelastic and thermal shape recovery alloys for orthodontic applications. Stainless steel archwires have been used as a remedial measure for misaligned teeth for several years.

Due to the minimal “stretch” and tensile characteristics of these wires, substantial forces are applied to teeth; however, this can cause a great degree of discomfort. When the teeth yield to the corrective forces applied, the stainless steel wire has to be re-tensioned. In the preliminary stages of treatment, visits to the orthodontist may be necessary to re-tension the wire every three to four weeks.

Superelastic wires are currently used for these corrective actions. Due to their elastic properties and extendibility, the level of discomfort can be decreased greatly as the SMA applies a continuous, mild pressure over a longer period. Visits to the orthodontist are reduced to maybe three or four annually.

This continuous, mild, and corrective force demonstrates the rather unusual elastic properties of superelastic SMAs. A graph illustrating extension plotted against load creates a straight, horizontal line after preliminary loading. The alloy is shown to be non-Hookean, unlike carbon steel and other springs, and constant forces can be derived from springs composed of Ni-Ti alloy.

Besides the tensioned archwires, other superelastic orthodontic devices are available that can move teeth linearly where uneven tooth spacing is present. Movements of 6 mm in six months are possible with reduced discomfort. There are also devices capable of applying torsional forces in the case of a “twisted” tooth.

Orthodontists use modular kits, wherein adhesively bonded brackets are fixed to the teeth and the archwire is subsequently fixed to and guided by the bracket. Other wire-forms can then be fitted to the brackets to push, twist, pull, or force other movements that assist in corrective measures for clinical or cosmetic reasons. Dental SMA devices like these have proved very effective in trials, and are commercially available in Europe.

Other related SMA devices are also being employed for healing broken bones—staples of the shape memory materials are fixed to the bone parts, and these staples apply a steady, well-defined force to pull two pieces together as the SMA is heated up by the body and begins to regain its original configuration. This force helps in joining the two pieces of bone back together. These smart “healing” powers are the reason why SMAs are in great demand for many applications in the dental, medical, and other fields in the future.