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Applications

Medical applications of shape memory alloys

Author: Date:6/7/2012 4:16:49 AM

The variety of forms and the properties of SMAs make them extremely useful for a range of medical applications. For example, a wire that in its “deformed” shape has a small cross-section can be introduced into a body cavity or an artery with reduced chance of causing trauma. Once in place and after it is released from a constraining catheter the device is triggered by heat from the body and will return to its original “memorised” shape.

Increasing a device’s volume by direct contact or remote heat input has allowed the development of new techniques for keyhole or minimally invasive surgery. This includes instruments that have dynamic properties, such as miniature forceps, clamps and manipulators. SMA-based devices that can dilate, constrict, pull together, push apart and so on have enabled difficult or problematic tasks in surgery to become quite feasible (See Table 2. for medical and other applications).

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

Aids for disabled

Aircraft flap/slat adjusters

Anti-scald devices

Arterial clips

Automotive thermostats

Braille print punch

Catheter guide wires

Cold start vehicle actuators

Contraceptive devices

Electrical circuit breakers

Fibre-optic coupling

Filter struts

Fire dampers

Fire sprinklers

Gas discharge

Graft stents

Intraocular lens mount

Kettle switches

Keyhole instruments

Key-hole surgery instruments

Micro-actuators

Mobile phone antennas

Orthodontic archwires

Penile implant

Pipe couplings

Robot actuators

Rock splitting

Root canal drills

Satellite antenna deployment

Scoliosis correction

Solar actuators

Spectacle frames

Steam valves

Stents

Switch vibration damper

Thermostats

Underwired bras

Vibration dampers

ZIF connectors

Stents

The property of thermally induced elastic recovery can be used to change a small volume to a larger one. An example of a device using this is a stent. A stent, either in conjunction with a dilation balloon or simply by self-expansion, can dilate or support a blocked conduit in the human body. Coronary artery disease, which is a major cause of death around the world, is caused by a plaque in-growth developing on and within an artery’s inner wall. This reduces the cross-section of the artery and consequently reduces blood flow to the heart muscle. A stent can be introduced in a “deformed” shape, in other words with a smaller diameter. This is achieved by travelling through the arteries with the stent contained in a catheter. When deployed, the stent expands to the appropriate diameter with sufficient force to open the vessel lumen and reinstate blood flow.

Figure 1. A reinforced graft for vascular application to replace or repair damaged arteries (25mm diameter).

The same technique can be employed in many of the body's conduits, including the oesophageus, trachea, biliary system and urinary system. The technology of self-expansion or balloon-assisted expansion is used for many millions of these stents each year and the numbers are steadily increasing.

Introducing a catheter directly through the complex arterial channels via a small external incision is generally not possible, owing to the relative rigidity and lack of steerability of the catheter alone. To ensure that the catheter gets to the correct site, a guide-wire must first be introduced. Superelastic Ni-Ti alloys are used very successfully for this application. Their torquability, deformability, recovery and low whipping effect allow the surgeon to get the highly flexible guide wire in place. The end of the guide wire is fed through a central or side hole in the catheter. The catheter can only go where the guide wire is positioned - it acts like a railway line. Often, the guide wire may be kept in place while other catheters for different tasks use the same guide wire.

Vena-cava Filters

Vena-cava filters have a relatively long record of successful in-vivo application. The filters are constructed from Ni-Ti wires and are used in one of the outer heart chambers to trap blood clots, which might be the cause of a fatality if allowed to travel freely around the blood circulation system. The specially designed filters trap these small clots, preventing them from entering the pulmonary system and causing a pulmonary embolism. The vena-cava filter is introduced in a compact cylindrical form about 2.0-2.5mm in diameter. When released it forms an umbrella shape. The construction is designed with a wire mesh spacing sufficiently small to trap clots. This is an example of the use of superelastic properties, although there are also some thermally actuated vena cava filters on the market.

Dental and Orthodontic Applications

Another commercially important application is the use of superelastic and thermal shape recovery alloys for orthodontic applications. Archwires made of stainless steel have been employed as a corrective measure for misaligned teeth for many years. Owing to the limited “stretch” and tensile properties of these wires, considerable forces are applied to teeth, which can cause a great deal of discomfort. When the teeth succumb to the corrective forces applied, the stainless steel wire has to be re-tensioned. Visits may be needed to the orthodontist for re-tensioning every three to four weeks in the initial stages of treatment.

Superelastic wires are now used for these corrective measures. Owing to their elastic properties and extendibility, the level of discomfort can be reduced significantly as the SMA applies a continuous, gentle pressure over a longer period. Visits to the orthodontist are reduced to perhaps three or four per year.

This continuous, gentle, corrective force illustrates the rather odd elastic properties of superelastic SMAs. A graph showing extension plotted against load produces a straight, horizontal line after initial loading. This shows the alloy to be non-Hookean, unlike carbon steel and other springs and constant forces can be derived from springs made of Ni-Ti alloy.

Apart from the tensioned archwires, other superelastic orthodontic devices exist which can move teeth linearly where there is uneven tooth spacing. Movements of 6mm in 6 months are possible with minimum discomfort. Devices also exist that can apply torsional forces in the case of a “twisted” tooth. Orthodontists have modular kits, in which adhesively bonded brackets are attached to the teeth and the arch wire is then attached to and guided by the bracket. Other wire-forms can then be fitted to the brackets to push, pull, twist or force other movements that facilitate corrective measures for cosmetic or clinical reasons.

Such dental SMA devices have proved very successful in trials and are being made commercially available in Europe. Other similar SMA devices are also being used for healing broken bones - staples of the shape memory materials are attached to each part of the bone, and these staples then apply a constant, well-defined force to pull the two pieces together as the SMA is warmed by the body and tries to return to its original configuration. This force helps knit the two pieces of bone back together. Such smart ‘healing’ powers are the reason why SMAs are being borne in mind for many applications in the medical, dentistry and other fields in the future.