Applications of Shape Memory Alloys in the medical field
 
Over the past few decades the development in material science was determined by the discovery of new material properties. Today Shape Memory Alloys (SMAs) are already commercially applied in several technical fields such as automotive, aerospace, medicine and they can be very successfully used as actuators or connectors in electronic devices.
The shape memory effect (SME) appears in some special alloys which show a crystallographically reversible, thermoelastic, martensitic transformation and refers to the ability to recover large strains after seemingly plastic deformation, only by a change of temperature (one-way effect). An external force has to be applied to reset the element to its strained condition if this effect should be used repeatedly.

When provided with a specific thermomechanical treatment (training), shape memory alloys develop an intrinsic two-way effect (TWSME), which means that shape changes between two forms purely as a result of changing the temperature and without the application of external stresses.

In 1970, the first successful demonstration of a shape memory alloy Cryofit tube coupling took place on a U.S. Navy F-14 fighter aircraft. This demonstration of the reliability of a shape memory device in a high pressure hydraulic system lead to the production of over a million couplings in the following years.

Since that beginning there have been many thousands of patents issued for every conceivable application for shape memory alloys, yet, remarkably, the list of truly commercially successful devices is quite small. In this sense, by commercially successful we imply the production of a significant volume in at least many thousands per year. The major applications for these alloys are in the field of medicine and orthodontics, with a few other areas of significance such as eyeglass frames, cellular phone antennas, women’s brassiere underwires and automotive devices. While most attention has been focused on the medical devices due to the attractive value of the business, the consumption of shape memory alloy materials in the consumer and industrial sections however far exceeds the usage in the medical field, and the list of commercial applications is growing at a rapid pace.

Shape-memory materials (SMM’s) are one of the major elements of intelligent/smart composites because of their unusual properties, such as the shape-memory efect (SME), pseudoelasticity or large recoverable stroke (strain), high damping capacity and adaptive properties which are due to the (reversible) phase transitions in the materials. SMMs may sense thermal, mechanical, magnetic or electric stimulus and exhibit actuation or some pre-determined response, making it possible to tune some technical parameters such as shape, position, strain, stifness, natural frequency, damping, friction and other static and dynamical characteristics of material systems in response to the environmental changes.

To date, a variety of alloys, ceramics, polymers and gels have been found to exhibit SME behaviour. Particularly, some SMMs can be easily fabricated into thin films, fibres or wires, particles and even porous bulks, enabling them feasibly to be incorporated with other materials to form hybrid composites.

Smart materials have been given a lot of attention mainly for their innovative use in practical applications. As mentioned earlier, one of the best examples of such materials is also the family of shape memory alloys (SMA) which are arguably the first well known and used smart material. Research activity in this field has been intense, and a number of alloys have been investigated, such as Cu-Zn, Cu-Zn-Al, Cu-Al-Ni, Cu-Sn, Cu-Au-Zn, Ni-Al, Ti-Ni, Ti-Ni-Cu, Ni-Ti-Nb and others.

Because these materials are relatively new, some of the engineering aspects of the material are still not well understood. Many of the typical engineering descriptors, such as Young’s modulus and yield strength, do not apply to shape memory alloys since they are very strongly temperature dependent. On the other hand, a new set of descriptors must be introduced, such as stress rate and amnesia. That is why numerous constitutive models have been proposed over the last 20 years to predict thermomechanical behavior.

These materials have been shown to exhibit extremely large, recoverable strains (on the order of 10%), and it is these properties as functions of temperature and stress which allow SMA’s to be utilized in many exciting and innovative applications. From a macroscopic point of view, the mechanical behavior of SMA’s can be separated into two categories: the shape memory effect (SME), where large residual (apparently plastic) strain can be fully recovered upon raising the temperature after loading and unloading cycle; and the pseudoelasticity or superelasticity, where a very large (apparently plastic) strain is fully recovered after loading and unloading at constant temperature.

Both effects are results of a martensite phase transformation. In a stress-free state, an SMA material at high temperatures exists in the parent phase (usually a body-centered cubic crystal structure, also referred as the austenite phase). Upon decreasing the material temperature, the crystal structure undergoes a self-accommodating crystal transformation into martensite phase (usually a face-centered cubic structure).

The phase change in the unstressed formation of martensite from austenite is referred to as ‘self-accommodating’ due to the formation of multiple martensitic variants and twins which prohibits the incurrence of a transformation strain. The martensite variants, evenly distributed throughout material, are all crystallographically equivalent, differing only by habit plane. The process of self-accommodation by twinning allows an SMA material to exhibit large reversible strains with stress. However, the process of self-accommodation in ordinary materials like stainless steel does not take place by twinning but via a mechanism called slip. Since slip is a permanent or irreversible process, the shape memory effect cannot occur in these materials.

Nickel-titanium alloys have been found to be the most useful of all SMA’s. Other shape memory alloys include copper-aluminum-nickel, copper-zinc-aluminum, and iron-manganese-silicon alloys. The generic name for the family of nickel-titanium alloys is Nitinol.

The properties of Nitinol are particular to the exact composition of the metal and the way it was processed. The physical properties of Nitinol include a melting point around 1240°C to 1310°C, and a density of around 6.5 g/cm3. Various other physical properties tested at different temperatures with various compositions of elements include electrical resistivity, thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity, magnetic susceptibility, and thermal conductivity. Mechanical properties tested include hardness, impact toughness, fatigue strength, and machinability.

Nitinol is being used in a variety of applications. They have been used for military, medical, safety, and robotics applications. The military has been using Nitinol couplers in F-14 fighter planes since the late 1960’s. These couplers join hydraulic lines tightly and easily.

Many of the current applications of Nitinol have been in the field of medicine. Nitinol is being used in robotics actuators and micromanipulators to simulate human muscle motion. The main advantage of Nitinol is the smooth, controlled force it exerts upon activation. Other miscellaneous applications of shape memory alloys include use in household appliances, in clothing, and in structures. A deep fryer utilizes the thermal sensitivity by lowering the basket into the oil at the correct temperature. Nitinol actuators as engine mounts and suspensions can also control vibration. These actuators can helpful prevent the destruction of such structures as buildings and bridges.

There are many possible applications for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical energy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use in car frames. Other possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls, and the control of a radiator blind ("to reduce the flow of air through the radiator at start-up when the engine is cold and hence to reduce fuel usage and exhaust emissions") SMA’s are "ideally suited for use as fasteners, seals, connectors, and clamps" in a variety of applications. Tighter connections and easier and more efficient installations result from the use of shape memory alloys.

Many potential uses and applications of shape memory alloys ensure a bright future for these metals. Research is currently carried out at many robotics departments and materials science departments. With the innovative ideas for applications of SMA’s and the number of products on the market using SMA’s continually growing, advances in the field of shape memory alloys for use in many different applications seem very promising.

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