Materials with shape memory

Shape memory is a specific thermodynamic characteristic. A cold sample of the material is deformed so that it changes its shape. However, after heating above a certain characteristic temperature, the sample "remembers" its original shape and spontaneously returns to it. When subjected to mechanical stress, such materials also exhibit other attractive properties such as superelasticity and pseudoplasticity. Depending on the temperature, the material can exist in several forms of internal arrangement. The form that the material has at lower temperatures is called martensite, at higher 'austenite. The temperature at which the transition between the two occurs is specific to the material and its composition.

History
Shape memory was first observed in brass in 1939, and since the 1960s interest in this area has been increasing. In 1962, an equiatomic alloy of Ni'  and Ti was investigated, in which an extremely pronounced shape memory was discovered. This discovery was made by William J. Buehler of the Naval Ordnance Laboratory in White Oak Maryland, USA. According to the composition and place of origin, this alloy is called Nitinol' . Other materials with shape memory include ceramic materials such as zirconium oxide (ZrO2), magnesium oxide (MgO), cerium oxide (CeO2), and also polyurethanes, and, some metal alloys such as copper-aluminum-nickel, copper-zinc-aluminum, iron-manganese-silicon.

Nitinol
Thanks to its biocompatibility and extraordinary physical properties, Nitinol has an exclusive position, especially in medicine. Among the outstanding properties of Nitinol are ' one-way and two-way shape memory, pseudo-elasticity, and high resistance.

Nitinol shape memory
Nitinol in the form of martensite shows the so-called zigzag structure, which is formed by bonding pairs between atoms in different variants. As martensite, Nitinol is relatively ``soft and ``easily deformable due to non-diffusible re-pairing. In form of ``austenite, on the other hand, has a ``stable cubic structure, which is why it is ``mechanically resistant and ``hard in this form. The Transition temperature'  of Nitinol ranges from -100 to 100 °C depending on the ratio of Ni and Ti and impurities, so in medicine, the transition temperature can be set to body temperature or slightly higher. Reversible deformation in the case of a one-way process' (elastic deformation) reaches more than 8%. In the case of the two-way process, the reversible deformation is only 1%, but the number of cycles is practically unlimited without the consequence of mechanical damage or physical changes. This deformation is achieved by ``repeated heating and ``cooling while simultaneously preventing reversible deformation to the original shape.

Some physical and mechanical properties of Nitinol

Pseudoelasticity
It is reported during mechanical loading and unloading at temperatures higher than the temperature for inducing shape memory. Under these conditions, the material exhibits classic elastic properties  , but with significant hysteresis. This is due to the growth of the transition temperature with the rate of material strain. As soon as the material stops being stressed, it behaves like austenite and returns to its initial shape. The pseudoelasticity of Nitinol is characterized by a significant range of deformations up to 10% and a distinct plateau phase, when under almost constant stress or at a constant reactive force, a significant range of deformations can be achieved changes. Thus, Nitinol has similar properties to some biological materials (hair, bones, tendons), so it can be used as a substitute''.

Uses of Nitinol in Medicine
As the only material with shape memory, Nitinol is used in medicine because it is considered a tolerant material (it is curious that although nickel itself is toxic, it has been shown that on the surface A protective layer of titanium dioxide (TiO2) is formed on nitinol, and this guarantees its biocompatibility as well as considerable stability and resistance to corrosion).

Orthodontics
In orthodontics, the use of Nitinol is the most widespread and also has the longest history. It is most often applied as straightening wires   for misaligned teeth and dental segments. Thanks to its pseudoelasticity effect and wide hysteresis, it is guaranteed that in a large range of reversible deformations, the orthodontic wire can exert approximately the same pressure on the corrected segment of the dental arch. Unlike similar wires made of classic materials (such as stainless steel, for example), with a Nitinol wire, it is not necessary to constantly re-suspend the wires after displacement of the dental segment. Nitinol is also used in endodontic instruments that are intended for the treatment of dental canals. Furthermore, in dentistry, Nitinol is applied within a tooth implant with an adjustable inclination of the protruding part, or within a nitinol crown'', which, after insertion and heating, perfectly surrounds the stump or superstructure.

Orthopedics
In orthopedics, we most often encounter shape memory in osteosynthetic staple constructions''. The staples expand in soft martensitic form so that they can be inserted into the pre-drilled holes at the ends of the fractured bone. With the subsequent increase in temperature, they contract on their own, thereby fixing parts of the broken bone. The jump material can be prepared in two ways. The grip temperature can be given by body temperature or a higher temperature. When body temperature is used, no additional artificial heating is needed; at a temperature higher than the body temperature, it is possible to interrupt the heating - and thus avoid the danger of excessive pinching and crushing of bones. Nitinol is also used in ``permanent implants'' in the form of rings, which can be a replacement for damaged intervertebral discs. Nitinol with a porosity of up to 70% has a wide range of elastic deformation and can therefore be used as a bone substitute.

Surgery
In surgery, nitinol wires are used to mark   and  localize'' tumor tissue in the breast. These wires are inserted into the tumors under X-ray using a needle. Due to pseudo-elasticity, the wire takes the shape of a hook and is fixed in the tumor after it is pulled out of the needle. Surgical procedures are then more precise and less invasive. Introduction wires of the same principle are used in catheters.

Nitinol is further used in mini-invasive surgery mainly for the production of medical preparations and special instruments. These are introduced in a compressed or packed form to the place of their application, where they take the desired predefined shape after being extruded and heated to body temperature (this is possible thanks to their shape memory).

Examples of applications are:
 * self-expandable stents and stent-grafts;
 * filters to catch thrombi;
 * urological "cups" for removing stones;
 * patches to close holes in the atrial septum.

Prosthetics
Nitinol is also used in prosthetics, namely in 'the constructions of actuators for limb replacements. The movement of the prosthesis is ensured by the deformation of the controllers, whareh is controlled by electrical impulses at the level of action potentials.

Literature

 * NAVRÁTIL, Leoš a Jozef ROSINA, et al. Medicínská biofyzika. 1. vydání. Praha : Grada, 2005. 524 s.  ISBN 80-247-1152-4.
 * SVOBODA, Emanuel, et al. Přehled středoškolské fyziky. 3. vydání. Praha : Prometheus, 2003.  ISBN 80-7196-116-7.