Nickel-titanium (Nitinol) alloys are rapidly becoming the materials of choice for self-expanding stents, graft support systems, filters, baskets and various other devices for minimally-invasive interventional and endoscopic procedures.
Nitinol alloys are most qommonly known for their superelasticity and thermal shape memory. The lerm ‘shape-memory’ is used to describe the phenomenon of restoring to a predetermined shape on heating, after having been ‘plastically’ deformed; ‘superelasticity’ refers to the enormous elasticity of these alloys. It can be 10 times more than the elasticity of the best stainless steels (SS) used in medicine today and follows a non-l inear path, characterised by a pronounced hysteresis (Figure 1). Although both effects are clearly spectacular, they are by no means the only important properties of the material.
The specific properties of Nitinol allow interesting solutions for superior medical devices :
- Elastic deployment
- Thermal deployment
- Kink resistance
- Constant stress
- Dynamic interference
- Stress hysteresis (biased stiffness)
- Temperature dependence of stress
Other, equally important properties, such as MRI compatibility, biocompatibility and corrosion .
Figure 1. Tensile behaviour of stainless steel and Nitinol
Elastic deployment
The enormous elasticity of Nitinol allows devices to be brought into the body through catheters or other delivery systems with a small profile. Once inside the body, the devices can be released from constraining means and unfold or expand to a much l arger size.
Thermal deployment
Most self-expanding implants, such as stents and filters, make use of the thermal shape-memory of Nitinol. A device with a transition temperature (Af) of 30°C can be compressed at room or lower temperature. It will stay compressed until the temperature is raised to >30°C. It will then expand toits preset shape. If this device could be kept cold during introduction into the body, it would not expand until at the desired location where body heat would warm it up. This is, of course, rather difficult to accomplish. All self-expanding devices, therefore, are constrained in the delivery systems to prevent premature deployment.
Kink resistance
To some extent this design property stems from the increased elasticity of superelastic Nitinol, but it is also a result of the shape of the stress-strain curve. When strains are locally increased beyond the plateau level, stresses increase markedly. This causes strain to partition to the areas of lower strain, instead of increasing the peak strain itself. Thus kinking, or strain localisation, is prevented by creating a more uniform strain than could be realised with a conventional material. The first applications to take advantage of this feature were guide-wires, which must be passed through tortuous paths without kinking . Steerability and torquability (the ability to translate a twist at one end of the guide-wire into a turn of nearly identical degree at the other end) of a guide-wire are directly affected by the straightness of the wire. Even very small permanent deformations will cause the wire to whip and destroy the ability to steer it through side branches or around sharp bends in the vasculature. Kink-resistant Nitinol wires play an important role in interventionai cardiology and radiology.
Constant stress
An important feature of superelastic Nitinol al loys is that their loading and unloading curves are substantially flat over large strains. This allows the design of devices that apply a constant stress over a wide range of shapes. The orthodontic archwire was the first product to make use of this property – more specifical ly the constant unloading stresses. SS and other conventional wires are tightened by the orthodontist regularly. As treatment continues, the teeth move and the force applied by SS wires quickly relaxes, according to Hook’s law. This causes treatment to slow, retard ing tooth movement. Nitinol wires, on the other hand, are able to ‘move with the teeth’, applying a constant force over a very broad treatment time and tooth position. Constant stress upon loading is used as ‘overload protection’ in hingeless graspers (or forceps) made from Nitinol. Hingeless instruments use the elasticity of spring materials, instead of pivoting joints, to open Deflection.
Dynamic interference
Dynamic interference refers to the long-range nature of Nitinol stresses and can be clearly illustrated using self-expanding stents as an example. Unlike balloonexpandable SS stents, self-expanding Nitinol stents will always expand to their pre-set diameters without recoil. Balloon-expandable stents, on the other hand, have to be over-expanded to achieve a certain diameter as a result of elastic spring-back after deflation. This spring-back, or loosening, is called acute recoil and is a highly undesirable feature. The over-expansion may damage the vessel and cause restenosis. Moreover, if the vessel diameter relaxes with time, or undergoes a temporary spasm, a SS stent will not fol low the vessel wall. The interference stresses will be reduced and the stent could embolise.The Nitinol stent will continue to gently push outward against the vessel wall after deployment and fo ll ows vessel movements. Typically, the pre-set diameter of a Nitinol stent is “‘1 – 2 mm greater than the target vessel diameter. It will therefore try to reach this diameter. Should the vessel increase in diameter, the Nitinol stent will also expand until it reaches its final diameter. A more complete description of this feature can be found elsewhere in this publication.
Biased stiffness (stress hysteresis)
The most unusual feature of Nitinol alloys is stress hysteresis . While in most engineering materials stress increases with strain upon loading in a linear way and decreases along the same path upon unloading, Nitinol exhibits a distinctly different
behaviour. Upon loading, stress first increases linearly with strain, up to “,1 % strain. After a first ‘yield point’, several percentage points of strain can be accumulated with only a little stress increase. The endof this plateau (‘loading plateau’) is reached at about 8% strain. After that, there is another linear increase of stress with strain. Unloading from the end of the plateau region causes the stress to decrease rapidly, until a lower plateau (‘unloading plateau’) is reached. Strain is recovered in this region with only a small decrease in stress. The last portion of the deforming strain is finally recovered in a linear fashion again. The unloading stress can be as low as 25% of the loading stress. The pronounced stress hysteresis can be utilised advantageously for a variety of medical devices.
Temperature dependent stiffness
The plateau stresses are strongly temperature dependent above the transition temperature of the alloy. As a result, superelastic devices become stiffer when temperature increases. The stiffness of a superelastic device of a given design at a specific temperature, body temperature for example, can be modified to some extent by adjusting the transition temperature of the Nitinol alloy used through a heat treatment. Lowering the transition temperature makes the device stiffer at body temperature. Plotting the loading plateau stress (at a defined strain) versus IH (body temperature minus transition temperature) D. Stoeckel yields a linear relationship as shown in Figure 16. with the stress increasing approx. 4.5 MPa per degree temperature difference for the most commonly used Nitinol alloy with Ti-50.8at% Ni.
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