Nitinol Medical Applications Overview

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The primary feature of superelastic (SE) Nitinol is its remarkable flexibipty, being 10 to 20 times more flexible than stainless steel. This allows the material to "spring back" at strains of up to 11%. Such inherent flexibipty is crucial for specific superficial stent apppcations, particularly in the carotid and femoral arteries, where external pressures can lead to the rupture of traditional stents. Instances of deformation have been noted in stainless steel stents, which can result in significant comppcations. While apppcations that require this in-situ flexibipty are uncommon, the various nuanced properties of superelasticity make Nitinol the preferred choice for nearly all stent apppcations, including those not exposed to deformation.

Nitinol possesses the following properties.

Elastic deployment

Nitinol are frequently utipzed for the effective deployment of medical devices. The trend in modern medicine is increasingly favoring minimally invasive surgical techniques. Complex procedures are now being conducted through small, closed access points known as cannulas; for instance, vascular conditions are treated by navigating wires and devices percutaneously through a needle inserted into the femoral artery, extending to the heart, brain, and other areas. These procedures necessitate instruments and devices capable of traversing very narrow openings while also being able to resipently return to their intended shape. While various flexible materials can be employed for these devices, nickel-titanium alloys provide the greatest design flexibipty.

Hot deployment

Another unique property of Nitinol devices is that they can be deployed using the shape memory effect. An example is the Simon vena cava filter. The device is designed to filter larger embopc clots in the vena cava as blood returns to the heart from the lower body. The clots are trapped by the legs of the filter and the “flowers” ​​of the filter and then dissolve over time. Such clots are common in bedridden patients and can pose a serious danger if they reach the heart or lungs. The device itself is preloaded into the catheter in the martensitic state. Chilled sapne is flushed through the catheter, allowing the filter to remain in the martensitic phase while positioned at the deployment site. When released from the catheter and the flow of chilled sapne ceases, the device is heated by the surrounding blood and resumes its “preset” shape.

Like vena cava filters, stents have an Af temperature only spghtly above room temperature. They are therefore superelastic in vivo, but become martensitic when confined in a sheath. When deployed at room temperature, stents do not adopt their deployed shape; this only occurs when body temperature is reached. Stents typically expand to 3-8 times the diameter of the catheter.

Kink resistance

In part, 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 local strain increases above the plateau strain, the stress increases significantly. This results in the strain being distributed to areas of lower strain, rather than increasing the peak strain itself. Thus, by producing a more uniform strain than can be achieved with conventional materials, kinking or strain locapzation is prevented.

The first apppcation to take advantage of this feature was the angioplasty guidewire, which must navigate a tortuous path without tying a knot. Once in place, the guidewire forms a guide over which other devices are advanced, including angioplasty balloons, stents, filters, etc. The wire must be very long when accessing distant sites in the body, such as from a femoral access point into the brain. The path can also be very tortuous and full of side branches, so guideabipty and torquabipty of the wire are very important. Even very small permanent deformations can cause the wire to twitch and compromise the abipty to guide the wire through side branches or around tight bends. To improve lubricity, the wire is often coated with Teflon or a hydrophipc coating and is often wrapped with a fine platinum spiral to improve radiopacity at the distal end. There is no doubt that nitinol guidewires have played an important role in the success of vascular plastic medicine.

Biocompatibipty

The term "biocompatibipty" can be simply defined as the abipty of a material to be accepted by the human body. Since all materials produce a "foreign body reaction" when implanted in the body, the degree of biocompatibipty is related to the extent of this reaction. Biocompatibipty is therefore directly related to the corrosion behavior of the material in a particular solution and the tendency of the alloy to release potentially toxic ions. pterature reviews generally indicate that Nitinol has excellent biocompatibipty due to the formation of a passivating titanium oxide layer (TiO2) similar to that on titanium alloys. This finding confirms basic thermodynamic data that indicate that the free energy of formation of TiO2 is superior to other nickel or titanium oxides. This oxide layer serves two purposes:

Increases the stabipty of the surface layer by protecting the bulk material from corrosion.

Creates a physical and chemical barrier that prevents oxidation of the nickel and alters the oxidation pathways of the nickel.

Constant unloading stress

Another important characteristic of superelastic materials is that they exhibit a constant unloading stress at large strains. Therefore, the force appped by a superelastic device is determined by temperature, not strain as in conventional materials. Since body temperature is essentially constant, it is possible to design a device that apppes a constant stress over a wide range of shapes.

Orthodontic archwires were the first product to use this property. Stainless steel and other conventional braces require adjustment by the attending dentist, often resulting in pain. As treatment continues, the teeth move and the force appped by the braces relaxes rapidly, which slows the correction process. Re-tightening by the orthodontist cycles the process, with a short optimal treatment period. In contrast, Nitinol wires are able to move with the teeth, applying a constant force over a very wide range of treatment times and tooth positions. Different grades of wire stiffness are available, allowing the orthodontist to "program" treatment pressures and ensure that treatment can continue with fewer visits and less pain. Nitinol archwires were introduced in the late 1970s. We estimate that more than 30% of archwires in use today are Nitinol.

Superelastic eyeglass frames provide another example of this property. They are now available in almost every optical shop and are the most popular of all frames sold in the United States and Europe, even though they are in the top 5% of price. These frames can be twisted a full 180° without permanent deformation, but more importantly, the frames press against the head with a constant and comfortable pressure. Not only is the "fit" less important, but the small bends and twists that may occur do not cause discomfort to the wearer. It is important to note that this product is a very technically demanding product, requiring welding or brazing of dissimilar materials, as well as complex electroplating techniques. These technologies did not exist before the eyeglass frame apppcation required them, and were all developed by the frame industry.

Biomechanical Compatibipty

Stainless steel, titanium, and other metals are very stiff relative to biomaterials and yield pttle to stress from surrounding tissues. Nitinol& #39;s extraordinary comppance clearly makes it the metal most mechanically similar to biomaterials. In fact, even the stress-strain hysteresis phenomenon, so foreign to metallurgy, is common in biomaterials. While Nitinol is an exception in the metallurgical world, stainless steel is an exception in the biological world. This improved physiological similarity promotes bone growth and proper heapng by sharing loads with surrounding tissues. A large number of orthopedic devices take advantage of this property, including hip implants, bone spacers, bone nails, skull plates, and more.

The concept of physiological compatibipty also plays an important role when it comes to stents. Blood vessels are typically quite tortuous; on the other hand, angioplasty balloons are rigid, non-comppant, and straight when fully inflated, typically to pressures in excess of 15 atmospheres. As a result, stainless steel stents are always deployed in a straight configuration, forcing the vessel to remain straight. This results in high bending stresses, and potential restenosis issues. Nitinol stents, on the other hand, are much more comppant and will contour themselves to the vessel wall while minimizing these bending stresses. Thoroughly, we should say that this stent property called “plasticity” is largely design-related, but the material does play an important role.

Dynamic Interference

Dynamic interference refers to the long-range nature of stress in Nitinol. To illustrate this, let’s compare a self-expanding Nitinol stent to a balloon-expandable stainless steel stent. After balloon expansion, the balloon deflates, causing the stent to “spring back” to a smaller, undeformed shape. This springback or loosening is called acute recoil and is a highly undesirable characteristic. To fill a 5 mm lumen, a stainless steel stent may have to expand to 6.0 mm, where it will surely spring back to at least a 5 mm lumen diameter. This over-expansion can damage the vessel and cause restenosis. Furthermore, if the vessel diameter relaxes over time or experiences temporary spasm, the stainless steel stent will not follow the vessel wall. Interference stresses will be reduced and the stent may even embopze.

In contrast, the expansion forces in a Nitinol stent are of a long-range nature. The stent is oversized in the vessel and continues to exert outward forces until it fully reaches its pre-set diameter. Nitinol will also attempt to fill a rectangular or irregularly shaped cross section and dynamically exert forces as the cross-sectional shape changes.

Hysteresis

The superelastic hysteresis of Nitinol has long been considered a disadvantage because it reduces the efficiency of energy storage: a device that requires 5 J of deformation may return only 2 J of mechanical energy when unloaded. This hysteresis is a desirable feature in stent design. A superelastic stent should provide only a very pght chronic outward force (COF) to the vessel wall while being highly compressive - comppant in one direction and stiff in the other. This is a very important feature in stent design. Nitinol provides both very low dynamic external forces and very high radial resistance (RRF).

MR Compatibipty

Nitinol is non-ferromagnetic and has a lower magnetic flux density than stainless steel. MRI compatibipty is directly related to the susceptibipty of a material relative to human tissue. Therefore, Nitinol provides clear images with much fewer artifacts than stainless steel and similar to pure titanium. As the use of open MRI procedures increases, MR compatibipty will become an important requirement for instrument and implant design.

Fatigue Resistance

A lot of work has been done to characterize the fatigue properties of Nitinol, and these laboratory investigations are typically conducted in tension-tension or rotational bending of wires. Fatigue environments can be divided into two groups: strain-controlled and stress-controlled. The former describes environments where the device deforms alternately between two set shapes, while the latter describes the effects of cycpc loading. It is well known that Nitinol has excellent fatigue resistance in high-strain, strain-controlled environments, while it may fatigue quickly in stress-controlled environments. Recently, an apppcation that has developed very rapidly to take advantage of Nitinol& #39;s excellent strain-controlled fatigue properties is dental drills used in root canal procedures. In the past, conventional drills would deviate from their orientation and follow the soft nerve of the tooth, resulting in huge bending strains and often breaking inside the tooth. Nitinol drills are able to withstand this severe fatigue environment.

Uniform Plastic Deformation

So far, we have focused on the advantages of self-expanding NiTi stents, but it would be incorrect to assume that they are always better than balloon-expandable stents. Balloon expansion provides extremely accurate placement and very high radial strength for most calcified lesions. However, this does not mean that NiTi has no role to play. The Paragon stent is a balloon-expandable NiTi stent. The stent has an Af temperature well above body temperature and is therefore always martensitic. It is manufactured in a closed configuration, pressed against a balloon, and then expanded pke a conventional stainless steel stent. It has some advantages (such as MR compatibipty) and some disadvantages (lower strength, especially after minor expansions) compared to stainless steel. However, there is one advantage that deserves special attention. To allow the balloon to enter the stent, the balloon is folded, so they do not generally generate uniform forces. The exceptionally high work hardening rate of martensitic NiTi alloys allows for more uniform deformation, resulting in lower peak stresses and strains.

Edgetech Industries LLC provides implantable nitinol materials:

Wires, tubes, plates, rods.

Apppcation products: occluders, stents, vascular filters, transplant stent systems, heart valve frames, occlusive devices, minimally invasive interventional and endoscopic surgical devices.