NiTi Shape Memory Alloy – NiTi alloys as smart materials for medical implants and devices

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Introduction of NiTi Shape Memory Alloy

According to one definition, “smart materials respond to external stimuli by changing key properties of the material,” and according to another definition, “smart materials often exhibit properties that are in some way unexpected or novel.” Along with piezoelectric ceramics, magnetostrictive materials and electrorheological fluids, shape memory materials have long been divided into smart materials or components of smart systems. This is thanks to their ability to change shape as they heat and cool, making them well-suited for advanced actuator applications. However, although this has been known for four decades and many applications have been proposed, only a limited number of industrial products containing “smart” shape memory actuators have appeared on the market. At the same time, the most popular metal shape memory material (SMA – shape memory alloy) – nickel-titanium alloy – has become the material of choice for many applications in the medical device industry. Although few of these applications have “smart systems,” according to the above definition, they are still eligible.

Ninool is a plasmonic or near-plasmonic intermetallic compound of nickel and titanium that undergoes solid-state transformation morethan a temperature range of -100°C to +100°C, depending on composition or processing history. Associated with this phase transition are significant changes in material properties. Its most significant performance is that after obvious plastic deformation, the material can return to its original shape after heating (thermal memory). Another related effect is so-called superelasticity, which is the ability of a material to recover its initial shape when unloaded after being subjected to significant strain (elastic memory). In addition to these amazing effects, shape memory alloys, especially Nitinol, exhibit some unusual but useful properties that make this material “smart.”

We then discuss Nitinol’s most successful medical application, the self-expanding stent, which is an impressive example of Nitinol’s intelligent behavior in a medical device. Finally, we’ll look at some recent advances in thin-film NiTi stents and filters.

Shape memory effect of nickel-titanium alloy

Shape memory and superelasticity are the result of thermoelastic martensite transformation. Above the transformation temperature, Nitinol (a nickel-titanium alloy containing approximately 50 at.% titanium) is austenitic. The crystal structure of austenite is cubic B2 or cesium chloride structure. Cooling under the transformation temperature transforms the B2 structure into a twinned monoclinic structure called martensite. This transformation does not occur with macroscopic shape changes. However, twinned martensite can be easily deformed by unconventional detwining mechanisms up to strains of approximately 8%. This deformation can be restored by heating the material to a temperature above the transition temperature, completing the shape memory cycle. As mentioned above, significant changes in material properties accompany this phase transition. Figure 1 illustrates a typical graph of the properties as a function of temperature, depicting the temperature As (austenite start), Af (austenite end), Ms (martensite start) and Mf (martensite end) with relevant definitions—characteristic hysteresis curve.

Figure 1 (left): Temperature hysteresis of NiTi alloy (schematic)

Figure 2 (right): Stress/strain characteristics and stress hysteresis of NiTi alloy (schematic)

At temperatures above Af, martensite can be stress-induced, that is, by converting austenite into martensite and immediately deforming it through detemperature, subjecting the material to deformation stress, producing recoverable strains of up to 8%, which is called superelasticity or elastic memory. Significant hysteresis can be found when plotting stress versus strain because the basic mechanism is the same as thermal memory. Figure 2 schematically shows the stress-strain curve of a NiTi alloy wire at a temperature of about 30 degrees above Af. When loading, stress first increases linearly with strain up to approximately 1% strain. After the first “yield point,” a few percent of strain can accumulate with only a small increase in stress. The end point of this plateau (the “loading plateau”) is reached at approximately 8% strain. After this, there is a linear increase in stress and strain. Dumping from completion of the plateau area leads to the stress to drop rapidly until a lower plateau is reached (the “unloading plateau”). Strain is restored in this region, while stress is reduced only a little. The last part of the deformation strain eventually recovers again in a linear fashion. Based on stress hysteresis, a device can exhibit so-called biased stiffness, high stiffness during loading and low stiffness during unloading. As will be shown later, this is a major aspect of the performance of self-expanding stents used to treat diseased vessels.

Figure 3: Tensile curves of superelastic nickel-titanium alloy wire at different temperatures

The stress-strain behavior of Nitinol alloys is closely related to temperature. Hysteresis shifts upward as the difference between Af and operating temperature increases, i.e., a device with a given Af becomes stiffer as temperature increases. On the other hand, lowering the temperature shifts the hysteresis downward. When the operating temperature is below Af, and more specifically below Mf, the deformation will no longer recover elastically. However, it can be hot restored (hot memory). Figure 3 shows the tensile curves of Af -10°C superelastic alloy at different temperatures.

Intelligent self-expanding stents and equipment

Undoubtedly, the most well-known application of nickel-titanium alloys in medical devices is in self-expanding stents. A stent is a stent-like structure that prevents blood vessels from re-closing after balloon dilation, tumor growth, or other obstructive effects. While most stents used in coronary arteries are still balloon-expandable, most stents used in nonvascular or peripheral vessels are self-expanding. The self-expanding stent has a diameter larger than the target vessel, is crimped and is restrained in a catheter-based delivery system. They are usually inserted into the body through small incisions or natural body openings under fluoroscopic or endoscopic guidance. The stent is released from the delivery system at the target site and elastically expands until it hits the vessel wall (Figure 4). Therefore, the performance of self-expanding stents depends on the material’s ability to store elastic energy when restrained in the delivery system, making NiTi alloys an ideal choice.

Figure 4: Self-expanding nitinol stent (left: half deployed from the delivery system)

The smart properties of NiTi stents become particularly apparent when examining the force balance in the stent vessel. As mentioned above, NiTi alloys’ stress hysteresis or path dependence results in a characteristic called skew stiffness. This concept is shown in Figure 5, which shows a schematic diagram of a typical superelastic stress-strain curve for a NiTi alloy, illustrating the nonlinear response and hysteresis. Using this diagram, we will follow the cycle of pressing the stent into the delivery system, deploying the stent, and allowing it to expand and interact with the blood vessel. To do this, the axis has been changed from stress-strain to hoop force-stent diameter. A stent of a given size that is larger than the blood vessel (point “a” in Figure 5 ) is crimped into the delivery system (point “b”) and then packaged, sterilized, and shipped. After insertion into the target site, the stent is released into the blood vessel and expands starting at “b” until movement is stopped due to collision with the blood vessel (point “c”). Based on this point, further expansion of the stent is prevented. Because the stent does not expand to its preset shape, it continues to exert a lower outward force, called a chronic outward force, or COF. However, it will resist recoil pressure or any other external compressive force determined by the loading curve from point “c” to “d,” which is steeper (stiffer) than the unloading line (toward “e”). These forces are called radial resistance, or RRF.

Figure 5: The concept of bias stiffness is the path dependence along the hysteresis force

Is the stand smart? It certainly meets the requirements of the definition. Nitinol’s unusual elastic hysteresis allows the sustained opening force of the stent acting on the vessel wall, known as the chronic outward force (COF), to remain very low even when the stent undergoes large deformations and oversizes. At the same time, the force generated by the stent to resist compression, known as radial resistance (RRF), increases rapidly with deflection until plateau stress is reached. Generally speaking, bracket designers strive for the highest possible RRF and the lowest possible COF.

Figure 6 shows a commercially available 10 mm nitinol stent (nominal diameter). The device was crimped to 2 mm and deployed to simulate a vessel diameter of 8.5 mm (data for diameters smaller than 4 mm were not recorded). At 8.5 mm, the RRF was recorded by crimping the stent back to 7.5 mm and then completely unloading it to its original diameter. The COF is fairly stable throughout the specified diameter range (8 to 9 mm), which is 0.035 N/mm. Withthe stent deforms from the equilibrium diameter, the RRF increases sharply, reaching 0.22 N/mm after one millimeter of deformation. Continuing deformation will show a plateau at approximately 0.24 N/mm.

Figure 6: Radial force test and offset stiffness diagram

Another notable and demonstrable attribute of nitinol stents is their crush recovery capabilities. Most, if not all, nitinol stents can be completely flattened and still elastically return to their original shape without clinically relevant loss of lumen diameter. This property is important for superficial indications susceptible to external compression, such as the carotid artery (Fig. 7).

Figure 7: Demonstration of crush recovery of NiTi alloy stent

The market size for nitinol stents for peripheral applications is estimated to be close to $500 million and is growing rapidly with the introduction and acceptance of new procedures. One area of particular interest is new devices for intracranial implantation, where small delivery dimensions are required. Most stents currently available are made by laser cutting nitinol tubes. However, tubes with wall thicknesses <0.05 mm required for neurovascular applications are very difficult to produce (Fig. 8). Another method for producing stents with very thin struts is vapor deposition, specifically sputter deposition of NiTi alloys. To create 3D shapes such as tubular films, Nitinol is sputter-deposited onto a highly polished substrate, heat treated to crystallize the material, and removed from the substrate. The process can be controlled to create films 1 to 5 microns thick that exhibit essentially the same shape memory, or superelasticity, as bulk Nitinol. The scaffold pattern is then created through photochemical etching methods. These devices have the potential to be delivered into the brain via microcatheters.

Figure 8: Laser cutting of stent from Nitinol tubing. The largest tube in the picture on the left has an outside diameter. 2mm, minimum 0.4 5mm

To capture blood clots that could lead to stroke during neurovascular interventional procedures, self-expanding filters can be placed distal to the treatment site. These filters prevent debris from entering the brain and allow blood clots and particles to be removed after surgery. Several different Nitinol filters are already on the market (Figure 9). Filters made from thin-film Nitinol could greatly improve surgery because it allows for smaller delivery systems and the potential to reach farther locations. The prototypes have been produced by sputtering a 4-micron-thick Nitinol film onto a cone-shaped substrate, photochemically etching the film through, and then removing the patterned film cone from the substrate (Figure 10).

Figure 9 (left): Different designs of distal protection filters (clockwise from top left: BSC, Cordis, Guidant, MicroVena)

Figure 10 (right): Thin film Nitinol conical filter prototype

Other applications of SMA in smart medical devices

The medical device industry has adopted nickel-titanium alloys as the material of choice for various devices. Extensive product reviews can be found in the proceedings of the International Shape Memory and Hyperelastic Technology Conference and previous conference proceedings. The primary benefit of using nickel-titanium alloys in medical devices is that they simplify design and support the overall trend toward minimally invasive therapies in medicine. Nitinol’s unique response to thermal or mechanical stimulation allows devices to be manufactured using fewer parts and smaller sizes while potentially reducing cost and having performance characteristics that are otherwise unobtainable. An early example is a hingeless instrument that consists of just one rather than the multiple complex, precision-machined parts and connections of traditional devices. In addition, Nitinol’s nonlinear stress/strain characteristics provide constant force clamping of large and small objects and built-in overload protection. Nitinol’s strong temperature-dependent stiffness can be used to make guidewires and catheters easier to maneuver by locally changing the component’s stiffness during surgery. This can be achieved by electrically heating the part or, as recently suggested, by sliding an optical fiber inside a nitinol microtube and heating the material with a laser.

Emerging thin film technologies will further expand the application of Nitinol in medical devices. In addition to stents and filters, producing all-metal grafts by vapor deposition and photochemical etching has also been proposed to replace traditional polymer grafts. MEMS containing thin-film nitinol actuators can be used in implantable drug pumps and valves.

Conclusion

The trend to minimally invasive procedures in medicine allows the design of new implants and instruments using smart shape memory alloys as key components. Driven by this demand, nickel-titanium alloys have grown exponentially over the past decade. Nitinol’s unique material properties, particularly thermal memory, nonlinear stress/strain behavior, stress hysteresis, and temperature-dependent stiffness, make this material and devices incorporating Nitinol components truly smart.

Supplier

TRUNNANO is a supplier of NiTi Alloy and their powder with over 12 years experience in nano-building energy conservation and nanotechnology development. It accepts payment via Credit Card, T/T, West Union and Paypal. Trunnano will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are looking for high quality NiTi Alloy and their powder, please feel free to contact us and send an inquiry.

Note: Source from Fulin Plastic