Nitinol

The Concentric Retriever (Concentric Medical Inc., Mountain View, CA), a flexible, nitinol wire with helical tapering coil loops (X5 and X6) that is used in conjunction with a balloon guide catheter (8 or 9 French) and a microcatheter, is the only device currently approved by the FDA for the endovascular treatment of stroke patients (Fig. 4.3). The second-generation devices (L5 and L6) differ from the X devices by the inclusion of a system of arcading filaments attached to a nontapering... 

The In-Time Retriever (Boston Scientific, Natick, MA) has four to six wire loops and tends to bow when opened but has no specific opening to capture the embolus. This device has been successfully used in a case of an MCA occlusion resistant to thrombolytics and balloon angioplasty, as well as in cases of basilar occlusion. The TriSpan (Boston Scientific, Natick, MA), a neck bridge device consisting of three nitinol loops originally designed to treat wide-necked aneurysms, has also been used to treat basilar occlusions. ... 

Nickel-tin-aluminum catalyst, 24 794 Nickel titanate, 25 47 Nickel-titanium (NiTi) alloy (Nitinol), 22 341, 712... 

Not much has been done in discussing ways to improve medical device surfaces using electrochemical coating methods in particular, with the aim of accomplishing the medically relevant goal, aside from discussion in the relevant literature, of the processing of nitinol (an alloy of nickel and titanium see Section 21.4). 

One of the most commonly used medical devices is the stent, (Fig. 21.1), small metallic structures that are expanded in blood vessels, functioning to maintain the patency (freedom from obstruction) of the vessel in which it is placed. Although the first use of stents was in vasculature (blood vessel systems), more recent applications include, for example, implantation between two vertebrae to increase the rigidity of the spine. A typical vascular stent is placed in its anatomical location and then either plastically deformed/expanded (stainless steel) or allowed to expand to a predetermined size, as a consequence of shape memory (nitinol). 

Figure 21.1. Stents are used to open arteries of the heart blocked by atherosclerotic plaques (A) a balloon and stent are placed across the plaque (B) the balloon is expanded, leaving the stent to prop open the artery (C) restenosis is the process wherein scar tissue builds up around the stent, again causing a flow restriction. A balloon is required for stainless steel, whereas a nitinol stent will expand on its own, due to the shape memory property of nitinol. (From Ref. 11, with permission.)...

Although the titanium oxide layer at the surface of the nitinol is highly biocompatible and protects the underlying substrate from electrochemical corrosion, the titanium oxide layer itself is mechanically very brittle. Under mechanical stress, such as the shear of blood flow in the aorta or under the bending moments of aortic pulsations, the titanium oxide surface layer can fracture, exposing the underlying metal to corrosion. Not only is corrosion undesirable in terms of biocompatibility (i.e., leaching of nickel and its... 

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

In an excellent study and review. Sun et al. (10) revealed the heterogeneity of nitinol under various temperature conditions, even in a simple lactated Ringer s solution. Lactated Ringer s solution is a mixture of salts and water meant to simulate the tonicity of blood. In this set of experiments, the surfaces were obtained from conunercial sources, and each sample had undergone similar surface processing prior to experimentation. When a nitinol sample was simply placed in the Ringer s solution at constant potential and a given temperature, current transients were seen, which represent breakdown and repassivation of the oxide film. 

The lag between the time that nitinol, was first produced and the time it was used commercially in medical devices was due in part to the fear that nickel would leach from the metal and not be tolerable as a human implant. As it turns out, with a correct understanding of the surface electrochemistry and subsequent processing, a passivating surface layer can be induced by an anodizing process to form on the nitinol surface. It is comprised of titanium oxide approximately 20 mn thick. This layer actually acts as a barrier to prevent the electrochemical corrosion of the nitinol itself. Without an appreciation for the electrochemistry at its surface, nitinol would not be an FDA-approved biocompatible metal and an entire generation of medical devices would not have evolved. This is really a tribute to the understanding of surface electrochemistry within the context of implanted medical devices. 

The most common SMAs are nickel-titanium alloys and copper alloys of various kinds. Nitinol, a specific alloy of nickel (Ni) and titanium (Ti), is probably the most widely used. (The word nitinol comes from the chemical symbols of its two metal components, along with an abbreviation for the Naval Ordnance Laboratory, where this alloy was discovered and studied in the early 1960s.) Although nickel and titanium alloys tend to be more expensive than copper materi-... 

The advantage in using an SMA over other materials is that SMAs can recover from greater strains. Nitinol can bend about 15-20 times more than steel before breaking. 

Catheter procedures avoid the need for surgeons to open the patient s chest. As a result, the patient experiences a lot less pain and needs far less time for recovery. These procedures have already become common in adults, but the bulky valves and catheters are not suitable for children. With nitinol s memory and flexibility, the valve can be fitted into a small space yet return to its needed size when placed in position. Nitinol is biocompatible—it does not harm tissues—so it causes no damage when implanted in the body. 

Wilham J. Buehler and his colleagues at the Naval Ordnance Laboratory in White Oak, Maryland, describe the shape-memory alloy known as nitinol. 

Gregory Carman, Lenka Stepan, and Daniel Levi design and build a heart valve made with nitinol. 

Baz, A., and T. Chen, Torsional stiffness of nitinol-reinforced composite drive shafts. Composites Eng., 3, 1119 (1993). 

Alloy with Memory. In seeking a way to reduce the brittleness of titanium, U.S. Navy researchers serendipitously discovered a nickel-titanium alloy having an amazing memory. Previously cooled clamps made of the alloy (nitinol) are flexible and can be placed easily in position. When warmed to a given temperature, the alloy hardware then exerts tremendous pressure. Use of conventional clamps for holding bundles of wires or cables in a ship or aircraft structure requires special tools. For this and other applications in industry and medicine, nitinol has been in demand. The alloy, however, is not easy to produce because only minor variations in composition can affect the snap back" temperature by several degrees of temperature. 

As previously mentioned, the nickel—titanium alloys have been the most widely used shape memory alloys. This family of nickel—titanium alloys is known as Nitinol (Nickel Titanium Naval Ordnance Laboratory in honor of the place where this material behavior was first observed). Nitinol have been used for military, medical, safety, and robotics applications. Specific usages include hydraulic lines capable of F-14 fighter planes, medical tweezers, anchors for attaching tendons to bones, eyeglass frames, underwire brassieres, and antiscalding valves used in water faucets and shower heads (38,39). Nitinol can be used in robotics actuators and micromanipulators that simulate human muscle motion. The ability of Nitinol to exert a smooth, controlled force when activated is a mass advantage of this material family (5). 

The discovery of the shape memory effect in TiNi by Buehler et al. at the Naval Ordinance Labs occurred during an investigation of the alloy for possible use as a corrosion-resistant knife for underwater activities. The investigators called the alloy nitinol for Nickel, Titanium, and Naval Ordinance Labs. 

Fig. 2. The Thermobile energy conversion device using two wheels and a strand of Nitinol wire loop Converting directly the thermal into mechanical energy (Wang, US Patent 4,275,561).

The temperature for the acoustic damping capacity change from Nitinol was found to be different for alloys that were prepared at different laboratories (even though both alloys have identical composition). Further, the shape memory response to temperature change, such as how fast and how much force, also varied a great deal from one alloy to another. 

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