NiTinol: A Review of Its Smart Properties That Make It a Smart Alloy and a Strong Ally in Endovascular Neurosurgery

Abstract

Nitinol is an equiatomic alloy of nickel and titanium. Its unique properties like “superelasticity” and “shape memory” have made it one of the most commonly used materials for manufacturing hardware in endovascular neurosurgery. The solid state of nitinol has two interconvertible (austenite and martensite) phases. With increasing temperature, the martensite phase gets transformed into the austenite phase (thermal phase transformation), and thus remembers the shape that it had in the austenite phase (shape memory effect). This forms the basis behind the use of nitinol in making self-expandable stents. The other property of “superelasticity” is attributed to the stress-induced phase transition that occurs when nitinol in its austenite phase is subjected to a deforming force. Once the force is removed, nitinol reverts back to its original shape. This forms the basis for the use of nitinol in making kink-resistant wires and catheters. The mechanism by which this phase transition is utilized to exploit these above-mentioned properties can be understood from the physical structure of the alloy. This in-depth knowledge of metallurgy is instrumental while choosing the right hardware for a procedure and shall help in further research at the basic science level. It will also lead to a better two-way communication between the biomedical engineer in the laboratory and the clinician in the operating room.

Keywords

nitinol - phase transition - shape memory alloy - smart alloy - superelasticity

Introduction

Nitinol is an equiatomic or near-equiatomic alloy of nickel and titanium. It is a unique material with interesting properties like “superelasticity” and “shape memory.” These are closely attributed to its peculiar microscopic architecture of crystals.[1] Over the decades, it has been widely used in diverse fields ranging from the production of virtually indestructible nitinol spectacles to the complex hydraulic systems in fighter jets. It has also been used in aerospace engineering, in our quest to find life in outer space, and the field of biomedical engineering, in our attempt to sustain life. As endovascular neurosurgeons, we come across implants and hardware that are made of nitinol. While maneuvering through the tortuous vascular territories of the brain, nitinol hardware is something we think about and ask for. The properties that make nitinol the most sought-after alloy are unique and form the basis of our success during endovascular procedures. Understanding the physical structure of these properties will not only help us in choosing our hardware with much more conviction but also pave the way for further research at the basic science level.

Nitinol Had a Smart Beginning

The etymology of the word “nitinol” stems from its composition and place of discovery (Ni: nickel, Ti: titanium, and NOL: Naval Ordnance Laboratory).[2] Its discovery dates back to 1959 when Dr. William J. Buehler, while working in the U.S. Naval Ordnance Laboratory, in an attempt to develop a material for the missile nose cone that would withstand heat, corrosion, and forceful impact, came across the unique and undiscovered properties of an equiatomic alloy of nickel and titanium. A couple of years later, at the laboratory committee meeting, a sample of nitinol was passed across the members and were asked to deform it. Although Beuhler wanted to demonstrate the fatigue-resistant property of nitinol, serendipitously one of them placed a pipe lighter under it and the deformed sample of the alloy spontaneously transformed back into its original shape, suggesting the property of “shape memory.” But this term was not new to the lexicon of metallurgy. About three decades earlier, Dr. Olander (a Swedish physicist) had reported the property of superelasticity in a gold–cadmium alloy. But it was Kurduinov and Khandros (1948) who explained the physical basis behind the phenomenon of superelasticity. In 1953, Chan and Read discovered that the “alloy of Olander” also had the property of “shape memory” besides “super elasticity.” Although nitinol was introduced to the world of metallurgy quite late, it became more appealing to the market as it was lot cheaper than the “alloy of Olander,” which had gold as one of the components. The property that first caught the eyes of the ever-evolving field of biomedical engineering was “superelasticity.” Over the decades, nitinol has been used in manufacturing kink-resistant guidewires. Soon biomedical engineers understood the shape memory property of nitinol, and thus began the beginning of the era of self-expandable stents.

Physical Properties of Nitinol That Make It a Smart Metal

Elementary school chemistry has taught us how the three states of matter get interconverted into each other. Some materials like nitinol, in their solid state, can exist in two distinctly different, reversible crystal forms (martensite and austenite phases).[3] As we increase the temperature, the martensite phase transforms into an austenite phase, before reaching the melting point. In the austenite phase (a phase or parent phase) that exists at a higher temperature, the atoms of nickel and titanium are aligned in a regular cuboidal shape.[4] As we cool nitinol, the alignment of crystals gets spontaneously transformed into a more complex structure called the martensite phase (M phase or daughter phase). This thermal conversion of the microscopic crystal forms is a reversible process ([Fig. 1]). Although the microscopic structure is transformed, the macroscopic shape remains the same as it was in the austenite phase. The “twinned” crystal structure of the martensite phase has the unique ability to undergo plastic deformation without breaking the atomic bonds. When external force is applied in the martensite phase, there occurs a plastic deformation of the shape. But microscopically, instead of undergoing permanent deformation, the atomic planes are rearranged (as if the atoms slide over each other) and the “twinned” martensite is transformed to a “detwinned” martensite structure. However, irrespective of the deformation status of the martensite phase, upon heating, there occurs phase transition to the austenite phase, and the alloy returns to the shape that it was given in the austenite phase ([Fig. 1]). In other words, nitinol remembers the shape that it had memorized in its austenite phase and regains it spontaneously upon heating. This phenomenon is called the “shape memory effect.”[5]

Fig. 1 Schematic representation of the two crystalline forms of nitinol. Below Mf, nitinol exists completely in the martensite phase. Under the effect of a deforming force, the twinned martensite gets transformed into detwinned martensite and this microscopic change is also accompanied by macroscopic alteration of shape. As we increase the temperature, the martensite phase gets progressively converted to the austenite phase. The first evidence of the austenite form is seen at As, and beyond Ms, there occurs no evidence of the martensite form. Above the Af temperature point, nitinol exists exclusively in the austenite phase. If the nitinol hardware is already in a plastically deformed detwinned martensite phase, on heating it not only gets microscopically transformed into the martensite phase but also regains the shape that it had in the austenite phase. The phenomenon is called “shape memory effect.” Af, full austenite; As, start austenite; Mf, full martensite; Ms, start martensite.

When we subject nitinol to a deforming force, while it is in the austenite phase, it gets temporarily deformed to a new shape (martensite phase), only to revert back into its original shape when the force is removed ([Fig. 2]). At the microscopic level, the atoms just slide over one another without actually breaking the bond in between them (called stress-induced phase transition).[6] [7] Compared with other metals, the degree of deformation that nitinol can tolerate is much higher, rendering an additional property of superelasticity to itself.

Fig. 2 A schematic diagram depicting the “superelastic” property of nitinol. When a deforming force is applied on nitinol in its austenite phase, it undergoes stress transformation transiently into its martensite phase. It immediately gets back to its original shape, both macroscopically and microscopically, once the force is removed. Af, full austenite; As, start austenite; Mf, full martensite; Ms, start martensite.

On heating, nitinol undergoes a phase transition from the martensite phase to the austenite phase over a wide range of temperatures ([Fig. 3]). At the temperature Mf (full martensite point), the whole of the alloy is in the martensite form.[8] As we start heating, the first evidence of the austenite form is seen at the As (austenite start point) temperature. Beyond this point, there occurs a gradual and progressive transformation of the martensite phase into the austenite phase. As we increase the temperature beyond Ms (martensite start point), there will not be any evidence of the martensite crystal form. Finally, at Af (full austenite point) and above, there occurs a complete phase transformation into the austenite crystal form. This forms the basis of the shape memory effect seen with all the smart alloys or intelligent alloys. Since phase transformation spontaneously occurs upon both heating and cooling, nitinol is said to be a two-way shape memory alloy.

Fig. 3 The temperature range over which phase transition occurs in a shape memory alloy like Nitinol. Af, full austenite; As, start austenite; Mf, full martensite; Ms, start martensite.

Nitinol is basically a 1:1 alloy of nickel (Ni) and titanium (Ti), and the phase transition temperature ranges between 20 and 50°C. However, with a slight manipulation in the percentage of nickel, we can alter this temperature range to suite the clinical scenario. With the increase in the nickel content, the temperature at which the martensite phase starts (Ms) decreases, so is the transition temperature.[9] Since the working temperature during an endovascular procedure is constant (i.e., body temperature), biomedical engineers could selectively harness the superelasticity or the shape memory property by altering the transition temperature of alloys. When the deforming force is applied above the transition temperature, the superelastic property of nitinol is explored. On the other hand, when the deforming force is applied at a temperature below the transition zone, the shape memory property of nitinol can be utilized by getting back to the original shape at body temperature.

“Superelasticity” and Kink-Resistant Guidewires

Elasticity refers to the ability of a deformed material to get back to its original shape and size once the deforming forces are removed. But there is a limit to this elastic deformation and when subjected to an external force beyond this limit, the material undergoes a plastic deformation and thus gets irreversibly deformed. Metals like copper and steel can hardly tolerate 0.1 and 0.5% of deformation, respectively, before getting deformed irreversibly. Nitinol has the unique property of tolerating a deformation up to 12% and still revert back to the original form once the deforming force is removed.[2] This superelasticity property of nitinol is harnessed to manufacture kink-resistant guidewires and microcatheters so as to be able to negotiate the tight curves of cerebral vasculature and without getting permanently deformed.[10] [11] It is the austenite phase that exhibits this property of superelasticity. For nitinol to be in a phase at body temperature, the Af point should be below the reference temperature (body temperature). As stated previously, by altering the composition of alloy, the phase transition temperature can be altered as per our clinical requirement. As a thumb rule, by increasing the nickel content we can bring down the transition temperature to below the body temperature/room temperature so that at the time of insertion of the guidewire or catheter, nitinol will already have an austenite crystal form and would be ready to exhibit its superelastic properties. The nitinol alloy that is frequently used to make guidewires has an Af point below 20°C. That means when we use wires at body temperature, they are already in their full austenite form. At the highly tortuous vascular territories, the nitinol wires are subjected to a deforming force that transiently transforms the austenite into the deformed martensite phase, which immediately reverts back to the austenite phase and regains its original shape as it comes back to straighter anatomy.

“Shape Memory Effect” and Self-Expandable Stents

Another significant impact that nitinol has made in the field of endovascular surgery is the development of self-expandable stents.[12] These stents utilize both the superelasticity and the shape memory properties of nitinol. The nitinol component in the alloy is so adjusted that the phase transition temperature is around 30°C. This has the advantage that once the stent is deployed at the desired site, at body temperature (above the transition temperature), its microscopic crystal form gets spontaneously transformed to the austenite phase and the stent unfolds back to its original imprinted shape (shape memory effect). During packaging, the stents are brought down to a temperature much below the transition temperature (martensite phase), crimped, and packed inside a retractable sheath. Once the delivery system is at the exact site of placement and the stent is deployed, it comes in contact with blood. At body temperature, there occurs the thermal phase transition into the austenite phase and it expands radially and tries to attain its original shape spontaneously. In this attempt to achieve its nominal diameter, it gets apposed to the vessel wall with a constant outward radial force. In routine practice, the size of the chosen stent is larger than the diameter of the target vessel, so that the stent does not open up completely to its preset size.[13] This results in a continuous outward radial force that not only keeps the stents opposed to the wall but also does not let recoil force or external compressive force to cause any kind of kinking of the stent. So the self-expansion of nitinol stents is attributed to the shape memory feature. Since the stent is now at body temperature, the superelasticity feature of the austenite crystal form comes into action and thus makes the stent kink resistant. This feature is very essential when stents are deployed in superficial vessels, such as the common carotid artery at the neck, which are prone to deformation by external forces.

How Is the Shape Training Given to Nitinol?

The shape that nitinol remembers is actually the shape that it had in the austenite phase. This imprinting of the shape is done by sequential heating and cooling of the wire into the desired shape. Nitinol, in its austenite phase, is first given the desired shape and restrained on a mandrel. It is then forged in a furnace at a very high temperature of 500°C for 1 to 5 minutes.[14] This procedure is called “shape training.” The restrained wire is then cooled to room temperature and the restraining mandrel is removed. The nitinol wire is still in the austenite phase, but the shape it has at this point gets imprinted or memorized. The next step is to store the device, and the technique depends on the property of nitinol that we would like to harness. If we want our device to show its superelasticity property, then we deform the wire and load it in a sheath by exerting a constant restraining force. But if we want our device to exhibit its shape memory property, then it is straightened and stored below the phase transition temperature. When we want our device to recall its memory, either we just remove the restraining force (to show stress elasticity) or by heating it in the martensite phase to above the phase transition temperature (to show thermo-elasticity or shape memory effect).[15]

Can Nitinol Coils Be Used for Embolization of Aneurysms?

In real-life scenarios, the selection of platinum coils during embolization of an aneurysm sac is mostly guided by the experience and conviction of the treating surgeon. An appropriately sized coil, especially the first framing coil, should perfectly fit onto the inner wall of the sac and form a basket for the later coils to fill in. Selection of a larger coil might lead to rupture of the sac or herniation of a loop of coil into the lumen of the parent vessel. This plays in our mind while selecting coils and most of the times we tend to err on the smaller side. This leads to the use of a greater number of coils.

Imagine we had a 3D model of the aneurysm that is to be treated days later. We can use nitinol coils to determine the exact size that would perfectly fit into the sac in one go. In the laboratory, we have the liberty to err and with trial and error we can achieve the desired shape and the best possible packing density. Nitinol in its austenite phase is superelastic and hence can be manipulated into the desired shape. Once we are satisfied with the sac obliteration achieved in the model, the particular coil that was used shall undergo shape training and the shape that was achieved in the 3D model gets imprinted or memorized. The coil can then be cooled below the phase transition temperature into the martensite phase and loaded inside a delivery catheter, much like what we do in self-expandable stents. On the day of the actual procedure, the same coil is placed in the sac of the real aneurysm. As the coil shall come out of the delivery catheter, it would undergo phase transition into the austenite phase upon exposure to blood at body temperature. In the austenite phase, it recalls the shape that it had memorized a few days back in the laboratory and fits into the aneurysm sac like the way it had done in the 3D model. This way we can actually predict the coil size and shape and thus achieve a greater packing density with lesser number of coils. But of course, a true clinical utilization of this novel concept shall require the expertise of biomedical engineers and well-designed study models.

In-depth knowledge of the material at hand and its metallurgy has its own benefits. First, it helps us to choose the exact hardware that we need to bail ourselves out of the difficult clinical scenarios. Second, we can have a better two-way communication with our biomedical engineers. It will help them understand our necessities and accordingly invent devices that can be shifted from their laboratory tables onto our operating tables with greater ease.

Conclusion

Nitinol has revolutionized the field of minimally invasive procedures, especially endovascular ones. Knowledge of metallurgy is of paramount importance in selecting a proper device. The composition of the nitinol alloy should be planned as per the requirement. For making guidewires and catheters where the “superelasticity” feature is required, the nickel composition is increased so that the phase transition temperature is much below the body temperature and the alloy is in the austenite phase while entering the vessels. The austenite form makes these wires kink resistant and thus can easily negotiate the tortuous vascular territory without getting permanently deformed. But for self-expandable stents, the phase transition temperature should be around the body temperature so that the shape memory property can be utilized during the deployment of stents. The game of permutations between the two solid phases and the phase transition temperature makes nitinol a smart alloy and a trusted ally in our battle against cerebrovascular diseases.

Conflict of Interest

None declared.

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Publication History

Article published online:
21 May 2025