Addressing the Challenges of Working with Nitinol

Nitinol describes a family of roughly 50% nickel and 50% titanium alloys that display unusual superelastic and shape memory properties where small compositional changes around this 50:50 ratio make dramatic changes in the operating characteristics of the alloy. Slightly nickel-rich alloys result in the effect known as "superelasticity" and it is this phenomenon that is used in the vast majority of medical applications.

FIGURE 1: The elastic strain range for superelastic nitinol compared to 316 stainless steel

Superelastic nitinol exhibits comparatively large yet fully recoverable strains. Although the term "elastic" is used to describe the effect, it is not, in fact, elastic in the true metallurgical definition of the word. Rather it is a martensitic phase transformation that results in spontaneously recoverable macro-deformations. The stress over which this transformation occurs is approximately constant and therefore a plateau is observed in the stress/strain curve. Figure 1 shows a typical stress/strain curve for nitinol when compared to 316 stainless steel (another popular choice for medical devices). If processed correctly, the nitinol may exhibit recoverable strains as high as 8.0%. The unique mechanical behaviour of nitinol and apparent biocompatibility has resulted in a number of interesting and often unique medical applications. Unfortunately, the same properties and effects that make it such a useful material for medical devices also make it difficult to process.

Understanding the precise processing routes for the alloy, however, offers a great deal of opportunity for optimising the performance of medical devices that use the alloy. Nitinol can be "shape set" by constraining the material in the desired final shape and heat treating the material at temperatures that are typically between 400 and 550°C. These shape setting, heat treatments essentially control what is known as the Af temperature, above which the nitinol displays the superelastic effect. For medical devices therefore, the Af temperature must always be below body temperature to exhibit superelastic properties. However, the lower the Af temperature, the higher the plateau stress shown in Figure 1. For instance, a final product that has an Af of for instance, 5°C, will have a considerably higher plateau stress and feel "stiffer" than a product that has an Af of 20°C. This property is often exploited in the manufacture of self-expanding nitinol stents to optimise the compliance of the stent with the vessel wall once deployed. It may also be exploited in guidewires to change the feel for the clinician, i.e. a stiffer feeling guidewire or a more compliant feeling guidewire.

FIGURE 2: A flexible cutting blade is suitable for arthroscopy, ophthalmic applications and minimally invasive therapies such as natural orifice translumenal endoscopic surgery.

In the past 10 years, nitinol has found widespread application in the cardiovascular industry, particularly in peripheral self-expanding stents, guidewires and embolic protection and filter devices. However, its application in other clinical areas has been limited by the difficulties associated with forming and working it. Machining nitinol is challenging and a barrier to wider device application, particularly in some specific clinical areas such as orthopaedics where standard machining processes such as turning and milling are essential. The superelastic properties of the alloy and high degree of work hardening, often results in high rates of wear on the tooling and poor workpiece quality. Localised heating can destroy the superleastic properties, stress induced phase transformations at the tool tip can lead to poor chip breaking on the workpiece and high work hardening rates can blunt the tool tip very quickly. Developed by Caragh Precision, a new process known as "MemoryCut" has been developed to address these issues.

An example of a nitinol product developed by the Caragh Precision Innovation Centre for one of the company's clients using the "MemoryCut" process is a superelastic cutting blade, which is much like a flexible cutting scalpel. Designed for application in arthroscopic orthopaedic procedures and minimally invasive surgery such as natural orifice translumenal endoscopic surgery, the blade is flexible and has been "shape-set" into a U bend configuration. The blade can be straightened for delivery through a device but will always tend to return to its U shape, unlike other alloys such as 316L stainless steel, which do not display this "superelastic" recovery.

FIGURE 3:  Scanning-electron microscope image of the nitinol blade edge

The precise nature of the blade edge is best viewed under a scanning electron microscope, Figure 3. A combination of electro-discharge machining, controlled heat treatments and chemical polishing result in a blade edge that is as sharp as a surgical scalpel yet highly flexible in the transverse direction.

The company also optimised the processing and shape setting of the blade to give variations on the feel and stiffness of the blade. Through optimisation of time and temperature, a blade has been produced that has an austenite transition finish temperature, Af of 4°C and another has been produced that has an Af of 24°C. As previously described, the blade with the higher Af temperature feels significantly more flexible than the blade with the lower Af, which feels stiffer. The feel of the blade can thus be optimised for both the clinician and the clinical indication for which it is intended.

Another clinical application that can utilise nitinol is spinal fixation, where rods and screws are used to stabilise the vertebrae of the spine. These rods are rigid and usually produced from titanium, which limits movement. The low effective modulus of nitinol, however, may provide a degree of flexibility and compliance with the bone and tissue. Through the development of the MemoryCut process, Caragh Precision has developed processes for prototype devices in this field. Some of the requirements include different diameter profiles, threaded sections, drilled and tapped holes.

FIGURE 4: Images of rod turned with threaded section, tapped hole and varying diameters along its length

“Since setting up the Innovation Centre, we have continually looked for new ways of doing things," says Innovation and Commercial Director, Richard Gribbons. "Our new nitinol capability and 'MemoryCut' processing, is an example where our ability to machine and process difficult materials such as cobalt-chrome alloys, can be applied to even the most challenging materials."  

Caragh Precision
Parkmore West Business Park,
Galway, Ireland
tel. +353 91 755 773
e-mail:  info@caragh.ie [8]
www.caraghprecision.com [9]