Nickel-rich nickel-titanium alloys known as Nitinol have what many medicaldevice applications require: superelasticity. Unfortunately, this same property makes Nitinol difficult to process.

exible cutting blade for arthroscopy, ophthalmic applications and minimally invasive therapies such as Natural Orifice Translumenal Endoscopic Surgery (NOTES).

Caragh Precision is addressing this challenge with its MemoryCut suite of machining capabilities –CNC milling, CNC turning, and EDM–for shape-memory materials such as Nitinol.

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%. The unique mechanical behavior of Nitinol and apparent biocompatibility has resulted in interesting and often unique medical applications.

Nitinol can be “shape set” by constraining the material into the desired final shape and heat treating the material at temperatures between 400°C and 550°C. These shape-setting heat treatments control what is known as the Af temperature, above which the Nitinol displays the superelastic effect. For medical devices, the Af temperature must always be below body temperature to exhibit superelastic properties.

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

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 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 selfexpanding Nitinol stents to optimize 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.

During 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 it.

Machining challenges

The challenges associated with machining Nitinol can limit broader applications in 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 result in high rates of wear on the tooling and poor workpiece quality. Localized 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.

An example of a Nitinol product developed by the Caragh Precision Innovation Centre for a client using the MemoryCut process is a superelastic cutting blade, much like a flexible cutting scalpel. Designed for application in arthroscopic orthopaedic procedures and minimally invasive surgery such as Natural Orifice Translumenal Endoscopic Surgery (NOTES), the blade is highly 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.

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