Hugh G. Willett and Erik Berndt

The use of shape memory alloys to manufacture implantable medical devices represents a high-growth portion of the medical materials market today. The ability of these specialized alloys to expand or contract provides significant opportunities for manufacturers to develop such surgical devices as stents, actuators, and fixators. However, maintaining the quality of such materials requires a careful evaluation of their properties and the environments in which they will be deployed.

Nitinol: The Most Popular Shape Memory Alloy

Nitinol is the most widely used shape memory alloy. Based on a combination of nickel and titanium, as shown in Figure 1, this unique material can undergo martensitic phase transformations induced by thermal or stress-based stimuli. This capability enables the material to display superelasticity, exhibiting recoverable strains as high as 10%—a level substantially higher than that of traditional alloys. Nitinol’s shape memory effect results directly from the reversible transformation in its crystalline structure. When exposed to environmental stimuli, it can change from its austenite to its martensite phase, as illustrated in Figure 2. The material also delivers high wear resistance.

In cardiovascular applications, nitinol is the material of choice for self-expanding stents because its elastic properties provide a mechanical scaffold that enhances vascular function. For example, nitinol stents can be expanded in place to support or help clear blood vessels. In orthopedic applications, it can be used to apply constant force to a fracture to promote faster healing and create interlocking intramedullary ‘nails.’ When cooled, such fasteners are surgically inserted into a cavity, whereby body heat causes them to bend into a preset shape and apply constant pressure in the axial direction of the bone. And because of its excellent malleability and ductility, nitinol can also be manufactured in the form of wires, ribbons, tubes, sheets, and bars.

The shape memory behavior of nitinol depends on the material’s composition and on the hot and cold processes that are used to manufacture it. The alloy exhibits a range of force/elongation behaviors that correlate directly with its exposure to different temperatures. In addition, Joule heating is typically used to electrically actuate nitinol-based actuators. To prevent unintended actuation by ambient heating, however, the environment surrounding the materials must be controlled.

When used as implantable-grade materials, shape memory alloys cannot be biotoxic, and they must also be corrosion resistant in order to withstand the harsh environment inside the body. Nickel is among the least toxic of shape memory alloys, while titanium has long been used in implantable devices because of its biocompatibility and the ease with which it can form a stable oxide surface layer.

Characterizing and Testing Shape Memory Materials

Shape memory alloys are subject to different types of fatigue. Structural fatigue, which is not unique to shape memory alloys, is caused by the accumulation of microstructural damage during cyclic loading. Cyclic loading, in turn, results in the initiation and propagation of a crack that eventually results in fracturing and catastrophic failure. Functional fatigue, in contrast, occurs when the alloy does not fail structurally but loses its ability to undergo a reversible phase transformation because of a combination of applied stress and/or temperature. For example, the working displacement in an actuator decreases with increasing cycle numbers.

Because all treatment processes and even environments can fundamentally change the behavior of shape memory alloys, the materials must be characterized and tested. The basic test methods for nitinol are described in ASTM F2516. While a standard has not yet been finalized for radial compression tests, an ASTM task group is engaged in the development of a draft. As the standard undergoes development, ASTM F2081 and ISO 25539 are often employed as substitutes.

The scope of testing for implantable devices made from shape memory alloys is driven by the applications for which they are intended. In the area of vascular stents, for example, platforms capable of measuring radial compression under controlled temperatures are required. The capacity of stents based on shape memory alloys to withstand radial compression forces is a key indicator of how well the product will perform its intended function, which typically involves mechanically holding a vessel open to dramatically improve blood flow.

Laser Extensometry

Because of the complex geometry of the test specimens and a stroke length on the order of millimeters for many tests, noncontact extensometers, such as the videoXtens and laserXtens from Zwick/Roell AG, can be utilized to calculate strain, as shown in Figures 3 and 4. Extensometers can detect changes in elongation as small as a fraction of a millimeter. Depending on the wall thickness of the sample, mandrels can be used to avoid collapsing the tube.

Product development engineers charged with the design of stents evaluate materials, geometrical mesh patterns, and production processes to arrive at a final design, such as that illustrated in Figure 5. The resulting product must exit the production process bearing precisely the material properties required in order to perform properly.

Stents must also exhibit an acceptable lifespan following surgical implantation. To determine a stent’s lifespan, numerical simulations using sophisticated CAD tools are employed as inputs. It is necessary to input mechanical properties into the CAD models to establish testing requirements. To characterize specimens as small as a few centimeters in length and thereby support the product development cycle, vascular stent manufacturers utilize testing equipment suitable for low forces, in combination with specialized grips.

measuring strain presents another. Some materials are so delicate that they can be damaged or stressed when they are tested using a contact-based extensometer. For example, nitinol’s superelasticity results from the imposition of stress. The use of a contact-based extensometer can stress the material, resulting in measurement inaccuracies.

In contrast, noncontact laser extensometers provide high precision, noncontact measurement of strain for delicate specimens that undergo extremely small changes in elongation. Implantable medical devices manufactured using shape memory alloys particularly benefit from testing using noncontact extensometry because their complex geometries can make conventional strain measurement difficult. In addition, such technology does not require measurement marks on the sample.

Laser extensometers utilize the unique structure of a specimen’s surface as a fingerprint to generate a virtual measurement mark. A laser directed at these measurement positions is reflected in various directions corresponding to the surface structure, creating a specific pattern of speckles. Selected measurement points are tracked and converted into direct extension values. The change in the surface structure, which is the basis for the speckle pattern, is continuously evaluated during specimen deformation. While allowing test labs to characterize materials, components, and subassemblies in quality control and R&D applications, this approach also supports tests on small specimens with small gauge lengths, which require high strain-measurement accuracy.

Conclusion

Because shape memory alloys exhibit properties that are heavily influenced by the processes they undergo during manufacturing, they must undergo mechanical testing. Evolving methods for gripping fragile test specimens and for calculating strain in applications that involve only millimeters of stroke are enabling design engineers and quality managers to make accurate measurements in this challenging field.

Hugh G. Willett is a technology journalist who frequently publishes articles on medical device technology and quality assurance testing. Reach him at [email protected].

Erik Berndt is the industry manager, medical, for Kennesaw, GA–based Zwick/Roell AG and head of the company’s competence center for the medical device industry. A mechanical and textile engineer, he was previously head of R&D for compression and immobilization products at Paul Hartmann AG, a provider of medical textile products for wound care, surgical, and patient diagnostic applications. Reach him at [email protected].