Understanding nitinol’s characteristics and challenges is essential for innovating with this uniquely useful medical device material.
By Matt Wanke, Cirtec Medical
By identifying the common mistakes and challenges of working with nitinol, engineers can better understand the steps they must take to properly handle the material, ensuring a smooth manufacturing process that will produce high-quality medical devices. [Image courtesy of Cirtec Medical]
It can return to a predetermined shape when heated above a certain temperature, which is useful for applications requiring precise actuation and control, including medical stents and actuators. Nitinol’s compatibility with biological tissues minimizes the risk of adverse reactions, making it suitable for medical implants like heart valve frames and devices like guidewires. Nitinol also exhibits excellent fatigue resistance, making it useful for springs, flexible couplings and other components subject to cyclic loading.
Processing nitinol into complex shapes and structures with manufacturing techniques such as laser cutting and additive manufacturing allow for innovative designs and customized solutions.
But to use nitinol effectively, engineers must navigate various complexities, including material selection, design optimization, manufacturing processes and stringent biocompatibility requirements. Proper handling and knowledge of these challenges are crucial for leveraging nitinol’s full potential for device innovation.
1. Failing to understand nitinol’s properties
One of the primary challenges in utilizing nitinol is ensuring a complete understanding of its material properties. Unlike many materials, not all nitinol is created equal. Variations in composition and processing can result in differences in flexibility, fatigue resistance and overall performance.
Some nitinol alloys are engineered to be more flexible, while others are designed to have greater strength or fatigue resistance. It is crucial for engineers to thoroughly understand these variations to ensure that the selected nitinol can withstand the specific demands of their project. This is particularly important regarding fatigue life (the number of stress cycles a material can withstand), which is critical in medical applications where devices undergo repetitive stress cycles and thermal effects.
Those working with nitinol must also pay attention to the grain structure on a flat surface before shaping the material. If nitinol is bent or shaped against the grain, it can potentially break during formation.
It is crucial to be wary of the percentage strain because it directly affects the material’s performance and longevity. The shape memory effect — which allows nitinol to return to a pre-defined shape when heated — is sensitive to the amount of strain applied during shaping.
Exceeding the recommended strain limits can lead to permanent deformation or loss of shape memory properties. To mitigate this, the austenite finish (AF) temperature must be dictated, which would also need to be tested before sending it out using the bend and free recovery (BFR) test method as described in ASTM F2082.
2. Creating nitinol microcracks with thermal effects
Thermal effects during the laser cutting and welding of nitinol can impact the performance and durability of the final product. These processes generate heat that can create heat-affected zones (HAZ) where the material properties are altered, often leading to the formation of microcracks.
The temperature above which nitinol is fully in its austenitic phase and exhibits super-elasticity is typically between 20°C to 40°C (68°F to 104°F). However, during laser cutting or welding, the intense heat applied to the nitinol can cause microcracks within the HAZ.
Microcracks can compromise the integrity and longevity of the device. If not addressed, microcracks can lead to premature failure under the cyclic loading conditions that are typical in many medical applications.
When cutting, femtosecond lasers are employed to reduce the HAZ, as femto cutting is considered cold cutting. Laser welding uses pulse-shaping techniques to control the laser’s energy output and reduce the amount of heat over time, thereby mitigating negative thermal effects.
Pulse-shaping involves carefully controlling the laser’s energy output over time to reduce the amount of heat introduced to the part. By delivering the laser energy in controlled pulses rather than a continuous stream, the HAZ can be minimized, reducing the risk of microcracks.
3. Failing to properly finish and treat the surface
Another critical challenge in the utilization of nitinol in medical device engineering is the oversight of surface finish and treatment. Surface treatment methods can further mitigate failures from microcracks.
Electropolishing smooths out microcracks, reduces surface roughness and enhances the overall biocompatibility of the device by removing surface contaminants. To electropolish a nitinol component, it is immersed in an electrolyte solution while direct current is applied to dissolve a thin layer of material from the surface.
Another treatment is chemical etching, which uses chemical solutions to selectively remove material from the surface of nitinol. Chemical etching can effectively eliminate microcracks, debris and other surface imperfections, improving the device’s mechanical properties and fatigue resistance. Chemical etching also allows for precise control over the material removal process, making it suitable for intricate designs and fine-tuned adjustments.
4. Getting in over your head with nitinol manufacturing processes
Maintaining nitinol’s integrity and performance requires proper handling and expertise in specialized manufacturing processes.
Comprehensive quality control measures will detect and address defects during manufacturing. Non-destructive testing methods such as ultrasonic testing, eddy current testing and visual inspections can identify issues before they lead to failures. Establishing stringent inspection protocols and regularly monitoring the manufacturing processes can ensure prompt correction of deviations to maintain the high quality and reliability of nitinol components.
Given the complexities involved in working with nitinol, continuous learning and improvement are vital. Collaborating with material scientists, process engineers, and industry experts can provide valuable insights and support in developing effective manufacturing processes. Proper documentation is also important to preserve this knowledge in a structured, accessible manner.
By identifying the common mistakes and challenges of working with nitinol, engineers can better understand the steps they must take to properly handle the material, ensuring a smooth manufacturing process that will produce high-quality medical devices.
Matt Wanke [Photo courtesy of Cirtec Medical]
How to submit a contribution to MDO
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design & Outsourcing or its employees.
