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The Importance of Transformation Temperature Testing of Nickel Titanium Wire used in Medical Devices, Part 3

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Controlling the Heat Treatment Process 

Heat treatment of Nitinol alloys is dependent on a variety of factors, and controlling and understanding their connection to each other is vital for repeatability of the process. Analyzing the wire manufacturer’s certification to assess ingot transformation temperatures is the first step in identifying the correct heat treatment for the application.

For medical devices, the industry standard for finished device transformation temperatures is below 22°C (typical operating room temperature). This is due in part to physician’s preference of seeing devices in their “active” state as soon as they are taken out of the package.

Temperature

Choosing the correct temperature for heat treatment is primarily based on the desired active Af temperature. Industry-wide, the generally recommended final heat treatment range is between 475°C and 550°C, though there are processes that may require a departure from this range. USBD process engineers have extensive experience in developing heat treatment specifications to target specifically desired Af temperature ranges and are able to select the appropriate process conditions for your application.

The tighter the control of the temperature during processing, the more repeatable the process will be. It is also important to ramp up quickly to the heat treatment temperature, and then after the appropriate dwell time, quench as fast as possible – water quench is recommended when possible. This maximizes the repeatability of the entire heat treatment process.

Tension

In the matter of tension, too much or too little will have an important impact on the final device performance. Not enough tension and the final shape of the stent will be compromised; too much tension incurs the risk of wire fracture during processing. Constant and controlled tension is key to the repeatability of the process.

Tooling

When selecting tooling for heat treating braided nitinol, consideration must be given to managing temperature ramp and quench time to achieve repeatable results. Consequently, material type and mass ratio are key components when designing forming mandrels. 

Equipment

The appropriate heat treatment equipment will maximize heat transfer to the mandrel and will ramp up to the appropriate temperature as fast as possible. Controlling this stage of heat-treatment is critical to the finished properties of the stent and for process repeatability.

When direct contact between the media and nitinol component is possible, a fluidized bed furnace or salt bath is the most recommended types of heat treatment equipment, due to excellent heat transfer and temperature control.

However, due to the fluidization of the media, not all braided stent-like device designs may be treated using a fluidized bed – especially fragile structures with ultra-fine wire. In such cases, an air furnace or vacuum furnace are often used in the industry.

Air furnaces provide an excellent non-contact heating modality for braided stent structures. Care must be taken in regards to appropriate airflow and minimizing ambient air contact through opening/closing of the furnace door.

Mandrels should not touch any of the walls or the floor of the furnace, as it could overheat a particular area of the nitinol braid and possibly degrade the overall performance.

Vacuum furnaces may be used to prevent/ minimize oxidation of the wire. However, nothing short of a “perfect” vacuum will prevent oxidation on the surface of the wire, negating the very reason to utilize this type of furnace. The ramp up time to temperature is a complicated step process under vacuum and quenching is much slower than optimal.

The slightest trace of oxygen will still oxidize non-passivated nitinol surfaces due to the high reactivity of titanium. Also, oxidization can change the surface color of nitinol alloys. Medical devices are sensitive to physicians’ perception, and color differences on a batch of nitinol devices may raise concerns about efficacy.

Heat treatment of braided nitinol devices is often accomplished in stages. An interim temperature treatment can be performed to relieve stress in the braid construct during processing. Additional treatments may be performed between interim forming or welding phases. A final heat treatment at higher temperature is used to set the final device properties. As previously discussed, transformation temperatures will generally increase during each heat treatment. The device manufacturer needs to exercise caution when applying multiple heat treatments so that the final device does not have a transformation temperature that is too high for the application.

Read the pdf here