Nitinol machining and finishing for medical devices
18 Jan 2024
Nitinol
machining and finishing are the final steps on this nickel-titanium alloy’s journey from the earth’s crust into the hands of doctors and bodies of patients as medical devices.
We’ve previously covered how medical nitinol is mined and melted into raw material and then processed into nitinol wires, tubes and sheets for device manufacturers to use in medical devices, components or parts.
Laser cutting nitinol tubes and sheets is the most common way to manufacture nitinol medical devices, components and parts. Laser cutting is used for common products, including stents, heart valves, inferior vena cava (IVC) filters and most other nitinol implants.
More advanced than fiber lasers, femtosecond lasers can cut medical grade nitinol with ultra precision without heating the alloy, which is sensitive to heat changes.
Other nitinol machining and manufacturing methods include:
Electric discharge machining (EDM): Used to turn large nitinol bars or plates into a shape such as a bone staple for orthopedics
Chemical etching: Corrosive chemicals to eliminate material, for example to make blades for surgical equipment
Wire winding: Nitinol wires are wound by hand or machine one at a time to make nitinol shape memory springs
Wire braiding: Similar to wire winding, except nitinol braiding weaves or braids multiple nitinol wires together by hand or machine for devices such as clot retrievers, stents and flow diverters
Computer numerical control (CNC) machining: Conventional CNC machining operations — turning, milling, lathing, etc. — are possible with nitinol, but very difficult without specialized techniques for this particular alloy and its properties.
Joining: Nitinol can be joined with nitinol or other metals such as stainless using welding, soldering, coining or riveting.
Laser ablation: Similar to laser cutting, laser ablation removes much smaller bits of material but leaves the bulk of the material, similar to Swiss turning for small, precision parts.
Grinding: Profile and centerless grinding are good methods of nitinol manufacturing because nitinol maintains its straightness and can be ground with precision, resulting in components like guidewires.
3D printing: Additive manufacturing methods with nitinol such as laser sintering are in very early stages
Medical nitinol shape setting and finishing
After laser cutting — or an alternative machining process — comes deburring (if needed) and then shape setting.
Nitinol shape setting uses a tooling fixture to compress or expand the nitinol into the desired final form, and heat from a salt bath, fluidized bed or furnace to treat the nitinol so it maintains that shape.
The heat applied in shape setting also affects the special shape memory and superelastic/pseudoelastic properties of nitinol. Different heat treatment temperatures and times will yield different properties, giving a stent radial force to resist crushing, for example, or allowing a minimally invasive implant to compress down into a catheter for delivery and then expand when placed inside the patient. The nitinol’s fatigue durability is another property that is affected by heat.
Shape setting is also a stage where nitinol can be given different colors and shades (like you might get by anodizing titanium) with different applications of time, temperature and atmosphere in the furnace. A traditional anodization process can also be used to give nitinol a colored finish, similar to what you might do with aluminum.
After shape setting, surface finishing for oxide removal is a necessary step for biocompatibility. This step prevents the harmful effects of long-term nickel exposure to the human body. There are benefits and drawbacks to different surface finishing treatments for nitinol.
Electropolishing and mechanical polishing can ensure biocompatibility and also remove blemishes that could become fatigue nucleation sites.
Electropolishing is a great way to achieve biocompatibility by removing surface oxide while softening sharp edges and creating smooth surfaces. Electropolishing doesn’t have the fatigue drawbacks of pickling or chemical polishing.
Pickling (or etching) uses acid to clean the surface, removing the oxide. Pickling can be used in a batch process by dipping parts into an acid bath to remove oxide for biocompatibility. However, pickling will leave microscopic changes to the surface topography that could introduce the risk of fatigue cracking. But that same surface topography could also enhance adhesion of polymer coatings on devices such as guide wires.
Chemical polishing is a similar acid bath batch process. It leaves a smoother surface than pickling for better fatigue resistance, but still offers some topography for coating applications.
Finally, visual and dimensional inspections verify the quality of the finished nitinol parts, components or devices.
More about medical nitinol manufacturing and devices:
What is nitinol and where is it used?
Medical nitinol manufacturing: How this nickel-titanium alloy is made for medical devices
Medical nitinol processing: How NiTi is turned into wire, tubes and sheets for medical devices
How Medtronic uses nitinol to improve the structure and effectiveness of heart devices
How J&J Medtech’s Cerenovus Embotrap stent retriever thrombectomy treats ischemic strokes
Why Affera’s cardiac ablation technology is worth $1B to Medtronic
After recall and relaunch, Medtronic wants to go global with its Harmony valve
Heart failure startup aims for nitinol shape memory breakthrough
Alcohol and nitinol needles could make renal denervation for hypertension faster and simpler
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