Shape Memory Alloys – Nitinol Wire for Medical Applications

Fort Wayne Metals specializes in supplying medical grade wires and cables, but also produces wire products from different grades of Nitinol or Nickel-Titanium that are utilized in medical applications. The company supplies all grades of Nitinol either as straightened super-elastic (SE) wire or in the cold worked condition set for heat treatment.

Nitinol Bend and Free Recovery Testing: Manual Recovery Method
Video credit: Fort Wayne Metals / YouTube

Nitinol Materials

Fort Wayne Metals has also developed a novel technique called Dynamic Plateau Strength (DPS) that produces special mechanical characteristics for Nitinol. This process is also available to customers. For customers opting for extremely smooth surface, the Silk™ Nitinol Wire provides a suitable option as it acquires a surface texture of 3µin RMS without electro or mechanical polishing.

All grades of Nitinol materials contain almost equal atomic weight percentage of titanium and nickel. NiTi#1, the most common medical grade from Fort Wayne Metals, surpasses chemistry requirements specified by ASTM F2063 for application in surgical implants.

Fort Wayne Metals inspects all incoming Nitinol raw materials in order to ensure that they fulfill internal specifications for ingot transformation temperature, chemistry, microstructure, and material homogeneity. All wire products use an advanced single and multi crystalline diamond drawing die technology and are optimized to induce a smooth and homogeneous surface finish quality.

When the Nitinol is subjected to a proper super-elastic heat treatment, it can accommodate strain up to 8% without any permanent kinking at body temperature. Mechanical properties of super-elastic alloys and shape memory alloys are listed in Tables 1 and 2. Table 3 shows the typical chemistry of Nitinol #1 for surgical implants.

Table 1. Super-elastic alloys – mechanical properties

  CW = (Cold Worked) SE = (Super-Elastic )
Product Ingot As (°C) UTS (psi) Elongation(%) UTS (psi) Elongation (%) Loading Plateau (psi) Unloading Plateau (psi) Active Af (°C)
Niti#1 -35 to -10 200,000 min >4% 180,000 min > 10% > 70,000 > 20,000 +10 to +18
Niti#2 -45 to -15 250,000 min >4% 210,000 min > 10% > 80,000 > 35,000 0 to +18
Niti#3 -80* 200,000 min >5% 190,000 min > 10% > 100,000 > 60,000 -15*
Niti#4 -10 to +10 220,000 min >4% 180,000 min > 10% > 65,000 > 7,000 +14 to +22
Niti#9 ≤ -35 220,000 min >4% 160,000 min > 10% > 75,000 > 25,000 ≤0

Permanent Set < 0.5% after strained to 8%

Notes : Results are typical for round wire diameters from 0.001" (0.025 mm) to ≤ 0.040" (1.016 mm). All Mechanical testing conducted at 22°C ± 2°C

*Typical target

Table 2. Shape memory alloys – mechanical properties

  CW = (Cold Worked) SM = (Shape Memory)
Product Ingot As(°C) UTS (psi) Elongation(%) UTS (psi) Elongation (%) Loading Plateau (psi) Active Af (°C)
Niti#5 ≥ +85 220,000 min >3% 160,000 min > 10% N/A ≥ +85
Niti#6 +35 to +85 220,000 min >3% 160,000 min > 10% N/A +40 to +80
Niti#8 +10 to +35 220,000 min >3% 160,000 min > 10% N/A +22 to +40

Permanent Set < 0.5% after strained to 8%

Notes : Results are typical for round wire diameters from 0.001" (0.025 mm) to ≤ 0.040" (1.016 mm). All Mechanical testing conducted at 22°C ± 2°C

Table 3. Nitinol #1 - typical chemistry for surgical implants

  ASTM F2063 FWM NiTi#1 (Average)
Nickel 54.5-57.0 wt% 56.0 wt%
Carbon 500 ppm 330 ppm
Oxygen 500 ppm 280 ppm
Hydrogen 50 ppm 25 ppm
Titanium Balance Balance

Applications of Nitinol Wire

Fort Wayne Metals’ Nitinol wire is utilized in a wide range of industrial and medical applications. Table 4 lists a few typical applications for each grade of Nitinol.

Table 4. Nitinol wire applications

Grade Application
Nitinol #1 Guidewires, stents, stylets, forming mandrels, stone retrieval baskets, orthodontic files, etc.
Nitinol #2* Ideal for applications that require a high loading and unloading plateau stresses at room temperature.
Nitinol #3** Applications requiring increased stiffness.
Nitinol #4 Provides the best cycling performances at body temperature (37°C), for example in orthodontic arch wires.
Nitinol #5 A common application for this alloy is high temperature actuators.
Nitinol #6 A common application for this alloy is high temperature actuators.
Nitinol #8 Ideal for applications that require a phase transformation at body temperature (37°C).
Nitinol #9 For use in very cold temperature environment applications.

*Typical chemical composition: ASTM F2063 Exc. Cr 0.2% to 0.3%
**Typical chemical composition: ASTM F2063 Exc. Co 1% to 2%

Nitinol Product Forms

The Nitinol product comes in various forms, such as rectangular, square, flat wire, round wire, stranded cables, and in a wide range of cross-sectional wires. Minimum thickness is 0.0003" and maximum width is 0.200. In addition, composites are produced with a clad jacket or Nitinol core.

Certification Data Package

Certification data package for medical grade Nitinol wires are as follows:

  • Chemical analysis
  • UTS-elongation
  • Fully annealed ingot transformation measured by DSC
  • Upper and lower plateau stress (SE only)
  • Multiple test temperatures available i.e. 22°C ±2°C or 37°C ±2°C
  • Wire transformation measured by Bend and Free Recovery (SE only)
  • Multiple gauge length and crosshead speeds
  • Permanent set after straining to 6% or 8% strain (SE only)

Surface Finishes

Surface finishes include dark oxide, light oxide and black oxide (diamond drawn surface); pickled; etch; and etched and mechanically polished. In etch finishing, the oxide layer is removed chemically to retain a smooth surface, while in pickled finishing, the oxide layer is removed chemically along with a small amount of base metal. This results in a surface with a rough texture.

In etched and mechanically polished process, chemical removal is carried out followed by mechanical polish, rendering the surface with stainless steel appearance; however, micro scratches will be visible at more than 40x magnification.

Conclusion

In addition to supplying medical grade Nitinol materials, Fort Wayne Metals also provides additional services such as continuous eddy current analysis, wire cleaning (alcohol wipe or hot alkaline), metallurgical lab testing, rotary beam fatigue testing, electron dispersive spectroscopy (EDS), transmission electron microscopy (TEM) , scanning electron microscope (SEM), X-ray diffraction (XRD), and auger electron spectroscopy (AES).

About Fort Wayne Metals

Fort Wayne Metals is a recognized leader in the research, development and production of fine grade medical wire. Our expertise lies in stainless steel, titanium and titanium alloys and specialty alloys such as Nitinol.

Our headquarters are in Fort Wayne, Indiana. We also have a production facility in Ireland as well as warehouse and sales facilities in Castlebar, Ireland to serve the European market.

Fort Wayne Metals is taking an active role in ASM International, a society dedicated to the exchange of material information. We are also active in ASTM, specifically the F-4 committee, which has authored many standards for medical wire.

Fort Wayne Metals

This information has been sourced, reviewed and adapted from materials provided by Fort Wayne Metals.

For more information on this source, please visit Fort Wayne Metals.

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