As patient sensitivities to materials rise and regulatory scrutiny increases, the medical design community is searching for alloy alternatives to common stainless steels or cobalt chrome molybdenum for new medical devices. According to the FDA Center for Devices and Radiological Health, an estimated 12–15 percent of the population in the United States is sensitive to nickel. It is difficult to accurately predict patient susceptibility to metal hypersensitivities, even in those with an established metal-allergy history pre-implant. Additionally, under new EU Medical Devices Regulation (EU MDR, 2017/745), medical devices that contain >0.01 percent cobalt are required to indicate on the device or a warning label to the presence of cobalt as a potential CMR (carcinogenic, mutagenic, reproductive toxin) substance.

Parallel to this, production techniques are advancing, and many device manufacturers are investigating additive manufacturing (AM) to capitalize on the possibilities of 3D printing for bionic designs, tailored mechanical properties, and mass customization. The versatile nature and high accuracy of powder bed fusion additive manufacturing allows for near-perfect replicas for patient matching implants, tailored surface finishes to enhance osseointegration, and geometric designs not previously achievable via traditional manufacturing methods.

BioDur® 108 can be used in FDA-approved designs for implantable medical devices and high-strength surgical instruments. (Credit: Carpenter Technology)

In response to these needs, a new alloy has emerged as a suitable alternative. BioDur 108 (ASTM F2229, UNS S29108) is an essentially nickel- and cobalt-free stainless alloy used in FDA-approved designs for implantable medical devices and high-strength surgical instruments. Its corrosion resistance, strength, fatigue, and nonmagnetic properties are advantageous to the medical design community looking for alloy solutions that are:

  • Suitable for patients with metal sensitivities, such as nickel.

  • Cobalt-free to address the increasing regulatory scrutiny with the EU MDR up-classification of cobalt as a Class IB RMR substance.1

  • Suitable for large-scale production of exceptionally high-strength medical components via additive manufacturing.

Performance Compared to Conventional Medical Device Materials

Although cobalt-containing alloys have a long history of safe clinical use, exposure to cobalt ions due to design deficiencies may lead to physiological complications. (Credit: Carpenter Technology)

Used for orthopedic, spine, trauma, and instrumentation applications, alloys such as BioDur 316LS (aka 316LVM) stainless, BioDur 734 stainless, and BioDur CCM (aka cobalt chrome molybdenum) are commonly utilized in the medical device industry. Exposure to nickel ions released from the normal wear of medical implants can lead to adverse side effects, such as local inflammation, aseptic loosening, and device failure. Although cobalt-containing alloys have a long history of safe clinical use, exposure to cobalt ions due to design deficiencies may lead to physiological complications. However, as seen in Table 1, the chemistry makeup of these alloys commonly used in the industry contain a substantial amount of either nickel or cobalt.

Table 1. Typical chemistries of BioDur 108, BioDur 316LS, BioDur 734, and BioDur CCM. BioDur 108 contains essentially no nickel for a patient-friendly alternative to traditionally used materials.

With a maximum value of 0.05 wt% of nickel and statistically irrelevant cobalt content, BioDur 108 is an alternative option for the medical device design community to meet or even exceed device performance previously achieved with traditional alloy systems.

In addition to a chemistry composition optimized around patients, BioDur 108 also exhibits improved mechanical properties over BioDur 316LS and BioDur 734. As seen in Table 2, both corrosion resistance and mechanical strength are higher with the essentially nickel-free alloy. As a replacement for BioDur CCM with cobalt content below EU MDR up-classification, the high-strength and fatigue properties are maintained, inferred by the close relationship between strength and fatigue in austenitic alloys.

Table 2. Typical mechanical properties of wrought bar product.

As noted in Table 2, only moderate levels of cold work are necessary to drive BioDur 108 up to 250 ksi yield strength (YS) and 300 ksi ultimate tensile strength (UTS). Although many applications exist that invoke the alloy’s capability for customization of properties, typical mechanical property targets for the industry are outlined in ASTM F2229.2

In studies with rotating-beam fatigue tests conducted on specimens prepared from annealed bar stock with an ASTM #5 grain size and an ultimate tensile strength of 930 MPa (135 ksi),3 the fatigue limit observed was approximately 380 MPa (55 ksi), or about 41 percent of the ultimate strength. Fatigue data for heavily cold worked BioDur 108 is not yet complete, but results are expected to be favorable with custom fatigue tests for specific applications available and ongoing. In a corrosion resistance study,4 BioDur 108 was calculated to have a pitting resistance equivalency number (PREN) of 31, compared to 30 for BioDur 734 and 28 for BioDur 316LS, where a higher number indicates better corrosion resistance.

Biocompatibility and nonmagnetic performance are prerequisite for medical device applications for which BioDur 316LS, BioDur 734, or BioDur CMM are currently used. Cytotoxicity, irritation, acute systemic toxicity, pyrogenicity, mutagenicity, implantation with histopathology, and hemocompatibility were all externally tested in BioDur 108. Each of these biocompatibility tests resulted in favorable results for the alloy.5 Nonmagnetic in all conditions and essentially free of ferrite phase, BioDur 108 is compatible in magnetic environments, such as MRI scanners. Ferrite certification testing is routinely performed and the results included in material test reports to give designers the confidence to innovate with new material solutions.

Additive Manufacturing Using BioDur 108

Typically, cold worked raw material feedstock is used in traditional subtractive manufacturing processes where high mechanical properties are required. Additive manufacturing offers design and production advantages over established subtractive manufacturing that designers are eager to capitalize on, but material performance cannot be sacrificed. Additive manufacturing of BioDur 108 presents a unique opportunity to achieve advanced mechanical properties not possible through additive using traditional stainless-steel materials, such as 316LS or 17-4.

Carpenter Technology has developed a combination of optimized powder chemistry and tailor printing parameter sets to achieve strength properties meeting 20 percent CW BioDur 108, compared to 48 percent CW 316L in an AM component without cold working. This far exceeds ASTM F3184 AM minimum requirements. High-pressure, nitrogen-atomized BioDur 108 powder demonstrated good processability in laser powder bed fusion (L-PBF) systems with high density (>99.8 percent) and no observed cracking. Several post processing conditions were explored, including as-built, stress relieved, annealed, hot isostatic pressed (HIPed), and HIPed plus annealed conditions. In most heat-treated conditions, the material was nearly 100 percent austenite to avoid electromagnetic interference with other medical equipment.

Table 3. Typical mechanical properties of additively manufactured components in BioDur 108 and the commonly used 316L stainless

Tensile mechanical properties were inline with ASTM F2229 wrought minimum values for Condition A (annealed) and as-built and stress relieved properties were in-line with Condition B cold work tensile properties (150 ksi ultimate tensile strength). When comparing AM material properties, as shown in Table 3, BioDur 108 offers a greater than 50 percent increase in strength over 316L while demonstrating superior corrosion resistance, nickel removal, and an expected increase in fatigue strength. BioDur 108 in the as-built or stress-relieved condition also has comparable ultimate and yield strength to 50 percent cold worked 316L properties.

High-pressure nitrogen-atomized BioDur 108 powder demonstrated good processability in laser powder bed fusion (L-PBF) systems with high density (>99.8 percent) and no observed cracking. (Credit: Carpenter Technology)

Conclusion

While medical implant allergies remain a diagnostic challenge, the aging population and increasing use of medical implants, an increase in allergy-related complications could be expected, and many medical device companies are seeking alternatives for materials optimized around patient outcomes. Yet optimized chemistries for new alloys only provide so much success to designers if they cannot be utilized with the most advanced production techniques available. BioDur 108 offers a solution to other stainless-steel options to be considered where nickel, cobalt, corrosion resistance, and strength are a concern. BioDur 108’s similarity to other accepted alloys and its own precedence of acceptability in several FDA approved medical applications, along with recent advancements in additive manufacturing can enable next generation of medical implant and instrumentation designs.

Reference

  1. Cobalt Institute
  2. ASTM F2229 , “Standard Specification for Wrought, Nitrogen Strengthened 23 Manganese-21Chromium-1Molybdenum Low-Nickel Stainless Steel Alloy Bar and Wire for Surgical Implants (UNS S29108)".
  3. ASM Tech Spotlight
  4. Properties of an Essentially Nickel-Free Stainless Alloy for Medical Implants,” BioDur 108 Alloy Carpenter Technology white paper.
  5. BioDur 108 Datasheet

This article was written by Gaurav Lalwani, PhD, This email address is being protected from spambots. You need JavaScript enabled to view it., Global Applications Engineering Lead – Medical, and Raymond DeFrain, This email address is being protected from spambots. You need JavaScript enabled to view it., Regional Metallurgist for Carpenter Technology, Philadelphia, PA. BioDur® is a registered trademark of CRS Holdings, Inc. For more information, click here .


Medical Design Briefs Magazine

This article first appeared in the October, 2020 issue of Medical Design Briefs Magazine.

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