Patient sensitivities to materials are rising and regulatory scrutiny continues to increase, causing the medical design community to search for alternatives to common stainless steels or cobalt chrome molybdenum alloys for new medical devices. According to the Center for Devices and Radiological Health, an estimated 12% to 15% of the population in the United States is sensitive to nickel. Under the new EU Medical Device Regulation (EU MDR, 2017/745), medical devices that contain >0.01% cobalt are required to indicate on the device or a warning label the presence of cobalt as a potential CMR (carcinogenic, mutagenic, reproductive toxin) substance.
Parallel to this, production techniques are advancing, furthering the use of additive manufacturing 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 or tailored surface finishes to enhance osseointegration.
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 cleared designs for implantable medical devices and high strength surgical instruments. Its corrosion resistance, strength, fatigue, and non-magnetic 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.
- Suitable for large-scale production of exceptionally high-strength medical components via additive manufacturing.
Performance Compared to Conventional Medical Device Materials
Exposure to nickel ions released from the normal wear of medical implants with commonly used alloys can lead to adverse side effects. Although cobalt containing alloys have a long history of safe clinical use, exposure to cobalt ions released due to progressive wear and tear of implants has been reported to be carcinogenic and neurotoxic. 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 commonly used medical alloys.
(Note, full chemistries not listed; please reference appropriate specification sheets.)
|BioDur 108||BioDur 316LS||BioDur 734||BioDur CCM|
|Chromium (Cr)||19 - 25||17 – 19||19.5 – 22.0||26 – 30|
|Nickel (Ni)||--||13 – 15||9.0 – 11.0||1.0 max|
|Molybdenum (Mo)||0.5 – 1.5||2.25 – 3.00||2.0 – 3.0||5.0 – 7.0|
|Nitrogen (N)||0.90||0.10 max||0.25 – 0.50||0.25 max|
|Manganese (Mn)||21 – 24||2.00 max||2.00 – 4.25||1.00 max|
In addition to a chemistry composition optimized around patients, BioDur 108 also exhibited improved mechanical properties over BioDur 316LS and BioDur 734. As seen in Table 2, both corrosion resistance and mechanical strength were higher with the nickel-free alloy. Eliminating cobalt content in situ as a replacement for BioDur CCM, the high-strength and fatigue properties are maintained, inferred by the close relationship between strength and fatigue in austenitic alloys. Only moderate levels of cold work are necessary to drive up the Yield Strength and Ultimate Tensile Strength of BioDur 108.
Table 2: Typical mechanical properties of wrought bar product
|BioDur 108||BioDur 316LS||BioDur 734||BioDur CCM|
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), the fatigue limit observed was approximately 380 MPa (55 ksi), or about 41% of the ultimate strength.
Biocompatibility and non-magnetic performance are prerequisite for medical device applications. BioDur 108 passes various biocompatibility tests and is non-magnetic in all conditions and essentially free of ferrite phase, thus compatible to magnetic environments, such as MRI scanners.
Material for Additive Manufacturing
Additive manufacturing of BioDur 108 presents a unique opportunity to achieve advanced mechanical properties not possible through additive using traditional stainless steel materials. BioDur’s combination of optimized powder chemistry and customized printing parameter sets achieves strength properties far exceeding ASTM F3184 AM minimum requirements.
Tensile mechanical properties were in-line 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. When comparing AM material properties, as shown in Table 3, BioDur 108 offers a greater than 50% 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% cold worked 316L properties.
Table 3: Typical mechanical properties of additively manufactured components
in BioDur 108 and the commonly used 316L stainless
|As-Printed||HIP + Anneal||As-Printed||HIP + Anneal|
|X and Y||Z||X and Y||Z||X and Y||Z||X and Y||Z|
|Tensile Strength, KSI||168||159||147||142||102||95||93||88|
|0.2 Yield Strength, KSI||143||128||83||85||80||71||41||41|
|Reduction Area, %||24||20||50||45||77||81||63||71|
|Hardness||34 HRC||34 HRC||25 HRC||24 HRC||93 HRB||94 HRB||84 HRB||80 HRB|
While metal allergies remain a diagnostic challenge for the aging population and increasing use of medical implants, an increase in material complications could be expected. Many medical device companies are seeking alternatives for materials optimized around patient outcomes that also enable them to capitalize on additive production. BioDur 108 offers a solution to other stainless steel options to be considered where nickel, cobalt, corrosion resistance and strength are a concern for the next generation of medical implant and instrumentation designs.
More about BioDur 108
Register for an upcoming webinar about BioDur 108 solutions for medical devices. Watch a video on BioDur 108 or our other low-cobalt stainless alloys to meet changing regulations.
Gaurav Lalwani, Ph.D. is Global Applications Engineering Lead – Medical at Carpenter Technology.