Nickel-free austenitic stainless steels for medical applications
Stainless steel and surgical steel (yes, there's a difference between the two) are very hygienic metal alloys and are considered hypoallergenic, making them suitable for a variety of different applications, including the production of non-stick stainless-steel cookware sets and biomedical applications, such as surgical tools and instruments.
However, for individuals with nickel allergies, stainless steel alloys aren't as safe as previously thought. In this article, we'll discuss nickel-free austenitic stainless steel for medical applications and other substitutes for more typical types of stainless steel.
Stainless Steel and Nickel
Stainless steel is a type of steel that contains, in addition to iron and carbon, also contains a high percentage of chromium and relatively reasonable nickel content. In fact, more than 70% of global nickel production is used to produce stainless steel.
The addition of chromium is what makes the steel stainless, as the alloying element bonds with the oxygen from the environment, preventing the chemical reaction between iron and oxygen. Additionally, the reaction between chromium and oxygen creates a thin passive layer of chromium oxide which prevents further oxidization of both the base metal and alloying elements.
However, it's the addition of nickel that allows stainless steel to become such a versatile metal alloy. In addition to marginally enhancing the corrosion resistance granted by chromium, nickel allows stainless steel to form and weld more easily and remain ductile at very low temperatures.
These properties make chromium-nickel stainless steel suitable for a very wide range of hygienic applications, such as the production of non-stick cookware sets such as pots and pans and various types of medical equipment such as tubing and surgical tools.
They're also suitable for producing industrial hardware such as leveling feet, but also stainless steel handle assemblies, and various types of knobs. However, the nickel content is known to cause allergic reactions within the human body, which is why they're not best suited for medical implants.
The Negative Effects of Nickel
Before we dive into the negative effects of nickel, it's important to discuss the corrosion-resistant properties of stainless steel. "Corrosion resistant" isn't the same as "corrosion-proof;" all stainless steel corrode and wears over time in the body's fluid environment.
Now, let's discuss nickel. When bonded with steel that develops a passive layer of chromium oxide, nickel poses no threat to human health. SAE 316L is a good representative of stainless steel used in orthopedics due to its desired properties.
However, it, too, is susceptible to corrosion within the human body, such as crevice, intergranular, and pitting corrosion, etc. This is a problem because not only can it lead to fracture or failure of a medical implant, but corrosion on its own may result in the release of harmful chemical elements into the body — including nickel.
Nickel is a known allergen, which may cause cutaneous inflammations on the skin (wedding, eczema, itching), allergy reaction, teratogenicity, and carcinogenicity in the human body.
Uses of Stainless Steel in the Medical Field
The healthcare industry has extremely precise criteria regarding the materials used to produce the instruments, equipment, storage, pipes, and other components that assist doctors and other medical workers keep their patients well.
Admittedly, very few materials are as perfectly aligned with the strict requirements of the medical industry as stainless steel. It's resistant to corrosion, it's non-porous, so it prevents bacterial buildup, and it can easily withstand extreme temperatures making it easy to sterilize.
Due to its versatility, stainless steel can be found in both patient care facilities and organizations and industries that serve the healthcare industry, such as the pharmaceutical industry and medical laboratories. Some of the common places you'll see stainless steel in use include:
- Surgical instruments
- Orthopedic implants
- Artificial heart valves
- Bone fixation equipment
- Prostheses
- Needles
- Catheters
- Wheelchairs and walkers
Benefits of Nickel-free Stainless Steel for Medical Applications
The negative effects of nickel in surgical stainless steel have come to the forefront of medical science, prompting those in the field of metallurgy to research nickel-free stainless steel replacements. As a result, researchers have been looking into high-nitrogen nickel-free austenitic steels, which are becoming an important medical metal material for various surgical and dental implants.
Nickel-free ferritic or martensitic stainless steel is ferromagnetic and has low ductility, even though nickel-free corrosion-resistant steels have been in development for decades now. However, since healthcare relies on non-magnetic instruments, austenitic stainless steel proved to be the best stainless steel for medical applications due to its non-ferromagnetic properties.
High levels of chromium, molybdenum, and nitrogen can be alloyed into austenitic stainless steel to make corrosion-resistant, nickel-free stainless steel. Although both carbon and nitrogen are strong and efficient austenite structure stabilizers, a minor carbon addition can reduce steel's resistance to corrosion and increase its propensity for precipitate formation.
As a result, less than 0.03% of medical stainless steel can contain carbon. Nitrogen proves to be the most effective and only appropriate element for austenitic phase stabilization in nickel-free austenitic stainless steels.
Properties of Nickel-free Austenitic Stainless Steel
Nickel-free austenitic stainless steel with high nitrogen content (over 0.90%) exhibits significantly stronger tensile strength than SAE 316L — it's actually 151.4% stronger than the conventionally used 316L. Besides increased strength, adding nitrogen also allows the alloy to develop very high levels of fatigue resistance.
Chromium, molybdenum, and nitrogen concentrations significantly correlate with austenitic alloy corrosion resistance. Nickel-free austenitic steel has high amounts of these elements, providing good resistance to pitting and crevice corrosion, which is essential for materials used in the production of numerous medical equipment.
Lastly, nickel-free austenitic steels have shown very high strength, ductility, and exceptional corrosion resistance, which have no negative impacts on cells growing directly on this type of steel. Furthermore, the austenitic structure would allow bone cells to penetrate the superficial passive layer of chromium oxide, resulting in better bonding and a more efficient biomechanical fixation responsible for the mechanical stability at the implementation site.
Summary
Nickel-free stainless steel for medical applications involves alloying the metal with other elements that would replace nickel contents in chromium-nickel steely alloys. If you want to learn more about Steel and its types, please visit Reid Supply, and refer to their Engineer Resource Guides or some of their guides on various types of steel, including stainless and surgical steel.