The Vital Role of Stainless Steel in Medical Devices and Instruments

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Stainless steel is an integral part of many industries thanks to its unique combination of strength and oxidation resistance. The medical field, in particular, uses different stainless-steel alloys for various specialized instruments, implants, surgical tools, and other applications. 

The main reason stainless steel is so important in the medical industry is its inertness. Stainless steel is non-corrosive and won’t react with body fluids, making it bio-compatible and generally safe for the human body. Moreover, stainless steel has a smooth and non-porous surface, meaning it is very easy to sterilize, a crucial factor that can prevent infections.

However, not all stainless-steel alloys are created equal, as most can’t be used in the medical field. Notably, regular stainless steel has a low carbon concentration and a high chromium concentration (more than 10.5%), which gives rust-free properties by reacting with the oxygen in the air and creating a stable bond. Crucially, this bond will be renewed even if the stainless steel is damaged.

However, medical-grade stainless-steel alloys also utilize nickel and molybdenum, which produce higher stiffness and hardness while retaining the alloy’s excellent corrosion resistance properties.

Medical-Grade Stainless Steel Alloys

Both austenitic and martensitic steels are used in the medical industry. Austenitic stainless steels offer superior corrosion resistance thanks to the high chromium and nickel content, though at the expense of strength and hardness. Meanwhile, martensitic steels offer higher strength and hardness but have lower corrosion resistance, ductility, and weldability.

Austenitic Stainless Steel Alloys in the Medical Industry

Austenitic stainless steel alloys have a face-centered cubic structure and are characterized by the high content of chromium and nickel. They are mainly used in corrosive environments and are highly formable and weldable.

300 Series austenitic stainless steels are the most widely used in the medical industry, including type 316 and type 316L. These alloys have a very high concentration of chromium (16-25%), which creates the chromium oxide layer, and nickel (20%), providing corrosion resistance and good formability. Stainless steel 316 and 316L also contain manganese and nitrogen for improved strength.

Notably, type 316 stainless steel also contains molybdenum (2-3%), which creates an even higher corrosion resistance in acidic environments and chlorides. Crucially, this allows operators to thoroughly clean and sterilize the instruments without worrying about degrading the material. As such, this alloy is widely used for surgical instruments like scalpels, forceps, retractors, and medical equipment requiring rigorous sterilization, like autoclaves and sterilizers.

Meanwhile, type 316L stainless steel is designed to be more weldable thanks to the miniscule carbon content (below 0.03%). This reduces carbide precipitation and, consequently, makes the weld more resistant to corrosion. The result is a highly bio-compatible alloy that can be used in orthopedic implants, such as bone plates and joint replacements, or cardiovascular devices, such as stents.

Martensitic Stainless Steel Alloys in the Medical Industry

Martensitic stainless steel alloys have a body-centered tetragonal crystal structure and a higher carbon content, which gives them higher strength, surface hardness, and wear resistance than their austenitic counterparts.

400 Series martensitic stainless steel alloys are typically used in the medical field, with the most common grades being 410, 420, and 440. These alloys have a chromium content of 12-14%, which provides good corrosion resistance, though in less invasive environments.

Still, the strength and hardness of these alloys make them an integral part of the medical industry, particularly for producing parts that require sharpness. For instance, type 410 stainless steel is used for dental tools, scalpels, clamps, and other cutting instruments.

Meanwhile, type 420 stainless steel, thanks to its higher carbon content, provides even higher hardness and wear resistance than type 410, making it ideal for high-precision surgical tools, such as scissors, blades, and cutters.

In addition, type 440 stainless steel has a higher chromium content (16-18%), which provides it with a higher corrosion resistance than type 410 and 420. However, type 440 also has a higher carbon content, enabling higher surface hardness and wear resistance. As such, type 440 stainless steel also finds use in manufacturing precision instruments.

Type 630 stainless steel (17-4 PH) is another martensitic stainless steel used in the medical industry. It can be precipitation hardened to achieve higher yield strength and contains roughly 17% chromium and 4% nickel, copper, and niobium. 

Thanks to its unique combination of strength, biocompatibility, and corrosion resistance, 17-4 PH has emerged as the material of choice for producing orthopedic implants, such as artificial joints, spinal fusion devices, and bone screws. However, its excellent hardness and wear resistance are also helpful in producing surgical and dental tools.

Conclusion

Thanks to the combination of corrosion resistance, biocompatibility, strength, hardness, and durability, stainless steel is a vital alloy in medical devices and instruments. 

In particular, the usage of medical-grade austenitic and martensitic stainless steel alloys will continue to grow, especially considering the latest advancements in body implants. 

Therefore, as medical science progresses, stainless steel will see an even more significant role in supporting medical treatments and patient care.

Ashton Henning