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Stainless steel is a class of special steel materials. The Cr content in this steel exceeds 12%, which makes it have strong corrosion resistance.
According to different corrosion resistance and strength requirements, according to the microstructure of stainless steel, it can be divided into austenite (C phase), ferrite (A phase), martensite (M phase), duplex (C+A phase, C+M phase, etc.) and precipitation hardening (M+ precipitated phase) and other types.
Medical stainless steel usually has good biocompatibility, good mechanical properties, excellent resistance to body fluid corrosion, and good processability. It is widely used in clinical medical implant materials and medical tool materials.
Medical stainless steel is widely used in:
In 1926, 18%Cr-8%Ni stainless steel (AISI304) was first used as an orthopedic implant material, and then it was also used in stomatology.
By 1952, AISI316 stainless steel containing 2% Mo was clinically applied and gradually replaced 304 stainless steel.
In order to solve the problem of intergranular corrosion of stainless steel, ultra-low carbon stainless steel AISI316L and AISI317L with good biocompatibility, mechanical properties, and better corrosion resistance began to be applied in the medical field in the 1960s.
At present, austenitic stainless steel represented by AISI316L and 317L is the most commonly used metal material for surgical implants.
Medical stainless steel is very different from ordinary industrial stainless steel.
Medical stainless steel needs to be done during production:
Therefore, the chemical composition requirements of medical stainless steel are very strict, such as:
The high density (about 718g/cm3), high strength (300~1000MPa) and high elastic modulus (about 200GPa) of medical stainless steel will be different from the mechanical properties of bone tissue. This will lead to insufficient matching of its mechanical compatibility, which will cause stress shielding effect, and easily lead to osteoporosis, bone resorption or bone atrophy.
At the same time, due to the lack of sufficient mechanical stress stimulation, bone tissue is not easy to form callus at the fracture site, and secondary fractures are prone to occur.
Medical stainless steel can have problems with corrosion or abrasion in biological environments.
The main corrosion form of medical stainless steel in the human body is crevice corrosion, followed by intergranular corrosion and pitting corrosion. In addition, fretting corrosion and stress corrosion cracking have also occurred.
In general, the longer the implant was used, the more severe the corrosion. Corrosion may have a strong impact on the mechanical properties and biocompatibility of stainless steel—not only will it affect the service life of the material or device, but it may also cause local necrosis and inflammation of the tissue around the implant due to metal leaching, resulting in inflammation , allergies and carcinogenic and other systemic reactions, which greatly affect the health of the host.
Medical austenitic stainless steel for implantation usually contains more than 10% Ni element to stabilize the austenitic structure of stainless steel.
A large number of clinical studies have proved that Ni is a potential sensitization factor to the human body. The common damage of Ni and its compounds to the human body is Ni contact dermatitis, which has a high incidence rate, and eczema occurs in those with strong allergies.
In addition, the enrichment of Ni ions in organisms may induce toxic effects, cell destruction and inflammation, and cause teratogenic and carcinogenic hazards to organisms.
In order to avoid adverse tissue reactions caused by the dissolution of Ni element, the research and development of medical low-Ni and Ni-free austenitic stainless steel has become a major development trend of medical stainless steel in the world.
The principle is to use cheap N element (or the combined effect of N and Mn) to replace the expensive Ni element in stainless steel to stabilize the austenite structure of stainless steel, so that stainless steel continues to maintain its excellent mechanical properties, corrosion resistance and biological properties. academic performance.
Why choose N elements?
Compared with Ni element. N element is both economical and harmless to the human body, and the addition of N can significantly improve the mechanical properties and corrosion resistance of stainless steel.
With the increase of N content in medical stainless steel, the strength of stainless steel is greatly improved, which is more than twice the strength of traditionally used 316L or 317L stainless steel, reaching the strength level of medical Co-Cr alloy, while the plasticity of stainless steel remains at a high level.
The research results show that, compared with the 316LCr-Ni stainless steel currently used clinically, the high-N Ni-free stainless steel exhibits better anti-platelet adhesion ability and longer dynamic coagulation initial coagulation time (25%-60% higher ). With the increase of N content in stainless steel, its anticoagulant performance is better.
The research results show that the mechanical properties and biological properties of high N Ni-free stainless steel and medical Co-Cr-Mo alloy (Co62-Cr28-Mo6, the remainder is Ni, etc.), we draw the following conclusions:
The mechanical properties of high-N Ni-free stainless steel are similar to those of Co-Cr-Mo alloys, but its pitting resistance and blood compatibility are significantly better than those of cobalt-chromium-molybdenum alloys, showing higher pitting points and longer dynamic Initial clotting time (about 25% higher) and better anti-platelet adhesion properties.
A large number of clinical studies have shown that compared with the medical 316L or 317L stainless steel widely used in clinical practice, high-N Ni-free austenitic medical stainless steel has more excellent mechanical properties, corrosion resistance, wear resistance and corrosion fatigue resistance. Performance, better biocompatibility, lower material cost, and good processability.
Because it does not contain Ni elements with potential toxic side effects, it has great application advantages as human implant materials, and will significantly improve the long-term use safety of medical metal implant materials.
Surface modification can not only effectively improve the corrosion resistance and wear resistance of medical stainless steel, but also further improve its biocompatibility, and even make the surface bioactive.
The methods currently applied to the surface modification of medical stainless steel mainly include: surface alloying, ceramicization, functionalization and other surface coating treatment technologies.
These technologies are mainly for the surface modification of stainless steel implants and stainless steel cardiovascular stents for hard tissues such as bones and teeth.
Coating a layer of polymer film or covering a layer of endothelial cell membrane with anticoagulant gene on the surface of stainless steel cardiovascular stent can improve the biological characteristics of the stent, effectively reduce thrombus formation, and improve the blood compatibility of the stent.
After a layer of diamond-like film was coated on the surface of the stainless steel cardiovascular stent, the amount of metal ion dissolution from the filmed stent decreased significantly. In addition, the drug-coated coating on the surface of stainless steel cardiovascular stent has achieved clinical application.
Ion implantation technology can not only improve the surface hardness and wear resistance of metal materials, but also improve the corrosion resistance and biocompatibility of medical metal materials.
The corrosion resistance and biocompatibility and hemocompatibility of stainless steel implants can be improved by preparing bioinert or active coatings on the surface of stainless steel substrates. Among them, diamond-like films, tantalum nitride films, silicon carbide films, hydroxyapatite coatings, polymer coatings, fiber coatings, biomimetic coatings and biological glass-ceramic coatings have been studied more.
Using bio-inert materials such as alumina, zirconia, and titanium nitride as the coating material on the surface of stainless steel can prevent the dissolution of harmful ions in the stainless steel matrix and inhibit the corrosion of the matrix, making it more biocompatible than the stainless steel matrix. sex.
At present, ceramic coating materials for metal implants that combine well with human bone tissue and soft tissue, such as hydroxyapatite and bioglass ceramics, have been used clinically. (updated in 2019)
In the clinical medical process, implanted medical devices must go through a series of very strict sterilization processes before use, but these measures cannot completely reduce the possibility of patients being infected by bacteria. As a common postoperative complication, bacterial infection caused by medical devices has also become an urgent problem to be solved in the field of medicine in the 21st century.
Related infections caused by implanted medical devices usually require long-term dependence on antibiotics or even multiple operations to be cured, which brings great pain to patients both mentally and physically.
Research and development of biomedical materials with anti-bacterial infection function, so that it has long-term automatic sterilization function, thereby reducing the infection process, reducing the infection rate, and reducing the use of antibiotics, has important clinical significance and broad application prospects.
In the late 1990s, Japanese steel companies took the lead in researching and developing stainless steel with antibacterial function in the world.
Japan’s Kawasaki Iron & Steel Co., Ltd. [40] first announced the development of Ag-containing antibacterial stainless steel R304-AB, R430-AB, R430LN-AB, the sterilization rate of Escherichia coli is above 99%, showing excellent antibacterial properties.
Japan’s Nisshin Steel Co., Ltd. has developed three series of Cu-containing antibacterial stainless steel NSSAM, lNSSAM, and NSS3 with good manufacturing and processing performance and antibacterial performance, which are effective against common bacteria such as Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Salmonella. Strong killing effect.
